Article pubs.acs.org/JPCA
In Situ Observation on the Dynamic Process of Evaporation and Crystallization of Sodium Nitrate Droplets on a ZnSe Substrate by FTIR-ATR Qing-Nuan Zhang,† Yun Zhang,† Chen Cai,† Yu-Cong Guo,† Jonathan P. Reid,‡ and Yun-Hong Zhang*,† †
Institute of Chemical Physics, Key Laboratory of Cluster Science, School of Chemistry, Beijing Institute of Technology, Beijing 100081, China ‡ School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS81TS, U.K. S Supporting Information *
ABSTRACT: Sodium nitrate is a main component of aging sea salt aerosol, and its phase behavior has been studied repeatedly with wide ranges observed in the efflorescence relative humidity (RH) in particular. Studies of the efflorescence dynamics of NaNO3 droplets deposited on a ZnSe substrate are reported, using an in situ Fourier transform infrared attenuated total reflection (FTIR-ATR) technique. The time-dependence of the infrared spectra of NaNO3 aerosols accompanying step changes in RH have been measured with high signal-to-noise ratio. From the IR difference spectra recorded, changes of the time-dependent absorption peak area of the O−H stretching band (ν-OH, ∼3400 cm−1) and the nitrate out-of-plane bending band (ν2-NO3−, ∼836 cm−1) are obtained. From these measurements, changes in the IR signatures can be attributed to crystalline and solution phase nitrate ions, allowing the volume fraction of the solution droplets that have crystallized to be determined. Then, using these clear signatures of the volume fraction of droplets that have yet to crystallize, the homogeneous and heterogeneous nucleation kinetics can be studied from conventional measurements using a steady decline in RH. The nucleation rate measurements confirm that the rate of crystallization in sodium nitrate droplets is considerably less than in ammonium sulfate droplets at any particular degree of solute supersaturation, explaining the wide range of efflorescence RHs observed for sodium nitrate in previous studies. We demonstrate that studying nucleation kinetics using the FTIR-ATR approach has many advantages over brightfield imaging studies on smaller numbers of larger droplets or measurements made on single levitated particles.
I. INTRODUCTION Aerosol particles have been attracting increasing attention in recent years due to their importance in atmospheric chemistry and physics, combustion science and their influences on human health.1−4 Aerosols are composed of inorganic and organic components with particle sizes spanning 4 orders of magnitude from 0.01 to 100 μm. The particle size and phase are determined by the relative humidity (RH) and temperature of the gas phase, with both size and phase playing important roles in determining aerosol properties such as sedimentation velocity and optical properties.5,6 Indirectly, aerosols influence climate by acting as cloud condensation nuclei (CCN) or ice nuclei (IN).7,8 Aerosols, which can absorb and concentrate air pollutants, mediate gas/liquid or gas/solid interface reactions and participate in heterogeneous chemical reaction directly.9,10 These multiphase or heterogeneous chemical reactions result in an evolving chemical composition that affects atmospheric composition and impacts on public health.11,12 Understanding the dynamic response of aerosols to changes in environmental conditions, such as temperature and RH, is crucial for interpreting the physical and chemical properties of aerosols. © 2014 American Chemical Society
In particular, sea salt aerosol undergoes heterogeneous reactions with HNO3, N2O5, and NO2 to form sodium nitrate aerosol.13−15 The physical states of NaNO3 particles can influence the radiative properties of the marine boundary layer.16 The related efflorescence and deliquescence of sodium sea salt aerosols has been previously studied,16,17 although measurements of the phase transitions and hygroscopicity of NaNO3 particles are ambiguous. Gysel et al. reported that submicrometer-sized NaNO3 droplets did not crystallize even at 6% RH by means of the hygroscopicity tandem differential mobility analyzer (H-TDMA) method.18 In contrast, Tang and co-workers used a single particle levitation technique (the electrodynamic balance, EDB) to study NaNO3 particles with diameter size of 6−16 μm, reporting an efflorescence relative humidity (ERH) in the range from 50 μm in radius) occurring at high RH, close to the saturation point (at 74.5% RH), while smaller droplets (
