Seasonal variation in the chemical composition of atmospheric aerosols and gaseous species in Sapporo, Japan

Abstract

Simultaneous samplings of atmospheric aerosols and gaseous precursors (SO 2, HNO 3,NH 3) were conducted during an 18-month period in Sapporo, Japan. The monthly mean mass concentration of particles less than 10 μm in diameter (suspended particulate matter: SPM) and their chemical composition were measured for aerosols collected at two sites located in northern and southwestern regions of the urban area. In the central part of the city, two-week mean concentrations of SO 2, HNO 3 and NH 3 were measured by using tandem filter packs. Seasonal variations of the SPM mass concentrations and composition are discussed along with the results of the gas measurements. In both northern and southwestern regions of the urban area, more than 85% of the mass concentration of SPM is characterized by considering eight major components (elemental carbon, particulate organics, SO 4 2− NO 3 −, Cl −, NH 4, Na + and soil particles). The concentration of elemental carbon indicates a winter maximum and summer minimum at both sites. Excess SO 4 2− concentrations at the two sites exhibit very similar values throughout the year. An analysis of the concentration of SO 2 suggests that the existence of secondarily formed sulfate in the atmosphere may contribute to this feature. Values of NO 3 − and NH 4 + exhibit marked seasonal variations: winter maxima and summer minima. By comparison with the concentration of total. inorganic nitrate ([NO 3] in aerosols + [HNO 3]) measured in the central part of the city, the variations of NO 3 − and NH 4 + are attributed to the change in the gas-particle phase between HNO 3, NH 3 and NH 4NO 3 The measurements of total ammonia ([NH 4 +] in aerosols + [NH 3]) suggest that, in urban area, significant ammonia sources exist other than the ground surface. From the molar equivalence of Na + and Cl − it was found that the Cl − in SPM aerosols is mostly derived from sea salt; the contribution from NH 4Cl is small, even in winter. Furthermore, the measured seasonal variation of Cl − was largely influenced by the chlorine loss from sea salt.