Harvard-China Project Q&A: Shaojie Song
“The Three Elements of Atmospheric Chemistry (Observations, Lab Experiments, and Modeling): Using an Organic Sulfur Compound as an Example”
Shaojie Song, Harvard-China Project Research Associate, is an atmospheric chemist who studies air quality in China and the chemistry of air pollution. He recently presented a Zoom talk (watch the talk here) on the ins and outs of atmospheric chemistry, differentiating the three components of research—observations, lab experiments, and modeling—and how they together help to build knowledge about the causes of air pollution and prospective solutions. He used his own work on the organic sulfur compound hydroxymethanesulfonate (HMS) to illustrate these research processes. The Harvard-China Project caught up with him afterwards to expand on his talk.
Harvard-China Project: In the wintertime over Beijing, there are alternating flows of cold and dry air from the North and warm and humid air from the South. Can you briefly explain how this impacts the chemistry and PM2.5 concentrations?
Shaojie Song: The wintertime weather patterns in the North China Plain are partly influenced by the Siberian High Pressure system. Northerly winds coming in from Siberia bring clean air to the North China Plain (Beijing is situated at its northern tip), leading to low PM2.5 concentrations. The weaker northerly winds or even southerly winds are usually associated with stable atmospheric conditions, allowing air pollutants and water vapor to accumulate within a few hundred meters above the ground. Strong human activities in the North China Plain emit a large amount of PM2.5 and reactive gases such as volatile organic compounds, nitrogen oxides and sulfur dioxide. The humid conditions facilitate the chemical reactions that produce PM2.5 from these reactive gases. Hydroxymethanesulfonate (HMS), produced by the aqueous reaction between formaldehyde and sulfur dioxide, is an example. In addition, higher PM2.5 concentrations may make the atmosphere more stable, which in turn increases the levels of PM2.5 near surface.
HCP: Shaojie, you explored the three components of studying Atmospheric Chemistry, including modeling, lab experiments, and observations. Which aspect are you personally most involved in, and how do these components work together to inform your overall findings when studying HMS?
SS: I have been involved in both observations and modeling of this organic sulfur compound called HMS. At the beginning, I had a hypothesis that the atmospheric conditions in northern China winter should favor the existence of HMS in PM2.5. This hypothesis was largely based on the knowledge learned from lab experiments and field observations in the 1980s when scientists studied acid rain.
When I decided to explore the idea of HMS in PM2.5, I believed that the most critical issue was to find observational evidence. Observation is the most fundamental component of atmospheric chemistry research because real-atmosphere processes are often too complex for models and lab experiments to account for all the key factors. HMS is not routinely measured in PM2.5 and methods to analyze it need to be carefully designed. As such research resources were not available to me, I used a special method to estimate HMS from the existing mass spectrometry data collected in Beijing by our research partners in China. This special method was indirect and based on several assumptions, but it seemed to give HMS results that were reasonable and comparable with direct observations published later by another research group.
After the confirmation of HMS observations in Beijing, it was a good time to use a model to explore its big picture such as the spatial and seasonal distributions. I have implemented the physical and chemical processes related to HMS into a global model named GEOS-Chem. The principle of modeling is to use the best available data from lab experiments. Thus, I critically reviewed the existing lab experiments and showed that the most commonly used data may not be the best available. The model simulations driven by these best-available data from lab experiments generally reproduce the available HMS observations in Beijing and other regions. Therefore, my research on HMS basically closed the loop — a reconciliation of observations, lab experiments, and modeling, at least for the currently available data.
HCP: You mentioned that HMS is a potentially important contributor to China’s haze that was overlooked until recently. Can you talk a bit about how a pollutant like this could go unrecognized despite so much research on China’s air pollution?
SS: I think there are three reasons. First, as I mentioned earlier, HMS is difficult to detect by common observational methods. One must know its possible existence in advance and design specific analytical methods. Second, the concentrations of HMS previously observed in other regions of the world have been very low. It was not apparent that the levels of HMS might be much higher in Beijing winter than seen in other locations. Third, HMS is produced by the reaction between formaldehyde and sulfur dioxide, but not much attention has been paid to formaldehyde in winter because of conventional understanding of its sources. The general understanding has been that the concentration of formaldehyde in winter is relatively low, but the amount of wintertime emissions, such as from the cold-starts of vehicle engines, is now being reconsidered.
HCP: You are trying to explore not only the finite data of HMS, but also the big picture of HMS and smog. What types of questions about HMS and smog are you hoping to answer through your current and future research?
SS: Based on my research and previous studies from other groups, we have learned that HMS serves as a tracer for aqueous and dark (without sunlight) chemistry. Therefore, HMS can tell us more details about the complex chemistry occurring in smog. One important scientific question for understanding China’s smog is the possibility of organic particles being produced by aqueous chemistry. Studies of HMS can offer help in answering this question. In addition, HMS is only one chemical compound in the family of hydroxyalkylsulfonates (HAS). Future studies should investigate the potential role of other HAS species.
RESEARCH CITED: Song, S., Ma, T., Zhang, Y., Shen, L., Liu, P., Li, K., Zhai, S., Zheng, H., Gao, M., Duan, F., He, K., and McElroy, M. B. Submitted. "Global modeling of heterogeneous hydroxymethanesulfonate chemistry." Atmos. Chem. Phys. Discuss. Manuscript available at ACPD for discussion.
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