Retrieved Extinction Profiles Research Articles

Evaluation of retrieved aerosol extinction profiles using as reference the aerosol optical depth differences between various heights

Aerosol extinction vertical profiles at Granada (Spain) are calculated with the GRASP (Generalized Retrieval of Aerosol and Surface Properties) code using as input Aerosol Optical Depth (AOD) and sky radiance measurements from AERONET (AEerosol RObotic NETwork) and ceilometer RCS (Range Corrected Signal) profiles, both corresponding to the Granada (Spain) station. This methodology is so called GRASPpac due to the combination of sun/sky photometer and ceilometer on GRASP. In order to evaluate the accuracy of these retrieved extinction profiles at Granada, two more nearby AERONET stations, located at different altitudes, are used. The AOD difference of the three choosen AERONET sun/sky photometers have been used to calculate the Integrated Aerosol Extinction (IAE) at different height layers. These three AERONET sun/sky photometers are used as a reference and compared against the integrated extinction at the same layers from the extinction profiles retrieved by GRASPpac. The differences between AERONET and GRASPpac retrieved IAE values indicate that GRASPpac aerosol extinction profiles are at least within the uncertainty of the sun/sky photometer measurements, but GRASPpac method overestimates the AERONET extinction at low altitudes and underestimates it at high levels. The most accurate and precise retrieved extinction correspond to the intermediate layer with a mean bias error (MBE ± standard deviation) of 0.00 ± 0.01 (0 ± 59%) for 1020 nm, and the worst integrated extinction results were obtained for the upper layers with a MBE of −0.01 ± 0.02 (28 ± 36%) for 1020 nm. In general these MBE values increases for shorter wavelengths. In order to obtain a complete characterization of this bias, the dependence of the obtained differences on the aerosol size and the solar zenith angle, among others, are analysed in detail. Finally, the behaviour of vertically-resolved aerosol extinction at Granada is evaluated using averages of the retrieved profiles from November of 2012 to December of 2017. The highest IAE values are found in Summer with mean values of 0.09 for the lower layers and 0.07 for the upper ones, both at 440 nm wavelength.

Automated Retrieval of Cloud and Aerosol Properties from the ARM Raman Lidar. Part II: Extinction

AbstractA feature detection and extinction retrieval (FEX) algorithm for the Atmospheric Radiation Measurement Program’s (ARM) Raman lidar (RL) has been developed. Presented here is Part II of the FEX algorithm: the retrieval of cloud and aerosol extinction profiles. The directly retrieved extinction profiles using the Raman method are supplemented by other retrieval methods developed for elastic backscatter lidars. Portions of features where the extinction-to-backscatter ratios (i.e., lidar ratios) can be obtained are used to infer the lidar ratios for the regions where no such estimate can be made. When neither directly retrieved nor an inferred value can be determined, a climatological lidar ratio is used. This best-estimate approach results in the need to use climatological lidar ratios for less than about 5% of features, except for thin cirrus at the ARM tropical western Pacific Darwin site, where above 12 km, about 20% of clouds use a climatological lidar ratio. A classification of feature type is made, guided by the atmosphere’s thermodynamic state and the feature’s scattering properties: lidar ratio, backscatter, and depolarization. The contribution of multiple scattering is explicitly considered for each of the ARM RL channels. A comparison between aerosol optical depth from FEX and that from collocated sun photometers over multiple years at two ARM sites shows an agreement (in terms of bias error) of about −0.3% to −4.3% (relative to the sun photometer).

The vertical distribution of BrO and aerosols in the Arctic: Measurements by active and passive differential optical absorption spectroscopy

[1] We present results from multiaxis differential optical absorption spectroscopy (MAX-DOAS) and long-path DOAS (LP-DOAS) measurements performed at the North Slope of Alaska from February to April 2009 as part of the Ocean-Atmosphere-Sea Ice-Snowpack Barrow 2009 campaign. For the first time, vertical profiles of aerosol extinction and BrO in the boundary layer were retrieved simultaneously from MAX-DOAS measurements using the method of optimal estimation. Even at very low visibility, retrieved extinction profiles and aerosol optical thickness are in good agreement with colocated ceilometer and Sun photometer measurements, respectively. BrO surface concentrations measured by MAX-DOAS and LP-DOAS are in very good agreement, and it has been found that useful information on the BrO vertical distribution can be retrieved from MAX-DOAS even in cases when blowing snow strongly reduces visibility. The retrieved BrO and extinction vertical profiles allow for a thorough characterization of the vertical structure of the boundary layer during numerous ozone depletion events observed during Barrow 2009. High BrO concentrations are usually present during the onset of ozone depletion events, and BrO disappears as ozone concentrations approach zero. The finding that elevated BrO concentrations occur mainly in the presence of high extinction near the surface strongly suggests that release of reactive bromine from airborne aerosols and/or ice particles at high wind speed plays an important role. Back trajectory calculations indicate that the particles were transported from the frozen ocean to the measurement site and that the release of reactive bromine from sea ice and/or frost flowers occurs when low temperatures (<250 K) prevail in the regions where reactive bromine is emitted.

Widespread Morning Drizzle on Titan

Precipitation is expected in Titan's atmosphere, yet it has not been directly observed, and the geographical regions where rain occurs are unknown. Here we present near-infrared spectra from the Very Large Telescope and W. M. Keck Observatories that reveal an enhancement of opacity in Titan's troposphere on the morning side of the leading hemisphere. Retrieved extinction profiles are consistent with condensed methane in clouds at an altitude near 30 kilometers and concomitant methane drizzle below. The moisture encompasses the equatorial region over Titan's brightest continent, Xanadu. Diurnal temperature gradients that cause variations in methane relative humidity, winds, and topography may each be a contributing factor to the condensation mechanism. The clouds and precipitation are optically thin at 2.0 micrometers, and models of "subvisible" clouds suggest that the droplets are 0.1 millimeter or larger.