Asian aerosols in North America: Frequency and intensity of transpacific transport, chemical composition, and mass concentrations.

摘要

Dust generated in the deserts and Loess Plateau of northern China causes severe pollution in China, Korea, Japan, and the western Pacific, appears as haze over North America, and deposits mineral material in ice and snow from Alaska to Greenland. Asian dust has significant impacts on atmospheric processes including modulation of Earth's radiation balance.; This report presents the first empirical assessment of the frequency and intensity of aerosol transport from Asia to midlatitude North America. Statistics on dust were derived for sites from Alaska to the Virgin Islands by using an elemental signature derived from intense dust events to probe a long-term set of aerosol samples. The analysis revealed consistent, frequent transport that contradicts the prevailing episodic characterization derived from short-term studies. Asian dust, although rare at sea level, is a regular component of the free troposphere above North America. Asian dust is concentrated in a zone from roughly 500 m to 3000 m MSL, consistent with patterns observed in the western Pacific.; Using the dust as a tracer for Asian air masses, the chemical composition for the whole Asian continental aerosol plume was found. Chemical compositions for days with Asian dust were found to be identical for a range of sites across North America. Furthermore, the dust concentrations are strongly correlated with combustion products, indicating that the pervasive, low concentration dust is unrelated to large dust storms, and suggesting that it is due to human activity.; These analyses demonstrate that: (1) Asian materials are a major factor in the aerosol budget at ground level at remote elevated sites in western North America and in the free troposphere across the continent; (2) the Asian continental aerosol plume is predominantly anthropogenic; and (3) the Asian contribution to Northern Hemisphere tropospheric aerosols has been underestimated in the past and Asia's role in global aerosol processes needs comprehensive reevaluation.