Asian continental aerosol persistence above the marine boundary layer over the eastern North Pacific: Continuous aerosol measurements from Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2)

Abstract

We report analyses of aerosols collected for the 2002 Intercontinental Transport and Chemical Transformation experiment (ITCT‐2K2). Sampling was conducted 15 April to 25 May 2002. Data are from three sites: a sea level site at Trinidad Head, California, a coastal mountain site 1 km altitude (Trinity Alps), and an inland mountain site near 2 km altitude (Mount Lassen). Aerosols were continuously collected in eight size bins (0.09 to 10 micrometers aerodynamic diameter) using eight‐stage rotating drum impactors. Samples were analyzed in 3‐hour time steps by synchrotron x‐ray fluorescence. We find the following. (1) Aerosol chemical composition at Trinidad Head was generally dominated by marine aerosols with varying minor contributions from local sources. Despite the presence of Asian continental aerosol above the marine boundary layer, significant concentrations of Asian aerosols were observed at sea level only during a strong frontal passage between 22 and 25 April. (2) At the elevated sites, aerosol elemental composition was predominantly Asian despite wide swings in concentration. Analysis of soil‐forming elements shows that Asian continental dust and associated combustion products overwhelmed local‐source aerosols through the first half of the sampling period; in the latter half, Asian aerosols present in the free troposphere were regularly delivered to the mountain sampling sites by nocturnal subsidence. (3) Asian aerosols in the lower free troposphere, although highly variable, were very persistent, not arriving only in discrete “transport events.” We conclude that throughout the experiment the aerosols in the lower free troposphere over the northeastern Pacific Ocean and western North America were dominated by continental outflow from Asia, with little marine or North American continental influence. Viewed in the context of previously published analyses of the long‐term aerosol history for Mount Lassen that showed frequent, strong Asian influence throughout spring, summer, and fall, the Asian impact appears likely to be quasi‐continuous for much of the year.