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Modeling greenhouse gas emissions from atmospheric observations with Lei Hu

Lei Hu
Lei Hu

In celebration of Women’s History Month, this article continues a series of interviews with NOAA Research employees and scientists. NOAA Global Monitoring Laboratory science communications specialist Xinyi Zeng conducted this interview.

Lei Hu is a CIRES research scientist working at the NOAA Global Monitoring Laboratory’s Halocarbon and other Trace Species Division and Carbon Cycle and Greenhouse Gas Division in Boulder, Colorado.

In her research, Lei uses atmospheric observations around the world and inverse modeling to quantify regional emissions and removals of greenhouse gases. These observation-based emission estimates help cross-check human-caused emission estimates based on inventory and also advance understanding of how natural ecosystems take up greenhouse gases from the atmosphere to mitigate the greenhouse effect on climate.

Additionally, part of Lei’s research focuses on ozone-destroying substances and their substitutes, which are also potent greenhouse gases. Her work on ozone-destroying substances not only tracks potential violations of the Montreal Protocol on Substances that Deplete the Ozone Layer but also examines climate implications of the transition from ozone-destroying substances to their substitutes.

Her path as a scientist started as a Chemistry major at the Ocean University of China. She then moved to the U.S. for her Ph.D. in Oceanography at Texas A&M University. She started her career at NOAA 10 years ago as a postdoc at the NOAA Global Monitoring Laboratory and has been working as a CIRES research scientist since her post-doc. Some of her previous work was funded by NOAA Climate Program Office

Our conversation follows.

What attracts you to your current field of study?

At first, I just thought it would be cool to work as a scientist in a research institution. I didn’t really know which research area I should go into when I was in college.

My undergraduate thesis advisor studied nitrous oxide, a greenhouse gas, and looked at how much of it is emitted from the ocean. In her marine biogeochemistry lab, I first learned about trace gases. Building on my undergraduate experience, I applied for Ph.D. programs in the U.S. and continue to study trace gas emissions from the ocean at Texas A&M University.

In the Earth’s atmosphere, any gases other than nitrogen, oxygen, and argon are trace gases. They only account for less than 0.1 percent of the air in the atmosphere but have substantial impacts on our living environment. These include what we commonly know as greenhouse gases and ozone-depleting substances. During my Ph.D. studies, my research focused on the marine emissions of ozone-depleting substances – methyl bromide and methyl chloride.

Another big portion of my Ph.D. research was to measure how much methane was emitted from deepwater gas hydrates from the Deepwater Horizon Oil Spill and natural gas hydrate seeps. This experience led me to become more interested in research on greenhouse gases.

How does your current research connect with your Ph.D. work and how do they differ from each other?

Quantifying trace gas emissions and their response to climate change from a large landscape is extremely important but quite challenging from local, short-term trace gas measurements. My Ph.D. work looks at emissions using short-term trace gas measurements made on a ship. Each measurement only represents emissions on a scale of less than a few hundred square meters. Understanding emissions from a larger landscape generally took hundreds of thousands of measurements.  Even with that, it is hard to characterize the temporal variability and trend of emissions very well.

What I do now still focuses on emissions but looks at a much larger landscape, generally on a national or a continental scale. This requires not only atmospheric measurements but also inverse modeling that derive emissions from atmospheric measurements. With an interest in studying larger-scale emission estimates, I came to the NOAA Global Monitoring Laboratory with the goal to learn more about inverse models.

Another difference is the trace gas I study. My Ph.D. work focuses more on ozone-destroying chemicals and now my work has shifted more towards greenhouse gases and the carbon cycle. Climate is a really important topic and I believe that it is very important to understand greenhouse gas emissions.

What were some of the challenges you faced in your early research career?

When I first started my post-doc, I had to learn quickly, but thoroughly, how inverse modeling works. Also, there wasn’t any existing common code that I could work with. So, I had to translate my mathematical understanding of inverse modeling into computer code.  All of this needed to start from scratch. 

After I developed the code, I had to exhaustively test the code to ensure the code worked properly and the algorithms took full advantage of the atmospheric measurements we had. I did gain a lot of support from other senior scientists in the lab who helped answer my questions, but that was still a challenging process.

It took me about three years to develop the model and that restrained me from being very productive in publication during my post-doc. However, now that I have the model developed, I have a tool ready to explore the data we collected around the globe and explore scientific questions. This is very rewarding.

What research areas are you looking to focus on in the near future?

I want to focus on three main areas.

The first thing is to provide timely observation-based emission estimates of greenhouse gases over the U.S. In the U.S., greenhouse gas emissions from human activities are estimated in the inventory each year based on activities reported to the U.S. Environmental Protection Agency (EPA). Observation-based estimates of emissions offer an objective evaluation of the values reported by the EPA and can help gauge the effectiveness of emission reduction practices.

Second, I want to better understand how plant communities respond to climate change. In my previous work, the combination of carbon dioxide and carbonyl sulfide revealed critical insights into how the Arctic plant communities respond to climate change. I want to explore the use of both trace gases in other regions.

Lastly, I am interested in incorporating satellite data into our existing regional modeling framework. Our current model uses only in-situ measurements of trace gases. However, in-situ measurements are sparse by nature, while satellite data, although containing some biases and lower signals of surface emissions, do have higher sampling density and a broader sampling coverage. Combining in situ with satellite observations could potentially help us better quantify greenhouse gas emissions.

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