THE CHEMISTRY OF BIOGENIC HYDROCARBONS AT A RURAL SITE IN EASTERN CANADA

An intensive field study was undertaken in southern Nova Scotia, on th e 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 co nditions prevailed during most of the study period, which allowed an e xamination of biogenic hydrocarbon chemistry with minimal influence fr om anthropogenic pollutants. Low NOx mixing ratios during the study ha d 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 concentrat ion of OH compared to conditions of higher [NOx]. Comparison to other studies indicate that there is a relationship between the ratio (MVK+M ACR)/isoprene and the mixing ratio of NOx. Biogenic hydrocarbons were the dominant reactive volatile organic compound (VOC) precursors to oz one 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(-1). The evidence confirms that ozone production at this sit e is very NOx-sensitive. Model simulations indicated that the ozonolys is of biogenic hydrocarbons is an important source of the hydroxyl rad ical at this site and that OH was, in fact, the dominant oxidant durin g the nighttime under the observed low NOx conditions. Although the OH source did affect the nighttime mixing ratios of biogenic hydrocarbon s, it could not fully explain the rapid nocturnal decay of isoprene ob served on most evenings.