Leona Charles

Analysis of the Interaction of Aerosol Transport Layers on Local Air Quality

In this paper, the usefulness of multi-wavelength lidar measurements to study the interaction of aerosols in the PBL with long range advected aerosol plumes is presented. Lidars measurements are used to determine the plume angstrom exponent, which is used 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, the correlation between the lidar derived PBL aerosol optical depth and surface PM2.5 is high, only if the optical depth from the upper level plumes is taken into account. In addition, the dynamic interaction of high altitude plumes interacting with the PBL result 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.

Aerosol Layer Properties and their Effect on Optical Depth Relations to PM2.5 Concentrations

To provide reasonable forecasts of near surface PM2.5 levels, it necessary that satellite measurements provide a reasonable estimator of PM2.5 which can be coupled to a transport model Unfortunately this requires that the aerosol be homogeneously mixed and that the extent of the PBL be sufficiently accurate. For example, the IDEA product (Infusing satellite Data into Environmental Applications) used by the EPA relies on a static relationship connecting PM2.5 to MODIS aerosol optical depth (AOD) which relies on a static model of the PBL aerosol height. In this paper, we show that the PBL height is far from static and by taking the variable PBL into account, a far better prediction of PM2.5 from the MODIS (AOD) measurements is obtained.

Examination of hygroscopic properties of aerosols using a combined multiwavelength elastic-Raman lidar

Water vapor is an important greenhouse gas due to its high concentration in the atmosphere (parts per thousand) and its interaction with tropospheric aerosols particles. The upward convection of water vapor and aerosols due to intense heating of the ground leads to aggregation of water particles or ice on aerosols in the air forming different types of clouds at various altitudes. The condensation of water vapor on aerosols is affecting their size, shape, refractive index and chemical composition. The warming or cooling effect of the clouds hence formed are both possible depending on the cloud location, cover, composition and structure. The effect of these clouds on radiative global forcing and therefore on the short and long term global climate is of high interest in the scientific world. A major interest is manifested in obtaining accurate vertical water vapor profiles simultaneously with aerosol extinction and backscatter in the meteorological and remote sensing fields all around the globe in an effort to understand the hygroscopic properties of aerosols In previous work, simultaneous measurements of RH with backscatter measurements from a surface nephelometer were used to probe the hygroscopic properties of aerosols. However, most of these measurements were not able to probe the high RH domain since such high RH is rare for surface altitudes. For this reason, experiments using a 355 nm raman water vapor and aerosol lidar at the ARM site were used. Capable of providing simultaneous backscatter and RH profiles, and performing the experiments under low altitude cloud decks insured stable well mixed layers as well as probing RH profiles to above 95% which is required for the differentiation of different aerosol hygroscopic models.

Improvement of MODIS retrieval of aerosols over urban areas using a regionally tuned ground albedo model

The collection 5 MODIS aerosol algorithm was designed to be more robust in handling surface reflection by differentiating different surfaces. When compared to the old collection 4 algorithm, we found that although there was some improvement, the MODIS retrievals were still biased high. This result was traced to the fact that the correlation coefficient in collect 5 was still too low compared to results from Hyperion. To address this issue, a regional surface albedo model is obtained based on matchups of CIMEL sunphotometer and MODIS retrievals. Given the optical depth measurments and utilizing the urban aerosol phase function (tuned by AOD) , we obtained a set of reflectance measurements which were than regressed against the scattering angle. Using this surface model, we recalculated the AOD using MODIS over a different set of measurments and saw a significant reduction in bias.

Atmospheric transport of Smoke and Dust Particulates and their interaction with the Planetary Boundary Layer as observed by multi-wavelength Lidar and supporting instrumentation

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.

Raman-Mie lidar measurements of low and optically thin cloud

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.

Backscatter properties of hygroscopic aerosols using models, combined multiwavelength Raman lidar, GPS, and radiosonds

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.