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

• Published in: Atmospheric chemistry and physics 2019-04, Vol.19 (7), p.4721-4740
• Main Authors: Caputi, Dani J, Faloona, Ian, Trousdell, Justin, Smoot, Jeanelle, Falk, Nicholas, Conley, Stephen
• Format: Article
• Language: English
• Subjects: Advection; Advection (Earth sciences); Air pollution; Air quality; Air quality models; Air quality standards; Analysis; Atmospheric chemistry; Atmospheric circulation; Boundary layers; Budgets; Coefficients; Corporate sponsorship; Data processing; Daytime; Deposition; Diffusion coefficients; Diffusivity; Dry deposition; Dynamics; Eddy diffusion; Eddy diffusivity; Flow (Dynamics); Fumigation; Horizontal advection; Hypotheses; Interactive systems; Low-level jets; Mountains; Night; Nitrogen dioxide; Nocturnal boundary layer; Organic chemistry; Outdoor air quality; Ozone; Photochemicals; Photochemistry; Pollutants; Pollution; Pollution control; Pollution monitoring; Radiation; Radiation (Physics); Sunrise; Synoptic conditions; Troughs; Turbulent diffusion; Vertical mixing; Violations; Vortices
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-19-4721-2019

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