Global_Environmental_Research_Vol.27No.1
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Generally, burning biomass like rice stubble releases incomplete various atmospheric pollutants due combustion of organic matter. Secondary particulate matter known as “PM2.5” is produced from precursors such as volatile organic carbon (VOC), nitrogen oxides (NOx), and sulfur oxides (SOx). As clarified by a well-known study of six cities in the U.S., PM2.5 poses a prominent risk to human health (Dockery et al., 1993). Therefore, PM2.5 is the most widely used indicator of the health hazards posed by air pollution. According to the WHO guideline updated in 2021, annual average concentrations of PM2.5 should not exceed 5 µg/m3, while 24-hour average exposures should not exceed 15 µg/m3 for more than three to four days per year. The daily observed values in Delhi from late October to early November, however, have often indicated levels in excess of 1,000 μg/m3. 2.2 Causes of Rice-Stubble Burning 3. Linkage between Stubble Burning in the Punjab and Air Pollution in Delhi reluctance to try new cropping systems. Despite the success of the Green Revolution in the Punjab in terms of improved productivity, it has been the subject of severe criticism for ecological reasons since the 1970s. The most prominent criticisms are the excessive use of groundwater, the soil degradation, the creation of a monoculture with a bias toward rice and wheat, and the excessive use of chemical pesticides; but in the 2000s, air pollution from the increasingly common practice of mass burning of rice stubble rapidly gained attention. A schematic diagram of background factors in rice-stubble burning is given in Fig. 2. Prior to the introduction of agricultural machinery in the 1980s, rice and wheat straw were manually cut close to the ground during harvest. With combine harvesters, conversely, the blades must be set high enough to prevent them from hitting the ground, leaving long stubble with scattered straw in the field. To get rid of this stubble, burning was reported to have begun in the late 1980s. Based on interviews with farmers in Punjab state, Gupta (2011; 2012) revealed that the most important factor determining whether rice-stubble burning was practiced was the use of combine harvesters. She noted that coarse rice producers were considerably more likely than basmati-rice producers to use combine harvesters. Basmati rice has a higher commercial value, and farmers tend to avoid using combine harvesters for fear of damaging the grain. Today, harvesting, including of basmati rice, appears to be done almost exclusively with combine harvesters, yet our research indicates that stubble burning is more common in coarse rice-growing areas than in basmati rice-growing areas. (Asada and Vatta, 2021). Meanwhile, groundwater levels have continued to fall unabated. In 2009, the Punjab state government, together with the Haryana state government, implemented the Preservation of Subsoil Water Act, which prohibited the transplanting of rice seedlings before the start of the monsoon season to conserve groundwater, thus delaying the start of rice planting. Promulgation of the Water Act induced important changes in the timing and manner of rice-stubble burning. In the wheat-rice double-cropping system, the wheat harvest must take place before the May heat wave arrives, so farmers must finish sowing wheat by late November. After the Water Act in 2009, rice transplanting was delayed, and, thus, so was harvesting. As a result, the period for dealing with rice stubble was shortened, and rice stubble has since been burned more intensively. Satellite-borne Moderate Resolution Imaging Spectroradiometer (MODIS) sensor observations of fires have significantly contributed to understanding the delay in the burning season (Sawlani et al., 2018; Jethva et al., 2018; Sembhi et al., 2020; Kant et al., 2022). Now, it is well known that large quantities of rice stubble are Tackling Air Pollution from Agricultural Residue Burning intensively burned during the short period in late October and early November. This period corresponds to a time of meteorological change; the winds become northwesterlies, which carry the pollutants directly toward the Delhi-NCT under low-temperature, stagnant conditions. It is assumed that this is closely related to the increasing severity of air pollution in the Delhi-NCT since the 2010s, especially in early November. To clarify the linkage between stubble burning and severe air pollution in Delhi, the Aakash Project aimed to conduct local air pollutant measurements during this period, taking into account meteorological conditions and the movement of air masses. several-hundreds µg/m3 The cause-and-effect relationship between stubble burning in the Punjab region and worsening air pollution in Delhi has not yet been established quantitatively. Though some previous studies (Cusworth et al., 2018; Beig et al., 2020) have conducted simulation-based studies, it is difficult to quantitatively assess the extent to which rice-stubble burning has impacted air pollution in Delhi, because of the lack of air pollution monitoring networks in rural areas. Most monitoring stations for air pollutants are located in urban areas, with no data available for rural areas. In 2022, the Aakash Project conducted intensive campaign-based air pollutant measurements during the stubble burning season to establish the link between stubble burning in the Punjab and air pollution in Delhi. The target area was divided into 30 grids (~60 × 40 km). A network of 32 compact PM2.5 and gas sensors (CUPI-Gs) and to PM2.5 (P-sensors) were established in rural and semi-rural areas of Punjab, Haryana and western Uttar Pradesh. Some sensors were also installed in the cities of Gurgaon and inside Delhi to obtain data to compare with the data from sensors seven to and sometimes even specific 7

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