Photochemical release characteristics of particle-bound mercury in the atmosphere and its influencing factors

This study investigates the photochemical reduction of mercury in urban total suspended particulates (TSP) and its key drivers, aiming to elucidate particulate-bound mercury (PHg) transformation under multiple environmental factors. Using year-round Qingdao field samples, the effects of light intensity, UV wavelength, ozone, particle moisture content, and ionic composition were examined through controlled experiments and multivariate analyses. The photoreduction rate of mercury in TSP exhibited pronounced seasonal variability, following the order summer > spring > autumn > winter, closely linked to solar radiation and particle composition. As light intensity increased from 480 W/m² to 1100 W/m², elemental mercury (Hg⁰) release increased by approximately fourfold. UV radiation generally promoted PHg photoreduction, whereas under UVB irradiation, Hg⁰ oxidation exceeded Hg²⁺ reduction due to the strong oxidative capacity of UVB. Increasing ozone concentrations from 0 to 100 ppb reduced Hg⁰ release by up to 30.7 %, while increasing particle moisture content from 0 % to 100 % enhanced the photoreduction ratio from 2.28 ± 0.62 % to 12.51 ± 2.16 %. Seasonal variations in water-soluble ions exerted complex influences on mercury reduction: low-solubility HgSO₄ facilitated PHg photolysis; elevated NO₃^- promoted Hg⁰ oxidation through ^·OH radical formation, suppressing net release; and Cl^- inhibited reduction via forming HgCl₄^2-. Overall, light intensity, UV wavelength, ozone, moisture content, and ionic composition are dominant factors of PHg photoreduction. This study highlights the critical role of photochemically formed secondary aerosols, particularly sulfates, in enhancing mercury reduction in particles, providing new insights into aerosol-driven mercury cycling and supporting improved regional pollution control and global mercury modeling.