Valentia Observatory

Aerosol Optical Depth Measurement Program at Valentia
Since the industrial revolution an increase in anthropogenic aerosols has caused a noticeable impact on direct radiative forcing. The Intergovernmental Panel on Climate Change (IPCC 1996) recognised that the global aerosol monitoring system of that time was inadequate for the level of precision needed for future study. This led to the formation of the World Optical depth Research and Calibration Centre (WORCC) based at the Physikalisch – Meteorologisches Observatorium Davos / World Radiation Centre (PMOD/WRC) in Switzerland. The WORCC was charged with the task of building a robust aerosol monitoring instrument capable of measuring over decadal time frames. The result was the Precision Filter Radiometer (PFR).

PFR (blue tube) positioned on Valentia Observatory’s solar tracker

A PFR accurately measures direct solar radiation transmitted in four narrow spectral bands(862, 500, 412 and 368nm). It is designed for continuous operation under a broad range of weather conditions with the ability to make accurate traceable measurements. From these measurements a series of algorithms can determine the atmospheric Aerosol Optical Depth (AOD), which is the primary objective of a PFR. Positioned on a solar tracker and requiring a cloud free line of sight to the sun, a global network of PFRs provides AOD measurements for the Global Atmospheric Watch (GAW) program. Valentia Observatory has joined this GAW-PFR network as an associate member.

AOD is a measure of the attenuation of solar radiation resulting from the scattering and absorption of light within an atmospheric column. An analogy often used to try and visualise optical depth is that of fog. Imagine that you are in a fog and an object is placed directly in front of you, which you can see clearly. The optical depth of the fog between you, the observer, and the object is zero. Now imagine this object moves away from you, as it does it becomes harder to see. The optical depth of the fog between you and the object increases up to the point where you can no longer see the object. In short, the larger the optical depth, the more suspended matter between observer (PFR) and object (sun or top of atmosphere) hence the less light reaching the observer. Knowing accurately the AOD, one can determine information about the quantity and size of aerosols overhead. This information can be used for numerous studies including: to monitor effects of natural and anthropogenic aerosols on climate change, provide estimates of visibilities for satellites or to generate integrated atmospheric water vapour values.

Schematic cross section through PFR instrument. Only 1 of 4 optical arrangements are drawn.
* Taken from PMOD/WRC PFR Operations Manual

Following an independent GAW review of Valentia Observatory (2006), a recommendation was made to start an AOD monitoring program at the site. It was proposed that this would complement the Observatory’s other extensive atmospheric monitoring programmes.

Examples of daily summary charts showing signal strength of 500nm channel.
(a) A cloudless sky at Valentia all day (b) Intermittent cloud during the course of the day.
The PFR signal provides a one minute resolution record when the Observatory is shadowed by cloud.

In the months following the successful installation of the PFR, Valentia embarked on developing its own unique improvements to the data management system. The measurement and calculation of AOD is complicated but can be summarised as follows. If one compares the solar extraterrestrial constant (i.e. the sun’s intensity at the top of the atmosphere) and the solar intensity at a point measured on the surface of the Earth (PFR at Valentia) one will find that the intensity at ground level is lower. This attenuation is primarily a factor of column air mass, scattering, absorption and AOD. If one measures or approximates all other parameters, then AOD can be calculated. The loss in intensity due to absorption is mainly due to ozone in the atmosphere. As a regional GAW station, Valentia operates Ireland’s only Brewer Spectrophotometer (and instrument used to measure total column ozone). In-house software written at Valentia, automatically extracts accurate daily ozone values from the Brewer, which is continuously measuring through the same atmospheric column as the PFR, and assimilates these accurate ozone values into the calculations of AOD. This provides Valentia with a very accurate AOD measurement which is incorporated into the GAW-PFR network.

AOD as measured at Valentia Observatory during 2008 using the 500nm channel.
AOD calculated using Brewer daily ozone values. Data processed by the WORCC

To further improve the availability of data, Valentia with the support of the WORCC, has recently further fine tuned its systems and have started automatically submitting raw daily PFR measurements to the WORCC, managers for the GAW-PFR network. Here these measurements are quality controlled, processed, AOD calculated and graphical output generated. Provisional daily graphs of Valentia PFR data, including diagnostic data, can be viewed from the ‘AOD Graphs’ tab at www.pmodwrc.ch/worcc/. Valentia definitive AOD data will soon be available on the World Data Centre for Aerosols.