Black carbon simulations using a size- and mixing-state-resolved three-dimensional model: 2. Aging timescale and its impact over East Asia

This study evaluates the aging timescale and the cloud condensation nuclei (CCN) activity of black carbon (BC) over East Asia and its outflow region using a size- and mixing-state-resolved three-dimensional model, the Weather Research and Forecasting model with chemistry (WRF-chem) with the Aerosol Two-dimensional bin module for foRmation and Aging Simulation (ATRAS) and the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC). The WRF-chem/ATRAS-MOSAIC model explicitly calculates BC aging (condensation and coagulation) and removal processes, with 12 size and 10 BC mixing state bins (128 bins in total). The model reveals large spatial and temporal variability of the BC aging timescale and the CCN activity of BC-containing particles over East Asia (spring 2009) with their strong size and supersaturation dependence. The BC aging timescale differs from 0.19 to 3.1days (period and domain average at an altitude of 1km), depending on the choice of size (mass or number) and supersaturation (1.0% or 0.1%). As a result, almost 100% of BC-containing particles are CCN-active at a supersaturation of 1.0%, whereas 20-50% of BC-containing particles are CCN-inactive at a supersaturation of 0.1%, with a strong size dependence. These results show the importance of resolving BC aging processes and their dependence on size and supersaturation in models for more accurate simulations of BC concentrations and their distribution and lifetime. A sensitivity simulation without resolving BC mixing state shows the underestimation of total BC mass concentrations by 5-10% and the BC mass concentrations in the CCN-inactive particles (at a supersaturation of 0.1%) by 40-60% over the outflow region (at 150 degrees E), compared with the simulation resolving BC mixing state. Because BC aging speed is very slow at 150 degrees E and eastward, the change in BC mass and its CCN activity by resolving BC mixing state will continue over long distances and may have a large impact on BC transport from East Asia to remote regions such as North America and the Arctic.