Formation of halogen-induced secondary organic aerosol (XOA)
2 Atmospheric Chemistry Research Laboratory, University of Bayreuth
P 3.8 in Ökosysteme: Funktion und Leistungen
Caused by their potential of stratospheric ozone depletion and surface ozone destruction, reactive halogen species (RHS) are a very important topic of atmospheric research. Crucial sources for halogens in the troposphere are halogen-release from sea-salt aerosol (Finlayson-Pitts, 2003) and heterogeneous reactions on those aerosol surfaces (Rossi, 2003). RHS seem to interact with secondary organic aerosol (SOA) precursors similarly to common atmospheric oxidizing gases like OH radicals and ozone. Although aerosol formation from reaction of RHS with typical SOA precursors was previously studied, e.g. by Cai et al. (2006), no data are available on bromine-induced aerosol formation from organic precursors yet. The potential interaction of RHS with performed SOA has recently been studied (Ofner et al., 2012).
An aerosol smog-chamber was used to examine the halogen-induced secondary organic aerosol (XOA) formation under atmospheric conditions using simulated sunlight. With a concentration of 10 ppb for the organic precursor and 2 ppb for molecular chlorine and 10 ppb for molecular bromine the experimental setup is close to ambient conditions. By combined measurements of the aerosol size distribution, the ozone and the NOx concentrations, as well as the decay of the aerosol precursor, determination of aerosol yields and aerosol growth factors could be achieved. Thereby the decay of the aerosol precursor was analyzed by capillary gas chromatography coupled with flame-ionization detection (GC-FID) and the aerosol size distribution was measured using a Scanning Mobility Particle Sizer (SMPS). We observed XOA formation even at very low precursor and RHS concentrations with a diameter mode at about 20 nm and a number concentration up to 10000 particles cm-3. While the XOA formation from chlorine is very rapid, the interaction of bromine with the organic precursors is about five times slower.
These studies may be related to natural XOA formation in sea-salt dominated environments, where we find organic species as precursors from biogenic or anthropogenic sources and photolyzed RHS from the surface or the sea-salt aerosol.
These studies may be related to natural XOA formation in sea-salt dominated environments, where we find organic species as precursors from biogenic or anthropogenic sources and photolyzed RHS from the surface or the sea-salt aerosol.
This work was funded by German Research Foundation (DFG) under grant HE 5214/5-1.
References:
Cai, X., and Griffin, R. J.: Secondary aerosol formation from the oxidation of biogenic hydrocarbons by chlorine atoms, J. Geophys. Res., 111, D14206/14201-D14206/14214, 2006.
Finlayson-Pitts, B. J.: The tropospheric chemistry of sea salt: A molecular-level view of the chemistry of NaCl and NaBr, Chem. Rev., 103, 4801-4822, 2003.
Ofner, J. Balzer, N., Buxmann, J., Grothe, H., Schmitt-Kopplin, Ph., Platt, U., and Zetzsch, C., Halogenation processes of secondary organic aerosol and implications on halogen release mechanisms, Atmos. Chem. Phys. Discuss. 12, 2975-3017, 2012.
Rossi, M. J.: Heterogeneous reactions on salts, Chem Rev, 103, 4823-82, 2003.