How Real-Time Source Apportionment Can Power NCAP

India, a rapidly developing country, has been witnessing increasing air pollution, and to address this, the government implemented the National Clean Air Programme (NCAP). Under this programme, the cities that were identified as non-attainment cities are required to reduce the levels of particulate matter (PM10) by 40% by 2025–26 from the 2019–20 levels or to reduce PM10 levels below that of the national ambient air quality standard (NAAQS) of 60 µg/m3.

Identifying the sources contributing to the increased pollution levels is critical to developing effective mitigation strategies. Therefore, NCAP recommends source apportionment studies before planning comprehensive regional- and city-level plans to mitigate air pollution. It has mandated all 131 non-attainment cities to carry out such studies to gain knowledge on the sources of air pollution in specific locations. The same-day availability of PM source apportionment results can help policymakers make prompt and informed decisions. For instance, identifying the sources contributing to hazardous air pollution peaks can lead to the institution and implementation of appropriate mitigation measures such as graded response action plans (GRAPs). This could also contribute to achieving the 15% improvement target for good air days (Air Quality Index below 200) set by the NCAP.

Traditionally, cities have relied on source apportionment approaches using laborious filter-based sampling of PM2.5 or PM10, followed by time-consuming laboratory analysis for determining the PM chemical composition. The resulting dataset is then analysed using source–receptor models such as the United Nations Environmental Protection Agency’s (EPA’s) Chemical Mass Balance (CMB) model. But CMB requires knowledge of local source profiles, which are not yet widely available for air pollution sources in India; the use of generic (e.g. EPA SPECIATE) source profiles can introduce significant error in the results. Further, few Indian laboratories can analyse filter samples, introducing further delays.

The challenges of delays with laboratory-based chemical composition analysis and high manual labour in collecting filter samples can be avoided by using PM2.5 chemical composition data from near-real-time instruments such as the Aerodyne Time-of-Flight Aerosol Chemical Speciation Monitor (ToF-ACSM), the SailBri Cooper Xact, and the Magee Aethalometer (AE33). The ToF-ACSM measures the chemical composition of non-refractory components such as organics, sulphate, nitrate, ammonium, and chloride. Black carbon from sources of fossil fuel and biomass burning can be measured using the Aethalometer. The Xact measures ambient concentrations of trace metals such as calcium, manganese, selenium, zinc, and lead. These instruments provide near-real-time measurements with a time resolution in the order of a few minutes.

Further, the need for source profiles can be avoided with the use of receptor models (e.g. Positive Matrix Factorisation [PMF]) instead of the CMB model. However, PMF requires a large number (~hundreds) of samples for the best results. This is difficult with traditional integrated (e.g. 24-h) filter-based sampling, but the near-real-time instruments mentioned above are ideal for PMF-based source apportionment.

Common packages such as the PMF Evaluation Tool (PET) and Source Finder (SoFi) facilitate receptor modelling of the high time-resolution PM chemical composition data. A further advancement is the Source Finder Real-Time (SoFi-RT) package, which can process the ACSM, Xact, and AE33 data, apportioning sources in near real time. This combination of high-time-resolution instruments with a package such as SoFi-RT is often called real-time source apportionment (RTSA).

The Air Quality Observations team at the Center for Study of Science, Technology and Policy (CSTEP) has been working on near-real-time source apportionment using SoFi-RT with ToF-ACSM and AE33 data, with plans underway to include the Xact monitor as well.

In this context, CSTEP is organising an (invitation-only) ACSM Users’ Workshop on 23 August 2025 at its Bengaluru office, following the India Clean Air Summit (ICAS) to be held from 20 to 22 August 2025. The workshop aims to bring together ACSM users from around the country to share their experiences and insights from laboratory and field campaigns, get feedback from Aerodyne scientists and their peers across India, and receive hands-on training at the CSTEP Air Quality Laboratory. The ICAS training day on August 20 will feature a track on the analysis of ACSM / Aerosol Mass Spectrometer (AMS) data (open to registered participants), while the opening day of ICAS (August 21) will feature presentations on the American Atmospheric Science and Chemistry mEasurement NeTwork (ASCENT) and European Aerosol, Clouds, and Trace Gases Research Infrastructure (ACTRIS) network.

Kavyashree N Kalkura (Former Analyst at CSTEP) co-authored this article.