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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-854
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Sep 2018

Research article | 12 Sep 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Residual Layer Ozone, Mixing, and the Nocturnal Jet in California's San Joaquin Valley

Dani J. Caputi1, Ian Faloona1, Justin Trousdell1, Jeanelle Smoot1, Nicholas Falk1, and Stephen Conley2 Dani J. Caputi et al.
  • 1Department of Land, Air, and Water Resources, University of California Davis, Davis, 95616, USA
  • 2Scientific Aviation, Inc., Boulder, 80301, USA

Abstract. The San Joaquin valley is known for excessive secondary 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 recent 70 ppb standard each year between 2012 and 2016. The nocturnal dynamics that contribute to these summertime high ozone events have yet to be fully elucidated. Here we investigate the hypothesis that on nights with a strong low-level jet (LLJ) ozone in the residual layer is more effectively mixed down into the stable boundary layer 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 jet will sustain residual layers that are more decoupled from the surface and thus lead to stronger fumigation of ozone in the mornings giving rise to higher ozone concentrations the following afternoon. We analyse aircraft data from a study sponsored by the California Air Resources Board (CARB) aimed at quantifying the role of residual layer ozone in the high ozone episode events in this area. By formulating nocturnal scalar budgets based on flights around midnight and just after sunrise the following days, we estimate the rate of vertical mixing between the residual layer (RL) and the nocturnal boundary layer (NBL), and thereby measure eddy diffusion coefficients in the top half of the NBL. The average depth of the NBL observed on the 12 pairs of flights was 210 (±50)m. Of the average −1.3ppbh−1 loss of the Ox family (here [Ox] = [O3] + [NO2]) in the NBL during the overnight hours from midnight to 06:00 PST, −0.2ppbh−1 was found to be due to horizontal advection, −1.2ppbh−1 due to dry deposition, −2.7ppbh−1 to chemical loss via nitrate production, and +2.8ppbh−1 from mixing into the NBL from the residual layer overnight. Based on the observed gradients of Ox in the top half of the NBL these mixing rates yield eddy diffusivity estimates ranging from 1.1–3.5m2s−1 that are found to inversely correlate with the following afternoon's ozone levels, and provide 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 has an important control on ozone. Additionally, we investigate the synoptic conditions that occasion strong nocturnal jets and find that on average, deeper troughs along the California coastline are associated with stronger jets. The LLJ had an average height of 340m, an average speed of 9.9ms−1 (SD = 3.1ms−1) and a typical peak timing around 23:00 ST. Seven years of 915MHz radio-acoustic sounding system and surface air quality network data show an inverse correlation between the jet strength and ozone the following day, suggesting that air quality models need to forecast the strength of this nocturnal feature in order to more accurately predict ozone violations.

Dani J. Caputi et al.
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Short summary
This paper covers the importance of understanding ozone pollution in California’s Southern San Joaquin Valley from the perspective of meteorological conditions that occur overnight. Our main finding is that stronger winds aloft allow ozone to be depleted overnight, leading to less ozone the following day. This finding has the potential to greatly improve ozone forecasts in the San Joaquin Valley. This study is primarily conducted with aircraft observations.
This paper covers the importance of understanding ozone pollution in California’s Southern San...
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