Diel variation in mercury stable isotope ratios records photoreduction of PM 2.5 -bound mercury

Abstract. Mercury (Hg) bound to fine aerosols (PM 2.5 -Hg) may undergo photochemical reaction that causes isotopic fractionation and obscures the initial isotopic signatures. In this study, we quantified Hg isotopic compositions for 56 PM 2.5 samples collected between 15 September and 16 October 2015 from Beijing, China, among which 26 were collected during daytime (between 08:00 and 18:30 LT) and 30 during night (between 19:00 and 07:30 LT). The results show that diel variation was statistically significant ( p  0.05 ) for Hg content, Δ 199 Hg and Δ 200 Hg , with Hg content during daytime ( 0.32±0.14 µ g g −1 ) lower than at night ( 0.48±0.24 µ g g −1 ) and Δ 199 Hg and Δ 200 Hg values during daytime (mean of 0.26 ‰±0.40 ‰ and 0.09 ‰±0.06 ‰ , respectively) higher than during nighttime ( 0.04 ‰±0.22 ‰ and 0.06 ‰±0.05 ‰ , respectively), whereas PM 2.5 concentrations and δ 202 Hg values showed insignificant ( p >  0.05 ) diel variation. Geochemical characteristics of the samples and the air mass backward trajectories (PM 2.5 source related) suggest that diel variation in Δ 199 Hg values resulted primarily from the photochemical reduction of divalent PM 2.5 -Hg, rather than variations in emission sources. The importance of photoreduction is supported by the strong correlations between Δ 199 Hg and (i)  Δ 201 Hg (positive, slope = 1.1), (ii)  δ 202 Hg (positive, slope = 1.15), (iii) content of Hg in PM 2.5 (negative), (iv) sunshine durations (positive) and (v) ozone concentration (positive) observed for consecutive day–night paired samples. Our results provide isotopic evidence that local, daily photochemical reduction of divalent Hg is of critical importance to the fate of PM 2.5 -Hg in urban atmospheres and that, in addition to variation in sources, photochemical reduction appears to be an important process that affects both the particle mass-specific abundance and isotopic composition of PM 2.5 -Hg.