Atmospheric wet and dry depositions of polycyclic aromatic compounds in a megacity of Southwest China

Abstract

The dry and wet depositions of polycyclic aromatic hydrocarbons (PAHs) and related derivatives have rarely been characterized separately. Parent, oxygenated and nitrated PAHs (PAHs, OPAHs and NPAHs) have been measured in monthly-averaged wet and dry deposition samples collected from January to December 2019 in urban Chongqing. The annual average concentrations of Ʃ17PAHs, Ʃ9OPAHs and Ʃ9NPAHs in wet deposition samples were 457 ± 375, 1311 ± 1416 and 8.25 ± 10.2 ng/L, respectively, with significant monthly variations introduced by rainfall and air-borne particle deposition. Most PAHs species were associated with the particulate phase in wet deposition, while OPAHs and NPAHs were mainly distributed in the dissolved phase, probably due to the lower octanol-water partitioning coefficient of N/OPAHs than that of PAHs. Annual deposition fluxes of PAHs, OPAHs and NPAHs in dry deposition were 25264, 25310 and 388 ng/m2/yr, respectively, higher than those in wet deposition (9869, 24083 and 207 ng/m2/yr). This indicates that PACs, especially PAHs, were removed from the atmosphere mainly via dry deposition. The contributions of wet deposition to total deposited PACs were larger for months with higher precipitation and for PACs with higher molecular weight. Composition pattern and temporal variation results indicated that wet deposition fluxes were mainly affected by precipitation-related particle deposition and chemical properties (e.g., water solubility), while dry deposition fluxes were affected more by factors such as gas/particle partitioning, particle size distribution and physicochemical properties of PACs. Principle component analysis combined with diagnostic ratios revealed that PACs in atmospheric deposition samples were from vehicle emission (48.6%), coal combustion (13.4%), petrogenic source (5.9%) and secondary formation (32.1%).