Pollution poses a profound challenge to sustainable human development by degrading natural resources (Nazeer et al., 2016; Shikwambana et al., 2020). Its detrimental impacts extend across various domains, manifesting in reduced labor productivity, disease proliferation, compromised agricultural yields, psychological distress, and escalating costs for socio-economic interventions (Rashid, 2017). Air pollution, in particular, is notably damaging as it has direct effect on life of every living thing. Emanating from both natural phenomena such as volcanic eruptions, forest fires, and dust storms, and anthropogenic activities including vehicular emissions, landfills, biomass burning, and industrial operations (Ialongo et al., 2016), air pollution warrants quantitative and qualitative assessment for effective air quality management. Addressing air pollution challenges necessitates collaborative efforts across climate, transport, trade, energy, and other relevant sectors, balancing environmental and economic growth objectives (Ali & de Oliveira, 2018).
World Health Organization (WHO) defined air quality based on six criteria pollutants. These are oxides of Nitrogen (NO, NO2 = NOx), Carbon Monoxide (CO), oxides of Sulphur (SO2, SO3 = SOx), Particulate Matter (PM2.5, PM10, PMtotal), Ozone (O3), Volatile Organic Compounds (VOCs) and heavy metals (Lead, Mercury etc.) (WHO, 2016). NOx is known to cause respiratory issues and inhibit plant growth, affecting crop productivity (Munawer, 2018). The prevalence of NOx contributes to respiratory issues in humans and can inhibit plant growth, thereby affecting crop productivity (Khan et al., 2022). CO exposure leads to severe health issues, including neurological and cardiovascular damage, but is less harmful to plant life due to its rapid oxidation to Carbon Dioxide (CO2), a key photosynthetic reactant (Munawer, 2018). The two trace gases also act as a precursor for photochemical smog resulting from complex atmospheric photochemical reactions (Kaplan et al., 2019; Qiao et al., 2019; Tanimoto et al., 2015). NO2 can additionally contribute to acid rain (Ul–Haq. et al., 2014). Anthropogenic sources, predominantly energy use and biomass burning, contribute significantly to global emissions of these gases. Natural sources including soil, lightning etc have global emissions of 13.5 Tg(N) per year. Whereas, anthropogenic sources account for 30.5 Tg(N) per year. Energy consumption & biomass burning each account for 500 Tg(CO) per year (Wallace & Hobbs, 2006).
The major sink for both NO2 and CO is OH radical, also known as the "cleansing agent" of the atmosphere (Lamsal et al., 2015; van der A. et al., 2008). The higher concentration of CO in the atmosphere depletes OH radical cleansing capacity. OH radical also converts atmospheric methane (CH4) to CO2, which reduces its global warming potential because methane is a stronger greenhouse gas (Sukitpaneenit & Kim Oanh, 2014).
The emergence of COVID-19 and the ensuing global response, including widespread lockdowns, presented an unprecedented opportunity to observe the impacts of reduced anthropogenic activity on air quality (Saadat et al., 2020). In South Asia, especially within the provinces of Punjab and Haryana across India and Pakistan, the pandemic response resulted in a notable albeit temporary improvement of air pollution levels. This has provided a unique backdrop for the spatio-temporal analysis of air pollutants, facilitating insights into the ramifications of human activities on atmospheric composition (Levelt et al., 2022).
Khokhar et al. (Khokhar et al., 2015) observed enhanced NOx levels using Ozone Monitoring Instrument (OMI) datasets from 2002-12 over urban areas in Pakistan due to multiple anthropogenic pollution sources. Using OMI dataset, (Ul–Haq. et al., 2014) identified an increasing trend of 3.29% per annum from December 2004 to November 2008 across Pakistan. Ul Haq also discussed CO variance from 2003 to 2012 using satellite-sensed (AIRS/AMSU) data along with their impacts over neighboring regions of Afghanistan, India, and Iran. (Ul-Haq et al., 2015). Various other studies by Ghude et al., 2011; Girach & Nair, 2014; Kumar et al., 2013; Worden et al., 2013 over South Asian region by Measurements Of Pollution In The Troposphere (MOPITT) infer that the anthropogenic activities are responsible for the CO concentrations at the planetary boundary during winters. The transportation of CO air masses regionally over the South Asian region plays a significant part in total anthropogenic CO over the whole region and adjoining regions than the local sources.
Advancements in satellite technology have opened new avenues for monitoring air pollution with greater efficiency (Rohn et al., 2014). Instruments such as the TROPOspheric Monitoring Instrument (TROPOMI) have become vital tools for tracking atmospheric pollutants, including Tropospheric NO2 and Total CO columns. This study uses TROPOMI data for a comprehensive spatio-temporal analysis over the Punjab and Haryana regions, covering the period from July 2018 to June 2023. Additionally, this study includes the COVID-19 Lockdown impact on air quality dynamics during strict lockdown measures from the two governments.
1.1 Study Area
This study conducts a detailed spatio-temporal examination of the tropospheric NO2 column and the total CO column density from July 2018 to June 2023. The focus is on the neighboring regions of Haryana in India and the Punjab region, spanning both India and Pakistan, as depicted in Fig. 1. The elevation of the region from sea level varies approximately from 80 meters to 450 meters. The topography also varies from plains in Southern Punjab to mountainous regions towards north. Whereas the Indian Punjab is also a planar area with mountains of Kashmir region towards the north. The climate in the study area is harsh. The summer temperatures can soar up to of 50ºC, while the winters are mostly dry with a minimum temperature dipping to -2 ºC (Abbas, 2013).
The etymology of Punjab—'the land of five rivers'—reflects its rich agricultural legacy, nurtured by its fertile soils. The region observes two principal farming seasons: the Rabi, or winter crop season, and the Kharif, or summer crop season. Wheat dominates the Rabi cropping cycle, with harvesting spanning late April to mid-May. Conversely, rice cultivation is prevalent in the Indian territories of Punjab and Haryana. Lahore and Gujranwala divisions are the principal rice-producing areas in Pakistan's Punjab. Cotton and sugarcane, with their distinct agro-climatic requisites, are cultivated primarily in Southern and Central Punjab of Pakistan respectively (Azhar et al., 2019).
National Institute of Population Studies (NIPS) Pakistan projected population figure for Punjab is 117.24 million (2020), with an approximate growth rate of 2.6%. According to the Population Census of India 2011 (Statistical Year Book (2018), Ministry of Statistics and Programme Implementation, Government of India, 2018), the projected population for Punjab, Haryana, Chandigarh, and Dehli is 79.86 million for the year 2020, with a population growth rate of approximately 1.4%. The population density for our study area in India is relatively higher at 830.4 people per km2, whereas the density is less for Punjab-Pakistan at 570.96 people/km2. The reason being 51.65% of the population in the Indian part of the study area live in cities (Population Census-2011), whereas Punjab-Pakistan has 36.71% urban population (Economic Survey of Pakistan 2020, Ministry of Finance, Pakistan, 2020).
Urban and industrial centers in the Punjab and Haryana provinces are annually challenged by air pollution, especially during the post-harvest period of the rice crop in winter. Time-series data analytics targeted six critical hotspots, encompassing three major cities—Delhi, Lahore, and Islamabad/Rawalpindi—and three industrial cities—Faisalabad, Gujranwala, and Panipat. These locales are further dotted with numerous minor hotspots associated with power plants and industrial zones, particularly influencing NO2 levels.
Major air pollutions determinants in these areas are population growth, industries vehicles emission, power producing stations and burning of rice straws. Pakistan and India (Punjab and Haryana Provinces) have registered vehicles of 30.75 million (Economic Survey of Pakistan 2020, Ministry of Finance, Pakistan, 2020) and 28.24 million (Statistical Year Book (2018), Ministry of Statistics and Programme Implementation, Government of India, 2018) with annual growth rate of 5.26 and 16.01 percent respectively. Thus, a big source of air pollution. Similarly study area of Pakistan and India have 23 and 36 power producing stations causing addition to air pollutants (Byers et al., 2018) Moreover, open rice straw burning is another reason for air pollutions in the neighboring countries due to cross border dispersal of air pollutants. It is reported that in India 0.0092 Gg of crop residues is burnt every year. Among it, 40% is made up of rice straw where crop resides share of wheat straw and sugarcane was 22% and 20% respectively (Bhuvaneshwari et al., 2019). In case of Pakistan wheat straw share is 48% followed by 23% share of rice and sugarcane each. A contribution of 6% from maize straw for total crops residues burnt is also considered (Azhar et al., 2019).