Chloramines as an important photochemical source of chlorine atoms in the urban atmosphere

Abstract

Monochloramine, dichloramine and trichloramine (NH 2 Cl, NHCl 2 , NCl 3 ) are measured in the ambient atmosphere, in downtown Toronto in summer (median 39, 15 and 2.8 ppt) and winter (median 11, 7.3 and 0.7 ppt). NCl 3 and NHCl 2 were also measured in summer (median 1.3 and 14 ppt) from a suburban Toronto location. Measurements at two locations demonstrate prevalence of chloramines in an urban atmosphere. At both sites, NCl 3 exhibits a strong diel pattern with maximum values during the night, and photolytic loss with sunrise. At the downtown site, a strong positive correlation between NH 2 Cl and NHCl 2 in the summer night indicates a common source, with daily average peak mixing ratios approaching 500 and 250 ppt, respectively. As a previously unidentified source of chlorine (Cl) atoms, we demonstrate that NCl 3 photolysis contributes 49 to 82% of the total local summertime Cl production rate at different times during the day with an average noontime peak of 3.8 × 10 5 atoms/cm 3 /s, with smaller contributions from ClNO 2 and Cl 2 . Photolysis of NH 2 Cl and NHCl 2 may augment this Cl production rate. Our measurements also demonstrate a daytime enhancement of chloroacetone in both the summer and winter, demonstrating the importance of Cl photochemistry. The results suggest that chloramines are an important source of Cl atoms in urban areas, with potential impacts on the abundance of organic compounds, ozone, nitrogen oxides, and particulate matter. Future studies should explore the vertical gradients of chloramines and their contribution to Cl production throughout the boundary layer.

Effects of Relative Humidity and Photoaging on the Formation, Composition, and Aging of Ethylbenzene SOA: Insights from Chamber Experiments on Chlorine Radical-Initiated Oxidation of Ethylbenzene

“…The chemistry of the phenoxy radical is important for accurately modeling nitrophenol formation and radical and ozone cycling in ambient environments . Cl will therefore likely impact this chemistry when high levels of precursors (Cl 2 , ClNO 2, NH x Cl y ) are present. ,,− , …”

“…Alkylaromatic oxidation can proceed through radical addition to the aromatic ring or hydrogen abstraction from the alkyl substituents. The former is the primary oxidation route for hydroxyl radical (OH) oxidation, particularly when only methyl substituents are present on the ring. ,− Chlorine radical (Cl), originating from a variety of sources − and observed in a variety of regions, − primarily initiates oxidation through H abstraction on the alkyl substituents. Cl concentrations are typically intermittent but will be substantially present under some regional conditions, at times reaching expected concentrations of 10–100× times less than typical OH concentrations. ,− This allows Cl to contribute substantially to VOC oxidation based on the ∼10–100× faster reaction rates for Cl compared to OH toward most VOCs .…”

Aqueous-phase chloramine production as a missing chain in atmospheric chlorine chemistry

“…The sensitivity of Cl 2 was derived using the previously reported relative sensitivity of Cl 2 to acetic acid in the same kind of instrument 66 . We assumed the sensitivities of the reactive chlorines were the same to each other, given that the calibrations in Beijing and Toronto 14 found similar sensitivities of NCl 3 , Cl 2 , and ClNO 2 . The estimated average Cl 2 (1.0 ppt) and ClNO 2 (12.9 ppt) mixing ratios in Delhi were comparable to those measured at the same site in the winter of 2019 67 .…”

“…S4). The average peak levels in (b) are averaged from reported field measurements at different sites in the US 8,9,44,45,[85][86][87][88][89] , China 11,23,48,70,[90][91][92][93][94] (this study included), Canada 14,36,95 , and Europe 13,47,96,97 . The mixing ratios in (c) -(f) are normalized to the value observed at the onset of sunlit.…”

“…This interconversion between atmospheric Cl 2 and NCl 3 within a day fills a missing part of the atmospheric chlorine cycle. Combining currently available field measurements of NCl 3 in this study (Beijing and Delhi) and literature (Toronto) 14 , we investigate how the significance of chloramine chemistry depends on the particle pollution levels, and discuss its long-term impacts with ongoing global efforts to mitigate air pollution.…”