Pollution concentrations in Delhi India during winter 2015-16: A case study of an odd-even vehicle strategy

Air pollution is a major threat to the health of the inhabitants of most large cities in the world including Delhi, the second most populated city. Delhi with 25 million people is among the most polluted. Meteorology as well as increasing anthropogenic emissions play important roles in Delhi's reduced air quality. Winter is marked by thick fog, low wind speeds, and low boundary layer heights that worsen air quality. In such an environment, this study monitored PM2.5, PM10, NOX (NO), CO, and O-3. To improve air quality, the Delhi Government implemented an Odd-Even (OE) Scheme based on license plate numbers to reduce traffic emissions. The improvements in air quality were not obvious due to the variability in the meteorological conditions during that period. Measured concentrations were compared among the Pre-OE, During-OE, and Post-OE periods and only a limited number of statistically significant differences were observed when all three periods were compared on a 24-hour or hourly bases. Passenger cars represented only a small fraction of the on-road traffic and exceptions were granted for many vehicles. For the limited duration of this trial, variations in dispersion conditions drove much of the observed period-to-period variability. The effect of the vehicle removal strategy to improve air quality was unobservable.