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Extensive airborne measurements of non-methane organic gases (NMOGs), methane, nitrogen oxides, reduced nitrogen-species, and aerosol emissions from US wild and prescribed fires were conducted during the 2019 NOAA/NASA Fire Influence on Regional to Global Environments and Air Quality campaign (FIREX-AQ). Here, we report the atmospheric enhancement ratios (ERs) and inferred emission factors (EFs) for compounds measured onboard the NASA DC-8 research aircraft for nine wildfires and one prescribed fire, which encompass a range of vegetation types. We use photochemical proxies to identify young smoke and reduce the effects of chemical degradation on our emissions calculations. ERs and EFs calculated from FIREX-AQ observations agree within a factor of 2 with values reported from previous laboratory and field studies for more than 80 % of the carbon- and nitrogen-containing species. Wildfire emissions are parameterized based on correlations of the sum of NMOGs with reactive nitrogen oxides (NOy) to modified combustion efficiency (MCE) as well as other chemical signatures indicative of flaming/smoldering combustion, including carbon monoxide (CO), nitrogen dioxide (NO2), and black carbon aerosol. The sum of primary NMOG EFs correlates to MCE with an R² of 0.68 and a slope of -296 ± 51 g kg-1, consistent with previous studies. The sum of the NMOG mixing ratios correlates well with CO with an R² of 0.98 and a slope of 137 ± 4 ppbv of NMOGs per ppmv of CO, demonstrating that primary NMOG emissions can be estimated from CO. Individual nitrogen-containing species correlate better with NO2, NOy, and black carbon than with CO. More than half of the NOy in fresh plumes is NO2 with an R² of 0.95 and a ratio of NO2 to NOy of 0.55 ± 0.05 ppbv ppbv-1, highlighting that fast photochemistry had already occurred in the sampled fire plumes. The ratio of NOy to the sum of NMOGs follows trends observed in laboratory experiments and increases exponentially with MCE, due to increased emission of key nitrogen species and reduced emission of NMOGs at higher MCE during flaming combustion. These parameterizations will provide more accurate boundary conditions for modeling and satellite studies of fire plume chemistry and evolution to predict the downwind formation of secondary pollutants, including ozone and secondary organic aerosol. [ABSTRACT FROM AUTHOR]

The availability of formaldehyde (HCHO) (a proxy for volatile organic compound reactivity) and nitrogen dioxide (NO2) (a proxy for nitrogen oxides) tropospheric columns from Ultraviolet-Visible (UV-Vis) satellites has motivated many to use their ratios to gain some insights into the near-surface ozone sensitivity. Strong emphasis has been placed on the challenges that come with transforming what is being observed in the tropospheric column to what is actually in the planetary boundary layer (PBL) and near to the surface; however, little attention has been paid to other sources of error such as chemistry, spatial representation, and retrieval uncertainties. Here we leverage a wide spectrum of tools and data to carefully quantify those errors. Concerning the chemistry error, a well-characterized box model constrained by more than 500 hours of aircraft data from NASA’s air quality campaigns is used to simulate the ratio of the chemical loss of HO2+RO2 (LROx) to the chemical loss of NOx (LNOx). Subsequently, we challenge the predictive power of HCHO / NO2 ratios (FNRs), which are commonly applied in current research, at detecting the underlying ozone regimes by comparing them to LROx / LNOx. FNRs show a strongly linear (R²=0.94) relationship to LROx / LNOx in the log-log scale. Following the baseline (i.e., ln(LROx / LNOx) = -1.0±0.2) with the model and mechanism (CB06, r2) used for segregating NOx-sensitive from VOC-sensitive regimes, we observe a broad range of FNR thresholds ranging from 1 to 4. The transitioning ratios strictly follow a Gaussian distribution with a mean and standard deviation of 1.8 and 0.4, respectively. This implies that FNR has an inherent 20 % standard error (1-sigma) resulting from not being able to fully describe the ROx-HOx cycle. We calculate high ozone production rates (PO3) dominated by large HCHO×NO2 concentration levels, a new proxy for the abundance of ozone precursors. The relationship between PO3 and HCHO×NO2 becomes more pronounced when moving towards NOx-sensitive regions due to non-linear chemistry; our results indicate that there is fruitful information in the HCHO×NO2 metric that has not been utilized in ozone studies. The vast amount of vertical information on HCHO and NO2 concentration from the air quality campaigns enables us to parameterize the vertical shapes of FNRs using a second-order rational function permitting an analytical solution for an altitude adjustment factor to partition the tropospheric columns to the PBL region. We propose a mathematical solution to the spatial representation error based on modeling isotropic semivariograms. With respect to a high-resolution sensor like TROPOspheric Monitoring Instrument (TROPOMI) (>5.5×3.5 km²), Ozone Monitoring Instrument (OMI) loses 12 % of spatial information at its native resolution. A pixel with a grid size of 216 km² fails at capturing ~65 % of the spatial information in FNRs at a 50 km length scale comparable to the size of a large urban center (e.g., Los Angeles). We ultimately leverage a large suite of in-situ and ground-based remote sensing measurements to draw the error distributions of daily TROPOMI and OMI tropospheric NO2 and HCHO columns. At 68 % confidence interval (1 sigma) errors pertaining to daily TROPOMI observations, either HCHO or tropospheric NO2 columns should be above 1.2–1.5×1016 molec.cm-2 to attain 20–30 % standard error in the ratio. This level of error is almost non-achievable with OMI given its large error in HCHO. The satellite column retrieval error is the largest contributor to the total error (40–90 %) in the FNRs. Due to a stronger signal in cities, the total relative error (<50 %) tends to be mild, whereas areas with low vegetation and anthropogenic sources (e.g., Rocky Mountains) are markedly uncertain (>100 %). Our study suggests that continuing development in the retrieval algorithm and sensor design and calibration is essential to be able to advance the application of FNRs beyond a qualitative metric. The availability of formaldehyde (HCHO) (a proxy for volatile organic compound reactivity) and nitrogen dioxide (NO2) (a proxy for nitrogen oxides) tropospheric columns from Ultraviolet-Visible (UV-Vis) satellites has motivated many to use their ratios to gain some insights into the near-surface ozone sensitivity. Strong emphasis has been placed on the challenges that come with transforming what is being observed in the tropospheric column to what is actually in the planetary boundary layer (PBL) and near to the surface; however, little attention has been paid to other sources of error such as chemistry, spatial representation, and retrieval uncertainties. Here we leverage a wide spectrum of tools and data to carefully quantify those errors. Concerning the chemistry error, a well-characterized box model constrained by more than 500 hours of aircraft data from NASA’s air quality campaigns is used to simulate the ratio of the chemical loss of HO2+RO2 (LROx) to the chemical loss of NOx (LNOx). Subsequently, we challenge the predictive power of HCHO / NO2 ratios (FNRs), which are commonly applied in current research, at detecting the underlying ozone regimes by comparing them to LROx / LNOx. FNRs show a strongly linear (R²=0.94) relationship to LROx / LNOx in the log-log scale. Following the baseline (i.e., ln(LROx / LNOx) = -1.0±0.2) with the model and mechanism (CB06, r2) used for segregating NOx-sensitive from VOC-sensitive regimes, we observe a broad range of FNR thresholds ranging from 1 to 4. The transitioning ratios strictly follow a Gaussian distribution with a mean and standard deviation of 1.8 and 0.4, respectively. This implies that FNR has an inherent 20 % standard error (1-sigma) resulting from not being able to fully describe the ROx-HOx cycle. We calculate high ozone production rates (PO3) dominated by large HCHO×NO2 concentration levels, a new proxy for the abundance of ozone precursors. The relationship between PO3 and HCHO×NO2 becomes more pronounced when moving towards NOx-sensitive regions due to non-linear chemistry; our results indicate that there is fruitful information in the HCHO×NO2 metric that has not been utilized in ozone studies. The vast amount of vertical information on HCHO and NO2 concentration from the air quality campaigns enables us to parameterize the vertical shapes of FNRs using a second-order rational function permitting an analytical solution for an altitude adjustment factor to partition the tropospheric columns to the PBL region. We propose a mathematical solution to the spatial representation error based on modeling isotropic semivariograms. With respect to a high-resolution sensor like TROPOspheric Monitoring Instrument (TROPOMI) (>5.5×3.5 km²), Ozone Monitoring Instrument (OMI) loses 12 % of spatial information at its native resolution. A pixel with a grid size of 216 km² fails at capturing ~65 % of the spatial information in FNRs at a 50 km length scale comparable to the size of a large urban center (e.g., Los Angeles). We ultimately leverage a large suite of in-situ and ground-based remote sensing measurements to draw the error distributions of daily TROPOMI and OMI tropospheric NO2 and HCHO columns. At 68 % confidence interval (1 sigma) errors pertaining to daily TROPOMI observations, either HCHO or tropospheric NO2 columns should be above 1.2–1.5×1016 molec.cm-2 to attain 20–30 % standard error in the ratio. This level of error is almost non-achievable with OMI given its large error in HCHO. The satellite column retrieval error is the largest contributor to the total error (40–90 %) in the FNRs. Due to a stronger signal in cities, the total relative error (<50 %) tends to be mild, whereas areas with low vegetation and anthropogenic sources (e.g., Rocky Mountains) are markedly uncertain (>100 %). Our study suggests that continuing development in the retrieval algorithm and sensor design and calibration is essential to be able to advance the application of FNRs beyond a qualitative metric. [ABSTRACT FROM AUTHOR]

The Arctic is a climatically sensitive region that has experienced warming at almost three times the global average rate in recent decades, leading to an increase in Arctic greenness and a greater abundance of plants that emit biogenic volatile organic compounds (BVOCs). These changes in atmospheric emissions are expected to significantly modify the overall oxidative chemistry of the region and lead to changes in VOC composition and abundance, with implications for atmospheric processes. Nonetheless, observations needed to constrain our current understanding of these issues in this critical environment are sparse. This work presents novel atmospheric in-situ proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) measurements of VOCs at Toolik Field Station (TFS, 68º38’ N, 149º36’ W), in the Alaskan Arctic tundra during May–June 2019. We employ a custom nested grid version of the GEOS-Chem chemical transport model, driven with MEGANv2.1 (Model of Emissions of Gases and Aerosols from Nature version 2.1) biogenic emissions for Alaska at 0.25 º × 0.3125º resolution, to interpret the observations in terms of their constraints on BVOC emissions, total reactive organic carbon (ROC) composition, and calculated OH reactivity (OHr) in this environment. We find total ambient mole fraction of 78 identified VOCs to be 6.3 ± 0.4 ppbv (10.8 ± 0.5 ppbC), with overwhelming (>80 %) contributions are from short-chain oxygenated VOCs (OVOCs) including methanol, acetone, and formaldehyde. Isoprene was the most abundant higher carbon containing biogenic emission identified. GEOS-Chem captures the observed isoprene (and its oxidation products), acetone, and acetaldehyde abundances within the combined model and observation uncertainties (±25 %), but underestimates other oxygenated VOCs including methanol, formaldehyde, formic acid, and acetic acid by a factor of 3 to 12. The negative model bias for methanol is attributed to underestimated biogenic methanol emissions for the Alaska tundra in MEGANv2.1. Observed formaldehyde mole fractions increase exponentially with air temperature, likely reflecting its biogenic precursors and pointing to a systematic model underprediction of its secondary production. The median campaign calculated OHr from VOCs measured at Toolik was 0.7 s-1, roughly 5 % of the values typically reported in lower-latitude forested ecosystems. Ten species account for over 80 % of the calculated VOC OHr, with formaldehyde, isoprene, and acetaldehyde together accounting for nearly half of the total. Simulated OHr based on median modelled VOCs included in GEOS-Chem averages 0.5 s-1 and is dominated by isoprene (30 %) and monoterpenes (17 %). The data presented here serve as a critical evaluation of our knowledge of BVOCs and ROC budgets in high latitude environments and represent a foundation for investigating and interpreting future warming-driven changes in VOC emissions in the Alaskan Arctic tundra. [ABSTRACT FROM AUTHOR]

Comprehensive aircraft measurements of volatile organic compounds (VOCs) covering the South Coast Air Basin (SoCAB) and San Joaquin Valley (SJV) of California were obtained in the summer of 2019. Combined with the CO, CH4, and NOx data, the total measured gas-phase hydroxyl radical reactivity (OHRTOTAL) was quantified. VOCs accounted for ~60%-70% of the OHRTOTAL in both basins. In particular, oxygenated VOCs (OVOCs) contributed > 60% of the OHR of total VOCs (OHRVOC) as well as the total observed VOC mixing ratio. Primary biogenic VOCs (BVOCs) represented a minor fraction (< 2%) of the total VOC mixing ratio but accounted for 21% and 6% of the OHRVOC in the SoCAB and SJV, respectively. Furthermore, the contribution of BVOCs to the OHRVOC increased with increasing OHRVOC in the SoCAB, suggesting that BVOCs was important ozone precursors during high ozone episodes. Spatially, the trace gases were heterogeneously distributed in the SoCAB with their mixing ratios and OHR significantly greater over the inland regions than the coast, while their levels were more evenly distributed in the SJV. The results highlight that a better grasp of the emission rates and sources of OVOCs and BVOCs is essential for a predictive understanding of the ozone abundance and distribution in California. [ABSTRACT FROM AUTHOR]

Large wildfires markedly alter regional atmospheric composition, but chemical complexity challenges model predictions of downwind impacts. Here, we elucidate key facets of gas-phase photochemistry and assess novel chemical processes via a case study of the 2013 California Rim Fire plume. Airborne in situ observations, acquired during the NASA Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) mission, illustrate the evolution of volatile organic compounds (VOC), oxidants, and reactive nitrogen over 12 hours of atmospheric aging. Measurements show rapid formation of ozone and peroxyacyl nitrates (PNs), sustained peroxide production, and prolonged enhancements in oxygenated VOC and nitrogen oxides (NO[sub x]). Measurements and Lagrangian trajectories constrain a 0-D puff model that approximates plume photochemical history and provides a framework for evaluating key processes. Simulations examine the effects of 1) previously -unmeasured reactive VOC identified in recent laboratory studies, and 2) emissions and secondary production of nitrous acid (HONO). Inclusion of estimated unmeasured VOC leads to a 250% increase in OH reactivity and a 70% increase in radical production via oxygenated VOC photolysis. HONO amplifies radical cycling and serves as a downwind NO[sub x] source, although two different HONO production mechanisms (particulate nitrate photolysis and heterogeneous NO[sub 2] conversion) exhibit markedly different effects on ozone, NO[sub x], and PNs. Analysis of radical initiation rates suggests that oxygenated VOC photolysis is a major radical source, exceeding HONO photolysis when averaged over the first 2 hours of aging. Ozone production chemistry transitions from VOC-sensitive to NO[sub x]-sensitive within the first hour of plume aging, with both peroxide and organic nitrate formation contributing significantly to radical termination. To simulate smoke plume chemistry accurately, models should simultaneously account for the full reactive VOC pool and all relevant oxidant sources. [ABSTRACT FROM AUTHOR]

Glyoxal (CHOCHO), the simplest dicarbonyl in the troposphere, is an important precursor for secondary organic aerosol (SOA) and brown carbon (BrC) affecting air-quality and climate. The airborne measurement of CHOCHO concentrations during the KORUS-AQ (KORea-US Air Quality study) campaign in 2016 enables detailed quantification of loss mechanisms, pertaining to SOA formation in the real atmosphere. The production of this molecule was mainly from oxidation of aromatics (59%) initiated by hydroxyl radical (OH), of which glyoxal forming mechanisms are relatively well constrained. CHOCHO loss to aerosol was found to be the most important removal path (69 %) and contributed to roughly ~20 % (3.7 μg sm-3 ppmv-1 hr-1, normalized with excess CO) of SOA growth in the first 6 hours in Seoul Metropolitan Area. To our knowledge, we show the first field observation of aerosol surface-area (4smf)-dependent CHOCHO uptake, which diverges from the simple surface uptake assumption as ^surf increases in ambient condition. Specifically, under the low (high) aerosol loading, the CHOCHO effective uptake rate coefficient, ^uptake, linearly increases (levels off) with ^uf thus, the irreversible surface uptake is a reasonable (unreasonable) approximation for simulating CHOCHO loss to aerosol. [ABSTRACT FROM AUTHOR]

Anthropogenic secondary organic aerosol (ASOA), formed from anthropogenic emissions of organic compounds, constitutes a substantial fraction of the mass of submicron aerosol in populated areas around the world and contributes to poor air quality and premature mortality. However, the precursor sources of ASOA are poorly understood, and there are large uncertainties in the health benefits that might accrue from reducing anthropogenic organic emissions. We show that the production of ASOA in 11 urban areas on three continents is strongly correlated with the anthropogenic reactivity of specific volatile organic compounds. The differences in ASOA production across different cities can be explained by differences in the emissions of aromatics and intermediate-and semi-volatile organic compounds, indicating the importance of controlling these ASOA precursors. With an improved modeling representation of ASOA driven by the observations, we attribute 340,000 PM2.5 premature deaths per year to ASOA, which is over an order of magnitude higher than prior studies. A sensitivity case with a more recently proposed model for attributing mortality to PM2.5 (the Global Exposure Mortality Model) results up to 900,000 deaths. A limitation of this study is the extrapolation from regions with detailed data to others where data is not available. Comprehensive air quality campaigns in the countries in South and Central America, Africa, South Asia, and the Middle East are needed for further progress in this area. [ABSTRACT FROM AUTHOR]

The absence of up-to-date emissions has been a major impediment to accurately simulate aspects of atmospheric chemistry, and to precisely quantify the impact of changes of emissions on air pollution. Hence, a non-linear joint analytical inversion (Gauss-Newton method) of both volatile organic compounds (VOC) and nitrogen oxides (NOx) emissions is made by exploiting the Smithsonian Astrophysical Observatory (SAO) Ozone Mapping and Profile Suite Nadir Mapper (OMPS-NM) formaldehyde (HCHO) and the National Aeronautics and Space Administration (NASA) Ozone Monitoring Instrument (OMI) tropospheric nitrogen dioxide (NO2) retrievals during the Korea-United States Air Quality (KORUS-AQ) campaign over East Asia in May-June 2016. Effects of the chemical feedback of NOx and VOCs on both NO2 and HCHO are implicitly included through iteratively optimizing the inversion. Emissions estimates are greatly improved (averaging kernels > 0.8) over medium- to high-emitting areas such as cities and dense vegetation. The amount of total NOx emissions is mainly dictated by values reported in the MIX-Asia 2010 inventory. After the inversion we conclude a decline in the emissions (before, after, change) for China (87.94 ± 44.09 Gg/day, 68.00 ± 15.94 Gg/day, -23 %), North China Plain (NCP) (27.96 ± 13.49 Gg/day, 19.05 ± 2.50 Gg/day, -32 %), Pearl River Delta (PRD) (4.23 ± 1.78 Gg/day, 2.70 ± 0.32 Gg/day, -36 %), Yangtze River Delta (YRD) (9.84 ± 4.68 Gg/day, 5.77 ± 0.51 Gg/day, -41 %), Taiwan (1.26 ± 0.57 Gg/day, 0.97 ± 0.33 Gg/day, -23 %), and Malaysia (2.89 ± 2.77 Gg/day, 2.25 ± 1.34 Gg/day, -22 %), all of which have effectively implemented various stringent regulations. In contrast, South Korea (2.71 ± 1.34 Gg/day, 2.95 ± 0.58 Gg/day, +9 %) and Japan (3.53 ± 1.71 Gg/day, 3.96 ± 1.04 Gg/day, +12 %) experience an increase in NOx emissions potentially due to risen number of diesel vehicles and new thermal power plants. We revisit the well-documented positive bias of the model in terms of biogenic VOC emissions in the tropics. The inversion, however, suggests a larger growth of VOC (mainly anthropogenic) over NCP (25 %) than previously reported (6 %) relative to 2010. The spatial variation in both magnitude and sign of NOx and VOC emissions results in non-linear responses of ozone production/loss. Due to simultaneous decrease/increase of NOx/VOC over NCP and YRD, we observe an ~ 53 % reduction in the ratio of the chemical loss of NOx (LNOx) to the chemical loss of ROx (RO2 + HO2) transitioning toward NOx-sensitive regimes, which in turn, reduces/increases the afternoon chemical loss/production of ozone through NO2 + OH (-0.42 ppbv hr-1)/HO2 (and RO2) + NO (+0.31 ppbv hr-1). Conversely, a combined decrease in NOx and VOC emissions in Taiwan, Malaysia, and the southern China suppresses the formation of ozone. Ultimately, model simulations indicate enhancements of maximum daily 8-hour average (MDA8) surface ozone over China (0.62 ppbv), NCP (4.56 ppbv), and YRD (5.25 ppbv) due to the non-linear ozone chemistry, suggesting that emissions standards should be extended to regulate VOCs to be able to curb ozone production rates. Taiwan, Malaysia, and PRD stand out as the regions undergoing lower MDA8 ozone levels resulting from the NOx reductions occurring predominantly in NOx-sensitive regimes. [ABSTRACT FROM AUTHOR]

Formaldehyde (HCHO) has been measured from space for more than two decades. Owing to its short atmospheric lifetime, satellite HCHO data are used widely as a proxy of volatile organic compounds (VOCs; please refer to Appendix A for abbreviations and acronyms), providing constraints on underlying emissions and chemistry. However, satellite HCHO products from different satellite sensors using different algorithms have received little validation so far. The accuracy and consistency of HCHO retrievals remain largely unclear. Here we develop a global validation platform for satellite HCHO retrievals using in situ observations from 12 aircraft campaigns with a chemical transport model (GEOS-Chem) as the intercomparison method. Application to the NASA operational OMI HCHO product indicates slight biases (-30.9 % to +16.0 %) under high-HCHO conditions partially caused by a priori shape factors used in the retrievals, while high biases (+113.9 % to +194.6 %) under low-HCHO conditions due mainly to slant column fitting and radiance reference sector correction. By providing quick assessment to systematic biases in satellite products over large domains, the platform facilitates, in an iterative process, optimization of retrieval settings and the minimization of retrieval biases. It is also complementary to localized validation efforts based on ground observations and aircraft spirals. [ABSTRACT FROM AUTHOR]

We apply a high-resolution chemical transport model (GEOS-Chem CTM) with updated treatment of volatile organic compounds (VOCs) and a comprehensive suite of airborne datasets over North America to i) characterize the VOC budget, and ii) test the ability of current models to capture the distribution and reactivity of atmospheric VOCs, over this region. Biogenic emissions dominate the North American VOC budget in the model, accounting for 70 % and 95 % of annually emitted VOC-carbon and reactivity, respectively. Based on current inventories anthropogenic emissions have declined to the point where biogenic emissions are the dominant summertime source of VOC reactivity even in most major North American cities. Methane oxidation is a 2× larger source of non-methane VOCs (via production of formaldehyde and methyl hydroperoxide) over North America in the model than are anthropogenic emissions. However, anthropogenic VOCs account for over half the ambient VOC loading over the majority of the region owing to their longer aggregate lifetime. Fires can be a significant VOC source episodically but are small on average. In the planetary boundary layer (PBL), the model exhibits skill in capturing observed variability in total VOC-abundance (R2 = 0.36) and reactivity (R2 = 0.54). The same is not true in the free troposphere (FT), where skill is low and there is a persistent low model bias (~ 60 %), with most (27 of 34) model VOCs underestimated by more than a factor of 2. A comparison of PBL : FT concentration ratios over the southeastern US points to a misrepresentation of PBL ventilation as a contributor to these model FT biases. We also find that a relatively small number of VOCs (acetone, methanol, ethane, acetaldehyde, formaldehyde, isoprene + oxidation products, methyl hydroperoxide) drive a large fraction of total ambient VOC reactivity and associated model biases; research to improve understanding of their budgets is thus warranted. A source tracer analysis suggests a current overestimate of biogenic sources for hydroxyacetone, methyl ethyl ketone and glyoxal, an underestimate of biogenic formic acid sources, and an underestimate of peroxyacetic acid production across biogenic and anthropogenic precursors. Future work to improve model representations of vertical transport and to address the VOC biases discussed are needed to advance predictions of ozone and SOA formation. [ABSTRACT FROM AUTHOR]

Organic aerosol (OA) is an important fraction of submicron aerosols. However, it is challenging to predict and attribute the specific organic compounds and sources that lead to observed OA loadings, largely due to contributions from secondary production. This is especially true for megacities surrounded by numerous regional sources that create an OA background. Here, we utilize in-situ gas and aerosol observations collected on-board the NASA DC-8 during the NASA/NIER KORUS-AQ (KORea United States-Air Quality) campaign to investigate the sources and hydrocarbon precursors that led to the secondary OA (SOA) production observed over Seoul. First, we investigate the contribution of transported OA to total loadings observed over Seoul, by using observations over the West Sea coupled to FLEXPART Lagrangian simulations. During KORUS-AQ, the average OA loading advected into Seoul was ~ 1-3 μg sm-3. Second, taking this background into account, the dilution-corrected SOA concentration observed over Seoul was ~ 140 μg sm-3 ppmv-1 at 0.5 equivalent photochemical days. This value is at the high end of what has been observed in other megacities around the world (20-70 μg sm-3 ppmv-1 at 0.5 equivalent days). For the average OA concentration observed over Seoul (13 μg sm-3), it is clear that production of SOA from locally emitted precursors is the major source in the region. The importance of local SOA production was supported by the following observations: (1) FLEXPART source contribution calculations indicate any hydrocarbons with a lifetime less than 1 day, which are shown to dominate the observed SOA production, mainly originate from South Korea. (2) SOA correlated strongly with other secondary photochemical species, including short-lived species (formaldehyde, peroxy acetyl nitrate, sum of acyl peroxy nitrates, dihydroxy toluene, and nitrate aerosol). (3) Results from an airborne oxidation flow reactor (OFR), flown for the first time, show a factor of 4.5 increase in potential SOA concentrations over Seoul versus over the West Sea, a region where background air masses that are advected into Seoul can be measured. (4) Box model simulations reproduce SOA observed over Seoul within 15% on average, and suggest that short-lived hydrocarbons (i.e., xylenes, trimethylbenzenes, semi- and intermediate volatility compounds) were the main SOA precursors over Seoul. Toluene, alone, contributes 9% of the modeled SOA over Seoul. Finally, along with these results, we use the metric ΔOA/ΔCO2 to examine the amount of OA produced per fuel consumed in a megacity, which shows less variability across the world than ΔOA/ΔCO. [ABSTRACT FROM AUTHOR]

Organic aerosol (OA) is one of the main components of the global particulate burden and intimately links natural and anthropogenic emissions with air quality and climate. It is challenging to accurately represent OA in global models. Direct quantification of global OA abundance is not possible with current remote sensing technology; however, it may be possible to exploit correlations of OA with remotely observable quantities to infer OA spatiotemporal variability. In particular, formaldehyde (HCHO) and OA share common sources via both primary emissions and secondary production from oxidation of volatile organic compounds (VOCs). We examine OA-HCHO correlations using data from summer-time airborne campaigns investigating biogenic (NASA SEAC4RS and DC3), biomass burning (NASA SEAC4RS) and anthropogenic conditions (NOAA CalNex and NASA KORUS-AQ). In situ OA correlates well with HCHO (r = 0.59-0.97) but the slope and intercept of this relationship vary with chemical regime. For biogenic and anthropogenic regions, the OA-vs-HCHO slope is higher in low NOx conditions, where HCHO yields are lower and aerosol yields are likely higher. The OA-vs-HCHO slope of wild fires is more than 9 times higher than that associated with biogenic and anthropogenic sources. An estimate of near-surface OA is derived by combining observed in situ relationships with HCHO column retrievals from NASA's Ozone Monitoring Instrument (OMI). We evaluate this OA estimate against OA observations from the US EPA IMPROVE network and simulated OA from the GEOS-Chem global chemical transport model. The OA estimate compares well with IMPROVE data obtained over summer months (e.g. slope = 0.62, r = 0.56 for August 2013), comparable to intensively validated GEOS-Chem performance (e.g. slope = 0.57, r = 0.56) and superior to the correlation with satellite-derived total aerosol extinction (r = 0.41). Improving the detection limit of satellite HCHO and expanding in situ airborne HCHO and OA coverage in future missions will improve the quality and spatiotemporal coverage of this OA estimate, potentially enabling constraints on the global OA distribution. [ABSTRACT FROM AUTHOR]

Recent increases in the Natural Gas (NG) production through hydraulic fracturing have called into question the climate benefit of switching from coal-fired to natural gas-fired power plants. Higher than expected levels of methane, Non-Methane Hydrocarbons (NMHC), and NOx have been observed in areas close to oil and NG operation facilities. Large uncertainties in the oil and NG operation emission inventories reduce the confidence level in the impact assessment of such activities on regional air quality and climate, as well as development of effective mitigation policies. In this work, we used ethane as the indicator of oil and NG emissions and explored the sensitivity of ethane to different physical parametrizations and simulation set-ups in the Weather Research and Forecasting with Chemistry (WRF-Chem) model using the U.S. EPA National Emission Inventory (NEI-2011). We evaluated the impact of the following configurations and parameterizations on predicted ethane concentrations: Planetary Boundary Layer (PBL) parametrizations, daily re-initialization of meteorological variables, meteorological initial and boundary conditions, and horizontal resolution. We assessed the uncertainties around oil and NG emissions by using measurements from the FRAPPÉ and DISCOVER-AQ campaigns over the Northern Front Range Metropolitan Area (NFRMA) in summer 2014. The sensitivity analysis shows up to 57.3 % variability in normalized mean bias of the near-surface modeled ethane across the simulations, which highlights the important role of model configurations on the model performance and ultimately the assessment of emissions. Comparison between airborne measurements and the sensitivity simulations indicates that the model-measurement bias of ethane ranged from −14.9 ppb to −8.2 ppb (NMB ranged from −80.5 % to −44 %) in regions close to oil and NG activities. Under-prediction of ethane concentration in all sensitivity runs suggests an actual under-estimation of the oil and NG emissions in the NEI-2011. Increase of oil and NG emissions in the simulations partially improved the model performance in capturing ethane and lumped alkanes (HC3) concentrations but did not impact the model performance in capturing benzene, toluene, and xylene which is due to very low emission rates of these species from oil and NG sector in the NEI-2011. [ABSTRACT FROM AUTHOR]

Evolution of organic aerosol (OA) and their precursors in the boundary layer of Colorado Front Range during the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ, July–August 2014) was analyzed by in-situ measurements and chemical transport modeling. Measurements indicated significant production of secondary OA (SOA), with enhancement ratio of OA with respect to carbon monoxide (CO) reaching 0.068 ± 0.004 μg m−3 ppbv−1. At background mixing ratios of CO, up to ~ 2 μg m−3 background OA was observed, suggesting significant non-combustion contribution to OA in the Front Range. The mean concentration of OA in plumes with a high influence of oil and natural gas (O&G) emissions was ~ 40 % higher than in urban-influenced plumes. Positive matrix factorization confirmed a dominant contribution of secondary, oxygenated OA (OOA) in the boundary layer instead of fresh, hydrocarbon-like OA (HOA). Combinations of primary OA (POA) volatility assumptions, aging of semi-volatile species, and different emission estimates from the O&G sector were used in the Weather Research and Forecasting model, coupled with Chemistry (WRF-Chem) simulation scenarios. The assumption of semi-volatile POA resulted in greater than a factor of 10 lower POA concentrations compared to PMF-resolved HOA. Including a top-down modified O&G emissions resulted in substantially better agreements in modeled ethane, toluene, hydroxyl radical, and ozone compared to measurements in the high O&G-influenced plumes. By including emissions from the O&G sector using the top-down approach, it was estimated that the O&G sector contributed to < 5 % of total OA, but up to 38 % of anthropogenic SOA in the region. The best agreement between the measured and simulated median OA was achieved by limiting the extent of biogenic hydrocarbon aging and consequently biogenic SOA (bSOA) production. Despite a lower production of bSOA in this scenario, contribution of bSOA to total SOA remained high at 40–54 %. Future studies aiming at a better emissions characterization of POA and intermediate volatility organic compounds (IVOCs) from the O&G sector are valuable. [ABSTRACT FROM AUTHOR]

Isoprene emissions from vegetation have a large effect on atmospheric chemistry and air quality. "Bottom-up" isoprene emission inventories used in atmospheric models are based on limited vegetation information and uncertain land cover data, leading to potentially large errors. Satellite observations of atmospheric formaldehyde (HCHO), a high-yield isoprene oxidation product, provide "top-down" information to evaluate isoprene emission inventories through inverse analyses. Past inverse analyses have however been hampered by uncertainty in the HCHO satellite data, uncertainty in the time- and NOx-dependent yield of HCHO from isoprene oxidation, and coarse resolution of the atmospheric models used for the inversion. Here we demonstrate the ability to use HCHO satellite data from OMI in a high-resolution inversion to constrain isoprene emissions on ecosystem-relevant scales. The inversion uses the adjoint of the GEOS-Chem chemical transport model at 0.25°×0.3125° horizontal resolution to interpret observations over the Southeast US in August–September 2013. It takes advantage of concurrent NASA SEAC4RS aircraft observations of isoprene and its oxidation products including HCHO to validate the OMI HCHO data over the region, test the GEOS-Chem isoprene oxidation mechanism and NOx environment, and independently evaluate the inversion. This evaluation shows in particular that local model errors in NOx concentrations propagate to biases in inferring isoprene emissions from HCHO data. It is thus essential to correct model NOx biases, which was done here using SEAC4RS observations but can be done more generally using satellite NO2 data concurrently with HCHO. We find in our inversion that isoprene emissions from the widely-used MEGAN v2.1 inventory are biased high over the Southeast US by 40% on average, although the broad-scale distributions are correct including maximum emissions in Arkansas/Louisiana and high base emission factors in the oak-covered Ozarks of Southeast Missouri. A particularly large discrepancy is in the Edwards Plateau of Central Texas where MEGAN v2.1 is too high by a factor of 3, possibly reflecting errors in land cover. The lower isoprene emissions inferred from our inversion, when implemented into GEOS-Chem, decrease surface ozone over the Southeast US by 1–3ppb and decrease the isoprene contribution to organic aerosol from 40% to 20%. [ABSTRACT FROM AUTHOR]

Widespread efforts to abate ozone (O3) smog have significantly reduced nitrogen oxides (NOx) emissions over the past two decades in the Southeast U.S. (SEUS), a place heavily influenced by both anthropogenic and biogenic emissions. How reactive nitrogen speciation responds to the reduction in NOx emissions in this region remains to be elucidated. Here we exploit aircraft measurements from ICARTT (July-August, 2004), SENEX (June-July, 2013), and SEAC4RS (August-September, 2013) and long-term ground measurement networks alongside a global chemistry-climate model to examine decadal changes in summertime reactive nitrogen species and ozone over the Southeast U.S. We find that most reactive nitrogen species, including NOx, peroxyacetyl nitrate (PAN) and nitric acid (HNO3) decline proportionally with decreasing NOx emissions in this region, leading to a similar decline in exported NOy. This linear response is in part due to the nearly constant summertime supply of biogenic VOC emissions in this region. Our model captures the observed relative change of reactive nitrogen species and surface ozone from 2004 to 2013. Model sensitivity tests indicate that further reductions of NOx emissions will lead to a continued decline in surface ozone and less frequent extreme ozone events. [ABSTRACT FROM AUTHOR]

Chemical models must accurately calculate the ozone formation rate, P(O3), to accurately predict ozone levels and test mitigation strategies. However, model chemical mechanisms can contain large uncertainties in P(O3) calculations, which can create uncertainties in ozone forecasts especially during the summertime when P(O3) can be high. One way to test mechanisms is to evaluate model P(O3) using direct measurements. During summer 2014, the Measurement of Ozone Production Sensor (MOPS) measured net P(O3) in Golden, CO, approximately 25 km west of Denver along the Colorado Front Range. Net P(O3) was compared to rates calculated by a photochemical box model using a lumped and a more explicit chemical mechanism. Observed P(O3) was up to a factor of two higher than that modeled during early morning hours when nitric oxide (NO) levels were high, contrary to traditional ozone chemistry theory. This disagreement may be due to model underestimation of hydroperoxyl (HO2) radicals relative to observations at high NO levels. These additional peroxyl radicals could come from the MOPS chamber chemistry or from missing volatile organic compounds co-emitted with NOx; additional cycling of OH into HO2 through reactions involving nitric oxide provides an alternate explanation for higher measured than modeled P(O3). Although the MOPS measurements are new, comparisons of observed and modeled P(O3) in NO space show a similar behavior to other comparisons between P(O3) derived from measurements and modeled P(O3). These comparisons can have implications for the sensitivity of P(O3) to nitrogen oxides and volatile organic compounds during morning hours, and can possibly affect ozone reduction strategies for the region surrounding Golden, CO in addition to other urban and suburban areas that are in non-attainment with national ozone regulations. [ABSTRACT FROM AUTHOR]

We present airborne measurements made in the Colorado Front Range aboard the NSF C-130 aircraft during the 2014 Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) project. Data on trace gases, non-refractory sub-micron aerosol chemical constituents, and aerosol optical extinction (βext) at λ = 632 nm in the presence and absence of a surface mesoscale circulation pattern, called the Denver Cyclone, were analyzed in three study regions of the Front Range: In-Flow, Northern Front Range (NFR), and Denver Metropolitan (DM). Pronounced increases in mass concentrations of organics, nitrate, and sulfate in NFR and DM were observed during the cyclone episodes (27–28 July) compared to the non-cyclonic days (26 July, 02–03 August). Organics (OA) dominated the mass concentrations on all evaluated days, with a 45 % increase in OA on cyclone days across all three regions while the increase during the cyclone episode was up to ~ 80 % for DM, from 3.78 ± 1.55 µg sm−3 to 6.78 ± 1.78 µg sm−3, where sm−3 is the STP unit of volume of air. Average nitrate mass concentrations were 0.26 ± 0.27 µg sm−3 vs. 1.03 ± 0.74 µg sm−3 followed by sulfate with an average of 0.58 ± 0.23 µg sm−3 vs. 1.08 ± 0.73 µg sm−3 on non-cyclone vs. cyclonic days, respectively. In the most aged air masses (NOx/NOy < 0.5), background OA over DM increased by a factor of ~ 4, from 0.93 ± 0.33 µg sm−3 to 3.70 ± 0.28 µg sm−3 due to transport from NFR. Furthermore, enhanced partitioning of nitric acid to the aerosol phase was observed during the cyclone episodes, mainly due to increased abundance of gas phase NH3. During the non-cyclone events, βext displayed strong correlations (r = 0.71) with OA and NO3− in NFR and only with OA (r = 0.70) in DM while correlation of βext during the cyclone was strongest (r = 0.86) with NO3− in DM. Mass extinction efficiency values (MEE) values in DM were similar under cyclone (2.85 ± 0.63 m2 g−1) and non-cyclone (2.72 ± 0.61 m2 g−1) days despite the dominant influence of different aerosol species on βext (non-cyclone: OA, cyclone; NO3−). [ABSTRACT FROM AUTHOR]

We present airborne measurements made in the Colorado Front Range aboard the NSF C-130 aircraft during the 2014 Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) project. Data on trace gases, non-refractory sub-micron aerosol chemical constituents, and aerosol optical extinction (βext) at λ = 632 nm in the presence and absence of a surface mesoscale circulation pattern, called the Denver Cyclone, were analyzed in three study regions of the Front Range: In-Flow, Northern Front Range (NFR), and Denver Metropolitan (DM). Pronounced increases in mass concentrations of organics, nitrate, and sulfate in NFR and DM were observed during the cyclone episodes (27-28 July) compared to the non-cyclonic days (26 July, 02-03 August). Organics (OA) dominated the mass concentrations on all evaluated days, with a 45% increase in OA on cyclone days across all three regions while the increase during the cyclone episode was up to ~ 80% for DM, from 3.78 ± 1.55 µg sm-3 to 6.78 ± 1.78 µg sm-3, where sm-3 is the STP unit of volume of air. Average nitrate mass concentrations were 0.26 ± 0.27 µg sm-3 vs. 1.03 ± 0.74 µg sm-3 followed by sulfate with an average of 0.58 ± 0.23 µg sm-3 vs. 1.08 ± 0.73 µg sm-3 on non-cyclone vs. cyclonic days, respectively. In the most aged air masses (NOx/NOy < 0.5), background OA over DM increased by a factor of ~ 4, from 0.93 ± 0.33 µg sm-3 to 3.70 ± 0.28 µg sm-3 due to transport from NFR. Furthermore, enhanced partitioning of nitric acid to the aerosol phase was observed during the cyclone episodes, mainly due to increased abundance of gas phase NH3. During the non-cyclone events, βext displayed strong correlations (r = 0.71) with OA and NO3- in NFR and only with OA (r = 0.70) in DM while correlation of βext during the cyclone was strongest (r = 0.86) with NO3- in DM. Mass extinction efficiency values (MEE) values in DM were similar under cyclone (2.85 ± 0.63 m² g-1) and non-cyclone (2.72 ± 0.61 m² g-1) days despite the dominant influence of different aerosol species on βext (non-cyclone: OA, cyclone; NO3-). [ABSTRACT FROM AUTHOR]

Summertime aerosol optical extinction (βext) was measured in the Colorado Front Range and Denver Metropolitan Area as part of the Front Range Air Pollution and Photochemistry Experiment (FRAPPÉ) campaign during July-August 2014. An Aerodyne Cavity Attenuated Phase Shift particle light extinction monitor (CAPS-PMex) was deployed to measure dry, βext of submicron aerosols at λ=632 nm at 1 Hz. Data from a suite of gas-phase instrumentation were used to interpret βext behavior under various categories of air masses and sources. Extinction enhancement ratios relative to CO (Δβext/ΔCO) were significantly increased in highly aged air masses compared to fresh air masses by 50-60%. The resulting increase in Δβext/ΔCO under highly aged air masses was accompanied by formation of secondary organic aerosols (SOA). In addition, the impacts of aerosol composition on βext in air masses under the influence of urban, natural oil and gas operations (O&G), and agriculture and livestock operations were evaluated. Estimated non-refractory mass extinction efficiency (MEE) values for different air mass types ranged from 1.83-3.30 m² g-1, with the minimum and maximum values observed in agriculture and urban + O&G influenced air masses, respectively. The mass distribution for organic, nitrate, and sulfate aerosols presented distinct profiles in different air mass types. During Aug. 11-12, regional influence of a biomass burning event was observed, increasing the background βext by 10-15 Mm-1 and the estimated MEE and Δβext/ΔCO values in the Front Range. [ABSTRACT FROM AUTHOR]

Summertime aerosol optical extinction (βext) was measured in the Colorado Front Range and Denver Metropolitan Area as part of the Front Range Air Pollution and Photochemistry Experiment (FRAPPÉ) campaign during July-August 2014. An Aerodyne Cavity Attenuated Phase Shift particle light extinction monitor (CAPS-PMex) was deployed to measure dry, βext of submicron aerosols at λ = 632 nm at 1 Hz. Data from a suite of gas-phase instrumentation were used to interpret βext behavior under various categories of air masses and sources. Extinction enhancement ratios relative to CO (Δβext/ΔCO) were significantly increased in highly aged air masses compared to fresh air masses by 50-60%. The resulting increase in Δβext/ΔCO under highly aged air masses was accompanied by formation of secondary organic aerosols (SOA). In addition, the impacts of aerosol composition on βext in air masses under the influence of urban, natural oil and gas operations (O&G), and agriculture and livestock operations were evaluated. Estimated non-refractory mass extinction efficiency (MEE) values for different air mass types ranged from 1.83-3.30 m² g-1, with the minimum and maximum values observed in agriculture and urban + O&G influenced air masses, respectively. The mass distribution for organic, nitrate, and sulfate aerosols presented distinct profiles in different air mass types. During Aug. 11-12, regional influence of a biomass burning event was observed, increasing the background βext by 10-15 Mm-1 and the estimated MEE and Δβext/ΔCO values in the Front Range. [ABSTRACT FROM AUTHOR]

Formaldehyde (HCHO) column data from satellites are widely used as a proxy for emissions of volatile organic compounds (VOCs), but validation of the data has been extremely limited. Here we use highly accurate HCHO aircraft 25 observations from the NASA SEAC4RS campaign over the Southeast US in August-September 2013 to validate and intercompare six operational and research retrievals of HCHO columns from four different satellite instruments (OMI, GOME2A, GOME2B and OMPS) and three different research groups. The GEOS-Chem chemical transport model provides a common intercomparison platform. We find that all retrievals capture the HCHO maximum over Arkansas and Louisiana, reflecting high emissions of biogenic isoprene, and are consistent in their spatial variability over the Southeast US (r=0.4-0.8 30 on a 0.5°×0.5° grid) as well as their day-to-day variability (r=0.5-0.8). However, all satellite retrievals are biased low in the mean by 20-51%, which would lead to corresponding bias in estimates of isoprene emissions from the satellite data. The smallest bias is for OMI-BIRA, which has the highest corrected slant columns and the lowest scattering weights in its air mass factor (AMF) calculation. Correcting the assumed HCHO vertical profiles (shape factors) used in the AMF calculation would further reduce the bias in the OMI-BIRA data. We conclude that current satellite HCHO data provide a reliable proxy for isoprene emission variability but with a low mean bias due both to the corrected slant columns and the scattering weights used in the retrievals. [ABSTRACT FROM AUTHOR]

Formaldehyde (HCHO) column data from satellites are widely used as a proxy for emissions of volatile organic compounds (VOCs), but validation of the data has been extremely limited. Here we use highly accurate HCHO aircraft observations from the NASA SEAC4RS campaign over the Southeast US in August-September 2013 to validate and intercompare six operational and research retrievals of HCHO columns from four different satellite instruments (OMI, GOME2A, GOME2B and OMPS) and three different research groups. The GEOS-Chem chemical transport model provides a common intercomparison platform. We find that all retrievals capture the HCHO maximum over Arkansas and Louisiana, reflecting high emissions of biogenic isoprene, and are consistent in their spatial variability over the Southeast US (r = 0.4-0.8 on a 0.5° × 0.5° grid) as well as their day-to-day variability (r = 0.5-0.8). However, all satellite retrievals are biased low in the mean by 20-51%, which would lead to corresponding bias in estimates of isoprene emissions from the satellite data. The smallest bias is for OMI-BIRA, which has the highest corrected slant columns and the lowest scattering weights in its air mass factor (AMF) calculation. Correcting the assumed HCHO vertical profiles (shape factors) used in the AMF calculation would further reduce the bias in the OMI-BIRA data. We conclude that current satellite HCHO data provide a reliable proxy for isoprene emission variability but with a low mean bias due both to the corrected slant columns and the scattering weights used in the retrievals. [ABSTRACT FROM AUTHOR]