NO ₃ radical measurements in a polluted marine environment: links to ozone formation

Nighttime chemistry in polluted regions is dominated by the nitrate radical (NO ₃) including its direct reaction with natural and anthropogenic hydrocarbons, its reaction with NO ₂ to form N ₂O ₅, and subsequent reactions of N ₂O ₅ to form HNO ₃ and chlorine containing photolabile species. We report nighttime measurements of NO ₃, NO ₂, and O ₃, in the polluted marine boundary layer southwest of Vancouver, BC during a three week study in the summer of 2005. The concentration of N ₂O ₅ was calculated using the well known equilibrium, NO ₃+NO ₂↔N ₂O ₅. Median overnight mixing ratios of NO ₃, N ₂O ₅ and NO ₂ were 10.3 ppt, 122 ppt and 8.3 ppb with median N ₂O ₅/NO ₃ molar ratios of 13.1 and median nocturnal partitioning of 4.9%. Due to the high levels of NO ₂ that can inhibit approach to steady-state, we use a method for calculating NO ₃ lifetimes that does not assume the steady-state approximation. Median and average lifetimes of NO ₃ in the NO ₃-N ₂O ₅ nighttime reservoir were 1.1–2.3 min. We have determined nocturnal profiles of the pseudo first order loss coefficient of NO ₃ and the first order loss coefficients of N ₂O ₅ by regression of the NO ₃ inverse lifetimes with the [N ₂O ₅]/[NO ₃] ratio. Direct losses of NO ₃ are highest early in the night, tapering off as the night proceeds. The magnitude of the first order loss coefficient of N ₂O ₅ is consistent with, but not verification of, recommended homogeneous rate coefficients for reaction of N ₂O ₅ with water vapor early in the night, but increases significantly in the latter part of the night when relative humidity increases beyond 75%, consistent with heterogeneous reactions of N ₂O ₅ with aerosols with a rate constant k_het=(1.2±0.4)×10 ⁻³ s ⁻¹−(1.6±0.4)×10 ⁻³ s ⁻¹. Analysis indicates that a correlation exists between overnight integrated N ₂O ₅ concentrations in the marine boundary layer, a surrogate for the accumulation of chlorine containing photolabile species, and maximum 1-h average O ₃ at stations in the Lower Fraser Valley the next day when there is clear evidence of a sea breeze transporting marine air into the valley. The range of maximum 1-h average O ₃ increase attributable to the correlation is ΔO ₃=+1.1 to +8.3 ppb throughout the study for the average of 20 stations, although higher increases are seen for stations far downwind of the coastal urban area. The correlation is still statistically significant on the second day after a nighttime accumulation, but with a different spatial pattern favouring increased O ₃ at the coastal urban stations, consistent with transport of polluted air back to the coast.