Biomass burning plumes from Indochina towards southern China: the predominant synoptic weather processes

The Indochinese Peninsula is one of the world's most active regions for biomass burning (BB). Springtime BB in this region, producing plumes rich in pollutants such as CO, ozone, black carbon, and organic aerosols, not only significantly impacts local air quality, but also plays a crucial role in weather and climate dynamics of East Asia. Previous studies have demonstrated that BB plumes from the Indochinese Peninsula can be transported towards southern China and the North Pacific Ocean during March and April in the free atmosphere. In this study, by employing objective weather classification methods using various reanalysis and satellite data, we identified three synoptic patterns conducive to the transport of BB plumes from the Indochinese Peninsula to inland China. These patterns are characterized by low pressure over the continent (46%), cold front passage (30%), and high pressure in the mid-latitudes (24%), respectively. Each pattern exhibits significant differences in synoptic evolutions, duration, and pollutant transport pathways, leading to varied horizontal and vertical distributions of pollutants. Notably, plumes formed under the continental low-pressure pattern are mainly transported in the lower and middle troposphere, while the cold front pattern can elevate pollutants to higher altitudes, above 700 hPa. Fire weathers over the source region during the synoptic processes are also discussed. Furthermore, we observed strong interannual variations in these weather types, which partly shape the distribution of pollutants such as CO and ozone on an interannual basis. This study highlights the importance of understanding the long-term variations in atmospheric composition caused by biomass burning, with a specific focus on synoptic weather processes.