STUDY OF NANO-SIZE PARTICLE DYNAMICS IN URBAN ROAD MICROENVIRONMENT IN DELHI

Abstract

Due to rapid urbanization, Delhi experiences frequent pollution events, and the particulate matter load exceeds the prescribed limit often. This study analyzes nanoparticle (10 to 1090 nm) during different emission scenarios, seasonal and meteorological conditions in two phases: April to June 2021 (Period I) and October to November 2021 (Period II). Period I experienced around 31% less concentration of particles (~2.4 × 104 cm-3 ) due to lockdown restrictions and, on the other hand, particle concentration increased by 35% compared to normal conditions due to the sudden rise in firework emissions in Period II. Except for the post-Diwali phase (104 cm-3 to 105 cm-3 ), the concentrations lie between 103 cm-3 and 105 cm-3 . The Aitken modes contribute 10 to 30% of total concentration in both periods. Particles in nucleation and accumulation modes contribute 30 to 40%, 20 to 30%, 15 to 25%, and 35 to 50% in Periods I and II, respectively. Concentration and behavior of nano particles in different seasons (winter, spring, summer, monsoon, and autumn) are examined, for the first time. Concentration of particles is classified into four different sizes as Nnuc (10 to 30 nm, nucleation), Nsatk (30 to 50 nm, small Aitken), Nlatk (50 to 100 nm, large Aitken), and Nacc (100 to 1000 nm, accumulation mode), and the total (10 to 1000 nm) particle number concentration (PNC) as Ntotal. PNC ranges between 104 cm-3 and 106 cm-3 over Delhi during the year, and the highest concentration occurs in winter. Winter concentration is 2 times higher than monsoon, summer, autumn and spring concentrations, respectively. Nnuc, Nsatk, Nlatk and Nacc and their respective contributions to total PNC exhibit significant seasonal variations. During winter Nlatk and Nacc contribute more to total PNC due to coagulation, with Nacc alone contributing >40% to total PNC. Nnuc, Nsatk, and Nlatk are higher in spring and summer during mid-day due to nucleation and/or ultrafine particle burst events. The direct primary emissions from engine exhaust produce a prominent double hump structure during morning and evening ix peak hours in winter and autumn. PNC and their contributions exhibit day-night variations as they are influenced by emission sources and variations in meteorological parameters (wind speed, relative humidity, temperature, solar radiation and boundary layer height) between day and night. Carbon monoxide correlates positively with Nacc in all seasons (R2~0.5) as fossil fuel emission is predominant source for gases and particles in study environment. The concentration of UFP size range particles is dominantly higher (70 to 80%) during peak hours than the non-peak hours. The variations in particle number concentration depend on the intensity and emissions of sources during peak, and non peak hours. UFP contributes ~60 to 80% to the total particle number concentration in the urban roadside microenvironment, and its contribution increases during peak hours. During the winter season, the average total particle number concentration was observed to be maximum (4.1 × 104 cm-3 ) with a higher surface area of particles of 3.5 × 10-3 mm2m 3 . Compared to the monsoon season, the concentration of NOX was 5 times higher in winter. The boundary layer height in the study region ranged from 600 to 2400 m during different seasons, and the maximum ventilation coefficient was observed to be >3000 m2s-1 during summer. Precipitation reduced the concentration of particles by half, from 2.2 x 104 to 1.1 x 104 cm-3 , due to wet scavenging. The study revealed that the concentrations of particles depend not only on primary emissions but also are influenced by local meteorology and other co-emitted pollutants. The Multiple Path Particle Dosimetry (MPPD) model simulated values show that the order of deposition goes as alveoli > bronchiole > bronchus. The deposition in the study area ranges between 10 and 18 million nanoparticles during different hours of the day, whereas the estimated inhalable particles vary between 0.5 to 1 billion. The concentration of total inhalable particles and the actual particles deposited in the lung varies. The seasonal sequence of deposition of nanoparticles is winter > monsoon > summer > autumn > spring. The x deposition of nanoparticle in adults is 30 to 40% higher than in children and infants, and further, the deposition is higher in the alveolar region than in the bronchiole and trachea regions. About 90% of the particles get deposited in the alveolar regions, 6 to 8% in the bronchiole region, and 2% in the trachea region. The estimated deposition of nanoparticles for an individual working 8 hours a day in the near road conditions is 338 µg/year in Delhi. The deposition increases almost linearly as a function of time, and is 3 times higher (1016 µg/year) for a person residing near the road throughout the day (24 h). The deposition fraction of particles ranges between 0.05 and 0.10 µg/day in alveolar region, <0.05 µg/day in the bronchiole region, and lies between 0.02 and 0.04 µg/day in the trachea. The nanoparticles deposited in the respiratory system can lead to the development of various diseases such as asthma, chronic obstructive pulmonary disease, and can lead to carcinogenicity. Number concentration-based studies are essential for estimating the potential impacts on human health due to air pollution. The study provides information regarding vehicle emission-based particle concentration under various emission scenarios in urban cities, which is crucial for estimation of emissions, health impact assessment, future policy formulation and strategy measures. These quantitative results on seasonal variations of air pollutants together with the knowledge on seasonal variations in meteorological parameters and atmospheric dynamics provide a foundation which can positively contribute to the planning and devising mitigation measures aimed at improving air quality and public health. The study provides new insights on inhalable particle concentration during the day that are crucial for strategy development, emission mitigation, and health hazard assessment for the citizens. Understanding the dynamics of atmospheric nanoparticles in urban roadside environments provides on the deposition of nanoparticles in humans residing near roadside conditions are crucial to estimate the xi human health risk potential, and to formulate mitigation measures for exposure reduction which can result in a better and sustainable future.