NO3 radical measurements in a polluted marine environment: links to ozone formation

Nighttime chemistry in polluted regions is dominated by the nitrate radical (NO3) including its direct reaction with natural and anthropogenic hydrocarbons, its reaction with NO2 to form N2O5, and subsequent reactions of N2O5 to form HNO3 and chlorine containing photolabile species. We report nighttime measurements of NO3, NO2, and O3, in the polluted marine boundary layer southwest of Vancouver, BC during a three week study in the summer of 2005. The concentration of N2O5 was calculated using the well known equilibrium, NO3+NO2↔N2O5. Median overnight mixing ratios of NO3, N2O5 and NO2 were 10.3 ppt, 122 ppt and 8.3 ppb with median N2O5/NO3 molar ratios of 13.1 and median nocturnal partitioning of 4.9%. Due to the high levels of NO2 that can inhibit approach to steady-state, we use a method for calculating NO3 lifetimes that does not assume the steady-state approximation. Median and average lifetimes of NO3 in the NO3-N2O5 nighttime reservoir were 1.1–2.3 min. We have determined nocturnal profiles of the pseudo first order loss coefficient of NO3 and the first order loss coefficients of N2O5 by regression of the NO3 inverse lifetimes with the [N2O5]/[NO3] ratio. Direct losses of NO3 are highest early in the night, tapering off as the night proceeds. The magnitude of the first order loss coefficient of N2O5 is consistent with, but not verification of, recommended homogeneous rate coefficients for reaction of N2O5 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 N2O5 with aerosols with a rate constant khet=(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 N2O5 concentrations in the marine boundary layer, a surrogate for the accumulation of chlorine containing photolabile species, and maximum 1-h average O3 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 O3 increase attributable to the correlation is ΔO3=+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 O3 at the coastal urban stations, consistent with transport of polluted air back to the coast

NO₃ radical measurements in a polluted marine environment: links to ozone formation

Nighttime chemistry in polluted regions is dominated by the nitrate radical (NO₃) including its direct reaction with natural and anthropogenic hydrocarbons, its reaction with NO₂ to form N₂O₅, and subsequent reactions of N₂O₅ to form HNO₃ and chlorine containing photolabile species. We report nighttime measurements of NO₃, NO₂, and O₃, in the polluted marine boundary layer southwest of Vancouver, BC during a three week study in the summer of 2005. The concentration of N₂O₅ was calculated using the well known equilibrium, NO₃+NO₂&harr;N₂O₅. Median overnight mixing ratios of NO₃, N₂O₅ and NO₂ were 10.3 ppt, 122 ppt and 8.3 ppb with median N₂O₅/NO₃ molar ratios of 13.1 and median nocturnal partitioning of 4.9%. Due to the high levels of NO₂ that can inhibit approach to steady-state, we use a method for calculating NO₃ lifetimes that does not assume the steady-state approximation. Median and average lifetimes of NO₃ in the NO₃-N₂O₅ nighttime reservoir were 1.1–2.3 min. We have determined nocturnal profiles of the pseudo first order loss coefficient of NO₃ and the first order loss coefficients of N₂O₅ by regression of the NO₃ inverse lifetimes with the [N₂O₅]/[NO₃] ratio. Direct losses of NO₃ are highest early in the night, tapering off as the night proceeds. The magnitude of the first order loss coefficient of N₂O₅ is consistent with, but not verification of, recommended homogeneous rate coefficients for reaction of N₂O₅ 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 N₂O₅ with aerosols with a rate constant <i>k</i>_het=(1.2&plusmn;0.4)&times;10^&minus;3 s^&minus;1&minus;(1.6&plusmn;0.4)&times;10^&minus;3 s^&minus;1. Analysis indicates that a correlation exists between overnight integrated N₂O₅ concentrations in the marine boundary layer, a surrogate for the accumulation of chlorine containing photolabile species, and maximum 1-h average O₃ 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 O₃ increase attributable to the correlation is &Delta;O₃=+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 O₃ at the coastal urban stations, consistent with transport of polluted air back to the coast

Determination of tropospheric vertical columns of NO₂ and aerosol optical properties in a rural setting using MAX-DOAS

Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were performed in a rural location of southwestern Ontario during the Border Air Quality and Meteorology Study. Slant column densities (SCDs) of NO₂ and O₄ were determined using the standard DOAS technique. Using a radiative transfer model and the O₄ SCDs, aerosol optical depths were determined for clear sky conditions and compared to OMI, MODIS, AERONET, and local PM_2.5 measurements. This aerosol information was input to a radiative transfer model to calculate NO₂ air mass factors, which were fit to the measured NO₂ SCDs to determine tropospheric vertical column densities (VCDs) of NO₂. The method of determining NO₂ VCDs in this way was validated for the first time by comparison to composite VCDs derived from aircraft and ground-based measurements of NO₂. The new VCDs were compared to VCDs of NO₂ determined via retrievals from the satellite instruments SCIAMACHY and OMI, for overlapping time periods. The satellite-derived VCDs were higher, with a mean bias of +0.5–0.9×10¹⁵ molec cm⁻². This last finding is different from previous studies whereby MAX-DOAS geometric VCDs were higher than satellite determinations, albeit for urban areas with higher VCDs. An effective boundary layer height, BLH_eff, is defined as the ratio of the tropospheric VCD and the ground level concentration of NO₂. Variations of BLH_eff can be linked to time of day, source region, stability of the atmosphere, and the presence or absence of elevated NO_x sources. In particular, a case study is shown where a high VCD and BLH_eff were observed when an elevated industrial plume of NO_x and SO₂ was fumigated to the surface as a lake breeze impacted the measurement site. High BLH_eff values (~1.9 km) were observed during a regional smog event when high winds from the SW and high convection promoted mixing throughout the boundary layer. During this event, the regional line flux of NO₂ through the region was estimated to be greater than 112 kg NO₂ km⁻¹ h⁻¹