Chapter 2 Aerosol-Cloud Interactions

Surface Aerosol Properties Studied Using a Near-Horizontal Lidar

Studying near-surface aerosol properties is of importance for a better assessment of the aerosol effect on radiative forcing. We employ the data from a near-horizontal lidar to investigate the diurnal behavior of aerosol extinction and single scattering albedo (SSA) at 349 nm. The response of these parameters to ambient relative humidity (RH) is examined for the data from a one-month campaign conducted in Chiba, Japan, during November 2017, a transition period from fall to winter. The Klett method and adaptive slope method are used in deriving the aerosol extinction coefficient from the lidar data, while the SSA values are retrieved using an aethalometer. Also, a visibility-meter is used to examine the aerosol loading inside the atmospheric boundary layer. It is found that the aerosol growth during the deliquescence phase is more readily observed than the contraction in the efflorescence phase. The decrease of SSA before the deliquescence RH is found for approximately 46% of the deliquescence cases, presumably representing the particle shrinkage of soot particles.

Comparison of Aerosol Optical Depth Products from Multi-Satellites over Densely Populated Cities of Pakistan

Air pollution in Pakistan is causing damage to health, environment and quality of life. Air pollution in Pakistan is not effectively monitored due to heavy cost involved in setting up ground stations. However, Satellite remote sensing can effectively monitor the air pollution in terms of Aerosol Optical Depth (AOD) at regional as well as global level. However, algorithms used to derive AOD from different sensors have some inherited differences which can pose challenges in monitoring regional AOD at high temporal resolution using more than one sensor. Therefore, this study focuses on comparison of four major satellite based AOD products namely Moderate Resolution Imaging SpectroRadiometer (MODIS), Multi-angle Imaging SpectroRadiometer (MISR), Ozone Monitoring Instrument multiwavelength (OMI) aerosol product and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) with the ground based AErosol RObotic NETwork (AERONET) AOD which is only available from Lahore and Karachi in Pakistan. The correlation of various AOD products with AERONET AOD is estimated statistically through coefficient of determination (R2), Root Mean Square Error (RMSE), slope and intercept. It is noticed that MODIS is relatively accurate and reliable for monitoring air quality on operational bases over the land cover area of Lahore (R2 = 0.78; RMSE = 0.18 ), whereas MISR over the coastal areas of Karachi (R2 = 0.82; RMSE = 0.20 ). The results of the study will help the stakeholders in planning additional ground stations for operational monitoring of air quality at regional level.

Comparison of Aerosol Optical Depth Products from Multi-Satellites over Densely Populated Cities of Pakistan

Air pollution in Pakistan is causing damage to health, environment and quality of life. Air pollution in Pakistan is not effectively monitored due to heavy cost involved in setting up ground stations. However, Satellite remote sensing can effectively monitor the air pollution in terms of Aerosol Optical Depth (AOD) at regional as well as global level. However, algorithms used to derive AOD from different sensors have some inherited differences which can pose challenges in monitoring regional AOD at high temporal resolution using more than one sensor. Therefore, this study focuses on comparison of four major satellite based AOD products namely Moderate Resolution Imaging SpectroRadiometer (MODIS), Multi-angle Imaging SpectroRadiometer (MISR), Ozone Monitoring Instrument multiwavelength (OMI) aerosol product and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) with the ground based AErosol RObotic NETwork (AERONET) AOD which is only available from Lahore and Karachi in Pakistan. The correlation of various AOD products with AERONET AOD is estimated statistically through coefficient of determination (R2), Root Mean Square Error (RMSE), slope and intercept. It is noticed that MODIS is relatively accurate and reliable for monitoring air quality on operational bases over the land cover area of Lahore (R2= 0.78; RMSE = 0.18 ), whereas MISR over the coastal areas of Karachi (R2= 0.82; RMSE = 0.20 ). The results of the study will help the stakeholders in planning additional ground stations for operational monitoring of air quality at regional level.

Variability of Microphysical Parameters in High-Altitude Ice Clouds: Results of the Remote Sensing Method

The remote sensing method for retrieving vertical profiles of microphysical parameters in ice clouds from ground-based measurements taken by the Doppler radar and IR radiometer was applied to several cloud cases observed during different field experiments including FIRE-II, ASTEX, and the Arizona Program. The measurements were performed with the NOAA Environmental Technology Laboratory instrumentation. The observed ice clouds were mostly cirrus clouds located in the upper troposphere above 5.6 km. Their geometrical thicknesses varied from a few hundred meters to 3 km. Characteristic cloud particle sizes expressed in median mass diameters of equal-volume spheres varied from about 25 μm to more than 400 μm. Typically, characteristic particle sizes were increasing toward the cloud base, with the exception of the lowest range gates where particles were quickly sublimating. Highest particle concentrations were usually observed near the cloud tops. The vertical variability of particle sizes inside an individual cloud could reach one order of magnitude. The standard deviation of the mean profile for a typical cloud is usually factor of 2 or 3 smaller than mean values of particle characteristic size. Typical values of retrieved cloud ice water content varied from 1 to 100 mg m−3; however, individual variations were as high as four orders of magnitude. There was no consistent pattern in the vertical distribution of ice water content except for the rapid decrease in the vicinity of the cloud base. The relationships between retrieved cloud parameters and measured radar reflectivities were considered. The uncertainty of estimating cloud parameters from the power-law regressions is discussed. The parameters of these regressions varied from cloud to cloud and were comparable to the parameters in corresponding regressions obtained from direct particle sampling in other experiments. Relationships between cloud microphysical parameters and reflectivity can vary even for the same observational case. The variability diminishes if stronger reflectivities are considered. A procedure of “tuning” cloud microphysics–reflectivity regressions for individual profiles is suggested. Such a procedure can simplify the radar–radiometer method and make it applicable for a broader range of clouds.

Solar irradiance, air pollution and temperature changes in the Arctic

A highly significant decrease in the annual sums of global irradiance reaching the surface of the Arctic, averaging 0.36 W m -2 per year, was derived from an analysis of 389 complete years of measurement, beginning in 1950, at 22 pyranometer stations within the Arctic Circle. The smaller data base of radiation balance measurements available showed a much smaller and statistically non-significant change. Reductions in global irradiance were most frequent in the early spring months and in the western sectors of the Arctic, coinciding with the seasonal and spatial distribution of the incursions of polluted air which give rise to the Arctic Haze. Irradiance measured in Antarctica during the same period showed a similar and more widespread decline despite the lower concentrations of pollutants. A marked increase in the surface radiation balance was recorded. Possible reasons for these interpolar anomalies and their consequences for temperature change are discussed.