CARB Research Seminar

This page updated September, 7, 2018

Ozone in the Lower Atmosphere and its Contribution to High Ozone Concentrations at Ground-Level in the Southern San Joaquin Valley

Photo of Ian Faloona, PhD

Ian Faloona, Ph.D., University of California, Davis

September 17, 2018
Cal EPA Headquarters, 1001 "I" Street, Sacramento, CA

Introduction
Presentation
Video
Research Project

Overview

We collected an unprecedented airborne data set by extensively sampling the lower 1500 m of the atmosphere in the Southern San Joaquin Valley (SSJV) in order to characterize conditions aloft to help improve the state's ability to model/predict surface ozone concentrations. Sampling occurred nearly all the way to the surface at four local airports and throughout the entire day/night cycle over the course of five different, continuous 2-6 day periods during the summer when ozone air pollution is at its worst. The experimental design of the flights allowed for an empirical estimation of midday entrainment rates of air aloft mixing into the valley boundary layer, which is a critical parameter in controlling ventilation of surface air in the valley. We found entrainment dilution to be, on average, nearly 10% of the photochemical production rate during the midday near Fresno, and even greater near Bakersfield. Furthermore, daytime budgeting of methane and NOx provided estimates of regional emissions, and both were found to be nearly a factor of two larger than current inventories suggest. The midday photochemical production rate of ozone was also estimated and shown to correlate with day-to-day variations in the observed NOx concentrations indicating that the SSJV is predominantly NOx-limited on the high ozone days sampled.

Analysis of consecutive late night and early morning flight data further allowed for an empirical estimation of the vertical mixing rate between the layer aloft (the residual layer) and the shallow nocturnal boundary layer. The resultant eddy diffusivity values were found to be reasonable, but on the high side of other estimates reported in the literature for stable environments suggesting that the low level jet (the nocturnal up-valley wind associated with the so-called 'Fresno Eddy') is responsible for particularly vigorous mixing in the SSJV. Nocturnal Ox (?O3 + NO2) budgets reveal that chemical loss to nitrate production is nearly 2.5 times greater than dry deposition, but is highly uncertain because of the lack of knowledge about the ultimate fate of NO3 due to reaction with VOC's, NO, or its uptake on aerosols. We propose that future, concerted studies of the nocturnal chemistry of the nitrate radical are necessary to accurately quantify this critical loss in air quality models. Nevertheless, continuous surface wind profiler data at Visalia indicate that overnight mixing is indeed influenced by the strength of the low level jet and that a stronger jet results in more nighttime dry deposition loss at the surface, which ultimately impacts the following afternoon's peak ozone levels. Finally, considering the measurements on aggragate, a three-layer atmospheric system emerges wherein the daytime mixed layer is kept relatively shallow (~700 m) by mesoscale subsidence induced by the valley-mountain circulation, and entrainment into that layer takes place from a "buffer layer" (from 700 to ~ 2000 m) that is made up of a mixture of polluted air lofted by mountain/advective venting from the valley sidewalls and onshore flow of basline free tropospheric air over the coastal mountains. Thus entrainment dilution in the SSJV is significantly contaminated by the stagnating, regionally-polluted air aloft. This conceptualization recasts the notion of air quality problems in the SSJV resulting from the 'recirculation' of pollution with quantifiable parameters of inflow, lofting, and boundary layer entrainment, and this work provides specific rates which can be used to validate these processes in air quality models.

Speaker Biography

Ian Faloona is an associate professor at the University of California Davis. He studied physical chemistry at UC Santa Cruz, including summer research in computational chemistry at Los Alamos National Lab, and then earned a Ph.D. in meteorology from the Pennsylvania State University. For four years in between he worked as an air quality consultant with SECOR Inc. in Fort Collins, Colorado. Later, after a postdoc in the Advanced Study Program at the National Center for Atmospheric Research, he joined the atmospheric science faculty at UC Davis. His research interests include the airborne investigation of vertical mixing and near-field pollutant dispersion, observational emission estimates, the meteorology of coastal fog, planetary boundary layer dynamics, biogeochemical cycling, and atmosphere/ocean photochemistry.


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