Atmospheric Chemistry and Physics | 2019
Photochemical production of ozone and emissions of NO x and CH 4 in the San Joaquin Valley
Abstract
Abstract. Midday summertime flight data collected in the\natmospheric boundary layer (ABL) of California s San Joaquin Valley (SJV)\nare used to investigate the scalar budgets of NOx , O3 , and\n CH4 , in order to quantify the individual processes that control near-surface concentrations, yet are difficult to constrain from surface\nmeasurements alone: these include, most importantly, horizontal advection and\nentrainment mixing from above. The setting is a large mountain–valley system\nwith a small aspect ratio, where topography and persistent temperature\ninversions impose strong restraints on ABL ventilation. In conjunction with\nthe observed time rates of change this airborne budgeting technique enables\nus to deduce net photochemical ozone production rates and emission fluxes of\n NOx and CH4 . Estimated NOx emissions from our principal\nflight domain averaged 216 ( ±33 ) t\u2009d −1 over six flights\nin July and August, which is nearly double the California government s\n NOx inventory for the surrounding three-county region. We consider\nseveral possibilities for this discrepancy, including the influence of\nwildfires, the temporal bias of the airborne sampling, instrumental\ninterferences, and the recent hypothesis presented by Almaraz et al.\xa0(2018)\nof localized high soil NO emissions from intensive agricultural application\nof nitrogen fertilizers in the region and find the latter to be the most\nlikely explanation. The methane emission average was 438\u2009Gg\u2009yr −1 \n( ±143 ), which also exceeds the emissions inventory for the region by almost\na factor of 2. Measured ABL ozone during the six afternoon flights\naveraged 74\u2009ppb ( σ=9.8 \u2009ppb). The average midafternoon ozone rise\nof 2.8\u2009ppb\u2009h −1 was found to be comprised of −0.8 \u2009ppb\u2009h −1 due to\nhorizontal advection of lower O3 levels upwind, −2.5 \u2009ppb\u2009h −1 from\ndry deposition loss, −0.5 \u2009ppb\u2009h −1 from dilution by entrainment mixing,\nand 6.9\u2009ppb\u2009h −1 net in situ photochemical production. The O3 \nproduction 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 volatile organic compounds\nin the region, suggesting that the ozone chemistry was predominantly\n NOx -limited on the flight days. Additionally, in order to determine the\nheterogeneity of the different scalars, autocorrelation lengths were\ncalculated for potential temperature (18\u2009km), water vapor (18\u2009km), ozone\n(30\u2009km), methane (27\u2009km), and NOx (28\u2009km). The spatially diffuse\npatterns of CH4 and NOx seem to imply a preponderance of broad\nareal sources rather than localized emissions from cities and/or highway\ntraffic within the SJV.