Variability in aerosol properties observed during ISDAC

The Indirect and Semi-Direct Aerosol Campaign (ISDAC) was conducted in April, 2008, in the vicinity of Barrow, Alaska, near the U.S. Department of Energy (DOE) North Slope of Alaska (NSA) ground site. A large dataset was collected, including both aerosol physicochemical and cloud microphysical measurements. The focus of this work is on in-situ aerosol measurements from instruments onboard the National Research Council of Canada (NRC) Convair-580 aircraft, obtained when the air was free of cloud and/or precipitation. A Passive Cavity Aerosol Spectrometer Probe (PCASP-100X) mounted under the aircraft wing was used to measure the total aerosol number concentration (N_a) and particle size distribution in the 0.12-3 µm diameter size range. Complementary measurements were obtained using a single particle mass spectrometer (SPLAT II) located inside the aircraft, which sampled particles via an aerosol inlet and provided chemical composition, N_a, and vacuum aerodynamic size distribution in the 0.05-1 µm diameter size range.

The objective of this work is to demonstrate the variability in the aerosol physicochemical properties for three contrasting scenarios: a clean case, a biomass burning case, and a clean case with defined aerosol layers. This variability can have important implications for estimates of aerosol properties used in global climate model (GCM) simulations. Variability in N_a is considered for both vertical profiles and constant altitude (horizontal) flight legs. The data show that the Arctic aerosol is often characterized by vertical and horizontal filamentous structures with varying aerosol properties: concentrations, size distributions, and compositions. When aerosol layers were encountered, N_a rapidly increased from 25 cm^-3 up to 550 cm^-3 within the relatively clean air masses, and reached up to 2200 cm^-3 within the polluted air masses with biomass burning pollution characteristics. During polluted conditions with higher N_a, there tended to be larger particles present, with maxima in the particle number distribution around 0.3 µm diameter, as opposed to the maxima around 0.17 µmduring cleaner periods. Interestingly, nearly all air masses originated from similar regions in Asia and the Pacific Ocean.

The chemical composition measurements from SPLAT II show a large fraction of oxygenated organics in the clean case and a large biomass burning component in the polluted case. In the clean case with defined aerosol layers, the layers contain high percentages of sulfate and biomass burning aerosol, while the clean layers show a higher proportion of organics.

Averages of aerosol characteristics are difficult to interpret, especially during polluted cases, due to the variability that can occur over short distances and time frames. When averaging N_a over different distance scales, it was found that N_a=140 cm^-3 is a good average for the majority of the clean cases encountered, but would fail to capture the details of aerosol layers encountered in some flights. This would not be a good estimate for the multi-day biomass burning plume, where a mean value of N_a= 720 cm^-3 was determined for shorter averaging scales (less than about 70 km), but fails to capture the fine details of the aerosol number fluctuations. The marked changes in N_a in all air masses considered suggest that the averaging scales used to determine representative aerosol number concentrations for GCMs need to be chosen with caution.