Observing entrainment mixing, photochemical ozone production, and regional methane emissions by aircraft using a simple mixed-layer framework

Abstract. 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 1.5 cm s⁻¹, and a maximum of 2.4 cm s⁻¹. The entrainment velocities derived during the summer months near Bakersfield averaged 3 cm s⁻¹ (ranging from 0.9 to 6.5 cm s⁻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 (2.8 ± 0.7 ppb h⁻¹) to be about one-third as large as in the summer months (8.2 ± 3.1 ppb h⁻¹). Moreover, the summertime ozone production rates observed above Bakersfield–Arvin exhibit an inverse relationship to a proxy for the volatile organic compound (VOC) : NO_x ratio (aircraft [CH₄] divided by surface [NO₂]), consistent with a NO_x-limited photochemical environment. A similar budget closure approach is used to derive the regional emissions of methane, yielding 100 (±100) Gg yr⁻¹ for the winter near Fresno and 170 (±125) Gg yr⁻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  ∼  55 % of the reference values reported by the California Irrigation Management Information System (CIMIS) for nearby weather stations.