Abstract
Abstract. 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). At upper tropospheric temperatures, the thermodynamically stable phase of AN is different from that at 298 K, meaning that presently available room-temperature optical constants of AN, that is, the real and imaginary parts of the complex refractive index, cannot be applied for the quantitative analysis of these infrared measurements. In this work, we have retrieved the first low-temperature data set of optical constants for crystalline AN in the 800–6000 cm−1 wavenumber range with a spectral resolution of 0.5 cm−1. The optical constants were iteratively derived from an infrared extinction spectrum 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 fit aircraft-borne infrared measurements of ammonium nitrate particles in the ATAL.
Highlights
The term “Asian Tropopause Aerosol Layer” (ATAL) was established in 2011 when analyses of CALIPSO lidar measurements revealed the existence of a layer of enhanced aerosol concentrations at altitudes from 13 to 18 km during the Asian summer monsoon (Vernier et al, 2011)
3.1 The new low-temperature data set of optical constants for crystalline ammonium nitrate (AN)
The peak positions of the infrared absorption bands in our low-temperature k(ν ) spectrum, that is, 3235 cm−1 for ν3(NH+4 ), 3062 cm−1 for (ν2 + ν4)(NH+4 ), 1760 cm−1 for ν2(NH+4 ), 1055 cm−1 for ν1(NO−3 ), and 831 cm−1 for ν2(NO−3 ), are in good agreement with the tabulated values for ammonium nitrate films probed at 90 K (Koch et al, 1996)
Summary
The term “Asian Tropopause Aerosol Layer” (ATAL) was established in 2011 when analyses of CALIPSO lidar measurements revealed the existence of a layer of enhanced aerosol concentrations at altitudes from 13 to 18 km during the Asian summer monsoon (Vernier et al, 2011). To support the interpretation of the satellite and aircraftbased infrared measurements, laboratory infrared extinction spectra of crystalline AN particles and supercooled aqueous AN solution droplets have been recorded at temperature conditions of the upper troposphere in the aerosol and cloud simulation chamber AIDA (Aerosol Interaction and Dynamics in the Atmosphere) (Höpfner et al, 2019). These measurements confirmed that the signature at 831 cm−1 is due to AN particles in the crystalline state, because the respective ν2(NO−3 ) mode of supercooled aqueous AN solution droplets is shifted to 829 cm−1 and has a larger width of about 6 cm−1 (Höpfner et al, 2019). We demonstrate that this new data set is appropriate to accurately fit infrared limb observations of AN in the ATAL (Sect. 3.3)
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