Sequential oxidation products from tropospheric isoprene chemistry: MACR and MPAN at a NO_x-rich forest environment in the southeastern United States

Measurements of peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), and peroxymethacryloyl nitrate (MPAN) were made during the Southern Oxidants Study 1995 Nashville/Middle Tennessee Ozone Study at the Youth Inc. Ranch southeast of Nashville from June 29 to July 26. These measurements were made along with those of isoprene and its oxidation products methacrolein (MACR) and methyl vinyl ketone (MVK), other carbonyl compounds, and supporting measurements. This data set represents the first high-frequency, simultaneous measurements of MPAN and its precursor, MACR as well as PPN and its precursor propanal. The NO_x sensitivity of isoprene chemistry can be studied with data from this site because large and widely fluctuating levels of NO_x were experienced as a result of the proximity to the Nashville urban center. Mean mixing ratios of PAN, PPN, MPAN, and MACR were 485, 50, 30, and 290 parts per trillion by volume respectively. The mean diurnal cycle of MPAN closely tracks that of MACR and was found to be considerably sharper than the mean diurnal cycles of PAN, PPN, and O₃, showing that MPAN is closely dependent on the availability of MACR. Considerable levels of MPAN and MACR appear to develop at night above a nocturnal boundary layer and are partly responsible for a commonly observed morning increase. Early morning OH reaction with MACR also produces MPAN in the morning. With the high summer temperatures at Youth Inc., the MPAN lifetime is largely determined by thermal decomposition, although OH and O₃ chemistry could substantially reduce the ambient lifetime. Additional loss mechanisms for MPAN and dependence on a single source make MPAN more sensitive to photochemical activity and NO_x than is PAN. Unless quickly advected to colder, less photochemically active regions of the atmosphere, the contribution of MPAN to O₃ formation through long-range transport of NO_x is likely to be less significant in comparison with PAN and PPN. Its influence is more limited to local O₃ production, so MPAN can be a useful indicator of local-scale, active biogenic photochemistry.