Potential Errors in CMAQ NO:NO2 Ratios and Upper Tropospheric NO2 Impacting the Interpretation of TROPOMI Retrievals

Abstract

Although Chemical Transport Models (CTMs) such as the Community Multiscale Air Quality Model (CMAQ) have been used in linking observations of trace gases to emissions and developing vertical column distributions, there remain consistent biases between CTM simulations and satellite retrievals. Simulated tropospheric NO₂ vertical column densities (VCDs) are generally higher over areas with large NO_x sources when compared with retrievals, while an opposite bias is found over low NO_x regions. Artificial (i.e., numerical) dilution in the model, where emissions are mathematically dispersed uniformly within the originating CTM grid, can impact modeled NO:NO₂ ratios, while lower CTM VCD levels often observed over rural areas can be attributed to missing emission sources of NO_x or flawed horizontal/vertical transport. Potential causes of both low and high biases are assessed in this study using CMAQ and Tropospheric Monitoring Instrument (TROPOMI) NO₂ retrievals. It was found that more detailed modeling of NO_x plumes to assess the NO:NO₂ ratio in two power plant plumes can mitigate the effect of artificial computational dilution, reducing the bias and overall differences in the observed vs modeled plumes (errors reduced by 30%). Adjustments of upper tropospheric NO₂ led to overall improvements, with a reduction in CMAQ bias (−43% to −29%) and improved spatial correlation (0.81 to 0.86). This study highlights the importance of having accurate modeled NO:NO₂ ratios when comparing models to retrievals and the impact of unintentional numerical dilution.

Underestimates of upper tropospheric NO₂ coupled with artificial mixing of NO_x emissions in chemical transport models can lead to low and high biases in simulated NO₂ vertical column densities when compared to satellite retrievals.