Photochemical production of ozone and emissions of NO<sub><i>x</i></sub> and CH<sub>4</sub> in the San Joaquin Valley

Abstract

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: these include, 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 rates of change this airborne budgeting technique enables us to deduce net photochemical ozone production rates and emission fluxes of NOx and CH4. Estimated NOx emissions from our principal flight domain averaged 216 (±33) t d−1 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 Gg yr−1 (±143), which also exceeds the emissions inventory for the region by almost a factor of 2. Measured ABL ozone during the six afternoon flights averaged 74 ppb (σ=9.8 ppb). The average midafternoon ozone rise of 2.8 ppb h−1 was found to be comprised of −0.8 ppb h−1 due to horizontal advection of lower O3 levels upwind, −2.5 ppb h−1 from dry deposition loss, −0.5 ppb h−1 from dilution by entrainment mixing, and 6.9 ppb h−1 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 volatile organic compounds in the region, suggesting that the ozone chemistry was predominantly NOx-limited on the 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 patterns 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.

Spatial patterns in summertime surface ozone in the Southern Front Range of the U.S. Rocky Mountains

“…These observations reflect the controls on diurnal ozone variability discussed in similar studies, including nighttime ozone depletion via surface deposition and NO titration, rapid midmorning ozone increases via boundary layer entrainment followed by in situ photochemical production, and diurnal winds facilitating horizontal advection (Jaffe et al, 2018;Trousdell et al, 2019;Oltmans et al, 2019). These effects can be seen to different degrees across the range of site elevations.…”

“…Higher elevation sites can be above the nighttime inversion and so experience significantly less ozone loss at night (Aneja et al, 2000;Ambrose et al, 2011). Although our study design does not permit us to directly determine the magnitude of depositional loss processes or how it may vary across sites, Trousdell et al (2019) propose that in the San Joaquin Valley, much of the ozone loss at dusk, in the absence of photochemistry and entrainment, is in fact due to surface deposition and not simply titration by rush hour NO emissions. Caputi et al (2019) found overnight losses of approximately 1.2 ppb h -1 in California's San Joaquin Valley.…”

“…Meanwhile, rates of ozone increase occur at most sites beginning around 5:00-6:00 MST (Figure 6) due to boundary layer growth and daytime turbulence causing ozone from the residual layer to be mixed with air at the surface across at most sites (Brodin et al, 2010;Fine et al, 2015;Bien and Helmig, 2018;Oltmans et al, 2019;Trousdell et al, 2019). In the NFRMA and the San Joaquin Valley of California, entrainment has been observed to cause morning increases in ozone values with minimal to no input from photochemical production from 6:00 to 9:00 MST (Trousdell et al, 2016;Oltmans et al, 2019), and we assume similar dynamics govern the diurnal profiles in Colorado Springs.…”

Ozone Production in the Soberanes Smoke Haze: Implications for Air Quality in the San Joaquin Valley During the California Baseline Ozone Transport Study

“…Visalia is located about one third of the way from Fresno to Bakersfield, the two most populous cities in the San Joaquin Valley (SJV), and the TOPAZ lidar was colocated with a radar wind profiler and Radio Acoustic Sounding System operated by the San Joaquin Valley Air Pollution Control District (Bao et al, 2008) and a vertically staring Doppler lidar operated by NOAA. Ozone, CO 2 , CH 4 , and other species were also sampled by single-engine Mooney TLS Bravo and Ovation 2 research aircraft operated by the University of California, Davis (UC Davis) and Scientific Aviation (SciAv) (Trousdell et al, 2019) and by the NASA Alpha Jet Atmospheric eXperiment (AJAX) research aircraft (Yates et al, 2015). Details of the sampling techniques and intercomparisons between the lidar and airborne O 3 measurements are described in Leblanc et al (2018) and Langford et al (2019).…”

“…Widespread agricultural irrigation (Li et al, 2016) and subsidence created by the mountain-valley circulations and persistent North Pacific High (Trousdell et al, 2016) typically cap the afternoon mixed layers below 1 km 10.1029/2019JD031777 (Bianco et al, 2011), and some of the pollution carried into the mountains by the afternoon upslope winds (Panek et al, 2013) is recirculated back over the valley above the shallow boundary layers (De Young et al, 2005;Fast et al, 2012;Gohm et al, 2009;Rampanelli et al, 2004;Zardi & Whiteman, 2013). There, the pollution accumulates in a persistent "buffer layer" (Faloona et al, 2020;Trousdell et al, 2019) that can persist for days or even weeks until dispersed by passing cold frontal systems. During CABOTS, free tropospheric air flowing over the Coast Ranges also carried smoke from the Soberanes Fire into the buffer layer where it mingled with the anthropogenic pollution.…”