During the CalWater 2015 field campaign, ground-level observations of aerosol size, concentration, chemical composition, and cloud activity were made at Bodega Bay, CA, on the remote California coast. A strong anthropogenic influence on air quality, aerosol physicochemical properties, and cloud activity was observed at Bodega Bay during periods with special weather conditions, known as Petaluma Gap flow, in which air from California's interior is transported to the coast. This study applies a diverse set of chemical, cloud microphysical, and meteorological measurements to the Petaluma Gap flow phenomenon for the first time. It is demonstrated that the sudden and often dramatic change in aerosol properties is strongly related to regional meteorology and anthropogenically influenced chemical processes in California's Central Valley. In addition, it is demonstrated that the change in air mass properties from those typical of a remote marine environment to properties of a continental regime has the potential to impact atmospheric radiative balance and cloud formation in ways that must be accounted for in regional climate simulations.
The remote northern California coast experiences a Mediterranean climate
(Aschmann, 1973; Lentz and Chapman, 1989) and warm, dry summers. The vast
majority of yearly precipitation occurs during winter (Regonda et al., 2005),
when the North Pacific extratropical storm track extends southward and brings
periodic pressure falls and rain to the region (Gyakum et al.,
1989). Also during the winter
months, conditions known as channeled gap flow can transport air masses from
much further inland to the remote coast. These episodic periods result when
very low near-surface buoyancy and an onshore-directed gap-parallel pressure
gradient co-occur in one of several prominent gaps in the coastal mountain
ranges (Overland and Walter Jr., 1981; Neiman et al., 2006; Loescher et al.,
2006; Colle et al., 2006). One such prominent gap is located near the town of
Petaluma in Sonoma County, CA, and can act to channel air from the north San
Francisco Bay Area (SFBA), the Sacramento River Delta, and California's
Central Valley (CV) to coastal northern California (see schematic in Fig. 1;
Neiman et al., 2006 – hereafter N06). N06 described the regional weather
patterns and lower-tropospheric dynamic meteorology associated with Petaluma
Gap flow (PGF) using 62 cases observed during the multi-winter deployment of
a 915 MHz wind profiling radar to Bodega Bay, CA. N06 described PGF as a
near-surface shallow (
Regional map displaying the location of BML, the town of Petaluma, CA (PTL); San Francisco, CA (SF); the Central Valley; and the Coastal Ranges. The orange arrow depicts the direction of typical flow during PGF conditions. Line A–B traces a path across the Petaluma Gap. The inset at bottom displays the cross-gap terrain profile along line A–B.
Evidence presented in N06 for the proliferation of anthropogenic pollutants at the coast during PGF included horizontal coast-normal transects and low-troposphere vertical profiles of carbon monoxide (CO) mixing ratio from a trace gas analyzer on board just one research flight of the National Oceanographic and Atmospheric Administration (NOAA) P-3 aircraft. During the coast-normal transect CO mixing ratio doubled from 120 to 240 ppbv across a 20 km wide gradient that was located approximately 75 km offshore of Bodega Bay and Point Reyes, CA. It was inferred that, when the near-surface air mass during PGF episodes traveled from the polluted Central Valley region before arriving at the coast, the air mass would acquire properties commensurate with combustion, transportation, agriculture, and manufacturing (e.g., the observation of elevated CO concentration).
When transport occurs, PGF should cause large measurable impacts on the
coastal environment via an abrupt but significant change in trace gas and
aerosol chemistry. Expected impacts include the following:
An increase in absorption of solar shortwave radiation by black carbon
(BC) aerosol, which has much greater emission sources on the continental side of
the Petaluma Gap. An increase in black carbon mass may also be associated
with more freshly emitted soot. Together, these factors may lead to a
relative decrease in both externally mixed and internally mixed
organic : elemental carbon (OC : EC) ratios. (e.g., Chung et al., 2012; Cahill et al., 2012). A brightening in nearshore marine stratocumulus clouds through the cloud
albedo indirect effect (Twomey, 1977; Solomon, 2007), since the inferred PGF
air mass contains more numerous pollution aerosol particles, a portion of
which will serve as cloud condensation nuclei (CCN). Increased deposition of nitrogen-containing particulate matter to the local
ecosystem, which may lead to increased eutrophication along the coastal shelf
(Paerl, 1995, 1997), because particles transported during PGF may have formed
or have been aged in a nitrous-oxide- and ammonia-enriched environment
(Seinfeld and Pandis, 2012).
As part of the CalWater-2 experiment (Leung et al., 2014; Ralph et al., 2015), measurements of trace gas concentrations, aerosol physicochemical properties, and lower-tropospheric meteorology were taken at the University of California, Davis Bodega Marine Laboratory during January, February, and March 2015. Using this dataset, described in Sect. 2, we report the abrupt changes in trace gases and particulate matter observed during five PGF events and establish composite aerosol size distributions, aerosol chemical sources, trace gas concentrations, and cloud condensation nuclei activation curves. We also identify particle aging through the accumulation of ammonium and nitrate during PGF using detailed single-particle mass spectrometry measurements. The analysis methods presented in Sect. 3, and their results, presented in Sect. 4, verify the above hypotheses and present a nuanced picture of the secondary heterogeneous chemistry active in aerosol particles that travel to the coast in the PGF air mass. Fine details of particle aging are location specific, but conclusions drawn from the increase in aerosol number, changes in aerosol source, brightening of marine clouds, and impact on aerosol absorption are generally applicable to many other coastal regions that periodically experience channeled offshore flow through a mountain gap.
Measurements and samples were collected from 14 January to 9 March 2015 at
Bodega Marine Laboratory (BML; 38
The sampling site at BML included two instrumented trailers located
Size-resolved aerosol composition at Bodega Bay was measured with an
aerosol time-of-flight mass spectrometer (ATOFMS) and an ultrafine
aerosol time-of-flight mass spectrometer (UF-ATOFMS). The UF-ATOFMS used a
diluting stage with an approximate dilution of
Aerosol size distributions at BML were measured using a scanning mobility
particle sizer (SMPS, TSI Inc. Model 3936) and an aerodynamic particle sizer
(APS, TSI Inc. Model 3321). The SMPS was operated with a pump flow of
0.3 L min
PM
Size-resolved (also referred to as “diameter scan”) cloud condensation
nuclei concentrations were measured using a streamwise cloud condensation
nuclei counter (Droplet Measurement Technologies Inc., CCN-100) coupled with
an SMPS. The SMPS (TSI 3080 long column) was operated at a sheath-to-sample
flow rate of 5 to 1.3 L min
Concentrations of gas-phase pollutants were determined using a suite of
gas-phase instruments, collocated with the aethalometer, CCN counter, and the
sizing instruments. A NO–NO
Level 2 MODIS cloud products (Platnick et al., 2003) are used to estimate the range of marine stratocumulus cloud optical depth offshore from BML during PGF episodes with clear sky above low clouds, and to verify that the clouds in nearshore MODIS scenes are low-level cumulus or stratocumulus clouds.
Composite aerosol size distributions, trace gas and aerosol type concentrations, indicators of secondary chemical aging, and CCN activation spectra corresponding to PGF periods and control periods are derived as a primary tool for addressing the hypotheses posed in this study. Herein, we define a control period (CTL) to be any hourly period which does not fit the definition for flow through the Petaluma Gap arriving at BML found in N06 (hereafter mPGF) and does not occur during short-lived episodes of concentrated local anthropogenic pollution. In this study, mPGF periods that also meet a minimum threshold for concentrated non-local anthropogenic pollution will be called PGF. Observed causes of local anthropogenic pollution included nearby brush fires, vehicle activity at BML, and “sea breeze resampling”. During the third of those, high concentrations of anthropogenic pollution either from a local source or from further inland that was previously transported offshore returned to the measurement site via the afternoon sea breeze. Since the polluted air mass may have up to 18 h of modification by the nearby BML marine environment just before sea breeze resampling episodes, these were classified as local anthropogenic pollution and were removed from the PGF and CTL composites.
We followed the methodology of N06 in identifying Petaluma Gap flow using the BBY 449 MHz vertically profiling radar and 10 m anemometer (see Sect. 4a in N06). Briefly, this methodology includes identifying continuous periods at least 6 h in length during which wind speed and direction criteria are met both at the surface (10 m anemometer at BML) and in the lowest retrieved layer (approximately 100 to 350 m m.s.l.) of the BML 449 MHz radar. When the conditions from the N06 methodology were met, we declared the period mPGF. It is important to note that, while 449 MHz wind profiles are collected hourly, all other data from the study are collected more frequently; therefore we classified local conditions in hourly intervals.
To choose local conditions based on an indicator of anthropogenic pollution,
we examined CalWater 2015 observations of CO, NO
Decision tree used for filtering measurement periods, and the resulting number of hourly periods (/total) in each category.
In order to exclude local or sea breeze resampled anthropogenic pollutants from CTL periods, we imposed an additional requirement based upon CO concentration – hourly mean CO concentration must be above the CalWater 2015 mean plus two standard deviations (138.1 ppbv). Along with mPGF, this requirement forms the basis of a decision table (Table 1) that allows the separation of CalWater 2015 measurements into four composites. We choose CO concentration as our additional discriminator because its interquartile range during mPGF is entirely above the interquartile range from all other periods, because its overall variability is the lowest among peripheral measurements, and because elevated near-surface CO concentration was observed by aircraft during a PGF event reported in N06. Table 1 allows the compositing of observational period by PGF (mPGF and elevated CO conditions met), CTL (neither mPGF nor elevated CO conditions met), LOCAL (mPGF not met, elevated CO met), and diffuse (mPGF met, elevated CO not met). In this light, Fig. 2a–c can be seen as examples of CTL, LOCAL, and PGF periods, respectively.
Summary of particle types determined by ATOFMS and their characteristic ion markers.
The attenuation recorded by the aethalometer was used to derive the aerosol
absorption coefficient (
ATOFMS and UF-ATOFMS can provide information on size and chemical composition (via mass spectra) for an individual particle. Generally, positive spectra reveal particle source, while negative spectra provides information on the atmospheric processing a particle has undergone (Guazzotti et al., 2001; Sullivan et al., 2007; Prather et al., 2008). ATOFMS, but not UF-ATOFMS, spectra were filtered for periodic radio frequency interference caused by a sub-optimally operating instrument component. A total of 115 416 particles were scattered and hit during PGF events, and 1 835 387 during the control time periods (see Sect. 3.1 for definition of PGF and control periods).
Single-particle spectra and size data were loaded into Matlab (The MathWorks,
Inc.) and analyzed via the software toolkit YAADA
(
It is important to describe not only particle sources but also secondary
aging impacts as the aging mechanism will change the light absorption
cross section of carbonaceous aerosols. For instance, a sulfate coating can
increase the light-absorbing properties of soot by a factor of 1.6
(Moffet and Prather, 2009). Internally mixed EC and OC have greater
absorption profiles than homogeneously mixed particles of either
species (Schnaiter et al., 2005). Additionally, aging can increase particle
hygroscopicity through condensation and reaction of gases like NO
The ATOFMS is a powerful tool with which to measure particle aging because of its
ability to measure single-particle composition and directly determine the
type and extent of particle aging. For similar particles of the same type,
relative peak areas (RPAs) qualitatively reflect the amount of a species on a
particle in relation to other species (Bhave et al., 2002; Gross et al.,
2000; Prather et al., 2008) and thus can be used to investigate the mechanism
of aging (Cahill et al., 2012). During this study, the mixing state of single
particles with secondary markers was investigated by identifying and
comparing peak areas for ammonium (
Size distribution, hygroscopicity, and CCN concentration measurements were collated from periods classified as PGF and CTL. Cumulative CCN supersaturation spectra, defined as median CCN concentration as a function of supersaturation were constructed from the integrated CCN and size distribution data. The spectra were fit to a two-mode hypergeometric model (Cohard et al., 1998) to estimate cloud droplet number concentration (CDNC) for a range of updraft velocity.
The albedo change (
Conservative scattering: this assumption is commonly invoked in studies that
estimate cloud albedo susceptibility or change (Twomey, 1991; Platnick and
Towmey, 1994; Hill and Dobbie, 2008; Hill
et al., 2008, 2009; Chen et al., 2011). Liquid cloud particles are generally
conservative (single scattering albedo
PGF events observed during CalWater 2015 and their significant parameters following N06. Ranks are out of 67 (62 cases from N06 plus 5 from CalWater 2015).
Invariant asymmetry: for visible light, cloud droplet scattering asymmetry
varies weakly with particle size (Kokhanovsky, 2004). For liquid drops, the
variation is primarily approximately 5 % over the range of effective
radius from 6 to 19
Constant liquid water path: this is the least likely of the above-listed assumptions to be valid. Cloud albedo is susceptible to changes in both cloud droplet number concentration and cloud liquid water path. The latter can also vary with cloud droplet number concentration through cloud dynamic pathways including the so-called “evaporation entrainment” and “sedimentation entrainment” effects (Lu et al., 2005; Wood, 2007; Hill et al., 2009; Chen et al., 2011). The impact of these feedbacks to cloud albedo through the dynamics that control cloud liquid water path vary strongly with environmental conditions and in some cases can cancel the direct increase in cloud albedo resulting from an increase in cloud droplet number concentration. Environmental conditions during PGF (greater likelihood of very dry air above the marine boundary layer, and an increase in large-scale subsidence and thus increased low-level static stability) have been found to favor competing effects on susceptibility through the entrainment effects (e.g., Wood, 2007; Chen et al., 2011). The strength of the entrainment feedbacks is strongly dependent on sea surface temperature as well. PGF can occur under a wide range of sea surface temperatures arising from natural variability in the northeastern Pacific Ocean. To disentangle the total susceptibility which may arise from these competing liquid water path feedbacks, a series of large-scale eddy simulations, similar to those in Lu et al. (2005) and Chen et al. (2011), are required. This is beyond the scope of the current study; thus we will only estimate the so-called “Twomey effect” (or cloud albedo first aerosol indirect effect) on albedo which corresponds to the increase in cloud albedo due to an increase in CCN concentration when liquid water path is held fixed.
The MODIS level 2 cloud products provide swath-level retrievals of liquid cloud optical depth, liquid cloud effective radius, and cloud top pressure twice daily during daylight hours from descending (Terra – 10:15 local time) and ascending (Aqua – 13:45 local time) sun-synchronous orbits. The level 2 cloud products have a nominal spatial resolution of 20 km. For this study, daytime retrievals during PGF conditions from the expanded catalog (N06 PGF events plus Table 3 from this study) during the MODIS operational period (2002–present) were screened to remove pixels over land or more than 75 km from the coast (offshore extent of PGF air mass found by aircraft and reported in N06) and pixels which likely did not correspond to low-level cumulus or stratocumulus. We followed the cloud type definitions (e.g., Fig. 2 from Rossow and Schiffer, 1999) from the International Satellite Cloud Climatology Project (ISCCP) that rely upon thresholds of both cloud top pressure and cloud optical depth. Pixels for which no cloud information was retrieved were also discarded (no cloud present, or retrieval algorithm failed). The retrieved effective radius was also retained to judge the suitability of the invariant asymmetry assumption. The cloud albedo change reported is thus the estimate of the Twomey effect on albedo during PGF episodes when marine cumulus or stratocumulus are present with clear sky above marine low-level clouds.
Table 3 lists all cases which fit the mPGF requirements during CalWater 2015. Hereafter, these will be referred to as PGF(1–5). Some key parameters which describe the PGF layer flow measured by the 449 MHz radar are also summarized in Table 3, along with their ranking among 67 cases (62 cases from N06 plus 5 from CalWater 2015). Note that in all 5 cases both mPGF and elevated CO are met for a majority of the period; however the listed start time and duration in Table 3 is for mPGF, and in some cases the duration for PGF may be shorter than that listed when the additional elevated CO constraint is enforced.
Figure 3 shows a box-and-whisker plot for the peripheral instrument data. So
that all measurements fall on the same scale, each measurement has been
normalized according to its all-study mean (
Figure 3 also displays wind rose diagrams for ALL and CTL periods. The distribution of wind directions and speeds during CTL suggests that these periods are dominated by the land–sea breeze diurnal cycle (BML is situated just east of a shoreline oriented NNW to SSE). The wind rose for PGF is not shown, since wind direction was used in the algorithm for defining PGF.
Normalized aethalometer light absorption coefficient at seven wavelengths for hourly periods classified as CTL (blue), PGF (red), and LOCAL (black). Upper/lower box bounds represent upper/lower 25 % values, respectively. Upper/lower whiskers represent max/min values, respectively. Box middle represents median value. Also displayed are the AAE values found by regression during each period.
The range of normalized hourly
Figure 5 shows the average merged size distributions for PGF and CTL sampling
periods. CTL periods were marked by lower particle concentrations in the
submicron mode and higher particle concentrations in the coarse mode (
PGF events, in contrast, showed a large increase in the number of particles
with
Pie charts for sub- (top panels) and supermicron (bottom panels) particle types for CTL (left panels) and PGF (right panels). Description of particle classifications can be found in Table 2.
PGF conditions coincide with a shift in particle type away from marine and
towards continental. Figure 6 shows pie charts of the sub- and supermicron
particle populations for CTL vs. PGF. Percentages indicate the number
fraction of particles assigned to the corresponding particle type. The total
hit rate for all particles was 20.8 %. Panels a and b show the submicron
(0.2–1.0
The clearest delineation in particle type was observed in the supermicron
fraction (1.0–3.0
The dust and dust/bio types also increased during PGF. The CV, despite its agricultural production, is a semiarid environment and can be a significant source of dust. Conversely, BML air masses during CTL periods were heavily influenced by the Pacific Ocean and thus were not expected to contain much dust. The shift in supermicron particle composition away from marine particles to anthropogenic and dust particle types supports the conclusion that PGF air masses likely originate from the CV.
Figure 7 shows the sulfate : nitrate ratio (SN) of particles separated by
type. Unmodified peak ratios are dependent upon which peak is in the
denominator, i.e., whether or not the ratio is greater than 1, thus
potentially skewing the data. To account for this, we calculated the
normalized peak ratio by the following: ratio
In addition to probing the partitioning of acidic species, basic species like
amines and ammonium were investigated. Figure 8 shows the normalized ratio
for amines : ammonium ratio (AA). The AA for CTL particles shows fairly
equal partitioning for all particle types. During PGF the average peak area
of amine peaks (
As in Fig. 7 except that amines : ammonium ion ratio distributions are shown.
Previous studies (Cahill et al., 2012) have used the ATOFMS to determine the
internal mixing state of carbonaceous particles. Figure 9 shows the organic
carbon : soot ratio as calculated by the ATOFMS, seperated by particle
type. CTL particles appear to have relatively higher amounts of OC, most
notably in the amine particle types and, unsurprisingly, the OC type. Amines
consist of organic carbon chains bound to nitrogen atoms, so it is also
unsurprising that these particles would have high OC : EC ratios. The ratio
plot indicates that PGF particles contain more EC relative to CTL particles.
This is despite the appearance of the AN particle type, which had greater OC
character. These OC
As in Fig. 7 except that OC : soot ion ratio distributions are shown.
In summary this analysis shows that the preeminent aging mechanisms associated with PGF are the accumulation of ammonium and nitrate, in accordance with previous studies on Central Valley particle composition (Qin and Prather, 2006). Amine accumulation was also observed in ECOC and AN particles but was determined to not be as significant as ammonium. Accumulation of nitrogen species on aerosol particles is important as it increases the risk of nitrogen deposition into coastal waters, which can lead to ecosystem degradation (Ryther and Dunstan, 1971; Paerl, 1995, 1997). The shift toward internal mixtures containing elemental carbon and away from particulate matter containing primarily organic carbonaceous species during PGF suggests that gap flow may cause increased solar absorption by near-surface aerosols, especially in visible wavelengths. This potential impact is corroborated by the aethalometer PGF and CTL measurements.
Left: cumulative median CCN supersaturation spectrum PGF periods (blue) and CTL (yellow). Dashed lines approximate the interquartile range. Right: as in left but for predicted cloud droplet number concentration as a function of updraft velocity.
Figure 10 displays the cumulative CCN supersaturation spectrum (versus liquid
supersaturation) transformed from the size-resolved CCN data and the CDNC for updraft velocities between 0.1 and
10 m s
During expanded catalog (see Sect. 3) PGF episodes the observed cloud albedo
ranged from 0.01 to 0.63, with upper (lower) quartile values of 0.43 (0.17).
Using the observed
Measurements taken at Bodega Bay, CA, during the CalWater 2015 intensive observing period were used to investigate the impacts of Petaluma Gap flow on local air quality and marine cloud albedo. The kinematics of PGF and its relation to synoptic-scale weather patterns and the Central Valley cold pool have been perviously described in N06. This study is the first attempt to quantify the impact of PGF on the boundary layer air mass and particle chemistry.
Vertically resolved lower tropospheric wind observations and carbon monoxide concentration were used to identify PGF periods during the CalWater 2015 intensive observing period and separate these from CTL periods, during which the BML air mass was influenced neither by PGF nor by heavy pollutant loads from a local source. Five PGF events were identified during Calwater 2015 and were compared to the PGF catalog published in N06 by means of their local weather attributes.
During Calwater 2015 PGF periods, several measures of anthropogenic
pollution – including CO, NO
Single-particle chemical mixing state during PGF events was investigated using UF-ATOFMS and ATOFMS measurements. It was found that submicron particle populations change during PGF to favor ECOC, BB, AN, and EC types at the expense of SS types. The large difference in supermicron particle mixing state is likely related to the shift in prominent wind direction during PGF. The analysis of secondary aging also showed that carbonaceous particles are more likely to contain elemental carbon than organic carbon during PGF episodes. Aethalometer-derived AAE also suggested that observed soot was less aged during PGF periods, but total absorption and total black carbon mass were greater than during CTL. The above results reinforce the hypothesis that PGF could lead to an increase in absorption of solar shortwave radiation by black carbon aerosol, which may be associated with more freshly emitted soot.
PGF and CTL single-particle mass spectra relative peak area ratios were used
to investigate particle aging mechanism. PGF particles were much more likely
to acquire nitrate than CTL particles, which preferentially contained
sulfate. This was especially true for AN, ECOC, BB, and EC particle types
during PGF but may not apply to SS and aged SS, which showed a preference
for nitrate aging even during CTL periods. The aging of SS by nitrate is a
well-documented phenomenon that was also regularly observed during CalWater
2015. Relative peak area analysis also showed that particles are much more
likely to chemically age by ammonium than by amines during PGF. This tendency
appeared especially strong for BB, EC, and ECOC types. While OC type particles
increased in relative number during PGF episodes, they appeared to favor the
amine aging pathway even during PGF. Together the above results reinforce the
hypothesis that PGF could lead to increased deposition of nitrogen-containing
particulate matter to the local ecosystem near and offshore of Bodega Bay.
This result may also be true in other coastal locations which are
periodically influenced by offshore gap flow originating in a NO
Particle hygroscopicity, as shown by size-resolved CCN measurements, was nearly invariant between PGF and CTL periods. The model of Cohard et al. (1998) was used to estimate the cloud droplet number concentration resulting from the derived CCN activation curves (Sect. 3.5). The increase found in CDNC was stable across a wide range of updraft velocities. The marine cloud albedo change in response to PGF CCN was estimated using MODIS level 2 cloud products and Eq. (7) from Platnick and Twomey (1994). To first order (assuming constant liquid water path) it is estimated that nearshore marine clouds will brighten by 16 to 28 % (interquartile range) in visible wavelengths during PGF events. This finding supports the hypothesis that PGF conditions may lead to a brightening in nearshore marine stratocumulus clouds through the cloud albedo indirect effect.
The conclusions reached in addressing the three hypotheses posed in this study represent only the first attempt to characterize the impact of Petaluma Gap flow on the aerosol direct effect, aerosol indirect effect, and coastal environment in northern central California. Due to the relatively short CalWater 2 intensive observing campaign, these results were drawn from only five PGF events. The data necessary to investigate these hypotheses were drawn from a large multi-agency effort including many specialized and operator intensive measurements, which by nature must be short in duration. Longer-term observation, perhaps by less detailed but targeted chemical observations at similar locations, could significantly augment the findings presented here.
During this study, we attempted to detect inter-event differences in relative peak area ratios for secondary aging indicators, but no significant change was observed. In addition, the authors wish to comment that many of the assumptions made (e.g., constant liquid water path) in estimating the impact of PGF on marine cloud albedo change can only be discarded through airborne observations or modeling studies. These were considered beyond the scope of this study but may be valuable future investigations to fully describe the impact of polluted offshore-directed gap flow on marine cloud brightness.
The findings presented herein demonstrate that PGF can impact aerosol number, chemical aging pathways, shortwave absorption, and the number of CCN available to nearshore marine clouds. These findings are the first of their kind that result from direct observation of an intermittent weather phenomenon that brings anthropogenic pollutants to an otherwise remote region. While the findings follow from in situ observations representative of a small region, we note that the meteorological factors causing Petaluma Gap flow (pooling of cold continental air; an onshore, mountain-normal-directed pressure gradient; a narrow low-elevation gap in the coastal mountain range) certainly exist in other regions. Thus, the introduction of anthropogenically influenced continental air to remote marine environments may modify air quality and aerosol direct and indirect effects in other regions experiencing regular offshore gap flow as well. The authors argue that further study of the chemical composition of continental outflow in other regions is necessary to refine current understanding of the impact of human activities on the environment.
The data used in this study is available at
The authors thank all other CalWater and ACAPEX 2015 participants, including those from Pacific Northwest National Laboratories; The National Oceanic and Atmospheric Administration; NASA's Jet Propulsion Laboratory; the Naval Research Laboratory; University of California, Davis; Scripps Institution of Oceanography; Colorado State University; and North Carolina State University. The authors would also like to thank the UC Davis Bodega Marine Laboratory for the use of laboratory and office space, and shipping and physical plant support while collecting data, as well as the California Air Resources Board and the National Park Service for the trailers used for sampling. This research was funded by NSF award number 1451347 (ACM, GCC, KAM, KAP), NSF award number 1450690 (MDP, NR, HT), and NSF award number 1450760 (SAA, SMK, PJD).Edited by: H. Saathoff Reviewed by: two anonymous referees