Ozone pollution in the Southeast US involves complex chemistry driven by emissions of anthropogenic nitrogen oxide radicals (NO ≡ NO + NO) and biogenic isoprene. Model estimates of surface ozone concentrations tend to be biased high in the region and this is of concern for designing effective emission control strategies to meet air quality standards. We use detailed chemical observations from the SEACRS aircraft campaign in August and September 2013, interpreted with the GEOS-Chem chemical transport model at 0.25°×0.3125° horizontal resolution, to better understand the factors controlling surface ozone in the Southeast US. We find that the National Emission Inventory (NEI) for NO from the US Environmental Protection Agency (EPA) is too high. This finding is based on SEACRS observations of NO and its oxidation products, surface network observations of nitrate wet deposition fluxes, and OMI satellite observations of tropospheric NO columns. Our results indicate that NEI NO emissions from mobile and industrial sources must be reduced by 30-60%, dependent on the assumption of the contribution by soil NO emissions. Upper tropospheric NO from lightning makes a large contribution to satellite observations of tropospheric NO that must be accounted for when using these data to estimate surface NO emissions. We find that only half of isoprene oxidation proceeds by the high-NO pathway to produce ozone; this fraction is only moderately sensitive to changes in NO emissions because isoprene and NO emissions are spatially segregated. GEOS-Chem with reduced NO emissions provides an unbiased simulation of ozone observations from the aircraft, and reproduces the observed ozone production efficiency in the boundary layer as derived from a regression of ozone and NO oxidation products. However, the model is still biased high by 8±13 ppb relative to observed surface ozone in the Southeast US. Ozonesondes launched during midday hours show a 7 ppb ozone decrease from 1.5 km to the surface that GEOS-Chem does not capture. This bias may reflect a combination of excessive vertical mixing and net ozone production in the model boundary layer.

Abstract. Isoprene emitted by vegetation is an important precursor of secondary organic aerosol (SOA), but the mechanism and yields are uncertain. Aerosol is prevailingly aqueous under the humid conditions typical of isoprene-emitting regions. Here we develop an aqueous-phase mechanism for isoprene SOA formation coupled to a detailed gas-phase isoprene oxidation scheme. The mechanism is based on aerosol reactive uptake coefficients (γ ) for water-soluble isoprene oxidation products, including sensitivity to aerosol acidity and nucleophile concentrations. We apply this mechanism to simulation of aircraft (SEAC 4 RS) and ground-based (SOAS) observations over the southeast US in summer 2013 using the GEOS-Chem chemical transport model. Emissions of nitrogen oxides (NO x ≡ NO + NO 2 ) over the southeast US are such that the peroxy radicals produced from isoprene oxidation (ISOPO 2 ) react significantly with both NO (high-NO x pathway) and HO 2 (low-NO x pathway), leading to different suites of isoprene SOA precursors. We find a mean SOA mass yield of 3.3 % from isoprene oxidation, consistent with the observed relationship of total fine organic aerosol (OA) and formaldehyde (a product of isoprene oxidation). Isoprene SOA production is mainly contributed by two immediate gasphase precursors, isoprene epoxydiols (IEPOX, 58 % of isoprene SOA) from the low-NO x pathway and glyoxal (28 %) from both low-and high-NO x pathways. This speciation is consistent with observations of IEPOX SOA from SOAS and SEAC 4 RS. Observations show a strong relationship between IEPOX SOA and sulfate aerosol that we explain as due to the effect of sulfate on aerosol acidity and volume. Isoprene SOA concentrations increase as NO x emissions decrease (favoring the low-NO x pathway for isoprene oxidation), but decrease more strongly as SO 2 emissions decrease (due to the effect of sulfate on aerosol acidity and volume). The US Environmental Protection Agency (EPA) projects 2013-2025 decreases in anthropogenic emissions of 34 % for NO x (leading to a 7 % increase in isoprene SOA) and 48 % for SO 2 (35 % decrease in isoprene SOA). Reducing SO 2 emissions decreases sulfate and isoprene SOA by a similar magnitude, representPublished by Copernicus Publications on behalf of the European Geosciences Union. 1604 E. A. Marais et al.: Aqueous-phase mechanism for SOA formation from isoprene ing a factor of 2 co-benefit for PM 2.5 from SO 2 emission controls.

Abstract. This overview paper highlights the successes of the Ozone Monitoring Instrument (OMI) on board the Aura satellite spanning a period of nearly 14 years. Data from OMI has been used in a wide range of applications and research resulting in many new findings. Due to its unprecedented spatial resolution, in combination with daily global coverage, OMI plays a unique role in measuring trace gases important for the ozone layer, air quality, and climate change. With the operational very fast delivery (VFD; direct readout) and near real-time (NRT) availability of the data, OMI also plays an important role in the development of operational services in the atmospheric chemistry domain.

Formation of organic nitrates (RONO) during oxidation of biogenic volatile organic compounds (BVOCs: isoprene, monoterpenes) is a significant loss pathway for atmospheric nitrogen oxide radicals (NO), but the chemistry of RONO formation and degradation remains uncertain. Here we implement a new BVOC oxidation mechanism (including updated isoprene chemistry, new monoterpene chemistry, and particle uptake of RONO) in the GEOS-Chem global chemical transport model with ∼25 × 25 km resolution over North America. We evaluate the model using aircraft (SEACRS) and ground-based (SOAS) observations of NO, BVOCs, and RONO from the Southeast US in summer 2013. The updated simulation successfully reproduces the concentrations of individual gas- and particle-phase RONO species measured during the campaigns. Gas-phase isoprene nitrates account for 25-50% of observed RONO in surface air, and we find that another 10% is contributed by gas-phase monoterpene nitrates. Observations in the free troposphere show an important contribution from long-lived nitrates derived from anthropogenic VOCs. During both campaigns, at least 10% of observed boundary layer RONO were in the particle phase. We find that aerosol uptake followed by hydrolysis to HNO accounts for 60% of simulated gas-phase RONO loss in the boundary layer. Other losses are 20% by photolysis to recycle NO and 15% by dry deposition. RONO production accounts for 20% of the net regional NO sink in the Southeast US in summer, limited by the spatial segregation between BVOC and NO emissions. This segregation implies that RONO production will remain a minor sink for NO in the Southeast US in the future even as NO emissions continue to decline.