Diurnal variations in H2O2, O3, PAN, HNO3 and aldehyde concentrations and NO/NO2 ratios at Rishiri Island, Japan: Potential influence from iodine chemistry

The presence of iodine chemistry, hypothesized due to the overprediction of HO2 levels by a photochemical box model at Rishiri Island in June 2000, was quantitatively tested against the observed NO/NO2 ratios and the net production rates of ozone. The observed NO/NO2 ratios were reproduced reasonably well by considering the conversion of NO to NO2 by IO, whose amount was calculated so as to reproduce the observed HO2 levels. However, the net production rates of ozone were calculated to be negative when such high mixing ratios of IO were considered, which was inconsistent with the observed buildup of ozone during daytime. These results suggest that iodine chemistry may not be the sole mechanism for the reduced mixing ratios of HO2, or that “hot spots” for iodine chemistry were present. Diurnal variations in the mixing ratios of HCHO, CH3CHO, peroxy acetyl nitrate (PAN) and HNO3 observed during the study are presented along with the simulated ones. The box model simulations suggest that the effect of iodine chemistry on these concentrations is small and that important sources of CH3CHO and sinks of PAN are probably missing from our current understanding of the tropospheric chemistry mechanism.