Tropospheric Chemistry and Composition

The troposphere, the region of the Earth’s atmosphere in which we live and breath, is composed of many different kinds of gases. These components range from the abundant two-atom molecules O2 and N2, to larger and more reactive chemicals, which include vast numbers of different volatile organic compounds (VOCs). Concentrations of VOCs can be as high as one part per million by volume (10 , or ppmv) in source regions (cities and near fires) and below one part per trillion by volume (10 , or pptv) in remote regions. Many VOCs are important in tropospheric reactions even at low pptv concentrations. A lot has been learned in the last few decades about VOC sources and their distributions. However, there is much still to be determined about this very important class of atmospheric gases. Together with sunlight and nitrogen oxides (NOx 1⁄4 NO þ NO2), VOCs undergo a complicated series of chemical and physical transformations over a whole range of spatial scales. Various classes of VOCs have a large impact on atmospheric chemistry, contributing to localized urban smog, regional scale photochemical ozone pollution, acid deposition, and particle formation/visibility degradation, and ultimately global changes in the oxidizing capacity of the troposphere. VOCs are also useful as diagnostic tracers for atmospheric transport, chemical pathways, and distribution of sources. Photochemical smog has been detected in almost all of the world’s major urban and industrial centers, at levels that often exceed internationally agreed threshold values set to protect human health. Detailed understanding of the mechanism of photochemical smog formation has developed from pioneering studies of Los Angeles smog in the 1950s through the combination of laboratory studies, field experiments, air quality monitoring, and computer modeling studies. Man-made chlorinated hydrocarbons have reached the stratosphere, where the intense solar radiation activates their chlorine content into a potent depleting agent of the stratospheric ozone shield. Research into the important role played by VOCs in a range of environmental problems remains one of the most rapidly developing topics in the fast-developing field of atmospheric chemistry. VOCs comprise a large variety of vapor phase carbon-containing atmospheric gases in the reduced form, with a wide range of physical and chemical behaviors. Table 1 lists some important atmospheric organic species. Pure hydrocarbons containing only C and H, such as alkanes, alkenes, alkynes, and aromatics are, in turn a very important category of VOC and generally are referred to as nonmethane hydrocarbons (NMHCs). However, VOCs containing oxygen, chlorine, nitrogen, and other elements are also of significance. These gases include aldehydes, ketones, organic acids, alcohols (Table 1), and also ethers, esters, furans, nitriles, organic nitrates, chlorinated alkanes and alkenes, chlorofluorocarbons (CFCs), and hydrochlorofluorocarbons (HCFCs). Only carbon dioxide (CO2) and carbon monoxide (CO) are excluded. VOCs are typically classed as containing up to 12 carbon atoms per molecule, as those with more than 10 carbons tend to partition into atmospheric particles (aerosols). VOCs are released into the atmosphere from numerous types of sources, both natural and from various human activities (anthropogenic). Table 1 indicates that important source categories are combustion processes (e.g., vehicle exhausts and biomass burning); production, treatment, storage, and distribution of fossil fuels; and natural emissions from plants and the ocean. Other sources include organic solvents and landfills. In order to fully assess and control the effect that these gases have on our local, regional, and global air quality, it is important to compile detailed information on their emissions. However, the many types of sources, and their vast number, mean that preparing an inventory of emissions that includes all the diverse individual species is an extremely difficult task. The diverse natural and anthropogenic VOC emission sources, together with their relatively short atmospheric lifetimes (Table 1), mean that VOCs show significant variation in their spatial (latitudinal and vertical), and temporal (diurnal and seasonal) 0422-P0005