Dr. Jun Wang

Impact of relative humidity and aerosol mixing state on

satellite characterization of urban aerosols
 

References: Wang, J., and S. T. Martin, Satellite characterization of urban aerosols: Importance of including hygroscopicity and mixing state in the retrieval algorithms,  J. Geophys. Res., doi:10.1029/2006JD007705, accepted, 2007. (pdf file)

1. Motivation
 

Satellite retrieval of aerosols is an inversion problem. Assumption of surface reflectance and aerosol optical properties (such as single scattering albedo and phase function) have to be pre-described during the computing of look table that links the satellite-measured reflectance with those retrieval parameters such as aerosol optical thickness and effective radius. In this paper, we investigate how the retrieval accuracy can be affected by those assumptions, in particular, the assumption on urban aerosol optical properties.

Recent field experiments in India and Mexico city have shown that the urban aerosols are most likely composed of internally mixed black carbon and sulfate-nitrate aerosols, and their single scattering albedo can be as low as 0.80 or smaller and is sensitive the ambient relative humidity.  In here, we investigate how the aerosol mixing state and ambient relative humidity, which is neglected in the majority of aerosol retrieval algorithms, will affect the retrieval accuracy of aerosol optical thickness and size.

2. Results

(see details in the manuscript)

The following figure shows the isopleths of the ratio of the retrieved aerosol optical thickness assuming external mixing with optical properties at 70% RH to that assuming internal mixing with optical properties at either 70% RH or variable with RH. (a) Optical properties of variable RH in the case of internal mixing; variable scattering angle; BC/sulfate mass ratio of 5%. (b) Optical properties of variable RH in the case of internal mixing; scattering angle at 180°; variable BC/sulfate mass ratio. (c) Optical properties at 70% RH in the case of internal mixing; variable scattering angle; BC/sulfate mass ratio of 5%; shown for increasing surface reflectance. (d) Optical properties at 70% RH in the case of internal mixing, scattering angle at 180°; variable BC/sulfate mass ratio; shown for increasing surface reflectance. The calculations are for a wavelength of 0.67 µm, a solar zenith angle of 30°, and surface reflectance of 0.06 (panels a and b).  In the shaded region of panel d, the retrievals are physically unrealistic.