Simulations of black carbon over the Indian region: improvements and implications of diurnality in emissions

Abstract

With a view to improving the performance of WRF-Chem over the Indian region in simulating BC (black carbon) mass concentrations as well as its short-term variations, especially on a diurnal scale, a region-specific diurnal variation scheme has been introduced in the model emissions and the performance of the modified simulations has been evaluated against high-resolution measurements carried out over eight ARFI (Aerosol Radiative Forcing over India) network observatories spread across India for distinct seasons: pre-monsoon (represented by May), post-monsoon (represented by October) and winter (represented by December). In addition to an overall improvement in the simulated concentrations and their temporal variations, we have also found that the effects of prescribing diurnally varying emissions on the simulated near-surface concentrations largely depend on the boundary layer turbulence. The effects are perceived quickly (within about 2–3 h) during the evening–early morning hours when the atmospheric boundary layer is shallow and convective mixing is weak, while they are delayed, taking as much as about 5–6 h, during periods when the boundary layer is deep and convective mixing is strong. This information would also serve as an important input for agencies concerned with urban planning and pollution mitigation. Despite these improvements in the near-surface concentrations, the simulated columnar aerosol optical depth (AOD) still remains largely underestimated vis-à-vis the satellite-retrieved products. These modifications will serve as a guideline for further model-improvement initiatives at a regional scale.