Assessment of Sun photometer Langley calibration at the high-elevation sites Mauna Loa and Izaña
<p>The aim of this paper is to analyze the suitability of the high-mountain stations Mauna Loa and Izaña for Langley plot calibration of Sun photometers. Thus the aerosol optical depth (AOD) characteristics and seasonality, as well as the cloudiness, have been investigated in order to prov...
| Main Authors: | , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-10-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/14555/2018/acp-18-14555-2018.pdf |
| Summary: | <p>The aim of this paper is to analyze the suitability of the high-mountain
stations Mauna Loa and Izaña for Langley plot calibration of Sun
photometers. Thus the aerosol optical depth (AOD) characteristics and
seasonality, as well as the cloudiness, have been investigated in order to
provide a robust estimation of the calibration uncertainty as well as the
number of days that are suitable for Langley calibrations. The data used for
the investigations belong to the AERONET and GAW-PFR networks, which maintain
reference Sun photometers at these stations with long measurement records:
22 years at Mauna Loa and 15 years at Izaña. In terms of clear-sky and
stable aerosol conditions, Mauna Loa (3397 m a.s.l.) exhibits on average
377 Langley plots (243 morning and 134 afternoon)
per year suitable for Langley plot calibration, whereas Izaña
(2373 m a.s.l.) shows 343 Langley plots (187 morning and 155 afternoon) per
year. The background AOD (500 nm) values, on days that are favorable for
Langley calibrations, are in the range 0.01–0.02 throughout the year, with
well-defined seasonality that exhibits a spring maximum at both stations plus
a slight summer increase at Izaña. The statistical analysis of the
long-term determination of extraterrestrial signals yields to a calibration
uncertainty of ∼  0.25–0.5 %, this uncertainty being smaller in the
visible and near-infrared wavelengths and larger in the ultraviolet
wavelengths. This is due to atmospheric variability produced by changes in
several factors, mainly the AOD. The uncertainty cannot be reduced based only
on quality criteria of individual Langley plots and averaging over several
days is shown to reduce the uncertainty to the needed levels for reference
Sun photometers.</p> |
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| ISSN: | 1680-7316 1680-7324 |