[1] It is known that isoprene nitrate production can represent a significant sink for atmospheric NO x and free radicals, and therefore this chemistry is important to understanding tropospheric O 3 and the fate of NO x in forest-impacted environments. Although six structural isoprene nitrate isomers can be produced from OH reaction with isoprene in the presence of NO, these have not been separately quantified in the atmosphere. A zero-dimensional isoprene photochemistry model was developed based on the known gas-phase isoprene oxidation chemistry for comparison with isoprene nitrate ambient concentration data obtained from field and laboratory measurements. The model incorporates calculated individual branching ratios for each isomer as well as rate constants for the reaction of each isomeric nitrate with the OH radical and O 3 and losses from the boundary layer by dry deposition and vertical mixing. The model indicates that under atmospheric conditions, there should be three nitrate isomers that represent 86% of the total, while our ambient measurements indicate only two dominant nitrate isomers. This contrasts significantly with what is observed in the laboratory because of atmospheric conversion to other nitrogen-containing products. These secondary nitrates are likely to be a significant fraction of NO y in forest environments.Citation: Giacopelli, P., K. Ford, C. Espada, and P. B. Shepson (2005), Comparison of the measured and simulated isoprene nitrate distributions above a forest canopy,

[1] Peroxyacetyl nitrate (PAN) was measured in ambient and snowpack interstitial air at Summit, Greenland, in June and July of 1998 and 1999 and at a rural/forest site in the Keewenaw Peninsula of Michigan in January of 1999. At Summit, we found that PAN typically represented between 30 and 60% of NO y . In the summer of 1999, a significant diel variation in both PAN/NO y and NO x /NO y was observed, but this was much less pronounced in 1998. Experiments during SNOW99 near Houghton, Michigan, indicated that PAN undergoes weak uptake onto snow grain surfaces. At Summit, we found that PAN concentrations in the snowpack interstitial air were significantly elevated (by as much as 2 -5 times) relative to ambient levels, implying a flux of PAN out of the snowpack during the study period. We also observed evidence that PAN can be photochemically produced in snow that is exposed to polluted air. These observations indicate that interactions with the snowpack can have a significant impact on PAN concentrations in the boundary layer and point to potential difficulties associated with investigation of long-term changes in PAN uptake into ice cores because of the impact of postdepositional processes.

Production of organic nitrates from OH reaction with cyclohexane, cyclohexene, n-butane, 1-bromopropane, and p-xylene in the presence of NO was studied. The total organic nitrate yields for cyclohexane and n-butane were determined to be 17 ± 4 and 7 ± 2% respectively, which is in good agreement with previous determinations. Total yields for cyclohexene, 1-bromopropane, and p-xylene were 2.5 ± 0.5, 1.2 ± 0.3, and 3.2 ± 0.7 respectively. The yield for cyclohexene was five times smaller than that for cyclohexane. The 1-bromopropane yield is three times smaller that that for n-propane, but similar to that for propene, indicating that the effect of Br substitution in the reactant may be similar to that for OH substitution. The only nitrooxy product detected for p-xylene was 4-methylbenzylnitrate, which was formed following H abstraction from either methyl group. No organic nitrate was detected for peroxy radicals produced from OH addition to the ring, which accounts for 90% of the OH oxidation of p-xylene. The calculated k 3b /k 3 value for p-methyl benzyl peroxy radicals (0.32) was slightly smaller than for n-octyl peroxy radicals (0.39). These data imply that substituent inductive effects impact the k 3b /k 3 ratios. We found no significant difference in the k 3b /k 3 ratios for primary vs. secondary peroxy radicals of the same carbon chain. C 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: [675][676][677][678][679][680][681][682][683][684][685] 2005

[1] Peroxyacetyl nitrate (PAN) is a NO x reservoir compound that has the ability to transport NO x to remote environments, allowing for NO x photochemistry and/or deposition of nitrogen to these clean locations. Measurements of PAN have been made at Alert, Nunavut, and Summit, Greenland, aimed at understanding the impact of PAN chemistry on atmospheric nitrogen in the Arctic. These measurements show concentrations of PAN that are only slowly varying, even during ozone depletion events at polar sunrise, when free radical photochemistry is relatively active. We used a zerodimensional photochemical model incorporating known gas-phase chemistry to simulate the observed behavior of PAN at Alert, Nunavut, and Summit, Greenland. The model simulations suggest a substantial net production rate for PAN over sunlit surfaces, which is inconsistent with the measured gas-phase concentrations. These observations thus indicate a fundamental problem with our understanding of PAN chemistry in low-temperature, snow-covered environments. We explore the possibility that we are missing an important sink for atmospheric PAN above snow-covered surfaces. If the loss is caused by snowpack deposition, the data result in calculated deposition velocities ranging from 0.05 to 1 cm s À1 , which would represent a significant fraction of the unidentified total nitrate input to the snowpack and glacial ice at Summit, Greenland.