Air pollution, both of particulate and gaseous nature, has emerged as a serious environmental problem and concern of the century with severe health consequences as well as societal, economic and climate impacts. Air pollution has impacted Asian countries the most as they are facing challenges in the technological advancement and, at the same time, a rapid increase in population has led to a burgeoning demand for food and energy. The urbanisation and industrial development have raised the demand of more energy from fossil fuels which, in turn, has led to an increase in emission of atmospheric gaseous pollutants such as NOx, SO2 and CO as well as particulate matter concentrations in the atmosphere.
Atmospheric aerosols, particularly carbonaceous aerosols, over South and South East Asia have been a subject of extensive research over the past two decades because of their potential impact on the regional air quality, climate and hydrological cycle 1-4. Atmospheric aerosol is, so far, very well known to have vast impacts on atmospheric chemistry, biogeochemistry, radiative forcing and cloud formation. However, the extent of their impacts is highly dependent on their spatio-temporal variability and composition. This article provides an overview of datasets on chemical characterization (primary as well as secondary aerosols) and their impact on aerosol and rainwater chemistry as well as optical properties over India, and particulate over the Indo-Gangetic plain and adjoining areas.
Carbonaceous aerosols: Chemical characterization, source apportionment and optical properties over India
Carbonaceous aerosols, consisting of organic carbon (OC) and elemental carbon (EC, also referred as black carbon; BC), are the major components of atmospheric particulate matter (PM) and constitute ~30–70% of the fine aerosol mass over urban atmospheres of India 5-8. Carbonaceous aerosols, originating from a variety of anthropogenic emission sources (vehicular exhaust, biomass burning and fossil fuel emissions) are gaining considerable importance because of their potential impact on regional air quality, atmospheric chemistry and climate2,9-12. The measurements of aerosol chemical composition are crucial for improving the model parameterizations for air quality, visibility pattern, and to assess sources of these health-afflicting aerosols. There have been several researches on physico-chemical and optical properties of aerosols over India to understand the complex interaction between aerosols-chemistry-optical properties and climate. For example, the Indian Space Research Organisation-Geosphere Biosphere Programme (ISRO-GBP) conducted two major land campaigns over different parts of India. The first land campaign (LC-I) was conducted over the peninsular western India during February–March 2004 13. The second land campaign (LC-I) was dedicated to study aerosol composition and optical properties during 1st to 31st December 2004, which included seven different sampling locations representing urban (Hisar, Delhi, Agra, Allahabad and Kharagpur), rural (Jaduguda) and a high altitude site (Manora Peak) of northern India. These studies have provided a decent amount of data on chemical, optical and physical properties though were limited to smaller samplings 14-25. Apart from such land campaign studies, several Indian researchers have independently provided data sets pertaining to either chemical 5,6,26-31 or optical properties32-38 of aerosols over different parts of India. However, a complete characterization and data of physico-chemical and optical properties of aerosols are still lacking for entire India.
Secondary aerosols formation and its impact on Fog-haze formation
The fog/haze formation over the entire stretch of Indo-Gangetic Plan (IGP) and the Bay of Bengal (BoB) is a classic example of the manifestation and complex interplay between emission, atmospheric chemistry and meteorology, especially during wintertime. However, whether it is a result of manifestation of atmospheric chemistry, mainly secondary aerosol formation, leading to foggy/hazy condition or vice-versa, is not very well known. This is mainly due to the wide range of temporal scale variability (a few seconds to few hours or day) of secondary organic aerosol (SOA) formation and other relevant atmospheric chemical/physical processes. However, detailed and long-term studies of the phenomenon over India are lacking and, thus, are highly recommended not only for better source apportionment but also to reduce the gap between measurements and modelling studies of SOAs.
A significant fraction of organic aerosol (OA) can also be derived from secondary formation in the atmosphere (referred to as secondary organic aerosol; SOA). Most of the SOAs are soluble in water (referred to as water-soluble OC, WSOC) and thus, WSOC/OC ratios have been often used for understanding SOA formation 28. The formation mechanism, the aging of organic aerosols and their hygroscopicity during the transport depends on the amount of volatile organic compounds (VOCs) and oxidizing species (O3, OH, NOx radicals) at a given location39,40. Thus, measurements of VOCs and oxidizing species, low molecular weight carboxylic acids (C2–C6) can provide an insight for the evolution of organic aerosols over Indian regions41,42. The measurements of aerosol chemical composition by an aerosol mass spectrometer (AMS) can provide the information on the degree of oxygenation and secondary organic aerosol (SOA) formation. In absence of AMS data, we rely on the filter-based measurements of chemical composition and WSOC in bulk- and/or size-segregated fractions27-29. Therefore, future investigations on the emission inventories of POA and SOA, and the measurements of poly-aromatic hydrocarbon (PAHs) would be required to constrain the organic aerosol budget from the South Asian region.
Atmospheric Humic-Like substances (HULIS) and Brown Carbon (BrC)
Atmospheric humic-like substances (HULIS) are a class of macromolecular organic compounds prevalent in diverse environmental media (e.g., aerosol, rainwater, cloud water), which have been recognized as the main component of BrC 43-45. The atmospheric HULIS can be easily isolated from the aerosols and considered as the surrogate of water-soluble BrC 46. During the past few years, researches regarding the atmospheric HULIS have been carried out globally, e.g., in the Amazon basin where large-scale forests are distributed 47 in the urban and rural environment with intensive anthropogenic activities 48,49, and even in the marine aerosols and Arctic snowpack 50,51. Atmospheric HULIS can be emitted directly during the process of biomass burning 47,52. Biopolymer like cellulose and lignin could thermally breakdown to (semi-) volatile, low molecular weight substances, which could further condense to the higher molecular weight HULIS through the gas-phase reactions 45. In addition, soil may also contribute to the atmospheric HULIS under certain geographic and meteorological conditions 53.
Besides the primary sources, atmospheric HULIS could also be produced by complex secondary reactions (e.g., heterogeneous and photosensitized reactions) from the anthropogenic or biogenic volatile organic compounds (VOCs) 43,54. Recent studies have shown that certain types of organic aerosols such as brown carbon (BrC), apart from well-known black carbon (BC), also exert considerable light absorption capability for the solar radiation, particularly at shorter wavelengths. The radiative impact of BrC increases and radiative forcing can be equal to 24% of BC when BrC is deposited on snow and sea ice. However, radiative forcing of BrC strongly depends on its optical properties, which is still poorly understood due to its complicated composition, sources and various formation processes. There are only a few studies on chemical characterization, optical properties and radiative impact of BrC over India 55,56. The optical properties of BrC are believed to vary in different regions 43, leading to a large gap between the global and regional modeling results. Therefore, it highlights the necessity to conduct more fundamental researches in different geographic areas to study its chemical and optical properties of brown carbon. In addition, the presence of BrC may also have significant impact on the atmospheric chemical processes, because ultraviolet radiation promotes many photochemical reactions in the atmosphere. However, the quantification of BrC/WSOC ratio is still lacking over India and needs to be investigated on a spatio-temporal scale.
Following are the objectives of the proposed research work:
Rainwater chemistry over IGP and Himalayas
Wet precipitation is an important and efficient mechanism for the removal of particulate (i.e., aerosols) and gaseous pollutants from the atmosphere. In addition, the chemical composition of rainwater can reveal the sources and interaction processes whereas its deposition on earth’s surface can affect the ecosystem, monuments and sub-surface water chemistry. The pH of precipitation revealed its alkaline nature over the Himalayas 57 as well in the IGP and the east coast 58 due the mixture of anthropogenic as well as the natural chemical constituents. Among the ions, HCO3- (35%) makes the highest contribution indicating that the rainwater chemistry during summer monsoon over the region was influenced mainly by terrigenous components and transported dust. One of the surprising but an important observation revealed the significant contribution from NH4+ in precipitation samples of Varanasi. Although a detailed measurement of NH3 is still lacking over the IGP, application of fertilizers and animal and human excretions in the immediate vicinity are the potential reasons for high ammonium concentration. The presence of high concentrations of acid neutralizing species, such as Ca2+ and NH4+, are the likely reason for the absence of acid rain, despite high emissions of NOx and SO2 over Indian region. The Principle Component Analysis (PCA) suggests that the SO42- and NO3- are present in the Himalayan region in rainwater as a salt form and transported from continental polluted side produced from both man-made and natural sources.
Air pollution and health
Recent studies have suggested that PM2.5 Oxidative Potential are highly variable within the city 59 as well in different cities 60. The studies revealed the contributions of city-specific PM2.5 to differential in-vitro Oxidative Stress and Toxicity between Beijing and Guangzhou of China. A recent study concluded that mixture of toxic pollutants such as polyaromatic hydrocarbons (PAHs) and metals in complex environment jointly can produce cumulative effects. However, PAHs contributed approximately twice the share of the PM2.5 mixture effects as metals with Fe, Cu, and Mn as the dominant metals. Therefore, this study would be helpful in assessing the impact of pollutants on human health in the Indo-Gangetic Plain. Even the air pollution exposure between racial and ethnic communities is found to be very different 61. The induced toxicity may vary with aerosol size, mixing state and attachment of toxic metals on the surfaces of the particles. Because the toxic metal is externally attached to the surface, it may be readily available and can thus easily move into the body. A recent study also concluded that air pollution was responsible for more deaths than smoking in Europe 62. Toxicity depends on the aerosol composition and exposure to different species (e.g., BC vs metals in atmospheric dust) can have health effect. PM2.5 aerosols were found to be enriched in Fe and K along with natural crustal elements (e.g., Al, Si, O, Mg, and S) at an urban site in Varanasi 63. Moreover, the presence of heavy metals (e.g., Pb, Zn) and As in PM2.5, along with other toxic pollutants such as black carbon and PAHs, were noted in PM2.5 over the region. Therefore, the mixture of toxic pollutants, such as PAHs and heavy metals in complex environments, is likely to increase the toxic potential of PM2.5, thus imposing serious effects on human health.
Air pollution mitigation and the way forward
The COVID-19 pandemic and the resultant lockdowns imposed during 2020 and 2021 have provided an opportunity to assess the impact of anthropogenic aerosols on the air quality over India. In our recent research, we have found a significant decrease in concentrations of all pollutants, except O3, during lockdown periods (LP)s compared to those in pre-LPs in 2020. The Air Quality Index (AQI) values reduced by ~48 per cent, 42 per cent, 43 per cent, 32 per cent, 24 per cent and 21 per cent over Delhi, Kolkata, Bengaluru, Hyderabad, Mumbai and Chennai, respectively. Notably, the reduction in air pollution is more pronounced over land-locked cities (Delhi, Kolkata, Bengaluru and Hyderabad) compared to the coastal mega cities (Mumbai and Chennai). However, the benefits are temporary as reviving the economy after lockdown may again result in a deteriorated air quality. Although, a significant reduction in concentrations of CO, NO2, NOx and SO2 is observed, an increase in O3 concentration is observed over most of the study sites during the lockdown 2020. This increase may be attributed to a decrease in the NOx level along with PM concentrations during the lockdown. This suggests that attention should be extended towards adopting efficient mitigation measures to control the emissions of precursors to reactive secondary pollutants while controlling emissions of PM in India. We suggest that a significant improvement in air quality could be expected if strict execution of air quality control measures is implemented in India. A substantial reduction of aerosol concentration during lockdown might paradoxically reduce the percentage of mortalities due to PM exposure in India.
Acknowledgements
My sincere thanks to Department of Science and Technology, Govt. of India for providing financial support under SPLICE- Climate Change Programme (DST/CCP/Aerosol/87/2017) and ECRA (#ECR000490) programme.
Kirpa Ram
Institute of Environment and Sustainable Development
Banaras Hindu University, Varanasi
Email: kirpa81@gmail.com, ram.iesd@bhu.ac.in