Real refractive indices and volatility of secondary organic aerosol generated from photooxidation and ozonolysis of limonene, α-pinene and toluene

Abstract

Thermodenuding particles can provide insights into aerosol composition and may be a way to create particles in laboratory chambers that better mimic the atmosphere. The relative volatility of secondary organic aerosol (SOA) was investigated by evaporating organics from the particles using a thermodenuder (TD) at temperatures between similar to 60 and 100 degrees C. Volatility was influenced by the parent hydrocarbon, oxidation chemistry and relative humidity (RH). For SOA generated from ozonolysis, limonene had lower volatility than alpha-pinene, and OH scavengers had no influence on volatility. For photooxidation, alpha-pinene SOA was slightly more volatile than limonene SOA. Increasing RH also modestly increased volatility, while toluene SOA was unaffected by heating to 98 degrees C. For both alpha-pinene and limonene, the concentration of NOx and the HC/NOx ratio had no discernible effect on SOA volatility. Refractive indices for the original and denuded particles were retrieved from polar nephelometer measurements using parallel and perpendicular polarized 532 nm light. Retrievals were performed with a genetic algorithm method using Mie-Lorenz scattering theory and measured particle size distributions. Retrieved refractive indices for the SOA before thermodenuding varied between 1.35 and 1.61 depending on several factors, including parent hydrocarbon, oxidation chemistry, and SOA generation temperature. For high NOx SOA, as particles shrink, their refractive index returns to the value of the corresponding size particles before heating (limonene) or slightly higher (alpha-pinene). For low NOx however, the resulting refractive index is 0.05 +/- 0.02 lower than the corresponding size undenuded particles. Additionally, for alpha-pinene SOA from ozonolysis with OH radical scavenger, resulting refractive indices were higher by about 0.03 after heating. Consistent with no change in size, refractive indices of toluene SOA were unaffected by heating. Finally, refractive index data available to date are reviewed, leading to the suggestion that the most representative values for m(r) at lambda = 532 nm for biogenic and anthropogenic SOA are 1.44 and 1.55, respectively.