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:

  1. 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.

  2. Contribution to Smog Formation:

    NOx is a key component in the formation of smog, which reduces visibility and harms respiratory health.

  3. 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 GasGWP (100 years)Persistence Level (Years)CO2e Equivalent per Ton of EmissionTypical SourcesNotes
Carbon Dioxide (CO2)1Variable (up to thousands of years)1Fossil fuel combustion, deforestation, industrial processesReference gas for GWP
Methane (CH4)27.9 (fossil sources); 28-36 (range)~12 years27.9 - 36Livestock, rice paddies, landfills, fossil fuel extractionHigh GWP due to strong absorption of infrared radiation
Nitrous Oxide (N2O)273~114 years273Agricultural activities, fossil fuel combustion, industrial processesSignificant GWP due to long atmospheric lifetime and potent greenhouse effect
HFC-23 (CHF3)12,400~270 years12,400Refrigeration, air conditioning, industrial processesExtremely high GWP due to strong absorption and long persistence
HFC-134a (C2H2F4)1,300~14 years1,300Refrigeration, air conditioningCommonly used HFC with high GWP
HFC-152a (C2H4F2)138~1.5 years138Aerosol propellants, foam blowing agentsLower GWP compared to other HFCs, but still significant
Perfluoromethane (CF4)6,630~50,000 years6,630Aluminium production, electronics manufacturingExtremely long atmospheric lifetime
Perfluoroethane (C2F6)11,100~10,000 years11,100Aluminium production, electronics manufacturingVery high GWP and long atmospheric lifetime
Sulfur Hexafluoride (SF6)25,200~3,200 years25,200Electrical insulation, magnesium productionHighest GWP among common greenhouse gases due to its long persistence and strong infrared absorption

 

Solutions to Reduce NOx Emissions