北京市ヘイズ時PM_(2.5) の増加にかかわる局域輸送と二次成分生産の働き

摘要

Regional transport and chemical conversions are two major processes that lead to the severe haze pollution in China. Our observations in Beijing between February 19 and March 12 of 2014 show that the two processes played different roles as PM_(2.5) increased from the clean (<75 μg m-3) to the light-medium pollution level (75–150 μg m-3) and to levels of heavy (150–250 μg m-3) and severe (>250 μg m-3) pollution. In the initial twelve hours of each episode, the PM_(2.5) reached the light-medium level with an increase of approximately 120 μg m-3. At the same time, the particle (~10–700 nm) number concentration also showed a distinct increase accompanied by a rapid increase in the mean diameter. A light-medium PM_(2.5) occurred in the south areas prior to the haze occurrence in Beijing and the southerly winds were predominant, indicating that the rapid increase of PM_(2.5) in the initial stage was caused by the regional transport from the south. Subsequently, PM_(2.5) elevated to the heavy and severe levels when the wind was weak, relative humidity was high and ozone concentration was low. The increase of PM_(2.5) in the elevated stages was characterized by a high percentage (45% for the heavy level and 55% for the severe level) of secondary inorganic components, indicating the substantial contribution of the formation of secondary aerosols. In addition, the increases of the mean diameter (from 108 nm to 120 nm) and the total volume concentration (by 67%) are regarded as a consequence of heterogeneous reactions on the surfaces of aerosol particles because the particle number concentration remained nearly constant in this two stages. Our results indicate that, during the five winter haze episodes, the regional transport from the south was the major reason for the initial-stage PM_(2.5) increase, while heterogeneous reactions dominated the later elevation.