Examining TROPOMI formaldehyde to nitrogen dioxide ratios in the Lake Michigan region: implications for ozone exceedances

Abstract

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.

Vertical Evolution of Ozone Formation Sensitivity Based on Synchronous Vertical Observations of Ozone and Proxies for Its Precursors: Implications for Ozone Pollution Prevention Strategies

“…Therefore, polynomial rather than linearity is used to fit the FNR curve. This method has been also used by previous studies in China , and the United States . Through testing, the third-order polynomial showed the best performance and was chosen to determine the threshold of ozone formation sensitivity.…”

“…This method has been also used by previous studies in China 25 , 38 and the United States. 39 Through testing, the third-order polynomial showed the best performance and was chosen to determine the threshold of ozone formation sensitivity. This method assumes that the peak plateau of the fitted curve indicates the transition from a VOC-limited regime to a NO x -limited regime in the respective layer.…”

Ozone production over arid regions: insights into meteorological and chemical drivers

“…In addition, to demonstrate the more recent and higher-resolution FNR over cities, the daily FNR from TROPOMI CH 2 O and NO 2 tropospheric columns were also calculated. To define NO x -or VOC-limited regimes, [33] recommended thresholds that we used (<3.2 VOC-limited, 3.2-4.1 transition, and >4.2 NO x -limited), as successfully demonstrated in recent studies showing acceptable agreement with chemical regimes defined by surface measurements [36,37]. The use of a constant threshold for satellite-based ozone formation mechanisms across various regions of the world may increase uncertainty.…”

Inferring Near-Surface Ozone Production Regimes: Challenges with using Satellite Retrievals over the Contiguous US

“…Ozone production is considered NO x -limited when the loss of hydrogen oxide radicals (HO X ) primarily occurs due to the self-reaction of peroxy radicals, and is considered VOC-limited when the loss of HO x primarily occurs due to reactions between NO 2 and OH (Sillman, 1995(Sillman, , 1999. One of the most common methods for describing ozone production regimes (OPRs) uses the ratio of HCHO/NO 2 (formaldehyde to nitrogen dioxide ratio, or FNR) (Acdan et al, 2023;Duncan et al, 2010;Martin et al, 2004;Schroeder et al, 2017;Tonnesen & Dennis, 2000). FNR is a useful indicator of ozone sensitivity since both HCHO and NO 2 have similar lifetimes, and consequently their ratio can represent local ozone production very well.…”