Abstract
Abstract. Mercury (Hg) bound to fine aerosols (PM2.5-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 PM2.5 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, Δ199Hg and Δ200Hg, with Hg content during daytime (0.32±0.14 µg g−1) lower than at night (0.48±0.24 µg g−1) and Δ199Hg and Δ200Hg 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 PM2.5 concentrations and δ202Hg values showed insignificant (p > 0.05) diel variation. Geochemical characteristics of the samples and the air mass backward trajectories (PM2.5 source related) suggest that diel variation in Δ199Hg values resulted primarily from the photochemical reduction of divalent PM2.5-Hg, rather than variations in emission sources. The importance of photoreduction is supported by the strong correlations between Δ199Hg and (i) Δ201Hg (positive, slope = 1.1), (ii) δ202Hg (positive, slope = 1.15), (iii) content of Hg in PM2.5 (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 PM2.5-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 PM2.5-Hg.
Highlights
Atmospheric mercury (Hg) consists of three operationally defined forms including particle-bound Hg (PBM), gaseous oxidized Hg (GOM) and gaseous elemental Hg (GEM) (Selin, 2009)
Hg isotope analysis showed that δ202Hg values varied from −1.49 ‰ to 0.55 ‰, with the lowest value found in sample Oct-2-N
This study showed significant diel variations in Hg isotopic compositions for ambient PM2.5-Hg collected in the city of Beijing
Summary
Atmospheric mercury (Hg) consists of three operationally defined forms including particle-bound Hg (PBM), gaseous oxidized Hg (GOM) and gaseous elemental Hg (GEM) (Selin, 2009). GEM is the most abundant (about 90 %) and chemically stable form (Selin, 2009) and is transported at regional and global scales. Q. Huang et al.: Diel variation in mercury stable isotope ratios cal scales, thereby reflecting Hg pollution and cycling within short distances from emission sources (Selin, 2009; Subir et al, 2012). PBM has multiple sources and undergoes complex transport and transformation processes in the atmosphere (Subir et al, 2012)
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