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

Abstract

[1] 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, v (550 nm), for dust ranging from 0.99+ for D p < 1 mm to $0.90 at D p = 10 mm and a size-integrated campaign average near 0.97 ± 0.01. The typical mass scattering efficiency for the dust was $0.3 m 2 g À1 , and the mass absorption efficiency (MAE) was 0.009 m 2 g À1 . Less dust south of 25°N and stronger biomass burning signatures resulted in lower values for v of $0.82 in plumes aloft. Chemically inferred elemental carbon was moderately correlated with BC light absorption (R 2 = 0.40), while refractory soot volume between 0.1 and 0.5 mm was highly correlated (R 2 = 0.79) with absorption. However, both approaches yield an MAE for BC mixtures of $7 ± 2 m 2 g À1 and higher than calculated MAE values for BC of 5 m 2 g À1 . 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 v 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.

Modified Thermal-Optical Analysis Using Spectral Absorption Selectivity To Distinguish Black Carbon from Pyrolized Organic Carbon

“…For example, oxidation of metals in the dust, such as Fe(II) to Fe(III), may affect light absorption measurements at visible wavelengths, however the MAE of dust is a factor of one hundred less than the MAE of BC (21). Consequently the signal interference of oxidizing metals on BC should be negligible in most field samples.…”

Trace gas and aerosol interactions in the fully coupled model of aerosol‐chemistry‐climate ECHAM5‐HAMMOZ: 1. Model description and insights from the spring 2001 TRACE‐P experiment

“…BC concentrations were estimated from observed aerosol absorptions that were converted to mass concentrations with a mass absorption efficiency of 7 m 2 g À1 POZZOLI ET AL. : ECHAM5-HAMMOZ COUPLED MODEL, 1 [Clarke et al, 2004]. Observed BC concentrations are underestimated by 50% both in the northern and southern regions, which likely reflects too low emissions.…”

Sulfur dioxide in the tropical marine boundary layer: dry deposition and heterogeneous oxidation observed during the Pacific Atmospheric Sulfur Experiment

“…Drying minimizes water uptake by the aerosol so that the distributions preferentially reflect the aerosol components over the water component . During horizontal legs the RDMA and OPC operated with a thermal pre-conditioning unit that cycled the aerosol through 40°C, 150°C and 350°C to drive off the volatile and semi-volatile aerosol constituents, allowing inference of size-resolved aerosol chemistry (Clarke et al 2004).…”