ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-16-2575-2016On the effectiveness of nitrogen oxide reductions as a control over ammonium nitrate aerosol PusedeS. E. DuffeyK. C. ShustermanA. A. https://orcid.org/0000-0002-4450-5161 SalehA. LaughnerJ. L. https://orcid.org/0000-0002-8599-4555 WooldridgeP. J. ZhangQ. https://orcid.org/0000-0002-5203-8778 ParworthC. L. https://orcid.org/0000-0003-4760-9481 KimH. https://orcid.org/0000-0001-6138-6443 CappsS. L. https://orcid.org/0000-0002-6872-6604 ValinL. C. CappaC. D. FriedA. WalegaJ. NowakJ. B. https://orcid.org/0000-0002-5697-9807 WeinheimerA. J. HoffR. M. BerkoffT. A. BeyersdorfA. J. https://orcid.org/0000-0002-4496-2557 OlsonJ. CrawfordJ. H. CohenR. C. https://orcid.org/0000-0001-6617-7691 Department of Chemistry, University of California Berkeley, Berkeley, CA 94720, USA Department of Environmental Toxicology, University of California at Davis, Davis, CA 95616, USA Center for Environment, Health and Welfare Research, Korea Institute of Science and Technology, Seoul, Korea Department of Mechanical Engineering, University of Colorado Boulder, Boulder, CO 80309, USA Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA Department of Civil and Environmental Engineering, University of California at Davis, Davis, CA 95616, USA Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA Aerodyne Research, Inc., Billerica, Billerica, MA 01821, USA Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80307, USA Department of Physics, University of Maryland Baltimore County, Baltimore, MD 21250, USA NASA Langley Research Center, Hampton, VA 23681, USA Department of Earth and Planetary Science, University of California Berkeley, Berkeley, CA 94720, USA now at: Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904, USA Ronald C. Cohen (rccohen@berkeley.edu)2March2016 16 4 25752596 23August2015 7October2015 17February2016 18February2016 This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/.pdf

Nitrogen oxides (NOx) have fallen steadily across the US over the last 15 years. At the same time, NOx concentrations decrease on weekends relative to weekdays, largely without co-occurring changes in other gas-phase emissions, due to patterns of diesel truck activities. These trends taken together provide two independent constraints on the role of NOx in the nonlinear chemistry of atmospheric oxidation. In this context, we interpret interannual trends in wintertime ammonium nitrate (NH4NO3) in the San Joaquin Valley of California, a location with the worst aerosol pollution in the US and where a large portion of aerosol mass is NH4NO3. Here, we show that NOx reductions have simultaneously decreased nighttime and increased daytime NH4NO3 production over the last decade. We find a substantial decrease in NH4NO3 since 2000 and conclude that this decrease is due to reduced nitrate radical-initiated production at night in residual layers that are decoupled from fresh emissions at the surface. Further reductions in NOx are imminent in California, and nationwide, and we make a quantitative prediction of the response of NH4NO3. We show that the combination of rapid chemical production and efficient NH4NO3 loss via deposition of gas-phase nitric acid implies that high aerosol days in cities in the San Joaquin Valley air basin are responsive to local changes in NOx within those individual cities. Our calculations indicate that large decreases in NOx in the future will not only lower wintertime NH4NO3 concentrations but also cause a transition in the dominant NH4NO3 source from nighttime to daytime chemistry.

 Introduction

Aerosol abundances are decreasing across the US, improving air quality and affecting climate. These decreases have been broadly attributed to regulatory controls on the emissions of gas-phase precursors; however, it has proven difficult to link precursor reductions to observed changes in aerosol concentration via specific chemical mechanisms. Thus, there is limited knowledge of how impacts will scale in the future.

We present an analysis to identify driving chemical mechanisms and to quantify the effects of large reductions in nitrogen oxides (NOx) (e.g., Russell et al., 2012; McDonald et al., 2012) on secondary aerosol chemistry. We take advantage of decreased NOx emissions on weekends compared to weekdays, which occur mostly without changes in other gas-phase emissions (e.g., Dallmann et al., 2012), and couple these weekday–weekend patterns to long-term NOx reductions (Pusede and Cohen, 2012). The effect is that weekday NOx levels equal weekend NOx years earlier in the record (Fig. 1). We use this NOx constraint to interpret trends in observed wintertime ammonium nitrate (NH4NO3) concentrations over the last decade in the San Joaquin Valley (SJV) of California.

Wintertime (November–March) NO2 columns (molecules cm-2) in the SJV using the UC Berkeley OMI BEHR retrieval (Russell et al., 2011). The urban NO2 plumes of Fresno, Visalia, and Bakersfield are labeled to the left of their respective cities in panel (a). Panel (a) shows 2005–2006 weekdays (Tuesday–Friday). Panel (b) shows 2005–2006 weekends (Saturday–Sunday). Panel (c) shows 2012–2013 weekdays. Panel (d) shows 2012–2013 weekends.

The SJV experiences the most severe aerosol pollution in the US (American Lung Association, 2014). From 2001 to 2013 there were on average 44 exceedances each winter (November–March) of the 24 h National Ambient Air Quality Standard (NAAQS) of 35 µgm-3 in the cities of Bakersfield and Fresno, with as many as 70 per winter early in the record. High aerosol in the SJV is generally limited to the winter months, with few exceedances occurring in other seasons. In the SJV, 30–80 % of wintertime aerosol mass is NH4NO3 and the remaining portion is mostly organic material (Chow et al., 2006; Chen et al., 2007; Ge et al., 2012). Characteristics of the wintertime SJV that are conducive to high aerosol abundances include shallow boundary layers (Bianco et al., 2011); prolonged periods of stagnation (Smith et al., 1981); and large emissions of NOx (NOx ≡ NO + NO2), ammonia (NH3) (Goebes et al., 2003; Clarisse et al., 2010), and organic aerosol (Ge et al., 2012). These conditions pose challenges to accurately simulating secondary aerosol in the region, as models need to represent bidirectional NH3 exchange (Gilliland et al., 2006; Flechard et al., 2010; Pleim et al., 2013), variable local meteorology, complex airflows, and vertical stratification in the rates of NO2 oxidation to NO3- (Heald et al., 2012; Walker et al., 2012; Kelly et al., 2014; Schiferl et al., 2014; Markovic et al., 2014).

In this paper, we take an observational approach, combining the decade-long record of speciated aerosol concentrations and of gas-phase precursors in the region with detailed measurements collected during the DISCOVER-AQ experiment (Deriving Information on Surface Conditions from COlumn and VERtically resolved observations relevant to Air Quality, 14 January–14 February 2013). We show that wintertime NO3-, which we treat as a measured surrogate for NH4NO3, has been dependent only on the NO2 concentration over the last 12 years. We calculate observationally constrained nighttime and photochemical NO3- production rates and show that measured trends in wintertime NO3- can be explained by decreased nitrate radical-initiated production in nocturnal residual layers, which are unmonitored layers of the atmosphere that are effectively separated from surface emissions at night. We test the impacts of forthcoming NOx emission controls on the probability of future NAAQS exceedances, showing that NOx reductions will not only decrease the frequency of high aerosol days but also shift both the timing and the oxidation mechanisms that drive NH4NO3 production.

 Results from observations

Trends in wintertime (November–March) 24 h NO3- versus daytime (10:00–15:00 local time) NO2 are shown in Fig. 2 in the cities Fresno and Bakersfield on weekdays and weekends for the period 2001–2013. The source of these observations, the methods used for collection, and measurement biases are discussed in Appendix A. Weekdays are defined as Tuesday–Friday and weekends are Saturday–Sunday. We expect carryover to have an effect on the interpretation, as concentrations of NO2 and aerosol are not only influenced by present day processes but also have some memory of processes occurring on the preceding day, especially in the winter when surface winds are slow and disorganized and horizontal transport is weak. We exclude Monday from weekdays for this reason but retain Saturdays to improve weekend statistics. As a result, weekend medians reported here might be slightly higher than would be observed due to weekend emissions alone. We define the day as beginning and ending at sunrise, since nighttime NO3- production builds from reactants present in the atmosphere during the preceding daytime hours.

Observed NO3- (µgm-3) in PM10 versus daytime (10:00–15:00 LT) NO2 on weekdays (closed circles) and weekends (open diamonds). Data are 3-year medians of wintertime (November–March) data in Fresno (a) (2001–2012) and Bakersfield (b) (2001–2013). There are an average of 41 weekday days and 18 weekend days point-1. Uncertainties in NO3- are ±2032 % µgm-3 on weekdays and ±3040 % µgm-3 on weekends for NO3- and less than ±9 % on weekdays and ±13 % on weekends for NO2 (see text for details). Slopes are 0.5 µgm-3 NO3- ppb-1 NO2 in Fresno and 0.64 µgm-3 NO3- ppb-1 NO2 in Bakersfield and are calculated using a weighted linear least squares fit with errors assumed in both the x and y; weights are computed as counting errors derived from the number of observations.

In the wintertime SJV, persistent stagnant conditions are common and punctuated only by infrequent cold fronts accompanied by strong winds that remove accumulated pollution from the basin. Stagnation events are a few days to multiple weeks in duration, and, during these stable periods, surface winds are slow and disorganized, controlled largely by surface heating, with limited horizontal mixing (Smith et al., 1981). On any individual winter day, air stagnation and planetary boundary layer height are the dominant controls over gas and aerosol concentrations; however, considering the data separately by weekday and weekend and then comparing year-to-year changes instead draws attention to the effects of emissions and subsequent chemistry.

By this method, the NO3- mass concentration is observed to have depended on the previous day's daytime NO2 concentration with a sensitivity of 0.5 µgm-3 ppb-1 NO2 in Fresno and 0.64 µgm-3ppb-1NO2 in Bakersfield (slopes in Fig. 2). Uncertainties in the NO3- concentration are computed as counting errors, with N as the total number of wintertime data points (3-year average), and are ±20 % on weekdays and ±30 % on weekends. Errors in NO2 are computed in the same way and are less than ±9 % on weekdays and ±13 % on weekends in both Fresno and Bakersfield. We interpret the positive x intercept in Fig. 2 as consistent with the known low bias in NO3- measurements (Appendix A) and the shorter wintertime atmospheric lifetime of NO3- than NO2 (Sect. 4). Uncertainty estimates, including a low NO3- measurement bias of 25 %, are ±2032 % µgm-3 ppb-1 NO2 on weekdays and ±3040 % µgm-3 ppb-1 NO2 on weekends. One ppb NO2 corresponds to 2.56 µgm-3 NO3- after oxidation (at 25 ∘C and 1 atm); thus the observed correlation corresponds to a decrease in NO3- mass that is 20 % of the NO2 decrease. While the full budget for wintertime NOx loss is beyond the scope of this paper, Fig. 2 implies that on average in the wintertime, 20 % of each day's NOx emissions are converted to NO3- in 1–2 days.

The key idea is that present-day NO3- concentrations on weekdays are equal to what were seen on weekends a decade ago, i.e., the NO2 dependence of NO3- has been unchanged with time. This suggests that in the wintertime average, the only source of NO3- in the atmosphere has been oxidation of NO2 and that NH4NO3 production has been nitrate rather than ammonium limited. Agreement of NO3- in different years at identical NO2 implies that there has been little change over time in the chemical mechanism producing NO3- and hence NH4NO3.

Additional evidence comes from observations made during DISCOVER-AQ, in which the sum of gas-phase nitric acid and aerosol-phase NO3- (NO3-(g+p)) was measured onboard the NASA P-3B on six research flights with almost identical flight patterns (Fig. 3a). See Appendix A for a description of the NO3-(g+p) measurements and DISCOVER-AQ experiment. Comparing the spatial distribution of NO3-(g+p) within the fully developed (afternoon) boundary layer (see Appendix A for the boundary layer filtering procedure) to NO2 observed from the satellite (Fig. 1) and NOx from onboard the P3-B (Fig. 3b) suggests that NO3-(g+p) better follows spatial patterns in NO2 than gas-phase NH3 (Fig. 3c), the precursor of particulate-phase NH4+.

NO3-(g+p) (µgm-3) in PM2.5 (a), NOx (ppb) (b), and NH3 (ppb) (c) measured onboard the NASA P-3B below the fully formed afternoon boundary layer and at pressure altitude (a.s.l.) greater than 0 on the same days and averaged to a 0.05∘ × 0.05∘ grid.

In Fig. 3a, urban–rural gradients in NO3-(g+p) are steep. In Bakersfield, NO3-(g+p) was on average 18–20 µgm-3 near the city center, twice as high as just 20 km to the northeast. During DISCOVER-AQ, surface wind speeds were ∼ 2–3 m s-1 in the daytime (10:00–15:00 LT), ∼ 1–2 m s-1 in the morning (06:00–10:00 LT), and typically < 1 m s-1 at night. An air parcel moving within the surface layer at 3 m s-1 would require approximately ∼ 20 daytime h, equivalent to multiple days, to either reach Fresno from the upwind cities of Stockton or San Jose or to reach Bakersfield from Fresno. An additional transport mechanism is mixing by winds in nocturnal low-level jets, which are well documented in the SJV in the summertime (Bao et al., 2008). There are few measurements of these winds in the winter, but wind speeds of up to 1–8 m s-1 have been observed at 0.1–2 km a.g.l. (3 days of data), which are fast enough to mix species valley wide in 1–2 days (Chow et al., 2006). However, the measured spatial heterogeneity in NO3-(g+p) (Fig. 3a) indicates faster and/or more localized processes control a significant portion of the NO3- concentration in each city.

We infer from Figs. 2 and 3 that the oxidation of locally emitted NOx is the largest term affecting the production of NH4NO3, that NH3 is in excess, and that transport and mixing are too slow to fully homogenize the aerosol throughout the wintertime SJV.

 Chemistry in the dynamic near-surface atmosphere

Under the abundant NH3, low sulfur dioxide, high aerosol, and low temperature conditions found in the wintertime SJV, most NO3- is aerosol bound in the 24 h average and NH4NO3 abundances are driven by NO3- production (PNO3-). PNO3- occurs by distinct nighttime and daytime mechanisms, each of which is a nonlinear function of NO2.

Nitrate radical (NO3) is the most important nighttime oxidant (Brown and Stutz, 2012). It is formed via reaction of NO2 with O3 (Reaction 1). NO2+O3→NO3 NO3-initiated chemistry occurs mainly at night because NO3 photolyzes rapidly to NO2. After sunset, large NO emissions can titrate O3, altering the relative amounts of NO2 and O3 but conserving odd oxygen (Ox≡ NO2+ O3). NO3 radical production is a nonlinear function of NO2 for a given O3 concentration, increasing with NO2 at low NOx, maximizing when NO2 is equal to O3 at constant Ox, and decreasing at higher NOx, shown as NO2 (Appendix B, Fig. B1a).

In the evening, reduced sunlight diminishes the heating of the Earth's surface, leading to strong suppression of vertical mixing and the formation of a shallow nocturnal boundary layer (NBL). Between the NBL and the free troposphere, in the nocturnal residual layer (NRL), mixing is weak and further layering may occur (Brown et al., 2007). The initial concentrations of species in the NRL are determined by the concentrations observed at the point in time when the residual layer decouples from the NBL, around sunset. Afterwards, the strong surface inversion keeps fresh emissions from entering the NRL, yet vertical chemical gradients have been observed within layers (Brown et al., 2007). The NRL is seen by surface monitors in the morning when solar heating and turbulent mixing reincorporate what was the NRL into the growing daytime boundary layer (Fig. 4), a process that also alters the NRL composition. On nights when NO3 radical production in the NBL is 0 due to high NO emissions, NO3 chemistry may still be active in the dynamically decoupled NRL. Loss from the atmosphere is likewise affected by this vertical structuring, as deposition to the surface occurs during the daytime and during the night from the NBL but not during the night from the NRL. Figure 5 shows examples of enduring nocturnal structure seen via potential temperature, the vertical distribution of NO3-(g+p), and O3 during DISCOVER-AQ by the P-3B in the early mornings over Bakersfield. At least one NRL is apparent for each profile, evident in the potential temperature variability; however, due to a combination of extremely shallow surface inversions, intermittent NO3-(g+p) sampling, and science flight timing, it is unclear that the P-3B ever captured NO3-(g+p) concentrations in the NBL prior to the second flight circuit in the late morning (not shown) when significant atmospheric mixing had already taken place.

Simplified illustration of the diurnal evolution of the near-surface atmosphere as it relates to PNO3-. Boundary layer dynamics schematic adapted from Stull (1988).

Vertical profiles of NO3-(g+p) (µgm-3) in PM2.5 between 08:00 and 09:00 LT over the city of Bakersfield on four flights when visibility and air traffic permitted a missed approach. Altitude data are in units km a.g.l. The O3 (orange) and potential temperature (black) are also shown. The orange diamonds represent the mean O3 measured at the surface (08:00–09:00 LT).

Nitrate radical reacts with NO2 to form dinitrogen pentoxide (N2O5) and generally under atmospheric conditions of high NO2 and low temperature, N2O5≫ NO3 (Brown et al., 2009; Brown and Stutz, 2012). The lifetime of N2O5 to thermal decomposition to NO2 and NO3 is ∼ 10 min at 270 K. NO3 is also lost to reaction with certain organic species, especially compounds with unsaturated carbon–carbon bonds and aldehydes. When reaction times are long, for example during long, dark winter nights and when unsaturated hydrocarbon emissions are low, the most important loss of NO3 is via N2O5 uptake onto aerosols (e.g., Dentener and Crutzen, 1993; Macintyre and Evans, 2010; Wagner et al., 2013), whereupon N2O5 reacts with aerosol-phase water to give either two NO3- or, if NO2+ combines with Cl-, ClNO2+ NO3-. At sunrise, ClNO2 photolyzes within a few hours (Nelson and Johnston, 1981), releasing NO2 and Cl radical, the latter reacting rapidly with most gas-phase organic compounds. The heterogeneous loss rate of N2O5 is a function of the total aerosol surface area and of the fraction of gas-particle collisions resulting in N2O5 uptake. The latter is aerosol composition dependent, enhanced at higher aerosol water content (e.g., Hu and Abbatt, 1997; Hallquist et al., 2003; Thornton et al., 2003), reduced in the presence of organic coatings (e.g., Cosman and Bertram, 2008; McNeill et al., 2006), and inversely proportional to NO3- (e.g., Wahner et al., 1998; Hallquist et al., 2003; Bertram and Thornton, 2009; Wagner et al., 2013).

During the daytime, nitric acid (HNO3) is the gas-phase reaction product of the oxidation of NO2 and the hydroxyl radical (OH) (Reaction 2). NO2+OH+M→HNO3+M The production rate of HNO3 increases rapidly with increasing NOx at low NOx and converges at a limit set by the primary HOx (HOx≡ OH + HO2+ RO2) production rate at higher NOx. Major sources of HOx in the polluted troposphere are O(1D) + H2O, formaldehyde (CH2O), and nitrous acid (HONO). The functional form of the dependence of HNO3 production on NO2 (Fig. B1b) arises from the nonlinear effects of NOx on the OH abundance, as NOx both propagates and terminates the HOx catalytic cycle. Under the high NOx conditions of the wintertime SJV, HNO3 is the dominant daytime HOx termination product. Combined with excess NH3, partitioning to the aerosol phase is a function of ambient temperature and humidity.

Calculated wintertime median PNO3- (µg m-3 day-1) in the NRL versus daytime (10:00–15:00 LT) NO2 (ppb) in Fresno (a) and in Bakersfield (b). Calculations are shown separately for weekdays (closed circles) and weekends (open diamonds). Fresno and Bakersfield results include years 2001–2012 and 2001–2013, respectively. There was no significant difference between medians and means. The light blue line has a slope of 2.56, expected for unit conversion of NO2 to NO3- (ppb to µg m-3). The actual NO3- versus NO2 slope (gray line) is calculated using a weighted linear least squares fit with errors assumed in both the x and y and weights that are the counting errors derived from the number of observations. The direction of time can be inferred from the NO2 trends, as NO2 concentrations have generally decreased each year over the decade.

Trends in calculated wintertime PNO3- for the nighttime and daytime mechanisms, as constrained by the observations (calculations and data are described in Appendix B), suggest that PNO3- in the NRL is the largest source of chemistry that matches trends in NO3- (Fig. 2). Specifically, PNO3- in the NRL exhibits identical NO2 dependence as observed in the NO3- measurements – both over time and from weekday to weekend. In Fig. 6, the calculated annual wintertime daily-integrated PNO3- in the NRL is shown versus daytime NO2 on weekdays and weekends in Fresno and Bakersfield. PNO3- in the NRL has decreased by 0.9 µgm-3 day-1 ppb-1 NO2 in both cities, a rate approximately twice the trend observed in NO3- versus NO2. The total daytime PNO3- (Reaction R2), equal to the sum of PNO3- attributed to the HOx sources, O(1D) + H2O, HONO, and CH2O, has, by contrast, not significantly changed as a function of NO2 over the last decade. In fact, PNO3- linked to the HOx sources O(1D) + H2O and CH2O has increased by ∼ 5 % since 2001. PNO3- attributed to HONO has decreased since 2001 (Appendix B, Fig. B2), displaying comparable NO2 dependence to both measured NO3- concentrations (Fig. 2) and calculated PNO3- in the NRL (Fig. 6). However, we calculate that PNO3- in the NRL is approximately 5 times greater than PNO3- formed during the daytime from OH originating from HONO. In the NBL (not shown) PNO3- has increased from < 1 to 3–5 µgm-3 day-1 on weekdays and to 6–8 µgm-3 day-1 on weekends due to reduced NOx titration of O3 at sunset. Increases in NBL PNO3- are not reflected in Fig. 2, potentially because these changes have occurred within a small faction of the volume of the NRL and daytime boundary layer and because depositional loss from the NBL is not impeded. For example, for an NBL that is 10 % the NRL height, PNO3- in the NBL would need to exceed 3 times the NRL production in order to alter the daytime boundary layer concentration by 20 %, even if zero deposition is assumed.

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The concentration of NO3-(g+p) is a function of PNO3-, as well as loss and mixing. While high aerosol days in the SJV are in part attributed to persistent and severe stagnation, controls over the portion of aerosol mass that is NH4NO3 are more dynamic (e.g., Pandis and Seinfeld, 1990; Vayenas et al., 2005). In this section we show that the effects of loss and mixing on the NO3- concentration are consistent with observed NO3- trends over time, differences by day-of-week, accumulation rates during stagnation, and differences between Fresno and Bakersfield.

Time series of NO3- (µg m-3) in PM1 measured at the ground in Fresno during DISCOVER-AQ. Days are in blue and nights are in black. Select afternoon data (magenta) were fit to derive τNO3-.

On days when NH4NO3 exceeded 20–30 µgm-3, typical during stagnation periods, the diurnal variability of surface NO3- was characterized by a steep and substantial increase in NO3- in the morning, a slow decline through midday, and a rapid decrease in the afternoon (Fig. 7). On these mornings, the rise rate of NO3- was consistent with two PNO3- pathways: reincorporation of high-NO3- NRL air into the boundary layer and NO3- formed by daytime chemistry with HONO as the OH source. By contrast, OH-initiated production attributed to O(1D) + H2O and CH2O increased gradually throughout the day. At midday, OH-initiated PNO3-, atmospheric loss, and mixing by winds all play upon [NO3-]. In the afternoon, OH-initiated PNO3- was minimal due to attenuated evening radiation. On days exhibiting this pattern, the NO3- concentration was observed to decrease at a rate equal to 3.0 ± 1.3 µgm-3 h-1 (1σ), with individual rates determined as the slopes of a linear fit through the magenta data and identified as periods of steady decrease at least three hours long. Time windows were allowed to vary and the average window was 13:25–16:05 LT, spanning 10:30–18:30 LT. We use this afternoon rate of change, which is when loss dominates production and mixing, to derive the atmospheric NO3- lifetime τNO3-.

The atmospheric lifetime of aerosol in the boundary layer is determined by wet and dry deposition of aerosol, the wet and dry deposition of gases in equilibrium with aerosol, and mixing to the free troposphere where concentrations are much lower. Wet deposition occurs by interaction with rain, which scavenges aerosol and soluble gases, and leaves the valley relatively clear. Multi-day fog is common in the winter in the SJV (Holets and Swanson, 1981), enhancing NH4NO3 removal when fog leads to rain or drizzle (Jacob et al., 1986a, b), as inorganic ions readily partition into aqueous fog droplets (Waldman et al., 1982; Munger et al., 1983), but having little effect if fog dissipates. Fog has not been seen to accelerate the conversion of NO2 to NO3- in the SJV (Jacob et al., 1984).

There are few direct measurements of deposition rates of aerosol, HNO3, and NH3. HNO3 is theorized to deposit at a transport-limited rate, NH3 exchange is dependent on surface and meteorological conditions, and aerosol, especially smaller particles, to deposit slowly. PM1 to PM2.5 deposition velocities (υd) have been reported to be 0.001 to 0.1 cm s-1 (Sehmel, 1980; Slinn, 1982; Farmer et al., 2013), too slow to account for the observed afternoon loss rates in Fig. 7. To compute NO3- loss by deposition of gas-phase HNO3, HNO3(g) was modeled with ISORROPIA II (Nenes et al., 1998; Fountoukis and Nenes, 2007) run in forward mode, an approximation that was reasonable because during the wintertime temperatures were low, humidities were high, and NH3 was abundant. ISORROPIA II was initialized as [NO3- + HNO3] = [NO3-]AMS and [NH4++ NH3] = [NH4+]AMS. Calculated HNO3(g) was added back to [NO3-+ HNO3], while NH3(g) was added as 1.1 HNO3(g) (by mole) to ensure NH3 was in excess because we found NH4NO3 to be NO3- limited (Figs. 2 and 3). ISORROPIA II was solved iteratively until daytime HNO3(g) changed by < 2 % by mass. The phase state was set as metastable (e.g., Rood et al., 1989; Zhang et al., 2003; Vayenas et al., 2005). We assume that gases and aerosol are in equilibrium, that aerosols are homogenous and internally mixed, and that unaccounted-for factors do not influence the thermodynamics of the system (Vayenas et al., 2005). Calculated HNO3(g) mixing ratios were greatest in the afternoon (12:00–16:00 LT), at which time they were 2 ppb on average for the DISCOVER-AQ time period and up to 4–6 ppb on the warmest days only. In the 24 h average, HNO3(g) was 15 % of [NO3-+ HNO3] by mass and was 40 % (median) in the afternoon. High HNO3(g) was generally simultaneous with the magenta-highlighted NO3- data (Fig. 7). To compute the υd of HNO3, the equation ∂NO3-∂t=υdhC was solved, with ∂NO3-∂t equal to the observed afternoon loss rate on designated (magenta) days, C equal to the daily mean HNO3(g) over the same time windows, and h equal to the maximum boundary layer height (i.e., the afternoon height) visually identified according to aerosol backscatter estimates by a micro-pulse lidar (MPL) supplemented with a wide-field receiver system (Appendix A). In this way, we derived υd equal to 5 ± 2 cm s-1, in line with previous direct measurements of 1–10 cm s-1 (e.g., Huebert and Robert, 1985; Meyers et al., 1989; Sievering et al., 2001; Volpe Horii et al., 2005; Farmer et al., 2006) and constrained estimates of 6 cm s-1 (Vayenas et al., 2005). Given a υd of 5 cm s-1 (assumed constant), the hourly ∂NO3-∂t was computed for every hour of the day, with C equal to the time-varying HNO3(g) and h equal to the time-varying boundary layer height. We assume losses are from the entire boundary layer, see Appendix C for our reasoning. For the daily time-varying h: the NBL was estimated as 10 % of the maximum daytime boundary layer height measured by MPL; the morning increase was estimated as linear over 5 h and fully developed at 11:00; and the evening (18:00) collapse (also linear) was estimated as occurring in 2 h.

In this way and with respect to surface deposition alone, τNO3- was calculated to be 3 h (0.1 days) under daytime conditions. As a lower bound, if the true υd was at the slowest end of previous observations (1 cm s-1), then τNO3- would be 14 h under daytime conditions. Lifetimes in this range are shorter than typical stagnation periods, observed to be 5 ± 1.5 days (1σ) in both Fresno and Bakersfield (decadal average). By comparison, PM1–PM2.5υd yield τNO3- of 6–58 days. Such long lifetimes indicate the frequency of frontal passages controls the PM lifetime. Because the loss of NH4NO3 via HNO3 deposition is rapid and PNO3- is relatively large, high aerosol days are expected to be more responsive to changes in emissions than expected when considering loss only through particle deposition.

There are other observational constraints that an account of aerosol NO3- in the SJV should explain. Median NO3- (2001–2013) is 25 % higher in Bakersfield than Fresno (Fig. 2). However, the observationally constrained calculated total PNO3- (NRL plus OH-initiated) is 15 % lower in Bakersfield than Fresno (Fig. 6). We find that wintertime stagnation events, defined as continuous days with increasing 24 h PM2.5, are more severe in Bakersfield than in Fresno, meaning there is a greater increase in PM2.5 day-1 over each event. The median increase in PM2.5 day-1 event-1 over the last decade was 15 % greater in Bakersfield (7.9 µgm-3 day-1 event-1) than Fresno (6.7 µgm-3 day-1 event-1) leading to 23 % larger increases in PM2.5 event-1 in Bakersfield (32.5 µgm-3 event-1) than Fresno (25.1 µgm-3event-1). PM2.5 on the first day of the event was also 20 % higher in Bakersfield (8.6 µgm-3 event-1 versus 6.9 µgm-3 event-1). These differences between Bakersfield and Fresno may in part be attributed to the former's location in the southern end of the SJV, where the city is enclosed on three sides by the mountains, resulting in reduced losses to advection and mixing than in Fresno. Likewise, transport may carry a portion of aerosol produced elsewhere in the valley to Bakersfield, either by advection in the surface-mixed layer or by a nocturnal low-level jet. Weaker correlations (r2) in Fig. 2 in Bakersfield (0.6) than in Fresno (0.9) serve as evidence for enhanced influences of mixing and transport processes over NO3- concentrations in the southern SJV.

 Impacts of future NO<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mi>x</mml:mi></mml:msub></mml:math></inline-formula> reductions

California has committed to additional, sizable controls on NOx emissions, with decreases of at least 50 %, and potentially up to 75 % NOx, imminent over the next decade. California has implemented a retrofit/replacement program to accelerate impacts of federal rules on diesel engines, affecting weekday NOx (Dallmann and Harley, 2010; California Air Resources Board, 2012), and has both tightened standards on gasoline-powered vehicles and required one in seven new cars sold in the state be zero-emission or plug-in hybrids for model years 2017–2025 (Environmental Protection Agency, 2012), affecting weekday and weekend NOx.

Currently, average wintertime NOx concentrations are low enough that reductions of 50 and 75 % are calculated to decrease PNO3- in the NRL in Bakersfield on weekends by 40 and 70 %, respectively (Fig. 8a), with similar results in Fresno and Visalia. Recall, the NO3 radical production is nonlinear versus NO2 and, for a fixed Ox concentration, production is described by a single curve in Fig. B1a. When Ox is variable, NO3 radical production is described by multiple curves and is most sensitive to changes in O3 at NO2 concentrations which are at and/or greater than peak NO3 radical production. At low NOx (and high NOx), PNO3- that is limited by NO3 radical production is more sensitive to changes in NO2. Figure 6 suggests that as a direct result of decreases in NO2, the chemical sensitivity of PNO3- to NO2 has been altered such that future NOx controls are poised to more effectively slow PNO3- in the NRL in the next decade than over the last, at least at weekend NOx levels.

In Bakersfield: tethered 3-year weekday (closed circles) and weekend (open diamonds) medians of calculated wintertime NH4NO3 production (µg m-3 day-1) in the NRL (a) and during the daytime (b) versus NO2. Medians and means give the same result. Brighter data are observationally constrained 3-year medians at present (2010–2013). Pale points are observationally constrained 3-year medians at the start of the record (2001–2004). Predicted NH4NO3 production at -50 % weekend NOx are black-filled diamonds and -75 % weekend NOx are gray-filled diamonds. Weekend data were selected simply to expand the NOx range of individual curves; impacts on weekdays can be inferred. In panel (a), the light blue line is stoichiometric and the gray line is a fit to the annual observations as in Fig. 5. In panel (b), the gray line is the calculated HNO3 production with PHOx and organic reactivity equal to present-day values.

We compute that NOx reductions of 50 and 75 % are large enough that changes in the average wintertime NO3- are quantified via 2.56 : 1 line (3.31 including molar equivalent NH4+), the stoichiometric NO3- response to NO2, meaning the O3 feedback from reduced NOx on PNO3- is minimal. The highest NOx conditions in the SJV are present in the shallowest boundary layers of December and January; during DISCOVER-AQ, NOx concentrations were high enough that reduced weekend NOx (21–22 January) had the effect of increasing PNO3- in the NRL relative to the preceding weekdays, i.e., chemistry on these days was right of peak NO3 radical production.

Our calculation implies greater decreases in PNO3- have occurred in lower-NOx rural environments than in cities since 2001 given the same relative NOx reductions. During a previous aerosol experiment, CRPAQS (California Regional PM10/PM2.5 Air Quality Study), conducted December 1999–February 2001 (Watson et al., 2000) with a wintertime intensive (15 December 2000–3 February 2001), it was generally observed that high NH4NO3 was a valley-wide phenomenon. CRPAQS measurements of 24 h NO3- (November 2000–January 2001) were similar in rural locations and in Fresno and Bakersfield, while high organic aerosol concentrations were spatially correlated with the cities. From these data, it was hypothesized that nocturnal low-level currents efficiently transported NH4NO3 and NO3- precursors, distributing NH4NO3 and NH4NO3 production throughout the SJV (Chow et al., 2006). On the contrary, during DISCOVER-AQ, NO3- was observed to spatially correlate with cities. Long-term co-located NO3- and NO2 measurements do not exist at any rural location in the SJV; however, the discrepancy between spatial patterns during DISCOVER-AQ and during CRPAQS can be explained through a combination of NOx emission controls shrinking urban NOx plumes and low-NOx nighttime chemistry being more sensitive to changes in NO2.

For NOx emission changes to affect daytime PNO3-, they must be large enough to transition photochemistry into the NOx-limited regime, less than a few ppb in the wintertime SJV (Fig. B1b). In Bakersfield (Fig. 8b), at -50 % NOx from current levels we predict an increase in daytime NH4NO3 production of 2 µgm-3 day-1 (∼ 15 %), but at -75 % NOx, we predict a transition to low-NOx chemistry and a net decrease in NH4NO3 production of 1.5 µgm-3 day-1 (15 %). Initial enhancements in NH4NO3 are caused by NOx feedbacks on the HOx precursors, O3 and CH2O, which are both predicted to increase in response to decreases in NO2 (Fig. B2). In Fig. 7b, the gray line is the modeled NH4NO3 production day-1 calculated for present day conditions. The modeled points show the results of the calculations, for which the influence of NOx decreases on PHOx, and the subsequent feedbacks on NH4NO3 production, are accounted. Elevation of NH4NO3 production above the gray line is due to the NO2-PHOx precursor feedback. In Fresno (not shown), we compute an increase of 0.5 µgm-3 day-1 (< 5 %) at -50 % NOx and a decrease in NH4NO3 production of ∼ 3 µgm-3day-1 (20 %) at -75 % NOx.

Effects of three NOx emission control scenarios on wintertime 24 h PM2.5 NAAQS exceedances in Fresno, Visalia, and Bakersfield. Percentages are calculated according to days in which data exist, not total wintertime days. Rows 1–2: average exceedances winter-1 (November–March) in the last 3 years of the record, rounded up, and percentage of days in violation. Rows 3–9: number of exceedances predicted after a 50 % increase (back in time) and 50 and 75 % reductions in NOx, including the calculated percent change from present day. In row 3, the number in parentheses is the actual number of exceedances averaged for 2001–2004. In row 5, the number of calculated exceedances was rounded down to compute the percent.

Control Fresno Visalia Bakersfield Exceedances (winter-1) 34 21 34 Winter days in exceedance (%) 31 14 23 +50 % NOx Exceedances after the control (winter-1) 48 (60) 31 (31) 40 (53) % change in exceedances 40 46 18 % change explained by the NOx reduction 50 90 32 -50 % NOx Exceedances after the control (winter-1) 32 19 29 % change in exceedances -7 -10 -16 -75 % NOx Exceedances after the control (winter-1) 24 16 24 % change in exceedances -31 -27 -32

Combining our derived trends in NRL and daytime PNO3- (Table 1), we calculate impacts of past and future NOx controls on the frequency of wintertime 24 h PM2.5 NAAQS exceedances. Using data from a multi-year experiment in the early 2000s in Fresno (Appendix A), the 24 h NO3- concentration was observed to be an almost constant fraction of 24 h PM2.5 each winter when 24 h PM2.5 was greater than 15 µgm-3 and an even larger fraction at lower loadings, typically in March. During DISCOVER-AQ, surface aerosol in Fresno was 41 % (median) and 39 % (mean) NO3- in PM2.5, and 57 % (median) and 53 % (mean) NH4NO3 in PM2.5. Previous work has shown that NH4NO3 is a smaller portion of total PM2.5 in Fresno than in any other location in the SJV, including Bakersfield, with rural PM2.5 dominated by NH4NO3 (Zhang et al., 2010). We applied our calculated changes in PNO3- to 50 % of wintertime PM2.5 mass as a conservative estimate. We also assume that NH4NO3 has been and will continue to be 50 % of the aerosol mass over the entire past and future record. This simplification implies there have and will be only small changes in the heterogeneous loss rate of N2O5, in the relative speciated NO3 reactivity, and in the fractional product yields of N2O5 hydrolysis. Figure 2 suggests these factors have not substantially affected the NO2 dependence of NO3- over time, at least in the wintertime average in this location. We find that over the last decade, the impact of NOx controls on PNO3-, and hence NH4NO3, has been to reduce the number of 24 h PM2.5 exceedances by 18–46 % (Table 1). The primary mechanism for these changes has been decreased PNO3- in the NRL. Trends in PO3- account for 32–90 % of the total observed change. We hypothesize that controls on the organic portion of aerosol mass, for example district-level amendments to national home wood burning and fireplace rules (San Joaquin Valley Air Pollution Control Board, 2003), may have driven the other portion of reductions.

In the future, a 50 % decrease in NOx is predicted to decrease PNO3- in the NRL more efficiently and to the point where this source is approximately equal to OH-initiated PNO3- on weekends. If reductions of 75 % NOx are achieved, PNO3- in the NRL will decrease sufficiently that daytime OH-initiated HNO3 formation is anticipated to become the dominant source of wintertime NH4NO3 on all days of the week. We calculate that over the next decade the SJV will experience 7–16 % fewer exceedance days with a 50 % decrease in NOx and ∼ 30 % fewer exceedances with a 75 % decrease in NOx.

We have not considered the impact of NH3 controls because our data indicate PNO3- chemistry, not the NH3 abundance, drives NH4NO3 (Figs. 2 and 3), and because it has been shown that NH3 emissions in the SJV are too high for any reasonable NH3 control to affect wintertime NH4NO3 concentrations (Herner et al., 2006). While these NOx controls constitute a major improvement to the air quality in the SJV, it is evident that decreases in organic aerosol mass are also required to eliminate high aerosol days in the SJV. We have not quantified, but do expect, future NOx reductions to influence the production of secondary organic aerosol (SOA) mass. In the laboratory, it has consistently been observed that NOx concentrations, relative to gas-phase organic compounds, influence the molecular identity and volatility of oxidation products such that SOA yields are higher at low NOx and suppressed at high NOx (e.g., Presto et al., 2005; Ng et al., 2007; Kroll and Seinfeld, 2008; Chan et al., 2010). Recent summertime field measurements of aerosol-phase RONO2 in Bakersfield (Rollins et al., 2012) and at a forested field site in Colorado (Fry et al., 2013) found that NO3 radical-initiated SOA formation correlated with NO3 production and was proportional to NOx at low to moderate NOx levels. In Fresno during DISCOVER-AQ, SOA constituted 40 % of the organic fraction of PM1, or 22 % of total PM1 mass (Young et al., 2015). Reductions in NOx as large as 50 to 75 % are expected to influence this portion of the aerosol mass and likely in a way that affects the frequency of exceedances in the SJV; however, the magnitude and sign of the impact are beyond the scope of this work.

Additional benefits of NOx decreases include reductions in high summertime O3 throughout the SJV (Pusede and Cohen, 2012; Pusede et al., 2014) and decreases in both summertime inorganic nitrate aerosol (Markovic et al., 2014) and NO3-radical initiated SOA (Rollins et al., 2012). Because the US EPA has recently decided to strengthen the annual PM2.5 standard of 12 µgm-3 (Environmental Protection Agency, 2013), compliance with this NAAQS in the SJV will require reductions in aerosol concentrations in all seasons. Generally speaking, regulatory policies of valley-wide inter-pollutant trading of NOx for PM2.5 control aimed at wintertime NH4NO3 must be designed with knowledge of each nonlinear PNO3- mechanism versus NO2, instead of use of a single exchange rate, as urban (high-NOx) and rural (low-NOx) PNO3- are differently responsive to changes in NO2. Finally, because NO3- is concentrated over Fresno and Bakersfield, NOx reductions need to happen in those cities themselves, prioritizing localized interventions to maximize the public health benefit and probability of regulatory compliance.

 Conclusions

We derived trends in the wintertime production of NO3- (PNO3-) as calculated from measurements of gas-phase precursors over the last 13 years. We used these PNO3- trends to explain the observed NO3- sensitivity to NO2, which was -0.5 and -0.64 µgm-3 ppb-1 NO2 in the San Joaquin Valley cities of Fresno in Bakersfield, respectively. We found that reductions in NOx have both decreased and increased NH4NO3 formation rates by the various chemical pathways, but the net downward trend in NO3- has been driven by local changes in nighttime chemistry in residual layers decoupled from fresh surface emissions. We showed that high NH4NO3 abundances were a combined function of active chemical PNO3- and rapid atmospheric loss by deposition of gas-phase HNO3 (τNO3-∼3 daytime h); in contrast, the total aerosol mass lifetime was controlled by cold fronts that turnover valley air on average every 5 ± 1.5 days. We computed the impact of future NOx decreases on PNO3- from both nighttime and daytime mechanisms, finding the sign and magnitude of the changes are dependent on oxidation pathway, oxidant precursor, NO2 concentration, and, at night, altitude. We calculated that the SJV will experience 7–16 % fewer days in exceedance of the 24 h PM2.5 standard with a 50 % NOx reduction and ∼ 30 % fewer 24 h PM2.5 exceedance days with a 75 % NOx reduction. As an additional consequence of anticipated NOx controls, daytime rather than nighttime chemistry will drive NH4NO3 production in the SJV in the future. The observations and calculations presented here offer improved insight into the chemistry imbedded in the wintertime NH4NO3 diurnal cycles and suggest such long-term measurements would inform the absolute and relative contributions by vertically stratified NO3 chemistry and OH-initiated production, especially if a record that captured diurnal variability were put in place prior to the sizable NOx reductions that are forthcoming. The specific NOx constraints on NH4NO3 chemistry we described here likely inform the effects of NOx emission changes, both increases and decreases, on aerosol in other polluted cities.

 Measurements Long-term records

The long-term aerosol NO3- data are from 24 h integrated filter-based measurements of PM10 collected once every 3 to 6 days as part of the US EPA's Chemical Speciation Network program. The data were downloaded from the California Air Resources Board (CARB) archive (http://www.arb.ca.gov/aqmis2/aqdselect.php). We used measurements at Fresno-First Street (36.782∘ N, 119.773∘ W) and Bakersfield-5558 California Avenue (35.357∘ N, 119.063∘ W), as the two stations had mostly uninterrupted records and co-located observations of NO2 and O3. In Fig. 2, Bakersfield NO3- wintertime (November–March) medians include the years 2001–2013 with an average of 46 weekday and 18 weekend data points year-1. The Fresno-First Street station was closed in 2012 and so the Fresno NO3- wintertime medians include 2001–2012 with an average of 36 weekday and 17 weekend data points year-1.

P-3B TD-LIF NO3-(g+p) in PM2.5 versus PILS NO3- in PM3 below 0.5 km a.s.l. The gray line is one-to-one and the black line is a least squares fit assuming equal weights in x and y data.

Species, measurement accuracy, analytical technique, time resolution, location/platform, and reference for select DISCOVER-AQ observation included in our analysis. Many compounds are measured with higher precision than accuracy. See original references for details.

Species Accuracy (±%) Analytical technique Resolution Location Reference(s) NO3-(g+p) 20 TD-LIF 1 s P-3B Day et al. (2002) PM1 ions 20 AMS 20 min Fresno-Garland Drewnick et al. (2005); Ge et al. (2012) PM2.5 ions 20 PILS 20 min Fresno-Garland NO2 5 LIF 1 s P-3B Thornton et al. (1999) NH3 35 cavity ring down 8–20 s P-3B Picarro G2103 analyzer CH2O 4 IR absorption 1 s P-3B Weibring et al. (2006, 2007)

To make these NO3- measurements, ambient air is sampled through a denuder and aerosol are collected on a quartz fiber filter for 24 h, midnight to midnight. Water-soluble ions are then quantified by ion chromatography. NH4NO3 is semi-volatile and exists in thermal equilibrium with gas-phase HNO3 and NH3. Long sample collection times (24 h) and the presence of a denuder result in low biases due to loss of NO3- to the gas phase (Appel et al., 1981; Shaw Jr. et al., 1982; Hering and Cass 1999; Babich et al., 2000). This interference is well documented and observed to be large during summer months, when more than 80 % of NH4NO3 may be lost, but is estimated to be ∼ 20 % when relative humidities are high and temperatures are cold (Appel et al., 1981; Shaw Jr. et al., 1982; Hering and Cass, 1999). If we assume ambient conditions, as opposed to conditions internal to the instrument, drive the equilibrium (Appel et al., 1981; Shaw Jr. et al., 1982; Hering and Cass, 1999; Babich et al., 2000), we are able to estimate the interference using surface AMS observations of NO3-, NH4+, Cl-, and sulfate (SO42-) and a particle into liquid sampler (PILS) of potassium (K+) and magnesium (Mg+) in Fresno during DISCOVER-AQ to constrain the thermodynamic model ISORROPIA II (Nenes et al., 1998; Fountoukis and Nenes, 2007). To do this, we set the total gas plus particle concentration equal to the ion data, running ISORROPIA II in the forward mode to simulate the gas–aerosol partitioning after the air stream passed through a denuder, consistent with all gases being captured by the denuder and all aerosol depositing on the filter. During DISCOVER-AQ, the daily average temperature and relative humidity (RH) were 8.5 ∘C and 70 %, respectively. Median daytime (08:00–18:00) wintertime temperatures were within ∼ 1 ∘C and 1 % RH in Bakersfield and Fresno. When these conditions drive the interference, we predict that daily-integrated NO3- was biased low by 25 %, in line with other reports (Chow et al., 2005). The average wintertime (November–March) temperature and RH at the USDA Shafter Station from 2000 to 2013 were 10 ∘C and 79 %, respectively. Under these ambient conditions, we predict measurements of the daily-integrated NO3- are biased low by 23 %. Evaporative loss of NO3- of 25 % implies the true slope is 20 % greater with respect to the measured value. The observed correlation between NO3- and NO2 indicates ∼ 20 % of NO2 is oxidized to NO3- (Fig. 2), a 20 % NO3- error implies that 25 % of NO2 is oxidized to NO3-.

Hourly O3, 24 h total PM2.5, and NO2 data are from the CARB archive at the following sites: Fresno-First Street (2000–2011), Fresno-Garland (36.785∘ N, 119.773∘ W) (2011–2013), Visalia-North Church Street (36.333∘ N, 119.291∘ W) (2000–2013), and Bakersfield-5558 California Avenue (2000–2013). NO2 measurements are made by chemiluminescence coupled to a heated molybdenum catalyst and have a known but poorly quantified positive interference from higher oxides of nitrogen (Winer et al., 1974; Williams et al., 1998; Dunlea et al., 2007). This interference is largest in the summertime when weakly bound higher oxides are more abundant relative to NOx and minimal in the wintertime. These instruments sample ambient air through a filter, removing NO3- and likely a considerable fraction of gas-phase HNO3 and multifunctional organic nitrates, reducing the positive artifact. NO2 concentrations are decreasing across the valley at a rate similar to that observed from space by OMI, an instrument selective for NO2, suggesting that relative trends in NO2 are accurate (Russell et al., 2010).

Hourly solar radiation, temperature, and RH data were taken from the California Irrigation Management Information System archive (http://www.cimis.water.ca.gov) at the Shafter US Department of Agriculture (USDA) station (35.530∘ N, 119.280∘ W). Sunrise and sunset times in Bakersfield (35.357∘ N, 119.063∘ W) were downloaded from the United States Naval Observatory Naval Oceanography Portal (http://aa.usno.navy.mil/data/docs/RS_OneYear.php).

Nitrate ion observations with 10 min time resolution were available during select time periods and were used to determine the wintertime variability in the fraction of PM2.5 that was NO3-. There data were collected in Fresno in 2000–2001 and 2003–2005 (Watson et al., 2000) as part of the EPA PM Supersites program (http://www.epa.gov/ttnamti1/supersites.html). These measurements were made by flash volatilizing NH4NO3, reducing HNO3 across a heated catalyst to nitric oxide (NO), and detecting NO by chemiluminescence (Stolzenburg et al., 2003; Chow et al., 2008). It was reported that 24 h averages of these high-time-resolution observations were well correlated with, but 20–40 % lower than, 24 h PM2.5 filter samples (annual averages). This effect has been attributed to incomplete volatilization and/or incomplete catalytic conversion of NO3- to NO. Two identical instruments at the Fresno supersite yielded data that were also well correlated but different by 10–55 % in the annual average (Chow et al., 2008). As a result, we treat these observations as uncalibrated but internally consistent over time.

 DISCOVER-AQ observations

The DISCOVER-AQ experiment synchronized multiple ground sites and aircraft sampling of in situ and column measurements, producing a data set resolved in space, both horizontally and vertically, and in time. The DISCOVER-AQ sampling strategy was built on repeated sampling across urban–rural (horizontal) and vertical gradients and on connecting observations made from balloons, onboard aircraft, and from space to monitoring sites at the surface. The NASA P-3B aircraft flew only during daylight hours, completing 2–3 identical circuits day-1, alternating low-altitude (150 m a.g.l.), along the valley's western edge, medium-low (300 m a.g.l.), and high-altitude flight passes (2.6 km a.g.l.). Circuits included missed approaches at all cities and rural waypoints when visibility permitted. Missed approaches allowed the P-3B to reach altitudes as low as 20–40 m (a.g.l.) and were conducted over airstrips. Landing strips were often rural and little trafficked but may have experienced airport-related NOx enhancements in the cities of Fresno and Bakersfield.

Panel (a): nighttime NO3 radical production (µg m-3 h-1) versus NO2 (ppb) under four Ox conditions: 50 ppb Ox (black), 40 ppb Ox (purple), 30 ppb Ox (violet), and 20 ppb Ox (gray). The temperature is 282 K and NO3 radical production is scaled by two, i.e., all NO3 reacts with NO2 and N2O5 hydrolysis is rapid compared to NO3 formation. Panel (b): daytime production of HNO3 (µg m-3 h-1) as a function of NO2 computed with an analytical model at NO : NOx=0.3 and VOCR = 4 s-1 at 0.3 ppt s-1 PHOx (orange) and 0.15 ppt s-1 PHOx (golden).

The comprehensive suite of DISCOVER-AQ data is available to the public at https://www-air.larc.nasa.gov/cgi-bin/ArcView/discover-aq.ca-2013. Table A1 lists the measurement accuracy, analytical technique, platform and location, and associated references of species key to this analysis.

The NO3-(g+p) measurements were made by thermal dissociation laser induced fluorescence (TD-LIF). The TD-LIF operating principle is as follows: NO2 is detected by laser-induced fluorescence (LIF) (Thornton et al., 1999). A tunable dye laser is pumped by a Q-switched, frequency doubled Nd3+:YAG laser. The narrow band dye laser is etalon-tuned to a specific 585 nm rovibronic feature of NO2, alternating between this feature and the weaker continuum absorption. The resulting red-shifted photons are imaged onto a photomultiplier tube and collected using time-gated counting. The LIF technique is spectroscopically specific and accurate (±5 %). The system was calibrated in flight every ∼ 30 min with an NO2 reference standard added at the inlet. The higher oxides of nitrogen, peroxy nitrates (RO2NO2), alkyl nitrates (RONO2), and HNO3 were measured by thermal dissociation (TD) coupled to LIF (Day et al., 2002). Dissociation of thermally labile species into NO2 and a companion radical occurs at characteristic temperatures due to differing N–O bond strengths. Ambient air is pulled through heated quartz tube ovens followed by PFA sampling lines before reaching the NO2 detection cell. An unheated channel detects only NO2, a second channel (180 ∘C) measures NO2+ RO2NO2, a third channel (400 ∘C) measures NO2+ RO2NO2+ RONO2(g+p), and a fourth (600 ∘C) measures NO2+ RO2NO2+ RONO2(g+p)+ NO3-(g+p). Mixing ratios of each species are determined as the difference between adjacent channels, i.e., NO3-(g+p) equals the 600 ∘C channel minus the signal in the 400 ∘C channel. ClNO2 is thought to appear primarily in the 400 ∘C channel. Recent observations indicated that about 50 % of ClNO2 was observed in the 400 ∘C channel and 50 % in the 600 ∘C channel. Residence times in the ovens are sufficient to volatilize aerosol-bound nitrates, which is reflected in the RONO2(g+p)+ NO3-(g+p) subscripting (Day et al., 2002).

NO3(p)- was measured onboard the P-3B by a PILS system sampling at 4 min time resolution with a well-characterized ∼ 3 µm aerosol size cutoff. In Fig. A1, 4 min averaged NO3-(g+p) measured by TD-LIF is compared against PILS NO3-(p) when the aircraft was below 0.5 km a.s.l. The data are fit using a linear least squares model that assumes equally weighted errors in both measurements. Reported uncertainties in the TD-LIF and PILS observations are 15 and 20 %, respectively, and they correlate (y=0.8x+2.3) within combined uncertainties of 25 %. The TD-LIF is sensitive to HNO3(g), while the PILS is not. Using ISORROPIA II (described in Sect. 4.2) HNO3(g) is predicted to be up to a few ppb in the afternoon, corresponding to the highest temperatures but not to the highest NO3-(p) (Fig. 7). Additional details on the TD-LIF aircraft inlet configuration are found in Perring et al. (2009) and Wooldridge et al. (2010).

 Data and filtering for boundary layer sampling

Occasionally the height of the daytime boundary layer was observed near the altitude of the P-3B's low-level flight legs, requiring analysis to distinguish between free troposphere and boundary layer air. Within boundary layer sampling was identified using 1 s-1 measurements of NO and RH recorded by the aircraft data system, according to steep discontinuities in both tracers, being high in the boundary layer and low aloft. In some cases, H2O(v) measured by diode laser hygrometer and O3 were also considered. In Fig. 3 in addition to filtering for within boundary layer sampling, data were only plotted when the pressure altitude (a.s.l.) was greater than 0.

For our derivation of τNO3-, boundary layer depth was estimated with data from an MPL located in the town of Porterville during DISCOVER-AQ. The MPL was supplemented with a wide-field receiver system that allowed for improved near-range signal recovery of the 527 nm attenuated backscatter profiles that were recorded at 30 m vertical and 1 min time resolutions. For daytime mixed-layer conditions driven by convection, the aerosol gradient falls off and stable molecular scatter signal above the lowest mixed aerosol layer signal represents the boundary layer height. According to this aerosol gradient, boundary layer heights were observed to range from 300 to 700 m during DISCOVER-AQ.

 Calculating <inline-formula><mml:math display="inline"><mml:mi>P</mml:mi></mml:math></inline-formula>NO<inline-formula><mml:math display="inline"><mml:mrow><mml:msubsup><mml:mi/><mml:mn mathvariant="normal">3</mml:mn><mml:mo>-</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>

The chemistry producing NO3 radical at night and HNO3 in the daytime is shown in Fig. B1 as a function of NO2. The production of NO3 radical (µgm-3h-1) is calculated at four initial Ox (Ox≡ NO2+ O3) conditions: 50 ppb Ox (black), 40 ppb Ox (purple), 30 ppb Ox (violet), and 20 ppb Ox (gray). In Fig. B1b, the production of HNO3 is shown for two PHOx conditions: 0.3 ppt s-1 PHOx (orange) and 0.15 ppt s-1PHOx (golden). For the sake of the night–day comparison, in Fig. B1 NO3 radical production is scaled by two, which assumes all NO3 reacts with NO2 and that N2O5 hydrolysis is rapid compared to NO2+ O3. In this analysis, we compute PNO3- as NO3 radical production scaled by the observationally constrained NO3 radical reactivity yielding NO3- (see text below).

NO<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">3</mml:mn></mml:msub></mml:math></inline-formula>-initiated <inline-formula><mml:math display="inline"><mml:mi>P</mml:mi></mml:math></inline-formula>NO<inline-formula><mml:math display="inline"><mml:mrow><mml:msubsup><mml:mi/><mml:mn mathvariant="normal">3</mml:mn><mml:mo>-</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> in the nocturnal residual layer

Wintertime NH4NO3- production (µg m-3 day-1) by OH-initiated chemistry versus daytime (10:00–15:00 LT) NO2 (ppb) for each individual HOx precursor: O(1D) + H2O (red), HONO (turquoise), and CH2O (green). Data are tethered present-day 3-year medians on weekdays (closed circles) and weekends (open diamonds) in Fresno (a) and Bakersfield (b). Lighter tint data are tethered 3-year medians at the start of the record (2001–2004). There is no significant difference between medians and means. Predicted NH4NO3- production calculated at -50 % weekend NOx (NOx black-filled diamond) and -75 % weekend NOx (gray-filled diamond) are also shown.

Our time-dependent box model of NO3 radical production was run separately for each day over the time period 2000–2013. The model was initialized with hourly O3 measurements at the surface using the maximum O3 concentration 1–3 h before sunset and the median daytime (10:00–15:00 LT) NOx. This window was determined based on the timing of steep afternoon O3 titration observed in the hourly surface O3 data, both during DISCOVER-AQ and throughout the interannual record. O3 and NOx were treated as well mixed in the daytime boundary layer and kNO2+O3 was calculated based on the mean surface temperature at 18:00–20:00 LT. NO3 radical production was integrated from sunset to 1 h prior to sunrise. Differences were insignificant using mean O3 over this time window versus the daily maximum.

Although there is no long-term, vertically resolved measurement record of any species in the region, there is observational evidence that the rapid decline in afternoon O3 corresponds to the stratification of the daytime boundary layer. First, as part of CRPAQS, measurements of NO, O3, and NO3- were made simultaneously atop a tower (90 m a.g.l.) and at a lower level (7 m a.g.l.) in the SJV town of Angiola. Winter average (December 2000–January 2001) NO and O3 at the two sampling heights were comparable from 10:00 to 13:00 LT; however, by 14:00 LT the concentrations began to diverge, with O3 at 7 m falling rapidly (∼ 5 ppb h-1), concurrent with increases in NO, while O3 at 90 m remaining approximately constant until sunset, whereupon a decrease of ∼ 2 ppb h-1 was observed. NO at the 90 m level was near 0 throughout the night. A 7-day time series of NO3- data at 90 m showed clear increases in NO3- beginning at nightfall and persisting until sunrise of ∼ 10 µgm-3 (Brown et al., 2006). These diurnal patterns suggest that the NRL(s) decouple from the surface layer ∼ 3 h prior to sunset and that initial concentrations of the nocturnal chemistry reactants, NO2 and O3, are also represented by surface concentrations 3 h prior to sunset. During DISCOVER-AQ, full vertical profiling by the P-3B (including a missed approach) concluded in Fresno at 14:30–15:30, ∼ 2–3 h prior to sunset. Profiles of O3 and the conserved tracer Ox suggest that stratification of the daytime boundary layer had begun by this time on multiple afternoons, although day-to-day variability was observed. A comparison of mean O3 and Ox concentrations in the top 100 m (∼ 0.35–0.45 km a.s.l.) and bottom 100 m (∼ 0.15–0.05 km a.s.l.) of the atmosphere below the height of the daytime boundary layer, found small differences in Ox on all flight days but, on some days, large differences in O3. O3 variations equaled 25–30 % the mean O3 profile concentration on two days, 18 and 22 January 2013. At midday (12:00–13:00) at the same altitudes, small absolute differences were observed in both O3 and Ox.

Reaction fates of the NO3 radicals calculated by the time-dependent box model described above were determined according to the mean proportional NO3 reactivity contributions observed during DISCOVER-AQ. NO3 Reactions (BR3–BR5) are as follows: NO3+NO2+M→N2O5+M, NO3+alkene→RONO2, NO3+aldehyde or alcohol or dimethyl sulfide→HNO3+products. Each of the three pathways results in a different number of NO3- produced per NO3 radical. NO3 reactivities are defined as kN2O5KeqT[NO2] (Brown et al., 2003, 2009), ∑ikNO3+alkenei[alkenei], and ∑ikNO3+alcoholi[alcoholi]+kNO3+aldehydeialdehydei +kNO3+DMS[DMS], for Reactions (BR3), (BR4), and (BR5), respectively. kN2O5=0.25ν¯AγN2O5 is the N2O5 uptake coefficient and Keq(T) is Reaction (R3) equilibrium constant. For kN2O5, ν¯ is the N2O5 mean molecular speed, A is the aerosol surface area density, and γN2O5 is the N2O5 aerosol uptake coefficient. A was determined as the product of the dry surface area of particles 60–1000 nm in diameter measured by an ultra-high sensitivity aerosol spectrometer and the hygroscopic growth factor, f(RH), calculated from the signal difference of a two nephelometers, one sampling dried air (RH < 40 %) and a second sampling after humidification to 80 % RH (each measurement was made onboard the P-3B). During DISCOVER-AQ, below 0.4 km a.s.l., the mean dry surface area was 191.8 ± 75 µm2cm-3 (1σ) and the mean f(RH) was 1.7 ± 0.2 (1σ). γN2O5 was set equal to 0.006 as estimated from the parameterization in Bertram and Thornton (2009) following Wagner et al. (2013): 30 M aerosol water and an aerosol system including only water, NH4+, and NO3-. Using the aerosol water, NH4+, and NO3- that we computed by ISORROPIA II, NO3- constituted 45 % aerosol mass. We assume that all N2O5 was converted to 2NO3-. If ClNO2 formation is an important N2O5 hydrolysis product in the SJV, we have overestimated the total NO3- production. While there is limited observational insight into the extent of ClNO2 formation, because ClNO2 is predicted to thermally dissociate in the RONO2 channel of the TD-LIF instrument (reviewed in Perring et al., 2013), in the early morning, a portion of the measured quantity RONO2(g+p) is possibly due to ClNO2. On all days, the P-3B flights began at ∼ 08:00 LT, which is early enough to capture at least portion of nocturnal ClNO2 prior to photolysis. The mean RONO2(g+p) before 10:00 and below 0.4 m a.s.l is equivalent to 1.6 µgm-3 NO3-, or 15 % of measured NO3-(g+p). However, there is little discernable loss of RONO2(g+p) in the late morning, which would be expected if the RONO2(g+p) was due to ClNO2, suggesting 15 % is an upper limit.

NO3 reactivities were computed using the DISCOVER-AQ data set with daytime (13:00–17:00 LT) surface observations of NO2, organic compounds (whole air canister sampling), and dimethyl sulfide (DMS) (whole air canister sampling). CH2O measurements from onboard the P3-B were included in the speciated reactivity for Reaction (BR5). Little temporal variability was observed in the concentrations of organic compounds between 13:00 and 17:00 LT. If alkenes reactive with NO3 in NRL are oxidized by OH or O3 prior to nightfall, then ∑ikNO3+alkenei[alkenei] represents an overestimate. This will alter the absolute value of the calculated change in PNO3- but not the functional form of the dependence. To account for this, we decrease the concentrations of organic species by one e-fold prior to computing the NO3 reactivity.

In Fresno and Bakersfield the reaction of NO2 with NO3 represented ∼ 80 % of total NO3 reactivity, with negligible weekday–weekend differences. The mean NO3 reactivity values used in the model were 0.005 s-1 for addition to double bonds, yielding 0 HNO3; < 0.001 s-1 for hydrogen abstraction, yielding 1 HNO3; and 0.02 s-1 for reaction with NO2, which after heterogeneous conversion of N2O5 yields 2 HNO3. This gives 1.5 NO3- produced per NO3 on average. The integrated PNO3- was taken as the NO3 radical production scaled by the NO3 reactivity to NO2, which assumed reactions with alkenes and DMS were instantaneous. This result is similar to that of the wintertime NACHTT experiment at comparable relative NO2 concentrations (Wagner et al., 2013). During NACHTT NO3 and N2O5 were measured, the kinetics of N2O5 explicitly included in the calculation of PNO3-, and 1.6 HNO3 per NO3 radical produced was inferred.

 NO<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">3</mml:mn></mml:msub></mml:math></inline-formula>-initiated <inline-formula><mml:math display="inline"><mml:mi>P</mml:mi></mml:math></inline-formula>NO<inline-formula><mml:math display="inline"><mml:mrow><mml:msubsup><mml:mi/><mml:mn mathvariant="normal">3</mml:mn><mml:mo>-</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> in the nocturnal boundary layer

The production of NO3 radical was directly computed from surface measurements of hourly O3, NO2, NO, and temperature each day from 2000 to 2013. NO3 production was integrated between sunset and 1 h prior to sunrise and scaled by 1.1 NO3- produced per NO3, which is the result from the NO3 reactivity calculation described above with no organic reactivity loss. There were times that under conditions of very high NO that nighttime O3 was observed to be positive and constant at nonphysical values of 1–10 ppb for multiple hours. This offset was interpreted as a measurement artifact as excess NO titrates O3 completely. To account for this, when NOx was greater than 5 times the reported O3, O3 was set equal to 0 prior to computing NO3 production. However, concentrations of DMS and organic emissions, largely anthropogenic in origin in the wintertime SJV, are predicted to be higher in the NBL than in the daytime boundary layer.

 OH-initiated <inline-formula><mml:math display="inline"><mml:mi>P</mml:mi></mml:math></inline-formula>NO<inline-formula><mml:math display="inline"><mml:mrow><mml:msubsup><mml:mi/><mml:mn mathvariant="normal">3</mml:mn><mml:mo>-</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>

The integrated daily production of HNO3 was calculated for each day from 2000 to 2013 separately for each of the three HOx sources: O(1D) + H2O, HONO photolysis, and CH2O photolysis (Eq. B1). PNO3- versus NO2 attributed to each HOx source is plotted in Fig. B2. OH was modeled with an analytical model constrained to DISCOVER-AQ observations, built on the assumption that oxidizing radicals were in steady state (Eq. B2) and that RO2 and HO2 production are approximately equal, as are RO2 production and loss, giving Eq. (B3) for both RO2 and HO2 (Murphy et al., 2007). The symbol α is the RONO2 branching ratio. RO2NO2 are considered to be in thermal equilibrium with NO2 and peroxy radicals and therefore not to contribute to net radical formation. Calculated wintertime OH values were ∼ 106 molecules cm-3 at noontime and exhibited reasonable nonlinear NO2 dependence throughout the day. Observational inputs to the model were NO and NO2, the total organic reactivity to OH (VOCR), PHOx, α, and temperature. VOCR was computed as equal to ∑ikOH+VOCi[VOCi] using whole air samples of speciated organic molecules collected at the ground during DISCOVER-AQ and CH2O data from onboard the P-3B, as VOCR equal to ∑ikOH+VOCi[VOCi]. The daytime average was ∼ 4 s-1, consistent with a recent analysis of the temperature dependence of total VOCR in Bakersfield (Pusede et al., 2014), giving confidence that the majority of the reactivity was accounted for. The α is set equal to 2 %. Equations (B2) and (B3) are combined to solve for OH. PHOx=2jO3→O1D[O3]kH2OkH2O+kN2+O2+2jCH2OCH2O+jHONO[HONO] PHOx=LHOx=2kHO2+HO2HO22+2kHO2+RO2HO2RO2+2kRO2+RO2RO22+kNO2+OHNO2OH+αkNO+RO2NO[RO2] RO2∼[HO2]=VOCR[OH]1-αkNO+RO2[NO] Noontime jO3, jHONO, and jCH2O were computed with the TUV calculator, http://cprm.acd.ucar.edu/Models/TUV/Interactive_TUV (Madronich, 1987), on a clear-sky day (20 January, 2013), scaled by the ratio of the TUV jNO2 and a measurement of jNO2 made onboard the P-3B, and combined with the diurnally varying long-term record of solar radiation.

The trend in O(1D) + H2O was calculated from the observational record of O3, RH, and solar radiation.

No multi-year measurements of HONO have been reported in US cities. HONO is formed at night by a mechanism functionally equivalent to the conversion of two NO2 to one gas-phase HONO and one ground-surface adsorbed HNO3 molecule (Finlayson-Pitts et al., 2003). We computed HONO as equal to 4 % the nighttime (10:00–18:00 LT) mean NO2 (Stutz et al., 2004), yielding one HONO data point for each day. This HONO initialized a calculation wherein photolytic loss was computed, giving HONO concentrations at 1 h time resolution. We have not accounted for daytime formation; however, in total, daytime source(s) are weekday–weekend independent (Pusede et al., 2015).

The interannual trend in CH2O is also unconstrained with observations. In the winter, CH2O is not monitored at the surface and cannot be quantified from space due to low column concentrations and shallow daytime boundary layers. CH2O is a primary organic emission from agricultural activities associated with animal feeds (Howard et al., 2010), dairy cows (Shaw et al., 2007), and combustion. State inventories offer little insight into CH2O trends, as it is unknown whether a priori accounts are complete. CH2O is also the oxidation product of most organic molecules in the atmosphere. We calculated the CH2O concentration using a 0-D chemical model constrained to the complete 1 min DISCOVER-AQ data set. All 95 organic molecules measured by whole air sampling at the ground level at the Fresno site were included after scaling by a fit to aircraft carbon monoxide. Within the boundary layer, modeled CH2O typically captured 75 % of the CH2O measured on the P-3B, with the discrepancy most likely due to the local primary CH2O emissions. We ran the model under four NOx reduction scenarios, -75 % NOx, -50 % NOx, +50 % NOx, and +75 % NOx assuming the portion of CH2O not captured by our model remained constant. At 50 % higher NOx, i.e., at the start of the record, secondary CH2O was ∼ 10 % lower on weekdays and ∼ 5 % lower on weekends than in the base model (2013 conditions). At -50 % and -75 % NOx, in the next decade, secondary CH2O is predicted to increase by 15–25 % from 2013 weekend NOx levels, as reductions in NOx increase OH.

The integrated wintertime 24 h NH4NO3 production for each of the three HOx sources is shown in Fig. B2, along with the projected response of to changes in NOx of -50 % and -75 % from weekend concentrations. PNO3- attributed to O(1D) + H2O is 2–3 µgm-3day-1 (2000–2013) and has increased by ∼ 0.1 µgm-3ppb-1NO2 in all three SJV cities. It is an order of magnitude smaller than PNO3- in the NRL in the wintertime average. NH4NO3 production attributed to HONO has decreased with reduced NO2, i.e., the trend has the correct sign compared to NO3- concentrations (Fig. 2). Declines in HONO have resulted in a decrease in NH4NO3 production of 3–6 µgm-3day-1 over the entire NO2 range. We find that CH2O, observed to be 2.3 ± 1.1 ppb (1σ) in Fresno and 2.0 ± 0.9 ppb in Bakersfield, has been the largest contributor to photochemical production of NH4NO3 at ∼ 6 µgm-3day-1NO3-.

 Additional details regarding <inline-formula><mml:math display="inline"><mml:mrow><mml:msubsup><mml:msub><mml:mi mathvariant="italic">τ</mml:mi><mml:mi mathvariant="normal">NO</mml:mi></mml:msub><mml:mn mathvariant="normal">3</mml:mn><mml:mo>-</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>

The timing of decoupling between the NBL and the NRL(s) has implications for our derivation of τNO3-. Observed daily ∂NO3-∂t are consistent with the majority of NO3- lost via deposition of HNO3(g) from most of the daytime boundary layer. If loss occurred from only the lowest 50 m of the daytime boundary layer, then the observationally derived HNO3(g)υd would be only 0.4 cm s-1, below direct measurements (e.g., Huebert and Robert, 1985; Meyers et al., 1989; Sievering et al., 2001; Volpe Horii et al., 2005; Farmer et al., 2006). Gaseous HNO3 that does not deposit will repartition to the aerosol phase when temperatures fall and RH rises in the evening. Assuming an NBL height of 50–100 m, then a nighttime rise (black data in Fig. 7) of 10–5 fold is expected. An NO3- concentration of 10 µgm-3 shifted to the gas phase would increase by 100–50 µgm-3 at nightfall. By contrast, typical nighttime increases were 0–15 µgm-3.

NO3-(g+p) (µg m-3) in PM2.5 measured during 10 missed approaches on 5 days over Visalia. The left-hand flight track is the midday (12:00–13:00 LT) missed approach and the right-hand track, shifted east by 0.02∘, is the afternoon (15:00–16:00 LT) missed approach.

In Fig. C1 we show low-altitude (20–350 m a.g.l.) observations from the P-3B colored by NO3-(g+p) concentration over the city of Visalia on 5 flight days as evidence that afternoon decreases are net HNO3+ NO3- loss and not a shift in partitioning between the two species. In each panel the left-hand flight track was at midday (12:00–13:00 LT) and the right-hand track, shifted in space by 0.02∘ longitude for visual clarity, was in afternoon (15:00–16:00 LT). On each of the five flights, ∼ 2-fold higher concentrations of NO3-(g+p) were observed at midday compared to a few hours later. Additionally, the reduction in NO3-(g+p) is apparent at the higher altitudes, shown in the top and bottom third of each panel, suggesting the loss in NO3- measured by AMS at the surface in Fresno does extend up to at least 300–350 m a.g.l.

 Acknowledgements

This work was funded by NASA under grant NNX10AR36G. Q. Zhang and C. D. Cappa were also supported by the California Air Resources Board (contract no. 14-307). We acknowledge use of publicly available data maintained by the US EPA, California Air Resources Board, and California Irrigation Management Information System. We thank John Barrick for the jNO2 and RH (PDS) data, Glenn Diskin for the H2O(v) (DLH) data, Luke Ziemba and Lee Thornhill for the f(RH) and UHSAS data, and Donald Blake for the speciated organic compound data. We thank Melinda Beaver for assistance interpreting the long-term ozone data. We thank Steve S. Brown for his feedback during the review process. This analysis would not have been possible without the work of the pilots, crew, and engineers of the NASA P-3B. Edited by: N. M. Donahue

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At the same time, NO<sub><i>x</i></sub> concentrations decrease on weekends relative to weekdays, largely without co-occurring changes in other gas-phase emissions, due to patterns of diesel truck activities. These trends taken together provide two independent constraints on the role of NO<sub><i>x</i></sub> in the nonlinear chemistry of atmospheric oxidation. In this context, we interpret interannual trends in wintertime ammonium nitrate (NH<sub>4</sub>NO<sub>3</sub>) in the San Joaquin Valley of California, a location with the worst aerosol pollution in the US and where a large portion of aerosol mass is NH<sub>4</sub>NO<sub>3</sub>. Here, we show that NO<sub><i>x</i></sub> reductions have simultaneously decreased nighttime and increased daytime NH<sub>4</sub>NO<sub>3</sub> production over the last decade. We find a substantial decrease in NH<sub>4</sub>NO<sub>3</sub> since 2000 and conclude that this decrease is due to reduced nitrate radical-initiated production at night in residual layers that are decoupled from fresh emissions at the surface. Further reductions in NO<sub><i>x</i></sub> are imminent in California, and nationwide, and we make a quantitative prediction of the response of NH<sub>4</sub>NO<sub>3</sub>. We show that the combination of rapid chemical production and efficient NH<sub>4</sub>NO<sub>3</sub> loss via deposition of gas-phase nitric acid implies that high aerosol days in cities in the San Joaquin Valley air basin are responsive to local changes in NO<sub><i>x</i></sub> within those individual cities. 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