Nighttime variations in HO 2 radical mixing ratios at Rishiri Island observed with elevated monoterpene mixing ratios

Abstract

HO 2 radical concentrations were measured by a laser-induced fluorescence instrument for three nighttime periods during the intensive field campaign at Rishiri Island, Japan, in June 2000. The HO 2 mixing ratio had temporal variations around its average of 4.2±1.2 (1 σ) pptv and showed a positive correlation with the summed mixing ratio of four monoterpene species, α-pinene, β-pinene, camphene, and limonene, that sometimes reached 1 ppbv. Our model calculations suggested that ozonolysis reactions of monoterpenes were the main source of nighttime radicals and they explained 58% of measured HO 2 concentration levels. The model roughly reproduced the dependence of the HO 2 mixing ratio on the square root of the radical production rate due to the ozonolysis reactions of the monoterpenes. However, the absolute HO 2 mixing ratio was significantly underpredicted by the model. We discuss possible reasons in terms of misunderstood RO 2 chemistry, RO 2 interference with HO 2 observations, unknown radical production process associated by high NO 2 mixing ratio, and the contribution of unmeasured olefinic species to radical production via their reactions with ozone.