Examining TROPOMI formaldehyde to nitrogen dioxide ratios in the Lake Michigan region: implications for ozone exceedances
Authors: Acdan Juanito Jerrold Mariano , Pierce Robert Bradley , Dickens Angela F. , Adelman Zachariah , Nergui Tsengel 
Published: July 2023

Abstract. Surface-level ozone (O3) is a secondary air pollutant that has adverse effects on human health. In the troposphere, O3 is
produced in complex cycles of photochemical reactions involving nitrogen oxides (NOx) and volatile organic compounds
(VOCs). Determining if O3 levels will be decreased by lowering NOx emissions
(“NOx-sensitive”), VOC emissions (“VOC-sensitive”), or both (“the transition zone”) can be done by using the
formaldehyde (HCHO; a VOC species) to nitrogen dioxide (NO2; a component of NOx) concentration ratio
(HCHO/NO2; “FNR”). Generally, lower FNR values indicate VOC sensitivity, while higher values indicate
NOx sensitivity. Despite being a highly populated region with coastal O3 air quality issues, the Lake Michigan region in
the United States, including the Chicago, Illinois, metropolitan area (CMA), remains relatively understudied, especially from the satellite
perspective. In this work, we present the first study that utilizes TROPOspheric Monitoring Instrument (TROPOMI) satellite data over the Lake
Michigan region from 2019–2021 to assess changes in O3 precursor levels and the inferred O3 chemistry sensitivity between
(1) O3 season days and CMA O3 exceedance days and (2) weekdays and weekends. Higher NO2 vertical column densities
(VCDs), HCHO VCDs, and FNR values are seen throughout the study domain on exceedance days, indicating generally more
NOx-sensitive O3 chemistry. The largest change occurs in the areal extent of the transition zone, which decreases
by 40 % during exceedance days. Major urban cores in the domain (e.g., Chicago, Illinois; Gary, Indiana; and Milwaukee, Wisconsin) remain
VOC-sensitive on exceedance days as the higher NO2 VCDs in these areas counterbalance the regionally higher HCHO
VCDs. Utilizing 10 m wind analysis data, we show that the lake breeze circulation is stronger on exceedance days. The strengthening of the
lake breeze causes stronger convergence of the wind field along the southwestern Lake Michigan coastline, which can concentrate
NO2 emissions originating in this area. This finding provides a possible explanation for the higher TROPOMI NO2 VCDs over the
urban core of Chicago on exceedance days. Investigation of 2 m air temperature analysis data reveals that temperatures are higher on
exceedance days, which explains the stronger lake breeze circulation and provides a possible cause for the higher TROPOMI HCHO VCDs over the
entire region (due to increased temperature-dependent biogenic VOC emissions). Comparing weekdays and weekends, higher FNR values throughout
much of the region indicate increasingly NOx-sensitive O3 chemistry on weekends. These changes are driven by lower
NO2 VCDs in urban areas, particularly in Chicago, and higher HCHO VCDs in the southern part of the domain on weekends. Overall, our
analyses suggest that VOC emissions controls in major urban areas and NOx emissions controls throughout the entire domain
are necessary to decrease O3 levels in the Lake Michigan region.