Biomass burning emissions over northern Australia constrained by aerosol measurements: II-Model validation, and impacts on air quality and radiative forcing

This two-part series investigates the emission and transport of biomass burning aerosol (or particulate matter) across the Top End of the Northern Territory of Australia. In Part I, Meyer et al. [2008. Biomass burning emissions over northern Australia constrained by aerosol measurements: I-Modelling the distribution of hourly emissions. Atmospheric Environment, in press, doi:10.1016/j.atmosenv.2007.10.089.] used a fuel load distribution coupled with a satellite-derived imagery of fire scars and hotspots and the diurnal variation of a fire danger index to estimate hourly emission rates of particulate matter with an aerodynamic diameter of 2.5 μm or less (PM2.5) for the dry season April-November 2004 at a spatial resolution of 1 km×1 km. In the present paper, these emission rates are used in TAPM, a three-dimensional meteorological and air pollution model, and the modelled PM2.5 concentrations and aerosol optical depths are compared with satellite and ground-based measurements. This exercise also seeks to fine-tune and validate the emission calculation methodology, a process through which it is found that cases with hotspots without any corresponding fire scars (e.g. in mountainous terrain), which were initially ignored, need to be included to improve the accuracy of model predictions. Overall, the model is able to describe the measurements satisfactorily, considering the issues associated with the model resolution, emission uncertainty, and modelled meteorology. The model hindcasts numerous exceedences of the advisory maximum PM2.5 exposure limit across the study region, with large areas in excess of 30 exceedences during the study period. Estimated mean top of atmosphere direct radiative forcing due to aerosol shows a seasonal mean of −1.8 W m−2 with a region of strong enhancement over the western portion of the Top End.