The top row shows OMI HCHO/NO2 for 3 world regions in 2005, while the bottom row shows the same ratio for 2015. In the U.S. and Europe, major decreases in NOx emissions have caused this ratio to increase, indicating that ozone production is more sensitive now vs. a decade ago to further reductions in NOx emissions. The changes in the ratios over East Asia indicate a patchwork of emission changes and concomitant changes in ozone production sensitivities.
Currently, Aura overpasses once per day in the early afternoon, giving only a "snapshot" of ozone production sensitivity. The upcoming NASA TEMPO mission, which will be in geosynchronous orbit, will allow the hourly evolution of ozone production sensitivity to be studied.
Instruments onboard NOAA's Joint Polar Satellite System (JPSS-2 and -3) will continue the O3 record of MLS, but there are currently no sensors planned to continue the MLS HCl measurements once Aura terminates. Aura was launched in 2004.
References: Jin X., A.M. Fiore, L.T. Murray, L.C. Valin, L.N. Lamsal, B.N. Duncan, K.F. Boersma, I. De Smedt, G. Gonzalez Abad, K. Chance, and G.S. Tonnesen (2017). Evaluating a space-based indicator of surface ozone-NOx-VOC sensitivity over mid-latitude source regions and application to decadal trends. J. Geophys. Res., 122. https://doi.org/10.1002/2017JD026720.
Data Sources: The regime classification uses the ratio of monthly average OMI formaldehyde (HCHO) to nitrogen dioxide (NO2). The HCHO product is the Level-3 HCHO product developed by the the Belgian Institute for Space Aeronomy (BIRA-IASB, version 14). The NO2 product is the Level-3 NASA standard operational NO2 Product (version 2.1) developed at NASA/Goddard Space Flight Center. The HCHO/NO2 values marking the boundaries of the transition regime are derived from a global chemical transport model (GEOS-Chem) that simulates the relationship between the HCHO/NO2 ratio in the model surface layer and the surface O3 response to precursor emission reductions (i.e., two simulations relative to a base case: one in which nitrogen oxide (NOx) emissions are decreased by 20% and one in which non-methane volatile organic compound (NMVOC) emissions are decreased by 20% globally). The surface HCHO/NO2 values are then adjusted further based on the modeled column-to-surface relationships and the average differences between the modeled and satellite-derived columns of HCHO and NO2.