Diel variation in mercury stable isotope ratios records photoreduction of PM<sub>2.5</sub>-bound mercury

Abstract

<p><strong>Abstract.</strong> Mercury (Hg) bound to fine aerosols (PM<span class="inline-formula">_2.5</span>-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<span class="inline-formula">_2.5</span> 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<span class="thinspace"></span>LT) and 30 during night (between 19:00 and 07:30<span class="thinspace"></span>LT). The results show that diel variation was statistically significant (<span class="inline-formula"><i>p</i> <i>&amp;lt;</i> 0.05</span>) for Hg content, <span class="inline-formula">Δ¹⁹⁹Hg</span> and <span class="inline-formula">Δ²⁰⁰Hg</span>, with Hg content during daytime (<span class="inline-formula">0.32±0.14</span><span class="thinspace"></span><span class="inline-formula">µ</span>g<span class="thinspace"></span>g<span class="inline-formula">⁻¹</span>) lower than at night (<span class="inline-formula">0.48±0.24</span><span class="thinspace"></span><span class="inline-formula">µ</span>g<span class="thinspace"></span>g<span class="inline-formula">⁻¹)</span> and <span class="inline-formula">Δ¹⁹⁹Hg</span> and <span class="inline-formula">Δ²⁰⁰Hg</span> values during daytime (mean of <span class="inline-formula">0.26 ‰±0.40 ‰</span> and <span class="inline-formula">0.09 ‰±0.06 ‰</span>, respectively) higher than during nighttime (<span class="inline-formula">0.04 ‰±0.22 ‰</span> and <span class="inline-formula">0.06 ‰±0.05 ‰</span>, respectively), whereas PM<span class="inline-formula">_2.5</span> concentrations and <span class="inline-formula"><i>δ</i>²⁰²Hg</span> values showed insignificant (<span class="inline-formula"><i>p</i> <i>&amp;gt;</i> 0.05</span>) diel variation. Geochemical characteristics of the samples and the air mass backward trajectories (PM<span class="inline-formula">_2.5</span> source related) suggest that diel variation in <span class="inline-formula">Δ¹⁹⁹Hg</span> values resulted primarily from the photochemical reduction of divalent PM<span class="inline-formula">_2.5</span>-Hg, rather than variations in emission sources. The importance of photoreduction is supported by the strong correlations between <span class="inline-formula">Δ¹⁹⁹Hg</span> and (i) <span class="inline-formula">Δ²⁰¹Hg</span> (positive, slope<span class="thinspace"></span><span class="inline-formula">=</span><span class="thinspace"></span>1.1), (ii) <span class="inline-formula"><i>δ</i>²⁰²Hg</span> (positive, slope<span class="thinspace"></span><span class="inline-formula">=</span><span class="thinspace"></span>1.15), (iii) content of Hg in PM<span class="inline-formula">_2.5</span> (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<span class="inline-formula">_2.5</span>-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<span class="inline-formula">_2.5</span>-Hg.</p>