Abstract. A rapid nighttime decay of isoprene (2-methyl-1,3-butadiene) has been observed at several forest sites. Data from the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) have been carefully examined with respect to this phenomenon. Essentially every evening (at PROPHET), isoprene concentrations fall from several ppb to levels below 100 ppt, with an average lifetime of 2.7 hours. Since this decay rate exceeds that expected from established nighttime chemistry, other possible mechanisms are suggested and discussed. Reaction with ozone will not occur at a rate consistent with the observed decay. Calculations of nitrate radical concentrations reveal that this oxidant only becomes an important sink for isoprene after the majority of the isoprene decay has taken place. The isoprene flux data were not consistent with dry deposition playing a significant role in nighttime forest loss. On the basis of ambient measurements of OH radical concentrations at the PROPHET site, calculated isoprene decay rates were compared with observations. For some nights the observed decay can be fit strictly by OH consumption; however, the reported OH data overpredict the isoprene loss rate on most nights. We estimate that vertical mixing with isoprene-depleted air probably contributes to the fast isoprene decay observed; however, the measurements needed to support this suggestion have yet to be made.

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Abstract. An intensive field study was undertaken in southern Nova Scotia, on the east coast of Canada, for several weeks during the summer of 1996 as part of the North American Research Strategy for Tropospheric Ozone -Canada East ( NARSTO-CE ) 1996 field measurement campaign. Clean air conditions prevailed during most of the study period, which allowed an examination of biogenic hydrocarbon chemistry with minimal influence from anthropogenic pollutants. Low NOx mixing ratios during the study had an impact on the ratio of isoprene's oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) to isoprene. The effects include changes to the fate of isoprene peroxy radicals and to the concentration of OH compared to conditions of higher [NOx]. Comparison to other studies indicate that there is a relationship between the ratio (MVK+MACR)/isoprene and the mixing ratio ofNOx. Biogenic hydrocarbons were the dominant reactive volatile organic compound (VOC) precursors to ozone production in this region, although the net ozone production rate predicted by a box-model simulation of the measurement data was only < 1 ppbv h -•. The evidence confirms that ozone production at this site is very NOx-sensitive.Model simulations indicated that the ozonolysis of biogenic hydrocarbons is an important source of the hydroxyl radical at this site and that OH was, in fact, the dominant oxidant during the nighttime under the observed low NOx conditions. Although the OH source did affect the nighttime mixing ratios of biogenic hydrocarbons, it could not fully explain the rapid noctumal decay of isoprene observed on most evenings.

Abstract. Aerosol and trace gas measurements were made at Kejimkujik National Park, Nova Scotia, Canada, during the summer of 1996. A case study from July 7-8 provides evidence of nucleation and condensation of products related to the oxidation of different biogenic emissions. Particles from 5 nm to 50 nm in diameter evolved during the afternoon and early evening associated with variations in isoprene. Late in the evening the ix-and [5-pinene mixing ratios and the aerosol particle volume increased. Soon after, there was a sharp increase in RO2H/H202 that persisted until about 0100 LT. The initial increases in the pinenes and aerosols were strong and influenced by changes in winds. After 2200 LT, and into the early morning, the winds were relatively steady, and the ix- supersaturation were enhanced by the appearance of the 80-90 nm mode pointing to at least some of these forest-generated particles as being able to serve as nuclei for cloud droplets at common atmospheric supersaturations.

Abstract. The isoprene atmospheric oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR), along with CO, were measured in Toronto, Ontario in the winter and summer of 1996. These carbonyl compounds were highly correlated with CO in the winter, indicating that they are produced in significant amounts by automotive sources. Regression of the observed MVK and MACR concentrations against CO leads to emission factors relative to CO of 1.4(+0.3)x10 '4 and 7.3(+1.6)x10 -5 (mole/mole), respectively. Emission inventories for CO and isoprene allow us to estimate that, for Toronto in the summer, as much as 48(+30)% of the MVK and 36(+23)% of the MACR input into the atmosphere is derived from mobile source emissions. This source also has a strong influence on the MVK/MACR ratio, which in this environment remains relatively constant, with values typically ranging between 2.0-2.5. The impact of this is that MVK and MACR mixing ratios are not unambiguous indicators for isoprene chemistry'and its impact on ozone production for urban environments.