One year of aerosol refractive index measurement from a coastal Antarctic site


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Rolf.Weller [ at ] awi.de

Abstract

Climate change model evaluations need a better understanding of the atmo- spheric aerosols' optical properties and with this of the refractive index (RI) of atmospheric aerosols as well. Due to the remoteness of Antarctica only a very few data on the refractive index exists from there. In this paper we calculate the real refractive index of atmospheric aerosols from number size distribution measurements at a coastal Antarctic measurement site. In our calculations we used the overlapping size range (120-340 nm) of a scanning mobility sizer (SMPS), which sizes the particles by their electrical mobility, and a laser aerosol spectrometer (LAS), which sizes the particles by their optical scattering signal. Based on almost a complete year of measurement and 2439 measurement points, the average effective refractive index (RIeff) turned out to be 1.44. This is in a good agreement with the RI value of 1.47 which we derived from the chemical composition filter measurements. At our measurement site the aerosol has a very characteristic seasonal pattern in both number concentration and chemical composition. Despite this, we could not identify any significant sea- sonal variability in the effective refractive index, the monthly averages remain within the range of 1.40-1.50. Two austral winter months June and September has a slightly higher average values (1.50 and 1.47). We could not identify any in uence of the occurring wind direction on the retrieved RIeff either. For the few examples of north winds coming from the Neumayer station (occurs very rarely, this is the reason why the measurement site was built to the south), we don't see different values than for the other wind directions. During an artificial, high contamination episode, when diesel engines were operated right next to the measurement site, we had an hour of constant conditions such that one RI fit was possible. This fit resulted in an unusual high RI of 1.59, which is most probably due to the high black carbon content of the diesel engine emission. Therefore, we also assume that even during northerly wind directions we did not have significant in uence from the Neumayer station. During a shorter period between 2017 December and 2018 January we used the time averaged LAS and SMPS number size distributions to get some in- formation on the size dependency of the refractive index. The RI was fit in 5 different particle size ranges, and we have found a slight decrease of the re- fractive index with the particle size from 1.47 in the 116-168 nm to 1.37 in the 346-478 nm range.



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Article
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Peer revision
ISI/Scopus peer-reviewed
Publication Status
Published
Eprint ID
49320
DOI 10.5194/acp-19-14417-2019

Cite as
Jurányi, Z. /. Z. and Weller, R. /. R. (2019): One year of aerosol refractive index measurement from a coastal Antarctic site , Atmospheric Chemistry and Physics, 19 (22), pp. 14417-14430 . doi: 10.5194/acp-19-14417-2019


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