EXAMINATION OF THE PHOTOCHEMISTRY AND MESOSCALE METEOROLOGY ASSOCIATED WITH POOR AIR QUALITY IN THE U.S.

Abstract

Mesoscale meteorological processes including advection, vertical mixing, thermally-direct circulations (sea/bay breezes) combined with chemical processes and deposition dominate boundary-layer ozone (O3). While bay breezes (BBs) transport higher O3 over land on polluted days, they also advect humid air and induce low-level convergence, which can lead to haze and deep convection. Thunderstorms can vent pollution out of the boundary layer and entrain cleaner, mid-tropospheric air into it, reducing surface pollutant concentrations. Here, the net local effect of these two mesoscale forcings (BBs and thunderstorms) on O3 concentrations are quantified. First, case studies using vertical profiles and surface observations during the 2011 MD and the 2013 TX deployments of DISCOVER-AQ show the severity of bay/gulf breeze exacerbation of pollution. Next is a BB and thunderstorm climatology for a Chesapeake Bay coastal site (summer 2011-2016). BBs are identified by a data-driven automated detection algorithm customized for the complex coastline. Thunderstorm vs. non-thunderstorm days are analyzed using gridded lightning data within an influential radius of the site. These meteorological classifications are compared with O3 exceedance days. While the highest conditional mean O3 was on BB days and the lowest on thunderstorm only days, thunderstorms do not always terminate an O3 event, especially in combination with a BB. To further understand the dynamical mechanisms responsible for changes in O3 from BBs and thunderstorms, the Weather Research and Forecasting (WRF) model is run at fine resolution with water vapor nudging to capture air-mass thunderstorms forced by the BB in MD. The model compared well with DISCOVER-AQ observations and radar reflectivity. Finally, an observation-constrained box model was used to study photochemical processes along the flight track during the 2013 TX DISCOVER-AQ deployment. O3 production and its sensitivity to NOx and VOCs were calculated at different locations and times of day. Results indicate controlling NOx emissions will benefit the Houston area overall, but select areas will also benefit from controlling VOC emissions. These studies, which can also be applied to particulate matter, uncover how meteorology and photochemistry come together to generate smog events at coastal cities, and can help develop efficient, high resolution policies for cleaner air.