The Influence of Road Salt on Nitryl Chloride Formation from Aerosol and Urban Grime Surfaces in Urban, Continental, Wintertime Environments
EMMA MCLAY, Trevor VandenBoer, Nasrin Dashti, Cora Young, York University
Abstract Number: 182
Working Group: Aerosol Chemistry
Abstract
Chlorine radicals, which often react much quicker than hydroxyl radicals, can play a role in atmospheric oxidation pathways that can lead to ozone and secondary organic aerosol formation. Nitryl chloride (ClNO2), a precursor to chlorine radicals, is formed from heterogeneous reaction of chloride-containing surfaces with N2O5. ClNO2 formation is predominantly studied in coastal regions with sea-salt influence, and on chloride-containing aerosol surfaces. Despite this, ClNO2 has been observed in continental regions where road salt (i.e. NaCl) is used in the wintertime for de-icing. In urban areas, the prevalence of urban grime, the films on impervious outdoor surfaces containing deposited aerosols, could serve as a surface for heterogeneous reactivity. To investigate these understudied areas, aerosols and urban grime were sampled in Toronto, ON during a four-week winter period where cold and snowy conditions were experienced along with road salt application. Sampling took place at two locations contrasting in their distance from road salt sources. Ionic composition determination revealed evidence of chloride depletion relative to sodium, possibly indicative of ClNO2 formation, in all urban grime samples and some aerosol samples. More depletion was seen in the grime than in the aerosols indicating that chloride loss may occur post-deposition on the grime surface itself. As well, more depletion was seen relative to sodium, magnesium, and calcium combined, potentially indicating reactivity of other chloride salts (e.g. CaCl2 and MgCl2) that are sometimes used for de-icing. Fractions of nitrate, which would increase from ClNO2 production, were higher in grime at the location further from road salt sources likely because of longer aerosol transport times pre-deposition. Direct reaction of N2O5 with urban grime samples in a coated-wall flow tube reactor coupled with an iodide-adduct chemical ionisation mass spectrometer will be conducted to determine the potential ClNO2 production, as opposed to other chloride activation or displacement reactions.