Evaluating a Space-Based Indicator of Surface Ozone-NOx-VOC Sensitivity Over Midlatitude Source Regions and Application to Decadal Trends
Citation:
Jin, X., A. Fiore, L. Murray, L. Valin, L. Lamsal, B. Duncan, K. Boersma, I. De Smedt, G. Gonzalez Abad, K. Chance, AND G. Tonnesen. Evaluating a Space-Based Indicator of Surface Ozone-NOx-VOC Sensitivity Over Midlatitude Source Regions and Application to Decadal Trends. JOURNAL OF GEOPHYSICAL RESEARCH: ATMOSPHERES. American Geophysical Union, Washington, DC, 122(19):10439-10461, (2017).
Impact/Purpose:
Surface ozone (O3), the main component of photochemical smog, has adverse effects on public health [Kampa and Castanas, 2008], agriculture [Van Dingenen et al., 2009] and ecosystems [Yue and Unger, 2014]. The global premature mortality rate due to O3 pollution is estimated at 0.8 million/year [Lelieveld et al., 2013]. Surface O3 formation in urban areas is non linearly dependent on the availability of two classes of O3 precursors: oxides of nitrogen (NOx) and volatile organic compounds (VOCs). That is, depending on local relative abundances of NOx to VOCs, O3 formation can be mitigated by reducing NOx emissions (NOx-limited regime), by reducing VOC emissions (NOx-saturated or VOC-limited regime), or both (transitional regime). At regional and global scales, O3 production is largely NOx-limited, though urban areas with high NOx emissions are frequently NOx-saturated.
Description:
Determining effective strategies for mitigating surface ozone (O3) pollution requires knowledge of the relative ambient concentrations of its precursors, NOx, and VOCs. The space-based tropospheric column ratio of formaldehyde to NO2 (FNR) has been used as an indicator to identify NOx-limited versus NOx-saturated O3 formation regimes. Quantitative use of this indicator ratio is subject to three major uncertainties: (1) the split between NOx-limited and NOx-saturated conditions may shift in space and time, (2) the ratio of the vertically integrated column may not represent the near-surface environment, and (3) satellite products contain errors. We use the GEOS-Chem global chemical transport model to evaluate the quantitative utility of FNR observed from the Ozone Monitoring Instrument over three northern midlatitude source regions. We find that FNR in the model surface layer is a robust predictor of the simulated near-surface O3 production regime. Extending this surface-based predictor to a column-based FNR requires accounting for differences in the HCHO and NO2 vertical profiles. We compare four combinations of two OMI HCHO and NO2 retrievals with modeled FNR. The spatial and temporal correlations between the modeled and satellite-derived FNR vary with the choice of NO2 product, while the mean offset depends on the choice of HCHO product. Space-based FNR indicates that the spring transition to NOx-limited regimes has shifted at least a month earlier over major cities (e.g., New York, London, and Seoul) between 2005 and 2015. This increase in NOx sensitivity implies that NOx emission controls will improve O3 air quality more now than it would have a decade ago.
