Numerical Simulation Of The Impact Of Open Biomass Burning On Secondary Organic Aerosol Formation

Organic aerosols（OA）is a major component of fine particulate matter,which has a significant impact on atmospheric visibility,climate change,and human health.Open biomass burning（OBB）is an important source of primary organic aerosol（POA）and percursors of secondary organic aerosol（SOA）.OBB contributes significantly to ambient OA,especially for the formation of SOA from volatile-organic-compound（VOC）percursors.The mechanisms of SOA formation due to OBB,however,are still poorly understood.In order to decrease the uncertainty of VOC percursors in OBB emissions and clarify the impact of OBB to SOA formation,a high spatial and temporal resolution inventory of OBB was built and applied to the CMAQ model simulation study.The main results and conclusions are as follows:（1）A near-real-time hourly OBB emissions inventory with spatial resolution of 3km×3km was built by fusing fire radiative energy（FRE）retrievals from polar-orbiting and geostationary satellites.The high temporal resolution OBB emissions inventory built in this study could distinguish daytime and nighttime emissions and provide hourly emissions input to the air quality model.Daytime emissions accounted for 53%～64%of daily emissions,and the peak time of OBB emissions in southern China was usually in the afternoon.The uncertainties of OBB emissions in southern China were 53.63%～58.22%for VOCs（95%confidence intervals）.Owing to the improvement of the OBB activity level algorithm and the application of local VOC species emission factors（EFs）,the uncertainty of VOC emission in this study was lower than that in previous studies.Furthermore,the emissions of intermediate volatility organic compounds（IVOCs）from BB were estimated by the ratios of IVOC/POA,and the average annual IVOCs emissions of each province ranged from 9433 tons to 136403 tons.（2）OBB contributed significantly to terpene-related SOA(SOA_Terp).The contribution of OBB to ambient SOA_Terp was up to 70%in southern China.The oxidation of monoterpenes was the main pathway of SOA_Terp formation in the downwind area of the OBB plume.The chemical process of OBB contributed to ambient SOA_Terpwas significantly different between daytime and nighttime.The SOA_Terpcontributed by OBB mainly from the reaction of isoprene and monoterpenes with OH,O_3,and NO₃ in the daytime,and monoterpenes with NO₃ in the nighttime.Except for the terpenes which directly emitted from OBB,other pollutants（e.g.,NO_xand VOCs）could enhance the atmoshperic oxidizing capacity and promote the formation of SOA_Terp from local biogenic VOCs.OBB contributed up to 40%of ambient SOA_Terp by this oxidation,and this was an important pathway to ambient SOA_Terp from OBB contribution.（3）IVOC emissions from OBB contributed significantly to ambient SOA.After modifying the IVOCs aging pathway in AE7 according to the volatility basis set（VBS）approach,the contribution of OBB to ambient SOA in Nanling forest was increased by 5 times,and the percentage contribution was up to 30%.At the peak of OBB contribution,the simulated PM₁ organic aerosol concentration was closer to the observed value of HR-ToF-AMS.Compared with the original AE7 mechanism,the deviation between simulation and observation decreased from27%to 4%.Hence,the use of a high temporal resolution OBB emission inventory and the addition of IVOCs aging mechanism to AE7 could improve the performance of SOA simulation.