Homogeneous nucleation rates of nitric acid dihydrate (NAD) at simulated stratospheric conditions – Part I: Experimental results

[摘要] The low temperature aerosol chamber AIDA was used to study the nucleation ofnitric acid dihydrate (NAD) in super-cooled nitric acid aerosolsunder simulated stratospheric conditions in the temperature range 192 K–197 K.The nucleating solution droplets had median diameters between 225 and 290 nmand molar fractions of nitric acid between 0.26 and 0.28.Nucleation of solid particles was unambiguously observed intwo out of three experiments during time periods of up to five hours.The newly formed crystals could be clearly distinguished from the remainingliquid droplets by their increasing size with an optical particle spectrometer.The solid particles could be unequivocally identified as strongly asphericalnitric acid dihydrate crystals (α-NAD) by in-situ FTIR-spectroscopy.From our experimental data set there is no indication of direct nucleation of NAT ora conversion of NAD into NAT while having saturation ratios with respect toNAT of about 20–26.The temporal evolutions of the NAD particle concentrations wereused to derive individual nucleation rates for NAD.The measured volume nucleation rates ranged from3.9×10⁵ cm⁻³ s⁻¹ at 195.8 Kand X_NA=0.27 to1.9×10⁷ cm⁻³ s⁻¹ at 192.1 K andX_NA=0.28.The corresponding hypothetical surface nucleation rates of2×10⁰ to 1×10² cm⁻² s⁻¹are smaller than the parameterizationof Tabazadeh et al. (2002) by factors between 25 and >10³.

[学科分类] 大气科学