Research

Keywords: Volatile organic compounds (VOCs); Ozone; Source attribution of air pollutants; GEOS-Chem; Proton Transfer Reaction - Mass Spectrometry (PTR-MS); Long-term tall tower observations

Research tools: field measurements, satellite observations, chemical transport models, inverse modeling

Current Projects

Factors controlling global tropospheric ozone

      Ozone is central to our understanding of tropospheric oxidant chemistry through its driving of radical cycles. In surface air, ozone is toxic to human and vegetation. Ozone is also the 3rd most important greenhouse gas in the troposphere. Yet our understanding of factors determining its spatial distribution and long-term trend is still poor. As a postdoc at Harvard, I use the GEOS-Chem chemical transport model as a platform to test our current knowledge of key factors controlling global tropospheric ozone, focusing on the following three projects:

1) Improving our understanding of global tropospheric ozone using integrated recent advancement of knowledge in isoprene chemistry, tropospheric halogen chemistry, lightning NOx source, and deep convection. This research is focused on evaluating the most recent model simulation integrating the above major model developments, against in-situ observations from ozonesonde, aircraft, and satellite to diagnose and correct model weaknesses.

2) Modeling global ozone concentration at very-high-resolution to improve understandings of intercontinental transport of pollution and ozone climate forcing. This project aims to better understand the coupled effects of transport and chemical evolution on tropospheric ozone simulation over an unprecedented range of scales. For this work, we use the GEOS-Chem as a chemical module in NASA GEOS model at ~12×12 km2 resolution.

3) Investigating factors controlling the variability and trend of tropospheric ozone and OH radical over the last 30 years using the NASA Global Modeling Initiative (GMI) and GEOS-Chem chemical transport models.

Previous projects

Constraints on the sources and impacts of VOCs over North America from tall tower measurements

      Volatile organic compounds (VOCs) are air toxics, and play a key role in the atmosphere as precursors of ozone and secondary organic aerosol (SOA). However, their seasonality and sources are highly uncertain. My dissertation presented the first-ever in-situ tall tower measurements of VOC concentrations, spanning three full years, providing new constraints on seasonal and long-term controls on VOC sources and their atmospheric effects. Key findings are outlined below:

a) Benzene, toluene, ethylbenzene, and xylenes (BTEX, or aromatic VOCs)

     

      Aromatic compounds are hazardous air pollutants (HAPs), as they are known to cause serious health effects (e.g., benzene is carcinogenic to humans). They are also important anthropogenic precursors of secondary organic aerosol (SOA). An inverse analysis of the tall tower measurements suggests that: i) the US EPA's NEI08 inventory overestimates toluene fluxes by threefold, reflecting biased non-road emissions; ii) NEI08 is accurate about total annual emissions of benzene and C8 aromatics, but with strong seasonal biases; and iii) nearly half of US benzene wintertime abundance is due to sources outside North America. [Hu et al., 2015a][JGR-Atmospheres cover]

b) Isoprene and its oxidation products

     

      Isoprene is the most important biogenic VOC. We find that PTR-MS m/z 69 signals, which are conventionally considered as biogenic isoprene, are subject to sizeable anthropogenic interferences (~25% during summer daytime). Model-measurement comparisons imply that regional isoprene emissions are underestimated in the US Upper Midwest by the latest MEGAN biogenic emission inventory, reflecting heterogeneous land cover in this ecological transition region. Isoprene emissions play a key role in a strong seasonal shift between VOC-limited chemistry during spring and fall and NOx-limited chemistry during summer. [Hu et al., 2015b]

c) Acetone

     

      Acetone is a major precursor of PAN (Peroxyacyl nitrates, as a NOx reservoir) and a significant source of HOx radicals especially in the upper troposphere. An inverse analysis of the tall tower observations implies that the resulting North American acetone source of 11 Tg/y, including both primary emission (5.5 Tg/y) and secondary production (5.5 Tg/y), with roughly equal contributions from anthropogenic and biogenic sources, is nearly as large as the total continental VOC source from fossil fuel combustion. [Hu et al., 2013]

d) Methanol

   

      Methanol is the most abundant non-methane VOCs in the atmosphere and an important precursor of CO (carbon monoxide) and HCHO (formaldehyde). We find that biogenic emissions from vegetation account for ~90% of the ambient methanol level during summer. The seasonal cycle for methanol peaks one month too late in current models, reflecting an underestimated emission from younger versus older leaves. This biased seasonality means that the photochemical role for methanol early in the growing season is presently underestimated. [Hu et al., 2011]

      More details about the KCMP tall tower I used for the above project can be found here. My Ph.D dissertation summarizing these findings can be found here

U.S. background ozone, interstate transport, versus locally produced ozone at the state-level: a case study of Minnesota 

      As the federal air quality standards get stricter (current ozone standard at 70 ppb) and oil and gas production in many states rapidly expands, the impact of ozone and its precursors transported from neighboring states should be considered when designing ozone control strategies at the state-level. With a research award from the UMN Consortium on Law and Values in Health, Environment and the Life Sciences, we investigated the impact of local versus transported pollutants in Minnesota, using a combination of regulatory monitoring data and model experiments. This is the first study to quantitatively estimate the magnitude of transported ozone in the state Minnesota. Our study suggests that emission sources outside the state exert a significant influence on Minnesota’s air quality. Local sources and cross-state transport each contribute ~10-25% of the simulated summer ozone. Controlling only local pollution sources in Minnesota will not be sufficient to attain the air quality goal set in the future. [Luan, Hu, and Wells, 2014]