@A00176-1@ 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 @N04262@. Owing to their small size, Aitken particles contribute little to the total mass concentration of all @A00176-2@ particles; this is determined primarily by particles of diameter \(> 0.1\ \unicode[Times]{x3BC}\text{m}\).