Direct evidence for chlorine-enhanced urban ozone formation in Houston, Texas

Abstract

Urban air pollution is characterized by high ozone levels, formed when volatile organic compounds (VOCs) are oxidized in the presence of nitrogen oxides (NO x ). VOC and NO x emissions controls have traditionally been implemented to reduce urban ozone formation, however, a separate chemical species implicated in ozone formation in Houston, TX and possibly other urban areas is the chlorine radical (Cl Á ). Cl Á enhances tropospheric VOC oxidation, but is not included in models used to develop air quality attainment plans. We present results of a three-fold approach to elucidate the importance of Cl Á in urban ozone formation: (1) the first direct evidence of chlorine chemistry in the urban troposphere, (2) enhanced ozone formation (>75 parts per 10 9 (ppb/h) observed when small amounts of chlorine (Cl 2 ) are injected into captive ambient air, and (3) enhanced ozone formation (B16 ppb) predicted by regional photochemical models employing Cl Á chemistry. These results suggest that reducing chlorine emissions should be considered in urban ozone management strategies. r

Secondary organic aerosol from chlorine-initiated oxidation of isoprene

“…Outside of the MBL and polar regions, natural emissions of reactive halogen species have been observed in volcano plumes (Bobrowski et al, 2007) and over salt lakes (Kamilli et al, 2016;Stutz, 2002). Anthropogenic sources include industrial emissions (Chang and Allen, 2006;Riedel et al, 2013;Tanaka et al, 2003), oil and gas production , water treatment (Chang et al, 2001), biomass burning (Lobert et al, 1999), engine exhaust (Osthoff et al, 2008;Parrish et al, 2009), and NO x -mediated heterogenous reactions, notably the production of ClNO 2 via reactive uptake of N 2 O 5 onto particles containing Cl − (Thornton et al, 2010). Recent studies have found that models underpredict the abundance of reactive halogen species, suggesting incomplete understanding of their sources (Faxon and Allen, 2013;Faxon et al, 2015;Simpson et al, 2015;Thornton et al, 2010).…”

“…Studies have shown that chlorine-initiated oxidation of α-pinene (Cai and Griffin, 2006;Ofner et al, 2013), toluene (Cai et al, 2008;Huang et al, 2014;Karlsson et al, 2001), and polycyclic aromatic hydrocarbons (PAHs; Riva et al, 2015) leads to SOA formation, with SOA yields close to unity reported for select PAHs . Reactive chlorine species could also enhance OH-radical propagation , nocturnal NO x recycling (Riedel et al, 2012;Thornton et al, 2010), and ozone production (Tanaka et al, 2003), further increasing the oxidative capacity of the atmosphere.…”

“…Major gas-phase products include methyl vinyl ketone (MVK), methacrolein (MACR), chloroacetone, chloroacetaldehyde, hydrochloric acid, and isomers of 1-chloro-3-methyl-3-butene-2-one (CMBO), a unique tracer for atmospheric chlorine chemistry (Nordmeyer et al, 1997;Riemer et al, 2008;Tanaka et al, 2003). 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.…”

“…Previous studies that observed C 5 H 8 O during OH-initiated oxidation of isoprene identified it as 2-methylbut-3-enal, but its formation mechanism remains unclear (Brégonzio-Rozier et al, 2015;Healy et al, 2008) and is beyond the scope of this work. It should be noted that CMBO has long been identified as a unique gas-phase marker for isoprene-chlorine oxidation (Chang and Allen, 2006;Nordmeyer et al, 1997;Riemer et al, 2008;Tanaka et al, 2003). Degradation of CMBO by OH is implemented in some air quality models, though only with inferred reaction rates ).…”

Gaseous elemental mercury depletion events observed at Cape Point during 2007–2008

“…The gas phase reaction with ozone is too slow at ozone mole fractions observed at Cape Point even at the highest rate constant reported (Lin et al, 2006), and the reactions with OH and H 2 O 2 are too slow at their concentrations expected in the MBL. But the vegetation of the Cape Peninsula might emit volatile organic compounds that in combination with marine chlorine emissions might enhance the reactivity without O 3 destruction (Chang et al, 2002;Tanaka et al, 2003) and reduce the lifetime of elemental mercury. But this explanation does not fit the observations by Temme et al (2003) and neither those of Joachim Kuss (unpublished results), the latter with DEs far from any vegetated coast.…”