The short version

In my Ph.D. work, I have focused on the effects of temporal resolution of a priori profiles on NO₂ retrievals from the Ozone Monitoring Instrument (OMI). I found that using daily profiles, instead of monthly average profiles, can have a significant effect on the retrieved NO₂, and that it is especially important in certain approaches to constrain emissions and lifetime of NO_x from space (the paper). I designed a major upgrade to the Berkeley High Resolution OMI NO₂ retrieval that incorporated daily profiles, and also took advantages of developments in the WRF-Chem model to include the effect of lightning on the NO₂ profiles (the paper). Currently, I am using the new version of the retrieval to study how NO_x emissions and chemistry changed over the continental US between 2005 and 2015.

The full story

My research focuses on the development and application of new NO₂ retrievals for the Ozone Monitoring Instrument (OMI) on board the Aura satellite. Launched in 2004, OMI had the highest spatial resolution of any satellite capable of observing NO₂ until the launch of TROPOMI (the Tropospheric Monitoring Instrument) in October 2017. OMI's spatial resolution is great enough to allow us to resolve gradients in NO₂ column density between cities and the surrounding area. The ability to observe NO₂ concentrations at this resolution provides a powerful tool to understand how NO_x (the sum of NO and NO₂) emissions and chemistry vary from city to city. At this resolution, the rate of loss of NO_x as it is advected out of cities can be measured, which combined with knowledge of the wind speed, allows us to infer the rate of chemical loss. When that is combined with measurments of the total amount of NO_x in an urban plume, the emissions rate can be inferred by a mass balance approach.

However, getting the most benefit out of satellite observations at this spatial resolution poses some unique challenges. Satellite observations of NO₂ infer the amount of NO₂ in the column extending vertically from the Earth's surface to the top of the troposphere by measuring the absorbance of sunlight passing through the atmosphere in a similar manner to any UV-vis spectrometry experiment. However, NO₂ at different altitudes in the atmosphere affects the amount of outgoing reflected sunlight differently, and so the same physical amount of NO₂ will register as a different measured amount depending on its altitude, unless that is taken into account. To do so requires the use of two models, a radiative transfer model (RTM) and chemical transport model (CTM). The latter simulates vertical distributions of NO₂ for different locations around the world, and the former accounts for how that vertical distribution of NO₂ affects the top-of-atmosphere radiances observed by the satellite.

CTMs are computationally expensive to run at high spatial resolution, so standard, worldwide retrievals of NO₂ simulate the necessary vertical distributions at resolutions greater than 1° by 1°, which is about 10x coarser than a nadir OMI pixel. A number of groups have experimented with trading global coverage for better resolution of the NO₂ profiles, and find that simulating NO₂ profiles at comparable resolutions to OMI pixels increases NO₂ retrieved over high emission areas, like cities and oil or gas extraction sands, by up to 100%. My research took the next step: if spatial resolution matters that much, what about temporal resolution? These retrievals often use monthly average profiles; how much difference does using daily profiles make?

My research found that the temporal resolution of the NO₂ profiles does have an important effect on the retrieved NO₂ column densities, especially when constraining emissions and lifetimes with those retrievals. I found that switching from monthly averaged to daily profiles would alter the retrieved NO₂ column density by as much as 40%. Further, while averaging the satellite data over multiple months reduces the importance of daily profiles in most cases, I also found that in regions with significant lightning NO_x emissions, the temporal resolution even affects a multi-month average. Daily profiles are especially important when constraining emissions and lifetime, because in that application, NO₂ observations downwind of a source are chosen, rather than using all directions in a standard average, thus the day-to-day variation in wind direction becomes an important part of the NO₂ profiles used in the retrieval process.

This work led directly to the creation of a new version of the Berkeley High Resolution OMI NO₂ retrieval that incorporated daily NO₂ profiles, along with several other improvements. Now, I am using the upgraded retrieval to study how NO_x emissions across the continental United States have changed between 2005 and 2015 and what effects that has had on NO_x chemistry. The ability to probe changes in NO_x chemistry from space offers a powerful method to predict how future emissions controls will affect the concentrations of related pollutants, such as ozone.