Modelling the impact of elevated primary NO 2 and HONO emissions on regional scale oxidant formation in the UK

Abstract

Recent increases in the fractional contribution of NO 2 to NO x emissions from the road transport sector in Europe are well documented in the literature. A photochemical trajectory model has been used to simulate the impact of increasing the NO 2 fraction on the chemical evolution of air masses arriving at the TORCH field campaign site in the southern UK during late July and August 2003, a period which included a widespread photochemical pollution episode associated with a heat-wave. The impact of partial emissions of NO x in the form of nitrous acid (HONO) has also been considered. The model incorporates emissions of NO x , CO, SO 2, methane and a detailed speciation of non-methane volatile organic compounds (VOC), coupled with a comprehensive description of the chemistry of secondary pollutant formation. An increase in the fractional contribution of NO 2 to NO x emissions from 0% to 30% (v/v) results in a 2.49 ppb increase in the simulated campaign mean mixing ratio of oxidant (defined as the sum of O 3 and NO 2). This is almost exclusively in the form of O 3, and represents a ca. 7% increase in the simulated campaign mean O 3 mixing ratio. Consideration of 156 events, at 6-hourly resolution throughout the campaign period, indicates that oxidant increments during the heat-wave period are generally simulated to be greater than those for the remainder of the campaign, with a maximum increment of ca. 12 ppb. The increases in oxidant mixing ratios are shown to derive from both the direct effect of increased NO 2 input, and indirectly from the enhanced regional-scale chemical processing that this promotes. An illustrative increase in the fractional contribution of HONO to NO x emissions from 0% to 5% (v/v) results in increases in the simulated campaign mean mixing ratios of O 3 and oxidant of 1.51 and 1.15 ppb, respectively, with the smaller increment for oxidant reflecting a decrease in the NO 2 mixing ratio resulting from a notably enhanced NO x oxidation rate. The oxidant increments during the heat-wave period are once again simulated to be greater than those for the remainder of the campaign, with a maximum increment of ca. 11 ppb, which results exclusively from the enhanced regional-scale chemical processing that the HONO emissions promote. The impact of increased fractional NO 2 and HONO emissions on the rate of oxidation of NO x to nitrate is also illustrated and discussed.