Firecrackers accounted for 71% of fine particulate pollution in Delhi on Diwali night, shows a first-of-its-kind assessment by the country’s premier atmospheric sciences research institute.The assessment by the Indian Institute of Tropical Meteorology (IITM), Pune, based on real-time scientific data, shows that firecrackers contributed to 71.1% of PM2.5 pollution in Delhi’s air between 1 am and 9 am on November 9 (Monday), immediately after the Diwali celebrations wound down.The share of firecracker emissions in the PM2.5 concentration was much lower at 43.4% during the same hours on the night before Diwali, and even lower at 5.6% between 1 am and 9 am on November 7.The IITM data corroborate the sharp spikes in actual ambient PM2.5 levels recorded at several air pollution monitoring stations in the capital on Diwali (Sunday) night. Fine particulate matter concentrations peaked at 4,985 µg/m³ in Jahangirpuri and 2,808 µg/m³ in Nehru Nagar at 3 am on November 9.Also Read | Delhi wakes up to toxic post-Diwali air, 8 stations in ‘Severe’ zoneThe IITM assessment has provided a breakdown of the relative contributions of various sources to PM2.5, including vehicular emissions, biomass-burning, waste-burning, secondary aerosols, and dust. Before Diwali night, these sources accounted for a bigger share of PM2.5 than firecrackers.On the night of November 7 leading up to 9 am on Diwali morning for example, the major contributors to PM2.5 were secondary organic aerosols (20.8%), mixed or unresolved biofuel burning (18.1%), waste-burning (11.6%), secondary inorganic aerosols (10.3%), and vehicles (8.8%).This changed dramatically on Diwali night leading up to 9 am on Monday. Firecrackers became the dominant contributor, while the shares of vehicles, secondary inorganic aerosols, secondary organic aerosols, and waste-burning fell to 6.9%, 5%, 4.5%, and 1.1% respectively.Story continues below this adAlso Read | Young engineers, midnight calls, 59 monitoring stations: Inside Delhi’s war room after DiwaliFirecrackers continued to be the dominant contributor to Delhi’s PM2.5 during the study hours of Tuesday as well, at 29.8%, the IITM study said. Secondary organic aerosols and transport sector emissions were the two next biggest contributors. Delhi’s AQI was reported at 299 in the ‘Poor’ category on Tuesday.Firecracker emissions are by nature episodic, unlike relatively continuous sources such as vehicular emissions. Their concentrations can change sharply depending on the timing and intensity of their bursting, wind speed, atmospheric mixing, and the height of the planetary boundary layer, which influences whether pollutants disperse or accumulate close to the ground.The researchers flagged the uncertainties inherently associated with atmospheric processes, source characteristics, and observational limitations, and also the fact that their estimates take into consideration only the 1 am-to-9 am window for each date, and not full-day contributions.Identifying firecracker emissions presents scientific challenges also because some of their chemical constituents overlap with those released by other combustion sources. Potassium, for instance, is a commonly used tracer for fireworks but is also released during biomass-burning, including crop-residue and wood-burning.Story continues below this adThe challenges arising out of this overlap can increase as pollutants age in the atmosphere. A study published this year in the journal ‘Environmental Science & Technology’ found that fireworks-derived particles in Delhi’s Diwali haze became internally mixed with biomass-burning aerosols, resulting in a “complex, chemically evolved haze”.Another study, published in 2022, which examined PM2.5 at two locations approximately 15 km apart in Delhi, found substantial variations in the concentrations of elements associated with fireworks. The study highlighted that the impact of firecrackers can vary across locations depending on the intensity of bursting, the movement of pollutants, and local atmospheric conditions.“The source-apportionment estimates are derived using the best available source profiles, scientifically established methodologies, and practices accepted by the scientific community,” IITM said.Last month, the Advanced Atmospheric Chemistry and Aerosol Toxicity Analysis Laboratory (AATAL) was inaugurated at IITM’s Delhi branch. The lab is equipped with instruments to analyse the chemical composition of atmospheric gases and aerosols, including PM2.5, metals, and toxic elements. It is integrated with IITM’s Air Quality Warning and Integrated Decision Support System for Emissions (AIRWISE), which combines air-quality forecasts, observations, and source-apportionment information.Story continues below this adAfter the inauguration of AATAL on October 24, Dr Sachin Ghude, Project Director of the Metro Air Quality and Weather Service (MAQWS) of the Union Ministry of Earth Sciences, which is implemented by IITM, had said that the integrated system would provide early warnings and detailed information on sources contributing to deteriorating air quality, including stubble-burning, vehicular emissions, and construction and road dust, which would help identify the major sources of air pollution and plan targeted interventions.Dr Ghude told The Indian Express on Tuesday that the new IITM report marks the first time that firecracker emissions have been specifically included in an official source-apportionment estimate. In the absence of real-time measurements of firecracker-related aerosols, their impact has to be estimated indirectly.