Reactive uptake of nitric acid onto sub-micron sea salt across a wide range of relative humidities

Sea salt is the second largest component by mass of suspended matter in the global atmosphere, and is the dominant aerosol species above the oceans. In the marine boundary layer (MBL), the region of air up to 2 km above the world's oceans, sea salt aerosols (SSA) are constantly being generated and suspended by breaking waves and wind action on the ocean surface. The size of the SSA ranges from less than 100 nm to about 50 mum in diameter (d), while the SSA number concentration is dominated by fine particles, i.e., those with d < 2 mum. SSA quickly equilibrates with the relative humidity (RH) of their ambient surroundings and condenses or evaporates water as necessary to maintain thermodynamic equilibrium. In the MBL, SSA exists as either concentrated aqueous droplets, if the RH is above the deliquescence relative humidity (DRH), a common occurrence in the marine environment, or as solid particles with significant amounts of surface adsorbed water (SAW), if the RH is below the efflorescence relative humidity (ERH).;Investigations of heterogeneous reactions on SSA, which are rich in chloride, with gas-phase species have become increasingly important to the understanding of the chemistry of the troposphere. One such heterogeneous interaction on SSA that leads to aerosol nitrification and contributes to the tropospheric halogen budget, through release of HCl vapor, is the acid-displacement reaction with an oxide of nitrogen, such as HNO3. Experimental measurement of the rate of displacement of chloride by nitrate in SSA allows the initial uptake coefficient (gamma0) to be determined. gamma0 is defined as the fraction of gas-phase collisions that lead to the irreversible uptake of the gas. This thesis details the experimental development and determination of the kinetics, mechanism, and reactive uptake coefficients of nitric acid onto sub-micron SSA across a wide range of relative humidities. In comparison to previous studies, the novel aspects of this work include following distinctions. Firstly, the reaction was conducted on sub-micron (which constitutes the majority by number of naturally occurring SSA), size-selected SSA surrogates and actual pristine sea water. Secondly, the relative humidity is modulated in the experimental setup, allowing the size-selected SSA to condition to a wide range of relative humidities typically encountered in the MBL. Thirdly, the reactions are conducted in a flow tube reactor connected to a single particle mass spectrometer, which provides a real-time monitor of the chloride to nitrate ratio of the transformed particles. Finally, pseudo-first order conditions were maintained by keeping the nitric acid pressure constant and in excess (60 ppb) with respect to the total aerosol solute concentration. The results presented suggest (1) the reaction of nitric acid with sub-micron SSA is a significant contributor to the global halogen budget, (2) fine particles (diameter <1 mum) are likely to exhibit greater chloride to nitrate transformation than coarse particles (diameter >1 mum), and (3) aerosol water content is an important and facilitating factor in the heterogeneous uptake process. Depending on experimental conditions, the initial uptake coefficient can range from 0.02 to 0.21 +/-0.02 for 102 nm to 230 nm NaCl, 0.12 to 0.25 +/-0.05 for 102 nm to 230 nm NaCl+MgCl 2˙6H2O and 0.11 to 0.20 +/-0.05 for 75 nm to 200 nm SSA. The magnitude of uptake is greatest near the ERH for droplets, where the aerosol salinity (by volume) is highest and the abundance of chloride ions available for reaction is at a maximum. These observations suggest that the displacement of chloride by nitrate in submicron SSA is efficient over the entire range of conditions in the MBL and that conversion should be nearly complete within a few hours after creation, a timescale much shorter than their residence time in the atmosphere.