In situ flight data from two distinct campaigns during winter and summer seasons in the San Joaquin Valley (SJV) of California are used to calculate boundary-layer entrainment rates, ozone photochemical production rates, and regional methane emissions. Flights near Fresno, California, in January and February 2013 were conducted in concert with the NASA DISCOVER-AQ project. The second campaign (ArvinO3), consisting of 11 days of flights spanning June through September 2013 and 2014, focused on the southern end of the SJV between Bakersfield and the small town of Arvin, California - a region notorious for frequent violations of ozone air quality standards. Entrainment velocities, the parameterized rates at which free tropospheric air is incorporated into the atmospheric boundary layer (ABL), are estimated from a detailed budget of the inversion base height. During the winter campaign near Fresno, we find an average midday entrainment velocity of 1.5g cmg sg'1, and a maximum of 2.4g cmg sg'1. The entrainment velocities derived during the summer months near Bakersfield averaged 3g cmg sg'1 (ranging from 0.9 to 6.5g cmg sg'1), consistent with stronger surface heating in the summer months. Using published data on boundary-layer heights we find that entrainment rates across the Central Valley of California have a bimodal annual distribution peaking in spring and fall when the lower tropospheric stability (LTS) is changing most rapidly.Applying the entrainment velocities to a simple mixed-layer model of three other scalars (O3, CH4, and H2O), we solve for ozone photochemical production rates and find wintertime ozone production (2.8g ±g 0.7g ppbg hg'1) to be about one-third as large as in the summer months (8.2g ±g 3.1g ppbg hg'1). Moreover, the summertime ozone production rates observed above Bakersfield-Arvin exhibit an inverse relationship to a proxy for the volatile organic compound (VOC)g :g NOx ratio (aircraft [CH4] divided by surface [NO2]), consistent with a NOx-limited photochemical environment. A similar budget closure approach is used to derive the regional emissions of methane, yielding 100 (±100)g Ggg yrg'1 for the winter near Fresno and 170 (±125)g Ggg yrg'1 in the summer around Bakersfield. These estimates are 3.6 and 2.4 times larger, respectively, than current state inventories suggest. Finally, by performing a boundary-layer budget for water vapor, surface evapotranspiration rates appear to be consistently g1/4 g 55g % of the reference values reported by the California Irrigation Management Information System (CIMIS) for nearby weather stations.

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 (D 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 D 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.