My main research focus is developing the new version of the GGG retrieval used by the Total Carbon Column Observing Network (TCCON). The new version includes major updates to the spectroscopy and prior profiles, aimed at reducing known airmass dependent biases and errors from the assumed gradient of CO2 and other greenhouse gases in the stratosphere, respectively. We have also invested significant effort in reducing site-to-site biases, especially in the post-processing corrections, which will improve estimates of greenhouse gas fluxes made using multiple TCCON sites together.
One feature of the new GGG version is the addition of two new CO2 column measurements with differing vertical sensitivity. Since one is more sensitive to the lower atmosphere and the other more sensitive to the upper atmosphere, I will investigate how we can use these two new measurements along with the traditional TCCON XCO2 column to differentiate changes in boundary layer CO2 from those in the free troposphere and stratosphere. A boundary layer partial column measurement would be more sensitive to CO2 surface fluxes, and so could help provide important constraints on, for example, photosynthetic activity.
One way that we can monitor photosynthetic uptake is through diurnal cycles of CO2. During the growing season, there is a clear multi-ppm drawdown in column CO2 over the course of a day. The drawdown in a boundary layer partial column will be stronger, and so we should be able to detect smaller changes in photosynthetic uptake. In addition to exploring diurnal cycles measured from TCCON, I am interested in whether we can use measurements from the third Orbiting Carbon Observatory (OCO-3). Carried on the ISS, OCO-3's precessing orbit allows it to sample different times of day over about two months. Work I presented at the AGU 2019 Fall Meeting indicates that we should be able to estimate a diurnal cycle from OCO-3 data given 2 to 3 years of data. OCO-3 has been taking science data since August 2019, so I plan on testing this later in 2021.
As part of my Ph.D., I showed that we can use measurements from the OMI satellite instrument to infer changes in NOx lifetime on multi-year time scales. The newer TROPOMI instrument, launched in 2017, offers the chance to look at month-to-month changes in NOx lifetime. With the dramatic reductions in NOx emissions during 2020 due to the COVID-19 pandemic, we have a chance to learn how urban NOx chemistry will change as we move to lower and lower NOx emission scenarios.
We can classify what chemical regime dominates chemistry in a given location by looking at how NOx lifetime changes with NOx concentration. When lifetime decreases with concentration, we are in a NOx-limited regime, and when lifetime increases with concentration, we are in either a VOCx-limited regime or (if NOx concentrations are very small) an alkyl nitrate dominated one. Knowing this helps us plan how to reduce ozone concentrations; in a NOx-limited regime, reducing NOx is most effective at reducing ozone, and in a VOC-limited regime, reducing VOCs is. (If we make it to an alkyl nitrate dominated regime, ozone production will already be very small.)
In some very preliminary work, I found that NOx lifetime increased during the COVID-19 pandemic for many cities around the world. That suggests that as we find ways to make pandemic-like emissions reductions permanent (though without such massive personal and economic dislocation), these cities will see reductions in their ozone levels.