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

Abstract

Abstract. The San Joaquin Valley of California is known for excessive ozone air pollution owing to local production combined with terrain-induced flow patterns that channel air in from the highly populated San Francisco Bay area and stagnate it against the surrounding mountains. During the summer, ozone violations of the National Ambient Air Quality Standards (NAAQS) are notoriously common, with the San Joaquin Valley having an average of 115 violations of the current 70 ppb standard each year between 2012 and 2016. Because regional photochemical production peaks with actinic radiation, most studies focus on the daytime, and consequently the nocturnal chemistry and dynamics that contribute to these summertime high-ozone events are not as well elucidated. Here we investigate the hypothesis that on nights with a strong low-level jet (LLJ), ozone in the residual layer (RL) is more effectively mixed down into the nocturnal boundary layer (NBL) where it is subject to dry deposition to the surface, the rate of which is itself enhanced by the strength of the LLJ, resulting in lower ozone levels the following day. Conversely, nights with a weaker LLJ will sustain RLs that are more decoupled from the surface, retaining more ozone overnight, and thus lead to more fumigation of ozone the following mornings, giving rise to higher ozone concentrations the following afternoon. The relative importance of this effect, however, is strongly dependent on the net chemical overnight loss of Ox (here [Ox] ≡ [O3] + [NO2]), which we show is highly uncertain, without knowing the ultimate chemical fate of the nitrate radical (NO3). We analyze aircraft data from a study sponsored by the California Air Resources Board (CARB) aimed at quantifying the role of RL ozone in the high-ozone events in this area. By formulating nocturnal scalar budgets based on pairs of consecutive flights (the first around midnight and the second just after sunrise the following day), we estimate the rate of vertical mixing between the RL and the NBL and thereby infer eddy diffusion coefficients in the top half of the NBL. The average depth of the NBL observed on the 12 pairs of flights 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 midnight to 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 loss via nitrate production, and +2.8 ppb h−1 from mixing into the NBL from the RL. Based on the observed gradients of Ox in the top half of the NBL, these mixing rates yield eddy diffusivity estimates ranging from 1.1 to 3.5 m2 s−1, which are found to inversely correlate with the following afternoon's ozone levels, providing support for our hypothesis. The diffusivity values are approximately an order of magnitude larger than the few others reported in the extant literature for the NBL, which further suggests that the vigorous nature of nocturnal mixing in this region, due to the LLJ, may have an important control on daytime ozone levels. Additionally, we propose that the LLJ is a branch of what is colloquially referred to as the Fresno eddy, which has been previously proposed to recirculate pollutants. However, vertical mixing from the LLJ may counteract this effect, which highlights the importance of studying the LLJ and Fresno eddy as a single interactive system. The synoptic conditions that are associated with strong LLJs are found to contain deeper troughs along the California coastline. The LLJs observed during this study had an average centerline 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 of 915 MHz radioacoustic sounding system and surface air quality network data show an inverse correlation between the jet strength and ozone the following day, further suggesting that air quality models need to forecast the strength of the LLJ in order to more accurately predict ozone violations.

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

“…The differences between the buffer layer and mixed layer measurements suggest that the surface smoke was transported up the valley with the Pacific inflow and not mixed downward from the buffer layer. We note that the buffer layer‐boundary layer coupling may have been stronger in the southern SJV where the subsidence is stronger and vertical mixing by the nocturnal low‐level jet important (Caputi et al, 2019).…”

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

“…Despite their shorter duration and elongated domain, we include some analyses from these flights because they are more numerous and offer valuable complementary information to this study. Details of the RLO flights can be found in Caputi et al (2019).…”

“…Average trends are taken across each ∼ 2 h flight, as well as estimated in between flights, totalling eight estimates each day. Data are binned into three altitude layers: the lower boundary layer 0-200 m -within the nocturnal boundary layer, as discussed by Caputi et al (2019); the upper boundary layer 200-600 m (mainly the RL when it is present or within the afternoon ABL); and the buffer layer 600-2000 m. Looking at the nearsurface data from the RLO flights and Fig. 8 we see similarities: a peak (dominated by fumigation from the RL) around 08:00-09:00 PST, a zero crossing around 15:00 PST, and a max loss at 19:00 PST.…”

Satellite Mapping of PM2.5 Episodes in the Wintertime San Joaquin Valley: A "Static" Model Using Column Water Vapor

“…Flows in the San Joaquin can be greatly influenced by the nearby mountains, with flows day and night promoting some upslope transport of material which can recirculate, detached from mixing on following days. Consideration of subsidence of air into the San Joaquin mixed layer suggests a flow-through time for aerosol and water of 2-3 days for some situations (Caputi et al, 2018). Mixing of entrained and mixed layer air allows for continued accumulation of Particles and water vapor are emitted and accumulate in the same region, and they are mixed similarly each midday and afternoon by convective stirring.…”