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My current and recent projects address two broad research themes: air quality & global chemistry.



NOx emissions in China

I analyzed 15 years of satellite observations of nitrogen dioxide (NO2) and use an atmospheric chemistry model to understand the seasonal changes and trends in nitrogen oxides (NOx) over China. I found that the seasonal changes in NO2 occur due to changes in the NOx oxidation lifetime. We find that Chinese NOx emissions peaked in 2011 and then decreased by about 25% by 2018. But the decrease in NO2 in winter was larger, likely because of a simultaneous decrease in the NOx oxidation lifetime. This work shows that changes in NOx chemistry should be considered when inferring NOx emissions from satellite observations. Learn more...


Winter air quality in US

Sulfate and nitrate trends in eastern US

In the eastern United States, substantial cuts in sulfur dioxide and nitrogen oxides emissions have considerably lowered particulate sulfate and nitrate concentrations for all seasons except winter. I investigated this using measurements from a wintertime aircraft campaign (WINTER) and atmospheric chemistry modeling. I found that in the wintertime particulate sulfate and nitrate formation were limited by the amount of oxidants available and by high particle acidity. These limitations relax when emission reductions start to decrease the particulate concentrations, which in turn increases the efficiency with which these particles form, and ultimately weaken the effect of emission reductions. The implication is that larger emission reductions are necessary to make substantial improvements in wintertime air quality in the eastern United States. Learn more...


Secondary organic aerosols

I analyzed the WINTER campaign observations of organic aerosols over the northeastern United States during winter. I found that a majority of organic aerosols are secondary (SOA) formed in the atmosphere from gases emitted from pollution sources. Prior to these measurements, the expectation was that wintertime pollution sources of SOA were minor outside of urban areas. But my results showed that their influence is ubiquitous over the entire eastern United States, and a better understanding of the sources of SOA will help in developing effective policies to reduce wintertime air pollution. Learn more...



Cloud and precipitation pH

Cloud water pH affects atmospheric chemistry, and acid rain damages ecosystems. We use model simulations along with observations to present a global view of cloud water and precipitation pH. Sulfuric acid, nitric acid, and ammonia control the pH in the northern midlatitudes, but carboxylic acids and dust cations are important in the tropics and subtropics. The acid inputs to many nitrogen-saturated ecosystems are high enough to cause acidification, with ammonium as the main acidifying species. Learn more...


Mercury redox chemistry

Mercury over the southeast US

I was part of an aircraft campaign that collected speciated measurements of Hg over the southeastern USA. The observations showed higher concentrations of oxidized Hg in clean, dry air masses descending in the subtropical anti-cyclones. I then used a state-of-the-science atmospheric chemistry model (GEOS-Chem) to interpret these observations and found that the Hg oxidation rate is much faster than what the model predicted. These results shifted our view of Hg chemistry from that of a slow conversion of elemental Hg to oxidized Hg to that of rapid cycling between the two forms in the atmosphere.  Learn more...


Global deposition of mercury

I used the GEOS-Chem model of Hg chemistry and transport, along with ground- and aircraft-based observations, to show that oxidized Hg chemically produced in the free troposphere makes up much of the Hg depositing to the Earth's surface, through descent in the subtropical anticyclones. These findings imply that mercury chemistry in the free troposphere and subsequent transport in the subtropics are important processes that control how Hg emitted to the atmosphere is distributed globally before depositing to the surface.  Learn more...