Research

My current research examines how changing human emissions and increasing wildfires impact atmospheric aerosols in a changing climate, and their feedback effects on aerosol-chemistry-climate interactions. Specifically, I focus on studying the climate and chemical impacts of aerosols in the upper troposphere and lower stratosphere (UTLS).

UTLS aerosols play a crucial role in global radiative balance by scattering and absorbing radiation, as well as influencing ice cloud formation. They also affect the protective ozone layer by modulating heterogeneous reaction rates of halogen species on the particle surface. However, significant knowledge gaps remain regarding the properties of UTLS aerosols, including their microphysics, chemical composition, and optical properties. To address these gaps, my research combines in situ measurements, laboratory experiments, satellite remote sensing, rediative transfer models, and aerosol-chemistry-climate modeling.

I have also conducted comprehensive studies on urban PM2.5 pollution, employing a combination of field and laboratory measurements. Furthermore, I have explored biosphere-atmosphere interactions by leveraging unmanned aerial vehicle (UAV) measurements.

1. In situ characterization and sampling of aerosols in the UTLS
(In collaboration with DCOTSS and SABRE teams)

During the NASA DCOTSS and NOAA SABRE aircraft missions, I developed and deployed two aircraft instruments to: 1) measure UTLS aerosol concentration and size distribution (DPOPS instrument), and 2) collect UTLS aerosol samples for offline chemical composition and morphology analysis (high-altitude aircraft MOUDI system). My work yielded a valuable dataset of aerosol concentration, size distribution, composition, and morphology up to 22 km over North America. This dataset is essential for characterizing background UTLS aerosols and perturbations from volcanic and wildfire injections. For example, my instrument measured volcanic plumes from La Soufrière eruptions in April 2021 in the stratosphere, which I used for detailed analysis of their spatiotemporal evolutions and impacts on radiative forcing and ozone (Li et al., ACP, 2023). I also evaluated the radiative impacts of high-altitude wildfire smoke measured in the UT. Data from DCOTSS mission are publically available at NASA ASDC archive. Feel free to email me if you have any questions on the aerosol data.

2. Offline chemical imaging of UTLS aerosol samples
(In collaboration with Alex Laskin Group at Puerdue University; Swarup China and Zezhen Cheng at EMSL)

UTLS aerosol samples for offline chemical imaging analysis were collected by a cascade impactor (Mini-MOUDI 135, MSP) onboard the NASA ER-2 and WB-57 high-altitude research aircrafts. Preliminary analysis of DCOTSS samples revealed that organic-containing particles, especially organics from wildfire sources, are common in the summer stratosphere, highlighting the potential importance of halogen activation reactions on the organic surface.

3. Composition dependence of stratospheric aerosol radiative forcing
(In collaboration with Terry Deshler at CU Boulder)

While it is generally assumed that stratospheric aerosol is dominated by pure sulfuric acid plus water aerosols, recent in situ measurements and modeling studies suggest that organic matter makes up a significant fraction of lower stratospheric aerosol. The implications of this organic component are uncertain but may require significant revision of our understanding of the stratosphere’s climate influence. I investigated the effects of the refractive index of organics and their mixing state with sulfate on shortwave radiative forcing (RF) of stratospheric aerosols. Using long-term balloon-borne aerosol measurement records and radiative transfer calculations, I found that organics may have significant impacts (up to 100% change) on stratospheric aerosol shortwave RF during periods of minimal-moderate volcanic activities. Currently, however, there is very little data on the mixing state and refractive index of organic-containing stratospheric aerosols. (Li et al., GRL, 2021)

4. Optical properties of organic aerosols
(In collaboration with Pengfei Liu Group at Georgia Tech)

Accurate aerosol refractive index measurements are critical for modeling aerosol-radiation interaction, yet they are limited for ambient organic aerosols, leading to large uncertainties in estimating aerosol radiative effects. I developed a semiempirical model that predicts the real refractive index n of organic aerosol material from its widely measured oxygen-to-carbon (O:C) and hydrogen-to-carbon (H:C) elemental ratios. The model was based on the theoretical framework of Lorenz-Lorentz equation and trained with n-values at 589 nm (n_589nm) of 160 pure compounds. The predictions can be expanded to predict n-values in a wide spectrum between 300 and 1200 nm. The model was validated with newly measured and literature datasets of n-values for laboratory secondary organic aerosol (SOA) materials. Uncertainties of n_589nm predictions for all SOA samples are within ±5%. The model suggests that n_589nm-values of organic aerosols may vary within a relatively small range for typical O:C and H:C values observed in the atmosphere. (Li et al., GRL, 2023)

5. Probing biosphere-atmosphere interactions with UAV measurements of VOCs
(In collaboration with Qi Chen Group at PKU and Scot Martin Group at Harvard)

Volatile organic compounds (VOCs) are important air pollutants and play a critical role in biosphere-atmosphere interactions. Atmospheric sampling onboard a multicopter unmanned aerial vehicle (UAV), serving as an economical and flexible measurement technique, collects valuable VOC data at intermediate spatial scales of hundreds of meters. I developed an UAV-based VOC sampling apparatus. The sampler was deployed in a subtropical forest in South China to get vertical measurements of VOCs over the canopy. VOC samples were analyzed offline by thermal-desorption gas chromatography-mass spectrometry (TD-GC-MS). High aromatic VOCs concentrations and (methacrolein + methyl vinyl ketone) to isoprene ratios indicate a strong influence of anthropogenic pollution. Our measurements together with a gradient transport model suggested significant isoprene emission heterogeneity along the mountain slope at intermediate spatial scales, which has not yet been well represented in most biosphere emission models.(Li et al., ACS Earth and Space Chemistry, 2021)

6. Measurements of reactive organic carbon with Harvard PTR3 instruments
(In collaboration with Jesse Kroll Group at MIT)

Reactive organic carbon (ROC) species are all atmospheric organic species excluding methane; this includes volatile organic compounds (VOCs) and other lower-volatility organics such as particulate organic carbon. Chemical ionization mass spectrometry (CIMS) is an important analytical tool for measurements of ROC in the atmosphere. I have been using and maintaining two Harvard CIMS instruments (aka PTR3) and participated in several laboratory and field projects with PTR3 instruments. Harvard PTR3 instruments can be operated in one of two ionization modes: using either proton transfer reactions such as for PTR-MS (Breitenlechner et al., 2017) or ammonium ion ligand-switching reactions such as for NH4+ CIMS (Zaytsev et al., 2019). Employment of the two ionization modes significantly improves the measurement capability of the instruments and allows for detection of a vast array of compounds covering a wide range of volatilities from VOCs to ELVOCs. In addition, the PTR3 instruments can be equipped with a thermal desorption inlet to quantify particle-phase compounds. I have extensively utilized PTR3 instruments for: (1) indoor air quality implications of oxidation-based air cleaners (Ye et al., ES&T Letters, 2021) and Germicidal 222 nm Light (Barber et al., ES&T, 2023); (2) chemistry of dimethyl sulfide (DMS) and its oxidation products in the atmosphere (Ye et al., ACP, 2022); (3) Munich Urban Air Quality Campaigns.