Secondary Organic Aerosol (SOA) from Nitrate Radical Oxidation of Monoterpenes: Effects of Temperature, Dilution, and Humidity on Aerosol Formation, Mixing, and Evaporation

Nitrate radical (NO3) oxidation of biogenic volatile organic compounds (BVOC) is important for nighttime secondary organic aerosol (SOA) formation. SOA produced at night may evaporate the following morning due to increasing temperatures or dilution of semivolatile compounds. We isothermally dilute the oxidation products from the limonene+NO3 reaction at 25 degrees C and observe negligible evaporation of organic aerosol via dilution. The SOA yields from limonene+NO3 are approximately constant (similar to 174%) at 25 degrees C and range from 81 to 148% at 40 degrees C. Based on the difference in yields between the two temperatures, we calculated an effective enthalpy of vaporization of 117-237 kJ mol(-1). The aerosol yields at 40 degrees C can be as much as 50% lower compared to 25 degrees C. However, when aerosol formed at 25 degrees C is heated to 40 degrees C, only about 20% of the aerosol evaporates, which could indicate a resistance to aerosol evaporation. To better understand this, we probe the possibility that SOA from limonene+NO3 and beta-pinene+NO3 reactions is highly-viscous. We demonstrate that particle morphology and evaporation is dependent on whether SOA from limonene is formed before or during the formation of SOA from beta-pinene. This difference in particle morphology is present even at high relative humidity (similar to 70%).