Abstract. A one-dimensional canopy model was developed to study photochemical processes inside and above a mixed deciduous forest in southern Ontario. The Eulerian model made use of Lagrangian dispersion principles with a correction factor to incorporate the average ensemble time since emission to calculate atmospheric mixing; traditional diffusion methods were found to provide insufficient mixing to match the measurements. Neglecting chemical losses while making isoprene emission estimates was found to underestimate emission rates up to 40%. The ozone oxidation of biogenic and anthropogenic alkenes was found to be a potential source of hydroxyl (OH) and hydroperoxy (HO2) radicals during the morning and night. Reactions of HO 2 with organic peroxy radicals formed by the OH oxidation of isoprene during the day were shown to be a significant source of organic peroxide formation above the canopy. The primary pathways of methacrolein and methylvinylketone formation were shown to be ozone oxidation of isoprene and hydroxyl radical oxidation of isoprene, respectively. Local ozone formation was shown to be limited by low mixing ratios of nitrogen oxides, despite high levels of isoprene present at the site. Table 2) laboratory study data. These latter studies extensively update the knowledge of atmospheric chemistry, particularly with regards to the reactions involving organic radicals, and pathways leading to the production of oxygenated species. 3581

Experiments were conducted during the growing season of 1993 at a mixed deciduous forest in southern Ontario, Canada to investigate the atmospheric abundance of hydrocarbons from phytogenic origins, and to measure emission rates from foliage of deciduous trees. The most abundant phytogenic chemical species found in the ambient air were isoprene and the monoterpenes a-pinene and/3-pinene. Prior to leaf-bud break during spring, ambient hydrocarbon mixing ratios above the forest remained barely above instrument detection limit (~20 parts per trillion), but they became abundant during the latter part of the growing season. Peak isoprene mixing ratios reached nearly 10 parts per billion (ppbv) during mid-growing season while maximum monoterpene mixing ratios were close to 2 ppbv. Both isoprene and monoterpene mixing ratios exhibited marked diurnal variations. Typical isoprene mixing ratios were highest during mid-afternoon and were lowest during nighttime. Peak isoprene mixing ratios coincided with maximum canopy temperature. The diurnal pattern of ambient isoprene mixing ratio was closely linked to the local emissions from foliage. Isoprene emission rates from foliage were measured by enclosing branches of trees inside environmentcontrolled cuvette systems and measuring the gas mixing ratio difference between cuvette inlet and outlet airstream. Isoprene emissions depended on tree species, foliage ontogeny, and environmental factors such as foliage temperature and intercepted photosynthetically active radiation (PAR). For instance, young (< 1 month old) aspen leaves released approximately 80 times less isoprene than mature (> 3 months old) leaves. During the latter part of the growing season the amount of carbon released back to the atmosphere as isoprene by big-tooth and trembling aspen leaves accounted for approximately 2% of the photosynthetically fixed carbon. Significant isoprene mixing ratio gradients existed between the forest crown and at twice canopy height above the ground. The gradient diffusion approach coupled with similarity theory was used to estimate canopy isoprene flux densities. These canopy fluxes compared favorably with values obtained from a multilayered canopy model that utilized locally measured plant microclimate, biomass distribution and leaf isoprene emission rate data. Modeled isoprene fluxes were approximately 30% higher compared to measured fluxes. Further comparisons between measured and modeled canopy biogenic hydrocarbon flux densities are required to assess uncertainties in modeling systems that provide inventories of biogenic hydrocarbons.

Measurements of isoprene concentration and flux were made at a mixed deciduous forest in southern Canada during 1995 to characterize diel and seasonal emissions and thus deduce annual inventories. Isoprene inventories are necessary for inputs to modeling systems to study atmospheric chemistry and carbon budgets. Despite adequate environmental conditions to promote emissions, the onset of isoprene emission occurred two weeks after full leaf expansion, and two additional weeks were required for plants to emit isoprene at the maximum capacity. Such maximum isoprene emission was measured during July when canopy isoprene fluxes reached 40-60 nmol (isoprene)·m Ϫ2 (ground area)·s Ϫ1 . Isoprene emission precipitously declined in concert with autumnal leaf senescence, with fluxes reaching the detection limit before the forest became leafless. In addition to plant development controls on emissions, temperatures below 10ЊC strongly modulated isoprene emission. After plants were exposed to low temperatures, isoprene emission remained suppressed and did not correspondingly increase in the manner that temperature is known to influence isoprene biosynthesis. Using a one-dimensional model to vertically adjust temperature and visible solar radiation with depth in the canopy, coupled with a seasonally adjusted emission rate, we estimated that the forest produced 71 mmol isoprene/m 2 during 1995. For a deciduous forest with final leaf-area index of 4.1 and active isoprene biomass of 75 g (dry mass)/m 2 , on average such isoprene source accounted for 2% of the carbon fixed through photosynthesis. The percentage of carbon entering the atmosphere in the form of isoprene became as high as 10% during warm (Ͼ30ЊC) and dry conditions. The data set reported here demonstrates that constant emission rates are inadequate to characterize emission rates for the entire growing season. Improved isoprene-emission inventories can be achieved if emission factors are seasonally adjusted. In this study we adopted a method to express the emission rates as a function of degree days.

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Significant associations between firefighting and cancer have been reported; however, studies finding toxic products of combustion at municipal fires have been limited by (1) technical difficulties encountered at the scene of working fires, (2) the lack of a coherent sampling strategy, and (3) the absence of verified sampling methods. The objective of the present study was to characterize the presence of volatile organic compound (VOC) combustion products in fire smoke. Air samples from experimental fires burning various materials commonly found at structural fires were collected into evacuated Summa canisters and analyzed for 144 target VOCs using cryogenic preconcentration and gas chromatography/mass spectroscopy (GC/MSD) methodology. The resulting chromatograms were characterized by a small number of predominant peaks, with 14 substances (propene, benzene, xylenes, 1-butene/2-methylpropene, toluene, propane, 1,2-butadiene, 2-methylbutane, ethylbenzene, naphthalene, styrene, cyclopentene, 1-methylcyclopentene, isopropylbenzene) being found in proportionately higher concentrations in all experimental fires and accounting for 65% (SD = +/-12%) by mass of total measured VOCs. Benzene, toluene, 1,3-butadiene, naphthalene, and styrene were found at higher concentrations than most other VOCs and increased with the time of combustion together with increasing levels of carbon monoxide. Benzene was found in the highest concentrations, with peak levels ranging from 0.6 ppm to 65 ppm, while the levels of 1,3-butadiene, styrene, and naphthalene peaked at 0.1, 0.4, and 3 ppm, respectively. This study revealed that there were no new or novel, toxic nonpolar VOCs resulting from the burning of common building materials. This is important in view of the studies that have found associations between firefighting and various forms of cancer.