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Particle Number Concentrations and Size Distribution in a Polluted Megacity: the Delhi Aerosol Supersite Study

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Particle Number Concentrations and Size Distribution in a Polluted Megacity: the Delhi Aerosol Supersite Study Atmos. Chem. Phys., 20, 8533–8549, 2020 https://doi.org/10.5194/acp-20-8533-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Particle number concentrations and size distribution in a polluted megacity: the Delhi Aerosol Supersite study Shahzad Gani1, Sahil Bhandari2, Kanan Patel2, Sarah Seraj1, Prashant Soni3, Zainab Arub3, Gazala Habib3, Lea Hildebrandt Ruiz2, and Joshua S. Apte1 1Department of Civil, Architectural and Environmental Engineering, The University of Texas at Austin, Austin, Texas, USA 2McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas, USA 3Department of Civil Engineering, Indian Institute of Technology Delhi, New Delhi, India Correspondence: Joshua S. Apte ([email protected]) and Lea Hildebrandt Ruiz ([email protected]) Received: 2 January 2020 – Discussion started: 17 February 2020 Revised: 25 May 2020 – Accepted: 8 June 2020 – Published: 22 July 2020 Abstract. The Indian national capital, Delhi, routinely expe- that a reduction in mass concentration, especially in winter, riences some of the world’s highest urban particulate matter may not produce a proportional reduction in PN concentra- concentrations. While fine particulate matter (PM2:5) mass tions. Strategies that only target accumulation mode particles concentrations in Delhi are at least an order of magnitude (which constitute much of the fine PM2:5 mass) may even higher than in many western cities, the particle number (PN) lead to an increase in the UFP concentrations as the coagula- concentrations are not similarly elevated. Here we report on tion sink decreases. 1.25 years of highly time-resolved particle size distribution (PSD) data in the size range of 12–560 nm. We observed that the large number of accumulation mode particles – that con- stitute most of the PM2:5 mass – also contributed substan- 1 Introduction tially to the PN concentrations. The ultrafine particle (UFP; Dp < 100 nm) fraction of PNs was higher during the traf- Outdoor air pollution has detrimental health effects (Pope fic rush hours and for daytimes of warmer seasons, which and Dockery, 2006; Schraufnagel et al., 2019a) and is re- is consistent with traffic and nucleation events being major sponsible for more than 4 million deaths globally every year sources of urban UFPs. UFP concentrations were found to (Cohen et al., 2017), resulting in substantial global and re- be relatively lower during periods with some of the highest gional decrements in life expectancy (Apte et al., 2018). mass concentrations. Calculations based on measured PSDs There has been a continued growth in megacities (population and coagulation theory suggest UFP concentrations are sup- > 10 million), and as of 2018 there are 47 megacities (United pressed by a rapid coagulation sink during polluted peri- Nations, 2018). Many of the lower-income megacities expe- ods when large concentrations of particles in the accumu- rience persistently severe air pollution problems, and Delhi lation mode result in high surface area concentrations. A (population: 28 million) routinely experiences the highest an- smaller accumulation mode for warmer months results in nual average fine particle mass (PM2:5) concentrations of any an increased UFP fraction, likely owing to a comparatively megacity in the world (World Health Organization, 2018). smaller coagulation sink. We also see evidence suggestive of In addition to aerosol mass, there is now growing evidence nucleation which may also contribute to the increased UFP of potential health risks associated with ultrafine particles proportions during the warmer seasons. Even though coagu- (UFP; Dp < 100 nm). These particles can penetrate and de- lation does not affect mass concentrations, it can significantly posit deep within the lungs, may enter the bloodstream, and govern PN levels with important health and policy implica- can reach sensitive internal organs (Yeh and Schum, 1980; tions. Implications of a strong accumulation mode coagula- Oberdörster et al., 2005; Salma et al., 2015; Schraufnagel tion sink for future air quality control efforts in Delhi are et al., 2019b). While UFPs scarcely contribute to aerosol mass (PM), they have been observed to dominate particle Published by Copernicus Publications on behalf of the European Geosciences Union. 8534 S. Gani et al.: Particle number concentrations and size distribution in a polluted megacity number (PN) concentrations observed in urban environments (Hussein et al., 2004; Rodríguez et al., 2007; Wu et al., 2008). Atmospheric particle size distributions (PSDs) are of- ten categorized by the nucleation mode (3–25 nm), the Aitken mode (25–100 nm), and the accumulation mode (100–1000 nm). UFPs include both the nucleation and the Aitken modes. Most of the PM2:5 mass is usually contributed by the accumulation mode particles, whereas UFPs consti- tute most of the PNs (Seinfeld and Pandis, 2006). Given that different modes of the PSDs contribute disproportion- ately to aerosol mass (accumulation mode for PM2:5) and aerosol number (UFPs for PNs) concentrations, PN concen- trations do not necessarily share the same spatial or tem- poral dynamics as PM (Johansson et al., 2007; Puustinen et al., 2007; Reche et al., 2011). The accumulation mode Figure 1. Diurnal profiles of meteorological parameters (temper- particles in urban environments result from a wide range of ature, relative humidity, wind speed, wind direction, planetary anthropogenic sources (traffic, industrial, biomass burning, boundary layer height (PBLH), and ventilation coefficient) by sea- etc.), natural sources (wildfires, volcanoes, deserts, etc.), and son. Average values by season and hour of day are presented for all chemical processing of gases and aerosol (Robinson et al., parameters except wind direction. The median value is presented for 2007; Zhang et al., 2007; Jimenez et al., 2009; Calvo et al., wind direction. Ventilation coefficient (VC) D PBLH × wind speed. 2013). However, traffic (Zhu et al., 2002; Charron and Har- rison, 2003; Fruin et al., 2008; Wehner et al., 2009), cooking (Saha et al., 2019; Abernethy et al., 2013), and new parti- cities with similar sources and meteorology across the Indo- cle formation (Brines et al., 2015; Hofman et al., 2016) are Gangetic Plain (Kumar et al., 2017; Singh et al., 2015) and is generally considered the only major sources of urban UFPs. relevant for other highly polluted urban environments. In cities with high aerosol mass loadings (e.g., Beijing and Delhi), coagulation scavenging can be a major sink of 2 Methods UFPs – the numerous but smaller UFPs are “lost” to the fewer but larger particles that contribute most to the aerosol 2.1 Sampling site mass (Kerminen et al., 2001; Zhang and Wexler, 2002; Kul- mala, 2003; Kulmala and Kerminen, 2008). Specifically, co- We installed a suite of high-time-resolution online aerosol agulation plays a crucial role in the rapid removal of freshly measurement instrumentation at the Indian Institute of Tech- nucleated particles onto preexisting, larger particles (Kermi- nology Delhi (IIT Delhi) campus in South Delhi. The instru- nen et al., 2001; Kulmala, 2003). Short-term studies from ments were located on the top floor of a four-story building, Delhi have observed coagulation as a major sink for UFPs and the nearest source of local emissions is an arterial road (Mönkkönen et al., 2004a, 2005), similar to observations located 150 m away from the building. Delhi experiences from polluted cities in the USA in the 1970s (Husar et al., a wide range of meteorological conditions with large diur- 1972; Whitby et al., 1972; Willeke and Whitby, 1975) and nal and seasonal variations in temperature, relative humidity contemporary polluted cities in China (Cai and Jiang, 2017; (RH), and wind speed, among other parameters (Fig.1). Fur- Peng et al., 2014; Wu et al., 2008). Polluted megacities such thermore, shallow inversion layers also occur frequently, es- as Delhi often experience aerosol mass concentrations that pecially for cooler periods. The prevailing wind direction in are an order of magnitude higher than those experienced in Delhi is from the northwest, implying that the air we sampled cities in high-income countries (Pant et al., 2015; Jaiprakash would have traversed through ∼ 20 km of industrial, com- et al., 2017; Gani et al., 2019), yet PN concentrations are not mercial, and residential areas in Delhi before reaching our high in similar proportions (Mönkkönen et al., 2004b; Apte site. We discussed the meteorology of Delhi in further detail et al., 2011). in an earlier publication (Gani et al., 2019). As part of the Delhi Aerosol Supersite (DAS) study, we investigated Delhi's aerosol chemical composition and its 2.2 Instrumentation and setup sources (Gani et al., 2019; Bhandari et al., 2020). Here we use long-term observations from Delhi to present seasonal We measured PSDs using a scanning mobility particle and diurnal profiles of PSDs in this polluted megacity. We sizer (SMPS; TSI, Shoreview, Minnesota, USA) consist- also provide insights into the phenomena – such as coagu- ing of an electrostatic classifier (model 3080; TSI, Shore- lation – that influence PSDs in Delhi and interpret their im- view, Minnesota, USA), a differential mobility analyzer plications for future policy measures. This study has the po- (DMA; model 3081; TSI, Shoreview, Minnesota, USA), tential to help understand processes that drive PSDs in other an x-ray aerosol neutralizer (model 3088; TSI, Shoreview, Atmos. Chem. Phys., 20, 8533–8549, 2020 https://doi.org/10.5194/acp-20-8533-2020 S. Gani et al.: Particle number concentrations and size distribution in a polluted megacity 8535 Minnesota, USA), and a water-based condensation parti- cles of any size greater than that particle using the technique cle counter (CPC; model 3785; TSI, Shoreview, Minnesota, of Westerdahl et al.(2009). We calculate condensation sink USA). Chemical composition of nonrefractory PM1 (NR– (CS) for vapor condensing on the aerosol distribution fol- PM1) was obtained using an aerosol chemical speciation lowing Kulmala et al.(2001, 1998) and the growth rate (GR) monitor (ACSM; Aerodyne Research, Inc., Billerica, Mas- following Kulmala et al.(2012).
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