In this paper, we implement and compare two complementary methods for the measurement of low cloud optical depth
with a Raman-Mie lidar over the metropolitan area of New York City. The first approach, based on the method of S.
Young, determines the cloud optical depth by regressing the elastic signal against a molecular reference signal above
and below the cloud. Due to high aerosol loading below and above the low cloud, correction for aerosol influence was
necessary and achieved with the combined Raman-elastic returns. The second approach uses N2-Raman signal to derive
cloud extinction profiles and then integrate them to determine optical depth. We find excellent agreements between
these two retrievals for cloud optical depths as large as 1.5. Extinction-to-backscatter ratio within the low cloud is
obtained and is shown to be consistent to values calculated from liquid water cloud model. The varied lidar ratios at
cloud edge may imply the changes of cloud droplet size providing clues to the CCN seeding process. Furthermore,
multiple-scattering effects on retrieving cloud optical depths are estimated by using an empirical model and specific
lidar parameters.
In this paper, we present results showing the usefulness of multi-wavelength lidar measurements to study the interaction
of aerosols in the PBL with long range advected aerosol plumes. In particular, our measurements are used to determine
the plume angstrom exponent, which allows us to differentiate smoke events from dust events, as well as partitioning the
total aerosol optical depth obtained from a CIMEL sky radiometer between the PBL and the high altitude plumes.
Furthermore, we show that only if the optical depth from the upper level plumes is taken into account, the correlation
between the lidar derived PBL aerosol optical depth and surface PM2.5 is high. In addition, we also observe the
dynamic interaction of high altitude plumes interacting with the PBL, resulting in a dramatic rise in surface PM10
concentrations without a corresponding dramatic rise in PM2.5 concentrations. These observations strongly suggest the
deposition of large particulates into the PBL which is consistent with both lidar angstrom coefficient measurements and
back-trajectory analysis. Finally, we investigate the correspondence between surface PM2.5 concentrations and optical
backscatter coefficients as a function of altitude. To perform this study, our lidar system is replaced by a ceilometer
(Vaisala CL-31) which can determine backscatter to near surface level. In particular, we confirm that near surface
backscatter within the first 100 meters is a good proxy for PM2.5 but as altitude increases beyond 500 meters, the
correlations degrades dramatically. These studies are useful in identifying the vertical length scales in which spaced
based lidars such as Calipso can be used to probe surface PM2.5.
In this paper, we explore the possibility of determining the nature and variability of urban aerosol hygroscopic
properties using multi-wavelength Raman lidar measurements at 355nm, as well as backscatter measurements at 532nm
and 1064nm. The addition of these longer wavelength channels allow us to more accurately validate the homogeneity of
the aerosol layer as well as provide additional multiwavelength information that can be used to validate and modify the
aerosol models underlying the hygroscopic trends observed in the Raman channel. In support of our hygroscopic
measurements, we also discuss our calibration procedures for both the aerosol and water vapor profiles. The calibration
algorithm we ultimately use for the water vapor measurements are twilight measurements where water vapor radiosonde
data from the OKX station in NYS, are combined with total water vapor obtained from a GPS MET station. These
sondes are then time correlated with independent near surface RH measurements to address any bias issues that may
occur due to imperfect calibration due to lidar overlap issues and SNR limitations in seeing the water vapor at high
altitudes. In particular, we investigate the possibility of using ratio optical scatter measurements which eliminate the
inherent problem of variable particle number and illustrate the sensitivity of different hygroscopic aerosols to these
measurements.
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