Urban air quality monitoring stations have a particularly high installation and maintenance cost, as well as a bulky nature. For this reason, official monitoring networks, comprising only a limited number of fixed monitoring stations, have a sparse coverage in most cities. Helsinki Environmental Services Authority operates only 11 monitoring sites in the entire Helsinki metropolitan area. These conventional monitoring stations are reliable data sources and are able to provide a good overview of concentration changes of various air pollutants on city-scale, but not necessarily on neighbourhood-level and certainly not on street level. However, pollution concentrations can differ substantially street-by-street in an urban environment. A recent hyperlocal air quality mapping in Copenhagen has shown that the major roads of the Danish capital had on an average five times higher black carbon concentrations than less busy residential areas and it was also indicated that there can be huge concentration differences even within a single city block.
The sparse coverage of large traditional regulatory instruments leads to a lack of reliable high-resolution data that is necessary to develop targeted neighbourhood- and micro-scale interventions that effectively reduce exposure of individuals. The cost-effective air quality sensor technology, which became more and more widely used in the last decade, can offer certain benefits in this regard, providing data with high spatial resolution. Air quality sensors have numerous advantages. They have a relatively low cost, they are easy to install and use, allow fast measurements, and due to their low power consumption and since they rely on wireless data transfer, they can be set up freely in the urban environment. Still, there are concerns about the use of the technology. Air quality sensors have lower accuracy as compared to traditional regulatory instruments, moreover their accuracy degrades further over time.
As a response to the sparse coverage of the official monitoring stations, three comprehensive local networks of stationary mid-cost sensors were deployed under HOPE project in Helsinki. The aim was to gain better insight into the spatial and time-related variability of concentration of air pollutants in Helsinki, to test the applicability of the technology, and to demonstrate the added value of the combination of sensors with official monitoring stations. Altogether 25 stationary (Vaisala AQT530) sensors were installed in HOPE. They were distributed within the three target areas of the project. In Jätkäsaari 11 sensors were placed, four of these were installed in busy streets next to the harbour, five nearby construction sites, while two served as urban background. In Vallila six sensors were set up in street canyons to be able to assess the variability of traffic-related air pollution. In the third district, Pakila, eight sensors were deployed to evaluate their suitability to monitor emissions from wood combustion.