C-2-C-5 HYDROCARBON CONCENTRATIONS IN CENTRAL OSAKA

Hourly measurements of nine C-2-C-5 hydrocarbons (HCs) were made over a period of 15 months in the center of Osaka City, Japan. The measurem ents were made by using an automated system consisting of a combinatio n of a gas preconcentrator and a gas chromatograph. The concentrations of all components were high in early winter and low in summer. The hi ghest concentration was of C2H4, while C3H6 showed the lowest concentr ation throughout the year, and was rarely detected in summer. There we re small annual variations in the concentrations of C-4-C-5 HC The che mical reactions that affect HC concentrations were also examined. The seasonal C2H4/C2H2 and C3H6/C2H2 ratios were compared, taking into acc ount that C2H4, C2H2, and C3H6 are mainly from vehicles and the differ ences in their reactivities. Despite the high reactivity of C2H4, the C2H4/C2H2 ratio was low in winter and high in summer, with the result that the C2H4 consumption by chemical reaction seemed small in the cen ter of the city. The behavior of C3H6/C2H2 is opposite to that of C2H4 /C2H2, illustrating the more reactive characteristics of C3H6. Regardi ng the effect of photochemical reactions, it was observed that HC's le vels drop as NO2/NOx increases. Though the slope does not show a direc t proportionality to HC reactivity, reactivity of CBH,, which is the m ost reactive HC among the nine measured HCs, showed the largest declin es. The ratio of C-4-C-5 HC to the C-2-C-5 HC rose with increase in am bient temperature, while the ratio of C-2-C-5 HC to the nonmethane hyd rocarbons decreased. This phenomenon was correlated with ambient tempe rature, indicating higher emissions of higher molecular weight HCs in summer. (C) 1998 Elsevier Science Ltd. All rights reserved.