The Puijo measurement station in Kuopio joined the SMEAR network in 2009 and became the SMEAR IV station. At Puijo the idea is to study the interactions of aerosol particles and clouds, especially the activation of aerosol particles into cloud droplets. The instrumentation covers measurement of aerosol dynamics, cloud droplet properties, optical measurements, trace gases and meteorological parameters. The characteristic features at SMEAR IV are the elevated location in a semi-urban area and the site being frequently in cloud. The station is operated by the Atmospheric Observations group at the Kuopio Unit of Finnish Meteorological Institute (FMI) and the Aerosol Physics group at the Department of Applied Physics of the Kuopio campus of University of Eastern Finland (UEF).

The station is situated in two locations, Puijo and Savilahti. The Puijo site is located approximately 2 km northwest from the center of Kuopio on the top of an observation tower on Puijo hill. Measurements are made at the height of 306 m above sea level and 230 m above the surrounding lake level. The measurements cover aerosol size distribution and number concentration, light extinction (scattering and absorption) by aerosol particles, cloud droplet size distribution, trace gas concentration, and several meteorological parameters. The aerosol size distribution is measured through two inlets: the interstitial, which removes the cloud droplets and particles smaller than 1.0 µm in diameter, and the total air inlet, which allows aerosol paticles and cloud droplets smaller than 40 µm in diameter to enter the sampling line. In the total air inlet the cloud droplets are dried and both the remaining residual particles and interstitial are measured. The difference between the total aerosol and interstitial aerosol gives then the fraction of activated particles as a function of particle size.

The Savilahti site is at the Kuopio campus area of the UEF, approximately 2 km southwest from Puijo and approximately 2 km west from the center of Kuopio. The site is at the height of 87 m above sea level and 5 m above the surrounding lake level. At Savilahti, the FMI has an automated weather station measuring several meteorological parameters, including the cloud base height measured with a ceilometer. In addition to the weather measurements, aerosol optical properties are measured by using a sun photometer that belongs to the AERONET (AErosol RObotic NETwork) network maintained by the US National Aeronautics and Space Association (NASA). Measurements of the direct solar radiation provide information to retrieve the columnar aerosol optical depth, while the sky radiance measurements can be inverted to produce aerosol optical properties such as size distribution, single scattering albedo, phase functions, and the complex index of refraction. Furthermore, the global solar radiation is measured with a pyranometer that belongs to the SolRad-Net (Solar Radiation Network) network implemented as a companion to AERONET and also maintained by NASA. Spectral diffuse and direct components are measured with an ultraviolet multi-filter rotating shadowband radiometer.