Session G03: Atmospheric Mercury and Soot

Atmospheric Mercury: Successes and Failures with Measurements in the Parts-Per-Quadrillion Range

Mercury is a potent neurotoxicant. Human exposure to mercury most often occurs from consumption of contaminated fish, but most mercury pollution is emitted to the atmosphere. Once emitted, mercury can be chemically transformed and transported around the globe. Except in some highly-contaminated industrial areas, concentrations of atmospheric mercury are too low to be a direct health hazard. Instead, atmospheric mercury deposits to ecosystems, allowing it to bioaccumulate and biomagnify up aquatic and other food chains. Elemental mercury makes up the vast majority of what is in the atmosphere, with typical concentrations in the range of 1-2 ng m$^{\mathrm{-3}}$ (100-200 ppq). Measurements of oxidized mercury compounds are much less certain and vary more with location, meteorology, and chemistry, but typical measurements are in the range of 0-200 pg m$^{\mathrm{-3}}$ (0-15 ppq). Several robust, well-verified methods exist to measure gas-phase elemental mercury. Pre-concentration on gold traps followed by atomic fluorescence is the most common method, and it is very sensitive and stable. Measurements of oxidized mercury are fraught, however. Low concentrations typically require lengthy pre-concentration, but oxidized mercury compounds are so reactive that they are not usually able to survive pre-concentration, leading to measurement bias. Also, no established method exists that can measure oxidized mercury compounds directly, so it must be pyrolytically reduced to elemental mercury for analysis. This makes it impossible to know with certainty what oxidized mercury compounds exist in the ambient atmosphere. Finally, oxidized mercury compounds are semivolatile, and dynamic partitioning between the gas-phase and particle-phases makes quantitative separation of the phases extremely challenging. Commercial oxidized mercury measurement systems exist, but they suffer from all of these problems and have repeatedly been shown be biased. Several research groups are working to improve measurements of oxidized mercury. These improvements include (1) development of methods to measure specific oxidized mercury compounds, and (2) development of methods to measure total oxidized mercury in an accurate way, both with better measurement methods and with reproducible calibration systems. To our knowledge, no work is currently being done to develop quantitative measurement systems that accurately differentiate between particle-phase and gas-phase oxidized mercury.   

Is direct molecular analysis of atmospheric oxidized mercury possible?

Mercury is a persistent environmental pollutant, entering the atmosphere mostly in elemental form and leaving in various oxidized forms, commonly named gaseous oxidized mercury (GOM). Little is known about the molecular identity of GOM and this hinders the ability to evaluate the fate of mercury in the environment. All current detection methods pre-concentrate GOM before analysis, producing various artifacts. A direct method capable of molecular analysis of GOM is needed. This presentation will cover our recent work on the development of an analytical technique for direct molecular analysis of GOM, based on the ion drift - chemical ionization mass spectrometry (ID-CIMS). In this method, gaseous molecules react in a drift tube with an appropriate reagent ion to form well-defined product ions, which are detected by a mass spectrometer. Using quantum chemical calculations and ID-CIMS experiments, we show that surrogate GOM molecules, such as HgX$_{\mathrm{2}}$ and HgXY (where X and Y are halides) can be analyzed quantitatively via ion-molecule reactions with several negatively charged reagent ions. I will discuss challenges associated with detecting mercury-containing molecules, ways to overcome those challenges, and prospective of using ID-CIMS for direct detection of GOM in the atmosphere.   

Possible Exchange Reactions during Analysis of Gaseous Oxidized Mercury

Gaseous oxidized mercury (GOM) is formed in the atmosphere upon photochemical oxidation of gaseous elemental mercury. Due to low atmospheric concentration of GOM, its chemical analysis requires pre-concentration on various substrates, such as KCl or various membranes. We hypothesize that GOM species collected on those substrates can engage in exchange reactions with the substrate, each other, or with other co-adsorbed atmospheric chemicals, changing the chemical composition of GOM and potentially leading to analysis artifacts. Here we investigated the exchange reactions of several GOM surrogates (HgCl$_{\mathrm{2}}$, HgBr$_{\mathrm{2}}$, HgI$_{\mathrm{2}})$ in aqueous solutions and on surfaces. Exchange reaction products were analyzed using ion drift - chemical ionization mass spectrometry (ID-CIMS) and electrospray ionization - mass spectrometry (ESI-MS). We observed volatile HgBrCl, HgBrI, HgClI as a result of exchange in aqueous solutions. Our preliminary results also indicate that the exchange reactions can occur on surfaces, producing products that are not related to original GOM, but can be volatilized by thermal evaporation.   

Tracking the Evolution in~Soot Aggregate Optical Properties~Concurrently with its Morphology

Radiative forcing by soot is strongly dependent on particle morphology and mixing state, which are complex and subject to change during atmospheric aging. Our previous research showed that fractal soot aggregates can restructure in the presence of particularly thin coatings. Recently, we developed an algorithm to model aggregate restructuring and found that individual aggregates with the same initial fractal parameters can end up with varying levels of compactness. In this study, we examine the impact of structural evolution on the optical properties of thinly coated soot. We apply our restructuring algorithm to an ensemble of fractal aggregates with the same initial morphological parameters. As each aggregate restructures, we extract and save morphologies that match a set of predefined fractal parameters. Using Discrete Dipole Approximation, we compare the optical properties of aggregates that were generated fractal and then restructured against those of aggregates that were generated compact. Additionally, we perform optical calculations on those fractal and compact aggregates after adding the coating material, which can be distributed as a uniform layer or localized in junctions between individual spheres in the aggregate. By tracking the evolution in optical properties of coated soot, the outcome of our findings will help improve the accuracy of radiative forcing by soot in atmospheric models.   

The Interplay between Capillary Condensation and Full Encapsulation in the Collapse of Fractal Soot Particles

Combustion soot is made of fractal aggregates of small, nearly spherical monomers. Changes in the mixing state and morphology of soot aggregates strongly affect their optical properties and hence their climate impacts. Condensation of vapors on soot aggregates produces coatings, which can transform the aggregate morphology from fractal to compact. Here we studied the restructuring of soot aggregates subjected to vapors of intermediated volatility organic compounds, using supersaturations from zero and above. The measurements were performed using an in-house-built miniature electrostatic classifier that could be placed at any distance after the aerosol coating chamber to probe the coated particle mobility diameter. The dependence of the particle mobility diameter on vapor supersaturation showed three different regions. At a zero supersaturation, capillary condensation took place, resulting in a minor shrinkage of soot aggregates. At supersaturations slightly above zero, the shrinkage progressed due to an increased restructuring driven by additional condensation, with particle diameter decreasing to 80{\%} of its initial value. At higher supersaturations, the particle diameter increased several times, eventually corresponding to full encapsulation of soot by the coating droplet.   

An Algorithm for Soot Aggregate Restructuring

Soot aggregates, derived from the incomplete combustion of fossil fuels and biomass burning, are ubiquitous in the atmosphere and adversely impact air quality and global climate. The fractal-like structure of soot aggregates undergoes significant restructuring due to interaction with condensable trace-gas chemicals during atmospheric aging. This morphological change affects the properties of soot aggregates including their light scattering and absorption, surface chemistry, cloud nucleation efficiency and atmospheric lifetime. In this study, we develop an algorithm to simulate the condensation-induced restructuring of soot aggregates. The restructuring algorithm accounts for adhesion forces between soot monomers, capillary forces due to coating trapped in junctions between monomers, and viscous dissipation forces. We test our restructuring algorithm on an ensemble of initially fractal aggregates and track the morphological evolution of the aggregate backbone. We also compare our predictions of final aggregate morphology against compact aggregates generated directly via traditional Diffusion Limited Cluster Aggregation (DLCA) methods. The results of our findings will provide a framework that facilitates predictions of morphologically dependent soot properties.