Optical properties of a heated aerosol in an urban atmosphere: a case study

Abstract

Light absorption measurements most commonly rely on measurement techniques. These filter-based methods are disturbed by light scattering constituents in the aerosol phase deposited on the filters. Light absorption measurements most commonly rely on 5 filter-based measurement techniques. These methods are disturbed by light scattering constituents in the aerosol phase deposited on the filters. Most light scattering constituents in a sub-micron aerosol are volatile by their nature and they can be volatilized by heating the sample air. This volatilisation significantly alters the optical properties of the urban aerosol and was studied during a short field campaign with two groups of 10 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. With less light scattering constituents in the aerosol phase the mass absorption cross section (MAC) of soot was calculated to be 13.5±0.5 m2 g −1 at λ =545 nm. An oven was set to scan 15 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). At 50 ◦C 79±13% of the volume remained while only 46±8% remained at 150 ◦C and just 23±6% at 280 ◦C. At 50 ◦C ω0 was 0.65 ± 0.06, at 150 ◦C ω0 = 0.54±0.06 and at 280 ◦C ω0 = 0.33±0.06. We found that absorption 20 coefficients measured at different temperatures showed a temperature dependency possibly indicating initially different mixing states of the non-volatile constituents.