Aarhus University (The Contracting Authority) will be investing in a HTDMA. An HTDMA system is an instrument to measure the hygroscopic properties of atmospheric particles at subsaturated conditions. In principle, the instrument simulates the conditions that atmospheric particles are exposed to in the atmosphere when relative humidities between about 10 % to 90 % are prevailing. The particles then take up water from the atmosphere and change their size. This change is measured with the HTDMA instrument, which is an important parameter in climate science to understand the impact of atmospheric particles on climate. From HTDMA measurements, information on the chemical composition of particles can be deducted.