The aerosols emitted during the fire event is majorly composed of BC and OC. Emission of BC and OC from different vegetation types is studied from the year 2013-2019, respectively.
BC and OC emission of different vegetative types
The emission scenario of different plant cover types is shown in Supplementary Fig. (1). From this Fig., we observe that the major components burned in fires are croplands (agricultural and natural crops), which share 80% of the total emission. Forest was next to it with a share of 3-14% of the total burned area, contributing to aerosols emission. While shrubland involves emissions from burning, which is not significant and shows a shrinking trend from 2017 to 2019. The major contribution of cropland to the overall burned area is that India is one of the largest producing countries globally and also evident with its agricultural fire accounting for 43– 57% of all cropland fires50. A study on Greenhouse gas inventory in Thailand also indicated similar results of more than 80% of emissions by the agricultural sector51. The decrease in forest fires contribution maybe because of land clearing for collecting Mahua (Madhuca Indica) flowers and the shifting cultivation by the tribal community, which accounts for 23% of annual deforestation in India52.
Cropland ecosystem
Cropland emission of BC ranges from 5.16×103 kg/year to 15.26×1015 kg/year. The mean BC emission in cropland ecosystem is found to be 2.19×106 kg (2013), 1.31×1014 kg (2014) 1.01×106 kg (2015), 1.7×106 kg (2016), 9.66×1012kg (2017), 1.7×106 kg (2018) and 1.5×106 kg (2019) respectively. While OC emission varies as 2.56 ×107 kg (2013), 1.53×1015 kg (2014) 1.02×107 kg (2015), 1.98×107 kg (2016), 1.03×1013 kg (2017), 1.13×107 kg (2018) and 1.75×107 kg (2019) respectively. The observed variations show that a peak BC concentration is found during 2014 (5.09 ×1010 Kg), and the lowest BC concentration is observed in the year 2018 with an emission of 3.52 Kg. Similarly, mean OC ranges from 9.03×107 Kg to a maximum of 1.3×1015 Kg (2014). The reason for the highest recorded BC and OC emission is due to the dry fires53 and the meteorological condition with an annually-averaged temperature hike (0.69◦C), making 2014 the warmest year since 188854. Land under cultivation must be considered for reducing emission during 2015 and 2018 as 1.5%, and 2% of the land areas under cultivation were reduced in 2015 and 2018, respectively. It may be caused due to a reduction in rainfall, leading to crop loss during agricultural seasons55. A similar satellite study conducted between 1950-2015 across Asia has estimated mean BC (3.89Tg) and OC (6.92Tg), which vary by a factor of ten to the power of five compared to our results56. The relationship between BC and OC emission is studied using multiple regression analysis. The regression coefficient (R2) returns a value of fifteen percent showing a lesser significant contribution by OC towards BC emission, which is a characteristic of residential biofuels and agricultural is burning57. ANOVA test was carried out to verify the significance of the difference between groups. In the cropland ecosystem, there was a statistically significant difference between the groups of BC and OC groups during the study period as determined by one-way ANOVA (F (6, 14615) =79.702, p=0.001. The emission trend of mean BC and OC in the cropland ecosystem is shown in Fig 1.
Forest ecosystem
BC emission ranges from 2.16 ×107 Kg to 2.9×109 kg, and OC is from 1.86×108 kg to 2.5×1010 kg during the emission events in the study period. The mean BC emission in forests is increasing consistently with each year from 2.16×107 kg (2013) to 2.9×109 kg (2019), respectively. While mean OC emission is found to be 1.31×1010 kg with a high concentration of 2.5×1010 kg (2019) and minimum concentration in 1.86 ×108 kg (2013), respectively. Forest cover is observed to decline from 2013 (1.83%) - 2015(1.2%), which serves as a major reason for the decline in fire-related emissions58. Fire incidents in the forests tended to increase during 2015-2019 due to low priority in managing anthropogenic fire as funds to be spent on fires are reduced59. As per the Indian State of Forest Report, the major contributors of a forest during our study period were people who indulge in clearing activities for purposes like cultivation, non-timber forest produces collection, and hunting/poaching purposes. The annual average aerosol emission from forest fires in China was 2.7 Gg, and 27.4 Gg for BC and OC is significantly less than our dry seasonal average emissions60.In a continental context, a satellite study put forward the emission of BC (2.3×107kg/year) and OC (2.3×107kg/year) from open non-agricultural fires, which is significantly less than our obtained results of BC (4.16×108 kg/year) and OC (4.84×109 kg/year) respectively61. The emission returns a significant relationship between the organic and BC with a regression coefficient of R2=0.92, p=<0.01. It suggests that the co-emission ratio of OC and BC is high in India's forests. Further, the mean annual OC (1.31×109 kg) is found to be higher than BC (1.50×108 kg), and this ratio is a general characteristic of forest emission62. A study ANOVA test was carried out to verify the significance of the difference between group and in forest ecosystem there was a statically significant difference in the emission of BC and OC is observed during the study period as determined by one-way ANOVA (F (6, 1499) =149.026, p=0.001.The emission trend of mean BC and OC in a forest ecosystem is shown in Fig. 2.
Shrubland ecosystem
Emission of mean BC shows a high spike during the emission events in the year 2014 (3.02×109 kg) and 2017 (2.07 ×107 kg) and then attains a lower bound emission in 2013(1.2×104 kg) and 2015 (1.28×104 kg). While OC ranges from 2.8×108 kg to 8.62×104 kg with upper peak emission in 2017(1.17×108 kg) and bottom peak in 2019 (8.62×104 kg) during the dry season in 2013-2019, respectively. A similar inventory study on shrubland emission using a smaller grid satellite data observed BC (19.4×103 kg) and OC (13.76×105 kg) as their annual emissions, which are found to be ten times lesser than our emission63. The mean BC and OC emission have a healthy relationship of seventeen percent at a confidence level of 95 percent. We hypothesize that due to much lesser natural fires and a lack of human intervention in forest bush fires during the dry season in the study period. ANOVA test was carried out to verify the significance of the difference between groups and in the shrubland ecosystem. There was a statistically significant difference between the emission of BC and OC during the study period as determined by one-way ANOVA (F (6, 1499) =24.501, p=0.001. The emission trend of mean BC and OC in the shrubland ecosystem is shown in Fig. 3. A satellite-based global estimated value for Southeast Asia was 5.14×1020 kg/year (BC) and 5.3 ×1021 kg /year (OC), respectively64. While our results show 3.04×1010 kg/year (BC) and 3.53×1011 kg/year (OC), which is ten times less than the estimates as it covers the entire region of southeast Asia, but in individual terms, it offers a significant contribution.
A regional analysis of emissions
A Geospatial analysis
The spatial analysis of OC and BC emission in kilograms during 2013-2019 is done using the Geographical Information System (GIS), which is presented as Fig. 4 and 5. The emission is segregated into grids, with each grid covering 27.75 km across the country. The emission is segregated into low (1.09-2.6), medium (2.69-6.11) maximum (greater than 71.2), and no emission (0-1.9). The estimated mean BC and OC emission for India from total biomass burning is around 1.98×107Kg and 1.59×108 Kg for the base year 2013. A significant increase in species emission is observed in the advancing years with 5.73×1013 Kg of BC and then 3.06×1014 Kg of OC until 2017, which then showed a declining trend. Since 2016 stringent enforcement of a ban on stubble burning by the government made major agricultural regions reduce its stubble count from leading to a reduction in emission, which is estimated based on a satellite study by NASA (https://www.downtoearth.org.in/news/air/stubble-burning-down-in-punjab-haryana-up-since-2016-nasa-maps-68331).
Region-wise analysis of BC and OC emission by different vegetation types is presented in Supplementary Table 1. Overall state-wise analysis of BC and OC indicates that Andhra Pradesh has the maximum amount of emission, followed by Arunachal Pradesh, Assam, Bihar, Chhattisgarh, Gujarat, Jharkhand, Karnataka, and Kerala. Cropland fires are high in states like Gujarat, Chhattisgarh, Odisha, Madhya Pradesh, Bihar, Tripura, Uttar Pradesh, and Andhra Pradesh. These regions are cultivating six major crops produced in India65. The states that accounted for the high cropland emission are the states with high population density and hold the most fertile land for agriculture66. Forests share the next major emission of BC and OC in India with the increasing wildfires during the dry season every year. Manipur, Meghalaya, Uttarkhand, and Tripura shares maximum emissions during the study period. These regions come under moist deciduous forest types that are subjected to frequent fires67. Uttarkhand is the youngest mountain region of the Himalayas, mainly experiencing annual forest fires, which have worsened during our study period, especially in the year 201668. In a study, the highest concentration of emerging intensified fire hotspots is found in the northeast and central India, which is substantiated by our results69. Shrubland fires are frequently observed in the regions of Telangana, Himachal, Jammu& Kashmir, Bihar, and Tamil Nadu. Further, megacities like Mumbai, Delhi, Chennai, and Kolkata are also observed to have higher emissions of OC and BC, which is due to the industrial and high vehicular emissions70,71.
Temporal analysis of emission
BC and OC concentration during biomass burning events for, 2013-2019 are displayed in Supplementary Fig. 2 and 3. Similarly, BC's emission is studied during India’s fire season, i.e., January to June, shown in Supplementary Fig. 4 and 5. The highest BC emission is observed in March with 5.37×106 kg (2018), followed by 4.91×106 kg (2016). For April, it is pointed at 4.30×1014 kg (2014) and 4.80×106 kg (2017). While May records emissions of 1.94×106 kg (2016), 6.51×105 kg (2017), and 5.77×105 kg (2018), respectively. Similarly, OC records 2.68×107 kg to 1.93×107 kg between 2018 and 2014, respectively. For April, the mean OC emission ranges from 6.55×104 kg to 2.56×107 kg (2014 and 2017), and then it hikes to 9.64×106 kg (2016) 1.84×1015 kg (2017) in May. Overall, the carbonaceous aerosol emission is high from March to May, especially with higher fire events between 2014 and 2017. As per the India State of Forest Report, India experienced a jump in forest fires by 125% during 2014-2017 in which most of them are non-dense forest areas. This pattern can be explained by the seasons of India. For forest regions, deciduous trees shed their leaves in January, which primes to add fuel load availability to the prevailing dry weather fires72. A Subside in emission is observed in June, which may be attributed to the onset of monsoon season as soil moisture increases due to rainfall. The results in reducing wood-burning capacity by absorbing moisture in the woods52. However, for cropland fires, observed temporal variation of carbonaceous aerosol emission indicates that the emission is high during March (56%) to May (28%) for the entire study period, which is a stubble burning season in India, especially in northern parts of the country73.
Carbon sequestration
The estimated carbon sequestration in cropland was found to be highest at 8.23× 1013 tonnes of carbon per hectare in 2019 with the leakage in 2015 (-4.21 × 1011 t/ha) and 2017(-1.54 × 1012 t/ha). Intensification of the crop in agriculture may result in loss of carbon stock in agricultural lands as the land area was reduced approximately two percent from 2015 to 201755. A modelling study also estimated the loss of carbon stock due to land-use intensification or agricultural land change33. Agricultural activities such as fertilizer and manure addition contribute to the replenishment of carbon stock, favouring sequestration and long-term storage of OC in the cropland ecosystem during our study period74. While in forest carbon sink is observed to be 4.10 × 1010 t/ha (2013), 9.18 × 1010 t/ha (2014), -8.10 × 1010 t/ha (2015), 1.77 × 1011 t/ha (2016), -5.60 × 1010 t/ha (2017), and 2.88 × 1010 t/ha (2018), -5.96 × 1010 t/ha (2019). In the Indian forest ecosystem, the carbon stock is found to be reduced after 2017, which is due to drought experienced for three years consecutive years in major forest regions75,76. Besides, a reduction in dense forest cover of over 20,000 hectares, which is diverted for mining, thermal power projects, and wildfires, also serves as a major causative agent for the reduction in carbon stock after 201558. Similarly, in shrubland ecosystem carbon sink is found to increase consistently from 2013 (4.89 × 1010 t/ha), 2014 (1.06× 1010 t/ha), 2016 (2.15 × 1011 t/ha) and 2018 (2.76 ×1011 t/ha) with leakage in 2015(-1.47 × 1011 t/ha), 2017(-2.78 × 1011 t/ha) and 2019(-1.01 × 1011 t/ha) respectively. Grasslands stock lays majorly on above-ground biomass, which dips down mainly because of fire, poor management, deforestation (direct effect), and shifting agriculture (indirect effect). For instance, 31% of shrublands are lost in a decade by fire, deforestation, and fodder production as demand for feeding livestock has grown by 65%77. Besides these, more than eighty percent of India's shrubland is in the poor range class that it is easily affected by weather and soil erosion78. The carbon sink of different biomes is represented in supplementary Fig. 6. Carbon sequestration of the burned area accounts for enhancing or degrading the carbon deposits of a region. The estimated current annual carbon stock (CACS) varied between different ecosystems, plotted in Supplementary Table 2. Cropland deposits of carbon decrease consistently from the base year 2013 until 2015 and then the stock seems to improve gradually until 2019 from 1.25×1011t of C per hectare to 1.13×1013 t of C per hectare. Similarly, in the shrubland ecosystem, carbon stock increased from 6.70×109 t C/ha to 3.78×1011t of C /ha until 2018 and then reduced to 137% in 2019. A higher variability is observed in forest ecosystem carbon stock reduced from 5.29×109 t of C per hectare to -8.16×1010t of C per hectare during 2013-2019. The total mean carbon stock of the vegetative ecosystem increased from 48% to 234% during 2013-2019, with dips in 2014-2015 (236%) and 2016-2017 (244%). Overall, the average carbon sink variability differs from yearly in our study, which agrees with a global study conducted in the terrestrial ecosystem using models79.
Deposition flux of BC and OC
The size and properties of the BC particles determine the residence time and deposition flux. The deposition flux of BC particles is estimated for different land cover emissions and is calculated with a range of upper and lower limits. The estimated lower limit means deposition flux was observed to be highest in the cropland ecosystem with 0.47 kg of BC deposited per kilometre and 11.9 kg/km (OC) in a day for an atmospheric mean concentration 5.85 ×1010 kg. While upper limit extents to 0.55 kg/km/day (BC) and 11.9 kg/km/day (OC). Interestingly, minimum means deposition flux is observed in shrublands for an average BC concentration (1.11×105 kg/km/day) and OC concentration (6.28×106kg/km/day), which has a mean flux of 1.04×10-6 kg/km/day for BC, OC has 5.86×10-6 kg/km/day as the lower limit and 1.93×10-5 (BC) and 1.09 ×10-4 kg/km/day (OC) as the upper limit. Dry deposition flux is most suitable for particles having a higher aerodynamic size (<2.5µm), and the washout mechanism majorly removes sub-micron particles. The dry deposition value of small micron particles in the range of 0-2.5 and 1-2.5µm was 35±3% and 21%80. The BC and OC coarse particles are not considered.
Mean fluxes of BC deposition for seven years from 2013-2019 were 2.77×101Tg/year (BC) and 1.57×102 Tg/year (OC) for shrubland, 9.1×101 Tg/year (BC) and 1.41×103Tg/year (OC) for the forest, and 1.41×106 Tg/year (BC) and 3.1×107 Tg/year (OC) for cropland ecosystem respectively. Overall, the mean deposition flux of BC and OC decreases with years with an exception in 2014. As our study is carried out during the summer season, where the low frequency of rainfall is observed leading to lesser deposition of carbonaceous aerosols and such a declining trend in dry deposition rate is observed across the globe. Global annually averaged dry deposited BC-based on model experiments vary from 0.66 to 1.66 Tg/year, which significantly differs with a ratio of 17.3:1 for shrubland, 56.8:1 for the forest more than a percent difference in cropland. This large variation may be attributed because discrepancies exist in modelling the submicron particles47.
Aerosol particles may have individual varying residence time, and it fluctuates with space and time. BC's mean residence time constantly stood for the study period with 0.7 days for upper estimates in the cropland ecosystem while lower estimates fluctuated between 0.1day (4.4×103 kg/km/day) to 23 days (6.4×103 kg/km/day). However, shrubland and forest ecosystem emitted aerosols had a similar mean atmospheric residence time of 8.6 days. Deposition flux of BC over grassland and shrubland is estimated to be between 7-11 days, which best fits our results81. However, the global BC residence time to be 7.85 days, which is very much less than our prediction82,83. The deposition flux of BC and OC is presented in Tables 1 and 2.