Seasonal Variation of PM2.5 Components Observed in an Industrial Area of Chiba Prefecture, Japan

In order to survey the seasonal variation of the chemical composition of particulate matter of 2.5 μm or less (PM2.5), PM2.5 was sampled from 8 February 2013 to 31 March 2014 in an industrial area of Chiba Prefecture, Japan. Chemical measurements of the sample included: ionic components (Na+, NH4+, Ca2+, Mg2+, K+, Cl-, NO3- and SO42-), carbonaceous components - organic carbon (OC) and elemental carbon (EC), and water-soluble organic carbon (WSOC). Also, secondary organic carbon (SOC) was measured based using the EC tracer method, and char-EC and soot-EC were calculated from the analytical results. The data obtained were interpreted in terms of temporal variation. Of the overall mean value of PM2.5 mass concentration obtained during the study period, ionic components, OC and EC accounted for 45.3%, 19.7%, and 8.0%, respectively. NO3- showed a unique seasonal distribution pattern due to a dependence on temperature and absolute humidity. It was estimated that an approximate temperature of 14°C, and absolute humidity of 7 g/m3 were critical for the reversible reaction of NH4NO3 (p) ⇋ NH3 (g)+HNO3 (g). The amount of OC and EC contributing to the monthly PM2.5 mass concentration was higher in autumn and winter compared to spring and summer. This result could be attributed to the impact of burning biomass, since WSOC and the ratio of char-EC/soot-EC showed a similar pattern during the corresponding period. From the comparison of monthly WSOC/OC values, a maximum ratio of 83% was obtained in August (summer). The WSOC and estimated SOC levels derived from the EC tracer method correlated (R=0.77) in summer. The high occurrence of WSOC during summer was mainly due to the formation of SOC by photochemical reactions. Through long-term observation of PM2.5 chemical components, we established that the degree to which the abovementioned factors influence PM2.5 composition, fluctuates with seasonal changes.