Source Attribution Of Black Carbon During Severe Haze Events In The Beijing-Tianjin-Hebei Region

The Beijing-Tianjin-Hebei?BTH?region is one of the main areas where severe haze events occurs in China.With the implementation of effective emission control measures,in recent years,the days of severe haze events(observed daily mean concentrations of surface fine particulate matter PM_2.5larger than 150?g m^–3)have shown a downward trend overall,but severe haze events still occur frequently.The occurrence of severe haze events is often accompanied by several typical large-scale synoptic patterns.Black carbon?BC?,as an important PM_2.5component and a primarily emitted species,has attracted extensive attention due to its strong absorption of solar radiation and short lifespan.It can be used as a tracer for investigating sources and formation mechanisms leading to severe haze events.We apply GEOS-Chem model and its adjoint with a high model resolution of 0.25°×0.3125°and the Multi-Resolution Emission Inventory of China?MEIC?to quantify the source contributions to BC concentrations during typical types of severe haze events from 2013 to 2017 in BTH.Model results reasonably capture the daily variations of BC measurements at three ground sites in BTH.The correlation coefficients between simulated and observed daily averaged surface BC concentrations are 0.40 at Xianghe station,0.61 at Peking University station,and 0.58 at Beijing Weather Modification Office station.The principal component analysis method?T-PCA?was used to classify the severe haze days?167 days?into four categories according to the large-scale synoptic patterns from April 2013 to December 2017,which mainly occurred in December-January-February?DJF,62.3%?and September-October-November?SON,26.3%?.When a severe haze event occurred in BTH,northern China is mainly controlled by Siberian high pressures,and Xinjiang,Mongolia are located at the center of high pressures.Eastern China is controlled by the uniform pressure field and the wind speed is small,which are not conducive for pollution diffusion.The adjoint method attributes BC concentrations to emissions from different source sectors and from local versus regional transport at the model spatial and temporal resolutions.By source sectors,the adjoint method attributes the daily BC concentrations during typical severe haze events in Beijing largely to residential emissions?48.1–62.0%?in DJF,followed by transportation?16.8–25.9%?and industry?19.1–29.5%?.In terms of regionally aggregated source influences,local emissions in Beijing?59.6–79.5%?predominate the daily surface BC concentrations,followed by emissions from Hebei?12.7–24.2%?and emissions outside BTH?4.1–13.7%?.Emission contributions from Beijing,Hebei,and outside BTH regions are comparable to the daily BC concentrations at the top of the planetary boundary layer??200–400m?,with contributions of 35.9–61.9%,23.7–39.1%,and 9.2–23.0%,respectively.BC sources emitted during the event itself and the previous 2 days account for 89.8–98.8%of daily BC concentrations during severe haze events.Emitted BC can transport 0–2 days from Beijing and Tianjin,0–3 days from Hebei,and 0–5 days outside BTH,respectively,to arrive at Beijing.GEOS-Chem sensitivity simulations show that,emissions from BTH account for 71.6–91.5%of surface BC concentrations in Beijing in 2014,and the contributions from local emissions in Beijing,Tianjin and Hebei,and regions outside BTH are41.0–57.4%,29.8–34.1%,and 12.3–25.3%,respectively.Our analyses would provide a scientific support for joint regional and targeted control policies on effectively mitigating the particulate pollution when the dominant synoptic weather patterns predicted.