Instrument Development and Characterization of Atmospheric Aerosol Physical Properties Through Airborne Measurement

Citation

Wang, Jian (2003) Instrument Development and Characterization of Atmospheric Aerosol Physical Properties Through Airborne Measurement. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/GNFA-V973. https://resolver.caltech.edu/CaltechETD:etd-06092005-132829

Abstract

Atmospheric aerosol has significant impact on climate. It influences radiative transfer by scattering and absorbing sunlight and by changing the microphysical structure, lifetime, and amount of the clouds. Due to its short lifetime, the spatial and temporal distributions of tropospheric aerosol are highly inhomogeneous. Aircraft have proven to be an effective platform in characterizing the atmospheric aerosol. To maximize the potential and to reduce the artifacts associated with aircraft sampling, both improvements in existing instruments and developments of new instruments are required.

To increase the speed of submicron aerosol size distribution measurements, a mixing condensation nucleus counter (MCNC) has been developed. By carefully designing the mixing chamber and condenser, the response time of the MCNC was significantly reduced. Our experiments demonstrate that a differential mobility analyzer (DMA) coupled with the developed MCNC can measure complete aerosol size distributions in as little as 2 seconds.

The effects of bends and elbows on the diffusional losses of particle in nanometer range were studied. The results show that the effect of bends and elbows on particle diffusion loss is significant, and for Reynolds number smaller than 250, the enhancement of diffusion losses due to bends and elbows is sensitive to both the relative orientations of the bends and elbows and the lengths of straight tubing between them. Because of this sensitivity, direct calibration or simulation is required to assess nanoparticle penetration efficiencies for any flow system containing bends or elbows at low Reynolds number. When the Reynolds number exceeds 250, the enhancement is insensitive to the actual flow configurations. Experimental results are presented, which can be used for design of aerosol flow systems at Reynolds number larger than 250.

To minimize the airborne sampling bias, an advanced differential mobility analyzer (DMA) system for measuring submicron aerosol size distribution at ambient relative humidity, with special attention to implementation on aircraft, has been developed. The system includes an active RH controller, a cylindrical differential mobility analyzer (CDMA), and a condensation nucleus counter. A cascade controller consisting of two PID modules maintains the RH inside the CDMA at ambient RH by actively adding or removing water vapor from the air stream. The flows are controlled with feedback PID controllers, which compensate for the variation of pressure as the aircraft changes altitude. This system was integrated into the CIRPAS Twin Otter aircraft and used to measure ambient size distributions during the Aerosol Characterization Experiment-Asia (ACE-Asia), carried out from March to May, 2001, in Japan.

During the ACE-Asia experiment, the above DMA system, together with an aerodynamic particle sizer (APS), was used to characterize aerosol size distributions in East Asia during 19 flights on board of CIRPAS Twin Otter aircraft. Besides providing the aerosol size characteristics, the data were combined with chemical composition and aerosol mixing state measurements to predict the vertical profile of aerosol extinction, which was compared with those derived from simultaneous direct measurements of aerosol optical depth by the NASA 14-channel sunphotometer. Agreement between the predicted and derived aerosol extinction varies for different scenarios, but the discrepancies were generally within the calculated uncertainties.

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