Characterization of the Physical Properties of Atmospheric Aerosols through Airborne Sampling

Abstract

Atmospheric aerosols are highly variable both in their characteristics and their concentration. Aircraft-based sampling provides a means of characterizing such a complex and variable constituent. To maximize the potential of aircraft sampling, improvements in existing aerosol instrumentation and development of new instruments is necessary. A new approach to inversion of differential mobility analyzer (DMA) data has been developed that improves the accuracy with which accelerated size distribution measurements can be made. Extensive testing of this inversion has demonstrated that it accurately recovers initial distributions of actual aerosols sampled directly by a commonly used detector, actual aerosols analyzed with a scanning DMA, and test-case aerosols. To expand upon the measurable size range of the DMA, a new technique has been employed in which the associated flow rates are varied in conjunction with the applied voltage. The performance of the instrument was evaluated theoretically through detailed flow and trajectory modeling, and experimentally through comparison with a constant flow instrument. By varying the flow rates by an order of magnitude, it was shown that measurable size range could be increased by a factor of four. During the Second Aerosol Characterization Experiment (ACE-2), aerosol size distributions were measured using a DMA and two optical particle counters (OPCs) during 21 missions flown on the CIRPAS Pelican. These data were combined with chemical composition measurements to derive a range of associated optical properties, which were compared with simultaneous direct measurements by a sunphotometer and three nephelometers. Agreement between derived and measured quantities varied, but was generally within calculated uncertainties. A similar payload to that used during ACE-2 was employed for the Southern California Ozone Study (SCOS). Physical and chemical aerosol properties were analyzed to provide a three-dimensional description of the Los Angeles aerosol. Pronounced aerosol layers aloft were found to exist over the majority of the study area, and over a range of altitudes. In addition to the complex vertical structure of the aerosol observed, horizontal gradients were found to be sufficient to cause more than 50% variability in aerosol concentration over 5 km. The sampled aerosol was estimated to enhance photolysis rates of important gas-phase species by up to 5%.