(160g) Effect of Humidity On Secondary Organic Aerosol (SOA) Formation From Biogenic Hydrocarbons and Nitrate Radicals

Ng, N. L., Georgia Institute of Technology
Boyd, C. M., University of South Carolina
Xu, L., Georgia Institute of Technology

The reactions of biogenic volatile organic compounds (BVOCs) with nitrate radicals (NO3) represent a direct link of anthropogenic and biogenic emissions. Half of the aerosol lifetime is in the dark where NO3 radicals and ozone can be the prevalent oxidants. Compared to SOA formation from ozonolysis and photooxidation chemistry, SOA formation and evolution involving NO3 radicals have received far less study. Water is ubiquitous in the atmosphere and water vapor can affect the mechanisms, chemical composition, and physical and chemical properties of aerosols formed. This can be especially important for nighttime chemistry (where NO3 radicals can dominate) where ambient RH is high. Here we report a series of experiments to study SOA formation from the reaction of different biogenic hydrocarbons (isoprene, a-pinene, and b-caryophyllene) with NO3 radicals. Experiments are performed in the newly constructed Georgia Tech indoor chamber facility, which consists of two 10 m3 Teflon chambers suspended in a temperature controlled enclosure (4-40 oC). The experiments are performed in the dark using thermal decomposition of N2O5 as a source of NO3 radicals. Experiments are conducted under both dry and humid conditions: dry (RH< 5%) and 80% RH. Aerosol growth and composition are monitored by Scanning Mobility Particle Sizer (SMPS) and High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). The SOA yields (and parameterizations) for these experiments will be presented and the effect of water vapor on SOA formation mechanism and composition will be discussed.