High concentrations of PM
2.5 were observed in the Kanto area from November 2–6, 2011. The PM
2.5 concentrations were compared from all over Japan, and high concentrations were only observed in the Kanto area. Therefore, this episode was mainly caused not by long-range transport or trans-boundary pollution, but by local pollution. The atmosphere was generally stagnant during this period, and the atmospheric stability was stable on November 3–4 due to the formation of an inversion layer, and was neutral on November 5–6. These conditions were one of the reasons for this episode. NO
3- and OC were dominantly high among the observed chemical species in the PM
2.5. NO
3- was particularly high on November 5–6 probably due to remarkable HNO
3 production from NO under a very high humidity condition during the nighttime. NO was also irregularly high on November 3–4, which possibly implies the influence of the combustion of agricultural residues (biomass burning). The chemical species indicators of biomass burning such as K
+, char-EC, and levoglucosan were also high. Thus it was suggested that the influence of biomass burning on the PM
2.5 was predominant during this period. However, the influence of fossil fuel burning was observed in the southern part of the Kanto area based on the SO
42- and V. Although the PM
2.5 concentration is measured by methods decreasing the water content, it was estimated that the water content affected the high concentration of the observed PM
2.5 during this period in comparison to the reconstructed PM
2.5 concentration from the main chemical species.
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