Observations of sulfur dioxide uptake and new particle formation in a midlatitude cumulus cloud

Abstract. Airborne measurements, obtained during the Asian Aerosol Characterisation Experiment (ACE-Asia), of SO₂ and condensation nuclei (CN) concentrations were made in the local environment of a cumulus cloud band. Conserved quantities, wet equivalent potential temperature θ_q, and total water content Q, were used to identify the sources of air detrained on the downwind side of the cumulus band. It was found that ~65% of the detrained air originated from below cloud base and the remainder was air that had been entrained from the free troposphere upwind of the cloud and subsequently been detrained. Calculation of the sources of the detrained air parcels enabled a prediction of the concentration of SO₂ and CN, assuming that SO₂ and CN experienced no processing within cloud. A comparison of the predicted concentration of SO₂ and CN was made with those observed. The concentration of SO₂ observed was less than predicted and the amount of SO₂ scavenged within cloud was calculated. The CN concentration observed was also less than predicted and, moreover, inclusion of the loss of CN to cloud droplets due to Brownian scavenging resulted in an enhanced decrease of the number concentration of CN predicted. Clear air regions around the cloud exhibited no indication of being a major source of new particles. It was concluded that new particles were formed within cloud.