Aerosol water uptake in the atmosphere alters aerosol size distributions, changes aerosol optical properties, and consequently impacts the aerosol radiative forcing. The parameterization of water uptake in aerosol transport models is empirical and unchecked, largely because of the lack of available measurements. We have developed a retrieval of the aerosol liquid water path (LWP) and hygroscopic growth using the AERONET database, and apply our results at over 200 AERONET locations. The real refractive index of prevalent atmospheric aerosols at the 500 nm wavelength ranges from 1.50 for sea salt to 1.56 for dust. Liquid water, on the other hand, has a real refractive index of 1.33 at 500 nm. We use this disparity in refractive indices to determine the equivalent water fractions for aerosol mixtures associated with the AERONET refractive index retrievals. Once the aerosol water fraction is known, we can also determine the hygroscopic growth factor, aerosol LWP, and the dry aerosol loading in the atmospheric column. Our aerosol LWP results are highly correlated with the following parameters: fine mode volume concentration, aerosol optical thickness (440 nm), aerosol water fraction, and coarse mode volume concentration (average correlation coefficients for all 200 sites ranges from 0.47 for the coarse mode concentration to 0.78 for the fine mode concentration). The regressions of these parameters have different slopes at each of the AERONET sites, which suggests a LWP sensitivity to aerosol composition. Aerosol liquid water path is not highly correlated with the mode radii or the fine mode volume fraction (average correlation coefficients for all sites range from 0.11 for the coarse mode median radius to 0.27 for the fine mode volume fraction). We compiled monthy-averaged statistics at over 200 AERONET locations based upon the daily-average AERONET retrievals; our results are consistent with regional and seasonal expectations. Aerosol water fractions range from a low value of 0.14 at the Etosha Pan site (Namibia) to a high value of 0.73 at the Anmyon site (South Korea) in August, which is consistent with the arid and humid climates of those two sites. Likewise, the hygroscopic growth factor ranges from 1.05 at Etosha Pan to 1.69 at Anmyon for that month. The aerosol water fraction does not necessarily dominate the aerosol LWP (the effective radius and aerosol loading are also important), so these two sites do not represent the extreme aerosol LWPs. Rather, Beijing had the highest average LWP for August, with a value of 120 mg m-2, and Tinga Tingana in Australia had the lowest August average with a value of 5 mg m-2. We will present the regional and seasonal variability of these retrieved parameters at several representative AERONET sites. We will also discuss the sensitivity of this retrieval to a variety of aerosol mixtures.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 0305 Aerosols and particles (0345;
- 0394 Instruments and techniques