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Understanding Nitrogen Oxides (NOx)
Understanding Nitrogen Oxides (NOx) and Their Impact on Air Quality
What is NOx?
Nitrogen Oxides (NOx) refer to a group of gases primarily composed of Nitric Oxide (NO) and Nitrogen Dioxide (NO2). These gases are produced during combustion processes where the flame temperature exceeds 1300°C, causing nitrogen and oxygen molecules in the air to bond.
Common sources of NOx include vehicle engines, power plants, and industrial activities.
Why is NOx a Problem?
NOx emissions significantly impact both environmental and human health. Here’s why:
- Formation of Tropospheric Ozone:
NOx reacts with volatile organic compounds (VOCs) in the presence of sunlight to form tropospheric ozone (ground-level ozone). This ozone is a potent greenhouse gas that contributes to global warming and air pollution.
Exposure to high levels of tropospheric ozone can lead to respiratory issues, reduced lung function, and exacerbation of conditions such as asthma and bronchitis, making it a significant public health concern. - Contribution to Smog Formation:
NOx is a key component in the formation of smog, which reduces visibility and harms respiratory health. Impact on Climate Change:
NOx indirectly affects climate change by contributing to the formation of ozone and fine particulate matter (PM2.5), both of which are potent greenhouse gases.
Global Warming Potential (GWP) and CO2e Values
Understanding the Global Warming Potential (GWP) of different greenhouse gases helps in comparing their impacts on climate change.
The table below summarises the GWP values, persistence levels, and CO2e equivalents for various gases as per the latest IPCC Sixth Assessment Report (AR6):
| Greenhouse Gas | GWP (100 years) | Persistence Level (Years) | CO2e Equivalent per Ton of Emission | Typical Sources | Notes |
|---|---|---|---|---|---|
| Carbon Dioxide (CO2) | 1 | Variable (up to thousands of years) | 1 | Fossil fuel combustion, deforestation, industrial processes | Reference gas for GWP |
| Methane (CH4) | 27.9 (fossil sources); 28-36 (range) | ~12 years | 27.9 - 36 | Livestock, rice paddies, landfills, fossil fuel extraction | High GWP due to strong absorption of infrared radiation |
| Nitrous Oxide (N2O) | 273 | ~114 years | 273 | Agricultural activities, fossil fuel combustion, industrial processes | Significant GWP due to long atmospheric lifetime and potent greenhouse effect |
| HFC-23 (CHF3) | 12,400 | ~270 years | 12,400 | Refrigeration, air conditioning, industrial processes | Extremely high GWP due to strong absorption and long persistence |
| HFC-134a (C2H2F4) | 1,300 | ~14 years | 1,300 | Refrigeration, air conditioning | Commonly used HFC with high GWP |
| HFC-152a (C2H4F2) | 138 | ~1.5 years | 138 | Aerosol propellants, foam blowing agents | Lower GWP compared to other HFCs, but still significant |
| Perfluoromethane (CF4) | 6,630 | ~50,000 years | 6,630 | Aluminium production, electronics manufacturing | Extremely long atmospheric lifetime |
| Perfluoroethane (C2F6) | 11,100 | ~10,000 years | 11,100 | Aluminium production, electronics manufacturing | Very high GWP and long atmospheric lifetime |
| Sulfur Hexafluoride (SF6) | 25,200 | ~3,200 years | 25,200 | Electrical insulation, magnesium production | Highest GWP among common greenhouse gases due to its long persistence and strong infrared absorption |
Solutions to Reduce NOx Emissions