Investigating the Chemical and Climatic Mechanisms Driving Extreme Air Pollution Episodes
Moch, Jonathan Manley
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CitationMoch, Jonathan Manley. 2020. Investigating the Chemical and Climatic Mechanisms Driving Extreme Air Pollution Episodes. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
AbstractFine particulate matter (PM2.5) is a significant cause of global mortality and an important component of anthropogenic interference with the climate. Also called aerosols, PM2.5 can affect climate by reflecting or absorbing light that otherwise would have reached the surface, or by influencing the formation of clouds. PM2.5 is a significant health risk as these particles can penetrate deep into the lungs, where they can cause or exacerbate a variety of respiratory and cardiovascular problems. Extreme air pollution episodes, when PM2.5 concentrations can exceed 200 μg m-3, may have dramatic impacts on both health and regional climate in a relatively short period of days to weeks. However, the chemical, meteorological, and climatic mechanisms behind extreme air pollution episodes are complex, making prediction of these episodes and an understanding of the public health and climatic consequences challenging. Using China as a case study, I investigate chemical and climatic mechanisms driving extreme air pollution episodes and the implications for climate and global air quality. With regard to chemical mechanisms, I focus in particular on sulfur-formaldehyde chemistry, demonstrating the importance of this chemistry both for extreme pollution events and for global air quality.
In China, a key uncertainty has been the source of sulfur compounds in PM2.5. During extreme haze in Beijing, levels of particulate sulfur are usually much higher than can be explained with traditional photochemical oxidation chemistry. This particulate sulfur has in the past been assumed to be in the form of sulfate. Using a 1-D chemistry model and the GEOS-Chem global chemical transport model, we show that a large fraction of the sulfur compounds in extreme Beijing haze may, rather than sulfate, in fact be a sulfur molecule called hydroxymethanesulfonate (HMS), which forms via the aqueous phase chemical reaction of formaldehyde and sulfur. Our results suggest a key role for formaldehyde in controlling levels of PM2.5 in Beijing extreme haze. We additionally show that HMS can be easily misinterpreted as sulfate in observations, implying that some previous reports of “sulfate” may have overlooked the contribution of HMS to particulate sulfur. Our results suggest that reductions of formaldehyde and other volatile organic compounds (VOCs) may provide an effective tool for reducing particulate air pollution in Beijing.
We also demonstrate that the sulfur-formaldehyde chemistry that forms HMS is important globally, not just during extreme haze events in Beijing. Using the GEOS-Chem global chemical transport model with HMS chemistry, we find that HMS may comprise over 25% of particulate sulfur in many polluted regions around the world. By analyzing the chemical conditions for HMS formation we find that formaldehyde has an important role in limiting particulate sulfur pollution in multiple regions globally and especially during boreal winter. By reanalyzing old observations and conducting new ones, we also find an ubiquitous global presence of HMS in observations that has been previously overlooked. Again these results imply that reducing emissions of formaldehyde and other VOCs may have a co-benefit of decreasing particulate sulfur worldwide.
Using GEOS-Chem with HMS chemistry, we then investigate climatic mechanisms behind extreme pollution episodes in China. To do this we couple GEOS-Chem aerosols to radiation in the GEOS earth system model. Using this coupled model we investigate the effects of aerosol-radiation interactions on local and regional climate, which in turn can influence surface PM2.5 concentrations. We hypothesize that as Chinese emissions declined in recent years, the magnitude of aerosol-radiation feedbacks has also declined, meaning that regional climate and meteorology has been altered as a consequence of improved air quality. In preliminary results, we find that this changing aerosol influence on meteorology has provided an additional benefit for air quality by reducing the likelihood of meteorological conditions conducive to the formation of extreme air pollution episodes.
Citable link to this pagehttps://nrs.harvard.edu/URN-3:HUL.INSTREPOS:37365746
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