Size distributions and mixtures of dust and black carbon aerosol in Asian outflow: Physiochemistry and optical properties

Abstract

During Transport and Chemical Evolution over the Pacific (TRACE-P) and Asian Aerosol Characterization Experiment (ACE-Asia) we measured the dry size distribution of Asian aerosols, their state of mixing, and the optical properties of dust, black carbon (BC) and other aerosol constituents in combustion and/or dust plumes. Optical particle sizing in association with thermal heating extracted volatile components and resolved sizes for dust and refractory soot that usually dominated light absorption. BC was internally mixed with volatile aerosol in ∼85% of accumulation mode particles and constituted ∼5-15% of their mass. These optically effective sizes constrained the soot and dust size distributions and the imaginary part of the dust refractive index, k, to 0.0006 ± 0.0001. This implies a single-scatter albedo, (550 nm), for dust ranging from 0.99+ for Dp < 1 μm to ∼0.90 at Dp = 10 μm and a size-integrated campaign average near 0.97 ± 0.01. The typical mass scattering efficiency for the dust was ∼0.3 m2 g-1, and the mass absorption efficiency (MAE) was 0.009 m2 g-1. Less dust south of 25°N and stronger biomass burning signatures resulted in lower values for of ∼0.82 in plumes aloft. Chemically inferred elemental carbon was moderately correlated with BC light absorption (R2 = 0.40), while refractory soot volume between 0.1 and 0.5 μm was highly correlated (R2 = 0.79) with absorption. However, both approaches yield an MAE for BC mixtures of ∼7 ± 2 m 2 g-1 and higher than calculated MAE values for BC of 5 M2 g-1. The increase in the mass fraction of soot and BC in pollution aerosol in the presence of elevated dust appears to be due to uptake of the volatile components onto the coarse dust. This predictably lowered for the accumulation mode from 0.84 in typical pollution to ∼0.74 in high-dust events. A chemical transport model revealed good agreement between model and observed BC absorption for most of SE Asia and in biomass plumes but underestimated BC for combustion sources north of 25°N by a factor of ∼3. Copyright 2004 by the American Geophysical Union.