Highly time-resolved chemical speciation and source apportionment of organic aerosol components in Delhi, India, using extractive electrospray ionization mass spectrometry

Abstract

Abstract. In recent years, the Indian capital city of Delhi has been impacted by very high levels of air pollution, especially during winter. Comprehensive knowledge of the composition and sources of the organic aerosol (OA), which constitutes a substantial fraction of total particulate mass (PM) in Delhi, is central to formulating effective public health policies. Previous source apportionment studies in Delhi identified key sources of primary OA (POA) and showed that secondary OA (SOA) played a major role but were unable to resolve specific SOA sources. We address the latter through the first field deployment of an extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF) in Delhi, together with a high-resolution aerosol mass spectrometer (AMS). Measurements were conducted during the winter of 2018/19, and positive matrix factorization (PMF) was used separately on AMS and EESI-TOF datasets to apportion the sources of OA. AMS PMF analysis yielded three primary and two secondary factors which were attributed to hydrocarbon-like OA (HOA), biomass burning OA (BBOA-1 and BBOA-2), more oxidized oxygenated OA (MO-OOA), and less oxidized oxygenated OA (LO-OOA). On average, 40 % of the total OA mass was apportioned to the secondary factors. The SOA contribution to total OA mass varied greatly between the daytime (76.8 %, 10:00–16:00 local time (LT)) and nighttime (31.0 %, 21:00–04:00 LT). The higher chemical resolution of EESI-TOF data allowed identification of individual SOA sources. The EESI-TOF PMF analysis in total yielded six factors, two of which were primary factors (primary biomass burning and cooking-related OA). The remaining four factors were predominantly of secondary origin: aromatic SOA, biogenic SOA, aged biomass burning SOA, and mixed urban SOA. Due to the uncertainties in the EESI-TOF ion sensitivities, mass concentrations of EESI-TOF SOA-dominated factors were related to the total AMS SOA (i.e. MO-OOA + LO-OOA) by multiple linear regression (MLR). Aromatic SOA was the major SOA component during the daytime, with a 55.2 % contribution to total SOA mass (42.4 % contribution to total OA). Its contribution to total SOA, however, decreased to 25.4 % (7.9 % of total OA) during the nighttime. This factor was attributed to the oxidation of light aromatic compounds emitted mostly from traffic. Biogenic SOA accounted for 18.4 % of total SOA mass (14.2 % of total OA) during the daytime and 36.1 % of total SOA mass (11.2 % of total OA) during the nighttime. Aged biomass burning and mixed urban SOA accounted for 15.2 % and 11.0 % of total SOA mass (11.7 % and 8.5 % of total OA mass), respectively, during the daytime and 15.4 % and 22.9 % of total SOA mass (4.8 % and 7.1 % of total OA mass), respectively, during the nighttime. A simple dilution–partitioning model was applied on all EESI-TOF factors to estimate the fraction of observed daytime concentrations resulting from local photochemical production (SOA) or emissions (POA). Aromatic SOA, aged biomass burning, and mixed urban SOA were all found to be dominated by local photochemical production, likely from the oxidation of locally emitted volatile organic compounds (VOCs). In contrast, biogenic SOA was related to the oxidation of diffuse regional emissions of isoprene and monoterpenes. The findings of this study show that in Delhi, the nighttime high concentrations are caused by POA emissions led by traffic and biomass burning and the daytime OA is dominated by SOA, with aromatic SOA accounting for the largest fraction. Because aromatic SOA is possibly more toxic than biogenic SOA and primary OA, its dominance during the daytime suggests an increased OA toxicity and health-related consequences for the general public.

Night-time NO emissions strongly suppress chlorine and nitrate radical formation during the winter in Delhi

“…Relative humidity ranged from 30% to 100%. More detail on the campaign site itself and the aerosol and gas-phase instrumentation can be found in other publications from this campaign (Lalchandani et al, 2021;Rai et al, 2020;Singh et al, 2021b;Wang et al, 2020;Kumar et al, 2022).…”

“…CC BY 4.0 License. inlet for gases and aerosols (FIGAERO-CIMS) in January and February 2019, as part of a larger-scale effort to characterise physical and chemical properties of Delhi's urban haze (Lalchandani et al, 2021;Rai et al, 2020;Singh et al, 2021a;Kumar et al, 2022). During the campaign, we observed particulate chloride loadings in excess of 100 µg m -3 on several occasions.…”

A one-year ACSM source analysis of organic aerosol particle contributions from anthropogenic sources after long-range transport at the TROPOS research station Melpitz

“…AMS is commonly limited to short time periods due to the high maintenance of the AMS measurements and their high operating costs. As a result, only a few studies run AMS continuously (e.g., see Kumar et al, 2022 andO'Dowd et al, 2014). However, there was still a strong need for such a long-term analysis.…”

“…From all ASCM/AMS stations (22 stations;Chen et al, 2022) only Melpitz as a rural-background site and Krakow as an urbanbackground site showed the coal combustion emissions with the maximum contribution during winter for both sites (Krakow: 18.2 % and Melpitz: 23 %) compared to summer (Krakow: 4.5 % and Melpitz: 8.7%). The drastic seasonal changes in Krakow are attributed to the common use of coal burning for residential heating reasons during the winter time(Casotto et al, 2022;Tobler et al, 2021), while in Melpitz, as discussed above, coal combustion is affected by both surrounding and transported emissions from other sites.…”

Quantification of primary and secondary organic aerosol sources by combined factor analysis of extractive electrospray ionisation and aerosol mass spectrometer measurements (EESI-TOF and AMS)

“…Source apportionment studies in Zurich using an EESI-TOF identified SOA factors from monoterpene oxidation in summer (Stefenelli et al, 2019) and oxidation of biomass-burning emissions in winter (Qi et al, 2019). EESI-TOF measurements identified SOA factors related to solid fuel combustion and aqueous-phase processes in Beijing (Tong et al, 2021) and SOA factors with aromatic and biogenic origins in Delhi (Kumar et al, 2022). However, to date, the factor concentrations returned by PMF analyses using these instruments are not quantitative.…”