Abstract
Large concentrations of small aerosols have been previously observed in the vicinity of anvils of convective clouds. A 3D cloud-resolving model (CRM) including an explicit size-resolving aerosol module has been used to examine the origin of these aerosols. Five different types of aerosols are considered: nucleation mode sulfate aerosols (here defined by 0 ≤ d ≤5.84 nm), Aitken mode sulfate aerosols (here defined by 5.84 nm ≤ d ≤ 31.0 nm), accumulation mode sulfate aerosols (here defined by d ≥ 31.0 nm), mixed aerosols, and black carbon aerosols.
The model results suggest that approximately 10% of the initial boundary layer number concentration of Aitken mode aerosols and black carbon aerosols are present at the top of the convective cloud as the cloud reaches its decaying state. The simulated average number concentration of Aitken mode aerosols in the cloud anvil (∼1.6 × 104 cm−3) is in the same order of magnitude as observations. Thus, the model results strongly suggest that vertical convective transport, particularly during the active period of the convection, is responsible for a major part of the appearance of high concentrations of small aerosols (corresponding to the Aitken mode in the model) observed in the vicinity of cloud anvils.
There is some formation of new aerosols within the cloud, but the formation is small. Nucleation mode aerosols are also efficiently scavenged through impaction scavenging by precipitation. Accumulation mode and mixed mode aerosols are efficiently scavenged through nucleation scavenging and their concentrations in the cloud anvil are either very low (mixed mode) or practically zero (accumulation mode).
In addition to the 3D CRM, a box model, including important features of the aerosol module of the 3D model, has been used to study the formation of new aerosols after the cloud has evaporated. The possibility of these aerosols to grow to suitable cloud condensation or ice nuclei size is also examined. Concentrations of nucleation mode aerosols up to 3 × 104 cm−3 are obtained. The box model simulations thus suggest that new particle formation is a substantial source of small aerosols in the upper troposphere during and after the dissipation of the convective cloud. Nucleation mode and Aitken mode aerosols grow due to coagulation and condensation of H2SO4 on the aerosols, but the growth rate is low. Provided that there is enough OH available to oxidize SO2, parts of the aerosol population (∼400 cm−3) can reach the accumulation mode size bin of the box model after 46 h of simulation.
Corresponding author address: Annica Ekman, Department of Meteorology, Stockholm University, SE-106 91 Stockholm, Sweden. Email: annica@misu.su.se
