VOC reactivity and its effect on ozone production during the HaChi summer campaign

Abstract

Abstract. Measurements of ozone and its precursors conducted within the HaChi (Haze in China) project in summer 2009 were analyzed to characterize volatile organic compounds (VOCs) and their effects on ozone photochemical production at a suburban site in the North China Plain. Ozone episodes, during which running 8-h average ozone concentrations exceeding 80 ppbv lasted for more than 4 h, occurred on about two thirds of the observational days during the 5-week field campaign. This suggests continuous ozone exposure risks in this region during the summer. The average concentrations of nitrogen oxides (NOx) and VOCs are about 20 ppbv and 650 ppbC, respectively. Total VOC reactivity is dominated by anthropogenic VOCs, including aromatics, alkanes and most alkenes. The contribution of biogenic VOCs to total ozone-forming potential, however, is also considerable in the daytime. 2-butenes, isoprene, trimethylbenzenes, xylenes, 3-methylhexane, n-hexane and toluene are key species associated with ozone photochemical production. Formation of ozone is found to be NOx-limited as indicated by measured VOCs/NOx ratios and further confirmed by a sensitivity study using a photochemical box model NCAR_MM. The Model simulation suggests that ozone production is also sensitive to changes in VOC reactivity under the NOx-limited regime, although this sensitivity depends strongly on how much NOx is present.