PM2.5 levels rising in proportion to stubble burning, finds Nasa study

NEW DELHI: A new study by Nasa scientists has concluded that there is a strong link between agricultural fires in Punjab and Haryana and PM2.5 levels in Delhi during the post-monsoon months of October and November.

PM2.5 concentrations in Delhi, which is downwind to Punjab and Haryana, show a coherent increase — rising often from as low as 50 micrograms per cubic metres (µg/ m3) before the onset of the burning season to 300µg/m3 at the beginning of November when burning is under way.

During the crop burning season of 2016, the seven-day average peaked to a record high of 550µg/m3. This could also explain the severe smog episode in November 2016 when PM2.5 concentration peaked to more than 700µg/ m3 on November 5, 2016. But the study also cautions that the impact of emissions from local sources, including more than 9.5 million vehicles, industries and construction activities, cannot be ruled out. However, this study could give the government clues to averting smog episodes. While the link between crop burning and winter pollution peak was being speculated for long, this is the first scientific study to correlate fire counts in Punjab and Haryana with wind direction and parallel rise in PM2.5 concentrations in Delhi.

The study that analysed satellite data for 15 years (2002-2016) also revealed an increasing trend in crop fires. It found that there is a shift in the distribution of fires from October to November. This study, which has been accepted recently to be published in the journal of Aerosols and Air Quality Research (AAQR), uses fire data from satellites, pollution data from Nasa’s A-train sensors and PM2.5 concentrations measured at the US Embassy in Chanakyapuri.

Scientists also conducted aback trajectory (path taken by air pollution particles in the past) using a National Oceanic and Atmospheric Administration (NOAA) model. Compared to paddy stubble burning in October-November, fire counts in May (wheat stubble burning season) were found to by lower by a factor of six to eight.

As far as post-monsoon pollution is concerned, just by excluding PM2.5 data for October and November, the annual mean concentrations decreased by 10%, 7%, 12% and 13% for 2013, 2014, 2015 and 2016, respectively. “Overall, the fire counts and PM2.5 concentrations showed a linear relationship, which yields an increase in PM2.5 of 33µg/m3, given an increment in fire counts by about 100 …” the study concluded.

Back-trajectory modelling also gave scientists a clue as to why Delhi may be worst affected by crop fires. Days with extreme levels of PM2.5 were mostly associated with northwesterly winds from crop burning areas, “suggesting a profound impact of smoke transport on modulating air quality over Delhi”, the study said.

While concluding that crop fires are on the rise in Punjab and Haryana, satellite data also reveal that the number of fires was higher in October between 2002 and 2009. But this trend has now changed and a majority of fires are being recorded in November.

The number of fire counts detected between 2009 and 2016 was found to be 32% higher than those found between 2002 and 2009 by one of the satellites. But, in 2016, fires were at a record high, which also possibly explains the severe smog episode in 2016 in Delhi. “The crop burning season of 2016 has been most anomalous with maximum fire hotspots (17,804) detected over the Punjab-Haryana region … compared to average fire counts (14,310) detected between 2011 and 2015, the total fire counts in 2016 were 24% higher,” the study added.

In 2016, fire counts were up by 44% compared to the average count recorded between 2002 and 2015.


The study was led by Hiren jethva, scientist from the Universities Space Research Association at the Nasa Godard Space Flight Center; co-authors include scientists Duli Chand, Omar Torres, Pawan Gupta, Alexei Lyapustin and Falguni Patadia. The Pacific Northwest National Laboratory and Morgan State University were also part of the study.