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Daytime atmospheric boundary layer (ABL) dynamics—including potential temperature budgets, water vapour budgets, and entrainment rates—are presented from in situ flight data taken on six afternoons near Fresno in the San Joaquin Valley (SJV) of California during July/August 2016. The flights took place as a part of the California Baseline Ozone Transport Study aimed at investigating transport pathways of air entering the Central Valley from offshore and mixing down to the surface. Midday entrainment velocity estimates ranged from 0.8 to 5.4 cm s−1 and were derived from a combination of continuously determined ABL heights during each flight and model-derived subsidence rates, which averaged -2.0 cm s−1 in the flight region. A strong correlation was found between entrainment velocity (normalized by the convective velocity scale) and an inverse bulk ABL Richardson number, suggesting that wind shear at the ABL top plays a significant role in driving entrainment. Similarly, we found a strong correlation between the entrainment efficiency (the ratio of entrainment to surface heat fluxes with an average of 0.23 ± 0.15) and the wind speed at the ABL top. We explore the synoptic conditions that generate higher winds near the ABL top and propose that warm anomalies in the southern Sierra Nevada mountains promote increased entrainment. Additionally, a method is outlined to estimate turbulence kinetic energy, convective velocity scale (w*), and the surface sensible heat flux in the ABL from a slow, airborne wind measurement system using mixed-layer similarity theory.

The San Joaquin Valley of California is known for excessive ozone air pollution owing to local production combined with terrain-induced flow patterns that channel air in from the highly populated San Francisco Bay area and stagnate it against the surrounding mountains. During the summer, ozone violations of the National Ambient Air Quality Standards (NAAQS) are notoriously common, with the San Joaquin Valley having an average of 115 violations of the current 70&ppb standard each year between 2012 and 2016. Because regional photochemical production peaks with actinic radiation, most studies focus on the daytime, and consequently the nocturnal chemistry and dynamics that contribute to these summertime high-ozone events are not as well elucidated. Here we investigate the hypothesis that on nights with a strong low-level jet (LLJ), ozone in the residual layer (RL) is more effectively mixed down into the nocturnal boundary layer (NBL) where it is subject to dry deposition to the surface, the rate of which is itself enhanced by the strength of the LLJ, resulting in lower ozone levels the following day. Conversely, nights with a weaker LLJ will sustain RLs that are more decoupled from the surface, retaining more ozone overnight, and thus lead to more fumigation of ozone the following mornings, giving rise to higher ozone concentrations the following afternoon. The relative importance of this effect, however, is strongly dependent on the net chemical overnight loss of Ox (here [Ox] ĝ‰i [O3] + [NO2]), which we show is highly uncertain, without knowing the ultimate chemical fate of the nitrate radical (NO3). We analyze aircraft data from a study sponsored by the California Air Resources Board (CARB) aimed at quantifying the role of RL ozone in the high-ozone events in this area. By formulating nocturnal scalar budgets based on pairs of consecutive flights (the first around midnight and the second just after sunrise the following day), we estimate the rate of vertical mixing between the RL and the NBL and thereby infer eddy diffusion coefficients in the top half of the NBL. The average depth of the NBL observed on the 12 pairs of flights for this study was 210( ± 50)&m. Of the average ĝ'1.3&ppb&hĝ'1 loss of Ox in the NBL during the overnight hours from midnight to 06:00&PST, ĝ'0.2&ppb&hĝ'1 was found to be due to horizontal advection, ĝ'1.2&ppb&hĝ'1 due to dry deposition, ĝ'2.7&ppb&hĝ'1 to chemical loss via nitrate production, and +2.8&ppb&hĝ'1 from mixing into the NBL from the RL. Based on the observed gradients of Ox in the top half of the NBL, these mixing rates yield eddy diffusivity estimates ranging from 1.1 to 3.5&m2&sĝ'1, which are found to inversely correlate with the following afternoon's ozone levels, providing support for our hypothesis. The diffusivity values are approximately an order of magnitude larger than the few others reported in the extant literature for the NBL, which further suggests that the vigorous nature of nocturnal mixing in this region, due to the LLJ, may have an important control on daytime ozone levels. Additionally, we propose that the LLJ is a branch of what is colloquially referred to as the Fresno eddy, which has been previously proposed to recirculate pollutants. However, vertical mixing from the LLJ may counteract this effect, which highlights the importance of studying the LLJ and Fresno eddy as a single interactive system. The synoptic conditions that are associated with strong LLJs are found to contain deeper troughs along the California coastline. The LLJs observed during this study had an average centerline height of 340&m, average speed of 9.9&m&sĝ'1 (σ Combining double low line 3.1&m&sĝ'1), and a typical peak timing around 23:00&PST. A total of 7 years of 915&MHz radioacoustic sounding system and surface air quality network data show an inverse correlation between the jet strength and ozone the following day, further suggesting that air quality models need to forecast the strength of the LLJ in order to more accurately predict ozone violations.

Midday summertime flight data collected in the atmospheric boundary layer (ABL) of California's San Joaquin Valley (SJV) are used to investigate the scalar budgets of NOx , O3, and CH4, in order to quantify the individual processes that control near-surface concentrations, yet are difficult to constrain from surface measurements alone: These include, most importantly, horizontal advection and entrainment mixing from above. The setting is a large mountain- valley system with a small aspect ratio, where topography and persistent temperature inversions impose strong restraints on ABL ventilation. In conjunction with the observed time rates of change this airborne budgeting technique enables us to deduce net photochemical ozone production rates and emission fluxes of NOx and CH4. Estimated NOx emissions from our principal flight domain averaged 216 (33) t d-1 over six flights in July and August, which is nearly double the California government's NOx inventory for the surrounding three-county region. We consider several possibilities for this discrepancy, including the influence of wildfires, the temporal bias of the airborne sampling, instrumental interferences, and the recent hypothesis presented by Almaraz et al. (2018) of localized high soil NO emissions from intensive agricultural application of nitrogen fertilizers in the region and find the latter to be the most likely explanation. The methane emission average was 438 Gg yr-1 (143), which also exceeds the emissions inventory for the region by almost a factor of 2. Measured ABL ozone during the six afternoon flights averaged 74 ppb (D 9:8 ppb). The average midafternoon ozone rise of 2.8 ppb h-1 was found to be comprised of-0:8 ppb h-1 due to horizontal advection of lower O3 levels upwind,-2:5 ppb h-1 from dry deposition loss,-0:5 ppb h-1 from dilution by entrainment mixing, and 6.9 ppb h-1 net in situ photochemical production. The O3 production rates exhibited a dependence on NO2 concentrations (r2 D 0:35) and no discernible dependence on methane concentrations (r2 0:02), which are correlated with many of the dominant volatile organic compounds in the region, suggesting that the ozone chemistry was predominantly NOx-limited on the flight days. Additionally, in order to determine the heterogeneity of the different scalars, autocorrelation lengths were calculated for potential temperature (18 km), water vapor (18 km), ozone (30 km), methane (27 km), and NOx (28 km). The spatially diffuse patterns of CH4 and NOx seem to imply a preponderance of broad areal sources rather than localized emissions from cities and/or highway traffic within the SJV.

Maaz et al. argue that inconsistencies across scales of observation undermine our working hypothesis that soil NO x emissions have been substantially overlooked in California; however, the core issues they raise are already discussed in our manuscript. We agree that point measurements cannot be reliably used to estimate statewide soil NO x emissions-the principal motivation behind our new modeling/airplane approach. Maaz et al.'s presentation of fertilizer-based emission factors (a nonmechanistic scaling of point measures to regions based solely on estimated nitrogen fertilizer application rates) includes no data from California or other semiarid sites, and does not explicitly account for widely known controls of climate, soil, and moisture on soil NO x fluxes. In contrast, our model includes all of these factors. Finally, the fertilizer sales data that Maaz et al. highlight are known to suffer from serious errors and do not offer a logically more robust pathway for spatial analysis of NO x emissions from soil.

Maaz et al. argue that inconsistencies across scales of observation undermine our working hypothesis that soil NO x emissions have been substantially overlooked in California; however, the core issues they raise are already discussed in our manuscript. We agree that point measurements cannot be reliably used to estimate statewide soil NO x emissions-the principal motivation behind our new modeling/airplane approach. Maaz et al.'s presentation of fertilizer-based emission factors (a nonmechanistic scaling of point measures to regions based solely on estimated nitrogen fertilizer application rates) includes no data from California or other semiarid sites, and does not explicitly account for widely known controls of climate, soil, and moisture on soil NO x fluxes. In contrast, our model includes all of these factors. Finally, the fertilizer sales data that Maaz et al. highlight are known to suffer from serious errors and do not offer a logically more robust pathway for spatial analysis of NO x emissions from soil.

Aerosol and tropospheric ozone transport to North America has important implications for regional air quality and climate. We present data from Chews Ridge, a remote mountaintop site in California, in February–September 2012, which identifies separate Asian dust and Asian combustion aerosol signals and quantifies exogenous ozone. We use positive matrix factorization of size-fractionated aerosol X-ray fluorescence data and 206Pb/207Pb and 208Pb/207Pb isotope measurements to isolate aerosol signals of Asian origins and find good agreement between both types source attribution techniques. Power spectra of Asian dust and Asian combustion signals reveal two distinct peaks, one seasonal (~90 days) and one monthly (~25–30 days). Overall, 89% of the Asian dust signal occurs in March–May, compared with June–September, whereas only 51% of the Asian combustion signal occurs in March–May. Nearly half the Pb observed at Chews Ridge was derived from Asia (46%; 8.3% from Asian dust, 23% from Asian combustion, and 15% of the Asian Pb signal was not explained by the positive matrix factorization). A positive correlation between deseasonalized ozone concentrations and the Asian factors suggests that ozone is elevated by 6.3 ± 0.8 ppb due to transpacific transport. Scanning mobility particle size spectra were also measured from May to September and indicate that new particles are often produced and grow throughout the afternoon during onshore flow. We hypothesize that particle nucleation on the windward edge of the continent is an important source of new particles.

Nitrogen oxides (NO x = NO + NO2) are a primary component of air pollution-a leading cause of premature death in humans and biodiversity declines worldwide. Although regulatory policies in California have successfully limited transportation sources of NO x pollution, several of the United States' worst-air quality districts remain in rural regions of the state. Site-based findings suggest that NO x emissions from California's agricultural soils could contribute to air quality issues; however, a statewide estimate is hitherto lacking. We show that agricultural soils are a dominant source of NO x pollution in California, with especially high soil NO x emissions from the state's Central Valley region. We base our conclusion on two independent approaches: (i) a bottom-up spatial model of soil NO x emissions and (ii) top-down airborne observations of atmospheric NO x concentrations over the San Joaquin Valley. These approaches point to a large, overlooked NO x source from cropland soil, which is estimated to increase the NO x budget by 20 to 51%. These estimates are consistent with previous studies of point-scale measurements of NO x emissions from the soil. Our results highlight opportunities to limit NO x emissions from agriculture by investing in management practices that will bring co-benefits to the economy, ecosystems, and human health in rural areas of California.

Nitrogen oxides (NO x = NO + NO2) are a primary component of air pollution-a leading cause of premature death in humans and biodiversity declines worldwide. Although regulatory policies in California have successfully limited transportation sources of NO x pollution, several of the United States' worst-air quality districts remain in rural regions of the state. Site-based findings suggest that NO x emissions from California's agricultural soils could contribute to air quality issues; however, a statewide estimate is hitherto lacking. We show that agricultural soils are a dominant source of NO x pollution in California, with especially high soil NO x emissions from the state's Central Valley region. We base our conclusion on two independent approaches: (i) a bottom-up spatial model of soil NO x emissions and (ii) top-down airborne observations of atmospheric NO x concentrations over the San Joaquin Valley. These approaches point to a large, overlooked NO x source from cropland soil, which is estimated to increase the NO x budget by 20 to 51%. These estimates are consistent with previous studies of point-scale measurements of NO x emissions from the soil. Our results highlight opportunities to limit NO x emissions from agriculture by investing in management practices that will bring co-benefits to the economy, ecosystems, and human health in rural areas of California.

In situ flight data from two distinct campaigns during winter and summer seasons in the San Joaquin Valley (SJV) of California are used to calculate boundary-layer entrainment rates, ozone photochemical production rates, and regional methane emissions. Flights near Fresno, California, in January and February 2013 were conducted in concert with the NASA DISCOVER-AQ project. The second campaign (ArvinO3), consisting of 11 days of flights spanning June through September 2013 and 2014, focused on the southern end of the SJV between Bakersfield and the small town of Arvin, California - a region notorious for frequent violations of ozone air quality standards. Entrainment velocities, the parameterized rates at which free tropospheric air is incorporated into the atmospheric boundary layer (ABL), are estimated from a detailed budget of the inversion base height. During the winter campaign near Fresno, we find an average midday entrainment velocity of 1.5g cmg sg'1, and a maximum of 2.4g cmg sg'1. The entrainment velocities derived during the summer months near Bakersfield averaged 3g cmg sg'1 (ranging from 0.9 to 6.5g cmg sg'1), consistent with stronger surface heating in the summer months. Using published data on boundary-layer heights we find that entrainment rates across the Central Valley of California have a bimodal annual distribution peaking in spring and fall when the lower tropospheric stability (LTS) is changing most rapidly.Applying the entrainment velocities to a simple mixed-layer model of three other scalars (O3, CH4, and H2O), we solve for ozone photochemical production rates and find wintertime ozone production (2.8g ±g 0.7g ppbg hg'1) to be about one-third as large as in the summer months (8.2g ±g 3.1g ppbg hg'1). Moreover, the summertime ozone production rates observed above Bakersfield-Arvin exhibit an inverse relationship to a proxy for the volatile organic compound (VOC)g :g NOx ratio (aircraft [CH4] divided by surface [NO2]), consistent with a NOx-limited photochemical environment. A similar budget closure approach is used to derive the regional emissions of methane, yielding 100 (±100)g Ggg yrg'1 for the winter near Fresno and 170 (±125)g Ggg yrg'1 in the summer around Bakersfield. These estimates are 3.6 and 2.4 times larger, respectively, than current state inventories suggest. Finally, by performing a boundary-layer budget for water vapor, surface evapotranspiration rates appear to be consistently g1/4 g 55g % of the reference values reported by the California Irrigation Management Information System (CIMIS) for nearby weather stations.

This study aims to enhance the understanding and application of the Aerodyne high-resolution aerosol mass spectrometer (HR-AMS) for the comprehensive characterization of organic nitrogen (ON) compounds in aerosol particles and atmospheric droplets. To achieve this goal, we analyzed 75 N -containing organic compounds, representing a diverse range of ambient non-organonitrate ON (NOON) types, including amines, amides, amino acids, N heterocycles, protein, and humic acids. Our results show that NOON compounds can produce significant levels of NHx+ and NOx+ ion fragments, which are typically recognized as ions representative of inorganic nitrogen species. We also identified the presence of CH2N+ at m/z = 28.0187, an ion fragment rarely quantified in ambient datasets due to substantial interference from N2+. As a result, the utilization of an updated calibration factor of 0.79 is necessary for accurate NOON quantification via the HR-AMS. We also assessed the relative ionization efficiencies (RIEs) for various NOON species and found that the average RIE for NOON compounds (1.52 ± 0.58) aligns with the commonly used default value of 1.40 for organic aerosol. Moreover, through a careful examination of the HR-AMS mass spectral features of various NOON types, we propose fingerprint ion series that can aid the NOON speciation analysis. For instance, the presence of CnH2n+2N+ ions is closely linked with amines, with CH4N+ indicating primary amines, C2H6N+ suggesting secondary amines, and C3H8N+ representing tertiary amines. CnH2nNO+ ions (especially for n values of 1–4) are very likely derived from amides. The co-existence of three ions, C2H4NO2+ , C2H3NO+ , and CH4NO+ , serves as an indicator for the presence of amino acids. Additionally, the presence of CxHyN2+ ions indicates the occurrence of 2 N -heterocyclic compounds. Notably, an elevated abundance of NH4+ is a distinct signature for amines and amino acids, as inorganic ammonium salts produce only negligible amounts of NH4+ in the HR-AMS. Finally, we quantified the NOON contents in submicron particles (PM1) and fog water in Fresno, California, and PM1 in New York City (NYC). Our results revealed the substantial presence of amino compounds in both Fresno and NYC aerosols, whereas concurrently collected fog water in Fresno contained a broader range of NOON species, including N -containing aromatic heterocycle (e.g., imidazoles) and amides. These findings highlight the significant potential of employing the widespread HR-AMS measurements of ambient aerosols and droplets to enhance our understanding of the sources, transformation processes, and environmental impacts associated with NOON compounds in the atmosphere. [ABSTRACT FROM AUTHOR]

Midday summertime flight data collected in the atmospheric boundary layer (ABL) of California's San Joaquin Valley (SJV) are used to investigate the scalar budgets of NOx, O3, and CH4 in order to quantify the individual processes that control near surface concentrations yet are difficult to constrain from surface measurements alone: most importantly, horizontal advection and entrainment mixing from above. The setting is a large mountain-valley system with a small aspect ratio where topography and persistent temperature inversions impose strong restraints on ABL ventilation. In conjunction with the observed time rate of change this airborne budgeting technique enables us to deduce net photochemical ozone production rates and emission fluxes of NOx and CH4. Measured NOx emissions from our principal flight domain were 216 (±33) metric tons/day averaged over six flights in July and August, which is nearly double the California government's NOx inventory for the surrounding three county region. We consider several possibilities for this discrepancy including the influence of wildfires, the temporal bias of the airborne sampling, instrumental interferences, and the recent hypothesis presented by Almaraz et al. (2018) of localized high soil NO emissions from intensive agricultural application of nitrogen fertilizers in the region and find the latter to be the most likely explanation. The methane emission average was 438 Gigagrams/year (±143), which exceeds an emissions inventory for the region by almost a factor of two as well. Measured ABL ozone during the six afternoon flights averaged 74ppb (σ=9.8ppb). The average mid-afternoon ozone rise of 2.8ppb/h was found to be comprised of -0.8ppb/h due to horizontal advection of lower O3 levels upwind, -2.2ppb/h from dry deposition loss, -0.5ppb/h from dilution by entrainment mixing, and 6.7ppb/h net in-situ photochemical production. The O3 production rates exhibited a dependence on NO2 concentrations (r2=0.35), and no discernible dependence on methane concentrations (r2~0.02) which are correlated with many of the dominant VOC's in the region, suggesting that the ozone chemistry was predominantly NOx-limited on these flight days. Additionally, in order to determine the heterogeneity of the different scalars autocorrelation lengths were calculated for potential temperature (18km), water vapor (18km), ozone (30km), methane (27km), and NOx (28km). The spatially diffuse pattern of CH4 and NOx seem to imply a preponderance of broad areal sources rather than localized emissions from cities and/or highway traffic within the SJV. [ABSTRACT FROM AUTHOR]

The San Joaquin valley is known for excessive secondary air pollution owing to local production combined with terrain-induced flow patterns that channel air in from the highly populated San Francisco Bay area and stagnate it against the surrounding mountains. During the summer, ozone violations of the National Ambient Air Quality Standards (NAAQS) are notoriously common, with the San Joaquin Valley having an average of 115 violations of the recent 70 ppb standard each year between 2012 and 2016. The nocturnal dynamics that contribute to these summertime high ozone events have yet to be fully elucidated. Here we investigate the hypothesis that on nights with a strong low-level jet (LLJ) ozone in the residual layer is more effectively mixed down into the stable boundary layer where it is subject to dry deposition to the surface, the rate of which is itself enhanced by the strength of the LLJ, resulting in lower ozone levels the following day. Conversely, nights with a weaker jet will sustain residual layers that are more decoupled from the surface and thus lead to stronger fumigation of ozone in the mornings giving rise to higher ozone concentrations the following afternoon. We analyse aircraft data from a study sponsored by the California Air Resources Board (CARB) aimed at quantifying the role of residual layer ozone in the high ozone episode events in this area. By formulating nocturnal scalar budgets based on flights around midnight and just after sunrise the following days, we estimate the rate of vertical mixing between the residual layer (RL) and the nocturnal boundary layer (NBL), and thereby measure eddy diffusion coefficients in the top half of the NBL. The average depth of the NBL observed on the 12 pairs of flights was 210 (±50) m. Of the average -1.3 ppb h-1 loss of the Ox family (here [Ox] = [O3] + [NO2]) in the NBL during the overnight hours from midnight to 06:00 PST, -0.2 ppb h-1 was found to be due to horizontal advection, -1.2 ppb h-1 due to dry deposition, -2.7 ppb h-1 to chemical loss via nitrate production, and +2.8 ppb h-1 from mixing into the NBL from the residual layer overnight. Based on the observed gradients of Ox in the top half of the NBL these mixing rates yield eddy diffusivity estimates ranging from 1.1-3.5 m² s-1 that are found to inversely correlate with the following afternoon's ozone levels, and provide support for our hypothesis. The diffusivity values are approximately an order of magnitude larger than the few others reported in the extant literature for the NBL, which further suggests that the vigorous nature of nocturnal mixing in this region due to the LLJ has an important control on ozone. Additionally, we investigate the synoptic conditions that occasion strong nocturnal jets and find that on average, deeper troughs along the California coastline are associated with stronger jets. The LLJ had an average height of 340 m, an average speed of 9.9 m s-1 (SD = 3.1 m s-1) and a typical peak timing around 23:00 ST. Seven years of 915 MHz radio-acoustic sounding system and surface air quality network data show an inverse correlation between the jet strength and ozone the following day, suggesting that air quality models need to forecast the strength of this nocturnal feature in order to more accurately predict ozone violations. [ABSTRACT FROM AUTHOR]

In situ flight data from two distinct campaigns during winter and summer seasons in the San Joaquin Valley (SJV) of California are used to calculate boundary layer entrainment rates, ozone photochemical production rates, and regional methane emissions. Flights near Fresno, California in January and February 2013 were conducted in concert with the NASA DISCOVER-AQ project. The second campaign (ArvinO3), consisting of eleven days of flights spanning June through September 2013 and in June 2014 focused on the southern end of the SJV between Bakersfield and the small town of Arvin, California, a region notorious for frequent violations of ozone air quality standards. Entrainment velocities, the parameterized rates at which free tropospheric air is incorporated into the atmospheric boundary layer (ABL), are estimated from a detailed budget of the inversion base height. During the winter campaign near Fresno, we find an average midday entrainment velocity of 1.5 cm s−1, and a maximum of 2.4 cm s−1. The entrainment velocities derived during the summer months near Bakersfield averaged 3 cm s−1 (ranging from 0.9-6.5 cm s−1), consistent with stronger surface heating in the summer months. Using published data on boundary layer heights we find that entrainment rates across the Central Valley of California have a bimodal annual distribution peaking in spring and fall when the lower tropospheric stability (LTS) is changing most rapidly. Applying the entrainment velocities to a simple mixed-layer model of three other scalars (O3, CH4, and H2O), we solve for ozone photochemical production rates and find wintertime ozone production (2.8 ± 0.7 ppb h−1) to be about one-third as large as in the summer months (8.2 ± 3.1 ppb h−1). Moreover, the summertime ozone production rates observed above Bakersfield/Arvin exhibit an inverse relationship to a proxy for the VOC : NOx ratio (aircraft [CH4] divided by surface [NO2]), consistent with a NOx-limited photochemical environment. A similar budget closure approach is used to derive the regional emissions of methane, yielding 100 Gg yr−1 for the winter near Fresno and 170 Gg yr−1 in the summer around Bakersfield. These estimates are 3.6 and 2.4 times larger, respectively, than current state inventories suggest. Finally, by performing a boundary layer budget for water vapour, surface evapotranspiration rates appear to be consistently ~ 55 % of the reference values reported by the California Irrigation Management Information System (CIMIS) for nearby weather stations. [ABSTRACT FROM AUTHOR]

The article presents a study on the quantification of nitrogen oxide (NOx) emissions from agricultural soils in California. Topics include the contribution of soil NOx in air quality issues, management of NOx emissions from agriculture, and NOx emissions in areas with high nitrogen (N) fertilizer applications.