Crop residue burning in South Asia: A review of the scale, effect, and solutions with a focus on reducing reactive nitrogen losses

This paper reviews the literature on crop residue burning - a widespread practice in many regions in South Asia. Specifically, we examine evidence from studies highlighting the scale of the practice in South Asia, the environmental implications, the drivers of the practice and the remedies to the problem. The studies provide evidence that the Indo-Gangetic Plain (IGP) is a hot-spot for atmospheric pollutants, with seasonal crop residue burning being a major contributor. The burning of crop residue is reported to degrade the soil, increase the risk of erosion, and increase the soil temperature, consequently decimating soil microorganisms. This subsequently impacts the monetary cost involved in recovering the soil fertility and the potential for further pollution through the increased use of fertilizer. The review shows that farmers' reasons for burning crop residues are mainly the high cost of incorporating, collecting, transporting, and processing crop residues in South Asia. Labour shortages, the marketability of the crop residue and the short time interval between harvest and next cropping seasons also influence farmers decision to burn crop residue. To address this problem, there is the need to encourage the use of agricultural machines capable of sowing crops in standing stubble, adopting in-situ practices and changing crop varieties to those with short duration. In addition, education and awareness are needed to change beliefs and perceptions on crop residue burning. Crucially, when promoting alternative sustainable uses of crop residue, the economic benefits should be prioritized, and support towards initial investments that accompany the adoption of alternative practices should be provided.

The Impact of Long-Range Transport of Biomass Burning Emissions in Southeast Asia on Southern China

The long-range transport of biomass burning pollutants from Southeast Asia has a significant impact on air quality in China. In this study, the Moderate Resolution Imaging Spectroradiometer (MODIS) fire data and aerosol optical depth (AOD) products and the Tropospheric Monitoring Instrument (TROPOMI) carbon monoxide (CO) data were used to analyze the impact of air pollution caused by biomass burning in Southeast Asia on southern China. Results showed that Yunnan, Guangdong and Guangxi were deeply affected by biomass burning emissions from March to April during 2016–2020. Comparing the data for fires on the Indochinese Peninsula and southern provinces of China, it is obvious that the contribution of pollutants emitted by local biomass burning in China to air pollution is only a small possibility. The distribution of CO showed that the overall emissions increased greatly from March to April, and there was an obvious transmission process. In addition, the MODIS AOD in areas close to the national boundary of China is at a high level (>0.6), and the AOD in the southwest of Guangxi province and the southeast of Yunnan Province is above 0.8. Combined with a typical air pollution event in southern China, the UVAI combined with wind direction and other meteorological data showed that the pollutants were transferred from the Indochinese Peninsula to southern China under the southwest monsoon. The PM2.5 data from ground-based measurements and backward tracking were used to verify the pollutant source of the pollution event, and it was concluded that the degree of pollution in Yunnan, Guangxi and Guangdong provinces was related to the distance from the Indochinese Peninsula. Results indicate that it is necessary to carry out in-depth research on the impact of cross-border air pollution transport on domestic air quality as soon as possible and to actively cooperate with foreign countries to carry out pollution source research and control.

Large contribution of biomass burning emissions to ozone throughout the global remote troposphere

Ozone is the third most important anthropogenic greenhouse gas after carbon dioxide and methane but has a larger uncertainty in its radiative forcing, in part because of uncertainty in the source characteristics of ozone precursors, nitrogen oxides, and volatile organic carbon that directly affect ozone formation chemistry. Tropospheric ozone also negatively affects human and ecosystem health. Biomass burning (BB) and urban emissions are significant but uncertain sources of ozone precursors. Here, we report global-scale, in situ airborne measurements of ozone and precursor source tracers from the NASA Atmospheric Tomography mission. Measurements from the remote troposphere showed that tropospheric ozone is regularly enhanced above background in polluted air masses in all regions of the globe. Ozone enhancements in air with high BB and urban emission tracers (2.1 to 23.8 ppbv [parts per billion by volume]) were generally similar to those in BB-influenced air (2.2 to 21.0 ppbv) but larger than those in urban-influenced air (-7.7 to 6.9 ppbv). Ozone attributed to BB was 2 to 10 times higher than that from urban sources in the Southern Hemisphere and the tropical Atlantic and roughly equal to that from urban sources in the Northern Hemisphere and the tropical Pacific. Three independent global chemical transport models systematically underpredict the observed influence of BB on tropospheric ozone. Potential reasons include uncertainties in modeled BB injection heights and emission inventories, export efficiency of BB emissions to the free troposphere, and chemical mechanisms of ozone production in smoke. Accurately accounting for intermittent but large and widespread BB emissions is required to understand the global tropospheric ozone burden.

Air Pollution From Forest and Vegetation Fires in Southeast Asia Disproportionately Impacts the Poor

Forest and vegetation fires, used as tools for agriculture and deforestation, are a major source of air pollutants and can cause serious air quality issues in many parts of Asia. Actions to reduce fire may offer considerable, yet largely unrecognized, options for rapid improvements in air quality. In this study, we used a combination of regional and global air quality models and observations to examine the impact of forest and vegetation fires on air quality degradation and public health in Southeast Asia (including Mainland Southeast Asia and south-eastern China). We found that eliminating fire could substantially improve regional air quality across Southeast Asia by reducing the population exposure to fine particulate matter (PM2.5) concentrations by 7% and surface ozone concentrations by 5%. These reductions in PM2.5 exposures would yield a considerable public health benefit across the region; averting 59,000 (95% uncertainty interval (95UI): 55,200-62,900) premature deaths annually. Analysis of subnational infant mortality rate data and PM2.5 exposure suggested that PM2.5 from fires disproportionately impacts poorer populations across Southeast Asia. We identified two key regions in northern Laos and western Myanmar where particularly high levels of poverty coincide with exposure to relatively high levels of PM2.5 from fires. Our results show that reducing forest and vegetation fires should be a public health priority for the Southeast Asia region.

Spectral Derivatives of Optical Depth for Partitioning Aerosol Type and Loading

Quantifying aerosol compositions (e.g., type, loading) from remotely sensed measurements by spaceborne, suborbital and ground-based platforms is a challenging task. In this study, the first and second-order spectral derivatives of aerosol optical depth (AOD) with respect to wavelength are explored to determine the partitions of the major components of aerosols based on the spectral dependence of their particle optical size and complex refractive index. With theoretical simulations from the Second Simulation of a Satellite Signal in the Solar Spectrum (6S) model, AOD spectral derivatives are characterized for collective models of aerosol types, such as mineral dust (DS) particles, biomass-burning (BB) aerosols and anthropogenic pollutants (AP), as well as stretching out to the mixtures among them. Based on the intrinsic values from normalized spectral derivatives, referenced as the Normalized Derivative Aerosol Index (NDAI), a unique pattern is clearly exhibited for bounding the major aerosol components; in turn, fractions of the total AOD (fAOD) for major aerosol components can be extracted. The subtlety of this NDAI method is examined by using measurements of typical aerosol cases identified carefully by the ground-based Aerosol Robotic Network (AERONET) sun–sky spectroradiometer. The results may be highly practicable for quantifying fAOD among mixed-type aerosols by means of the normalized AOD spectral derivatives.

Nitrate sources and formation of rainwater constrained by dual isotopes in Southeast Asia: Example from Singapore

Emission of reactive nitrogen species has a major impact on atmospheric chemistry, ecosystem and human health. The origin and formation mechanisms of wet-deposited nitrate are not well understood in Southeast Asia (SEA). In this study, we measured stable isotopes of nitrate (δ¹⁵N and δ¹⁸O) and chemical compositions of daily rainwater from May 2015 to July 2017 in Singapore. Our results showed that δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- varied seasonally with higher values during the Inter-monsoon period (April-May and October-November) than during Northeast (December-March) and Southwest monsoon (June-September). Bayesian mixing modeling, which took account of the isotope fractionation, indicated that traffic emission (47 ± 32%) and lightning (19 ± 20%) contributed the most to NO₃^- with increased traffic contribution (55 ± 37%) in the Northeast monsoon and lightning (24 ± 23%) during the Inter-monsoon period. Biomass burning and coal combustion, likely from transboundary transport, contributed ∼25% of nitrate in the rainwater. Monte Carlo simulation of δ¹⁸O-NO₃^- indicated that oxidation process by hydroxyl radical contributed 65 ± 14% of NO₃^-, with the rest from hydrolysis of N₂O₅. Wind speed had large effect on δ¹⁸O-NO₃^- variations in the atmosphere with more involvement of hydroxyl radical reactions when wind speed increased. Our study highlights the key role of isotopic fractionation in nitrate source apportionment, and the influence of meteorological conditions on nitrate formation processes in SEA.

Ambient Particulate Matter Exposure and Under-Five and Maternal Deaths in Asia

The Asian region is one of the major emission sources of air pollution. Although ambient PM_2.5 has been linked to several health risks in high-, low-, and middle-income countries, the further analysis of type impact is still rare but significant. The PM_2.5 distribution retrieved from MODIS (Moderate Resolution Imaging Spectroradiometer) aerosol optical depth products within 16 years thus explored the associations between under-five and maternal mortality for 45 countries in Asia. Both the nonparametric (Generalized Additive Mixed-Effect) and parametric (Generalized Linear Mixed-Effect) models were employed to analyze the collected datasets. The results show that the levels of PM_2.5 in Asian sub-regions were higher than the Global Air Quality Standards. Biomass PM_2.5 concentrations was associated with increased the rate of under-five (Incidence Rate Ratio, IRR = 1.29, 95% CI, 1.13–1.47) and maternal (IRR = 1.09, 95% CI: 1.08–1.10) deaths in Asia. Anthropogenic PM_2.5 was associated with increased rate of under-five deaths in Asia by 12%. The nonparametric method revealed that dust PM_2.5 was positively associated with the under-five (β = 0.04, p < 0.001) and maternal (β = 0.07, p < 0.001) deaths in Asia. The rate of maternal deaths was increased by biomass/dust (IRR = 1.64, 95% CI: 1.63–1.65) and anthropogenic/dust (IRR = 1.22, 95% CI: 1.19–1.26) mixture types. In summary, long-term exposure to different types of ambient PM_2.5 in high concentration increased the rate of under-five and maternal deaths, suggesting that policies focusing on preventive and control measures is imperative for developing an improved maternal, newborn, and child health in Asia.

Modeling the effect of VOCs from biomass burning emissions on ozone pollution in upper Southeast Asia

We used a Weather Research and Forecasting Model with Chemistry (WRF-CHEM) model that includes anthropogenic emissions from EDGAR-HTAP, biomass burning from FINN, and biogenic emissions from MEGAN to investigate the main volatile organic compound (VOC) ozone precursors during high levels of biomass burning emissions in March 2014 over upper Southeast Asia. A comparison between the model and ground-based measurement data shows that the WRF-CHEM model simulates the precipitation and 2 m temperature reasonably well, with index of agreement (IOA) values ranging from 0.76 to 0.78. Further, the model predicts O3, NO2, and CO fairly well, with IOA values ranging from 0.50 to 0.57. However, the magnitude of the simulated NO2 concentration was generally underestimated compared to OMI satellite observations. The model result shows that CO and VOCs such as BIGENE play an important role in atmospheric oxidation to surface O3. In addition, biomass burning emissions are responsible for increasing surface O3 by ∼1 ppmv and increasing the reaction rate of CO and BIGENE by approximately 0.5 × 106 and 1 × 106 molecules/cm3/s, respectively, in upper Southeast Asia.

Size distribution of bioaerosols from biomass burning emissions: Characteristics of bacterial and fungal communities in submicron (PM1.0) and fine (PM2.5) particles

The North China Plain is the agricultural heartland in China. High PM_2.5 levels and elevated chemical pollutants have been observed during crop harvest seasons due to open biomass burning. Biomass burning in the wheat-harvest season may significantly deteriorate the regional air quality. The harmful ingredients in smoke particles also have severe implications for toxicity and health effects. Previous studies have illustrated the potential role of bioaerosols as ice-nuclei and cloud condensation nuclei and highlighted their influence on biochemical cycles and human health effects. In a monthly field observation campaign of biomass burning conducted at the summit of Mount Tai in July 2015, we reported the composition, potential role, size distribution of microorganisms in particulate matters PM_1.0, PM_2.5, and estimated their contribution to particles. The wide-range particle spectrometer suggested that the predominant particles were distributed in submicron particles (PM_1.0), which resulted in a similar community structure for bacteria and fungi in PM_1.0 and PM_2.5. Among bacteria, the predominant Pseudomonas accounted for 18.06% and 21.29% in PM_1.0 and PM_2.5, respectively. Alternaria covered up to 69.01% and 72.76% of the fungal community in PM_1.0 and PM_2.5, respectively. A disparity between bacterial communities was identified by the abundance of rare species, such as Bacilli being higher in PM_1.0 (2.4%) than in PM_2.5 (1.8%), and Defluviicoccus being higher in PM_2.5 (2.5%) than in PM_1.0 (0.5%), which may be related to cell size and cell growth patterns. Quantitative PCR revealed that microbial cell numbers in PM_2.5 were higher than in PM_1.0, and that the bacterial cell number was about an order of magnitude greater than the fungal cell number. However, the mass concentration and contribution of fungi to particulate matter was much higher than that of bacteria, suggesting the underestimated role of fungi in atmospheric aerosols. Airborne microorganisms in alpine areas remained less characterized. The findings presented here illustrated the potentially important impacts on air quality and bioaerosol pollution by biomass burning, which provides an essential reference for understanding the transmission and health effects of bioaerosols.

Effect of biomass burning on black carbon (BC) in South Asia and Tibetan Plateau: The analysis of WRF-Chem modeling

The focus of this study is to evaluate the impact of biomass burning (BB) from South Asia and Southeast Asia on the glaciers over the Tibetan Plateau. The seasonality and long-term trend of biomass fires measured by Terra and Aqua satellite data from 2010 to 2016 are used in this study. The analysis shows that the biomass burnings were widely dispersed in the continental of Indian and Southeast Asia and existed a strong seasonal variation. The biomass burnings in winter (January) were relatively weak and scattered and were significantly enhanced in spring (April). The highest biomass burnings located in two regions. One was along the foothill of Himalayas, where is a dense population area, and the second located in Southeast Asia. Because these two high biomass burning regions are close to the Tibetan Plateau, they could have important effects on the BC deposition over the glaciers of the Tibetan Plateau. In order to study the effect of BB emissions on the deposition over the glaciers in the Tibetan Plateau, a regional chemical model (WRF-Chem; Weather Research and Forecasting Chemical model) was applied to simulate the BC distributions and the transport from BB emission regions to the glaciers in Tibetan Plateau. The result shows that in winter (January), due to the relatively weak BB emissions, the effect of BB emissions on BC concentrations was not significant. The BC concentrations resulted from BB emissions ranged from 0.1 to 2.0 μg/m³, with high concentrations distributed along the foothill of Himalayas and the southeastern Asia region. Due to the relative low BC concentrations, there was insignificant effect of BB emissions on the deposition over the glaciers in the Tibetan Plateau in winter. However, the BB emissions were highest in spring (April), producing high BC concentrations. For example, along the Himalayas Mountain and in the southeastern Asia region, The BC concentrations ranged from 2.0 to 6.0 μg/m³. In addition to the high BC concentrations, there were also west and south prevailing winds in these regions. As a result, the BC particles were transported to the glaciers in the Tibetan Plateau, causing significant deposition of BC particles on the snow surface of the glaciers. This study suggests that the biomass burning emissions have important effects on the BC deposition over the glaciers in the Tibetan Plateau, and the contaminations of glaciers could have significant impact on the melting of snow in the Tibetan Plateau, causing some severe environmental problems, such as the water resources.

Spatio-Temporal Characteristics of Tropospheric Ozone and Its Precursors in Guangxi, South China

The temporal and spatial distributions of tropospheric ozone and its precursors (NO2, CO, HCHO) are analyzed over Guangxi (GX) in South China. We used tropospheric column ozone (TCO) from the Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS) onboard the Aura satellite (OMI/MLS), NO2 and HCHO from OMI and CO from the Measurements of Pollution in the Troposphere (MOPITT) instrument in the period 2005–2016. The TCO shows strong seasonality, with the highest value in spring and the lowest value observed in the monsoon season. The seasonal variation of HCHO is similar to that of TCO, while NO2 and CO show slightly different patterns with higher values in spring and winter compared to lower values in autumn and summer. The surface ozone, NO2 and CO observed by national air quality monitoring network sites are also compared with satellite-observed TCO, NO2 and CO, showing good agreement for NO2 and CO but a different seasonal pattern for ozone. Unlike TCO, surface ozone has the highest value in autumn and the lowest value in winter. To reveal the difference, the vertical profiles of ozone and CO from the measurement of ozone and water vapor by airbus in-service aircraft (MOZAIC) observations over South China are also examined. The seasonal averaged vertical profiles of ozone and CO show obvious enhancements at 2–6 km altitudes in spring. Furthermore, we investigate the dependence of TCO and surface ozone on meteorology and transport in detail along with the ECMWF reanalysis data, Tropical Rainfall Measuring Mission (TRMM) 3BV42 dataset, OMI ultraviolet index (UV index) dataset, MODIS Fire Radiative Power (FRP) and back trajectory. Our results show that the wind pattern at 800 hPa plays a significant role in determining the seasonality of TCO over GX, especially for the highest value in spring. Trajectory analysis, combined with MODIS FRP suggests that the air masses that passed through the biomass burning (BB) region of Southeast Asia (SEA) induced the enhancement of TCO and CO in the upper-middle troposphere in spring. However, the seasonal cycle of surface ozone is associated with wind patterns at 950 hPa, and the contribution of the photochemical effect is offset by the strong summer monsoon, which results in the maximum surface ozone concentration in post-monsoon September. The variations in the meteorological conditions at different levels and the influence of transport from SEA can account for the vertical distribution of ozone and CO. We conclude that the seasonal distribution of TCO results from the combined impact of meteorology and long-term transport.

Tropospheric ozone enhancement during post-harvest crop-residue fires at two downwind sites of the Indo-Gangetic Plain

In the present study, surface ozone (O₃), nitrogen oxides (NO_x), and carbon monoxide (CO) levels were measured at two sites downwind of fire active region in the Indo-Gangetic Plain (IGP): Agra (27.16° N, 78.08° E) and Delhi (28.37° N, 77.12° E) to study the impact of post-harvest crop-residue fires. The study period was classified into two groups: Pre-harvest period and Post-harvest period. During the post-harvest period, an enhancement of 17.3 and 31.7 ppb in hourly averaged O₃ mixing ratios was observed at Agra and Delhi, respectively, under similar meteorological conditions. The rate of change of O₃ was also higher in the post-harvest period by 56.2% in Agra and 39.5% in Delhi. Relatively higher O₃ episodic days were observed in the post-harvest period. Fire hotspots detected by Moderate Resolution Imaging Spectroradiometer (MODIS) along with backward air-mass trajectory analysis suggested that the enhanced O₃ and CO levels at the study sites during the post-harvest period could be attributed to crop-residue burning over the North-West IGP (NW-IGP). Satellite observations of surface CO mixing ratios and tropospheric formaldehyde (HCHO) column also showed higher levels during the post-harvest period. Graphical abstract.

Identification of sources contributing to PM2.5 and ozone at elevated sites in the western U.S. by receptor analysis: Lassen Volcanic National Park, California, and Great Basin National Park, Nevada

The proposed revision of the United States (US) air quality standard for ozone will result in violations in sparsely populated remote rural areas in the Western US. Replicating air quality as measured at surface monitoring sites by modeling is particularly difficult in this region due to complex terrain, poorly represented in regional and global models, and uncertainties in emission rates and timing at all scales (locally as well as hundreds to thousands of km upwind). As an alternative method, a fully empirical, receptor-based scheme using in situ aerosol composition and simple meteorological variables to simulate ozone (O3) measurements was tested and found to produce O3 simulation results comparable in uncertainty to regional modeling, and supporting trajectory-based identification of O3 source regions. This approach was tested using two widely-separated (650 km) high altitude (approx. 2 km above sea level) monitoring sites, Lassen Volcanic National Park, in northern California (LAVO) and Great Basin National Park in eastern Nevada (GRBA). Comparing correlations between observed O3 and aerosols, and examining back-trajectories associated with peak concentrations for the two sites permitted distinguishing among local, distant North American, and Asian sources of particulate matter (PM2.5) and O3. This analysis indicates that anthropogenic enhancement of O3 at LAVO is primarily due to transport from Asia. Asia is also the dominant source of anthropogenic O3 at GRBA in spring, but regional North American sources of O3 appear to drive additional ozone peaks in late summer and fall at this more interior site.

Observations of ozone and carbon monoxide at Mei-Feng mountain site (2269 m a.s.l.) in Central Taiwan: seasonal variations and influence of Asian continental outflow

Continuous measurements of ozone (O(3)) and carbon monoxide (CO) were carried out at Mei-Feng (24.05°N, 120.10°E, 2269 m above sea level), a remote mountain site in central Taiwan, to investigate the influence of long-range transported air pollution on O(3) and CO variations in the subtropical Pacific region. Data collected from March 2009 to September 2010 revealed average mixing ratios of 37±14 ppb for O(3) and 188±82 ppb for CO at this remote site. Diurnal variations for both O(3) and CO were observed as well in all seasons. The higher levels for O(3) and CO in the afternoon were attributed to transport of boundary layer pollution to the site during daytime upslope flow. Monthly means of both O(3) and CO showed maxima in spring and in the continental air masses from Southeast Asia, coastal China, and Korea/Japan. On the contrary, the lower O(3) and CO levels found in summer were due to the marine air masses originating from the Philippine Sea and Pacific Ocean. The relationship between O(3) and CO was analyzed, using nighttime data to minimize any local influence. The results showed a fairly good correlation between O(3) and CO from March to September. The contribution of CO from the Asian outflow reached a maximum in spring (88 ppb) and had a minimum in summer (27 ppb). The photochemical buildup of O(3) resulting from anthropogenic emissions in continental Asia was estimated to be 15 ppb in spring, while its production was insignificant, with an average of 4 ppb, in summer. A positive correlation between O(3) and CO plus high ozone levels in springtime suggested that the enhancements of O(3) were likely due to O(3) which was photochemically produced over this region.