Evaluation of Photochemical Pollution during Transport of Air Pollutants in Spring over the East China Sea

Abstract

We conducted intensive observations of ozone, CO, NOx (=NO and NO2), NOy (total odd nitrogen species including particulate nitrate) and total nitrate (the sum of gaseous HNO3 and particulate nitrate) at Cape Hedo, Okinawa, Japan, from 19 March to 3 April, 2009, to investigate ozone production during long-range transport from the Asian continent. Ozone production efficiency (OPE) was used to evaluate photochemical ozone production. OPE is defined as the number of molecules of ozone produced photochemically during the lifetime of a NOx molecule. OPE is calculated by the ratio of the concentration increase of ozone to that of NOz (=NOy-NOx). Average OPE during observation was estimated to be 12.6±0.5, but concentrations of ozone increased nonlinearly with those of NOz. This non-linearity suggests that OPE depends on air mass origin and NOz concentrations. There were very different values of OPE for the same air mass origin, so that only the air mass origin alone does not control OPE. OPE was low when NOz concentration was high. We examined the correlation between NOz and CO/NOy ratios, which we used instead of the ratio of non-methane hydrocarbons (NMHCs) to NOx. The CO/NOy ratios decreased with increasing NOz concentrations. These results indicate that competition reactions of OH with NMHCs and NO2 are the rate determining steps of photochemical ozone production during long-range transport from the Asian continent to Cape Hedo, for high concentrations of nitrogen oxides.

Keywords

• ozone production efficiency
• total odd nitrogen species
• long-range transport
• east asia
• non-methane hydrocarbons