Relationship and variations of aerosol number and PM10 mass concentrations in a highly polluted urban environment—New Delhi, India

Measurements of the aerosol number concentration and the PM10 mass concentrations of urban background aerosols in different seasons were performed in New Delhi 2002, including the simultaneous measurements of NO2, SO2 and CO concentrations. The results indicate an interesting relationship between the aerosol number and the PM10 mass concentrations. The number concentration increases with the mass concentration up to 300 μg m−3. However, after this point, the number concentration decreases even if the mass concentration increases. An explanation for this nonlinear behavior is proposed through a dynamic model involving the coagulation sink concept. The linear relationship between the mass and the number concentration in ambient air is valid if the mass concentration is relatively low. A high sink, however, means that the number concentration cannot be high—resulting in a decline in the number vs. mass plot. Clear diurnal, weekly and seasonal variations in concentrations were observed. High number concentrations of aerosols were observed in the mornings (7–8 a.m.) and in the evenings (7–10 p.m.). The number concentration was highest in March and lowest in June and the mass concentration was highest in November and lowest in June. The number concentration was higher during weekdays, but the mass concentration was higher during weekends. The number concentration correlates with the NO2 concentration, which indicates that one major source of aerosol particles for the monitored site in New Delhi may be vehicular emission, but also domestic use of fossil fuels and biofuels cannot be neglected.