Characteristics of Nitrogenous Air Pollutants at Urban and Suburban Forested Sites, Western Japan

摘要

Nitrogenous air pollutants including nitrogen dioxide (NO_2), nitric acid (HNO_3), nitrate (NO_3~-), ammonia (NH_3), ammonium (NH_4~+), and nitrous acid (HONO) were characterized at an urban forested (UF) site in Hiroshima and at a suburban forested (SF) site in Fukuoka, western Japan, using an annular denuder system for 1 year from May 2006 to May 2007 to compare the concentrations and chemical species of atmospheric nitrogenous pollutants between UF and SF sites. The proximity of the urban area was reflected in higher NO_2 concentrations at the UF site than at the SF site. NO_2 was more oxidized at the SF site because it is farther from an urban area than the UF site, which was reflected in higher concentrations of HNO_3 at the SF site than the UF site. HNO_3 and acidic sulfate is neutralized by NH3, existing as ammonium nitrate (NH_4NO_3) and ammonium sulfate [(NH_4)_2SO_4] at the UF site. At the SF site, acidic sulfate is neutralized by NH_3, existing as (NH_4)_2SO_4, but NH_4NO_3, had scarcely formed at the SF site. A much higher HONO concentration was observed at the UF site than at the SF site, especially in winter and spring at night, which could be explained by higher NO_2 concentrations at the UF site because of its proximity to an urban area and stagnant meteorological conditions. Atmospheric HONO determination was critical in evaluating the possibility of damage to trees in UF areas.