Current World Environment

The growing demand of energy and food has resulted in increased consumption of fossil fuels resulting in huge emissions of NOx and NH3 gases. Both these gases are important reactive nitrogen species. Coal, petrol and diesel are the major sources of NOx while urea fertilizer is the major source of NH₃ ^1-2. Urea is produced through Haber-Bosch process during which inert nitrogen (N₂) is converted into urea. When we apply urea in the agricultural fields, its thermal dissociation gives rise to NH₃ and CO₂ in air. For every one molecule of NH₃, one molecule of CO₂ is emitted.³ Globally, urea fertilizer consumption has increased from 50 million tons in 1961 to 215.37 million tons in 2019. Similarly, in India, the urea consumption is increased from 1 million ton in 1960s to around 33.5 million tons in 2019-20 (https://factly.in/data-chemical- fertilizer-consumption-increased-by-about-16-in-the-last-six-years/). Global consumption of fossil fuels such as petroleum oil has increased from 17790 TWh in 1965 to 51170 TWh in 2021, coal has increased 16140 TWh in 1965 to 44473 TWh in 2021 and gas from 6304 TWh in 1965 to 40375 TWh in 2021 (https://ourworldindata.org/fertilizers). Similarly, India has also showed increasing trends of energy consumption which are reflected in the study reporting increasing atmospheric NOx trends in different regions in India.⁴ Most of south Asian countries have geared up their fossil fuel usage for energy production after 1990.

Both NH₃ and NO_x are deposited from the atmosphere through wet and dry deposition processes and are generally estimated as NH⁴⁺ and NO_3- respectively. Chemical composition of atmospheric deposition is an indicator of air pollution of the region. In the process of wet deposition, the pollutants are deposited through rain, snow and hails.  In the dry deposition process, the pollutants are removed through dry deposition process during dry weather conditions. Dustfall is also a kind of dry deposition of coarse particles taking place during dry weather conditions. Dustfall is very common phenomenon in the south Asian region.⁵ The atmospheric dust being alkaline in nature, controls acid rain occurrence through buffering action.⁶ However, high loadings of particulate matter due to atmospheric dust Is considered for poor air quality. 

According to the reports, the Himalayan snow is highly affected by the impact of long range transport of  reactive nitrogen species.⁷ Singh and co-workers⁸ have reports that NO_3- content in rain water increased by 11.7 times in 2011 as compared to 1994 values. Significantly large increase in ambient NOx in air and NO₃^- concentrations in rain water in  Delhi during past two decades indicates the increase in fossil fuel consumption. Among NH₃^-N NH₄⁺N and NO₃^-N, gaseous NH₃ contributes maximum fraction of Nr in the atmosphere in the region. The Indo-Gangetic region experiences the highest wet deposition of NH₄⁺ due to very high population density and related anthropogenic activities. At rural sites, the high NH₃ levels can be attributed to the fertilizers and biomass burning while at urban sites, major sources of gaseous NH₃ include human excreta, municipality waste and vehicular traffic etc..⁹ In addition, the high temperature of the tropics and the alkaline nature of aerosols also favour the building up of NH₃ in the atmosphere in this region.¹⁰ Abundance and phase distribution studies show that particulate NH₄⁺ is noticed lower than gaseous NH₃ in Delhi during all the seasons.

A total of 1.97 Tg of wet deposition and 1.67 Tg of dry deposition of Nr species takes place in India.¹¹ Emissions vs deposition budget still needs more measurements of atmospheric deposition through systematic dense network of sites. In general, the number of reactive nitrogen studies of wet and dry deposition from South Asia is very limited. Also, there are very few measurements about the through fall deposition in the south Asian region. However, there has been an appreciable development about the measurements of nitrogen assessment from individual groups in the region. At present, UKRI-GCRF-South Asian Nitrogen Hub, WMO-Global Atmospheric Watch (GAW) and DRSNet-India programs are carrying out measurements of atmospheric deposition of reactive nitrogen species in south Asia. Earlier atmospheric deposition studies in the region have been carried out under Composition of Asian Deposition (CAD), Composition of Aerosol and Precipitation in India and Nepal (CAAP) programs of Swedish Development Authority (SIDA).^12-14

The long term data limited to rain water chemistry are available from the WMO GAW sites.¹⁵ Other important reports in this regard include the studies reported by Dentener and co-workers¹⁶ and other groups.^17-18  Deposition fluxes of coarse mode NH₄₊ and NO_3- particles on the indoor plants have been reported highlighting their impacts on the biochemical properties of the plants.¹⁹ `The Indian Nitrogen Assessment’ is a very recent compilation of various studies about different aspects of reactive nitrogen in India.²⁰ However, forest N is not much covered in this report. In Nepal, the precipitation is found to have significant influence of pollution even at a remote site in the Khumbu region of the Himalayas where relatively high value of scavenging ratio of NO₃ was noticed as compared to other ions.²¹ NO^3- concentrations in aerosols were one third of NH₄₊ but in precipitation, the NO^3- concentrations were greater than NH⁴⁺ due to the presence of gaseous HNO₃ in the air. This feature was more dominant when air was blowing from more polluted areas. 

Model vs measurements comparison studies are very important for developing deposition prediction capabilities. In a pioneering effort, the measurements of the reactive nitrogen species such NH₄₊ and NO_3- in rain water in India have been compared with the modeling output by Kulshrestha and co-workers¹⁴ by using the MATCH model.²² The output of the model was helpful in providing an interpretation of the observational data of NH⁴⁺ and NO^3-. However, such exercises are further needed to reduce the uncertainties of the model outputs.

This is to mention that there have been some issues of QA/QC of the data in south Asia region mainly due to the problems associated with sampling and chemical analysis of the reactive nitrogen species such as NH⁴⁺ and NO_3-. Kulshrestha and co-workers have reviewed the precipitation chemistry data in the region.¹⁴ These workers found that in most of the studies, NH⁴⁺ was under estimated due to analytical problems such as delay in analysis, no addition of preservative, improper storage of the samples etc. while NO^3- was over estimated due to its additional contribution from  local soil. Ion balance and conductivity balance approaches are supposed to be reported to support the analytical assurance. But most of the earlier studies lack such estimates of errors. It is important to mention that in most of the measurements include F^-, Cl^-, NO^3-, SO₄^2-, Na⁺, K⁺, Ca²⁺, Mg²⁺, NH⁴⁺ in ionic and conductivity balance calculations. But, the pH of rain water in India is relatively high having high concentrations of HCO₃ ion due to the influence of calcareous soils of this region. Hence, ion balance and conductivity balance checks without HCO^3- do not really assures the data quality. Sometimes the results are influenced by the shape and material type (steel, plastic, glass) of the collection assembly. Improper transport, preservation and delay in analysis are also very important factors affecting the quality of the data.^23-24 In order to have good quality measurements, it is necessary to select sites which can represent larger area. Overall, an adherence to QA/QC of data is mandatory for ensuring reliable and robust data sets in the long run.

Gaps and recommendations

The south Asia region needs the following actions to fill the existing knowledge gaps related to N cycle-

1. Need to develop an integrated assessment plan for various Nr species including their emissions, abundance, transport, transformations, scavenging, impacts, forest pool etc. 
2. Good quality data availability is limited for Nr depositions in Indian region which needs a dedicated data and parameter specific protocol,  inter-lab comparison exercises and a reference standards development initiative.
3. A long term measurement network covering a number of sites of different charateristics with the aim to use its research findings in the policy making. 
4. In order to understand local, trans-boundary and long range transport of Nr species, the deposition studies need to be coupled with the trajectory analysis. It will be a good idea to constitute a program to monitor an import and export of pollution within south Asian countries under The South Asian Association for Regional Cooperation (SAARC).
5. We need to focus on the impact studies of various Nr species.
6. Budgets of dry deposition of gases and aerosols especially NH³ and NH₄^- in indoor environment will be highly useful.
7. In order to understand the process involving in gas-aerosol interactions, scavenging, transport, evapo-transpiration, deposition and uptake etc., we need to develop a separate task force. Appropriate emphasis needs to be given on dry deposition studies of Nr in order to reduce uncertainties in Nr budget of this region.
8. Also, we need to develop a common modeling group involving active scientist/groups. It would be more appropriate if we include some socio-economic expert(s) which will help in translating the scientific findings into an impactful report needed for the policy makers.

References

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