Intricate behavior of winter pollution in Hanoi over the 2006-2020 semi-climatic period

During the winter period (October to March), air quality in northern Vietnam is strongly impacted by the northeast monsoon cycles, that create periodic changes in the air masses pathways and the meteorological conditions. In this work, an original approach is proposed to identify the main sources contributing to the degradation of air quality in Hanoi and quantify the share of local and imported pollution under the influence of the northeast monsoon. Firstly, semi-climatic trends in particle emissions in East Asia and mainland Southeast Asia were studied using outputs from the NAAPS model, that assimilates space-borne Aerosol Optical Depth (AOD) observations. From winter 2006/07 to winter 2010/11, the pollution plumes were most frequently located over the Indochina Peninsula, and the smoke partial AOD showed biomass burning activities (natural and anthropogenic) to be responsible. From winter 2011/12 to winter 2019/20 however, the pollution plumes were mostly observed over eastern China and to a lesser extent, Northern Vietnam, and the sulfate partial AOD indicated they came from coal-based industrial activities. Secondly, the impact of the northeast monsoon on the air masses origin was studied using HYSPLIT back-trajectories with clustering over the 14 winter periods. During regular winters, northeastern air masses predominated, which can bring pollution from eastern China and the industrial region surrounding Hanoi (Red River Delta). However, the existence of El Nin similar to o perturbed the air masses trajectories during three winters (2014/15, 2015/16 and 2018/19), bringing more western air masses and thus, causing a temporary decrease of the sulfate AOD and simultaneous increase of the smoke AOD over Hanoi. Thirdly, a method to classify the winter days and highlight the influence of cold surges on PM2.5 vari-ability was built, relying only on Hanoi local wind and NAAPS AOD value. This classification allowed to estimate that long-range transport from China during the onset of cold surges caused an average increase of around 30% of the PM2.5 level in Hanoi. Additionally, the contribution of local pollution to the most severe pollution epi-sodes, occurring during the persistence of cold surges, was estimated to be around 40% of the average PM2.5.