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.
Highlights
The setting for this research is the San Joaquin Valley (SJV) which is the southern end of California’s Central Valley, one of the largest valleys in the world by area
We present a variety of inferences gleaned from the three scalar budgets performed for NOx to derive regional surface emissions (Sect. 4.1.1), for O3 to derive afternoon photochemical production rates (Sect. 4.1.2) and to see how that fits into the overall diurnal budget of ozone (Sect. 4.1.2), and for CH4 to derive regional emissions (Sect. 4.1.3)
We would expect the airborne sampling domain to be a subset of the three-county region; since ∼ 86 % of the NOx sources in the California Air Resources Board (CARB) inventory are mobile for these counties and our sampling occurred in the vicinity of each county’s major population center (Visalia, Fresno, and Hanford) and one of the SJV’s main traffic arteries (CA state Highway 99), it may be reasonable to expect the countywide NOx emissions to be mostly sampled by the flights
Summary
The setting for this research is the San Joaquin Valley (SJV) (see Fig. 1) which is the southern end of California’s Central Valley, one of the largest valleys in the world by area. The SJV is a complex mesoscale environment where the surrounding topography limits the low-level inflow to the valley and makes vertical mixing important to atmospheric boundary layer (ABL) ventilation, similar to the conditions in the Po Valley of Italy (Maurizi et al, 2013). Estimates of the coverage of mountainous terrain on the Earth’s land surface varies anywhere from ∼ 25 % to 70 % (Grab, 2000; Noppel and Fiedler, 2002; Rotach et al, 2014), depending on the subjective criterion used, and orograph-. Horizontal inhomogeneities in the Earth’s land surface affect the adjacent ABL in a variety of ways leading to pronounced changes in the EAE involving sea breezes (Miller et al, 2003), internal boundary layers (Garratt, 1990), and orographic effects (Rotach et al, 2015). Valleys are popular areas for human inhabitation due to lowland access, access to river waterways, and fertile soils for agriculture (Small and Cohen, 2004)
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