Hongming Cai

An improved dual-stage protocol to pre-concentrate mercury from airborne particles for precise isotopic measurement

The Hg isotopic signature may provide insight into tracking the sources and pathways of both airborne particulate matter (PM) and particle-bound Hg (PHg) in the atmosphere. However, separating and concentrating trace levels of PHg and accurately analyzing its isotope ratios remain a technical challenge. Here, we optimized a combustion-trapping dual-stage protocol specifically for the pre-concentration of Hg from a PM sample collected on a quartz fiber membrane (QFM) for high-precision Hg isotopic analysis. The protocol was validated by testing synthetic samples of varying concentrations and trapping solutions of different volumes, and by comparison with two conventional methods (acid digestion and column purification). Using the dual-stage protocol, an individual sample containing up to 570 ng of Hg can be combusted at programmed temperatures in an Hg-free O2 stream, and the volatilized gaseous Hg was trapped in a 5 mL acid (4 M HNO3 and 1.3 M HCl) mixture. The method results in a relatively lower procedural blank and quantitative recovery (99 ± 6%, 2SD, n = 90). Long-term measurements of three certified reference materials (CRM021, CRM024, and GBW07405) with complex matrices using the optimized protocol gave identical Hg isotopic ratios of both mass-dependent fractionation (MDF) and mass-independent fractionation (MIF), in agreement with the results obtained from the standard addition method. The protocol was applied to two PM2.5 samples collected on a 20.3 × 12.5 cm QFM. The result showed evident variations of both MDF and MIF, highlighting the importance of studying Hg isotopic compositions in PM of variable environments in order to fully understand the behaviors of Hg and its isotopes in the atmosphere.

Sequential samples reveal significant variation of mercury isotope ratios during single rainfall events

Although the investigation of mercury (Hg) isotopes in precipitation has largely improved our knowledge of the source and transformation of Hg in the atmosphere, rainwater investigated in previous studies were integrated samples collected over an event and could obscure key information about the physiochemical transformation and deposition dynamics of Hg (and its isotopes) in short precipitation events. In this study, we investigated Hg isotopic composition of filtered (HgF) and particulate Hg (HgPM) in sequential rain samples from three single rainfall events in Guiyang, China. All samples showed a decrease of total Hg concentration, as well as HgF and HgPM with time in each rainfall event, and large variation of both mass-dependent fractionation (MDF) and mass-independent fractionation of odd Hg isotopes (odd-MIF) for both phases. Isotopic data indicated variable contributions of different sources triggered by the instant change of meteorological conditions, rather than internal atmospheric processes. The rapid response of MDF and odd-MIF of precipitation samples to the incense burning on the Tomb Sweeping Day implied that Hg isotopic composition was very sensitive to the momentary anthropogenic emission, which could have at least a regional short-lived effect and should be taken into account in future studies. Hg isotopes are a powerful tool for investigating both atmospheric transformation and instant deposition dynamic of Hg, and like stable H and O isotopes, could provide useful information about local or regional meteorological changes.

Diel variation of mercury stable isotope ratios record 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 Sept. 15th and Oct. 16th, 2015 from Beijing, China, among which 26 were collected during the daytime (between 8:00 a.m. and 6:30 p.m.) and 30 during night (between 7:00 p.m. and 7:30 a.m.). The results show that diel variation was statistically significant ( p 199 Hg and Δ 200 Hg, with Hg content during the daytime (0.32 ± 0.14 µg g −1 ) lower than at night (0.48 ± 0.24 µg g −1 ) and Δ 199 Hg and Δ 200 Hg values during the daytime (mean of 0.26 ‰ ± 0.40 ‰ and 0.09 ‰ ± 0.06 ‰, respectively) higher than during the 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.