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

The San Joaquin valley is known for intractable air pollution, owing to local production combined with flow patterns that channel in air from the San Francisco Bay area, with surrounding mountains trapping the air inside. During the summer, ozone violations of the National Ambient Air Quality Standards (NAAQS) are notoriously common, with most stations obtaining an average of 20-60 exceedance days. Here we explore the dynamics of intermittent nocturnal turbulent mixing and its contribution to these summertime ozone events. During the summers of 2015 and 2016, a Mooney aircraft operated by Scientific Aviation Inc. collected 170 hours of airborne data between Fresno and Bakersfield, CA. Flights were performed throughout the full diurnal cycle, with abundant low-altitude measurements included on each flight. With this data we first investigate the hypothesis that on nights with a strong low-level jet, ozone in the residual layer (RL) may be more efficiently mixed down into the nocturnal stable boundary layer (NBL), where it is subject to chemical loss from local sources of NO_x (NO+NO₂) as well as dry deposition to the surface, resulting in lower ozone 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 and consequently higher afternoon ozone concentrations. We quantify nocturnal scalar budgets of O_x (defined here as O₃+NO₂) and thereby estimate eddy diffusion coefficients at the top of the stable boundary layer on each night. Of the average observed -1.3 ppb/hr loss of O_x in the NBL overnight, -0.2 is due to horizontal advection, -0.8 ppb/hr is due to dry deposition, -3.7 ppb/hr is chemical loss, and 3.8 ppb/hr from mixing into the NBL from the RL overnight. We relate the inferred eddy diffusivity to a product of the buoyancy length scale (ι_b, or the quotient of the σ_w and the Brunt-Visalia frequency), and the square root of horizontal Turbulent Kinetic Energy (TKE), which predicts a value of σ_w of 3 cm/s in the NBL. The wind variance is estimated from a recently developed low-cost system that measures horizontal wind at 1 Hz. Power spectra from the wind data indicates that the inertial subrange of turbulence is captured at spatial scales down to ~ 150 m, below which an extrapolation is carried out by integrating the power spectrum and using a Kolmogorov fit to estimate the amount of variance not being captured by the system. The nocturnal eddy diffusivity inferred from the scalar budget is negatively correlated to the ozone observed at sunrise the following morning from the 12 nights analyzed (r²=0.27, p=0.09). Additionally, 6 years of 915 MHz radio-acoustic sounding system and surface air quality network data is probed to find a relationship between low level jet speed and ozone the following afternoon, which also shows a negative correlation (r²=0.14, p<10^-5). These findings support the hypothesis that stronger mixing overnight from the RL into the NBL reduces ozone the following day, though a direct causal relationship must be explored with future modeling studies. The flight and sounding system data is also used to further probe the nature of the low-level jet. We find the non-dimensional quantity σ_u/U_max in our flight data to be as high as ~0.16, and horizontal TKE below the jet maximum to be between 0.5 and 2 m²/s², with the upper limit of this range comparable to that of a daytime convective boundary layer. For comparison, previous reports of these parameters in NBLs have ranged from 0.02 – 0.06 for σ_u/U_max, and from 0.2 – 1.1 m²/s² for TKE. Annual sounding data reveals the nocturnal jet maximum to be at a significantly lower altitude from April through August (~Z/Z_i ~ 0.4 +- 0.2) compared to December and January (~Z/Z_i ~ 1.1 +- 0.5), likely contributing to the wind shear at lower altitudes, including the NBL, during the summer. The average height of the low-level jet during our nocturnal flights was measured to be at ~300 m. In addition to shear-driven mixing, we note several localized thermodynamic instabilities (dθ_v/dz < -5 K/km) in our flight data. These instabilities are often observed well above the jet height (median = 970 m, SD = 527 m) and have average thicknesses of 20-30 m, though thicknesses greater than 70 m are observed.