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The emission of isoprene and monoterpenes from plants is influenced by light and leaf temperature, which account for almost all short‐term variations (minutes to days) and a large part of spatial and long‐term variations. The temperature dependence of monoterpene emission varies among monoterpenes, plant species, and other factors, but a simple exponential relationship between emission rate (E) and leaf temperature (T), E = Es [exp (β(T − Ts))], provides a good approximation. A review of reported measurements suggests a best estimate of β = 0.09 K−1 for all plants and monoterpenes. Isoprene emissions increase with photosynthetically active radiation up to a saturation point at 700–900 μmol m−2 s−1. An exponential increase in isoprene emission is observed at leaf temperatures of less than 30°C. Emissions continue to increase with higher temperatures until a maximum emission rate is reached at about 40°C, after which emissions rapidly decline. This temperature dependence can be described by an enzyme activation equation that includes denaturation at high temperature. Algorithms developed to simulate these light and temperature responses perform well for a variety of plant species under laboratory and field conditions. Evaluations with field measurements indicate that these algorithms perform significantly better than earlier models which have previously been used to simulate isoprene emission rate variation. These algorithms account for about 90% of observed diurnal variability and can predict diurnal variations in hourly averaged isoprene emissions to within 35%.

Vegetation provides a major source of reactive carbon entering the atmosphere. These compounds play an important role in (1) shaping global tropospheric chemistry, (2) regional photochemical oxidant formation, (3) balancing the global carbon cycle, and (4) production of organic acids which contribute to acidic deposition in rural areas. Present estimates place the total annual global emission of these compounds between approximately 500 and 825 Tg yr−1. The volatile olefinic compounds, such as isoprene and the monoterpenes, are thought to constitute the bulk of these emissions. However, it is becoming increasingly clear that a variety of partially oxidized hydrocarbons, principally alcohols, are also emitted. The available information concerning the terrestrial vegetation as sources of volatile organic compounds is reviewed. The biochemical processes associated with these emissions of the compounds and the atmospheric chemistry of the emitted compounds are discussed.

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The concentrations of ozone, nitrogen oxides, and nonmethane hydrocarbons measured near the surface in a variety of urban, suburban, rural, and remote locations are analyzed and compared in order to elucidate the relationships between ozone, its photochemical precursors, and the sources of these precursors. While a large gradient is found among remote, rural, and urban/suburban nitrogen oxide concentrations, the total hydrocarbon reactivity in all continental locations is found to be comparable. Apportionment of the observed hydrocarbon species to mobile and stationary anthropogenic sources and biogenic sources suggests that present-day emission inventories for the United States underestimate the size of mobile emissions. The analysis also suggests a significant role for biogenic hydrocarbon emissions in many urban/suburban locations and a dominant role for these sources in rural areas of the eastern United States. As one moves from remote locations to rural locations and then from rural to urban/suburban locations, ozone and nitrogen oxide concentrations tend to increase in a consistent manner while total hydrocarbon reactivity does not. hydrocarbon concentrations in four chemically distinct regimes of the atmospheric boundary layer, each having a distinct mix of anthropogenic and natural hydrocarbon and NOx emissions. These regimes are: I, the urban/suburban atmosphere, which is the regime most strongly impacted by anthropogenic emissions; II, the rural atmosphere, which is somewhat less impacted by anthropogenic emissions and more impacted by natural emissions than that of the urban atmosphere; III, the atmosphere over the remote, tropical forest which is essentially free of anthropogenic volatile organic compounds (VOC) and NOx emissions and strongly influenced by natural emissions; and IV, the remote, marine atmosphere, which is not only free of anthropogenic emissions but is also characterized by relatively small biogenic sources of VOC and NO x. Because we are most interested in the conditions that foster ozone episodes, our analysis concentrates on observations made during the daylight hours of the summer months.In the sections below we first briefly summarize the fundamentals of the photochemical smog mechanism and the nonlinearities inherent in this system and then discuss the concentrations of 03, NOx, and hydrocarbons typically observed in the four regimes listed above. PHOTOCHEMICAL SMOGWhile uncertainties remain in our understanding of tropospheric photochemistry, the basic set of reactions that lead to 03 production have been identified. These reactions, commonly referred to in the aggregate as the "photochemical smog" mechanism, involve the oxidation of hydrocarbons and other volatile organic compounds in the presence of nitrogen oxides (NOx) and sunlight [Haagen-Srnit, 1952; $einfeld, 1988]. Typical of this mechanism are reactions (R1) through (R7),RH + OH--> R + H20 (R2) R + 02 + M--> RO2 + M (R3) RO 2 + NO--> RO + NO 2 (R4) RO + 0 2 --> HO 2 + RCHO 6037