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



NOx emissions in China

We 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. We 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 had decreased by about 25‚ÄČ% by 2018. But the decrease in NO2 in winter was larger, likely because of a simultaneous decrease in the NOx oxidation lifetime. 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. Simulations that reproduce detailed airborne observations of wintertime atmospheric chemistry over the eastern United States (WINTER campaign) indicate that particulate sulfate and nitrate formation is limited by the availability of oxidants and by the acidity of fine particles, respectively. These limitations relax at lower ambient concentrations, forming particulate matter more efficiently, and weaken the effect of emission reductions. These results imply that larger emission reductions are necessary for substantial improvements in wintertime air quality in the eastern United States. Learn more...


Secondary organic aerosols

We used the WINTER observations of organic aerosols over the northeastern United States during winter. We observed that a majority of organic aerosols consist of secondary organic aerosols that form in the atmosphere from gases emitted from pollution sources. Prior to our measurements, the expectation was that wintertime pollution sources of secondary organic aerosols are minor outside urban areas. Our results show that their influence is ubiquitous over the entire eastern United States, and a better understanding of these sources can 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...


Mecury oxidation chemistry

Mercury over the southeast US

We collected airborne observations of mercury (Hg) over the southeastern USA during summer. We found higher concentrations of oxidized Hg in clean, dry air masses descending in the subtropical anti-cyclones. We then used an atmospheric chemistry model (GEOS-Chem) to interpret these observations. We found reasonable agreement between the model and the observations only when the modeled oxidation of elemental mercury was increased, suggesting rapid cycling between the elemental and oxidized forms of Hg in the atmosphere. Learn more...


Global deposition of mercury

We 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 descends in the subtropical anticyclones and makes up much of the Hg depositing to the Earth's surface. Our findings imply that mercury chemistry in the free troposphere and transport in the subtropics are important links between global emissions and surface deposition of mercury. Learn more...