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Discussion papers
https://doi.org/10.5194/acp-2018-1188
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1188
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 23 Jan 2019

Research article | 23 Jan 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Photochemical Production of Ozone and Emissions of NOx and CH4 in the San Joaquin Valley

Justin F. Trousdell1, Dani Caputi1, Jeanelle Smoot2, Stephen A. Conley3, and Ian C. Faloona1 Justin F. Trousdell et al.
  • 1Department of Land, Air, and Water Resources, University of California Davis, USA
  • 2Department of Chemistry, University of California Davis, USA
  • 3Scientific Aviation, Inc., Boulder, Colorado, USA

Abstract. Midday summertime flight data collected in the atmospheric boundary layer (ABL) of California's San Joaquin Valley (SJV) are used to investigate the scalar budgets of NOx, O3, and CH4 in order to quantify the individual processes that control near surface concentrations yet are difficult to constrain from surface measurements alone: most importantly, horizontal advection and entrainment mixing from above. The setting is a large mountain-valley system with a small aspect ratio where topography and persistent temperature inversions impose strong restraints on ABL ventilation. In conjunction with the observed time rate of change this airborne budgeting technique enables us to deduce net photochemical ozone production rates and emission fluxes of NOx and CH4. Measured NOx emissions from our principal flight domain were 216 (±33) metric tons/day averaged over six flights in July and August, which is nearly double the California government's NOx inventory for the surrounding three county region. We consider several possibilities for this discrepancy including the influence of wildfires, the temporal bias of the airborne sampling, instrumental interferences, and the recent hypothesis presented by Almaraz et al. (2018) of localized high soil NO emissions from intensive agricultural application of nitrogen fertilizers in the region and find the latter to be the most likely explanation. The methane emission average was 438 Gigagrams/year (±143), which exceeds an emissions inventory for the region by almost a factor of two as well. Measured ABL ozone during the six afternoon flights averaged 74 ppb (𝜎 = 9.8 ppb). The average mid-afternoon ozone rise of 2.8 ppb/h was found to be comprised of −0.8 ppb/h due to horizontal advection of lower O3 levels upwind, −2.2 ppb/h from dry deposition loss, −0.5 ppb/h from dilution by entrainment mixing, and 6.7 ppb/h net in-situ photochemical production. The O3 production rates exhibited a dependence on NO2 concentrations (r2 = 0.35), and no discernible dependence on methane concentrations (r2 ~ 0.02) which are correlated with many of the dominant VOC's in the region, suggesting that the ozone chemistry was predominantly NOx-limited on these flight days. Additionally, in order to determine the heterogeneity of the different scalars autocorrelation lengths were calculated for potential temperature (18 km), water vapor (18 km), ozone (30 km), methane (27 km), and NOx (28 km). The spatially diffuse pattern of CH4 and NOx seem to imply a preponderance of broad areal sources rather than localized emissions from cities and/or highway traffic within the SJV.

Justin F. Trousdell et al.
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Short summary
Midday summertime flight data collected in the atmospheric boundary layer of California's San Joaquin Valley are used to investigate NOx, O3, and CH4 in order to quantify the individual processes that influence near surface concentrations most importantly, horizontal advection and vertical mixing from above. This enables us to deduce net photochemical O3 production rates and emissions of NOx and CH4. Budget breakdowns and their results are important for local air quality workers and modelers.
Midday summertime flight data collected in the atmospheric boundary layer of California's San...
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