FORMATION, GROWTH AND QUANTIFICATION OF ULTRAFINE PARTICLESIN SELECTED MICROENVIRONMENTS OF MEGACITY DELHI

Abstract

The present study investigates the particle number concentrations and size distributions in the ultrafine and fine-sized regimes over a polluted megacity, New Delhi (28.75° N, 77.12° E), India. The field experiments were conducted from November 2019 to June 2021 including the strict social and travel restrictions (lockdown) imposed by the Government of India aiming to control the spread of Coronavirus Disease 19 (COVID 19) pandemic. The different phases of the COVID-19 lockdown witnessed restrictions of varying magnitudes with the significant cessation of anthropogenic sources, viz., industrial, road, railways, and air traffic emissions. The impact of varying urban emissions on particle number size distributions and new particle formation events were examined during these restrictions. The mean total number concentrations were in the range of ~ (2 to 3.5) x 104 cm-3 and depicted a gradual increase (~26%) with progressive unlock of the anthropogenic activities. At the same time, accumulation particle concentrations were doubled. However, ultrafine particles (UFP) (diameter < 100 nm) dominated (50-88%) the total number concentrations during most of the days and several new particle formation (NPF) events resulted in elevated (2 - 5 fold) UFP concentrations. Subsequently, the particles grew to larger sizes with rates ~3.31- 8.37 nm hr-1 . The NPF events occurred during the daytime, and during the events, a clear enhancement in the concentrations of [H2SO4] proxy (2 to 3.5 x 107 molecules cm-3 ; 2-3 orders higher than the non-event values), suggesting the role of strong gas-phase photochemistry. Also, some of the NPF events were associated with increased odd oxygen concentrations [Ox = O3+NO2], indicating the regional nature of the precursors and participation of VOC precursors in nucleation/growth. Interestingly, different classes of NPF events were seen during the strictest lockdown period, whereas more frequent and well-defined NPF events were witnessed when anthropogenic activities were opened up with conditional relaxations. These events demonstrated the competition between source strengths of precursor vapors from anthropogenic activities and primary particles acting as condensation sink restricting NPF. This study highlighted that urban pollution mitigation viii policies must consider ultrafine particles emanating from the secondary aerosol formation process from traffic emissions. Since the air pollution and noise generated from fireworks are related to air quality and human health, the regulatory bodies had implemented the eco-friendly "Green Crackers" in megacity Delhi, India, to celebrate Diwali 2019 with the permission of a specific time slot (8:00 PM to 10:00 PM). The present study was conducted on a residential, educational institute campus to evaluate the particle number size distribution (PNSD) of green cracker emissions. During the Diwali event period, the high peak of particle number concentration (PNC) reached 1.7 x 105 # cm-3 with a geometric mean diameter (GMD) of ~44 nm. The average PNC increment on Diwali day was 138 % and 97 % compared to pre (26th October 2019) and post (28th October 2019) Diwali period, respectively, including 468 %, 142 %, 65 %, 75 % on pre-Diwali and 485 %, 110 %, 32 %, 26 % on post-Diwali 2019 period in terms of Nucleation mode (10 nm < Dp < 20 nm), Small Aitken mode (20 nm < Dp < 50 nm), Large Aitken mode (50 nm < Dp < 100 nm), and Accumulation mode (100 nm < Dp < 1000 nm), respectively. Unlike traditional firework emissions, green crackers had a high UFP/Ntotal ratio of 0.72, including Nucleation mode-0.35, Aitken mode-0.30, and Accumulation mode 0.35, distinguishing it from other pre-and post-Diwali particle number size distribution-dN/dlogDp curves. These observations indicate that green crackers emit more particles with smaller diameters than traditional crackers. Recommendations for using green crackers for Diwali celebrations may be an option if lower size-diameter particle emission could be controlled by changing the material composition of the green cracker. More research studies need to be conducted to assess atmospheric emissions of green crackers and their health impacts to evaluate whether they are better or worse than traditional crackers. Research studies have proved that smaller-sized particulate matter is more dangerous to human health when its particle number concentration is high. In the urban transport microenvironment, vehicular emission is a significant source of smaller size particulate matter. The Odd-Even scheme was implemented in Delhi in 2019 on vehicle movement to curb air pollution during the winter season. This study selected three locations, namely Najafgarh, Pitampura, and Panchkuian road, to evaluate the Odd-Even scheme 2019. This study revealed a reduction of 24 to 26 % in PM1 particle number concentration along ix with their particle number size distribution as QuasiUFP (24 to 25.4%), sub-fine (26 to 42.7%) and fine (16.5 to 24.1%) in particle number concentrations. This study also covers the coarse particle size, with the lowest reduction (7 to 10%) at all the selected locations. Car & taxi, on which Odd-Even scheme was effective, has shown a reduction of 12.6 to 26.4%.