Measurement report: Diurnal variability in NO<sub>2</sub> and HCHO lower-tropospheric vertical profiles in southeastern Los Angeles
<p>Ground-level ozone in excess of the United States National Ambient Air Quality Standards remains a prevalent issue across Southern California, particularly in the summer months. To improve our understanding of the vertical distribution of ozone precursors in Southern California, we used gro...
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| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2025-07-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/25/7777/2025/acp-25-7777-2025.pdf |
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| Summary: | <p>Ground-level ozone in excess of the United States National Ambient Air Quality Standards remains a prevalent issue across Southern California, particularly in the summer months. To improve our understanding of the vertical distribution of ozone precursors in Southern California, we used ground-based multi-axis differential absorption spectroscopy (MAX-DOAS) measurements in Whittier, California, to simultaneously retrieve both near-surface mole fractions and vertical column densities (VCDs) of both <span class="inline-formula">NO<sub>2</sub></span> and HCHO. Ratios of HCHO to <span class="inline-formula">NO<sub>2</sub></span>, commonly referred to as FNR, derived from satellite-based measurements are used to diagnose ozone production chemistry over regions without consistent surface-based measurements. While VCDs of <span class="inline-formula">NO<sub>2</sub></span> are well correlated with TROPOMI observations over the study period (<span class="inline-formula"><i>R</i>=0.73</span>), HCHO VCDs and FNRs derived from MAX-DOAS observations are less well correlated (<span class="inline-formula"><i>R</i>=0.48</span> and 0.59, respectively). These observations also showed differing diurnal cycles between near-surface mixing ratios and VCDs due to variability in the vertical profile; this diurnal behavior will be increasingly critical to understand given the ongoing shift from Sun-synchronous to geostationary satellite observations. Using ground-based measurements, we determined FNRs using both surface mole fractions and VCDs, finding that FNRs derived from surface mole fractions are generally lower than those derived from column-based measurements. Evaluating the ozone exceedance probability as a function of FNR for both quantities suggests that the transition from volatile organic compound (VOC)-limited to NO<span class="inline-formula"><sub><i>x</i></sub></span>-limited regimes may begin at lower FNR values than those derived from satellite-based measurements in East Los Angeles. We find that these differences in FNRs derived from ground-based and satellite-based measurements are driven by variability in the vertical distribution of HCHO. These impacts are most pronounced in late afternoon, when ozone exceedances are most prevalent.</p> |
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| ISSN: | 1680-7316 1680-7324 |