Low levels of nitryl chloride at ground level: nocturnal nitrogen oxides in the Lower Fraser Valley of British Columbia

• Main Authors: H. D. Osthoff, C. A. Odame-Ankrah, Y. M. Taha, T. W. Tokarek, C. L. Schiller, D. Haga, K. Jones, R. Vingarzan
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-05-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/6293/2018/acp-18-6293-2018.pdf

The nocturnal nitrogen oxides, which include the nitrate radical (NO₃), dinitrogen pentoxide (N₂O₅), and its uptake product on chloride containing aerosol, nitryl chloride (ClNO₂), can have profound impacts on the lifetime of NO_<i>x</i> ( =  NO + NO₂), radical budgets, and next-day photochemical ozone (O₃) production, yet their abundances and chemistry are only sparsely constrained by ambient air measurements.</p><p>Here, we present a measurement data set collected at a routine monitoring site near the Abbotsford International Airport (YXX) located approximately 30 km from the Pacific Ocean in the Lower Fraser Valley (LFV) on the west coast of British Columbia. Measurements were made from 20 July to 4 August 2012 and included mixing ratios of ClNO₂, N₂O₅, NO, NO₂, total odd nitrogen (NO_<i>y</i>), O₃, photolysis frequencies, and size distribution and composition of non-refractory submicron aerosol (PM₁).</p><p>At night, O₃ was rapidly and often completely removed by dry deposition and by titration with NO of anthropogenic origin and unsaturated biogenic hydrocarbons in a shallow nocturnal inversion surface layer. The low nocturnal O₃ mixing ratios and presence of strong chemical sinks for NO₃ limited the extent of nocturnal nitrogen oxide chemistry at ground level. Consequently, mixing ratios of N₂O₅ and ClNO₂ were low ( &lt;  30 and  &lt;  100 parts-per-trillion by volume (pptv) and median nocturnal peak values of 7.8 and 7.9 pptv, respectively). Mixing ratios of ClNO₂ frequently peaked 1–2 h after sunrise rationalized by more efficient formation of ClNO₂ in the nocturnal residual layer aloft than at the surface and the breakup of the nocturnal boundary layer structure in the morning. When quantifiable, production of ClNO₂ from N₂O₅ was efficient and likely occurred predominantly on unquantified supermicron-sized or refractory sea-salt-derived aerosol. After sunrise, production of Cl radicals from photolysis of ClNO₂ was negligible compared to production of OH from the reaction of O(¹D) + H₂O except for a short period after sunrise.