Levels of ambient air pollution in many of the world's largest cities are alarming. According to the World Health Organization (WHO), 90% of the population living in cities in 2014 was exposed to concentrations of fine particulate matter (PM 2.5) exceeding WHO air quality guidelines. Most Indian cities have noxious urban air quality for much of the year. Among the mega-cities of the world (those with more than 14 million inhabitants), Delhi has the highest level of suspended particulate matter (PM10) and Kolkata has the fourth-highest.
In response to court mandates and public pressure, several Indian states have passed legislation to improve air quality through the use of cleaner fuels, an improved vehicle fleet and traffic restrictions. The Kolkata high court, under pressure from the Supreme Court, ordered the state government to implement legislation that would control air pollution in the city. Starting in July 2009, all auto-rickshaws were required to have 4-stroke engines and use LPG. In addition, buses older than 15 years were ordered off city roads. There was considerable resistance to this by private players, who eventually had to comply. Delhi, was the first to move to CNG, but the deterioration in air quality over the years led to an experiment with driving restrictions over two fortnights in 2016, January 1-15 and April 15-30.
We use data from ambient air pollution monitoring stations in these two most polluted Indian cities to estimate the effect of air quality legislation on major pollutants. For Kolkata, we find that the introduction of LPG and the phasing out of old vehicles systematically reduced levels of carbon monoxide and particulate matter. Air quality continued to improve over each of the six quarters after June 2009, when the new law came into force, suggesting that there were considerable lags in its implementation. In spite of reporting improved health, many of the auto-drivers vehemently opposed the legislation due to the financial burden it imposed. A small increase in the cost of new car registrations in Kolkata would have been sufficient to compensate the auto-drivers in Kolkata for the switch towards clean fuel. Had this been done, the path from clean-fuel to clean air could have been much smoother.
In Delhi, we find that driving restrictions did improve air quality and reduce traffic in the first phase, but these effects were much smaller in the second phase. Levels of air pollution vary considerably by season and the smaller effects in the second phase could result from these extraneous factors. We conduct surveys of Delhi citizens during the second phase of the restrictions and find that they did not switch to public transport but rather to permissible private cars and taxis. A more integrated approach to urban air and transport that prices congestion and simultaneously expands public transport is necessary to achieve sustained improvements to city air.