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Abstract
The hygroscopic behavior of atmospheric aerosols complicates modeling and measurements of aerosol properties adding significant uncertainty to our best estimates of the direct effect aerosols exert on the radiative balance of the atmosphere. Airborne measurements of aerosol hygroscopicity are particularly challenging but critically needed. This motivated the development of a new system designed to measure the dependence of the aerosol light scattering coefficient (σ sp) on relative humidity (RH), known as f(RH), in real-time on an aerial platform.
The new instrument has several advantages over existing systems. It consists of three integrating nephelometers and humidity conditioners for simultaneous measurement of the σ sp at three different RHs. The humidity is directly controlled in exchanger cells without significant temperature disturbances and without particle dilution, heating, or loss of volatile compounds. The single-wavelength nephelometers are illuminated by LED-based light sources thereby minimizing heating of the sample stream. The flexible design of the RH conditioners, consisting of a number of specially designed exchanger cells (driers or humidifiers), enables us to measure f(RH) under hydration or dehydration conditions (always starting with the aerosol in a known state) with a simple system reconfiguration. These exchanger cells have been characterized for losses of particles using latex spheres and laboratory generated ammonium sulfate aerosols. The performance of this instrument has been assessed aboard DOE's G-1 research aircraft during test flights over California, Oregon, and Washington.
Copyright 2013 American Association for Aerosol Research
Acknowledgments
The authors would like to thank Nels Laulainen (PNNL), David Covert (UW), and Patrick Sheridan (NOAA) for extensive technical discussions; the flight and ground crew of the G-1 research aircraft for support and flight execution. We also would like to acknowledge the thorough and thought-provoking reviews of two anonymous reviewers, which have significantly improved this article. Financial support for the design and development of this instrument was provided by the American Recovery and Reinvestment Act through the US Department of Energy's Atmospheric Radiation Measurement Program.
[Supplementary materials are available for this article. Go to the publisher's online edition of Aerosol Science and Technology to view the free supplementary files.]