Past Projects

Formaldehyde (HCHO) is a trace gas primarily found in the planetary boundary layer (PBL). It is a first-generation oxidation product of a wide range of volatile organic compounds (VOCs) during their reaction with hydroxyl radicals (OH) and serves as a key precursor for major air pollutants, including surface ozone (O₃) and secondary organic aerosols (SOA). Despite its significance, HCHO remains poorly constrained in North America, particularly in the northern high latitudes.

My Ph.D. research focuses on elucidating the variability of HCHO across the Arctic and the contiguous United States (CONUS), spanning temporal scales from diurnal cycles to interannual variability, and identifying the key drivers in these regions.

1. Source and variability of formaldehyde (HCHO) at northern high latitude

We use satellite observations of formaldehyde (HCHO) vertical column densities (VCD) from the TROPOspheric Monitoring Instrument (TROPOMI), aircraft measurements, combined with a nested regional chemical transport model (GEOS-Chem at 0.5 × 0.625 resolution), to better understand the variability and sources of summertime HCHO in Alaska. We first evaluate GEOS-Chem with in-situ airborne measure- ments during the Atmospheric Tomography Mission 1 (ATom-1) aircraft campaign. We show reasonable agree- ment between observed and modeled HCHO, isoprene, monoterpenes and the sum of methyl vinyl ketone and methacrolein (MVK+MACR) in the continental boundary layer. In particular, HCHO profiles show spatial ho- mogeneity in Alaska, suggesting a minor contribution of biogenic emissions to HCHO VCD. We further ex- amine the TROPOMI HCHO product in Alaska in summer, reprocessed by GEOS-Chem model output for a priori profiles and shape factors. For years with low wildfire activity (e.g., 2018), we find that HCHO VCDs are largely dominated by background HCHO (58 %–71 %), with minor contributions from wildfires (20 %–32 %) and biogenic VOC emissions (8 %–10 %). For years with intense wildfires (e.g., 2019), summertime HCHO VCD is dominated by wildfire emissions (50 %–72 %), with minor contributions from background (22 %–41 %) and biogenic VOCs (6 %–10 %). In particular, the model indicates a major contribution of wildfires from direct emissions of HCHO, instead of secondary production of HCHO from oxidation of larger VOCs. We find that the column contributed by biogenic VOC is often small and below the TROPOMI detection limit, in part due to the slow HCHO production from isoprene oxidation under low NOx conditions. This work highlights challenges for quantifying HCHO and its precursors in remote pristine regions.

2. Interannual variability of summertime formaldehyde (HCHO) vertical column density and its main drivers at northern high latitudes

The northern high latitudes (50–90° N, mostly including boreal-forest and tundra ecosystems) have been undergoing rapid climate and ecological changes over recent decades, leading to significant variations in volatile organic compounds (VOC) emissions from biogenic and biomass burning sources. Formaldehyde (HCHO) is an indicator of VOC emissions, but the interannual variability of HCHO and its main drivers over the region remains unclear. In this study, we use the GEOS-Chem chemical transport model and satellite re- trievals from the Ozone Monitoring Instrument (OMI) and the Ozone Mapping and Profiler Suite (OMPS) to examine the interannual variability of HCHO vertical column density (VCD) during the summer seasons span- ning from 2005 to 2019. Our results show that, in 2005–2019 summers, wildfires contributed 75 %–90 % of the interannual variability of HCHO VCD over Siberia, Alaska and northern Canada, while biogenic emissions and background methane oxidation account for ∼ 90 % of HCHO interannual variability over eastern Europe. We find that monthly solar-induced chlorophyll fluorescence (SIF) from the Orbiting Carbon Observatory-2 (OCO-2), an efficient proxy for plant photosynthesis, shows a good linear relationship (R = 0.6–0.7) with the modeled biogenic HCHO column (dVCD_Bio,GC) in eastern Europe, Siberia, Alaska and northern Canada, in- dicating the coupling between SIF and biogenic VOC emissions over the four domains on a monthly scale. In Alaska, Siberia and northern Canada, SIF and dVCDBio,GC both show relatively lower interannual variabilities (SIF: CV = 1 %–9 %, dVCD_Bio,GC: CV = 1 %–2 %; note that CV stands for coefficient of variation) in com- parison to wildfire-induced HCHO (CV = 8 %–13 %), suggesting that the high interannual variabilities of OMI HCHO VCD (CV = 10 %–16 %) in these domains are likely driven by wildfires instead of biogenic emissions.

Current Projects

In the new geostationary era, the atmospheric chemistry can be studied in hourly basis across North America, Europe and East Asia. Ground-based spectrometer network is a powerful tool to provide validation for geostationary satellites and models.

I am trying to investigate the reason of model bias in surface ozone, with insights from vertical resolved ozone precursor observations in lower troposphere.

The variation and driver of HCHO diurnal cycle across the contiguous US
Excessive HCHO vertical diffusion contributes significant model bias in surface ozone during urban heatwaves