No abstract

Gas-phase chemical mechanisms are vital components of prognostic air quality models. The mechanisms are incorporated into modules that are used to calculate the chemical sources and sinks of ozone and the precursors of particulates. Fifty years ago essential atmospheric chemical processes, such as the importance of the hydroxyl radical, were unknown and crude air quality models incorporated only a few parameterized reactions obtained by fitting observations. Over the years, chemical mechanisms for air quality modeling improved and became more detailed as more experimental data and more powerful computers became available. However it will not be possible to incorporate a detailed treatment of the chemistry for all known chemical constituents because there are thousands of organic compounds emitted into the atmosphere. Some simplified method of treating atmospheric organic chemistry is required to make air quality modeling computationally possible. The majority of the significant differences between air quality mechanisms are due to the differing methods of treating this organic chemistry. The purpose of this review is to present an overview of atmospheric chemistry that is incorporated into air quality mechanisms and to suggest areas in which more research is needed.

h i g h l i g h t sNighttime concentrations of the nitrate radical (NO 3 ) were measured using a DOAS. Process analysis of 0-D simulations showed NO 3 accounted for 85% of a-pinene loss. Several nights of the campaign were significantly impacted by wildfires upwind. a b s t r a c tNighttime concentrations of the gas phase nitrate radical (NO 3 ) were successfully measured during a four week field campaign in an arid urban location, Reno Nevada, using long-path Differential Optical Absorbance Spectrometry (DOAS). While typical concentrations of NO 3 ranged from 5 to 20 ppt, elevated concentrations were observed during a wildfire event. Horizontal mixing in the free troposphere was considerable because the sampling site was above the stable nocturnal boundary layer every night and this justified a box modeling approach. Process analysis of box model simulations showed NO 3 accounted for approximately half of the loss of internal olefins, 60% of the isoprene loss, and 85% of the a-pinene loss during the nighttime hours during a typical night of the field study. The NO 3 þ aldehyde reactions were not as important as anticipated. On a polluted night impacted by wildfires upwind of the sampling location, NO 3 reactions were more important. Model simulations overpredicted NO 2 concentrations for both case studies and inorganic chemistry was the biggest influence on NO 3 concentrations and on nitric acid formation. The overprediction may be due to additional NO 2 loss processes that were not included in the box model, as deposition and N 2 O 5 uptake had no significant effect on NO 2 levels.