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Abstract. The San Joaquin Valley of California is known forexcessive ozone air pollution owing to local production combined withterrain-induced flow patterns that channel air in from the highly populatedSan Francisco Bay area and stagnate it against the surrounding mountains.During the summer, ozone violations of the National Ambient Air QualityStandards (NAAQS) are notoriously common, with the San Joaquin Valley havingan average of 115 violations of the current 70 ppb standard each yearbetween 2012 and 2016. Because regional photochemical production peaks withactinic radiation, most studies focus on the daytime, and consequently thenocturnal chemistry and dynamics that contribute to these summertime high-ozone events are not as well elucidated. Here we investigate the hypothesisthat on nights with a strong low-level jet (LLJ), ozone in the residuallayer (RL) is more effectively mixed down into the nocturnal boundary layer(NBL) where it is subject to dry deposition to the surface, the rate ofwhich is itself enhanced by the strength of the LLJ, resulting in lowerozone levels the following day. Conversely, nights with a weaker LLJ willsustain RLs that are more decoupled from the surface, retaining more ozoneovernight, and thus lead to more fumigation of ozone the following mornings,giving rise to higher ozone concentrations the following afternoon. Therelative importance of this effect, however, is strongly dependent on thenet chemical overnight loss of Ox (here [ Ox ] ≡ [ O3 ] + [ NO2 ]), which we show is highly uncertain, without knowing theultimate chemical fate of the nitrate radical ( NO3 ). We analyzeaircraft data from a study sponsored by the California Air Resources Board(CARB) aimed at quantifying the role of RL ozone in the high-ozoneevents in this area. By formulating nocturnal scalar budgets based on pairsof consecutive flights (the first around midnight and the second just aftersunrise the following day), we estimate the rate of vertical mixing betweenthe RL and the NBL and thereby infer eddy diffusion coefficients in the tophalf of the NBL. The average depth of the NBL observed on the 12 pairs offlights for this study was 210(±50 ) m. Of the average −1.3  ppb h −1 loss of Ox in the NBL during the overnight hours from midnightto 06:00 PST, −0.2  ppb h −1 was found to be due to horizontal advection, −1.2  ppb h −1 due to dry deposition, −2.7  ppb h −1 to chemical lossvia nitrate production, and +2.8  ppb h −1 from mixing into the NBLfrom the RL. Based on the observed gradients of Ox in the top half ofthe NBL, these mixing rates yield eddy diffusivity estimates ranging from1.1 to 3.5 m 2  s −1 , which are found to inversely correlate with thefollowing afternoon's ozone levels, providing support for our hypothesis.The diffusivity values are approximately an order of magnitude larger thanthe few others reported in the extant literature for the NBL, which furthersuggests that the vigorous nature of nocturnal mixing in this region, due tothe LLJ, may have an important control on daytime ozone levels.Additionally, we propose that the LLJ is a branch of what is colloquiallyreferred to as the Fresno eddy, which has been previously proposed torecirculate pollutants. However, vertical mixing from the LLJ may counteractthis effect, which highlights the importance of studying the LLJ and Fresnoeddy as a single interactive system. The synoptic conditions that areassociated with strong LLJs are found to contain deeper troughs along theCalifornia coastline. The LLJs observed during this study had an averagecenterline height of 340 m, average speed of 9.9 m s −1 ( σ=3.1  m s −1) ,and a typical peak timing around 23:00 PST. A total of 7 years of915 MHz radioacoustic sounding system and surface air quality network datashow an inverse correlation between the jet strength and ozone the followingday, further suggesting that air quality models need to forecast thestrength of the LLJ in order to more accurately predict ozone violations.

Residual layer ozone, mixing, and the nocturnal jet in California's San Joaquin Valley