Role of meteorological regime in mitigating biomass induced extreme air pollution events
Graphical abstract
Introduction
Biomass burning is a significant source of air pollutants leading to poor air quality worldwide (Chen et al., 2017). India is an agrarian country in South Asia. Biomass in the form of crop residues is often burned in field before the next crop, especially in the Northern part of India. As a result, crop residue combustion plays a significant role in haze pollution in nearby cities and surrounding regions (Zhang et al., 2017). Rice is an important food crop in India and plays a critical role in food security in South Asia leading to large quantity of residue generation (Li et al., 2020). The capital of India, Delhi, is one of the largest megacities and also one of the worst affected capitals in the world for asthma (Dandona et al., 2017). The geographical and meteorological misfortune of Delhi mixed with high levels of anthropogenic emissions (Beig et al., 2019) keeps air pollution at critical level to a considerable extent, especially during wintertime. The extreme air pollution events resulting from fire smoke are highly related to morbidity and mortality (Horton et al., 2014; Lepeule et al., 2012). A significant population in Northern part of India take rice as their preferred food which is largely grown around Delhi's neighboring states of Punjab and Haryana, during monsoon (known as Kharif crop) and harvested in post monsoon season (Vadrevu et al., 2011).
The residue generated after the harvesting are required to be disposed of in order to empty the field for sowing the next crop. Average available time between the rice harvesting and wheat sowing is in the range of 20–30 days (Reddy et al., 2019) depending on the varieties of rice crop. In the limited time available, open stubble burning is considered to be the fastest method by farmers to ready fields as it also improve their efficiency (Kaskaoutis et al., 2014). However, the harvesting season coincides with weather conditions that favour stagnation and weak surface north-westerly winds in Delhi (Kaskaoutis et al., 2014; Singh and Naseema Beegum, 2013) which leads to frequent episodes of extreme pollution events based on the weather conditions during which concentration of PM2.5 touches hazardous level posing a serious health risk. As a rapidly developing capital of India, Delhi featured among the top 10 most polluted cities in the world as reported by the World Health Organization (WHO, 2016). The cumulative exposure of urban residents to PM2.5 and the corresponding health burdens are estimated by Chen et al. (2020a) for four Indian cities including Delhi based on in-situ observations during 2015–2018. Chen et al. (2020a) also quantified the health risks of short-term exposure during Diwali firework festival in Delhi for the first time. The results of this study are valuable for modeling studies and helpful in implementation of mitigation policies on a city level. Balakrishnan et al. (2019) reported that Delhi had the highest annual population-weighted mean exposure to ambient particulate matter PM2.5 in the country. They estimated that if the air pollution level in India were less than the minimum causing health loss, the average life expectancy in 2017 would have been higher by ~1.7 years. A comprehensive analysis of epidemiological transition, disease burden, and risk factors across the states of India from 1990 to 2016 has also been provided by Dandona et al. (2017).
It is hard to avoid burning of residue as farmers find it most economical and easy way to empty fields. Authorities are in search of possible alternative solution so that extreme events can be avoided. Incentives to farmers by means of several methods are going on for years but with huge economic cost. To protect the environment from the air pollution, Government rolled out a new scheme on Promotion of agricultural mechanization for in-situ management of crop residue in the states of Punjab, Haryana, Uttar Pradesh and NCT of Delhi in 2018 (Reddy et al., 2019). Crop residue management equipment's such as Rotavators, Super straw Management System, Happy Seeder, Rotary Mulcher, Shrub Master, Paddy Straw Chopper/Shredder have been developed and successfully demonstrated to the farmers, fields (GOI, 2018). It provided for higher level of subsidy to them for purchasing a bouquet of machineries for in-situ crop residue management. As a result, about 11.01% and 41.69% reduction in number of burning events were observed in 2018 as compared to that in 2017 and 2016 detected by thermal remote sensing and burnt area estimated by high resolution remote sensing data (GOI, 2019).
Rice and wheat usually produce majority of crop residue in Northern India where this study is focused. The large scale cultivation of these crops is likely to increase significantly in future due to boost by government policy on productivity. This is likely to generate more and more crop residue burning activities in coming year. In the present work, a modeling cum satellite /ground measurement based attempt has been made to assess the impact of stubble related PM2.5 emission in two different scenarios and its role in modulating the extreme air pollution events over megacity Delhi's air quality. Based on the pilot study of 2018, we have proposed an alternative mitigation strategy to minimize the impact of stubble burning induced intensity of extreme events, thereby improving the air quality over megacity Delhi as one of the policy related solutions.
Section snippets
Observations
The work has been carried out under the project System of Air quality and Weather Forecasting and Research (SAFAR) of Indian Government and a pilot project of World Meteorological Organization (Beig et al., 2015). The SAFAR consists of dense network of 10 monitoring stations spread in and around Delhi. The mass concentration of PM2.5 was continuously monitored by online analyzer based on the industry-proven principle of beta-ray attenuation method using suspended particulate beta gauge monitor
Result and discussion
The SAFAR model results are routinely validated for Delhi region for normal case as well as for extreme events being an operational service (Beig et al., 2013, Beig et al., 2019; Srinivas et al., 2016a, Srinivas et al., 2016b). The validation of the model results with that of observational network is shown in Fig. 2 for the study period 10th September to 30th November' 2018 along with standard deviation in the observed data. The verification on a day to day basis is found to be reasonable and
Conclusion
Present finding assent to 2018 demonstrate that if the Kharif crop is sowed just about a month earlier resulting in advancing stubble burning by the same period then national capital megacity Delhi can minimize the emergency episodes created by residue burning under the proposed scenario. The finding although requires feasibility study along with policy level intervention and limited to 2018 but if realized and implemented then it may lead to significant improvement in winter time air quality
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge with thanks the Director, IITM and Secretary, Ministry of Earth Sciences for their support. The funding for research is from the mainstream project SAFAR of IITM under the Ministry of Earth Sciences, Govt of India which also includes PROMOTE (Process analysis, observations and modelling) project of APHH (Air Pollution and Human Health).
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