Volatile organic compound measurements point to fog-induced biomass burning feedback to air quality in the megacity of Delhi

We report the first ambient measurements of thirteen VOCs for investigations of emissions and air quality during fog and non-fog wintertime conditions at a tower site (28.57° N, 77.11° E, 220 m amsl) in the megacity of Delhi. Measurements of acetonitrile (biomass burning (BB) tracer), isoprene (biogenic emission tracer in daytime), toluene (a traffic exhaust tracer) and benzene (emitted from BB and traffic), together with soluble and reactive oxygenated VOCs such as methanol, acetone and acetaldehyde were performed during the winters of 2015-16 and 2016-17, using proton transfer reaction mass spectrometry. Remarkably, ambient VOC composition changes during fog were not governed by solubility. Acetaldehyde, toluene, sum of C8-aromatics (e.g. xylenes), sum of C9-aromatics (e.g. trimethyl benzenes) decreased by ≥30% (>95% confidence interval), whereas acetonitrile and benzene showed significant increases by 20% (>70% confidence interval), even after accounting for boundary layer dilution. During fog, the lower temperatures appeared to induce an emissions feedback from enhanced open BB within Delhi for warming, releasing both gaseous and aerosol pollutants with consequences for fog chemistry, sustenance and intensity. The potential feedback is important to consider for improving current emission parametrizations in models used for predicting air quality and fog in such atmospheric environments.

Characterization and source identification of PM2.5 and its chemical and carbonaceous constituents during Winter Fog Experiment 2015-16 at Indira Gandhi International Airport, Delhi

Data on mass concentration of PM_2.5 and its carbonaceous and water soluble inorganic chemical ions were compiled through sampling of PM_2.5 at Indira Gandhi International Airport, Delhi during Dec. 16, 2015-Feb. 15, 2016 under Winter Fog Experiment (WIFEX) program of the Ministry of Earth Sciences (MoES) and analysing the samples. The data so generated were interpreted in terms of their variation on different time scales and apportioning their sources. It is found that mass concentration of PM_2.5 averaged over the whole period of observation was 198.6±55.6. The concentration of organic carbon (OC) and elemental carbon (EC) was 24.7±9.4 and 11.7±4.7μg/m³ respectively with no any trend of increase or decrease over the observational period. SO₄^2-, Cl^- and NO₃^- dominated over other anions with their overall average concentration 34.0±23.1, 32.7±16.1 and 13.3±8.7μg/m³ respectively. Among cations, NH₄⁺ showed highest concentration with an average value of 21.0±10.6μg/m³. Variation of daily average mass concentration of these parameters over the period of observation matched well with the variation of PM_2.5 mass concentration indicating thereby to be the major contributors to the PM_2.5 mass. NH₄⁺ mostly occurred as NH₄Cl and NH₄NO₃ and poorly as (NH₄)₂SO₄ or NH₄HSO₄. H⁺ ion mostly occurred as H₂SO₄ and occasionally as HNO₃. Carbonaceous aerosols and NO₃^- were mainly generated from fossil-fuel combustion. NH₄⁺ and anthropogenic Cl^- were mostly generated by biomass burning. The source of SO₄^2- was found to be industries and thermal power plants. Continental Ca²⁺ and Mg²⁺ originated from thermal power plants and soil dust.