Airborne Lidar Measurements of Ozone and Aerosol Profiles Over Major US Metropolitan Areas

During field missions in 2021 and 2023, the airborne NASA Langley Research Center High Spectral Resolution Lidar‑2 (HSRL‑2) measured the temporal and spatial evolution of ozone and aerosol distributions impacting urban air quality over the four most populated cities in the United States. HSRL-2 measurements in 2021 were acquired over the Houston metropolitan region, including Galveston Bay and the Houston Ship Channel, as part of the NASA Tracking Aerosol Convection Experiment – Air Quality (TRACER-AQ) mission conducted in collaboration with the Department of Energy. HSRL-2 measurements were acquired over Los Angeles, Chicago, and New York City in 2023 as part of the NASA Synergistic TEMPO Air Quality Science (STAQS) mission conducted in collaboration with the NOAA Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) mission. HSRL‑2 provided nadir vertical profiles of ozone, aerosol backscatter, extinction, and depolarization as the aircraft flew lawnmower type patterns at 9 km for several hours over these urban areas. HSRL-2 measured profiles of aerosol extinction and aerosol optical depth (AOD) via the HSRL technique at 355 and 532 nm and profiles of aerosol backscatter and depolarization at 355, 532, and 1064 nm. Mixed Layer Heights (MLH) were derived by locating sharp vertical gradients in the profiles of aerosol backscatter.

The flights were comprised of up to three repeating lawnmower patterns over each city showing the evolution of the ozone and aerosol distributions from the morning through the afternoon. The HSRL-2 measurements reveal ozone enhancements near the surface as well as in the free troposphere above the mixed layer. Some lidar measurements over Chicago and New York City show the daytime boundary layer growing into elevated layers of biomass burning aerosol. These layers complicate efforts to use column-integrated satellite measurements to infer surface air quality. As expected, mixed layer height (MLH) typically increased significantly during the day; however, during some flights, particularly over the Houston area, MLH also showed large spatial variability associated with changes in surface cover and/or small‑scale circulations. Often HSRL-2 measurements of AOD also showed large spatial and temporal variability throughout the day over these cities. We discuss how the ozone and aerosol profiles are averaged over different vertical and horizontal scales near the surface for use in assessments of regional air quality models and near-surface ozone retrievals from NASA’s recently launched Tropospheric Emissions: Monitoring Pollution (TEMPO) satellite.