2006: Inter-comparison campaign at Ny-Ålesund (Norway)

Accuracy requirements for aerosol optical depth (AOD) in polar regions are much more stringent than those usually encountered in established sun photometer networks, while comparability of data from different archive centres is a further important issue. Therefore, an intercomparison campaigns was held during spring 2006 at Ny-Ålesund (Svalbard) with the participation of various research institutions routinely employing different instrument models (Fig. 1) at Arctic and Antarctic stations. A common algorithm was used for data analysis with the aim of minimizing a large part of the discrepancies affecting the previous studies (Fig. 2). During the Ny-Ålesund campaign, the results confirmed that sun photometry is a valid technique for aerosol monitoring in the pristine atmospheric turbidity conditions usually observed at high latitudes (Mazzola et al., 2012).

2008: Inter-calibration campaign ai Izaña (Spain)

Building on the success of the 2006 Inter-comparison campaign, participants reconvened in 2008 for a second workshop and to inter-calibrate their instruments (Fig. 3) in preparation for the International Polar Year 2007-2008. Among the line-up of instruments at the Izana Atmospheric Observatory (2367 meters elevation) were many Cimel systems used by AERONET and also a Star Photometer.

2015: Lunar Workshop in Valladolid (Spain)

Studies using highly resolved spectral Aerosol Optical Depth (AOD) data acquired at high latitudes have shown the value of such data. The increasing oil and gas extraction and shipping activities in the Arctic are a concern due to their contribution to pollution and black carbon (BC) deposited on sea ice. Retrieving Arctic aerosol properties using satellite data is challenging due to deficiencies in AOD climatologies and a lack of validation data during winter when traditional Sun photometry is not possible. Long polar nights are a major obstacle to completing an AOD climatology. Lidar observations can detect atmospheric column aerosols at night, but Lidar systems are limited in the Arctic due to harsh environmental conditions. Measuring irradiance from stars or reflected Moonlight has been proposed as a solution to fill gaps in the Arctic AOD climatology. Lunar photometry is an emerging technology that has demonstrated success at high latitudes, and with high-precision exo-atmospheric lunar irradiance available, there is an urgent need to establish common goals and procedures to assure data consistency and quality for optimizing network activity. The focus of the workshop was on evaluating lunar irradiance models (ROLO – Fig. 4 – vs others) in the frame of nocturnal measurements of AOD, whose results were published by Barreto et al. (2019) (Fig.5).

2017: Inter-calibration campaign at Izaña (Spain)

This was a multi-instrument AOD inter-comparison campaign held at Izaña Observatory (Tenerife, Spain) in June 2017. The focus was on evaluating lunar irradiance models, ROLO versus others, in the frame of nocturnal measurements of AOD. Although AERONET (https://aeronet.gsfc.nasa.gov/) has deployed a large number of modified Cimels, (the CIMEL Electronique CE318 multiband sun photometer) very few operate in Polar regions and these have been prone to failure due to severe cold. Further improvements are being made to improve reliability and data recovery, with special attention given to thermal corrections and robust operations.

2019: Polar-AOD Lunar Retreat, North Canaan (USA)

In May 2019, a retreat was held in North Canaan, Connecticut, which focused on night-time AOD measurements. The workshop was attended by a diverse group of scientists and engineers from different countries and institutions who were actively involved in Polar-AOD. Bob Stone, one of the earliest members of Polar-AOD and resident of North Canaan, hosted the event with support from local citizens and the organizing committee. Subsequent to the Retreat, the Polar-AOD program received an official endorsement by the International Arctic Science Committee (IASC) as a participating organization during the T-MOSAiC campaign. As an outcome of the Canaan Retreat, two field campaigns were tentatively planned in the following year; an Arctic Night Campaign to inter-compare retrievals of AOD under actual winter conditions, and another to follow during summer to further evaluate multi-sensor capabilities.

2020: Lunar-AOD Inter-comparison campaign in Ny-Ålesund (Norway)

The Lunar AOD Intercomparison Campaign conducted in Ny-Ålesund in 2020 was a collaboration between CNR, NOAA, AWI and others. Aerosol Optical Depth (AOD) measurements using lidars can be challenging as they require pulsed lasers and fast data-acquisition electronics, and have difficulty measuring particles close to the ground. To address this, a novel technique called Camera Lidar or CLidar was used, which involves imaging a laser beam from the ground to the zenith using a CCD camera and wide-angle lens. CLidar provides complementary information to traditional AOD measurements by showing the altitude of aerosols. Results from the campaign revealed unexpected aerosol concentrations around 60 meters altitude, which may be explained by the displacement of ground-level air by colder, cleaner air from the glacier. The CLidar technique provided a valuable addition to traditional lidar measurements which cannot measure at very low altitudes.

Fig.1 The array of sun-photometers deploying during the 2006 inter-comparison campaign at Ny-Ålesund.
Fig. 2 Time-series of AOD at the 500 nm (upper part) and 870 nm wavelengths (lower part) as obtained on March 28, 2006, using all instruments taking part in the Ny-Ålesund campaign (Mazzola et al., 2012).
Fig. 3 AOD time-series at the 500 nm (upper part) and 870 nm (lower part) wavelengths as obtained on October 14, 2008, from all the measurements performed with the sun photometers taking part in the Izaña campaign (Mazzola et al., 2012).
Fig. 4  An example showing time series of ROLO-derived, one-minute resolved spectral exo-atmospheric irradiances during the September/October 2012 lunar cycle at Mauna Loa (Hawaii).
Fig. 5 AOD time series for master Cimel CE#971 (at 500.6 nm), LunarPFR (at 500.7 nm), stellar (at 500.7 nm) and daytime AERONET (at 500.8 nm) in the period 2–17 June 2017. AOD from lunar photometers have been extracted by means of (a) Lunar-Langley calibration with the RIMO model, and (b) Langley-plot calibration (Barreto et al., 2019).