Local incomplete combustion emissions define the PM2.5 oxidative potential in Northern India.

The oxidative potential (OP) of particulate matter (PM) is a major driver of PM-associated health effects. In India, the emission sources defining PM-OP, and their local/regional nature, are yet to be established. Here, to address this gap we determine the geographical origin, sources of PM, and its OP at five Indo-Gangetic Plain sites inside and outside Delhi. Our findings reveal that although uniformly high PM concentrations are recorded across the entire region, local emission sources and formation processes dominate PM pollution. Specifically, ammonium chloride, and organic aerosols (OA) from traffic exhaust, residential heating, and oxidation of unsaturated vapors from fossil fuels are the dominant PM sources inside Delhi. Ammonium sulfate and nitrate, and secondary OA from biomass burning vapors, are produced outside Delhi. Nevertheless, PM-OP is overwhelmingly driven by OA from incomplete combustion of biomass and fossil fuels, including traffic. These findings suggest that addressing local inefficient combustion processes can effectively mitigate PM health exposure in northern India.

Effect of Biomass Burning, Diwali Fireworks, and Polluted Fog Events on the Oxidative Potential of Fine Ambient Particulate Matter in Delhi, India.

We investigated the influence of biomass burning (BURN), Diwali fireworks, and fog events on the ambient fine particulate matter (PM2.5) oxidative potential (OP) during the postmonsoon (PMON) and winter season in Delhi, India. The real-time hourly averaged OP (based on a dithiothreitol assay) and PM2.5 chemical composition were measured intermittently from October 2019 to January 2020. The peak extrinsic OP (OPv: normalized by the volume of air) was observed during the winter fog (WFOG) (5.23 ± 4.6 nmol·min-1·m-3), whereas the intrinsic OP (OPm; normalized by the PM2.5 mass) was the highest during the Diwali firework-influenced period (29.4 ± 18.48 pmol·min-1·µg-1). Source apportionment analysis using positive matrix factorization revealed that traffic + resuspended dust-related emissions (39%) and secondary sulfate + oxidized organic aerosols (38%) were driving the OPv during the PMON period, whereas BURN aerosols dominated (37%) the OPv during the WFOG period. Firework-related emissions became a significant contributor (∼32%) to the OPv during the Diwali period (4 day period from October 26 to 29), and its contribution peaked (72%) on the night of Diwali. Discerning the influence of seasonal and episodic sources on health-relevant properties of PM2.5, such as OP, could help better understand the causal relationships between PM2.5 and health effects in India.

Diurnal variability in the spectral characteristics and sources of water-soluble brown carbon aerosols over Delhi.

It is well established that light-absorbing organic aerosols (commonly known as brown carbon, BrC) impact climate. However, uncertainties remain as their contributions to absorption at different wavelengths are often ignored in climate models. Further, BrC exhibits differences in absorption at different wavelengths due to the variable composition including varying sources and meteorological conditions. However, diurnal variability in the spectral characteristics of water-soluble BrC (hereafter BrC) is not yet reported. This study presents unique measurement hitherto lacking in the literature. Online measurements of BrC were performed using an assembled system including a particle-into-liquid sampler, portable UV-Visible spectrophotometer with liquid waveguid capillary cell, and total carbon analyzer (PILS-LWCC-TOC). This system measured the absorption of ambient aerosol extracts at the wavelengths ranging from 300 to 600 nm with 2 min integration time and water-soluble organic carbon (WSOC) with 4 min integration time over a polluted megacity, New Delhi. Black carbon, carbon monoxide (CO), nitrogen oxides (NOx), and the chemical composition of non-refractory submicron aerosols were also measured in parallel. Diurnal variability in absorption coefficient (0.05 to 65 Mm-1), mass absorption efficiency (0.01 to 3.4 m-2 gC-1) at 365 nm, and absorption angstrom exponent (AAE) of BrC for different wavelength range (AAE300-400: 4.2-5.8; AAE400-600: 5.5-8.0; and AAE300-600: 5.3-7.3) is discussed. BrC chromophores absorbing at any wavelength showed minimum absorption during afternoon hours, suggesting the effects of boundary layer expansion and their photo-sensitive/volatile nature. On certain days, a considerable presence of BrC absorbing at 490 nm was observed during nighttime that disappears during the daytime. It appeared to be associated with secondary BrC. Observations also infer that BrC species emitted from the biomass and coal burning are more absorbing among all sources. A fraction of BrC is likely associated with trash burning, as inferred from the spectral characteristics of Factor-3 from the PMF analysis of BrC spectra. Such studies are essential in understanding the BrC characteristics and their further utilization in climate models.

Particle size dynamics and risk implication of atmospheric aerosols in South-Asian subcontinent.

Presented here are size-resolved aerosol measurements conducted using cascade impactor set at breathing zone in indoor-outdoor residential microenvironments. PM2.5 contributed about 64-80% of PM10 in which over 29% of mass was shared by PM0.25. Total PM concentration varied from 261 ± 22 µg/m3 (indoors) to 256 ± 64 µg/m3 (outdoors) annually; whilst summer and monsoon demonstrated 1.2- and 1.9- times lower concentration than winters. The measured metals ranged between 9% (in PM2.5-10) to 18% (in PM1-2.5) of aerosol concentration; whereby crustal elements dominated coarse fractions with relatively higher proportion of toxic elements (Ba, Cd, Co, Cr, Cu, Ni) in ultrafine range. Considering lognormal particle size distribution (PSD), accumulation mode represented the main surface area during entire monitoring period (Mass Median Aerodynamic Diameter (MMAD) < 1). PSD of metal species reflected their different emission sources with respect to season integrated samples. High air exchange conditions permitted the shift of indoor PSD pattern closer to that of outdoor air while low ventilation in winters reflected modal shift of metals (Pb, Mg. K) towards larger size particles. Relative surge towards smaller diameter size of soluble metal fraction relative to the total concentration of toxic elements was noted on an annual basis with high infiltration capacity of smaller size particulates (Finf =1.36 for ultrafine particles in summers) identified through indoor-outdoor regression analysis. Principal Component Analysis identified sources such as vehicular traffic, combustion, crustal emission with activities viz. smoking and those involving use of electric appliances.