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 PM 2.5 (PM with particle
diameters less than 2.5  µm ), for the first time, we derive PM 2.5 
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 µg m−3 ) between the averaged nighttime
CALIOP-derived PM 2.5 and ground-based PM 2.5 (with a lower r2 
of ∼0.21 for daytime; mean bias of −0.4 µg m−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 PM 2.5 studies. Lastly, the e -folding correlation
length for surface PM 2.5 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 PM 2.5 studies.