Apart from the traditional mass concentration, the monitoring also focuses on the number concentrations of airborne particles. Monitoring number concentration serves to gain information on what kind of concentrations there are in different kinds of surroundings and how they have changed. The monitoring helps keep tabs on, for instance, how the tightened emission norms for cars in 2015 influence air quality. HSY monitors the number of particles in a monitoring range of 9─1000 nanometres (nm) on the Kallio sports ground. The annual average in the Helsinki Metropolitan Area has been 5 000─25 000 particles per cubic centimetre of air (particle/cm3). Local emissions such as car traffic can be seen in the results.