24 Feb 2023
 | 24 Feb 2023
Status: this preprint is currently under review for the journal ACP.

Direct observations of NOx emissions over the San Joaquin Valley using airborne flux measurements during RECAP-CA 2021 field campaign

Qindan Zhu, Bryan Place, Eva Y. Pfannerstill, Sha Tong, Huanxin Zhang, Jun Wang, Clara M. Nussbaumer, Paul Wooldridge, Benjamin C. Schulze, Caleb Arata, Anthony Bucholtz, John H. Seinfeld, Allen H. Goldstein, and Ronald C. Cohen

Abstract. Nitrogen oxides (NOx) are principle components of air pollution and serve as important ozone precursors. As the San Joaquin Valley (SJV) experiences some of the worst air quality in the United States, reducing NOx emissions is a pressing need, yet quantifying current emissions is complicated due to a mixture of mobile and agriculture sources. We performed airborne eddy covariance flux measurements during the Re-Evaluating the Chemistry of Air Pollutants in CAlifornia (RECAP-CA) field campaign in June 2021. Combining footprint calculations and land cover statistics, we disaggregate the observed fluxes into component fluxes characterized by three different land cover types. On average we find emissions of 2.95 mg m-2 h-1 over highways, 1.24 mg m-2 h-1 over urban areas and 0.79 mg m-2 h-1 over croplands. The calculated NOx emissions using flux observations are utilized to evaluate anthropogenic emission inventories and soil NOx emission schemes. We show that two anthropogenic inventories for mobile sources, EMFAC (EMssion FACtor) and FIVE (Fuel-based Inventory for Vehicle Emissions), yield similar agreement with emissions derived from measured fluxes over urban regions with 24 % and 22 % low bias, respectively. Three soil NOx schemes, including MEGAN v3 (Model of Emissions of Gases and Aerosols from Nature), BEIS v3.14 (Biogenic Emission Inventory System) and BDISNP (Berkeley Dalhousie Iowa Soil NO Parameterization), show substantial underestimates over the study domain. Compared to the cultivated soil NOx emissions derived from measured fluxes, MEGAN and BEIS are lower by more than one order of magnitude and BDISNP is lower by a factor of 2.7. Despite the low bias, observed soil NOx emissions and BDISNP present a similar spatial pattern as well as temperature dependence. We conclude that soil NOx is a key feature of the NOx emissions in the SJV and that a state-of-the-science model of these emissions is needed to simulate emissions for modeling air quality in the region.

Qindan Zhu et al.

Status: open (until 07 Apr 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Qindan Zhu et al.

Qindan Zhu et al.


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Short summary
Nitrogen oxides (NOx) is a hazardous air pollutant, and it is the precursor of short-lived climate forcers like tropospheric ozone and aerosol particles. While NOx emission from transportation has been strictly regulated, soil NOx emission is overlooked. We use the airborne flux measurements to observe NOx emissions from highways, urban and cultivated soil land cover types. We show a non-negligible soil NOx emission, which is significantly underestimated in current model simulations.