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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

& Volatile organic compound (VOC) emission rate factors are estimated for 49 tree genera based on a review of foliar emission rate measurements. Foliar VOC emissions are grouped into three categories: isoprene, monoterpenes and other VOCs. Typical emission rates at a leaf temperature of 30°C and a light intensityof1OOO~molm~'s~'rangefrom~0.1to70~gCg~'h~'forisoprene.<0.1to3~gCg~'h~'for monoterpenes, and < 0.5 to 5 Icg C g ' h-' for other VOCs. Isoprene emission factors are given for biogenic emission models that incorporate canopy shading effects and thus require leaf-level emission rates and for emission models that do not include a canopy model and therefore require branch-level isoprene emission factors which already account for some shading. Landscape-level emission rates are estimated by combining emission rate factors determined for tree genera with species composition and foliar mass data. Landscape emission rate factors are determined for each of the 91 woodland landscapes in the high resolution (I. I km) gridded land-cover database compiled by the EROS Data Center (EDC) from satellite and ancillary data. This database covers the entire contiguous United States of America. Landscape emission rates are also be determined using gridded tree distribution data. based on aerial photographs and ground measurements, such as that available in the U.S. Forest Service (USFS) Eastwide Forest Inventory Database (EFID). Emission rates are reported for 41 of the 65 tree genera in the EFID including all of the most common genera. Total VOC emission rate factors for the 91 EDC woodland-cover types range from 0.8 to II mgCm-'h-' at a standard condition of 30°C and 1OOO~~molm~'s~'. These landscape factors are based on branch-level emission factors and thus already incorporate canopy shading effects. The estimated fluxes of isoprene and monoterpenes are in relatively good agreement with field measurements of areaaveraged fluxes if accurate species composition data (e.g. from the EFID) are available. Total VOC emission rate estimates range from 0.8 to 4.3 mgCm-'h-l for scrub woodlands and 2.2 to 11 mgCm-'h-i for mixed deciduous/coniferous woodlands. The chemical composition of the VOC flux ranges from 8 IO 91% isoprene. 1 to 56% for monoterpenes and R to 73% for other VOC. On an area-weighted. basis, the U.S. average total VOC emission rate factor of 5. I mg m-' h-i for all woodlands is comprised of 58% isoprene, 18% monoterpenes and 24% other VOC. In comparison to previous estimates, these emission rates are generally higher for isoprene and lower for monoterpenes. KEY word it&v: Volatile organic compound, isoprene. terpene, biogenic, natural emissions.

emergence. Leaves that emerged in July and developed in hot, midsummer temperatures emitted isoprene within 6 days. Leaves that had emerged during the cool spring, and had grown for several weeks without emitting isoprene, could be induced to emit isoprene within 2 h of exposure to 32°C. Continued exposure to warm temperatures resulted in a progressive increase in the isoprene emission rate. Thus, temperature appears to be an important determinant of the early season induction of isoprene emission. The seasonal pattern of isoprene emission was examined in trees growing along an elevational gradient in the Colorado Front Range (1829-2896 m). Trees at different elevations exhibited staggered patterns of bud-break and initiation of photosynthesis and isoprene emission in concert with the staggered onset of warm, springtime temperatures. The springtime induction of isoprene emission could be predicted at each of the three sites as the time after bud break required for cumulative temperatures above 0°C to reach approximately 400 degree days. Seasonal temperature acclimation of isoprene emission rate and photosynthesis rate was not observed. The temperature dependence of isoprene emission rate between 20 and 35°C could be accurately predicted during spring and summer using a single algorithm that describes the Arrhenius relationship of enzyme activity. From these results, it is concluded that the early season pattern of isoprene emission is controlled by prevailing temperature and its interaction with developmental processes. The late-season pattern is determined by controls over leaf nitrogen concentration, especially the depletion of leaf nitrogen during senescence. Following early-season induction, isoprene emission rates correlate with photosynthesis rates. During the season there is little acclimation to temperature, so that seasonal modeling simplifies to a single temperature-response algorithm.

Acid deposition and photochemical smog are urban air pollution problems, and they remain localized as long as the sulfur, nitrogen, and hydrocarbon pollutants are confined to the lower troposphere (below about 1-kilometer altitude) where they are short-lived. If, however, the contaminants are rapidly transported to the upper troposphere, then their atmospheric residence times grow and their range of influence expands dramatically. Although this vertical transport ameliorates some of the effects of acid rain by diluting atmospheric acids, it exacerbates global tropospheric ozone production by redistributing the necessary nitrogen catalysts. Results of recent computer simulations suggest that thunderstorms are one means of rapid vertical transport. To test this hypothesis, several research aircraft near a midwestern thunderstrom measured carbon monoxide, hydrocarbons, ozone, and reactive nitrogen compounds. Their concentrations were much greater in the outflow region of the storm, up to 11 kilometers in altitude, than in surrounding air. Trace gas measurements can thus be used to track the motion of air in and around a cloud. Thunderstorms may transform local air pollution problems into regional or global atmospheric chemistry problems.