Photochemical evolution of submicron aerosol chemical composition in the Tokyo megacity region in summer

Abstract

We investigate the chemical transformation of submicron aerosol in the Tokyo megacity region in summer. An Aerodyne quadrupole aerosol mass spectrometer (AMS) was deployed both at an urban site in Tokyo (35°39′N, 139°40′E) and a suburban site (downwind site) in Saitama (36°05′N, 139°33′E) in the summer of 2004. The temporal evolution of size‐resolved chemical compositions of submicron (PM1) aerosols during photochemical smog episodes are investigated using the photochemical age derived from the combination of alkyl nitrate‐to‐hydrocarbon ratio and NOz/NOy ratio (where NOz is defined as the total reactive nitrogen oxides (NOy) excluding nitrogen oxides (NOx)). The photochemical age observed at the downwind site was about 12 h in most aged air. Organic aerosols (OA) and sulfate (SO42−) were major constituents of PM1 aerosols (40–50% and 20–30%, respectively) at both sites during the observation period and their fractions showed no large variation with the NOz/NOy ratio. Mass ratios of OA and SO42− to black carbon (BC) largely increased with the NOz/NOy ratio (by factors of ∼3 and ∼2, respectively), indicating the significance of secondary formation of these compounds in controlling PM1 mass concentrations. We also investigate the photochemical evolution of OA mass spectra observed by the AMS. The mass‐to‐charge ratio (m/z) peaks of organic compounds relative to BC mass generally showed an increase with the NOz/NOy ratio. These increasing trends vary significantly for different m/z peaks, suggesting the complexity of the temporal evolution of organic functional groups. The m/z 44 and 45 peaks, which are good markers of carboxylic groups in organic particles, showed larger increases than any other m/z peaks, suggesting an efficient formation of carboxylic functional groups on a timescale of hours during the measurement period.