Nighttime variations in HO2 radical mixing ratios at Rishiri Island observed with elevated monoterpene mixing ratios

HO2 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 HO2 mixing ratio had temporal variations around its average of 4.2±1.2 (1σ) 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 HO2 concentration levels. The model roughly reproduced the dependence of the HO2 mixing ratio on the square root of the radical production rate due to the ozonolysis reactions of the monoterpenes. However, the absolute HO2 mixing ratio was significantly underpredicted by the model. We discuss possible reasons in terms of misunderstood RO2 chemistry, RO2 interference with HO2 observations, unknown radical production process associated by high NO2 mixing ratio, and the contribution of unmeasured olefinic species to radical production via their reactions with ozone.