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Infrared Spectrum of Nitric Acid Dihydrate:Influence of Particle Shape

机译:二水合硝酸的红外光谱:颗粒形状的影响

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摘要

In situ Fourier transform infrared (FTIR) extinction spectra of airborne a-NAD microparticles generated by two different methods were recorded in the large coolable aerosol chamber AIDA of Forschungszentrum Karlsruhe.The extinction spectrum of alpha-NAD crystals obtained by shock freezing of a HNO_3/H_2O gas mixture could be accurately reproduced using Mie theory with published refractive indices of alpha-NAD as input.In contrast,Mie theory proved to be inadequate to properly reproduce the infrared extinction spectrum of alpha-NAD crystals which were formed via homogeneous nucleation of supercooled HNO_3/H_2O solution droplets,evaporating slowly on a time scale of several hours at about 195 K.Much better agreement between measured and calculated extinction spectra was obtained by T-matrix calculations assuming oblate particles with aspect ratios greater than five.This indicates that strongly aspherical alpha-NAD crystals are obtained when supercooled nitric acid solution droplets freeze and grow slowly,a process which has been discussed as a potential pathway to the formation of crystalline polar stratospheric cloud (PSC) particles.
机译:在Forschungszentrum Karlsruhe的大型可冷却气溶胶室AIDA中记录了通过两种不同方法生成的机载a-NAD微粒的原位傅里叶变换红外(FTIR)消光光谱。通过HNO_3 /的冷冻冻结获得的α-NAD晶体的消光光谱使用Mie理论,以已公布的α-NAD折射率为输入,可以精确地复制H_2O气体混合物。相反,Mie理论被证明不足以正确复制通过过冷的均匀成核形成的α-NAD晶体的红外消光光谱。 HNO_3 / H_2O溶液液滴在约195 K的几小时时间尺度上缓慢蒸发。通过假设长宽比大于5的扁球形颗粒,通过T-矩阵计算获得的消光光谱与测量值和计算值之间的一致性更好。当过冷的硝酸溶液液滴冻结时,获得非球面α-NAD晶体并缓慢生长,这一过程已被讨论为形成结晶极平流层云(PSC)粒子的潜在途径。

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