Development and aerosol optical depth ...

Development and aerosol optical depth measurements with a Lunar photometer at Ny-Ålesund Natalia Kouremeti1, Stelios Kazadzis1, Mauro Mazzola2, Georg Hansen3, Kerstin Stebel3, Julian Gröbner1 1Physikalisch-Meteorologisches

Observatorium Davos and World Radiation Center (PMOD/WRC), Davos, Switzerland 2National Research Council of Italy, Bologna, Italy, 3NILU – Norwegian Institute for Air Research, Tromsø, Norway Atmospheric aerosols are known to impact the climate, and they represent one of the largest uncertainties in climate change studies. Night-time aerosol optical depth (AOD) measurements could provide valuable information on the climatology of aerosols at high latitude stations, where direct sun measurements are not possible throughout the year. For example, in the northern hemisphere they can be used to monitor the arctic haze. In passive remote sensing techniques, the moon or the stars are used as the light source in order to calculate the AOD. The growing interest in night-time observations of AOD has led to the development of a Lunar Precision Filter Radiometer (PFR) at PMOD/WRC. In addition to the retrieval of nocturnal AOD, the radiometric calibration of the lunar PFR aims to provide absolute lunar irradiance measurements with an expanded uncertainty of less than 5% (k = 2). The development stages and instrument modifications are described below. A lunar PFR was installed at Ny-Ålesund, Norway in October 2014 and it provides measurements each year from October to February.

Lunar PFR characterization

Instrument upgrade and modifications

The instrument has been fully characterized using the radiometric calibration laboratory facilities of PMOD/WRC. Specifically the filter functions (Figure 4) and the instrument linearity (Figure 5) have been measured using the ATLAS (ESA funded project) tunable laser system. Differences of the order of 0.5 nm were found from the initial nominal central wavelength values. The absolute responsivity (figure 6) of the lunar PFR was measured on the direct irradiance calibration setup, consisting of a reference irradiance source (1000W FEL type lamp source) and fully motorized XYZ, azimuth and zenith stages. The calibration factors were determined with an expanded uncertainty of 3.5% (k=2) including uncertainties in the reference plane determination, non-linearity correction and the reference irradiance used.

The Lunar Precision Filter Radiometer (LunarPFR) is a standard PFR instrument that that has been developed at PMOD /WRC based on the sunPFR experience, with enhanced sensitivity and measures at four wavelengths 862, 675, 500 and 412 nm while the sensor is stabilized at 20oC.

Lunar PFR 2014-2015 (before) • Enchantment of 2 amplification stages • Wavelengths 368 nm replaced by 675nm • Detector aperture- Increased from 3 mm to 6 mm • Data Acquisition System Campbell Logger CR10 (13bit)

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Lunar PFR 2015-now (after) • Hardware improvement of PFR Internal Gains circuit • Increased amplification for channels 862, 500 and 412 nm • Data Acquisition System : SACRAM OWEL/PMOD (±5V - 22 bit) Linearity better than 0.5‰ • Direct communication with the instrument • Full flexibility in optimizing the measurement

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Fig. 4: Filter functions of the 4 channels of the Lunar PFR measured with the ATLAS tunable laser system.

Measurements at Ny-Ålesund

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PMOD/WRC has participated in “The Lunar Arctic project” (March 2014 - June 2015) funded by the Svalbard Science Foundation, with six collaborating institutions (NILU (NO), ISAC/CNR (IT), AWI (DE), NOAA/ CIRES (US) and IGF/PAS (PL)) , that aimed to close the gap in the annual cycle of the arctic aerosol climatology and to develop Svalbard, Norway as satellite validation site. The role of PMOD/WRC in the project was the modification of an existing PFR and its deployment at Ny Ålesund. The lunar PFR is a standard PFR instrument that is used in the GAW-PFR network, with enhanced. The instrument was installed at Ny-Ålesund, Norway (78.9°N, 11.9°E), from October 2014 to February 2015 and measured 6 lunar cycles. After 2015 the instrument has been transferred every winter to Ny-Ålesund to monitor the night time AOD.

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While the solar irradiance changes slowly over the course of a year, lunar irradiance may change during a given observation night. Project partners at ISAC have successfully implemented a semi-empirical model (Kiefer and Stone, 2005) to calculate the instantaneous lunar irradiance.

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Fig. 7: LunarPFR signal (mV) as measured at Ny-Ålesund during various Lunar cycles (2014-2018). The cloudy conditions is the main limiting factor for continuous measurements over the moon cycles.

Fig. 8: The AOD timeseries of daytime measurements (daily & monthly means) along with the night-time AOD from retrievals (daily mean values) over the period 2014-2017 (Mazzola et al., 2013)

Lunar AOD Intercomparison campaign at Izaña, Spain The first multi-instrument nocturnal aerosol optical depth intercomparison campaign was held at the high mountain Izaña Observatory (Tenerife, Spain) in June 2017, involving two different types of lunar photometers (Cimel CE318-T and Moon Precision Filter Radiometer, LunarPFR) and one stellar photometer. Langley calibrations and the open-access ROLO and the ROLO-modified, RIMO was used in this field campaign to retrieve AOD. The comparison of raw signal measurements at coincident wavelengths between the participating lunar photometers (Cimel and LunarPFR) showed consistent measurements within the combined uncertainties at 870 nm and 675 nm, while slightly larger deviations were observed at 500 nm, pointing to some unaccounted instrumental variations (e.g. dark current) during the measurement period. The AOD retrieved by applying a Langley-based calibration for each night showed very good agreement between the lunar photometers of better than 0.01 at 870 nm, 675 nm, and 500 nm. In contrast, when applying the Lunar-Langley calibration using the RIMO model, AOD variations of up to 0.01 (for 870 nm and 675 nm spectral bands) and 0.04 (for 500 nm) were found, with these variations increasing with phase angle. (Barreto et al., 2018, under review).

Fig. 9: Example of Angstrom exponent and AOD measurements retrievals based on the ISAC lunar irradiance model (Mazzola et al., 2013)

Main conclusions  Instrument development The PFR instrument has been modified in order to measure the moon reflected irradiance, including: measured wavelength modifications, detector aperture increase, increased amplification, change of the data acquisition system.  Instrument characterization A full instrument characterization (linearity, filter functions) using a tunable laser system ATLAS and calibration has been performed in PMOD/WRC.  AOD results The instrument has been measuring since winter 2014 at Ny-Ålesund as part of the “Lunar Arctic project” (2014-2015 and will continue its operation under the “Svalbard Integrated Arctic Earth Observing SystemInfrastructure development of the Norwegian node ” Atmospheric module projects for the next 5 years. The current time series of night time AOD together with the daytime (spring-summer) ones help on completing an full-annual AOD series at Ny Ålesund.  Lunar AOD assessment CIMEL/AERONET, Lunar PFR and Stellar photometers have participated in an AOD Lunar based campaign held in Tenerife, Spain during June, 2017. Lunar PFR and CIMEL AOD measurements showed quite good agreement. Uncertainties linked with calibration, moon modeled irradiances, instrument technical aspects and post processing techniques have been identified and assessed. Similar intercomparisons at high AOD locations are needed. References Kazadzis, S., Kouremeti, N., Nyeki, S., Gröbner, J., and Wehrli, C.: The World Optical Depth Research and Calibration Center (WORCC) quality assurance and quality control of GAWPFR AOD measurements, Geosci. Instrum. Method. Data Syst., 7, 39-53, https://doi.org/10.5194/gi-7-39-2018, 2018. Kiefer H. H. and Stone T. C.: 2005, The spectral irradiance of the Moon, Astron. Journal, 129:2887–2901. M. Mazzola, V. Vitale, A. Lupi, C. Tomasi, R. S. Stone, T. A. Berkoff, T. C. Stone, J. Wendell, D. Longenecker, C. Wehrli, N. Kouremeti, S. Nyeki, K. Stebel: Development of first moon photometric measurements in Arctic, oral presentation at the Aerosol and Cloud workshop, Bremen, Germany, December 16-19, 2013 África Barreto, Roberto Román, Emilio Cuevas, Daniel Pérez-Ramírez, Alberto J. Berjón, Natalia Kouremeti, Stelios Kazadzis, Julian Gröbner, Mauro Mazzola, Carlos Toledano, Jose A. Benavent-Oltra, Lionel Doppler, Jakub Juryšek, A. Fernando Almansa, Stephane Victori, Fabrice Maupin, Carmen Guirado-Fuentes, Ramiro González, Vito Vitale, Philippe Goloub, Luc Blarel, Lucas Alados-Arboledas, Emma Woolliams, Claire Greenwell, Sarah Taylor, Juan Carlos Antuña, and Margarita Yela, Evaluation of night-time aerosols measurements and lunar irradiance models in the frame of the first multi-instrument nocturnal intercomparison campaign, Atmospheric Chemistry and Physics, under review.

Fig. 10 Evaluation of night-time aerosol measurements and lunar irradiance models in the frame of the first multiinstrument nocturnal intercomparison campaign, Barreto et al., 2018, under review, acp-2018-534

Acknowledgments

The author would like to thank Anne-Cathrine Nilsen, Leif Arild Håhjem, Filip Heitmann and Olav Ljøkjel for their continuous support in the daily operation and maintenances of the Lunar & Sun PFRs at Ny-Ålesund station.