Investigating sources of secondary organic aerosols and linking laboratory and field studies with a rapid-flow reactor
Authors
Jose-Luis Jimenez — University of Colorado
Brett Palm — University of Colorado
Amber V. Ortega — University of Colorado, Boulder
Rui Li — State University of New York, Albany
Douglas A. Day — University of Colorado, Boulder
Michael Jelfs Cubison — Cooperative Institute for Research in Environmental Sciences
Joost deGouw — NOAA - Earth System Research Laboratory
William R. Brune — Pennsylvania State University
Category
Aerosol Properties
Description
Recent field studies reveal large formation of secondary organic aerosol (SOA) under urban polluted ambient conditions, and there are indications of strong synergy between anthropogenic pollution and biogenic volatile organic compounds (VOCs) in increasing SOA formation. Analysis of previous field studies depend on assumptions about vertical mixing, regional dispersion, and photochemical age in order to relate the evolution of SOA versus its precursors and oxidants, which limits the scientific insights achievable under many conditions. To directly study SOA formation in ambient air in real-time, our group has demonstrated a Potential Aerosol Mass (PAM) photo-oxidation flow reactor in conjunction with an Aerodyne aerosol mass spectrometer (AMS), a scanning mobility particle sizer (SMPS), and a proton-transfer-reaction mass spectrometer (PTRMS). We have used this system to characterize SOA formation in (a) urban air during CalNex-LA-2010 in the Los Angeles area of California, (b) forest air at the USFS Manitou Forest in Colorado during BEACHON-RoMBAS-2011, and (c) biomass smoke in at the USFS Fire Science Lab in Missoula, Montana, during FLAME-3. The PAM reactor uses mercury lamps to create OH concentrations up to 10,000 times ambient levels. High oxidant concentrations accelerate the processing of volatile organic compounds and inorganic gases and their growth into the aerosol phase. PAM photochemical processing can represent up to approximately 20 days of equivalent atmospheric aging in the span of 4 minutes of residence time in the reactor, and PAM-processed aerosols have shown aging signatures and sulfate and SOA yields similar to ambient and large chamber studies. In some campaigns we used a gas-phase denuder to study heterogeneous OH processing of the pre-existing aerosol or injected O3 or N2O5 in PAM without lights to investigate SOA formation from O3 or NO3 oxidation. In all cases PAM OH photoxidation enhances SOA at intermediate exposure but results in net loss of OA at very long exposures. SOA formation greatly exceeds that calculated from the measured precursors in urban and forest air. PAM oxidation also results in a similar slope in the Van Krevelen diagram than ambient oxidation. A model of the radical and oxidation chemistry has been developed to characterize the reactor under different conditions and understand its sensitivities. Lab experiments are used to determine SOA yields for key precursors of the above campaigns and also to study SOA formation under conditions simulating the 2010 Gulf of Mexico oil spill.
