This project will investigate lightweight oxidation products from anthropogenic and biogenic volatile organic compounds (VOCs) as a source for secondary organic aerosol (SOA) in urban Atlanta, Georgia. The mechanism through which SOA may be formed involves partitioning of low carbon-number, water-soluble, secondary VOCs to the particle aqueous phase where they are heterogeneously oxidized to lower vapor pressure products. To investigate this process the Principal Investigator (PI) proposes to undertake online measurements of a suite of small organic acid and carbonyl compounds in the gas and aerosol phases, along with additional ancillary measurements. These experiments will be conducted over an extended period of time at a variety of sites to examine factors that influence the gas-particle partitioning and its linkage to hydrophilic/hydrophobic fractions and overall particle water-soluble organic carbon mass. The motivation for the project is the unexpectedly high SOA concentrations observed in urban atmospheres that cannot be explained by current SOA formation chemical mechanisms.
Research is showing that water-soluble organic components of ambient aerosols (often referred to as water soluble organic carbon, i.e., WSOC), play important roles in determining the extent of aerosol environmental impacts, including effects on visibility, climate forcing and human health. This project investigated the source and processes that control concentrations of WSOC in the southeastern United States and builds on our earlier research that identified a possible synergistic processes by which natural (biogenic) and anthropogenic emissons somehow interact to more effectively produce WSOC aerosol relative to each emission alone. This process could have major implications for aerosol control strategies in urban regions with large biogenic emissions and may lead to important feedbacks between aerosols and climate. By developing new instrumentation to directly measure both WSOC gas and particle phases simultaneously, and in near real-time, this project identified processes that lead to WSOC aerosol. Extensive ambient measurements employing this novel method showed that heterogeneous reactions in particle water are an important route to generating WSOC aerosol in environments where biogenic gases are mixed with pollutants. At relative humidities (RH) over 70% we estimated that in Atlanta, on average 0.3 to 0.9 µg/m3 of WSOC were formed by this route, a substantial fraction of the total WSOC. The unique biogenic/anthropogenic process was verified by not observing a similar liquid water gas-particle partitioning process in Los Angeles, a region soleley dominated by anthropogenic emissions. In the Southeast, and in regions with similar emissions, we postulated that a liquid phase aerosol formation route could lead to unique interations between natural processes and anthropogenic emissions, such as vegitation provides both the gas phase organic compounds that form WSOC, and also liquid water, through contributions from evapotranspiration, which provides the medium for the chemical reactions. The project also investigated ancillary aerosol properties that may result from this heterogeneous aerosol formation process. Optical properties of aerosols formed in the southeast urban environment were found to differ than those formed in anthropogenic-dominated environments; the WSOC in Atlanta was much whiter in color compared to brown WSOC observed in Los Angeles. The light absorbing properties of WSOC can affect photochemical processes and regional climate. This project also contributed to research by other investigators on a number of topics. The phase properties of Atlanta WSOC as a function of RH were found to transition from a liquid to glassy phase as RH dropped. Factors that influence soluble aerosol iron were also studied. Iron can play an important role in heterogeneous oxidation reactions (e.g., fenton reactions) and so may impact WSOC formation. We found no evidence for this, instead water-soluble iron was only found to be linked to aerosol acidity (pH).
