@[email protected] particles below \(0.1\ \unicode[Times]{x3BC}\text{m}\) in diameter. These generally are the most numerous among all particles in the air. Their concentrations can be determined with the Aitken counter which measures total particle @[email protected] Owing to their small size, Aitken particles contribute little to the total mass concentration of all @[email protected] particles; this is determined primarily by particles of diameter \(> 0.1\ \unicode[Times]{x3BC}\text{m}\).