Airborne measurements of atmospheric carbonaceous aerosols during ACE-Asia

Abstract

Airborne levels of carbonaceous aerosols were measured using the Twin Otter aircraft during the Aerosol Characterization Experiment (ACE)-Asia. Particles were collected using a newly developed honeycomb denuder sampler and organic carbon (OC), elemental carbon (EC), and carbonate (CC) carbon levels determined using a thermal–optical carbon analyzer. During some flights, atmospheric layers could be identified as marine boundary, pollution dominated, or mineral dust dominated. Ångstrom exponent (å) values, calculated based on data from an onboard three-wavelength nephelometer, were used to discern the nature of some individual layers. Values of å for individual layers ranged from 0.2 to 2, corresponding to dust- and pollution-dominated layers, respectively. OC and EC concentrations below 3 km ranged from 0.58 to 29 μg C m⁻³ and from 0.20 to 1.8 μg C m⁻³, respectively. In general, for a given type of atmospheric layer, higher levels of total carbon (TC) were observed during ACE-Asia than those observed during ACE-2, Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) and Indian Ocean Experiment (INDOEX). Mixed layers of dust and pollution were found on some occasions. CC was detected in samples taken from layers in which å = 1.6, indicating that significant amounts of dust can be present even though å > 0.2. A linear regression of light absorption coefficient σ_(ap) (Mm⁻¹) versus EC concentration had an r² of only 0.50, indicating that parameters other than the mass of EC significantly affected the value of σ_(ap). The mass absorption coefficient E_(abs) (m² g⁻¹) varied by as much as a factor of 8 between sampling events, and the average value of 11 m² g⁻¹ (±5.0) agrees well with previous published values.

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