Observing Lower-Tropospheric Ozone Spatiotemporal Variability With Airborne Lidar and Surface Monitors in Houston, Texas

Abstract

Surface-level ozone is a trace gas regulated by the Environmental Protection Agency as its oxidizing properties are detrimental to air quality, impacting human and environmental health. Satellite observations provide spatially continuous intraurban ozone distributions, potentially filling in gaps within monitoring networks. However, near-surface ozone is difficult to retrieve from columns due to the large signal in the stratosphere and lack of sensitivity to the lower troposphere in the ultraviolet wavelengths. Airborne lidar measurements of ozone profiles present the opportunity to assess vertical, geospatial, and temporal variability of lower tropospheric (0–2 km) near-surface ozone subcolumn products for air quality analyses. This study uses the first city-wide airborne-lidar measurements collected by the National Aeronautics and Space Administration High-Spectral Resolution Lidar-2 instrument over Houston, Texas during the September 2021 Tracking Aerosol Convection ExpeRiment–Air Quality campaign alongside surface-monitoring and ozone-sonde measurements to examine ozone diurnal variability within a city. In situ ground measurements and lidar subcolumns were well correlated (r = 0.87) with 2× larger differences observed in the morning than afternoon reflecting the impacts of chemical titration at the surface. Matched ozone-sonde and airborne-lidar subcolumns are also well correlated (r = 0.96, bias = 1.3 ppb) suggesting biases between surface and subcolumn ozone reflect vertical distribution variability not instrument biases. Finally, if the Tropospheric Emissions: Monitoring of Pollution instrument achieves its precision requirement, we find this product may be able to detect enhanced ozone over a city like Houston with up to 55% of near-surface subcolumns capturing ozone variability, particularly during exceedance events.