Impacts of volatilisation on light scattering and filter-based absorption measurements: a case study
Abstract. Aerosol light absorption measurements most commonly rely on filter-based techniques. These methods are influenced by light scattering constituents in the aerosol phase deposited on the filters. The coating of soot by non-absorbing constituents changes the mixing state of soot as the aerosol ages and increase light absorption by the aerosol. Most light scattering constituents in a sub-micron aerosol are volatile by their nature due to their chemical composition and can be volatilized by heating the sample air. The initial mixing state is lost but the remaining light absorption by the aerosol should be by non-coated soot alone. This was studied during a short field campaign with two groups of equipment measuring in parallel for six days in April 2009 at the SMEAR III station in Helsinki. When heated, the light scattering constituents were evaporated thus reducing the single-scattering albedo (ω0) of the aerosol by as much as 0.4. An oven was set to scan different temperatures which revealed the volatility of the urban aerosol at different temperatures as well as the single-scattering albedo's dependence on the non-volatile volume fraction remaining (NVFR). The NVFR was 0.72 ± 0.13, 0.42 ± 0.06 and 0.22 ± 0.05 at 50, 150 and 280 °C respectively. ω0 behaved analogically, it was 0.71 ± 0.05, 0.62 ± 0.06 and 0.42 ± 0.07 at the respective temperatures. We found that absorption coefficients measured at different temperatures showed a temperature dependency possibly indicating initially different mixing states of the non-volatile constituents. By heating the aerosol the mode of the size distribution gets shifted to smaller sizes which in turn changes the filter-based instrument's response due increased penetration depth into the filter by the smaller residual particles. This was compensated for by using size distribution data.