1999 : Birth of a network
The concept of developing a network of Sun photometers to monitor Aerosol Optical Depth (AOD) was first conceived during a meeting that took place in Bologna, Italy, October 1999 between Italian and U.S. groups already engaged in making measurements at a few Polar locations, Terra Nova Bay and South Pole in the Antarctic and Barrow, Alaska in the Arctic. A cooperative program was established between CNR-ISAC and National Oceanic and Atmospheric Administration (NOAA) to expand their existing programs, share data and commence with analyses of that data. The idea was to be shared with other institutions with a stake in monitoring Polar climates. The Lead on the cooperative program at the time was Dr. Claudio Tomasi of CNR-ISAC (Fig. 1).
2002: Informal inter-comparison campaign
Individual groups began to initiate or update existing programs with the idea of creating a network of sites across the Arctic and Antarctic. Previously, during the austral summer of 2002/2003 a first inter-comparison campaign was carried out at the Italian Mario Zucchelli Station, in Terra Nova Bay, Antarctica. Jointly, NOAA and CNR-ISAC developed and tested a semi-automated system to measure AOD at eight wavelengths spanning the UV to near-infrared. Later, the system was successfully operated also at Concordia Station, Dome C, Antarctica. By then, similar systems were being operated at a number of other sites as the network began to take shape. In the beginning there were very a few stations equipped with Sun photometers that were being operated continuously for monitoring purposes.
2003: First POLAR-AOD workshop in Bologna (Italy)
A subsequent workshop held at CNR-ISAC in 2003 greatly expanded the list of participating institutes (Alfred Wegener Institute, University of Sherbrooke, Norwegian Institute for Air Research, University of Bremen, Finnish Meteorological Institute, University of Florence) by bringing together a number of experts in the field. The groundwork was laid for a more formal approach, Polar-AOD was officially adopted as a viable program with specific goals. At the time, no thought was given to night time monitoring but this was eventually found to be very important.
The goal was to deploy as many systems as possible at locations where other aerosol and radiation measurements were being made, specifically collocated with Global Atmospheric Watch (GAW) or Baseline Surface Radiation Network (BSRN) stations operating at high latitudes. Also, a new observatory was to be established at Tiksi (Russia), as part of a joint U.S.-Russian project under SEARCH (Study of Environmental Arctic Change).
When the Polar-AOD program was initiated there were several groups from an international community that assembled, each with its preferred device and manner of calibrating them and processing data.
2006: First inter-comparison campaign at Ny-Ålesund (Svalbard, Norway)
In the spring, the first official inter-comparison campaign was organized to operate their diverse set of instruments, side-by-side, under Arctic conditions. This campaign was conducted adjacent to the airport in Ny-Alesund during March/April, and was attended di scientists from 9 nations. This was prelude to operations during the upcoming International Polar Year. The Campaign was very successful and results were published by Mazzola et al. (2012).
IPY 2007-2008
By the time the International Polar Year (IPY) was announced, to take place beginning in 2007, and the POLAR-AOD program (IPY Project #171) was firmly established and recognized as a legacy project. It’s aim was to characterize the means, variability and trends of the climate-forcing properties of aerosols in Polar Regions. The proposal was adopted with the goals as follows:
Through coordinated activities data have been collected, archived and analyzed by participants from 40 research groups representing 22 countries. During IPY measurements were made at 15 Arctic and 16 Antarctic locations with support of established national programs. Archiving and data management, inter calibration efforts and research activities were coordinated primarily by ISAC. AOD measurements were used to characterize different aerosol types, infer their optical properties, and in conjunction with other observations quantify their direct radiative impact on the surface energy budget. Aerosol-induced perturbations to the surface-atmosphere thermal structure were investigated through a set of closure experiments using observations in conjunction with radiative transfer models. Natural and anthropogenic aerosols were distinguished and climatologies of their seasonal and regional patterns were established. Collectively, studies provided a basis to improve parameterizations of aerosol processes in climate models and thus reduce the uncertainty in climate predictions. An overview of the POLAR-AOD project is given in the context of the Global Climate Observing System (GCOS) and other existing programs; Global Atmosphere Watch, the U.S. Study of Environmental Arctic Change and the International Arctic Systems for Observing the Atmosphere (Project #196) that coordinated a number of interdisciplinary activities during IPY.
2008: First inter-calibration campaign at Izaña (Tenerife, Spain)
Partecipants of the first campaign in 2006 reconvened in 2008 at Izaña, Tenerife for a second workshop and to inter-calibrate their instruments in preparation for the IPY. Among the line-up of instruments were Cimel, POM02, SP1A, PFR, and also a Star Photometer.
2009: First POLAR-AOD aircraft campaign
First Polar-AOD sanctioned aircraft campaign as part of Germany’s contribution to the IPY; campaign involved 22 flight segments between Svalbard and Barrow, Alaska, named Pan-Arctic Measurements and Arctic Regional Climate Model Simulations (PAM-ARCMIP). During this campaign, the modified SP02 Photometer was fitted to the Polar-5 research aircraft to make semi-automated observations along flight tracks and during vertical profiling to determine the three-dimensional distribution of aerosols along the flight path (Stone et al, 2010).
2011: Secondo POLAR-AOD aircraft campaign
Second PAM-ARCMIP campaign took place. Again, a transect was flown but in reverse direction from that flown in 2009. During this campaign, a newly developed robotic Sun photometer was deployed but unfortunately few valid measurements were obtained due to condensation building up inside the glass dome fitted on top of Polar-5.
2012: First lunar-AOD measurements in the Arctic
Following the lead of AErosol RObotic NETwork (AERONET), the NOAA Global Monitoring Division developed their own thermally controlled, modified SP02 lunar photometer with four channels operating between 412 and 862 nm. The instrument was fabricated and tested at the Boulder, CO laboratory, sent to Mauna Loa Observatory (MLO) for calibration and then deployed to the NOAA Barrow Observatory (BRW), where it was operated through the 2012/2013 winter. While the prototype device proved the feasibility of making continuous (one-minute) observations the experiment also revealed where improvements were needed for reliable operational use.
2015: First Lunar Workshop in Valladolid (Spain)
This was the first Lunar Workshop, hosted by the GOA group of Valladolid University between June 24 and 26, and attended by representatives from many institutes and stake-holders comprising an expanded Polar-AOD community. The goals of the worshop were multiple: (i) connect ongoing activities if POLAR-AOD, (ii) promote development of instumentation and exchange of experience, and (iii) develop joint research actions aimed to assess accuracy and uncertainties of different methodologies, instrumentation, and data procdures.
2017: Second Polar-AOD inter-calibration Campaign at Izaña Atmospheric Observatory (Tenerife, Spain)
The focus of the second inter-calibration campaign was on evaluating lunar irradiance models (ROLO vs others) in the frame of nocturnal measurements of AOD, whose results were published by Barreto et al. (2019).
2019: Polar-AOD Lunar Retreat, North Canaan (Connecticut, USA)
A retreat was held in rural Connecticut during the May 2019 full Moon. Due to logistical constraints, attendance was limited to the most active and key members of the Polar-AOD community. Again, night-time AOD measurements were a focus. The Canaan Retreat also commemorated 20 years of Polar-AOD activity. Another topic discussed at the Canaan Retreat was how best to expand visibility of Polar-AOD and establish linkages to other established programs and new programs in planning, such as MOSAiC and T-MOSAiC.
2020: Lunar AOD Intercomparison Campaign and Workshop Ny-Ålesund (Svalbard, Norway)
During the January/February lunar cycle took place in Ny-Ålesund the lunar AOD inter-comparison campaign togheter with lunad-AOD workshop. 11 participants from five countries participated. During this campaign multiple systems were operated, in order to compare AOD retrievals (including those derived from a Raman Lidar, lunar and Star photometers, and a new Camera-Lidar system).
2023: Current status
In 2023, after the covid pandemic, the Institute of Polar Sciences (CNR-ISP) develop the official web page of the POLAR-AOD network resuming and developing the work done by Tomasi and colleagues began in 1999. The web page will be a repository for AOD measurements carried out at polar regions, both in the Arctic and Antarctica, by using different techniques and instruments by various research groups over the years.
