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<jats:p>Oxidation rates of trace gases in the troposphere depend on species-specific rate coefficients and are predominantly governed by the concentration of the hydroxyl radical (OH), the most important oxidizing molecule in the troposphere. The concentration of OH is in local photochemical steady state; it is, however, dependent on the concentration of trace gases such as ozone, water vapor, volatile organic compounds, and oxides of nitrogen. Diffusive and advective transport processes influence and change the concentrations of these trace gases. Oxidative and transport time scales are often of similar magnitude, which leads to coupling between tropospheric chemistry and transport.</jats:p>

<jats:p> Tropospheric air pollution has impacts on scales ranging from local to global. Reactive intermediates in the oxidation of mixtures of volatile organic compounds (VOCs) and oxides of nitrogen (NO <jats:sub>x</jats:sub> ) play central roles: the hydroxyl radical (OH), during the day; the nitrate radical (NO <jats:sub>3</jats:sub> ), at night; and ozone (O <jats:sub>3</jats:sub> ), which contributes during the day and night. Halogen atoms can also play a role during the day. Here the implications of the complex VOC-NO <jats:sub>x</jats:sub> chemistry for O <jats:sub>3</jats:sub> control are discussed. In addition, OH, NO <jats:sub>3</jats:sub> , and O <jats:sub>3</jats:sub> are shown to play a central role in the formation and fate of airborne toxic chemicals, mutagenic polycyclic aromatic hydrocarbons, and fine particles. </jats:p>

<jats:p>Atmospheric aerosols play important roles in climate and atmospheric chemistry: They scatter sunlight, provide condensation nuclei for cloud droplets, and participate in heterogeneous chemical reactions. Two important aerosol species, sulfate and organic particles, have large natural biogenic sources that depend in a highly complex fashion on environmental and ecological parameters and therefore are prone to influence by global change. Reactions in and on sea-salt aerosol particles may have a strong influence on oxidation processes in the marine boundary layer through the production of halogen radicals, and reactions on mineral aerosols may significantly affect the cycles of nitrogen, sulfur, and atmospheric oxidants.</jats:p>

<jats:p>Heterogeneous and multiphase reactions on solids and in liquids, respectively, have the potential to play a major role in determining the composition of the gaseous troposphere and should be included in models for understanding this region and assessing the effects of anthropogenic emissions. Making a distinction between reactions on solids (heterogeneous reactions) and those occurring in liquid droplets (multiphase reactions) is convenient for understanding, describing, and including them in models of the troposphere. Frameworks are available for including multiphase reactions in numerical models, but they do not yet exist for heterogeneous reactions. For most of these reactions, water not only provides the medium but it is also a reactant. Other substrates such as sulfate and organic and sea-salt aerosols may also be important, but their effects cannot currently be accurately assessed because of a lack of information on their abundance, nature, and reactivities. Our ability to accurately predict the composition of the troposphere will depend on advances in understanding the microphysics of particle formation, laboratory investigations of heterogeneous and multiphase reactions, and collection of field data on tropospheric particles.</jats:p>

<jats:p>Measurements of trace gases and photolysis rates in the troposphere are essential for understanding photochemical smog and global environmental change. Chemical measurement techniques have progressed enormously since the first regular observations of tropospheric ozone in the 19th century. In contrast, by the 1940s spectroscopic measurements were already of a quality that would have allowed the use of modern analysis techniques to reduce interference between gases, although such techniques were not applied at the time. Today, chemical and spectroscopic techniques complement each other on a wide range of platforms. The boundaries between spectroscopic techniques will retreat as more Fourier transform spectrometers are used at visible wavelengths and as wide-band lidars are extended, and combining chemical techniques will allow detection of more trace gases with better sensitivity. Other future developments will focus on smaller, lighter instruments to take advantage of new platforms such as unmanned aircraft and to improve the effectiveness of urban sampling.</jats:p>

<jats:p>Recent studies have shown that global radiative and hydrological fluxes are strongly linked to microphysical processes in clouds. The sensitivity of predictions of climate variations to assumptions about the microphysical processes has led to new approaches to atmospheric measurements and to heightened interest and progress in understanding the physical chemistry, radiative properties, and kinetics of small solid and liquid aqueous particles.</jats:p>