Tracking Down the Saharan Air Layer (SAL)

In the spring and fall, Mediterranean cyclones are not unusual. It’s then that people commonly find a yellow-red covering of dust on the windows of their cars and homes. These dust particles have traveled a long way: after being whirled up in the Sahara, these particles – with an average size of just a few thousandths of a millimeter – are spread across the world. The mineral dust serves as a fertilizer in regions low in nutrients, and at the same time also influences the earth’s thermal economy and cloud formation. On the other hand, these fine particulates have negative effects on our health and adversely affect air quality.

Saharan dust extractor for European aerosol research DNAAP

The DNAAP (Detection of Non-Anthropogenic Air Pollution) project collects data on non-anthropogenic (i.e., not caused by humans) air pollution in real time across Europe. Dust particles are measured using light absorption at selected locations in Cyprus, Spain and Slovenia, providing information about the mineral dust concentration, particularly that of Saharan dust, in the Mediterranean.

Aerosol d.o.o., leading project partner, uses its own Magee Scientific Aethalometer Model AE33 for measurements. To achieve optimal measurement results, a virtual impactor is connected upstream from the absorption photometer. This filters out black carbon, which distorts measurement results. Black carbon is mostly generated by slash-and-burn fires and the burning of fossil fuels, or in other words, it is usually caused by humans. In the DNAAP project, black carbon is an unwelcome travel companion. The dust extractor function in the impactor relies on pumps from KNF. This duty is served by the N 940; the series model was specially adapted to meet the needs of this project.

To attain the required flow rate of 100 l/min, parallel connected pump heads from a higher performance class were chosen. The pump heads, which are made of aluminum, have been anodized precisely for the purpose of achieving the durability that is required under the ambient conditions prevalent in the coastal regions where the measuring points are located. The measurement campaigns benefit from the longest possible, trouble-free service life of the measurement systems, which is why the pump has been equipped with a robust brushless DC motor. The maintenance-free diaphragm gas pump impresses due to its reliability.

Isolated gas flows for precise optical particle measurement

The pump transfers the ambient air into the virtual impactor where it is divided in separate streams. The impactor is classified as virtual because the necessary counter-flow is produced artificially. The volumetric gas flow is divided into a main flow of 98 l/min and a minor flow of 2 l/min. Saharan dust is rather coarse, so that the particles are inertially impacted into the minor flow and subsequently into the absorption photometer, the Aethalometer model AE33.

The data gathered using this optical measuring method provides information about the concentration of Saharan dust. The KNF N 86 diaphragm gas pump, which has been integrated into the Aethalometer, transfers a defined volume of the minor flow to the device’s unit of analysis. KNF drew on the flexibility of its modular system and production engineering to meet the requirements of the measuring device. Installing the pump in the devices from Aerosol d.o.o. is made quick and easy thanks to specially adapted plugs and sockets.

Preliminary data from the research project was already presented at the European Aerosol Conference 2020. This adds to the data from other projects which also rely on KNF’s flexible solutions. Examples include the mobile measurement of fine particulates performed in Google Street View cars in Copenhagen as well as data from a stationary research station at the Amazon Tall Tower Observatory (ATTO) in the Amazon rainforest.