Characteristics of atmospheric nitrogen and sulfur containing compounds in an inland suburban-forested site in northern Kyushu, western Japan

To evaluate the effects of the regional pollutant emission and long-range transport from East Asia of nitrogen and sulfur pollutants on air quality at an inland suburban-forested site in northern Kyushu, western Japan, nitrogen and sulfur pollutants were characterized from May 2006 to April 2007 using an annular denuder system. The annual mean concentration of HNO3 (2.16 μg m− 3) at the study site was comparable to concentrations reported in urban areas. The HNO3 concentration had a diurnal variation, being higher in daytime and lower at night. The diurnal variation of wind direction suggests air pollutants emitted from the regional urban area were transported to the study site during the daytime and caused a higher HNO3 concentration. The HNO3 concentration was significantly higher in summer and spring than in winter. The annual mean concentrations of SO2 and nss-SO42− (6.52 and 7.70 μg m− 3, respectively) were higher than concentrations determined in other urban areas and forested areas near urban areas in Japan and other countries. The SO2 concentration was significantly higher in winter and spring than in summer. Backward trajectory analysis indicates the long-range transport from East Asia partly contributed to the higher concentrations of SO2 in winter and spring. In contrast, there were no statistical differences in the nss-SO42− concentrations among seasons, which could be explained by increased oxidation of SO2 to SO42− in summer. The value of sulfur conversion ratio (Fs) being higher throughout the year at the study site (52.6%) than values determined in urban sites and there being no diurnal variation of Fs indicates the long-range transport of nss-SO42− not only from East Asia but also from other areas including Japan. The equivalent ratio of NH4+ and nss-SO42− at the study site (0.96 ± 0.22) indicates that acidic sulfate is neutralized by NH3, existing mainly as (NH4)2SO4 in fine particles. On the other hand, NH4NO3 had scarcely formed at the study site, unlike the case for urban areas. Ca2+ and Na+ are suggested to be the counter ions of NO3− in coarse particles, which is similar to the situation for the sites where the concentration of NH3 is insufficient for NH4NO3 to form.