Time-dependent uptake of NO3 by sea salt

Using a coated-insert flow tube reactor coupled to a low-energy electron-impact mass spectrometer with molecular beam sampling, we studied uptake of NO3 by sea salt at room temperature and [NO3] = 8⋅1011 − 4⋅1013 molecule cm−3. The radical uptake coefficient γ(t) is time dependent: its initial value (γini) decreases exponentially with the characteristic time (τ) to its steady-state value (γss) at given [NO3]. The parameters γini, γss and τ depend on [NO3], whereas γss is water vapor independent at [H2O] = 8⋅1012 − 1.6⋅1015 molecule cm−3 and RH ≤ 0.5 %. HCl and NO2 are uptake products detected in the gas phase. We used these findings to estimate γ values under tropospheric conditions for urban coastal and remote marine environments: at high NO3 (~90 ppt), the time dependence becomes important, and the γ value averaged over the aerosol lifetime is 4⋅10−3; at low NO3 (~1 ppt), the radical uptake is time independent and proceeds faster with γini = 8⋅10−3