Abstract As large wildfires become more frequent and severe in North America due to heightened fuel aridity and fire weather conditions driven by anthropogenic climate change, wildfire smoke has an increasingly important influence on air quality. Transported smoke can impact urban boundary layer ozone (O 3 ) directly, via the entrainment of O 3 formed in smoke, or indirectly, through the influence of pyrogenic volatile organic compounds (VOCs) and aerosols on local O 3 production. Widespread smoke impacts from the record‐breaking 2023 Canadian wildfire season coincided with the July–August 2023 Airborne Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) aircraft‐based field campaign, which provided extensive in situ observations of urban chemistry in North American megacities. Vertically resolved trace gas and aerosol measurements from the August 2 AEROMMA research flight in Chicago indicated the entrainment of diffuse smoke from an aged, lofted wildfire plume into the urban boundary layer and were used to constrain a zero‐dimensional photochemical model and a radiative transfer model. We perturbed the models across a range of smoke densities and urban nitrogen oxide emissions and found net pyrogenic O 3 enhancements of 3–17 ppbv, with the largest contribution from transported smoke O 3 (65%–84%), followed by locally produced photochemical O 3 from smoke VOCs (18%–46%), and finally smoke aerosol shading (−12%–0%). Although these results are specific to the observed August 2 smoke plume and Chicago urban chemistry, the modeling framework presented could be applied to other smoke‐impacted locations to disentangle the various effects of wildfire smoke on urban O 3 .

Abstract Satellite megaconstellations (SMCs) are driving rapid increases in rocket launch and re‐entry rates, emitting pollutants throughout the atmosphere. The environmental impact of SMCs lacks characterization to determine the need for regulation. We utilize a global 3D emission inventory of recent (2020–2022) space activity that distinguishes SMC and non‐SMC emissions. We calculate a decade of emissions using trends in propellant consumption and re‐entry mass to project growth rates in SMC (28% a−1) and non‐SMC (<20% a−1) propellant and re‐entry mass. We implement this in the GEOS‐Chem chemical transport model coupled to a radiative transfer model to characterize impacts of SMCs and all mission types on atmospheric composition and climate. By 2029, global chemical loss of stratospheric ozone from all missions, dominated by chlorine from solid propellant, is small (0.02%) compared to regulated sources (2%). SMC missions predominantly use kerosene‐fueled rockets that do not emit chlorine, so account for only 9% of all‐mission ozone depletion. Kerosene is a large source of black carbon (BC) that induces positive instantaneous radiative forcing per mass unit BC emitted that is more than 500 times greater than BC forcing from Earth‐bound sources. Unlike surface sources, BC from rockets are released above the tropopause, so behave like potential solar geoengineering strategies: positive instantaneous forcing (6.47 mW m−2), negative stratospherically adjusted forcing (−6.40 mW m−2). SMCs account for over half (56%) the instantaneous forcing and 42% of the stratospherically adjusted forcing. Ambient measurements and laboratory studies are critically needed to constrain and validate our model findings.

Airborne observations from ACCLIP on 2 August 2022, combined with Lagrangian particle dispersion model back trajectories, reveal that SO2 mixing ratios at 14-16 km were enhanced by a factor of 4-6 in regions influenced by tropical cyclones (TCs). These enhancements are linked to rapid lofting of marine dimethyl sulfide (DMS) into the upper troposphere (UT). GEOS-Chem simulations indicated that on 31 July 2022, TC-scale circulation injected DMS into the UT within hours, with a mean flux of 9.4 kg hr-1 across 0.5-12 km and 8.4% of emissions penetrating above 12 km, consistent with observations of elevated DMS at the same altitudes. Because of its low solubility and longer UT lifetime (59.2 vs. 5.7 hr at the surface), DMS sustains SO2 production that is largely resistant to wet scavenging. This TC-driven pathway provides a significant natural SO2 source in the UT, with implications for aerosol-cloud-climate interactions.

The University of York's laser-induced fluorescence (LIF) instrument for measuring sulfur dioxide (SO2) was compared to a commercial pulsed fluorescence (PF) analyser and iodide chemical ionisation mass spectrometer (I-CIMS) aboard the UK FAAM research aircraft in both remote and ship-polluted marine environments. In high SO2 concentration plumes, the LIF instrument and PF analyser compared well, but LIF was the only instrument capable of SO2 measurements in the remote marine boundary layer due to its campaign limit of detection (LoD, 3 sigma) of 0.07 ppb at 10 s compared with 0.4 ppb for the PF analyser. Quantification of SO2 using I-CIMS was challenging due to a significant interference, but good signal correlation with the other instruments was observed in polluted air masses. A comparison of response time was also made, for which the I-CIMS and LIF instrument proved much faster than the PF analyser with 3 e-folding times of 0.6, 2 and 17 s respectively. This work demonstrates the importance of sensitive instrumentation like the LIF system for quantifying low concentrations of SO2, such as over remote marine environments, at the time resolutions required for a fast moving platform. This is particularly relevant now as a result of more stringent sulfur emission regulations for shipping, and likely more so in the future as anthropogenic SO2 concentrations continue to decline.

The operation of geostationary (GEO) instruments such as the Tropospheric Emissions: Monitoring of Pollution (TEMPO) provides unprecedented hourly nitrogen dioxide (NO2) observations compared to the once-daily data from a low-Earth orbit (LEO) platform like the TROPOspheric Monitoring Instrument (TROPOMI). This study investigates the performance and challenges of using TEMPO versus TROPOMI measurements to constrain anthropogenic nitrogen oxides (NOx) emissions. The accuracy of TEMPO and TROPOMI NO2 tropospheric columns are assessed using Pandora observations, finding a low bias of 9%-12.3% in TEMPO, and TROPOMI data during August 2023, while TEMPO midday and late afternoon observations are less of low bias. Top-down NOx emissions derived by midday TEMPO and TROPOMI data are generally consistent over urban areas, being 5%-20% lower than bottom-up emissions provided by the 2021 GReenhouse gas And Air Pollutants Emissions System (GRA2PES), and align with 2023 GRA2PES emissions, demonstrating the reliability of using satellite data for timely updates of bottom-up inventories. However, assimilating additional morning/late afternoon TEMPO data leads to the poorest top-down NOx emissions, likely resulting from larger negative measurement biases. NOx emission inversions effectively mitigate NOx overprediction, though the top-down NOx emissions might be over-corrected in urban cores. NOx emissions optimization also improves ozone forecasts by reducing the model's positive biases, especially when assimilating midday TEMPO data. Our study suggests that TEMPO midday observations provide better constraints on the magnitude and spatiotemporal variation of anthropogenic NOx emissions than TROPOMI, while morning TEMPO v3 data should be used cautiously due to potential negative impact on NOx emissions inversion.

Declining nitrogen oxide (NOx = NO + NO2) emissions have transformed oxidation pathways in urban atmospheres, with implications for air quality. Organic peroxy radicals (RO2), key intermediates in volatile organic compound oxidation, typically react with NO to form ozone (O3). Under lower-NO conditions, alternative RO2 fates, including isomerization forming highly oxidized organic molecules (HOMs), can enhance secondary organic aerosol (SOA) production. We combine aircraft observations over four major North American cities with geostationary satellite data to characterize isoprene-derived RO2 fate across urban environments. We infer RO2 bimolecular lifetimes (τbi) as a proxy for isomerization potential, finding longer τbi (17 ± 11 seconds) in New York, Chicago, and Toronto compared to Los Angeles (7 ± 6 seconds). Satellite measurements reveal that long τbi is widespread across urban North America, suggesting that declining NOx is likely to lead to greater HOM formation in urban regions. These findings indicate that atmospheric models omitting RO2 isomerization chemistry may incorrectly simulate organic oxidation and the subsequent oxidation state of volatile organic compounds and SOA.

Analysis of recent in situ data reveals a persistent mode of organic-rich aerosol particles in the stratosphere below 19 kilometers at nitrous oxide (N2O) > 270 parts per billion by volume, with a number geometric mean diameter of ~0.03 to 0.11 μm (0.08 to 0.2 μm in surface and 0.11 to 0.3 μm in volume). This mode, composed mostly of organic-rich particles transported from the troposphere, is poorly sensed by satellites and most balloon-borne optical measurements but dominates the surface area for heterogeneous reactions and the sink for condensable vapors. These small particles grow in size and decrease in concentration as they mix with older stratospheric air. A global chemistry-climate model fails to replicate the characteristics of these particles, suggesting that model improvements are necessary for accurate assessment of proposed geoengineering efforts.

Abstract Airborne observations from ACCLIP on 2 August 2022, combined with Lagrangian particle dispersion model back trajectories, reveal that SO 2 mixing ratios at 14–16 km were enhanced by a factor of 4–6 in regions influenced by tropical cyclones (TCs). These enhancements are linked to rapid lofting of marine dimethyl sulfide (DMS) into the upper troposphere (UT). GEOS‐Chem simulations indicated that on 31 July 2022, TC‐scale circulation injected DMS into the UT within hours, with a mean flux of 9.4 kg hr −1 across 0.5–12 km and 8.4% of emissions penetrating above 12 km, consistent with observations of elevated DMS at the same altitudes. Because of its low solubility and longer UT lifetime (59.2 vs. 5.7 hr at the surface), DMS sustains SO 2 production that is largely resistant to wet scavenging. This TC‐driven pathway provides a significant natural SO 2 source in the UT, with implications for aerosol–cloud–climate interactions.

Free tropospheric (FT) nitrogen dioxide (NO2) plays a critical role in atmospheric oxidant chemistry as a source of tropospheric ozone and of the hydroxyl radical (OH). It also contributes significantly to satellite-observed tropospheric NO2 columns, which should be considered when using these columns to quantify surface emissions of nitrogen oxide radicals (NOx ≡ NO + NO2). But large uncertainties remain in the sources and chemistry of FT NO2 because observations are sparse. Here, we construct a cloud-sliced FT NO2 (700 to 300 hPa) product from the Tropospheric Emissions: Monitoring of Pollution (TEMPO) geostationary satellite instrument over North America. This product provides higher data density and quality than previous products from low Earth orbit instruments, including the first observations of the FT NO2 diurnal cycle in different seasons. Combined with coincident observations from the Geostationary Lightning Mapper, the TEMPO data imply that lightning is the dominant source of FT NOx in nonwinter seasons. Comparison of TEMPO FT NO2 data with the Goddard Earth Observation System-Composition Forecasts (GEOS-CF) atmospheric chemistry model shows overall consistent magnitudes, seasonality, and diurnal variation, with a midday minimum in nonwinter seasons from photochemical loss. However, there are major discrepancies that we attribute to GEOS-CF's use of a standard cloud-top-height-based scheme for the lightning NOx source. We find that this scheme underestimates offshore lighting flash density and misrepresents the diurnal cycle of lightning over land. Our FT NO2 product provides a unique resource for improving the lightning NOx parameterization in atmospheric models and the ability to use NO2 observations from space to quantify surface NOx emissions.

Volatile chemical products (VCPs) and other non-traditional anthropogenic sources, such as cooking, contribute substantially to the volatile organic compound (VOC) budget in urban areas, but their impact on ozone formation is less certain. This study employs Lagrangian box modeling and sensitivity analyses to evaluate ozone response to sector-specific VOC and nitrogen oxide (NOx) emissions in two Los Angeles (LA) Basin cities during the summer of 2021. The model simulated the photochemical processing and transport of temporally and spatially gridded emissions from the FIVE-VCP-NEI17NRT inventory and accurately simulates the variability and magnitude of O3, NOx, and speciated VOCs in Pasadena, CA. VOC sensitivity analyses show that anthropogenic VOCs (AVOC) enhance the mean daily maximum 8 h average ozone in Pasadena by 13 ppb, whereas biogenic VOCs (BVOCs) contribute 9.4 ppb. Of the ozone influenced by AVOCs, VCPs represent the largest fraction at 45 %, while cooking and fossil fuel VOCs are comparable at 26 % and 29 %, respectively. NOx sensitivity analyses along trajectory paths indicate that the photochemical regime of ozone varies spatially and temporally. The modeled ozone response is primarily NOx-saturated across the dense urban core and during peak ozone production in Pasadena. Lowering the inventory emissions of NOx by 25 % moves Pasadena to NOx-limited chemistry during afternoon hours and shrinks the spatial extent of NOx saturation towards downtown LA. Further sensitivity analyses show that using VOCs represented by a separate state inventory requires steeper NOx reductions to transition to NOx sensitivity, further suggesting that accurately representing VOC reactivity in inventories is critical to determining the effectiveness of future NOx reduction policies.

The North American component of the Geo-Ring for Air Quality, the Tropospheric Emissions: Monitoring of Pollution (TEMPO) instrument, began collecting measurements on August 2, 2023. Multiple airborne field-intensives were conducted over the US during this TEMPO first-light period, including the NOAA Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) and Coastal Urban Plume Dynamics Study (CUPiDS) campaigns, coordinated with the NASA Synergistic TEMPO Air Quality Sciences (STAQS) campaign. The North American cities targeted included New York City, Los Angeles, Chicago, and Toronto. Here, we present an overview of the AEROMMA / CUPiDS collected summer 2023 datasets, relevant for calibration and validation activities of TEMPO, including for ozone (O3), nitrogen dioxide (NO2), formaldehyde (CH2O), sulfur dioxide (SO2), aerosol optical depth (AOD), and aerosol layer height (ALH). Ground-based lidar and airborne in-situ vertical profiling by the NASA DC-8 and NOAA Twin Otter aircraft are available for evaluating TEMPO Level 2 O3 profile products (tropospheric and 0-2 km column retrievals). Airborne measurements of NO2 (photolytic conversion of NO2 into NO followed by laser-induced fluorescence, cavity enhanced spectroscopy, and multi-axis differential optical absorption spectroscopy (MAX-DOAS)) are available for evaluating TEMPO Level 2 NO2 vertical column density products. Airborne measurements of formaldehyde and glyoxal (in-situ and MAX-DOAS remote sensing) can be evaluated similarly as other volatile organic compounds (VOCs). Lastly, a wide array of aircraft-based in-situ measurements of composition, size distribution, optical properties can be utilized to derive aerosol optical depth (AOD) and aerosol extinction profiles for evaluating TEMPO AOD and ALH products, along with TROPOMI and stereoscopic aerosol layer height products from GOES-16/18. Preliminary evaluation of TEMPO NO2 will be presented as an initial calibration / validation test case, employing best practices to facilitate direct comparisons between airborne data with TEMPO Level 2 observations.

The Asian Summer Monsoon (ASM) has garnered attention in recent years for its impacts on the composition of the upper troposphere and lower stratosphere (UTLS) via deep convection. A recent observational effort into this mechanism, the Asian Summer Monsoon Chemical and CLimate Impact Project (ACCLIP), sampled the composition of the ASM UTLS over the northwestern Pacific region during boreal summer 2022 using two airborne platforms. In this work, we integrate Lagrangian trajectory modeling with convective cloud top observations to diagnose ASM convective transport which contributed to ACCLIP airborne observations. This diagnostic is applied to explore the properties of convective transport associated with prominent ASM sub‐systems, revealing that for species ranging in lifetime from days to months, transport from convection along the East Asia Subtropical Front was generally associated with more UTLS pollutants than transport from convection over South Asia. The convective transport diagnostic is used to isolate three convective transport events over eastern Asia which had distinct chemical tracer relationship behaviors, indicating the different economical behaviors of the contributing source regions. One of these transport events is explored in greater detail, where a polluted air mass was sampled from convection over the Northeast China Plain which may have been high enough in altitude to impact the composition of the stratosphere. Overall, the presented diagnosis of convective transport contribution to ACCLIP airborne sampling indicates a key scientific success of the campaign and enables process studies of the climate interactions from the two ASM sub‐systems.

The CH3SCH2OO radical is a key intermediate formed in the gas-phase oxidation of dimethyl sulfide (CH3SCH3, DMS). In this study, the rate coefficient, k1(T), for the gas-phase CH3SCH2OO + NO reaction was measured using a pulsed laser photolysis-iodide chemical ionization mass spectrometry (CIMS) detection competitive reaction method over the temperature range 313-413 K. Hydroperoxymethyl thioformate (HOOCH2SCHO, HPMTF) formed following a H-shift reaction of the CH3SCH2OO radical was monitored using CIMS as a function of added NO concentration. The k1(T) results are described by the Arrhenius expression k1(313-413 K) = (1.43 +/- 0.29) x 10-12 exp((510 +/- 160)/T) cm3 molecule-1 s-1, where the quoted uncertainties are 2 sigma and the pre-exponential coefficient uncertainty includes estimated systematic errors. An extrapolation to room temperature yields k1(298 K) = 7.9 x 10-12 cm3 molecule-1 s-1. Results from this study are compared with previous room temperature and temperature dependent (261-400 K) studies. The reaction rate coefficient obtained in this work is recommended for use in atmospheric chemistry and climate models.

Hydroxyl radical (OH) reactivity, which is the inverse lifetime of the OH radical, provides information on the burden of air pollutants, since almost all air pollutants react with OH. OH reactivity measurements from field experiments can help to identify gaps in the measurement of individual reactants and serve as a proxy for the potential formation of secondary pollutants, including ozone and particles. However, OH reactivity is not regularly measured specifically on airborne platforms due to the technical complexity of the instruments and/or the need for careful instrumental characterisation to apply accurate correction factors to account for secondary chemistry in the instruments. The method used in this work, based on the time-resolved measurement of OH radicals produced by laser flash photolysis in a flow tube, does not require corrections as secondary chemistry in the instrument is negligible for typical atmospheric conditions. However, the detection of OH radicals by laser-induced fluorescence is challenging. In this work, an OH reactivity instrument has been further developed specifically for airborne measurements. The laser system used to detect the OH radicals has been simplified compared to previous setups, thereby significantly reducing the need for user interaction. The improved sensitivity allows measurements to be made with a high time resolution on the order of seconds and a measurement precision of 0.3 s−1. The OH reactivity measurements were validated using a propane gas standard, which allowed the determination of the reaction rate constant of the OH reaction with propane. The values are in excellent agreement with literature recommendations within a range of 4 % to 8 %. Deviations are well within the combined uncertainties. The accuracy of the OH reactivity measurements is mainly limited by the determination of the instrumental zero, which has a typical maximum uncertainty of 0.5 s−1. The high sensitivity of the improved instrument facilitates the data acquisition on board an aircraft as demonstrated by its deployment during the AEROMMA (Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas) campaign in 2023.

Surface ozone (O 3 ) mixing ratios exceeding the National Ambient Air Quality Standard were measured at rural monitors along the Colorado Front Range on 17 April 2020 during the COVID‐19 lockdown. This unusual episode followed back‐to‐back upslope snowstorms and coincided with the presence of a deep stratospheric intrusion, but ground‐based lidar and ozonesonde measurements show that little, if any, of the O 3 ‐rich lower stratospheric air reached the surface. Instead, the statically stable lower stratospheric air suppressed the growth of the daytime boundary layer and trapped nitrogen oxides (NO x = NO + NO 2 ) and volatile organic compounds (VOCs) emitted by motor vehicles and oil and natural gas (O&NG) operations near the ground where the clear skies and extensive snow cover triggered a short‐lived photochemical episode similar to those observed in the O&NG producing basins of northeastern Utah and southwestern Wyoming. In this study, we use a combination of lidar, ozonesonde, and surface measurements, together with the WRF‐Chem and Goddard Earth Observing System composition forecast models, to describe the stratospheric intrusion and characterize the boundary layer structure, HYSPLIT back trajectories to show the low‐level transport of O 3 and its precursors to the exceedance sites, and surface measurements of NO x and VOCs together with a 0‐D box model to investigate the roles of urban and O&NG emissions and the COVID‐19 quarantine in the O 3 production. The box model showed the O 3 production to be NO x saturated, such that the NO x reductions associated with COVID‐19 exacerbated the event rather than mitigating it.

The UK research aircraft is being equipped with flux capable measurements of NO, NO2, SO2 and O3 in the Laser based Observation of Key Inorganics (LOKI) instrument rack. All instruments within the rack are being made in-house. This presentation will discuss the aspects of the new instrumentation as well as the sample inlets used that allow high time resolution (10 Hz), high precision measurements to be made over the working envelope of the aircraft. The need for this improvement in the gas sensing capabilities on board is driven by a number of factors: the increasing desire to use flux measurements to calculate emission rates and identify sources, smaller changes in concentration being of interest (SO2 depletion in clouds) as well as reductions in absolute ambient concentrations of some of these species below the detection limits of currently deployed instrumentation. Laser Induced Fluorescence is used for NO (Rollins et al., 2020), NO2 (converted to NO), SO2 (Rollins et al., 2016), with O3 being monitored using Broadband Cavity Enhanced Absorption Spectroscopy (BBCEAS) (Hannun et al., 2020).

Carbonyl sulfide (OCS) is an important atmospheric sulfur species that plays a dominant role in the formation of (nonvolcanic) stratospheric sulfate aerosol in the middle stratosphere. Major uncertainties in surface sources and sinks and inconsistent model representation of vertical transport limit understanding of OCS distribution, particularly in the sparsely sampled upper atmosphere. During the 2022 Asian Summer Monsoon Chemical and CLimate Impact Project (ACCLIP) campaign, in situ measurements of OCS in the Upper Troposphere and Lower Stratosphere (UTLS) at the eastern edge of the Asian summer monsoon anticyclone (ASM), showed significant OCS enhancements (>750 ppt) near the tropopause from convectively influenced air parcels. Here, we compare these novel Asian UTLS measurements with long‐term satellite observations and regional measurements to broaden understanding of OCS trends and its transport by the ASM. Trajectory analysis identifies northern China as the main source region for deep convective lofting of OCS‐enriched parcels and demonstrates ASM entrainment in the UTLS, allowing evaluation of global model predictions for OCS's stratospheric influence. The ACCLIP data set provides vital in situ validation of limited vertically resolved OCS data in a region of significant anthropogenic emissions, which serves to enhance our understanding of the global sulfur budget.

New particle formation in the free troposphere is a major source of cloud condensation nuclei globally. The prevailing view is that in the free troposphere, new particles are formed predominantly in convective cloud outflows. We present another mechanism using global observations. We find that during stratospheric air intrusion events, the mixing of descending ozone-rich stratospheric air with more moist free tropospheric background results in elevated hydroxyl radical (OH) concentrations. Such mixing is most prevalent near the tropopause where the sulfur dioxide (SO 2 ) mixing ratios are high. The combination of elevated SO 2 and OH levels leads to enhanced sulfuric acid concentrations, promoting particle formation. Such new particle formation occurs frequently and over large geographic regions, representing an important particle source in the midlatitude free troposphere.

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 on board 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 parts per million by volume (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.