Halogenation processes of secondary organic aerosol and implications on halogen release mechanisms

Abstract

Abstract. Reactive halogen species (RHS), such as X·, X 2 and HOX containing X = chlorine and/or bromine, are released by various sources like photo-activated sea-salt aerosol or from salt pans, and salt lakes. Despite many studies of RHS reactions, the potential of RHS reacting with secondary organic aerosol (SOA) and organic aerosol derived from biomass-burning (BBOA) has been neglected. Such reactions can constitute sources of gaseous organohalogen compounds or halogenated organic matter in the tropospheric boundary layer and can influence physicochemical properties of atmospheric aerosols.Model SOA from α-pinene, catechol, and guaiacol was used to study heterogeneous interactions with RHS. Particles were exposed to molecular chlorine and bromine in an aerosol smog-chamber in the presence of UV/VIS irradiation and to RHS, released from simulated natural halogen sources like salt pans. Subsequently, the aerosol was characterized in detail using a variety of physicochemical and spectroscopic methods. Fundamental features were correlated with heterogeneous halogenation, which results in new functional groups (FTIR spectroscopy), changes UV/VIS absorption, chemical composition (ultrahigh resolution mass spectroscopy (ICR-FT/MS)), or aerosol size distribution. However, the halogen release mechanisms were also found to be affected by the presence of organic aerosol. Those interaction processes, changing chemical and physical properties of the aerosol are likely to influence e.g. the ability of the aerosol to act as cloud condensation nuclei, its potential to adsorb other gases with low-volatility, or its contribution to radiative forcing and ultimately the Earth's radiation balance.

Secondary organic aerosol from chlorine-initiated oxidation of isoprene

“…The rate constant of the isoprene-chlorine reaction at 25 • C (2.64-5.50 × 10 −10 molecules −1 cm 3 ; Fantechi et al, 1998;Orlando et al, 2003;Ragains and Finlayson-Pitts, 1997) is much larger than the rate constant of the isoprene-OH reaction (1.00 × 10 −10 molecules −1 cm 3 ; Atkinson and Arey, 2003), suggesting that isoprene-chlorine chemistry could compete with isoprene-OH chemistry under certain conditions. Moreover, reactions between chlorine and isoprene or isoprene-derived SOA could serve as a reactive chlorine sink in the atmosphere, as has been proposed for reactions between chlorine and biogenic SOA (Ofner et al, 2012). To our knowledge, SOA formation from chlorine-initiated oxidation of isoprene has not been reported in the literature.…”

“…3. The oxidizing capacity of chlorine radicals has also been demonstrated for select biogenic SOA derived from α-pinene, catechol, and guaiacol, where halogenation led to significant SOA aging, formation of high-molecular-weight compounds, and particle growth (Ofner et al, 2012). High reactivity of chlorine radicals towards isoprene and its reaction products meant that extensive SOA processing could be easily achieved within laboratory timescales.…”

“…Photolysis of reactive halogen species produces halogen radicals that can react with O 3 , hydrocarbons, SOA, and other radicals in the atmosphere. Reactions with hydrocarbons and organic aerosols serve as chlorine and bromine radical sinks (Buxmann et al, 2015;Ofner et al, 2012;Platt and Hönninger, 2003), especially in high-NO x environments where halogen recycling via HO x and XO reaction pathways is suppressed (Edwards et al, 2013;Riedel et al, 2014;Simpson et al, 2015).…”

Application of FTIR Spectroscopy in Environmental Studies

“…The role of halogens in the aging process of organic aerosols was determined by Ofner and coworkers (2012) using long-path FTIR spectroscopy (LP-FTIR), attenuated-total reflection FTIR (ATR-FTIR), UV/VIS spectroscopy, and ultrahigh resolution mass spectroscopy (ICR-FT/MS). They concluded that the aerosol-halogen interaction might strongly contribute to the influence of organic aerosols on the climate system (Ofner et al, 2012).…”

“…The role of halogens in the aging process of organic aerosols was determined by Ofner and coworkers (2012) using long-path FTIR spectroscopy (LP-FTIR), attenuated-total reflection FTIR (ATR-FTIR), UV/VIS spectroscopy, and ultrahigh resolution mass spectroscopy (ICR-FT/MS). They concluded that the aerosol-halogen interaction might strongly contribute to the influence of organic aerosols on the climate system (Ofner et al, 2012). Khalizov and coworkers (2010) investigated the heterogeneous reaction of nitrogen dioxide (NO2) on fresh and coated soot surfaces to assess its role in night-time formation of nitrous acid (HONO) in the atmosphere using ATR-FTIR (Khalizov et al, 2010).…”