Role of isoprene in secondary organic aerosol formation on a regional scale

Creators: Zhang, Yang; Huang, Jian-Ping; Henze, Daven K.; Seinfeld, John H.

Abstract

The role of isoprene as a source of secondary organic aerosol (SOA) is studied using laboratory-derived SOA yields and the U.S. Environmental Protection Agency regional-scale Community Multiscale Air Quality (CMAQ) modeling system over a domain comprising the contiguous United States, southern Canada, and northern Mexico. Isoprene is predicted to be a significant source of biogenic SOA, leading to increases up to 3.8 μg m^(−3) in the planetary boundary layer (PBL, defined as 0–2.85 km) and 0.44 μg m^(−3) in the free troposphere over that in the absence of isoprene. While the addition of isoprene to the class of SOA-forming organics in CMAQ increases appreciably predicted fine-particle organic carbon (OC_(2.5)) in the eastern and southeastern U.S., total OC_(2.5) is still underpredicted in these regions. SOA formation is highly sensitive to the value of the enthalpy of vaporization of the SOA. The role of isoprene SOA is examined in a sensitivity study at values of 42 and 156 kJ mol^(−1); both are commonly used in 3-D aerosol models. Prediction of ambient levels of SOA in atmospheric models remains a challenging problem because of the importance of emissions inventories for SOA-forming organics, representation of gas phase atmospheric chemistry leading to semivolatile products, and treatment of the physics and chemistry of aerosol formation and removal.

Additional Information

© 2007 American Geophysical Union. Received 20 March 2007; revised 3 June 2007; accepted 24 July 2007; published 23 October 2007. The work at NCSU was supported by NASA Award NNG04GJ90G, NSF Career Award Atm-0348819, and the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement DW13921548. The work at Caltech was supported by the Electric Power Research Institute. Thanks are due to Warren Peters, the U.S. EPA/OAQPS, and George Pouliot, Ken Schere, and Tom Pierce, the U.S. NOAA/EPA, for providing meteorological fields, emission inventories, initial and boundary conditions for the 2001 simulations and helpful discussions on BEIS v3.12 and v3.13; to Krish Vijayaraghavan, AER, Inc., for conducting baseline CMAQ simulation with the default SOA module; to Eric Edgerton and Rick Saylor, ARA, Inc., for providing the SEARCH OC measurement data; to Steve Howard and Alice Gilliland, the U.S. NOAA/EPA, for providing the Fortran code for extracting data from CMAQ and the CASTNet, IMPROVE, and AIRS-AQS observational databases; and to Shaocai Yu, the U.S. NOAA/EPA, for providing the Fortran code for statistical calculations. Thanks are also due to Prakash Bhave, the U.S. NOAA/EPA, for calculating SOA yields from monoterpenes at different values of DHn, 298, absorbing organic mass, and temperatures as well as insightful diagnostic analyses and discussions for low SOA formation at low values of DHn, 298 from CMAQ, and to Jesse Kroll, Aerodyne Research, Inc., and Edward Edney, the U.S. EPA, for helpful discussions on SOA formation mechanism from isoprene.

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