SEASONAL AND DIURNAL-VARIATION OF ISOPRENE AND ITS REACTION-PRODUCTS IN A SEMIRURAL AREA

Isoprene and its reaction products, methyl vinyl ketone (MVK), methacrolein (MAC) and 3-methyl furan (3-MF), in the atmosphere were measured in a semi-rural area in Japan. Measurements were conducted by an automated gas chromatograph/mass spectrometer with a small Tenax TA trap. 1877 data sets were obtained during the period July-December 1991. MVK and MAC showed high intercorrelation (R = 0.95) and with isoprene (R = 0.70 and 0.74, respectively). To elucidate the destructive pathways of isoprene after emission, the relative ratios among the concentrations of isoprene and its products were studied, based on their reactivity with ozone, OH radicals and NO3 radicals. In summer, the lower ratios of MVK and MAC to isoprene in the daytime were explained by predominant OH reaction as well as high isoprene emission. An increase of MVK/isoprene and MAC/isoprene ratios up to 5 times at night was related with the remaining ozone. Possible explanations for this anticorrelation of ozone and the ratio of products/isoprene are (1) rapid decay of isoprene through the reaction with NO3 which is formed from the NO2 + O-3 reaction, and (2) vertical transport of air which affects the distribution of isoprene and its products as well as O-3. In late autumn, the diurnal variation of isoprene was completely different from that in summer, and isoprene was often most abundant in the evening and was anticorrelated with ozone. The ratio of(MAC + MVK) to isoprene was positively correlated with ozone, and its high value of up to 4-6 in daytime could not be explained by OH chemistry alone. These relationships of isoprene and its products with ozone may also be explained by meteorology and/or by NO3 chemistry.