A bulk-mass-modeling-based method for retrieving particulate matter pollution using CALIOP observations

Abstract

In this proof-of-concept paper, we apply a bulk-mass-modeling method using observations from the NASA Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument for retrieving particulate matter (PM) concentration over the contiguous United States (CONUS) over a 2-year period (2008-2009). Different from previous approaches that rely on empirical relationships between aerosol optical depth (AOD) and PM2:5 (PM with particle diameters less than 2.5 μm), for the first time, we derive PM2:5 concentrations, during both daytime and nighttime, from near-surface CALIOP aerosol extinction retrievals using bulk mass extinction coefficients and model-based hygroscopicity. Preliminary results from this 2-year study conducted over the CONUS show a good agreement (r2 ∼ 0:48; mean bias of -3:3 μgm-3) between the averaged nighttime CALIOPderived PM2:5 and ground-based PM2:5 (with a lower r2 of ∼ 0:21 for daytime; mean bias of-0:4 μgm-3), suggesting that PM concentrations can be obtained from active-based spaceborne observations with reasonable accuracy. Results from sensitivity studies suggest that accurate aerosol typing is needed for applying CALIOP measurements for PM2:5 studies. Lastly, the e-folding correlation length for surface PM2:5 is found to be around 600 km for the entire CONUS (∼ 300 km for western CONUS and ∼ 700 km for eastern CONUS), indicating that CALIOP observations, although sparse in spatial coverage, may still be applicable for PM2:5 studies.