What efforts were made to reduce air pollution caused by vehicles in Delhi? Has the air quality in Delhi improved?

(N/A) There are two main sources of air pollution. According to $1990$ data,Delhi ranked $4$th among the most polluted cities in the world. Due to the high volume of vehicular traffic,the level of air pollution in Delhi was the highest in the country. Considering the severity of pollution in Delhi,a Public Interest Litigation $(PIL)$ was filed in the court. Subsequently,the Government of India was ordered to take appropriate measures,including replacing the entire fleet of public transport.
Key measures taken include:
$(i)$ Use of $CNG$ instead of diesel in public transport buses; all buses were converted to $CNG$ by $2002$.
$(ii)$ Phasing out of old vehicles.
$(iii)$ Use of unleaded petrol.
$(iv)$ Use of low-sulfur diesel/petrol.
$(v)$ Installation of catalytic converters in vehicles.
$(vi)$ Mandatory $PUC$ (Pollution Under Control) certification for vehicles.
By following these regulations,it is estimated that the levels of carbon dioxide and sulfur dioxide in Delhi decreased significantly between $1997$ and $2005$.