Retrieving PM2.5 Concentrations over the Contiguous United States through the Use of CALIOP and HSRL Observations

Particulate matter with diameters less than 2.5 µm (PM_2.5) adversely affects air quality and human health. In the past, satellite-based PM_2.5 studies mostly concentrated on deriving surface PM_2.5 concentrations using correlative relationships between satellite retrieved aerosol optical depth (AOD) and PM_2.5 concentrations. Such studies are fundamentally flawed, as AOD is a column-integrated quantity and PM_2.5 is a surface-based parameter. Using a bulk-mass-modeling-based method, we demonstrate the feasibility of deriving surface PM_2.5 concentrations using near-surface aerosol extinction retrievals from the NASA Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument, bulk mass extinction efficiencies, and model-based hygroscopicity. The newly developed method is applied to CALIOP aerosol extinction retrievals using nearly the entire CALIOP data record (2007-2018), and the derived PM_2.5 values were further evaluated using ground-based PM_2.5 measurements from the U.S. Environmental Protection Agency (EPA). The potential of applying similar methods to measurements from the NASA Langley airborne High Spectral Resolution Lidar (HSRL) is also investigated. Lastly, we evaluate uncertainties in our method to examine the subsequent uncertainties in derived PM_2.5 as one way to assess the usefulness of future space-based lidars for PM_2.5 monitoring and forecasting.