Over the years, source apportionment studies by multiple research institutions have differed on the relative contribution of traditional and emerging sources to particulate matter in Delhi’s air. Assessments of the share of vehicles in the total pollution have varied widely, as have assessments of the contributions of different types of vehicles to the total vehicular pollution.

The first major study was carried out in 2008 by the Central Pollution Control Board (CPCB) with the National Environmental Engineering Research Institute (NEERI). It identified road dust as the biggest contributor (52.5%) to particulate matter in Delhi’s air, followed by industries (22.1%). The study attributed only 6.6% of particulate emissions to vehicles. For NOx, the study found industries contributed 79% and vehicles 18%; vehicles were the main source for CO and hydrocarbons: 59% and 50% respectively.

The CPCB-NEERI findings were by and large reiterated by another government study two years later by the System of Air Quality Forecasting and Research (SAFAR) under the central Ministry of Earth Sciences. Results of the study that ascertained the sources of particulate matter during the Commonwealth Games in New Delhi in 2010, were published in the peer-reviewed journal Atmospheric Environment. It reached the “surprising” conclusion that “the unattended source of windblown dust from paved and unpaved roads” was the major contributor to PM 10 emissions. SAFAR Program Director Dr GufranBeig said, “We found the majority of PM 10 came from road dust. This was in contradiction to previous published studies which blamed vehicular pollution as the major source.”

The study found the contribution of windblown dust to be more than all other sources — including vehicles, industries and cooking fuel — put together, and more than four times the share of transport emissions, which was the second biggest contributor. This, despite 2010 experiencing the wettest monsoon in Delhi since 1978, which would be expected to cause road dust to settle. Vehicular and industry sources continued to be the dominant sources of PM 2.5 in this study, followed by construction.

Both the 2008 amd 2010 studies disagreed with the white paper on air pollution in Delhi that the union Ministry of Environment and Forests published in 2003. The white paper reported that between 1970-71 and 2000-01, the contribution of vehicles to Delhi’s particulate matter had increased from 23% to 72%.

Several other studies for the 1990-2008 period, and up to even 2010 and 2012, identified vehicles as the primary contributors to Delhi’s emission inventories. A study, Emission Estimates and Trends (1990-2000) for Megacity Delhi and Implications published in Atmospheric Environment in October 2004 found that over 80% of emissions of major pollutants including particulates, NOx and CO, came from the transport sector.

In 2013, Dr SarathGuttikunda published a study in Atmospheric Environment that concluded that vehicular emissions contributed 90%, 54% and 33% to NOx, Total Suspended Particulate Matter and SO2 emissions respectively.

The same year, a study led by Dr Pramila Goyal of IIT-Delhi’s Centre for Atmospheric Studies flagged emissions from two wheelers as a “matter of concern”. Two wheelers contribute between 40% and 60% of the total pollution from vehicles, the study said. It identified Heavy Commercial Vehicles (HCVs) to be the major contributors to particulates (92%).

Other studies since 2007 have said that trucks contribute the lion’s share of pollutants like CO, SO2 and PM 2.5 in Delhi’s air.

Also in 2013, a UN Environment Program study by IIT Delhi and Dr Guttikunda that studied pollution inventories in three cities including Delhi, noted that “the steady increase and higher pollution levels (in the NCR) can be attributed to the exhaust emissions from trucks which are allowed to pass through the city after 9 PM.

“The high concentrations observed at night tend to linger during the rush hours through 11 AM combined with the passenger traffic and further exacerbating the exposure times and related health concerns along the major corridors.” The transport sector was found to contribute 17% and 10% of PM 2.5 and PM 10 respectively.

A 2015 IIT-Delhi and IIT-Bombay study by Dr Guttikunda and Dr Rahul Goel on vehicle exhaust emissions, which included transport policy recommendations for 2015-30, pointed out that trucks contribute more than 60% of total vehicular pollutants PM 2.5, CO and SO2 in the “greater Delhi region”.

The most recent study by IIT Kanpur, a two-year analysis submitted to the Delhi government last month, identified seasonal variations in contributions to particulates. In winter, the study attributed 46% particulate emissions to trucks, and 33% to two wheelers. Four wheelers trailed behind at 10%, followed by buses (5%) and Light Commercial Vehicles (4%). But in summer, the four top contributors of PM 10 are road dust, concrete batching plants, industrial point sources and vehicles. The top four contributors for PM 2.5 are dust, vehicles, domestic fuel burning and industrial pollution, the results of the study showed.