Study Aerosol Humidity Effects Using the ARM Measurements

Li, Z., University of Maryland

Radiation Processes

Aerosol

Jeong, M.-J., Z. Li, E. Andrews, and S.-C. Tsay (2007). Effect of aerosol humidification on the column aerosol optical thickness over the Atmospheric Radiation Measurement Southern Great Plains site, J. Geophys. Res., 112, D10202, doi:10.1029/2006JD007176.


(a)-(j) Column-mean aerosol humidification factor as functions of the weighted column mean RH, . The ambient RH at one of the ten level-legs of the In-situ Aerosol Profile (IAP) measurements is replaced with RH equals 99%.


Comparison of estimated column R(RH) following six different methods (M1~M6). Gray solid lines and black dashed lines are linear fit and one-to-one lines, respectively.


(a)-(j) Column-mean aerosol humidification factor as functions of the weighted column mean RH, . The ambient RH at one of the ten level-legs of the In-situ Aerosol Profile (IAP) measurements is replaced with RH equals 99%.

Comparison of estimated column R(RH) following six different methods (M1~M6). Gray solid lines and black dashed lines are linear fit and one-to-one lines, respectively.

Aerosol optical thickness (AOT) is a vertically integrated (column) quantity whose magnitude depends on aerosol mass loading, scattering, and absorption efficiencies. In addition to these inherent properties, AOT also varies with the ambient humidity. Quantifying the humidification effects of aerosols has important implications. To date, many studies investigating the influence of humidity on the AOT are based on the measurements made for short time periods, reporting the range of contribution of humidity effects. It is necessary to study the behavior of AOT in response to changes in humidity variables throughout the atmospheric column using data sets incorporating the simultaneous vertical distributions of humidity and aerosols together.

This study investigates the aerosol humidification effect (AHE) using 70 profiles of the aerosol scattering coefficients at high (~80%) and low (~40%) relative humidity (RH) levels and an absorption coefficient at a low RH level. The measurements were obtained by a light aircraft (Cessna C-172N) over the Southern Great Plains (SGP) site from April 2003 to June 2004. The column aerosol humidification factor, R(RH), defined as the ratio of AOT at the ambient RH to that at RH, equals 40% throughout the column and rarely exceeded 1.3 (mean, 1.09 ± 0.12) over the SGP site. However, for an atmospheric column of a constant RH that equals 85%, R(RH) is greater than 1.5 for the majority of cases (mean, 1.57 ± 0.28). R(RH) was fitted to a function of column RH based on this unique aerosol data set. Several methods were proposed to estimate R(RH) for use when direct measurements of R(RH) are not available.

It was found that for the relationship between R(RH) and aerosol extinction coefficient the weighted column-mean RH works best. Performance of other methods depends on the measurements available. Sensitivity of R(RH) to a very humid (RH equals 99%) layer with varying thickness values (0.1–0.3 km) is examined. The results indicate that, on average, the AHE on the AOT over the SGP site is not likely to exceed 50%. The methods and results of this study may be utilized with caution to remove the AHE from the AOT retrieved using satellite or automated Sun photometer measurements, which will be useful for studies on aerosol indirect effect or quantifying cloud contamination in aerosol retrievals.