Measuring street dust

Helsinki has an issue with air quality, especially with particulate matter pollution, every spring. Traffic and sanding of the walking ways generate particles, but they are held in place in snow and ice during the winter. However, a lot of the particles go airborne when the snow and ice melt away and surfaces dry. This generates a relatively short but acute period of bad air quality. Some people really feel it, and it generates health risks for all.

Air quality monitoring produces quantified data to visualize the situation. Good data help in engaging preventive actions timely and in the worst affected neighborhoods. The road dust data is also of interest to citizens; some people want to monitor the situation and avoid affected areas whenever the situation is at its worst.

The monitoring is nowadays done in reference stations which are accurate but big and need a lot of attention. Hefty investment and running costs of these stations mean that there are only five in the Helsinki area. They are placed in “hot spots” and provide valuable information, but it may not be true in your immediate surroundings.

Vaisala has now used its optical measurement know-how to develop a laser particle counter which can reliably measure particles from the small 0.3 µm to “large” 10 µm. The same technology is used in many airports to measure visibility and weather conditions. The laser particle counter is inside AQT560, a new compact air quality sensor. These compact sensors cost a fraction of the reference sensors and require little human attention to operate.

We placed three AQT560 sensors next to a reference station in Helsinki to verify the measurement accuracy. See the table to observe how well they agree with each other and with the reference. This period includes the 2023 spring and some days of large PM10 concentrations. Deterioration of the air quality is mostly due to road dust particles. Especially these larger particles have been difficult for compact air quality sensors to measure, but no more.

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PM10 and especially the figure above which involves only particles larger than 2.5 µm, does not tell what size particles there are in the air, only their combined mass. Figure below is the mass distribution of the particles in the same data set.

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In road dust case the mass value is strongly dominated by particles with a diameter above 4 µm. This kind of accuracy and data has previously been only available from reference or equivalent sensors. AQT560 provides the same data reliably and accurately with a fraction of the cost.