Atmospheric wet deposition of trace metal elements: Monitoring and modelling.

Trace elements (TEs), a group of atmospheric pollutants, have attracted considerable attention from scientists and government administrators worldwide. The wet deposition fluxes of nineteen trace elements (NTE) were monitored at Wanqingsha, a coastal site in the Pearl River Delta, for three consecutive years (2016.9-2019.8). Significant seasonal differences in NTE between wet and dry seasons were observed. The fluxes of crustal elements (Ca, Na, Al, Mg, K, Fe, Zn and Ba) were significantly higher than those of anthropogenic elements, accounting for over 99 % of the total annual wet deposition of 19 elements. Analysis of PM2.5 and rainfall samples reveals that both the fraction of each TE in the PM2.5 (CQ) and the Apparent Scavengance Ratio for TE (ASR, defined as the concentration ratio in rain and PM2.5) follow lognormal distributions. The logCQ variation for each element is relatively small but shows substantial differences, with means ranging from -5.48 to -2.03, while the logASRs for all elements show similar means (varying from 5.86 to 7.64) and an extremely wide range of variation. The influences of meteorological factors on CQ and ASR were also investigated. A simple box model framework was constructed to reasonably simplify the TE removal process by precipitation. The corresponding regression analysis showed significant correlations between NTE and the precipitation rate, PM2.5 concentration, ASR, and CQ, with R2 ranging from 0.711 to 0.970. By substituting the effects of environmental factors on ASR and CQ into the above relationship, temporal variations in NTE can be predicted. The reliability of the model was demonstrated by comparing model simulations with observations over three years. For most elements, the models can predict the temporal variation of NTE quite accurately, and even for the worst predictions, such as Al, Mg, K, Co and Cd, where predictions exceed observations by only an order of magnitude.

Oxidation Flow Reactor Results in a Chinese Megacity Emphasize the Important Contribution of S/IVOCs to Ambient SOA Formation.

Oxygenated volatile organic compounds (OVOCs) and secondary organic aerosol (SOA) formation potential of ambient air in Guangzhou, China was investigated using a field-deployed oxidation flow reactor (OFR). The OFR was used to mimic hours to weeks of atmospheric exposure to hydroxyl (OH) radicals within the 2-3 min residence time. A comprehensive investigation on the variation of VOCs and OVOCs as a function of OH exposure is shown. Substantial formation of organic acids and nitrogen-containing OVOC species were observed. Maximum SOA formation in the OFR was observed following 1-4 equiv days' OH exposure. SOA produced from known/measured VOC/IVOC precursors such as single-ring aromatics and long-chain alkanes can account for 52-75% of measured SOA under low NOx and 26-60% under high NOx conditions based on laboratory SOA yield parametrizations. To our knowledge, this is the first time that the contribution (8-20%) of long-chain (C8-C20) alkane oxidation to OFR SOA formation was quantified from direct measurement. By additionally estimating contribution from unmeasured semivolatile and intermediate volatility compounds (S/IVOCs) that are committed with C8-C20 alkanes, 64-100% of the SOA formation observed in the OFR can be explained, signifying the important contribution of S/IVOCs such as large cyclic alkanes to ambient SOA.

Ambient PM2.5-bound polycyclic aromatic hydrocarbons (PAHs) in rural Beijing: Unabated with enhanced temporary emission control during the 2014 APEC summit and largely aggravated after the start of wintertime heating.

For human health benefits it is crucial to see if carcinogenic air pollutants like polycyclic aromatic hydrocarbons (PAHs) are reduced accordingly along with the control of the criteria pollutants including fine particles (PM2.5). A number of studies documented that enhanced temporary emission control during the 2014 Asia-Pacific Economic Cooperation summit (APEC) in Beijing resulted in substantial drops of observed ambient PM2.5, as well as PAHs, in urban areas of Beijing, yet it is not clear whether PM2.5-bound PAHs in the rural areas were also lowered during the APEC. Here filter-based PM2.5 samples were collected at a rural site in northeast of Beijing, and analyzed for 25 PAHs before (Oct. 27-Nov. 2, 2014), during (Nov. 3-12, 2014) and after (Nov. 13, 2014-Jan. 14, 2015) the APEC. Observed concentrations of PM2.5, OC and EC during the APEC dropped by about 30%, however, average PM2.5-bound PAHs and their incremental lifetime cancer risk (ILCR), 25.65 ng/m3 and 3.2 × 10-4, remained almost unchanged when compared to that of 25.48 ng/m3 and 3.5 × 10-4, respectively, before the APEC. After the APEC with the start of wintertime central heating in urban Beijing on Nov. 15, 2014, average total concentration of PAHs and their ILCR highly elevated and reached 118.25 ng/m3 and 1.5 × 10-3, respectively. Source apportioning by positive matrix factorization (PMF) revealed that coal combustion was the largest source that contributed 63.2% (16.1 ng/m3), 78.5% (20.1 ng/m3) and 56.1% (66.3 ng/m3) to the total PAHs before, during and after the APEC, respectively. Uncontrolled residential coal use during the APEC was found to be the reason for unabated levels of PAHs, and the largely aggravated PAHs after the APEC was resulted from increased coal consumption for wintertime residential heating. Our results suggested reducing emission from residential coal combustion is crucial to mitigate carcinogenic PAHs in ambient air, especially in rural areas.