In this analysis we focus on analysis of diel profiles, averaged over the duration of each field campaign. This method highlights the general differences between the years but does not distinguish between different meteorological conditions within a campaign. In Fig. , we show whole-campaign diel averages of the ozone levels at the Horsepool ground site for the winters of 2012, 2013, and 2014. The dotted line shows the NAAQS level of 75 ppbv. On average, ozone levels were 2.5 times higher in 2013 than in 2012. Additionally, ozone production during midday (between the dotted lines at 09:45 and 14:30 mountain standard time) was 2.7 ppbv h-1 in 2012 and 6.9 ppbv h-1 in 2013, a factor of 2.6 higher. In 2014, the ozone levels were intermediate, with the daily increase at 4.8 ppbv h-1. Although the ozone increase is affected by both chemical production and dilution due to the changing boundary layer, chemical ozone production accounts for most of this increase at this site. For 2012, when atmospheric conditions were least stable, chemical production was estimated to account for 70–85 % of the observed average diel rise in surface O3. These estimates were derived from comparison of the model to the measured surface level rise and from measurements of the diel average O3 profile at different heights up to 500 m from a tethered balloon. .

The top plot in Fig.  shows the diurnally averaged total reactive nitrogen (NOy). The NOy in 2013 is on average a factor of 2.5 higher than 2012, with 2014 again at intermediate levels. However, the middle plot of Fig.  shows that the total NOx concentrations are consistently similar for all three years, despite significantly different meteorological conditions and ozone production rates. The bottom plot shows the ratio NOx / NOy, a measure of the rate of oxidation of reactive nitrogen independent of dilution, whereby a lower ratio implies more oxidation. The large differences in this ratio (a factor of 2.6 on average between 2012 and 2013) instead indicates large differences in levels of NOx oxidation caused by changes in ambient chemistry, which caused the similarity in NOx levels between the measurement years.

We examine the oxidation pathways and products in order to understand the different levels of NOx oxidation for the various years. Figure  shows the partitioning of NOz (≡ NOy - NOx) for 2012 and 2013. In 2012, since NOx makes up approximately 80 % of NOy, the subtraction to calculate NOz results in a noisy trace with large uncertainty relative to the amount of NOz present, and we instead take the sum of components to define total NOz. This is not the case in 2013, and the “missing” part of NOz is likely organic nitrates (RONO2) for which we do not have a measurement.

Ammonium nitrate might be measured partially in the acid CIMS and the NOy instrument due to heated inlets, and its contribution to NOz has not been included in this analysis. Measurements of aerosol nitrate, which would include coarse-mode aerosol whose source might not be exclusively photochemical, present an average upper limit of 0.4 ppbv in 2012 and 1 ppbv in 2013. Nitrous acid, HONO, was measured as a small fraction (2.4 %) of NOz in 2012. Its mixing ratio was measured by both the acid CIMS and DOAS measurements, which both showed maximum values smaller than 120 pptv average at night and smaller during the day, with agreement to within a factor of 2. During 2013, the acid CIMS was the only measurement available. It showed very large signals at the mass normally interpreted as HONO with a distinct, daytime maximum. As described in , HO2NO2 mixing ratios were observed to reach an average daytime maximum of approximately 4 % of NOz. Unpublished laboratory results suggest that a large fraction of the HO2NO2 is detected as HONO using the acid CIMS, resulting in a positive daytime bias in the 2013 measurements. Based on the similarity of DOAS HONO measurements in 2012 and 2014, HONO for 2013 was set equal to that from 2012. For further details on comparisons of HONO measurements, please see .

In 2012, N2O5 and ClNO2 make up about half of the total NOz budget at night, whereas they form a small percentage in 2013. Nitric acid (HNO3) and PAN, however, make up about 75 % of total NOz throughout the whole diel cycle in 2013, with the inferred organic nitrates making up most of the remainder. The major oxidation pathways that produce these compounds during the day are NO2+OH+M⟶HNO3+M,NO2+PA+M⟶PAN+M,NO+RO2+M⟶αRONO2+M, where PA is the peroxyacetyl radical and includes all acyl peroxy radicals, with CH3C(O)O2 being the most important. RO2 includes all other organic peroxy radicals, and α is the temperature-dependent yield of organic nitrates from the reaction of organic peroxy radical with NO, where the majority of this reaction produces an alkoxy radical and NO2 . At night, when NO3 is photochemically stable, the main pathway for NOx oxidation is NO2+O3⟶NO3+O2,NO3+NO2+M⟶N2O5+M. This N2O5 can then further react heterogeneously to form nitric acid and nitryl chloride. N2O5+H2O⟶het2HNO3N2O5+HCl⟶hetHNO3+ClNO2

Calculating the reaction rates of Reactions (R1)–(R5) allows us to compare NOx loss rates (rates of conversion to NOz) through these different pathways. The reaction rate constants are known, and the concentrations of OH and PA are supplied by a box-model simulation using the Master Chemical Mechanism (MCM), as is the production rate of organic nitrates. The MCM utilizes greater than 104 reactions, and the base run accurately reproduces an ozone buildup event in 2013 . Additionally, the OH concentrations agree with OH inferred from VOC ratios with average midday maximum OH levels calculated by the model to be approximately 1×106 cm-3. During the 2012 study, calculated midday OH was 7×105 cm-3 . Although PAN can thermally dissociate, the long lifetime at wintertime temperatures (> 10 h below 10 ∘C) means we can effectively consider only the forward reaction. The limiting step in Reactions (R4)–(R5) is the NO2+O3 reaction and we assume that the sequence of reactions in Reactions (R4)–(R7) quantitatively converts NO2 to stable products, mainly HNO3, at night in 2013 (we calculate N2O5 lifetimes to be < 2 h; see below). The NOx loss rate due to Reaction (R4) is doubled, because the sum of Reactions (R4) and (R5) would lead to NOx loss at twice the rate of Reaction (R4). The reaction pathway to make N2O5 is negligible during daylight hours due to photodissociation of NO3 together with the fast reaction of NO3 with NO, and has been set to zero. The resulting 2013 NOx loss rates due to Reactions (R1)–(R5) are shown in Fig. .

Separating the daytime and nighttime partitioning in Fig.  highlights the species that are long-lived at night and short-lived during the day (N2O5 and ClNO2), demonstrating the role of the nighttime species in reactive nitrogen chemistry. Nitric acid, PAN, and organic nitrates, on the other hand, are long-lived compared to a diel cycle, and we do not expect the nighttime or daytime average to reflect chemical production that is restricted to these periods. It instead represents an average not just over a diel cycle but over the whole campaign.

Integrating the diurnally averaged loss rates gives total daily calculated production of the three major components of NOz, with the simplifying assumption that all N2O5 is converted to nitric acid (we estimate the ClNO2 yield for 2012 and 2013 to be 11 and 2 %, respectively). In Fig.  we compare the partitioning of these integrated production rates with the measured partitioning of HNO3, PAN, and inferred organic nitrates for 2013. Production rates and observed concentrations should not necessarily be proportional, depending on the loss mechanisms. For example, HNO3 will be lost via dry deposition to the ground or snow surface such that its measured contribution to nitrogen partitioning may be smaller than that inferred from its production rate. However, the agreement between production rates and observations illustrates that our methods of treating the reactive nitrogen in the current analysis and in the MCM box model are self-consistent.

Reactions (R1) and (R4)–(R7) result in formation of HNO3, which makes up the bulk of NOz in 2013. Furthermore, the integrated nighttime loss toward nitric acid is 5.9 times greater than during the day. Therefore much of the difference in NOz between the low-ozone year of 2012 and the high-ozone year of 2013 must be due to a large difference in nighttime N2O5 reactivity, which we analyze below.

When the sinks of NO3 are small compared to those of N2O5, and assuming an equilibrium state between NO2, NO3, and N2O5, the ratio of the N2O5 concentration to the production rate of NO3 equals the N2O5 lifetime (τN2O5), τN2O5=[N2O5]k⋅[NO2]⋅[O3], where k is the rate coefficient for Reaction (R4) . An analysis of the resulting lifetimes, which can be considered a measure of N2O5 reactivity, is shown with the solid lines in Fig. . Since Eq. () assumes a steady state in NO3 and N2O5, the relevant period when this lifetime interpretation will be most valid is at the end of the night. However, a simple five-reaction chemical box model including NO3 and N2O5 production and first-order loss shows that it would take > 20 h to reach a steady state in 2012. After the 14 h of night, we predict that the lifetime calculated using Eq. () gives us 77 % of the actual lifetime. In 2013, the model predicts that the system reaches 90 % of steady state in 1.8 h. The lifetimes in 2012 are a factor of approximately 2 times longer than in 2013, or 2.6 times if we use calculated equilibrium values. have suggested an alternate method for lifetime analysis that explicitly takes the time derivative of N2O5 into account to correct its lifetime for failure to reach steady state. Figure  also shows the steady-state lifetime calculated using this method using a smooth fit function for the N2O5 diel profile to calculate the derivative. Since the reaction of N2O5 occurs heterogeneously via uptake onto surfaces, the difference in lifetime between the two years could conceivably be due to higher aerosol surface area or faster ground deposition. The average value of the product of the NO3-N2O5 equilibrium constant, Keq(T), and the NO2 concentration (Keq[NO2]), equal to the predicted ratio of N2O5 to NO3, was 115 and 440 during nighttime hours in 2012 and 2013, respectively. Late night average NO3 of 2.2 pptv agreed well with the predicted equilibrium. Average predicted NO3 of less than 0.5 pptv in 2013 could not be accurately measured. The late night average steady-state lifetime of NO3 in 2012 was approximately 100 s, while in 2013 it was 13 s. Under these cold conditions, the very short NO3 lifetimes do not represent the reactivity of the NO3–N2O5 system, which is dominated by heterogeneous loss of N2O5, and we provide them here for reference only.

Lifetimes due to aerosol can be calculated separately using measurements of aerosol surface area and the equation for heterogeneous uptake, assuming no limitation for gas phase diffusion (valid for small particle size and small to moderate uptake coefficients, and consistent with conditions from both 2012 and 2013): τN2O5=14γc¯SA-1, where γ is the uptake coefficient, c¯ the mean molecular speed, and SA the surface area density of the aerosol. The aerosol surface area density was calculated from number size distributions measured using a scanning mobility particle sizer for particles between 20 and 500 nm geometric diameter, and a aerodynamic particle sizer for particles between 0.7 and 10.37 µm. Size distribution measurements were taken at relative humidity < 25 %, and a hygroscopic growth factor was calculated using measurements of ambient humidity and aerosol composition . There are few determinations of N2O5 uptake coefficients in winter. During winter measurements in Colorado, determined an average γ=0.02 under similar conditions of temperature and relative humidity, and at a site with nearly identical latitude and elevation. Using γ=0.02, we calculate the lifetimes of N2O5 due to aerosol uptake for 2012 and 2013, plotted as dashed lines in Fig. . The 2012 lifetime includes a 10 % correction from the contribution of losses due to VOCs (see below). On average, lifetimes calculated from aerosol uptake were a factor of 4.1 higher in 2012 than 2013, compared to the factor of 2.6 change in lifetime calculated from the N2O5 steady state of Eq. () and the box model. However, an uptake factor of γ=0.026 in 2012 would bring the lifetimes calculated using these two methods into agreement. Since we did not perform eddy covariance flux measurements, we do not know the deposition rate, and the γ values derived from comparison to the steady-state lifetimes thus represent an upper limit. Additionally, since the lifetime of N2O5 is longer in 2012, the influence of deposition to the ground surface might be greater if it were roughly constant relative to other sinks that increased between 2012 and 2013. The change in aerosol uptake between the two years is in part due to the higher relative humidity measured in 2013, which increased the aerosol surface area through hygroscopic growth. The increased relative humidity in 2013 caused frequent and persistent fog. Due to the difficulty in extrapolating a hygroscopic growth factor near saturation, data during periods of relative humidity above 95 % have been excluded in this analysis. Hygroscopic growth associated with the higher relative humidity contributed a factor of approximately 1.3 to the difference in lifetime between the two years.

One condition of Eq. () is that the major sink of NO3 is through aerosol uptake via N2O5 instead of reactions with volatile organic compounds (VOCs). Previous studies in regionally polluted areas have shown that loss of NO3 and N2O5 can be dominated by NO3–VOC reactions, N2O5 uptake, or a combination of the two . Given the high VOC concentrations in the Uintah Basin , we performed an analysis of NO3 reactivity to quantify the contribution of NO3 chemistry to the lifetime of N2O5. The loss due to VOC is simply the sum of all the NO3–VOC rate constants (ki) times the measured VOC concentrations klossNO3=∑ikiVOCi. This first-order loss rate coefficient for NO3 can be compared to the first-order loss rate coefficient for uptake of N2O5 to aerosol by dividing the former by the equilibrium ratio of N2O5 / NO3 . VOC measurements by proton transfer reaction mass spectrometry and gas chromatography in 2012 provided measurements of a more extensive VOC suite than the measurements in 2013, so VOC ratios from 2012 were used to estimate some compounds missing from 2013 measurements, as was done by . The calculations show that with an N2O5 uptake coefficient of 0.02, NO3 losses due to reactions with VOCs were approximately 10 times less than N2O5 uptake to aerosol in 2012, and approximately 40 times less in 2013. A lower N2O5 uptake coefficient would increase the fraction of the NO3 and N2O5 reactivity attributable to NO3–VOC chemistry. However, the comparisons of Fig.  suggest that the average N2O5 uptake coefficient is not appreciably smaller than 0.02. Figure  shows the relative loss rates, as well as the breakdown of reactivity with different classes of VOCs. During both years, reactivity with alkanes form the major part of NO3 loss to VOCs (45–51 %). To our knowledge, this is the first instance in which alkanes have been determined as the largest single component of NO3–VOC reactivity in ambient air. For example, studies in other locations, such as Houston, Texas, show that alkanes contribute approximately 1 % to ambient NO3 reactivity . Despite their very slow rate constants for reaction with NO3, alkanes make up an overwhelming fraction of the measured VOC composition in the Uintah Basin, leading to an unusually large contribution to NO3 reactivity. Isoprene and dimethyl sulfide (DMS) are collectively labeled “biogenic” according to convention, but due to winter conditions we anticipate no biogenic source for these compounds. Rather, we assume both to be emissions from oil and gas operations. For example, an anthropogenic source of isoprene may be emitted in small quantities in vehicle exhaust , while DMS may be a component of the reduced sulfur emissions from natural gas. In any case, the measured concentrations of both compounds are small (2 and 0.7 pptv, respectively, nighttime average in 2013), and their contribution to NO3 reactivity represents the fast NO3 rate constant with these species. It is possible that other highly reactive but unmeasured VOCs contribute to the NO3 reactivity. For example, report an important role for reduced sulfur species other than DMS in loss of NO3 radicals near an oil refinery. Such measurements were unavailable for the UBWOS studies.

Since N2O5 uptake to the ground can also affect lifetimes, one has to consider differences in inlet height and ground composition between different years. In 2012, N2O5 was measured from a scaffold tower at a height of 11 m, whereas in 2013, the lack of such a tower limited the sampling height to 4 m. To investigate a possible N2O5 gradient, we alternately sampled from 14 and 1 m during the final weeks of the 2014 campaign, spanning the sample heights of the 2012 and 2013 inlets. In 2014, the ground was snow-covered, and conditions generally resembled 2013 more than 2012. The resulting lifetime calculations using NO3 production rates (Eq. ) are shown in Fig.  with black solid and dotted lines. We measured roughly twice the N2O5 lifetime at the high inlet as compared to the low inlet. This difference results solely from differences in N2O5 concentrations; measurements of NO2, O3, and aerosol surface area between 4 and 14 m did not show significant differences at night and were assumed to be equal for the lifetime calculation. Ground deposition of N2O5 can form an important contribution to the lifetime , but the year-to-year variability is a significantly larger effect than the measured N2O5 gradient. This suggests that nighttime aerosol uptake of N2O5 could play a major role in NOx oxidation and contributes to keeping NOx levels similar between the three years.