Satellite observations of the Earth’s atmosphere allow global measurements of various constituents in the troposphere (the bottom ~10 km of the atmosphere). These data are important for detecting anthropogenic and natural emission sources, and for quantifying the global distribution and transport of pollutants and gases important for climate and air quality.

Satellite instruments observing ultraviolet and visible radiation backscattered from the surface and atmosphere are particularly sensitive to the atmosphere near the surface of the Earth. Using spectral features measured by these instruments, we are able to derive amounts of several gases of importance to air quality, including sulfur dioxide, nitrogen dioxide, ozone, formaldehyde, and glyoxal.



TEMPO logo

Our group at the Smithsonian Astrophysical Observatory leads the development of the TEMPO satellite instrument, which is NASA's next-generation air quality mission. TEMPO will make observations of NO2, ozone, formaldehyde, SO2 and aerosols from geostationary orbit on a hourly basis during the daytime, at resolutions on the order of 2 x 4.5 km2. I work on the development of retrieval algorithms for TEMPO, which are used to derive trace gas amounts from measured spectra.


Airborne Remote Sensing

GeoTASO NO2 over Houston 20130913
Nitrogen dioxide over downtown Houston, Texas on 13 September 2013 as measured by GeoTASO during the DISCOVER-AQ campaign [Nowlan et al. 2016]
The GeoTASO airborne instrument was developed by Ball Aerospace as a test-bed instrument for the GEO-CAPE and TEMPO satellite missions. GeoTASO has participated in the DISCOVER-AQ air quality field campaigns in Texas (2013) and Colorado (2014) and the KORUS-AQ campaign in South Korea (2016). My work with GeoTASO involves the development of trace gas retrieval algorithms for the aircraft data, which are used to derive trace gas amounts from spectra in combination with radiative transfer modeling using data from chemical transport models. These GeoTASO data have produced some of the first 2-dimensional maps of urban air pollution over cities in the United States at spatial resolutions on the order of 250 x 250 m2.




Sulfur Dioxide (SO2)

power plantSO2 is emitted from anthropogenic (human) sources (~70%) and natural sources (~30%). Anthropogenic sources are mainly fossil fuel burning (coal and oil) and from the smelting of sulfur-containing ores. Natural emissions are primarily from the oxidation of dimethyl sulfide from phytoplanckton, and from volcanoes. The impacts of SO2 in the atmosphere are diverse, and range from acid rain and effects on human health to modifications of the global climate from the formation of sulfate aerosols.

SO2 can be detected from space in the ultraviolet. I developed an optimal estimation algorithm to determine SO2 from GOME-2 on the Metop-A satellite, which is also capable of determining volcanic plume height under very high SO2 loading [Nowlan et al., 2011].


Space-Based Estimates of Dry Deposition

Sulfur dioxide dry deposition estimated from OMI and EPA CASTNET network [Nowlan et al. 2014]
Dry deposition occurs when gases or aerosols are transferred to the Earth's surface (soil, water, vegetation, or other surfaces) through the motions of air. Excess nitrogen and sulfur can lead to acidification, eutrophication of ecosystems and loss of biodiversity and influence the climate by perturbing the carbon cycle. Estimates of dry deposition of NO2 and SO2 often depend entirely on models, or on point measurements of these gases. We have used measurements from the OMI satellite instrument to estimate global surface concentrations of NO2 and SO2, and inferred the global dry deposition of NO2 and SO2 using deposition rates from the GEOS-Chem chemical transport model. These data are available for download here.