OMI HCHO/NO2 ratio, August 2006. Credit: Bryan Duncan.
Using satellite data to infer sensitivity of ozone production chemistry to NOx versus VOC
Information on nitrogen oxides (NOx) and volatile organic compounds (VOCs) from satellite instruments can be used to infer trends in the sensitivity of ozone production to these precursors as both are necessary ingredients for the formation of unhealthy levels of surface ozone. However, one ingredient is typically limiting the production of ozone, so that reducing emissions of the other has little influence on net ozone formation. An example of this is given by Duncan et al. The figure above shows that ozone production in most of the U.S. is limited by the availability of NOx, particularly in the eastern U.S., which has lots of natural VOC emissions from trees. Some of these natural VOCs oxidize to HCHO, which we can measure from space. Over the last decade, NO2 levels have decreased by 20-50% in large U.S. cities, strengthening the NOx limitation of ozone production.
Sillman (1995) developed “indicator ratios” of observed atmospheric constituents (e.g., peroxides and nitric acid) to diagnose the chemical sensitivity of ozone formation. Martin et al. (2004) demonstrated that the ratio of formaldehyde and NO2 column densities can be an effective indicator ratio. (Satellite instruments measure the total amount between the surface and space; for NO2, the amount in the stratosphere is subtracted from the total to obtain the tropospheric column amount, used here).
Duncan et al. (2010) used the ratio of OMI formaldehyde to NO2 data to show that the chemical sensitivity of ozone formation is becoming more sensitive to NOx levels in U.S. cities, even cities that were typically considered to be more sensitive to VOCs (e.g., Los Angeles), as a result of substantial reductions in NOx emissions during the 2000s. Research on this topic should be revisited, considering the recent improvements in both the formaldehyde and NO2 datasets, the now decadal time records of the data, and the limitations of previous studies.
Below is a recent update taking advantage of the decade-long record of OMI observations. Analysis of HCHO/NO2 reveals a transition from VOC-limited to NOx-limited regimes during at least some summer months in some major eastern U.S. cities. The figure below highlights the region around New York City as falling in the transitional regime during the warm season (May through September average) in 2005 but NOx-limited in 2015. Evidence for additional NOx limitation is evident throughout the broader region in 2015 versus 2005.
The following graphs zoom in to the urban regions of New York City and Chicago to show monthly average HCHO/NO2 (solid line for June-July-August) from 2005 to 2015. The average HCHO/NO2 has increased by about 50% in the past decade. Ozone sensitivity was in a transitional regime (HCHO/NO2 between 1 and 2) throughout the summer of 2005, but NOx-limited (HCHO/NO2 > 2) since 2010 for most summer months in NYC and all summer months in Chicago.
Download or View NO2 Data Products
NO2 products from the European TEMIS project (KNMI, Netherlands)
NASA OMI NO2 L3 gridded product
NO2 trends (U.S. and Global Cities)
Download or View HCHO Data Products:
HCHO products from European TEMIS project (KNMI, Netherlands)
References
Duncan, B., Y. Yoshida, J. Olson, S. Sillman, C. Retscher, R. Martin, L. Lamsal, Y. Hu, K. Pickering, C. Retscher, D. Allen, and J. Crawford, 2010: Application of OMI observations to a space-based indicator of NOx and VOC controls on surface ozone formation, Atmos. Environ., 44, 2213-2223, doi:10.1016/j.atmosenv.2010.03.010.
Martin, R.V., A.M. Fiore, and A. Van Donkelaar, 2004: Space-based diagnosis of surface ozone sensitivity to anthropogenic emissions, Geophys. Res. Lett., 31, L06120, doi:10.1029/2004GL019416.
Sillman, S. (1995), The use of NOy, H2O2, and HNO3 as indicators for ozone-NOx -hydrocarbon sensitivity in urban locations, J. Geophys. Res., 100(D7), 14175–14188, doi:10.1029/94JD02953.
Contributors: Gabriele Pfister, Bryan Duncan, Xiaomeng Jin, Russ Dickerson, Tracey Holloway, Arlene Fiore (AQAST)
