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

Nighttime chemistry in polluted regions is dominated by the nitrate radical (NO sub(3)) including its direct reaction with natural and anthropogenic hydrocarbons, its reaction with NO sub(2) to form N sub(2)O sub(5), and subsequent reactions of N sub(2)O sub(5) to form HNO sub(3) and chlorine containing photolabile species. We report nighttime measurements of NO sub(3), NO sub(2), and O sub(3), in the polluted marine boundary layer southwest of Vancouver, BC during a three week study in summer of 2005. The concentration of N sub(2)O sub(5) was calculated using the well known equilibrium, NO sub(3)+NO sub(2) reversible reaction N super(2)O sub(5). Median overnight mixing ratios of NO sub(3), N sub(2)O sub(5) and NO sub(2) were 10.3 ppt, 122 ppt and 8.3 ppb with median N sub(2)O sub(5)/NO sub(3) molar ratios of 13.1 and median nocturnal partitioning of 4.9%. Due to the high levels of NO sub(2) that can inhibit approach to steady-state, we use a method for calculating NO sub(3) lifetimes that does not assume the steady-state approximation. Median and average lifetimes of NO sub(3) in the NO sub(3)-N sub(2)O sub(5) nighttime reservoir were 1.1-2.3 min. We have determined nocturnal profiles of the pseudo first order loss coefficient of NO sub(3) and the first order loss coefficients of N sub(2)O sub(5) by regression of the NO sub(3) inverse lifetimes with the [N sub(2)O sub(5)]/[NO sub( 3)] ratio. Direct losses of NO sub(3) are highest early in the night, tapering off as the night proceeds. The magnitude of the first order loss coefficient of N sub(2)O sub(5) is consistent with recommended homogeneous rate coefficients for reaction of N sub(2)O sub(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 sub(2)O sub(5) with sea salt and/or other aerosols with rate constant k sub(het)=1.210 super(&minus ; 3) s super(− 1). Analysis indicates that a correlation exists between overnight integrated N sub(2)O sub(5) concentrations in the marine boundary layer, a surrogate for the accumulation of chlorine containing photolabile species, and maximum 1-h average O sub(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 sub(3) increase attributable to the correlation is Delta O sub(3)=+1.1 to +8.3 ppb throughout the st