The atmospheric behavior of nitrous acid, HONO, was discussed on the basis of field measurements by an annular-denuder or a filter-pack method at four different nature sites in northern Japan, Sapporo (urban), Moshiri (rural), Rishiri (coastal remote) and Teshio (mountainous remote), in terms of their atmospheric chemistry parameters including nitrogen oxides, nitric acid, nitrate aerosol and ozone. The annual mean concentrations of HONO in Sapporo ranged from 36 to 41 nmol m
-3 where the concentration level was 11 to 31 times higher than those for the other sites. A different seasonality of the HONO concentration was detected at the different nature site: the urban site showed a maximum in the winter, the remote in the summer, and the rural in the winter and the summer. A heterogeneous reaction between NO
2 and H
2O on the surface of airborne particles and the ground surface is likely to play a significant role in the nitrous acid formation because the HONO/NOx ratio in ambient air is higher than those by direct emission as reported by Kurtenbach et al. (2001). In the case of the ground-surface heterogeneous reaction, the concentration gradient of HONO is expected to negative correlate with that of NO
2 because the deposition of NO
2 would induce emissions of HONO into the atmosphere. Actually, in Teshio, the difference between the concentrations of HONO at the heights of 30 m and 10 m positive correlated with that for NO
2. Furthermore, the Rishiri measurements showed a significant correlation between the concentrations of HONO and PM
10. In addition, Rishiri and Moshiri measurements showed significant correlations between the concentration ratio, HONO/NO
2, and absolute humidity. These results strongly support the view that the heterogeneous reactions on the airborne particles prevails in the formation of HONO in the rural and remote areas with considerably low concentrations of NO
2. On the other hand, the heterogeneous reaction on the ground-surface reaction plays a significant role in Sapporo, especially in the winter with high NO
2 concentrations. In Sapporo, negative correlations were detected in the winter between the concentrations of HONO and NO, and between of HNO
3 and O
3, which would be attributable to the insufficient conversion of NO
2 to HNO
3, and further to nitrate species, under urban conditions because of the low concentration of ozone along with high concentrations of NO. These field measurements would indicate that the behavior of nitrous acid is controlled by the chemical reactions of NO
2 with H
2O, and that this heterogeneous reaction was affected by the NO
2 concentration, absolute humidity and O
3 concentration.
View full abstract