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Correlations are presented between model predictions for O3‐NOx‐hydrocarbon sensitivity and afternoon concentrations of four “indicator species”: NOy, O3/(NOy‐NOx), HCHO/NOy, and H2O2/HNO3. The indicator species correlations are based on a series of photochemical simulations with varying rates of anthropogenic and biogenic emissions and meteorology. Hydrocarbon‐sensitive chemistry in models is shown to be linked to afternoon NOy > 20 ppb, O3/(NOy ‐ NOx) < 7, HCHO/NOy < 0.28, and H2O2/HNO3 < 0.4. Lower NOy and higher ratios correspond with NOx‐sensitive ozone. The correlation between NOx‐hydrocarbon sensitivity and indicator species remains, even when model emission rates and hydrocarbon/NOx ratios are changed by a factor of 2. Methods are developed for evaluating the goodness of fit between model NOx‐hydrocarbon sensitivity and indicator values. Ozone chemistry is also analyzed in terms of fundamental properties of odd hydrogen, and theoretical criteria for the transition between NOx‐ and hydrocarbon‐sensitive regimes are derived. A theoretical correlation between O3 and H2O2 + NOy ‐ NOx is developed as a way to extend rural O3‐NOy correlations into urban locations. Measured indicator values during pollution events in Los Angeles, Atlanta, and rural Virginia are used to illustrate the range of observed values under different environmental conditions.

This paper evaluates the current status of global modeling of the organic aerosol (OA) in the troposphere and analyzes the differences between models as well as between models and observations. Thirty-one global chemistry transport models (CTMs) and general circulation models (GCMs) have participated in this intercomparison, in the framework of AeroCom phase II. The simulation of OA varies greatly between models in terms of the magnitude of primary emissions, secondary OA (SOA) formation, the number of OA species used (2 to 62), the complexity of OA parameterizations (gas-particle partitioning, chemical aging, multiphase chemistry, aerosol microphysics), and the OA physical, chemical and optical properties. The diversity of the global OA simulation results has increased since earlier AeroCom experiments, mainly due to the increasing complexity of the SOA parameterization in models, and the implementation of new, highly uncertain, OA sources. Diversity of over one order of magnitude exists in the modeled vertical distribution of OA concentrations that deserves a dedicated future study. Furthermore, although the OA/OC ratio depends on OA sources and atmospheric processing, and is important for model evaluation against OA and OC observations, it is resolved only by a few global models. The median global primary OA (POA) source strength is 56 Tg a(-1) (range 34-144 Tg a(-1)) and the median SOA source strength (natural and anthropogenic) is 19 Tg a(-1) (range 13-121 Tg a(-1)). Among the models that take into account the semi-volatile SOA nature, the median source is calculated to be 51 Tg a(-1) (range 16-121 Tg a(-1)), much larger than the median value of the models that calculate SOA in a more simplistic way (19 Tg a(-1); range 13-20 Tg a(-1), with one model at 37 Tg a(-1)). The median atmospheric burden of OA is 1.4 Tg (24 models in the range of 0.6-2.0 Tg and 4 between 2.0 and 3.8 Tg), with a median OA lifetime of 5.4 days (range 3.8-9.6 days). In models that reported both OA and sulfate burdens, the median value of the OA/sulfate burden ratio is calculated to be 0.77; 13 models calculate a ratio lower than 1, and 9 models higher than 1. For 26 models that reported OA deposition fluxes, the median wet removal is 70 Tg a(-1) (range 28-209 Tg a(-1)), which is on average 85% of the total OA deposition. Fine aerosol organic carbon (OC) and OA observations from continuous monitoring networks and individual field campaigns have been used for model evaluation. At urban locations, the model-observation comparison indicates missing knowledge on anthropogenic OA sources, both strength and seasonality. The combined model-measurements analysis suggests the existence of increased OA levels during summer due to biogenic SOA formation over large areas of the USA that can be of the same order of magnitude as the POA, even at urban locations, and contribute to the measured urban seasonal pattern. Global models are able to simulate the high secondary character of OA observed in the atmosphere as a result of SOA formation and POA agi...

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We examine the sensitivity of ozone concentrations in rural areas of the United States to emissions of NOx and hydrocarbons using a regional photochemical model. Ozone production in rural areas appears to be limited by the availability of NOx. Rural ozone is strongly dependent on emission rates for NOx but is almost independent of hydrocarbons. This relationship is quite different from that in urban air, where ozone levels depend on both NOx and hydrocarbons. The predicted relationship between ozone and nitrogen oxides appears to be consistent with observations in rural air. For the low NOx regime (< 2 ppb) in rural areas, increases in NOx lead to increases in OH and to corresponding increases in the oxidation rate of hydrocarbons and in levels of ozone. Ozone concentrations in urban plumes appear to be related to regional scale production in addition to production within the plume.