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. Infrared spectroscopic observations have shown that crystalline ammonium nitrate (AN) particles are an abundant constituent of the upper tropospheric aerosol layer which is formed during the Asian summer monsoon period, the so-called Asian Tropopause Aerosol Layer (ATAL). Atupper tropospheric temperatures, the thermodynamically stable phase of AN isdifferent from that at 298 K, meaning that presently availableroom-temperature optical constants of AN, that is, the real and imaginaryparts of the complex refractive index, cannot be applied for thequantitative analysis of these infrared measurements. In this work, we haveretrieved the first low-temperature data set of optical constants forcrystalline AN in the 800–6000 cm−1 wavenumber range with a spectral resolution of 0.5 cm−1. Theoptical constants were iteratively derived from an infrared extinctionspectrum of 1 µm sized AN particles suspended in a cloud chamber at 223 K. The uncertainties of the new data set were carefully assessed in a comprehensive sensitivity analysis. We show that our data accurately fitaircraft-borne infrared measurements of ammonium nitrate particles in theATAL.

High-resolution optical constants of crystalline ammonium nitrate for infrared remote sensing of the Asian Tropopause Aerosol Layer