Impact of CNG on emissions of PAHs and PCDDs/Fs from the road transport in Delhi

In this paper we present the first estimates and inventory of polycyclic aromatic hydrocarbon (PAH) emissions from mobile sources in megacity Delhi, India for the period 1999–2006. The “COPERT 4” model was used to estimate 23–species of PAHs and 5–congeners of polychlorinated dibenzo–p–dioxins (PCDDs) and dibenzo–furans (PCDFs) from the gasoline, diesel, and CNG (compressed natural gas) fuelled vehicles.Our study shows that the total annual emissions of ∑23–PAHs from road transport has increased ~4 times and emissions of Napthalene (Nap) emerged as the most prominent (8 times), whereas a two-fold increase was seen for the carcinogen benzo[a]pyrene (BaP) and benzo[a]pyrene equivalence (BaPeq) emissions between 1999 and 2006 from the road transport alone. Further increase in total PCDDs and PCDFs by ~3 times can made air quality even worse. Estimated emission share of low molecular weight PAHs (2–ring) has increased (from 43%–85%), whereas vice-versa for ones with high molecular weights. Switch–over to CNG especially for public transport resulted into an offset of 21% emissions of ∑23–PAHs, 14% in BaP, and 15% in BaPeq for the year 2006. It is also observed that the PAH emissions from CNG fuelled vehicles have decreased, but overall increase in the share of private vehicles (1.5 times) has outweighed this benefit.