Aerosol optical properties and the influence of particulate organic matter

This study summarizes (1) the aerosol's optical properties related to climate forcing along the northeast coast of North America, as part of the New England Air Quality Study - Intercontinental Transport and Chemical Transformation (NEAQS-ITCT) 2004 field campaign, (2) a closure evaluation of the hygroscopic response in the aerosol's real refractive index (n) between experimental measurements and a semi-empirical model for select aqueous solutions containing inorganic salts, dicarboxylic acids, and their mixtures, (3) an overall closure study, which uses results from the above refractive index closure study, on aerosol's optical parameters at dry and controlled RH conditions with an emphasis on the influence of POM, and (4) the impacts of POM on aerosol's optical and hygroscopic properties based on the results from ambient aerosols and laboratory aerosols.;During the NEAQS-ITCT 2004 field campaign, humidogram curve structure for the ambient aerosols was dominated by hygroscopic growth without hysteresis (76% frequency). Dependence of the aerosol's total light scattering (sigma sp) values with changes in RH,fsigmasp(RH, RHRef), was observed to decrease with increasing mass fraction of particulate organic matter (POM, FO). Statistical analyses indicated that increasing resulted in a less hygroscopic aerosol, while increasing molar equivalence ratio (ER) resulted in lower hysteresis factors based on the aerosols' optical responses at a 0.95 confidence level.;A closure evaluation of the hygroscopic response in n was completed with measurements and a semi-empirical model using aqueous solutions containing inorganic salts, dicarboxylic acids, and their mixtures. Measurements of n were made for subsaturated aqueous solutions at select mass fractions of solute with an ellipsometer and a refractometer. The semi-empirical model was based on the Zdanovskii-Stokes-Robinson (ZSR) mixing rule, the partial molar refraction (PMR) method and previously published thermodynamic data. Closure was obtained within 0.75% for n values obtained from the ellipsometry and refractometry measurements when compared to results from the ZSR-PMR model for select aqueous solutions that contain inorganic salts and POM. It was observed that the hygroscopic response of n on RH could be described by a linear equation. Sensitivity of the top of the atmosphere aerosol radiative forcing to changes in n, and the strengths and limitations for the ellipsometer, the refractometer, and the ZSR-PMR method are also discussed.;As an extension of the refractive index closure, an overall closure study was completed for the first time by comparing measured and modeled values of the optical and hygroscopic properties of the same test aerosols. It was observed that the humidograph-measured deliquescence RH (DRH) and crystallization RH (CRH) values were within the ranges of the EDB-based measurements, and the fsigmasp(RH, RHRef) values at each RH value decreased when the mass ratio of the tested organic acids increased. Comparisons between predicted and measured sigmasp values revealed good agreement (within 8%) at dry RH conditions for all test aerosols. In addition, the agreement between measured and modeled values of f sigmasp(RH, RHRef), b, and a at low RH and RH = 82% was also evaluated and the average absolute error (AAE) values between corresponding measured and modeled results ranged from 0.1% to 30.8%. The artifacts due to temperature, RH uncertainties and sample heating were also evaluated. Comparing the total relative uncertainty for sigmasp with the AAE values between measured and modeled sigmasp values indicated that the closure on sigmasp was achieved for all of the laboratory aerosols tested in this study.;Based on the results from this research on ambient and laboratory aerosols, it can be concluded that (1) presence of organic compounds (i.e., glutaric and succinic acids) decrease the hygroscopic growth of aerosol particles when compared with aerosol particles composed of pure inorganic compounds (i.e., sodium chloride and ammonium sulfate) and that increasing the mass ratio of these organic species results in a reduction in the aerosol's optical hygroscopicity. (Abstract shortened by UMI.)...