Building a low-cost monitoring network to track vertical transport of particulate matter along valley slope: the benefits of low-tech in mountainous environment

Low-cost aerosol sensors give the opportunity to build air quality monitoring network, allowing for increased spatial resolution of particulate matter (PM) measurements [1]. This type of network has already been used in urban environments to capture variation of PM at the city scale [2]. Another environment where PM can change quickly with space is alpine valleys. Indeed, the vertical stratification of air caused by nighttime ground cooling induces important PM concentration variation along a slope. While it is well known that aerosol concentration is greater at the valley floor, the aim of this study is to describe how PM concentration evolves with altitude at the ground and what meteorological phenomenon can change the vertical distribution of PM. Indeed, complex topography influences vertical transport of aerosols through upward and downward motions as well as through local to regional sources of PM.

In the development of a monitoring station for mountainous regions, it is necessary to incorporate specific constraints, such as challenging access to measurement sites, a lack of phone network, and limited access to power. Optical particle counters are low-cost PM sensors that are both small and energy-efficient, enabling the development of portable and autonomous stations. These are key features for building a monitoring network in an alpine valley. We designed monitoring stations based on these constraints. The stations have several sensors that measure the temperature, humidity, and PM_10, PM_2.5, and PM₁ levels. Every fifteen minutes, a microcontroller gathers data and stores them on an SD card. The energy is provided by a battery recharged by a  solar panel recessed at the top of the 3D-printed framework. The overall size of the stations is a cube with a 30 cm side length.

These laboratory-made stations have been used to study PM concentration in the Arve River valley, located in the northern French Alps and encompassing cities like Sallanches, Passy, and Chamonix. The network is composed of 12 monitoring stations and 6 monitored sites positioned on the slopes around the Passy basin. One monitoring site is located on the valley floor, on an air quality agency monitoring station containing a TEOM-FDMS. The discussion will focus on the comparison of low-cost stations with the reference measurements, the benefits of using a low-cost monitoring network to study PM concentrations in mountainous terrain, and the limitations inherent to low-cost sensors and autonomous stations.

[1] Bagkis, E., Hassani, A., Schneider, P., DeSouza, P., Shetty, S., Kassandros, T., Salamalikis, V., Castell, N., Karatzas, K., Ahlawat, A., Khan, J. Evolving trends in application of low-cost air quality sensor networks: challenges and future directions. npj Clim Atmos Sci 8, 335 (2025). https://doi.org/10.1038/s41612-025-01216-4

[2] Feinberg, S. N., Williams, R., Hagler, G., Low, J., Smith, L., Brown, R., Garver, D., Davis, M., Morton, M., Schaefer, J. & Campbell, J. Examining spatiotemporal variability of urban particulate matter and application of high-time resolution data from a network of low-cost air pollution sensors. Atmospheric Environment 213, 579–584 (2019). https://doi.org/10.1016/j.atmosenv.2019.06.026