A Novel Method For Retrieving Aerosol Physical and Optical Properties From Aerosol Size Distribution Measurements During the Big Bend Regional Aerosol and Visibility Study (BRAVO)

摘要

As part of the Big Bend Regional Aerosol Visibility and Observation study (BRAVO) in July through October of 1999, dry aerosol size distributions were measured over the size range of 0.05 < D{sub}P < 20μm, using a TSI differential mobility analyzer (DMA), PMS LASAIR 1003 optical particle counter (OPC), and TSI aerodynamic particle sizer 3320 (APS). The experimental set-up was designed to maximize counting statistics by minimizing particle losses and measurement biases. Aerosol measurements were made at low relative humidity in order to avoid uncertain particle sizing from changing relative humidities both in the ambient and sampled air. Calibrations were performed to characterize the response of the OPC and APS to particles of different size and composition. Constructing a complete size distribution using data from all three instruments requires that distributions be reconciled in the overlap size regions. We describe a new method developed to align size distributions in the instrument overlap regions that does not require assumptions regarding aerosol composition, and allows for the retrieval of aerosol real refractive index and effective density. To validate the method, retrieved particle real refractive index was compared with volume-weighted model estimates based on measured PM{sub}2.5 chemical composition. The study average retrieved real refractive index was m{sub}r = 1.566±0.012 and the average computed PM{sub}2.5 refractive index was m{sub}r = 1.56±0.02; the agreement is well within experimental uncertainties. The average value of computed PM{sub}2.5 bulk density was 1.85±0.14 g cm{sup}(-3). The average value of retrieved effective density, a function of particle dynamic shape factor, was 1.56±0.12 g cm{sup}(-3). The comparisons of effective density with computed bulk density suggested an average particle dynamic shape factor of χ = 1.2. Sensitivity studies showed that real refractive index could be retrieved with uncertainties on the order of 2-3%, and effective density was retrieved with uncertainties on the order of 20-30%.