Atmospheric Chemistry & Climate Change

Ke's research interest is contributing to a deeper understanding of atmospheric chemistry, climate change, and their environmental impacts. -Tropospheric air pollutants, such as ozone and fine particles, threaten human health and also contribute to perturbing earth radiation balance. -Climate change has the potential to abate or worsen air pollution by influencing the formation, transport, and removal of air pollutants. Current projects:1) Response of atmospheric pollutants to the ongoing China Clean Air Action; 2) How does weather/climate change affect air quality



1. Ground-level ozone pollution


(1) A two-pollutant strategy for improving ozone and particulate air quality 

Previous model simulations have suggested that the increase in ozone could be driven by the decrease in PM2.5. Li et al. (Nature Geoscience, 2019) show observational evidence for this effect. The observations show suppression of ozone pollution at high PM2.5 concentrations, consistent with a model simulation in which PM2.5 scavenging of HO2 and NOx depresses ozone concentrations by 25 ppb relative to PM2.5-free conditions. PM2.5 chemistry makes ozone pollution less sensitive to NOx emission controls, emphasizing the need for controlling emissions of VOCs, which so far have not decreased in China. The new 2018–2020 Clean Air Action plan calls for a 10% decrease in VOC emissions that should begin to reverse the long-term ozone increase even as PM2.5 continues to decrease. Aggressive reduction of NOx and aromatic VOC emissions should be particularly effective for decreasing both PM2.5 and ozone.


(2) Drivers of increasing summer surface ozone

Drastic air pollution control in China since 2013 has achieved sharp decreases in fine particulate matter (PM2.5), but ozone pollution has not improved. Li et al. (PNAS, 2019) find that surface ozone has increased in megacity clusters of China, notably Beijing and Shanghai, after removing the effect of meteorological variability. The increasing trend cannot be simply explained by changes in anthropogenic precursor [NOx and volatile organic compound (VOC)] emissions, particularly in North China Plain (NCP). The most important cause of the increasing ozone in NCP appears to be the decrease in PM2.5, slowing down the sink of hydroperoxy radicals and thus speeding up ozone production. Decreasing ozone in the future will require a combination of NOx and VOC emission controls to overcome the effect of decreasing PM2.5. (Reported by Harvard SEAS, The Independent, UPI, World Economic Forum, C&EN, South China Morning Post, The Korea TimesThe Straits Times, Youtube, and a poetry named by Particulate Behaviour)

ozone trend

2. Particulate Matter Pollution 

(1) Effects of Climate Change on Haze Pollution in China

Haze pollution featuring a high concentration of PM2.5 is one of the most concerning environmental issues in China. Conducive weather conditions are an important ingredient of severe haze episodes. In Cai and Li et al., (Nature Climate Change, 2017), we project a 50% increase in the frequency and an 80% increase in the persistence of conducive weather conditions similar to those in January 2013, in response to climate change. In Li et al. (Geophysical Research Letters, 2018), We further show that greenhouse warming-driven circulation change has already significantly increase the probability of the occurrence of such similar atmospheric patterns by, at least, about 45% in January 2013, and 27% in December 2015, respectively. Thus, more strict emission reduction measures are needed to improve air quality under a continuing anthropogenic warming in the upcoming decades and global effort to reduce greenhouse gas emissions can decrease the risk of severe haze over eastern China. (covered by Nature Climate Change, highlighted by Nature, and reported by Nature Climate ChangeNew York TimesChina Daily, CNBC, CarbonBriefCAS et al. )

Weather conditions causing winter severe haze in Beijing   PM

(2) Future Air Quality in China under Different Emission Scenarios

In Li et al. (Journal of Geophysical Research, 2016), we applied the nested-grid version of the chemical transport model (GEOS-Chem) to quantify 2000–2050 changes in PM2.5 air quality and related direct radiative forcing (DRF) in China, based on future emission changes under the representative concentration pathway (RCP) scenarios of RCP2.6, RCP4.5, RCP6.0, and RCP8.5. We find that Beijing-Tianjin-Hebei (BTH) wintertime PM2.5 concentrations less than 35 µg m-3 occur after 2040 under RCP2.6, RCP4.5, and RCP8.5, and summertime PM2.5 concentrations reach this goal by 2030 under RCP2.6 and RCP4.5. The difficulty in controlling future PM2.5 concentrations relates to unmitigated high levels of nitrate, although NOx and SO2 emissions show substantial reductions during 2020–2040. When considering both health and climate effects of PM2.5 over China, for example, PM2.5 concentrations averaged over east China under RCP4.5 (RCP2.6) decrease by 54% (43%) in 2050 relative to 2000, but at the cost of warming with DRF of 1.88 (1.22) W m-2 . (Reported by Asian Scientist, CASEurekAlert).

Simulated 2000-2050 PM2.5 concentration

(3) Simulation of Black Carbon and its radiative forcing in China

Black carbon (BC) as an air pollutant has harmful impacts on human health, and it also plays an important role in climate change through its strong absorption of sunlight. BC emissions in China accounted for about 25% of the annual global total emission of BC in recent years. In Li et al., (Atmospheric Environment, 2016), We quantify the contributions from five domestic emission sectors (residential, industry, transportation, energy, and biomass burning) and emissions outside of China (non-China) to concentration and direct radiative forcing (DRF) of BC in China for year 2010 using a nested-grid version of the global chemical transport model (GEOS-Chem) coupled with a radiative transfer model. 

Simulated direct radiative forcing of black carbon in China