Eastern Indo-Gangetic Plain Research Articles

Effect of Anthropogenic Aerosols on Wheat Production in the Eastern Indo-Gangetic Plain

The Indo Gangetic Plain (IGP) is a food basket of South Asia and is considered a hotspot for air pollution due to persistently high emissions of anthropogenic aerosols. High levels of aerosols in the IGP not only affect the health of people but also the health of the natural system and the climate of the region. Aerosol effects on crop production in the IGP is an emerging area of interest for policymakers and the scientific community due to their possible effect on the food security and livelihood of millions of people in the region. To investigate the effect of anthropogenic aerosols on wheat production in the eastern IGP, we used a calibrated and validated Agricultural Production System Simulator (APSIM) model at nodes in Bangladesh, India and Nepal, 2015–2017. The effects of anthropogenic aerosols on wheat production were examined by running the APSIM model under three conditions: firstly, the condition with anthropogenic aerosols, using the observed meteorological data; secondly, the condition without anthropogenic aerosols, considering only the radiative effect of anthropogenic aerosols (adding the reduced radiation due to anthropogenic aerosols on the observed data); thirdly, the condition without anthropogenic aerosols, considering the radiation as well as temperature effects (by adding the reduced solar radiation and temperature due to anthropogenic aerosols on the observed data). The study revealed that, on average, anthropogenic aerosols reduced the wheat grain yield, biomass yield, and crop evapotranspiration by 11.2–13.5%, 21.2–22%, and 13.5–15%, respectively, when considering the 2015–2017 seasons at the target sites of eastern IGP. The study also showed an average reduction of more than 3.2 kg per capita per annum of wheat production in the eastern IGP due to anthropogenic aerosols, which has a substantial effect on food security in the region. Moreover, the loss of wheat grain yield due to anthropogenic aerosols in the eastern IGP is estimated to be more than 300 million USD per annum during the study period, which indicates a significant effect of anthropogenic aerosols on wheat production in the eastern IGP.

Seasonal variations in aerosol acidity and its driving factors in the eastern Indo-Gangetic Plain: A quantitative analysis

This study employs ISORROPIA-II for the evaluation of aerosol acidity and quantification of contributions from chemical species and meteorological parameters to acidity variation in the Indian context. PM2.5 samples collected during summer (April–July 2018), post-monsoon (September–November 2018), and winter (December 2018–January 2019) from a rural receptor location in the eastern Indo-Gangetic Plain (IGP) were analyzed for ionic species, water-soluble organic carbon (WSOC), and organic and elemental carbon (OC, EC) fractions. This was followed by estimation of the in situ aerosol pH and liquid water content (LWC) using the forward mode of ISORROPIA-II, which is less sensitive to measurement uncertainty compared to the reverse mode, for a K+-Ca2+-Mg2+-NH4+-Na+-SO42--NO3--Cl--H2O system. Aerosol pH was moderately acidic (summer: 2.93 ± 0.67; post-monsoon: 2.67 ± 0.23; winter: 3.15 ± 0.34) and was most sensitive to SO42− and total ammonium (TNH3) variation. The LWC of aerosol showed an increasing trend from summer (16.6 ± 13.6 μg m−3) through winter (32.9 ± 10.4 μg m−3). With summer as the baseline, the largest changes in aerosol pH during the other seasons was driven by SO42− (ΔpH: −0.70 to −0.82 units), followed by TNH3 (ΔpH: +0.25 to +0.38 units) with K+ and temperature being significant only during winter (ΔpH: +0.51 and + 0.46 units, respectively). The prevalent acidity regime provided three major insights: i) positive summertime Cl− depletion (49 ± 20%) as a consequence of SO42− substitution increased aerosol pH by 0.03 ± 0.20 units and decreased LWC by 2.4 ± 5.9 μg m−3; ii) the rate of strong acidity (H+str) neutralization and the [H+str]/[SO42−] molar ratio suggested the existence of bounded acidity in ammonium-rich (winter) conditions; and iii) significant correlations between LWC, WSOC, and secondary organics during post-monsoon and winter pointed towards a possible indirect role of WSOC in enhancing LWC of aerosol, thereby increasing pH. Given the inability of proxies such as H+str and charge ratios to accurately represent aerosol pH as demonstrated here, this study emphasizes the need for rigorous thermodynamic model-based evaluation of aerosol acidity in the Indian scenario.

Conservation agriculture practice influences soil organic carbon pools in intensive rice‐based systems of the Eastern Indo‐Gangetic Plain

AbstractStudies of rice‐based systems in the Indo‐Gangetic Plain (IGP) have demonstrated the beneficial effects of Conservation Agriculture on soil organic carbon (SOC) status, along with increased soil health and crop productivity. However, it remains unclear as to the time for such treatments to have a positive effect. In this study of lentil‐mung bean‐rice and wheat‐mung‐rice rotations in Bangladesh positive effects of strip planting or bed planting, along with residue return, on SOC pools were apparent after 1.5 years, compared with intensive conventional tillage and limited residue return. Conventional tillage resulted in higher CO2 emission compared with strip planting or bed planting as did high residue return. In the cereal‐dominated rotation, the strip planting system sequestered carbon at a rate of 0.24–0.53 Mg C ha−1 year−1 (at 0–0.15 m depth) while conventional tillage was associated with a carbon loss of 0.52–0.82 Mg C ha−1 year−1. In the legume‐dominated rotation, neither practice sequestered SOC. Under strip planting, a minimum annual crop residue input of 1.7 Mg C ha−1 for the cereal‐dominated system and 5.2 Mg C ha−1 for the legume‐dominated system was required to maintain SOC at equilibrium. We conclude that strip planting with high levels of crop residue return can be an effective and quick strategy in either slowing the loss of SOC or improving C sequestration in the intensive rice‐based systems of the Eastern IGP.

Enhancement in free-tropospheric aerosol loading over India

Free-tropospheric aerosol loading and its changes assume importance as the diabatic heating due to radiation absorption by aerosols is more intense at higher altitudes. The present study quantifies trends in free-tropospheric aerosol loading over Indian landmass and examines the role of local production and long range transport, using multi-satellite observations and reanalysis data. Increasing trends in free-tropospheric aerosol abundance are seen over most of the regions in almost all the seasons. Strongest positive trend over the Indian landmass is seen in post-monsoon, with mean rate of increase in free-tropospheric aerosol optical depth (AODFree Troposphere) 0.063dec−1. While dust aerosol loading in the free-troposphere decreases, smoke aerosol concentration increases over the Indian landmass. While increase in columnar and free-tropospheric aerosol loading is seen over larger regions of the Indian landmass, including regions of northwest India, Indo Gangetic Plain (IGP), central India and southern peninsula, enhancement in aerosol production is observed mainly over the regions of northwest India in post-monsoon. Due to the prevailing westerly winds, aerosols over northwest India are transported through IGP to its central and eastern regions. Increase in aerosol production over northwest India and strengthening of updrafts over the central and eastern IGP lead to vertical uplift of more aerosols into the free-tropospheric altitudes. These free-tropospheric aerosols are subsequently transported, by the prevailing anti-cyclonic system, to the regions of central India and southern peninsula and cause the enhancement in free-tropospheric aerosol loading over these regions in post-monsoon. Though strengthening of updraft is observed over the regions of central India and southern Peninsula, net aerosol production is not seen to be increasing over these regions. The observed enhancement in free-tropospheric aerosols over Indian landmass is important as they can affect the thermal structure and stability of the atmosphere and modify the properties and distribution of clouds.

Conservation agriculture effects on yield and profitability of rice-based systems in the Eastern Indo-Gangetic Plain

Abstract Sustaining productivity of the rice-based cropping systems in the Eastern Indo-Gangetic Plain (EIGP) requires practices to reverse declining soil fertility resulting from excessive tillage and crop residue removal, while decreasing production costs and increasing farm profits. We hypothesize that the adoption of conservation agriculture (CA), involving minimum tillage, crop residue retention and crop rotation, can address most of these challenges. Therefore, the effects of crop establishment methods – strip planting (SP), bed planting (BP) and conventional tillage (CT); and levels of crop residue retention – high residue (HR) and low residue (LR) on individual crop yield, system yield and profitability were evaluated in a split-plot design over three cropping seasons in two field experiments (Alipur and Digram sites) with contrasting crops and soil types in the EIGP. The SP and BP of non-rice crops were rotated with non-puddled rice establishment; CT of non-rice crops was rotated with puddled transplanted rice. In the legume-dominated system (rice-lentil-mung bean), lentil yields were similar in SP and CT, while lower in BP in crop season 1. A positive effect of high residue over low residue was apparent by crop season 2 and persisted in crop season 3. In crop season 3, the lentil yield increased by 18–23% in SP and BP compared to CT. In the cereal-dominated system (rice-wheat-mung bean), significant yield increases of wheat in SP and BP (7–10%) over CT, and of HR (1–3%) over LR, were detected by crop season 3 but not before. Rice yields under CA practices (non-puddled and HR) were comparable with CT (puddled and LR) in both systems. Improved yield of lentil and wheat with CA was correlated with higher soil water content. The net income of SP increased by 25–28% for dry season crops as compared to CT and was equal with CT for rice cropping systems. Conservation agriculture practices provide opportunities for enhancing crop yield and profitability in intensive rice-based systems of the EIGP of Bangladesh.

Characteristic dissimilarities during high aerosol loading days between western and eastern Indo-Gangetic Plain

This study investigates the long-term (2003–2019) variations of high aerosol loading days and their radiative impacts over the western Indo-Gangetic Plain (IGP) and eastern IGP during pre-monsoon season (March-April-May-June). The Aerosol Optical Depth (AOD) climatology from MODIS (Terra and Aqua) and MERRA-2 reanalysis shows high aerosol burden across the IGP region during the pre-monsoon season. The high aerosol loading days are identified based on a standardized AOD anomaly approach, from MODIS and MERRA-2. The frequency of high aerosol loading days over the western IGP is roughly twice that of the total number of high aerosol loading days over the eastern IGP. The area-averaged percentage differences in AOD between high aerosol loading days and normal days over western IGP is always higher, about 6–8%, than eastern IGP from Terra, Aqua and MERRA-2. The natural (mainly dust) and anthropogenic aerosols (particularly sulfate, black carbon and organic carbon) are majorly contributed to total AOD over western IGP and eastern IGP. Furthermore, the MERRA-2 and ERA5 composite surface and 850 hPa wind anomalies show that strong westerly winds dominate, transporting dust aerosols from arid regions to the western IGP. On the other hand, weak prevailing winds and background pre-monsoonal cyclonic circulations over eastern IGP favor the accumulation of regionally emitted aerosols. During high aerosol loading days, the decrease in ventilation coefficient indicates the high aerosol burden (less dispersion) over both the regions, leading to the deterioration of air quality. The enhanced aerosol loading induced potential atmospheric radiative forcing (19.78 Wm-2 over western IGP and 20.77 Wm-2 over eastern IGP) during high aerosol loading days compared to normal days (11.12 Wm-2 and 12.9 Wm-2).

Energy use pattern of diversified cropping systems under different nutrient and crop residue management practices in Eastern Indo-Gangetic plain

Aim: Assessment of energy input output relationship, greenhouse gases emission and carbon footprint of diversified jute-rice cropping systems under different nutrients and crop residue management practices. Methodology: The inventory was prepared for all inputs required for crop cultivation and outputs of crops in cropping systems. These inputs and outputs were converted into energy by multiplying with energy equivalent coefficient and CO2 emission coefficient following standard procedure. Results: Jute-rice-baby corn cropping system recorded significantly higher net energy (324 GJ ha-1) and energy use efficiency (8.02). Among different nutrient and crop management (NCRM) practices, significantly higher energy output (336.9 GJ ha-1) and net energy (291.4 GJ ha-1) recorded 100% NPK with crop residue. The highest carbon footprint recorded with rice-rice (0.44 kg COe kg-1 economic yield) and the lowestwith jute-rice-pea (0.29 kg COe kg-1 economic yield) cropping system. Among different NCRM practices, higher carbon footprint was (0.38 kg COe kg-1 economic yield) recorded with 100% NPK with crop residue. Interpretation: The energy efficient and low input required cropping systems which include legume crops like garden pea and mungbean should be considered for cultivation for diversifying the existing rice-rice cropping system in Eastern India.

Measurement report: Altitudinal variation of cloud condensation nuclei activation across the Indo-Gangetic Plain prior to monsoon onset and during peak monsoon periods: results from the SWAAMI field campaign

Abstract. Vertical distributions (altitude profiles) of condensation nuclei (CN) and cloud condensation nuclei (CCN) and their spatial variations across the Indo-Gangetic Plain (IGP) have been investigated based on airborne measurements carried out during the SWAAMI field campaign (June to July 2016) capturing the contrasting phases of the Indian monsoon activity in 2016 just prior to its onset and during its active phase. Prior to the monsoon onset, high concentrations of CN and CCN prevailed across the IGP, and the profiles revealed frequent occurrence of elevated layers (in the altitude range 1–3 km). Highest concentrations and elevated peaks with high values occurred over the central IGP. The scenario changed dramatically during the active phase of the monsoon, when the CN and CCN concentrations dropped (CN by 20 % to 30 % and CCN by 6 % to 25 %) throughout the IGP with more pronounced changes at altitudes higher than 3 km where decreases as high as > 80 % were observed. These reductions have an east-to-west decreasing gradient, being most remarkable in the eastern IGP and very weak over the western IGP where the CN concentrations above 3 km increased during the monsoon. The activation ratios (ARs) showed contrasting features, increasing with increase in altitude, prior to the onset of monsoon, reversing the trend to decrease with increase in altitude during the active phase of the monsoon. The supersaturation spectrum became flatter during the active phase of the monsoon, indicating an increase in the hygroscopicity of aerosols following the mixing of surface-based emissions with the advected marine air mass.

Wintertime direct radiative effects due to black carbon (BC) over the Indo-Gangetic Plain as modelled with new BC emission inventories in CHIMERE

Abstract. To reduce the uncertainty in climatic impacts induced by black carbon (BC) from global and regional aerosol–climate model simulations, it is a foremost requirement to improve the prediction of modelled BC distribution, specifically over the regions where the atmosphere is loaded with a large amount of BC, e.g. the Indo-Gangetic Plain (IGP) in the Indian subcontinent. Here we examine the wintertime direct radiative perturbation due to BC with an efficiently modelled BC distribution over the IGP in a high-resolution (0.1∘ × 0.1∘) chemical transport model, CHIMERE, implementing new BC emission inventories. The model efficiency in simulating the observed BC distribution was assessed by executing five simulations: Constrained and bottomup (bottomup includes Smog, Cmip, Edgar, and Pku). These simulations respectively implement the recently estimated India-based observationally constrained BC emissions (Constrainedemiss) and the latest bottom-up BC emissions (India-based: Smog-India; global: Coupled Model Intercomparison Project phase 6 – CMIP6, Emission Database for Global Atmospheric Research-V4 – EDGAR-V4, and Peking University BC Inventory – PKU). The mean BC emission flux from the five BC emission inventory databases was found to be considerably high (450–1000 kg km−2 yr−1) over most of the IGP, with this being the highest (> 2500 kg km−2 yr−1) over megacities (Kolkata and Delhi). A low estimated value of the normalised mean bias (NMB) and root mean square error (RMSE) from the Constrained estimated BC concentration (NMB: < 17 %) and aerosol optical depth due to BC (BC-AOD) (NMB: 11 %) indicated that simulations with Constrainedemiss BC emissions in CHIMERE could simulate the distribution of BC pollution over the IGP more efficiently than with bottom-up emissions. The high BC pollution covering the IGP region comprised a wintertime all-day (daytime) mean BC concentration and BC-AOD respectively in the range 14–25 µg m−3 (6–8 µg m−3) and 0.04–0.08 from the Constrained simulation. The simulated BC concentration and BC-AOD were inferred to be primarily sensitive to the change in BC emission strength over most of the IGP (including the megacity of Kolkata), but also to the transport of BC aerosols over megacity Delhi. Five main hotspot locations were identified in and around Delhi (northern IGP), Prayagraj–Allahabad–Varanasi (central IGP), Patna–Palamu (mideastern IGP), and Kolkata (eastern IGP). The wintertime direct radiative perturbation due to BC aerosols from the Constrained simulation estimated the atmospheric radiative warming (+30 to +50 W m−2) to be about 50 %–70 % larger than the surface cooling. A widespread enhancement in atmospheric radiative warming due to BC by 2–3 times and a reduction in surface cooling by 10 %–20 %, with net warming at the top of the atmosphere (TOA) of 10–15 W m−2, were noticed compared to the atmosphere without BC, for which a net cooling at the TOA was exhibited. These perturbations were the strongest around megacities (Kolkata and Delhi), extended to the eastern coast, and were inferred to be 30 %–50% lower from the bottomup than the Constrained simulation.

Impact of COVID-19 lockdown on aerosol optical and radiative properties over Indo-Gangetic Plain

Aerosol optical and radiative properties, measured with the ground based sun-sky radiometer (under AERONET), were explored at Kanpur in the central and Gandhi College (Ballia) in the Eastern Indo-Gangetic Plain (IGP) in Northern India during January to July 2019 (non-lockdown) and 2020 (lockdown). This year 2020 is marked an overall reduction of 15.79% and 3.39% in aerosol optical depth (AOD) than year 2019 at Kanpur and Gandhi College respectively. Lockdown 1 (23rd March to 14th April 2020) showed a reduction of 52.52% and 42.22% in AOD compared to pre-lockdown condition at Kanpur and Gandhi College respectively. Higher values of angstrom exponent measured at Eastern region (1.13 ± 0.17) than central region (0.97 ± 0.26) indicating the dominance of fine particles at Gandhi College. There was an overall reduction of 1.31% in single scattering albedo (SSA) at Kanpur while 4.34% reduction was observed at Gandhi College in year 2020 than 2019. The resultant atmospheric forcing and heating rate shows 11.86% reduction at Kanpur while 35.27% increase at Gandhi College during the lockdown period in 2020 compared to 2019. However, these values were found to decrease by 23.48% and 15.07% at Kanpur and Gandhi College respectively during the strict lockdown period compared to year 2019.

Improving regional air quality predictions in the Indo-Gangetic Plain – case study of an intensive pollution episode in November 2017

Abstract. The Indo-Gangetic Plain (IGP) experienced an intensive air pollution episode during November 2017. Weather Research and Forecasting model coupled to Chemistry (WRF-Chem), a coupled meteorology–chemistry model, was used to simulate this episode. In order to capture PM2.5 peaks, we modified input chemical boundary conditions and biomass burning emissions. The Community Atmosphere Model with Chemistry (CAM-chem) and Modern-Era Retrospective analysis for Research and Applications Version 2 (MERRA-2) global models provided gaseous and aerosol chemical boundary conditions, respectively. We also incorporated Visible Infrared Imaging Radiometer Suite (VIIRS) active fire points to fill in missing fire emissions in the Fire INventory from NCAR (FINN) and scaled by a factor of 7 for an 8 d period. Evaluations against various observations indicated the model captured the temporal trend very well although missed the peaks on 7, 8, and 10 November. Modeled aerosol composition in Delhi showed secondary inorganic aerosols (SIAs) and secondary organic aerosols (SOAs) comprised 30 % and 27 % of total PM2.5 concentration, respectively, during November, with a modeled OC/BC ratio of 2.72. Back trajectories showed agricultural fires in Punjab were the major source for extremely polluted days in Delhi. Furthermore, high concentrations above the boundary layers in vertical profiles suggested either the plume rise in the model released the emissions too high or the model did not mix the smoke down fast enough. Results also showed long-range-transported dust did not affect Delhi's air quality during the episode. Spatial plots showed averaged aerosol optical depth (AOD) of 0.58 (±0.4) over November. The model AODs were biased high over central India and low over the eastern IGP, indicating improving emissions in the eastern IGP can significantly improve the air quality predictions. We also found high ozone concentrations over the domain, which indicates ozone should be considered in future air quality management strategies alongside particulate matter.

Comparative assessment of energy flow, carbon auditing and eco-efficiency of diverse tillage systems for cleaner and sustainable crop production in eastern India

Identification of the appropriate tillage production system having lower energy use and carbon-emission, and better crop productivity is becoming increasingly important to maintain the environmental sustainability. In the present study, a comprehensive system analysis was performed for four consecutive years (2016–2019) in three major agroecosystems of eastern India: eastern Indo-Gangetic plain, coastal agroecosystem and hill & plateau region. Six rice-based production systems with different levels of farm mechanization viz., a) fully mechanized tillage, b) partly mechanized tillage and c) traditional tillage were considered in the analysis. The main aim was to assess the energy flow and carbon-balance of diverse tillage production systems. Among the different sources of total input energy, chemical fertilizer accounted for the highest energy used in partly mechanized tillage (44%) and mechanized tillage (38%) followed by diesel, irrigation water, plant protection chemical, seed and electricity. Seed, human, animal energy and farmyard manure accounted for 21, 20, 16 and 16%, respectively, of the total energy input in traditional tillage. Maximum energy input (52161 MJ ha−1) was noted in mechanized tillage and minimum with traditional tillage (16879 MJ ha−1). Cropping systems followed in eastern Indo-Gangetic plain were more energy-intensive (50908 MJ ha−1) compared to coastal-ecosystem (27459 MJ ha−1). On an average, the total energy output in mechanized tillage (395245 MJ ha−1) were 0.3 and 2.4 times higher over partly mechanized and traditional tillage, respectively. Overall, the present results indicated that partly mechanized tillage and coastal agroecosystem were the most energy-efficient with an energy ratio of 8.88 and 9.81, respectively. Mechanized tillage was 0.24 and 1.66 times more carbon-intensive in comparison to partly mechanized and traditional tillage system. Mechanized tillage had higher carbon efficiency (3.75), carbon-sustainability index (2.75), carbon-footprint in spatial scales (4342 kg CO2eq. ha−1), but had 34% less carbon-footprint in yield scales compared to traditional tillage. Mechanized tillage showed 22 and 73% higher system productivity compared to partly mechanized and traditional tillage, respectively. Partly mechanized tillage had a 23% lower cultivation cost than mechanized tillage. Thus, the present study suggests that partly mechanize tillage was the most appropriate energy and carbon-efficient production system in eastern India.

Sources and atmospheric processing of brown carbon and HULIS in the Indo-Gangetic Plain: Insights from compositional analysis

We present here spectroscopic compositional analysis of brown carbon (BrC) and humic-like substances (HULIS) in the Indian context under varying conditions of source emissions and atmospheric processing. To this end, we study bulk water-soluble organic matter (WSOM), neutral- and acidic-HULIS (HULIS-n and HULIS-a), and high-polarity (HP)-WSOM collected in the eastern Indo-Gangetic Plain (IGP) with respect to UV–Vis, fluorescence, FT-IR, 1H NMR and 13C characteristics under three aerosol regimes: photochemistry-dominated summer, aged biomass burning (BB)-dominated post-monsoon, and fresh BB-dominated winter. Absorption coefficients (babs_365 nm; Mm−1) of WSOM and HULIS fractions increase by a factor of 2–9 during winter as compared to summer, with HULIS-n dominating total HULIS + HP-WSOM absorption (73–81%). Fluorophores in HULIS-n appear to contain near-similar levels of aromatic and unsaturated aliphatic conjugation across seasons, while HULIS-a exhibits distinctively smaller-chain structures in summer and post-monsoon. FT-IR spectra reveals, among others, strong signatures of aromatic phenols in winter WSOM suggesting a BB-related origin. 1H NMR-based source attribution coupled with back trajectory analysis indicate the presence of secondary and BB-related organic aerosol (SOA and BBOA) in the post-monsoon and winter, and marine-derived OA (MOA) in the summer, which is supported by 13C measurements. Overall, these observations uncover a complex interplay of emissions and atmospheric processing of carbonaceous aerosols in the IGP.

Airborne in situ measurements of aerosol size distributions and black carbon across the Indo-Gangetic Plain during SWAAMI–RAWEX

Abstract. During the combined South-West Asian Aerosol–Monsoon Interactions and Regional Aerosol Warming Experiment (SWAAMI–RAWEX), collocated airborne measurements of aerosol number–size distributions in the size (diameter) regime 0.5 to 20 µm and black carbon (BC) mass concentrations were made across the Indo-Gangetic Plain (IGP), for the first time, from three distinct locations, just prior to the onset of the Indian summer monsoon. These measurements provided an east–west transect of region-specific properties of aerosols as the environment transformed from mostly arid conditions of the western IGP (represented by Jodhpur, JDR) having dominance of natural aerosols to the central IGP (represented by Varanasi, VNS) having very high anthropogenic emissions, to the eastern IGP (represented by the coastal station Bhubaneswar, BBR) characterized by a mixture of the IGP outflow and marine aerosols. Despite these, the aerosol size distribution revealed an increase in coarse mode concentration and coarse mode mass fraction (fractional contribution to the total aerosol mass) with the increase in altitude across the entire IGP, especially above the well-mixed region. Consequently, both the mode radii and geometric mean radii of the size distributions showed an increase with altitude. However, near the surface and within the atmospheric boundary layer (ABL), the features were specific to the different subregions, with the highest coarse mode mass fraction (FMC∼72 %) in the western IGP and highest accumulation fraction in the central IGP with the eastern IGP in between. The elevated coarse mode fraction is attributed to mineral dust load arising from local production as well as due to advection from the west. This was further corroborated by data from the Cloud-Aerosol Transport System (CATS) on board the International Space Station (ISS), which also revealed that the vertical extent of dust aerosols reached as high as 5 km during this period. Mass concentrations of BC were moderate (∼1 µg m−3) with very little altitude variation up to 3.5 km, except over VNS where very high concentrations were seen near the surface and within the ABL. The BC-induced atmospheric heating rate was highest near the surface at VNS (∼0.81 K d−1), while showing an increasing pattern with altitude at BBR (∼0.35 K d−1 at the ceiling altitude).

Effect of conservation tillage and rice-based cropping systems on soil aggregation characteristics and carbon dynamics in Eastern Indo-Gangetic Plain

Traditional rice–wheat cropping system, which follows wet puddling in rice and conventional tillage in wheat, is deteriorating soil health resulting yield stagnation in the Indo-Gangetic Plains of South Asia. Conservation agriculture which endorses minimum soil disturbance, residue retention and crop diversification not only improves soil health but also reduces the CO2 concentration in atmosphere. We hypothesized that adoption of conservation agriculture could improve the soil health and soil organic carbon in comparison with conventional practices. A field experiment was conducted during 2012–2015 to observe the effects of different tillage practices and cropping systems on soil aggregation and carbon dynamics. The experiment comprised of three cropping systems, viz. rice–wheat, RW; rice–maize, RM; rice–lentil, RL, practiced in three tillage practices, viz. conventional tillage, CT; reduced tillage, RT; reduced tillage with 30% residue, RT30 in factorial randomized block design. Adoption of RT and RT30 resulted in higher macroaggregate content of 51.7 and 61.2%, respectively, in comparison with CT. Total water stable aggregates and mean weight diameter (MWD) of aggregates were considerably higher in RT and RT30 treatments, and the effect was most pronounced in the upper 0–15 cm soil layer. The rice–maize cropping system registered the highest macroaggregate content, water stable aggregates (WSA) and MWD of 55.6%, 80.0% and 2.28 mm, respectively, in the upper surface soil. The rice–maize cropping systems under RT30 recorded the highest total soil organic carbon (SOC) stock (51.0 Mg ha−1) in comparison with other systems after 3 years of experimentation. Reduced tillage and residue management resulted in positive changes in soil infiltration rate. The effect of tillage operations and cropping systems on different soil properties (aggregate distribution, WSA, MWD and geometric mean diameter of aggregates, SOC stock, Bulk density) was mostly limited to surface layer of soil.

Optical properties of aerosol brown carbon (BrC) in the eastern Indo-Gangetic Plain.

We report here measurements of aerosol black carbon (BC) and aqueous and methanol-extractable brown carbon (BrCaq and BrCme) from a receptor location in the eastern Indo-Gangetic Plain (IGP) under two aerosol regimes: the photochemistry-dominated summer and biomass burning (BB) dominated post-monsoon. We couple time-resolved measurements of BC and aerosol light absorption coefficients (babs) with time-integrated analysis of BrC UV-Vis and fluorescence characteristics, along with measurements of total and water-soluble organic carbon (OC and WSOC), and ionic species (NH4+, K+, NO3-). In the BB regime, BC and its BB-derived fraction (BCBB) increased by factors of 3-4 over summertime values. In comparison, babs_365_aq and babs_365_me (absorption coefficients of BrCaq and BrCme at 365nm) increased by a factor of 5 (9.7±7.8 vs 2.1±1.4Mm-1) and 2.5 (17.2±9.0 vs 6.9±2.9Mm-1), respectively, in the BB period over summer, and were highly correlated (r=0.82-0.87; p<0.01) with the BB-tracer nssK+. The wavelength dependence of babs_BrC (Ångstrom exponent: 5.9-6.2) and the presence of characteristic fluorescence peaks at 420-430nm suggested presence of humic-like substances (HULIS) in the aged BB aerosol, while significant association between BrCaq and NO3- (r=0.73; p<0.01) possibly indicated formation of water-soluble nitroaromatic compounds. BrCaq contributed 55% to total BrC absorption at 300-400nm while that for the water-insoluble component (WI-BrC) increased from 41% at 340nm to ~60% at 550nm, suggesting formation of water-insoluble polycyclic aromatic hydrocarbons (PAHs) and/or N-PAHs. Mass absorption efficiencies at 365nm (MAE365) of BrCaq and BrCme in the BB regime (0.95±0.45 and 1.17±0.78m2g-1, respectively) were in line with values expected from photobleaching of BB source emissions after transport to the eastern IGP. Overall, BrCaq and BrCme were significant components of light absorbing aerosol in the BB regime, with contributions of 9±5% and 16±7%, respectively, to radiative forcing vis-à-vis BC in the 300-400nm range.

Foliar Spray of Synthetic Osmolytes Alleviates Terminal Heat Stress in Late-Sown Wheat

Wheat sowing often gets delayed and the crop faces terminal heat stress, which is severely impacting the crop productivity. Synthetic osmolytes maintain structural integrity of cell membrane and protect oxidative damage of cells under stress conditions. Hence, foliar application of these compounds is likely to alleviate the negative effects of high temperature on crop plants. However, field response of these chemicals on late-sown wheat in the subtropical environment (eastern Indo–Gangetic Plain) is still uncertain. A field experiment was conducted for three years (2013–2016) to envisage the field efficacy of two synthetic compounds viz., potassium nitrate (KNO3) (1.0 and 0.5%) and glycine betaine (GB) (100 and 50 mM), each sprayed either at respective higher dose at booting or anthesis, or at half the dose at both booting and anthesis for improving grain yield of two contrasting wheat cultivars (DBW14 and K307) facing terminal heat stress. Foliar application of 0.5% KNO3 at both booting and anthesis stages resulted in the maximum increase (13–16%) in grain yield over no-spray treatment closely followed by 1% KNO3 at anthesis (10–14%), and 50 mM GB at both booting and anthesis stages (6–9%). However, foliar spray of KNO3 and GB at only booting stage had a non-significant effect on grain yield. The increase in grain yield with the osmolytes application was attributed to an increase in the number of grains ear− 1 (9–16%) and thousand-grain weight (3–10%), demonstrating an increased sensitivity of sink capacity and grain development of wheat to terminal heat stress. The degradation of flag leaf chlorophyll content (34–36%) and increase in the relative membrane permeability (29–52%) during anthesis to grain filling period was observed as the critical yield-limiting factors in late-sown wheat. Foliar spray of osmolytes had a regulatory effect on these physiological anomalies being highest with the treatment 0.5% KNO3 foliar spray at both booting and anthesis stages followed by 1% KNO3 foliar spray at anthesis. At the grain filling stage, significant relationships between grain yield and flag leaf chlorophyll (r = 0.43–0.68) and relative membrane permeability (r = − 0.47 to − 0.53) were observed. Thus, foliar application of 0.5% KNO3 at both booting and anthesis stages could be recommended to alleviate the adverse effect of terminal heat stress and improve productivity of late-sown wheat in subtropical environments.

Altitude profiles of cloud condensation nuclei characteristics across the Indo-Gangetic Plain prior to the onset of the Indian summer monsoon

Abstract. Concurrent measurements of the altitude profiles of the concentration of cloud condensation nuclei (CCN), as a function of supersaturation (ranging from 0.2 % to 1.0 %), and aerosol optical properties (scattering and absorption coefficients) were carried out aboard an instrumented aircraft across the Indo-Gangetic Plain (IGP) just prior to the onset of the Indian summer monsoon (ISM) of 2016. The experiment was conducted under the aegis of the combined South-West Asian Aerosol–Monsoon Interactions and Regional Aerosol Warming Experiment (SWAAMI–RAWEX) campaign. The measurements covered coastal, urban and arid environments. In general, the CCN concentration was highest in the central IGP, decreasing spatially from east to west above the planetary boundary layer (PBL), which is ∼1.5 km for the IGP during pre-monsoon period. Despite this, the CCN activation efficiency at 0.4 % supersaturation was, interestingly, the highest over the eastern IGP (∼72 %), followed by that in the west (∼61 %), and it was the least over the central IGP (∼24 %) within the PBL. In general, higher activation efficiency is noticed above the PBL than below it. The central IGP showed remarkably low CCN activation efficiency at all altitudes, which appears to be associated with high black carbon (BC) mass concentration there, indicating the role of anthropogenic sources in suppressing the CCN efficiency. These first-ever CCN measurements over the western IGP, encompassing “the Great Indian Desert” also known as “the Thar Desert”, showed high CCN efficiency, ∼61 % at 0.4 % supersaturation, indicating the hygroscopic nature of the dust. The vertical structure of CCN properties is found to be air mass dependent, with higher activation efficiency even over the central IGP during the prevalence of marine air mass. Wet scavenging associated with precipitation episodes seems to have reduced the CCN activation efficiency below cloud level. An empirical relation has emerged between the CCN concentration and the scattering aerosol index (AI), which would facilitate the prediction of CCN from aerosol optical properties.

Geochemistry of PM2.5 aerosols at an urban site, Varanasi, in the Eastern Indo-Gangetic Plain during pre-monsoon season

Abstract The knowledge of actual morphological features and composition of aerosols are very important to understand atmospheric chemistry, mixing state and radiative forcing on a global scale. Daily and weekly PM2.5 samples were collected during pre-monsoon season (March–May 2015) from an urban site, Varanasi, located in the eastern Indo-Gangetic Plain to study geochemical and morphological features of aerosols. PM2.5 samples were analyzed for their elemental concentration by means of an Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) whereas particle morphology and composition were ascertained by means of a Scanning Electron Microscope (SEM) coupled with an Energy Dispersive X-ray (EDX) spectroscopy. PM2.5 concentrations ranged between 22.2 and 70.5 μgm–3 in daily samples, whereas it varied between 52.0 and 106.2 μgm–3 in weekly samples. PM2.5 concentrations, except in one daily sample, were found to be higher than the 24-h threshold limit of World Health Organization (WHO) standards (25 μgm–3). Elemental concentration of Pb and Zn were as high as ~2000 ppm and 3700 ppm, respectively highlighting impact of heavy metal pollution in Varanasi. The mass concentration of Al was the highest amongst all the measured elements followed by K, Fe, Zn, Ti, Pb, Mn, As, Cd. Enrichment Factors (EF) 5 for K, Zn, As, Pb and Cd suggesting anthropogenic sources as a major contributor to these elements at the study site. The cluster analysis suggests that biomass burning emissions and air mass traversing through northern and western parts of India majorly contributed to high PM2.5 concentration at Varanasi. In contrast, wind patterns did show lower velocities and different directions (from NW India to Eastern parts of India) during the days with relatively lower PM2.5 concentrations. Morphological features of individual particles reveal an irregular, aggregated and flaky morphology whereas the elemental composition revealed the dominance of aluminosilicates, soot and tarball with inclusion of anthropogenic elements (e.g. Pb) at Varanasi.

Spatio-temporal variability of CO over the Eastern Indo-Gangetic Plain (IGP) and in parts of South-East Asia: a MERRA-2-based study

The present study revolves around the distribution of surface CO concentrations around the industrial belts of the Eastern Indo-Gangetic Plains (IGP) and in parts of Southeast Asia over a period of 10 years from January 2007 to December 2016. MERRA-2 reanalysis fields are analysed to study the long-term spatial and seasonal behaviour of CO. The focus is on three CO hotspot areas in this region, denoted by A, B and C, which are further subdivided into eight smaller divisions based on topographical and regional characteristics as discussed in details in “Study Region & Meteorology” section. The monthly average distribution of CO in all the eight zones ranges from 750 (C1) to 3781(A2) ppbv. Very high values are consistently observed in the Indo-Gangetic Plains as compared with other regions during the entire period of study. Seasonal variation of surface CO concentration indicates high values in the post-monsoon or winter season. Analysis of spatio-temporal variation alongside emission, chemical production and loss reveals that presence of surface CO is predominantly due to combustion of fossil fuels and burning of organic matters. Fire hotspot data from FIRMS also support these results. Statistical analysis performed using the non-linear regression model further elucidates the sources of CO in the hotspot areas. The coefficient of determination (R2) values between surface CO concentration (response variable) and emission, chemical production and loss (the independent process variables) for all the zones show interesting results. MERRA-2 does not consider any kind of convective transport as influx and outflux of the hypothetical control volume (box) and as such slightly overestimates the net CO concentration as compared with the ground-based measurements under steady-state scenarios.