Observations on aerosol optical properties and scavenging during cloud events

Abstract

Long-term statistics of atmospheric aerosol and especially cloud scavenging were studied at the Puijo measurement station in Kuopio, Finland, during October 2010–November 2014. Aerosol size distributions, scattering coefficients at three different wavelengths (450, 550, and 700 nm), and absorption coefficient at wavelength 637 nm were measured with a special inlet system to sample interstitial and total aerosol in clouds. On average, accumulation mode particle concentration was found to be correlated with temperature with the lowest average concentrations of 200 cm−3 around 0 ∘C increasing to 800 cm−3 at 20 ∘C. The scavenging efficiencies of both scattering and absorbing material were observed to have a slightly positive temperature correlation in in-cloud measurements. At 0 ∘C, the scavenging efficiencies of scattering and absorbing material were 0.85 and 0.55 with slopes of 0.005 and 0.003 ∘C−1, respectively. Scavenging efficiencies were also studied as a function of the diameter at which half of the particles are activated into cloud droplets. This analysis indicated that there is a higher fraction of absorbing material, typically black carbon, in smaller sizes so that at least 20 %–30 % of interstitial particles within clouds consist of absorbing material. In addition, the PM1 inlet revealed that approximately 20 % of absorbing material was observed to reside in particles with ambient diameter larger than ∼ 1 µm at relative humidity below 90 %. Similarly, 40 % of scattering material was seen to be in particles larger than 1 µm. Altogether, this dataset provides information on the size-dependent aerosol composition and in-cloud scavenging of different types of aerosol. The dataset can be useful in evaluating how well the size-dependent aerosol composition is simulated in global aerosol models and how well these models capture the in-cloud scavenging of different types of aerosol in stratus clouds.