The chemistry of biogenic hydrocarbons at a rural site in eastern Canada

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 NO_x 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 [NO_x]. Comparison to other studies indicate that there is a relationship between the ratio (MVK+MACR)/isoprene and the mixing ratio of NO_x. 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^-1. The evidence confirms that ozone production at this site is very NO_x-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 NO_x conditions. Although the OH source did affect the nighttime mixing ratios of biogenic hydrocarbons, it could not fully explain the rapid nocturnal decay of isoprene observed on most evenings.