Residual layer ozone, mixing, and the nocturnal jet in California's San Joaquin Valley
Authors: Caputi Dani J. , Faloona Ian , Trousdell Justin , Smoot Jeanelle , Falk Nicholas , Conley Stephen 
Published: April 2019

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.