The atmosphere is an oxidizing environment. Gas-phase oxidants, including ozone, hydroxyl radicals (OH), nitrate radicals (NO₃), chlorine atoms (Cl), and bromine atoms (Br), can react with organic and inorganic pollutants to generate a myriad of gas- and condensed-phase oxidation products. With regards to atmospheric aerosols, OH is particularly important in initiating the oxidation of sulfur dioxide to generate sulfuric acid and initiating the oxidation of volatile organic compounds (VOCs) to generate low-volatility organic compounds that condense to form secondary organic aerosol (SOA). NO₃ is an important oxidant at nighttime and in some cases during the daytime. Significant Cl production occurs in regions such as the marine boundary layer, polluted coastal cities, and the Arctic atmosphere. Additionally, significant inland Cl production has been observed, bleach washing has been shown to initiate significant indoor chlorine chemistry, and both Cl and Br have been linked to enhanced secondary aerosol formation in China. 

To date, most laboratory SOA formation studies have used O₃, OH, and to a lesser extent NO₃, to mimic daytime and nighttime oxidation of hydrocarbons. A handful of studies that have measured yields of SOA obtained from Cl oxidation of VOCs have shown that Cl exposure generates SOA in yields that are comparable to, or exceed, OH oxidation of the same precursors. Prior to this project, SOA formed from Br oxidation of VOCs had not been studied. To investigate these knowledge gaps, we characterized the chemical composition and yield of laboratory SOA generated in an oxidation flow reactor (OFR) from the OH and Cl oxidation of 5 VOCs (isoprene, a-pinene, toluene, n-dodecane, decamethylcyclopentasiloxane), and the Br oxidation of isoprene and a-pinene. Because OFRs use residence times that are on the order of minutes and oxidant concentrations that are typically 100-1000 times higher than ambient levels, we also compared the chemical composition and mass yields of SOA obtained from OH and/or Cl oxidation of the aforementioned VOCs plus m-xylene, ethylbenzene and limonene in the OFR with previous chamber studies. To constrain the impacts of Cl-induced atmospheric aging on pollution in an unconventional oil and gas development (UOGD) source region, we deployed a Cl-OFR in Karnes City, Texas in Spring 2021. Aging ambient air in the Cl-OFR generated OVOC and chlorinated gas-phase species as well as secondary organic and chloride aerosols, including up to 80 μg m^-3 of SOA during one nocturnal plume of aromatic hydrocarbons associated with UOGD. Finally, four Cl and Br empirical exposure estimation equations were developed for Cl produced by photolysis of either molecular chlorine (Cl₂) or oxalyl chloride (C₂Cl₂O₂) and Br produced by photolysis of either molecular bromine (Br₂) or oxalyl bromide (C₂Br₂O₂).

Results from our experiments are summarized in 2 manuscripts that have been published or submitted to refereed journals, along with 3 additional manuscripts in preparation. In addition, the research team presented the work in 6 poster or platform presentations at American Association for Aerosol Research and Indoor Air conferences. Information disseminated in our manuscripts is being used by researchers in the field of atmospheric chemistry. This work provided and continues to provide key parameters needed to understand the role of halogen atoms in atmospheric chemistry and in global climate. The work related to OVOC and SOA formation is providing new basic information about interactions and transformations of these important pollutants.  The results of our studies also have implications for indoor air quality during bleach cleaning events that increased in response to the SARS-CoV-2 pandemic.  Our findings will provide guidance for research groups working to characterize basic indoor air chemistry and health effects associated with bleach-related halogen chemistry. 

Last Modified: 01/11/2024
Modified by: Lea Hildebrandt Ruiz