Black carbon and PM2.5 monitoring campaign on the roadside and residential urban background sites in the city of Tehran

Fine particulate matter characterized as PM2.5 is the most important criteria air pollutant in the city of Tehran. Tehran is one of the most polluted cities of the Middle East based on annual mean PM2.5 concentrations. Tehran emission inventory shows the large contribution of mobile sources to the total particles. PM2.5 source apportionment studies show large fraction of black carbon (BC) in the total mass of PM2.5, especially during the cold seasons. BC is the product of incomplete combustion that is mainly derived from diesel engines and rich-burned gasoline carburetor engines on scooters and light-duty vehicles. The present study shows the results of a large experimental campaign in which BC concentration data along with PM2.5 mass concentration data were collected at various sites in the city of Tehran for a period of a year. The effect of heavy-duty diesel vehicle traffic on fresh BC and on the mass fraction of BC to PM2.5 was observed. Nearly 17% of total mass of PM2.5 was identified as BC. During the night hours the BC/PM2.5 mass ratio was increased to more than 4 times than that of day hours due to heavy-duty diesel traffic restrictions during the day. More detailed analyses based on absorption characteristics of BC showed that by increasing heavy diesel vehicle traffic during the night hours, more fresh BC was collected compared to that of aged BC collected during the day time. A city wide temporary ban of heavy-duty diesel traffic during an air pollution episode resulted in a sudden reduction of BC concentration in Tehran air, while the total mass of PM2.5 remained constant. This shows the direct contribution of diesel fleet to BC. It shows the necessity of introducing more accurate and relevant pollutant indicators such as BC mass instead of total PM2.5 mass for better and more effective policies.