Effectively controlling hazardous airborne elements: Insights from continuous hourly observations during the seasons with the most unfavorable meteorological conditions after the implementation of the APPCAP

Concentrations of PM_2.5 and PM_2.5-associated elements were observed hourly using an X-ray fluorescence-based online elemental analyzer at a chosen location within urban Beijing from November 28, 2016, to May 30, 2017, when nearly the lowest atmospheric environmental capacity was recorded after enforcing the Air Pollution Prevention and Control Air Plan (APPCAP). Throughout the study period, the PM_2.5 concentrations were an average of 91.2 μg/m³, with 68 days exceeding the second tier of the Chinese ambient air quality standard (75 μg/m³). The PM_2.5-associated hourly hazardous airborne element (HAE) concentrations ranged between 56 ng/m³ and 4374 ng/m³, accounting for an average of 0.2 ± 0.1 % of PM_2.5, which were lower than or comparable with those reported in previous studies. Different from the traditional filter sampling offline analysis, dynamic variations and evolution of particulate HAEs were illustrated based on hourly observations, which showed that the concentrations of most HAEs declined with increasing mixed layer height (MLH) as expected, while slight increases in the Cr and Pb concentrations were observed when the MLH increased from 250 m to 500 m. Compared with the values in spring, As, Cr, Hg, Mn, Ni, Pb and Se increased by 135.0 %, 67.2 %, 110.1 %, 36.4 %, 32.4 %, 185.6 % and 63.9 % in winter, respectively. Based on hourly elemental data, the more robust source analysis showed that industrial emissions contributed substantially to Cr, Hg, Ni and Se (58.5 %, 42.7 %, 65.0 % and 62.0 %, respectively), vehicular sources primarily accounted for Mn (41.1 %) and almost all sources except for dust contributed equally to the Pb loading. A nonparametric wind regression analysis applied to 1-h HAE concentrations pinpointed distinct hot spots (high concentrations) for all HAEs except Cd in the northeast wind sector at wind speeds of approximately 0−6 km/h. Based on a potential source contribution function analysis, the areas in western China with intense coal consumption strongly contributed to the loading of As in Beijing. Cr, Hg, Ni, Pb and Se showed source areas that were similar to but larger than that of As; additionally, these elements exhibited moderate and strong hotspots in Shanxi Province with strong coal consumption and industrial production. Ni clearly originated from the coastal areas where the effect of shipping activities was intense.