A COMPARATIVE STUDY ON SIZE SEGREGATED ULTRAFINE PARTICLE CONCENTRATION AND ITS TEMPORAL DISTRIBUTION IN URBAN AND BACKGROUND REGIONS

Abstract

In developing countries like India, air pollutants have become a serious issue. Industrialization and urbanization have greatly boosted the Indian economy. On the other hand, they have also brought about a drastic change in the environment. Due to various natural and anthropogenic activities, different pollutants are released into the atmosphere. Air pollution is still a major problem faced by the whole world, resulting in 6.2 million deaths throughout the world and 6,20,000 premature deaths in India alone. The alarming increase in air pollutants, with its intensity on the higher side, has resulted in numerous health issues and lung diseases, including chronic respiratory disorders, pneumonia, acute asthma, and shortness of breath. Although technological advancements have improved our understanding of ultrafine particles and air quality, many environmental conditions and scientific problems remain unresolved. Ultrafine particles in the atmosphere are formed due to natural and anthropogenic activities, including photochemical reactions and combustion processes. Atmospheric aerosols persist in solid or liquid form and significantly impact a region’s air quality, contributing to climate change and affecting human health. The concentration of the ultrafine particles in a region is highly dependent on the intensity of the emission sources, represented by the number concentration. The ultrafine particle concentration is quantified in terms of particle number concentration (PNC) per unit volume of air, expressed as the number of particles per cubic centimeter (cm3). Ultrafine particles are considered a universal carrier of a wide variety of toxic chemicals to humans. The deposition of particles in the human respiratory system varies based on the size of the particles, which are categorized into nucleation, Aitken, and accumulation modes. Also, the concentration of these particles signifies the emission sources in a particular region. These can enter the deeper regions of the respiratory system, such as the alveoli, and can be transported to the circulatory system, allowing them to reach any organ in the body. This study analyzes the year-long measurements of particle number size distribution (10.21-1090.23 nm) in 2022 in the urban traffic corridor of the megacity Delhi. The study examined particle number concentration in the ultrafine (<100 nm) and accumulation (100-1000 nm) ranges during both day and night, in terms of the source's intensity, emission patterns, and local meteorological conditions. Further, the ultrafine particles (< 100 nm) are separated into three mode size fractions xi (Nnuc, Natk, and Nacc) were also considered, and the data set was analyzed for the different meteorological periods and day and night time in terms of sources and type of emission (from CNG, petrol, and diesel vehicular exhaust) and meteorology. Nucleation mode particles (Nnuc, 10-30 nm) contributed ~50% during the daytime in the summer period due to increased fresh traffic emissions. During the daytime, particle concentration was dominated by smaller sizes, such as nucleation mode particles (Nnuc, 30-100 nm), while at nighttime, the particles were found in the Aitken and Accumulation (Nacc, 100-1000 nm) modes. Nnuc and Natk were reduced by ~45% and 4% at night, while Nacc increased by ~34%. The mean normalized particle number size distribution confirmed these findings, showing clearly that Nnuc dominated during the daytime, especially during peak rush hours, and at night, particle concentration increased up to the size of 300 nm particles. The particle number size distribution and the changes experienced in the percentage concentration of the different particle size ranges highlight the role of engine exhaust emissions in the atmospheric particles contributed in urban areas. The study's findings help identify the intensity and behavior of different sources during the day and nighttime, which can inform policy formulation and decision-making to mitigate air quality and its associated health impacts. The study measured and analysed the particle number concentration of particles ranging from 10 nm to 800 nm in urban and background sites. The aim of the study is to estimate the role of anthropogenic sources in urban regions in terms of number concentration. The study finds that the particle number concentration in an urban site is ~8 times higher than in the background sites. At the urban site, the Aitken and Nucleation modes contributed more to the total particle concentration, whereas at the background site, it was the Aitken and Accumulation modes. The annual average concentrations at the urban and background sites were 2.5 x 104, 2.9 x 103 cm-3, respectively. Aitken-mode particles dominated both in the urban and background sites, with the lowest concentrations observed in the summer (6.4 x 102 cm-3) and monsoon periods (1.09 x 102 cm-3), respectively. The diurnal concentration illustrates the role of transportation emissions at the urban site and the natural process of particle condensation at the background site. The long-range transportation of sources indicates that during the winter and autumn seasons, the westerly and north-westerly winds bring biomass burning emissions smoke to the urban region, which is less visible xii in the background conditions. Road traffic emissions are often identified as the leading source of ultrafine particles in urban environments. However, recent international studies have revealed that in cities with elevated solar radiation, the occurrence of new particle formation events may also significantly contribute to the levels of ultrafine particles. To assess the frequency and effects of these events, a comprehensive study was carried out in Delhi from January to December 2022. The analysis showed that the highest mean concentration of total particle number concentration (Ntot) followed the order of Post- Monsoon > Winter > Monsoon > Summer. The Aitken particle contribution (Natk) to Ntot was approximately 52% on event days and 46% on non-event days. Throughout the year, 23 new particle formation (NPF) events were identified, accounting for 6% of the total days, which included 2 nocturnal and 21 midday occurrences. During the nocturnal events, ammonia levels increased by about 34%, which aids in the growth of newly formed particles. The remaining NPF events predominantly occurred during daylight hours, when solar radiation was a crucial factor in particle generation. The highest growth rates were observed in winter (12.07 ± 1.07 nm h-1), while the condensation sink (cs) was highest in the post-monsoon period (0.087 ± 0.039 s-1). The presence of larger pre-existing particles during the post-monsoon season appeared to hinder formation rates but enhance particle growth. An increase in particle numbers was associated with a decrease in the condensation sink throughout the day. Winds from the northwest direction were conducive to new particle formation, coinciding with increased solar radiation and temperature, alongside a decrease in relative humidity. In summary, although traffic remains the predominant source of ultrafine particles (UFP) in urban areas, urban nucleation events also represent a significant source of UFP. A future decline in traffic-related particle concentrations is expected; however, an increase in nucleation events in urban areas is likely due to the reduced capacity of urban condensation sinks. The festival of Diwali, characterized by extensive fireworks displays in India, significantly contributes to the increase of atmospheric particles over a brief period, thereby compromising air quality. Implementing short-term measures, such as prohibiting the use of firecrackers during these celebrations, can enhance urban air quality. This study examined particle number concentrations ranging from 10 nm to xiii 1000 nm during the years 2021 and 2022. A notable decrease in particle number concentration was recorded, dropping from 3.8 x 104 cm-3 to 3.1 x 104 cm-3, following the ban on firecrackers in Delhi. The concentration range shifted from 105 cm-3 to 104 cm-3. The analysis of various size categories, Nucleation (10 nm to 30 nm), Aitken (30 nm to 100 nm), and Accumulation (100 nm to 1000 nm), revealed that on Diwali day, Accumulation mode particles accounted for approximately 60% to 83% of the total particle number concentration. Furthermore, the total inhalable particle concentration exposure on Diwali day was reduced by about 18%, equating to 1.6 million particles per day. The findings of this study indicate that significant reductions in emissions within urban environments can be achieved through effective policy implementation and active citizen engagement. Lowering particle emissions is crucial for enhancing air quality, minimizing health risks, and promoting sustainability. The overarching sustainability objectives emphasize the necessity of clean air for all, while health improvements in polluted areas represent achievable interim goals through the execution of appropriate mitigation strategies, which also contribute to combating climate change.