Abstract. 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 NOx (= NO + NO2), radical budgets, and
next-day photochemical ozone (O3) production, yet their abundances and
chemistry are only sparsely constrained by ambient air measurements. 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 (NOy), O3, photolysis frequencies,
and size distribution and composition of non-refractory submicron aerosol
(PM1). 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
(< 30 and < 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.
Low levels of nitryl chloride at ground level: nocturnal nitrogen oxides in the Lower Fraser Valley of British Columbia
