Comparison of real-time NMVOCs measurements using PTR-TOF-MS in two cities of IGB region, India: Sources identification and influence on SOA formation

Non-methane volatile organic compounds (NMVOCs) are emitted from various anthropogenic and biogenic sources. They act as precursors for the formation of tropospheric ozone and secondary organic aerosols (SOA) in the presence of sunlight and oxidizing radicals (OH, Cl, NO₃). The measurements of NMVOCs are essential to understand the formation of new gas-to-particle aerosols, leading to high air pollution episodes. The Indo-Gangetic Basin (IGB) of India, one of the world’s most polluted areas, has been experiencing high aerosols and NMVOCs loadings throughout the year. Delhi and Lucknow are the two main cities in the IGB region selected for the study. The main aim of this study is to compare the contributions of different source factors to NMVOCs concentrations and their role in SOA formation. A proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) was deployed to perform real-time chemical characterisation of NMVOCs during two different campaigns in Delhi (2019) and Lucknow (2020-2021), respectively. The High-resolution Time of Flight Mass Spectrometer (HR-ToF-MS), Aethalometer and other instruments were also deployed at both sites. A receptor modelling approach, positive matrix factorisation (PMF), was used with a robust multilinear engine (ME-2) for source apportionment analysis. The present study is a novel attempt to perform PMF over mass spectra of ~90 and ~170 NMVOCs in Delhi and Lucknow, respectively, for different seasons. Their associations with secondary organic aerosol formation using SOA yields were also analyzed. For Delhi, 8-factor solution was selected and resolved into two traffic-related factors: solid-fuel combustions (SFC), secondary VOCs (SVOCs), biogenic factor and solvent-use factor based on statistical parameters. Similarly, for Lucknow, a 6-factor solution was selected with traffic, 2 SVOC factors, 2 SFC factors and one volatile chemical products-related factor. The traffic factor has the presence of aromatics, non-aromatics and oxygenates, while the biogenic factor is marked by isoprene and its fragment (methyl vinyl ketone). The first, second, and third-order oxygenates show peaks in the SVOCs factor, while phenols, furans, and n-containing compounds are found in the SFC factor. It is observed that vehicular emissions (30%) contributed highest to NMVOCs concentrations in Delhi, while the SFC (28%) was a prominent factor in Lucknow. Interestingly, SFC factors contribute the highest to SOA formation at both cities. It is inferred that the agricultural residue burning episodes in neighbouring states, trash burning and solid fuel burning for cooking within and around the cities contributed to the emissions of NMVOCs and the formation of SOA during winter and post-monsoon periods.