An updated and expanded version of the Carbon Bond mechanism (CB05) has been incorporated into the Community Multiscale Air Quality (CMAQ) modeling system to more accurately simulate wintertime, pristine, and high-altitude situations. The CB05 mechanism has nearly 2 times the number of reactions relative to the previous version of the Carbon Bond mechanism (CB-IV). While the expansions do provide more detailed treatment of urban areas, most of the new reactions involve biogenics, toxics, and species potentially important to particulate formation and acid deposition. Model simulations were performed using the CB05 and the CB-IV mechanisms for the winter and summer of 2001. For winter with the CB05 mechanism, ozone, aerosol nitrate, and aerosol sulfate concentrations were within 1% of the results obtained with the CB-IV mechanism. Organic carbon concentrations were within 2% of the results obtained with the CB-IV mechanism. However, formaldehyde and hydrogen peroxide concentrations were lower by 25% and 32%, respectively, during winter with the CB05 mechanism. For the summer, ozone concentrations increased by 8% with the CB05 mechanism relative to the CB-IV mechanism. The aerosol sulfate, aerosol nitrate, and organic carbon concentrations with the CB05 mechanism decreased by 8%, 2%, and 10%, respectively. The formaldehyde and hydrogen peroxide concentrations with the CB05 mechanism were lower by 12% and 47%, respectively, during summer. Model performance with the CB05 mechanism improved at high-altitude conditions and in rural areas for ozone. Model performance also improved for organic carbon with the CB05 mechanism.

Laboratory and field experiments have revealed that uptake of dinitrogen pentoxide (N₂O₅) on aerosols containing chloride produces nitryl chloride (ClNO₂) and nitric acid. We incorporate heterogeneous ClNO₂ formation into the hemispheric Community Multiscale Air Quality model. This heterogeneous chemistry substantially enhances ClNO₂ levels in several areas of the Northern Hemisphere and alters the composition of airborne reactive nitrogen, comprising more than 15% of monthly mean values in some areas. Model results suggest that this heterogeneous chemistry reduces monthly mean total nitrate by up to 25% and enhances monthly mean daily maximum 8 h ozone by up to 7.0 ppbv. The pathway also enhances hydroxyl radical by more than 20% in some locations which in turn increases sulfate and other secondary pollutants. The largest ClNO₂ concentrations and impacts occur over China and Western Europe, two areas in which few relevant field measurements have been made.

We compare Community Multiscale Air Quality (CMAQ) model predictions with measured nitrous acid (HONO) concentrations in Beijing, China, for December 2015. The model with the existing HONO chemistry in CMAQ severely underestimates the observed HONO concentrations with a normalized mean bias of −97 %. We revise the HONO chemistry in the model by implementing six additional heterogeneous reactions in the model: the reaction of nitrogen dioxide (NO₂) on ground surfaces, the reaction of NO₂ on aerosol surfaces, the reaction of NO₂ on soot surfaces, the photolysis of aerosol nitrate, the nitric acid displacement reaction, and the hydrochloric acid displacement reaction. The model with the revised chemistry substantially increases HONO predictions and improves the comparison with observed data with a normalized mean bias of −5 %. The photolysis of HONO enhances daytime hydroxyl radical by almost a factor of 2. The enhanced hydroxyl radical concentrations compare favorably with observed data and produce additional sulfate via the reaction with sulfur dioxide, aerosol nitrate via the reaction with nitrogen dioxide, and secondary organic aerosols via the reactions with volatile organic compounds. The additional sulfate stemming from revised HONO chemistry improves the comparison with observed concentration; however, it does not close the gap between model prediction and the observation during polluted days.

We incorporate the recently developed Regional Atmospheric Chemistry Mechanism (version 2, RACM2) into the Community Multiscale Air Quality modeling system for comparison with the existing 2005 Carbon Bond mechanism with updated toluene chemistry (CB05TU). Compared to CB05TU, RACM2 enhances the domain-wide monthly mean hydroxyl radical concentrations by 46% and nitric acid by 26%. However, it reduces hydrogen peroxide by 2%, peroxyacetic acid by 94%, methyl hydrogen peroxide by 19%, peroxyacetyl nitrate by 40%, and organic nitrate by 41%. RACM2 enhances ozone compared to CB05TU at all ambient levels. Although it exhibited greater overestimates at lower observed concentrations, it displayed an improved performance at higher observed concentrations. The RACM2 ozone predictions are also supported by increased ozone production efficiency that agrees better with observations. Compared to CB05TU, RACM2 enhances the domain-wide monthly mean sulfate by 10%, nitrate by 6%, ammonium by 10%, anthropogenic secondary organic aerosols by 42%, biogenic secondary organic aerosols by 5%, and in-cloud secondary organic aerosols by 7%. Increased inorganic and organic aerosols with RACM2 agree better with observed data. Any air pollution control strategies developed using the two mechanisms do not differ appreciably.

This paper presents model estimates of the effect of chlorine emissions on atmospheric ozone concentrations in the eastern United States. The model included anthropogenic molecular chlorine emissions, anthropogenic hypochlorous acid emissions from cooling towers and swimming pools, and chlorine released from sea-salt aerosols. The release of chlorine emissions from sea-salt aerosols was modeled using heterogeneous reactions involving chloride ions in aerosols and three gas-phase species. The gas-phase chlorine chemistry was combined with the Carbon Bond Mechanism and incorporated into the Community Multiscale Air Quality modeling system. Air quality model simulations were performed for July 2001 and the results obtained with and without chlorine emissions were analyzed. When chlorine emissions were included in the model, ozone concentrations increased in the Houston, Texas, and New York–New Jersey areas. The daily maximum 1-h ozone concentrations increased by up to 12 parts per billion by volume (ppbv) in the Houston area and 6 ppbv in the New York–New Jersey area. The daily maximum 8-h ozone concentrations increased by up to 8 ppbv in the Houston area and 4 ppbv in the New York–New Jersey area. The monthly average daily maximum 1-h ozone concentration increased by up to 3 ppbv in the Houston area, but the increases in the monthly average daily maximum 1-h ozone concentration in the New York–New Jersey area were small. Chlorine emissions and chemistry enhanced the volatile organic compound oxidation rates and, thereby, increased the ozone production rate.