Anthropogenic air pollution observed near dust source regions in northwestern China during springtime 2008

摘要

Trace gases and aerosols were measured in Zhangye (39.082°N, 100.276°E, 1460 m a.s. 1.), a rural site near the Gobi deserts in northwestern China during spring 2008. Primary trace gases (CO: 265 ppb; SO_2: 3.4 ppb; NO_y~*: 4.2 ppb; hereafter results given as means of hourly data) in the area were lower than in eastern China, but still indicative of marked anthropogenic emissions. Sizable aerosol mass concentration (153 μg/m~3) and light scattering (159 Mm~(-1) at 500 nm) were largely attributable to dust emissions, and aerosol light absorption (10.3 Mm~(-1) at 500 nm) was dominated by anthropogenic pollution. Distinct diurnal variations in meteorology and pollution were induced by the local valley terrain. Strong daytime northwest valley wind cleaned out pollution and was replaced by southeast mountain wind that allowed pollutants to build up overnight. In the afternoon, aerosols had single scattering albedo (SSA, 500 nm) of 0.95 and were mainly of supermicron particles, presumably dust, while at night smaller particles and SSA of 0.89-0.91 were related to pollution. The diverse local emission sources were characterized: the CO/SO_2, CO/NO_y, NO_y/SO_2 (by moles), and BC/CO (by mass) ratios for small point sources such as factories were 24.6-54.2, 25.8-35.9, 0.79-1.31, and 4.1-6.1 x 10~(-3), respectively, compared to the corresponding inventory ratios of 43.7-71.9, 23.7-25.7, 1.84-2.79, and 3.4-4.0 x 10~(-3) for the industrial sector in the area. The mixing between dust and pollution can be ubiquitous in this region. During a dust storm shown as an example, pollutants were observed to mix with dust, causing discernible changes in both SSA and aerosol size distribution. Further interaction between dust and pollutants during transport may modify the properties of dust particles that are critical for their large-scale impact on radiation, clouds, and global biogeochemical cycles.