Use of Photochemical Box Modeling and Field Data to Investigate the Impact of VOC and NOx on Photochemical Ozone

Abstract:

Ground level ozone is a secondary pollutant regulated by the US Environmental Protection Agency. The chemistry of photochemical O₃ formation is complex and can be driven by either VOCs or NO_x. Given the non-linear response of O₃ to VOCs and NO_x, accurate estimation of their relationship is critical to creating effective control strategies for tropospheric O₃. Through ambient measurements and modeling techniques, a clearer understanding of how ground level O₃ reacts to changes in VOC and NO_x emissions can be formed, providing a foundation for policymaking targeting O₃ pollution reduction.

Winter-time field studies have identified Oil and Gas (O&G) upstream operations to be one of the primary sources for VOC emissions in northeastern Oklahoma.¹ In this work, the impact of such emissions on regional air quality has been investigated. Employing the Framework for 0-D Atmospheric Modeling (F0AM) photochemical box model, this study aims to understand how VOC and NO_x emissions influenced photochemical O₃ formation during the winter of 2017.

A simple box model was set-up where VOCs and NO_x mixing ratios were constrained to their diurnal averages observed during the 2017 field study. The model used the SAPRC-07 chemical mechanism to simulate atmospheric reactions leading to photochemical O₃ production. While the model lacked a sophisticated treatment of atmospheric dynamic processes, atmospheric dilution was represented using a simple dilution rate coefficient.

The model was able to reproduce observed O₃ levels. The effect of NO_x on photochemical O₃ was also studied by varying the initial NO_x mixing ratios in the simulation. The results indicate that O₃ production in this case was in the transition zone in between NO_x sensitive and NO_x saturated regimes (Figure 1). VOC sensitivity, including the effect of both total VOC and VOCs from O&G emissions, were also examined. The results and their significance will be discussed in the presentation.

References

1. Ghosh, B., Impact of Changes in Oil and Gas Production Activities on Air Quality in Northeastern Oklahoma: Ambient Air Studies in 2015–2017. Sci. Technol. 2018, 52, (5), 3285-3294.