Assessing Sun-Photometer Performance: Insights from an Intercomparison Campaign in Valladolid, Spain

In May-June 2024, an intercomparison campaign is scheduled to take place in Valladolid, Spain, comparing different photometer models. Specifically, the campaign will involve the PREDE POM-02L from the Institute of Polar Sciences (CNR-ISP), the CIMEL CE318-T from the Atmospheric Optics Group (GOA) at Valladolid University, and the PFR from the Physikalisch-Meteorologisches Observatorium Davos (PMOD). The primary objective of this campaign is to assess the effectiveness of a personalized algorithm for determining Aerosol Optical Depth (AOD) and Angstrom Exponent (α), while also evaluating the quality of these measurements by comparing them with data from the AERONET station operated by the GOA. AOD measurements will be conducted at various wavelengths: 440, 500, 675, and 870 nm for CIMEL; 400, 500, 675, and 870 nm for PREDE; and 440, 500, and 870 nm for PFR; α will be evaluated between 500 and 870 nm for all the photometers. For the multi-instrument intercomparison, the recommendations defined in 2005 by the WMO during the event ’WMO/GAW expert workshop on a global surface network for long-term observations of columnar aerosol optical properties' will be followed. These consist of: (i) acquiring more than 1000 points over at least 5 days of measurements, with AOD500 between 0.04 and 0.20; (ii) ensuring traceability with 95% uncertainty within 0.005 + 0.01/m; (iii) using the 500±3 nm and 865±5 nm channels, with a bandwidth of 15 nm or less, since these channels are mostly free from absorption effects. Valladolid, a medium-size city in the northern Castilian plateau, typically experiences continental aerosols as the predominant aerosol type. AOD values tend to be lowest during winter, gradually increasing to peak levels in late spring and summer. Maximum AOD values in Valladolid are often associated with the presence of dust particles from Saharan dust outbreaks, which are characterized by low α values. This intercomparison campaign, spanning late spring and early summer, provides an opportunity to observe the first significant desert dust transport events at the site. Additionally, biomass-burning events, associated with high AOD and high α values, may also be observed during this period. Detecting and studying these events will be another key focus of the campaign. In conclusion, this campaign offers us a chance to assess how well different sun-photometers work. Additionally, we’ll be able to study how desert dust and biomass burning events affect the site during this period.