---
title: "NOx / SOx / CO"
description: "NOx (nitrogen oxides — NO and NO₂), SOx (sulphur oxides — primarily SO₂ with smaller SO₃), and CO (carbon monoxide) are the principal regulated gaseous emissions from combustion plants, alongside particulate matter. All three are measured continuously by CEMS and permit-limited under most jurisdictions' emission codes."
canonical_url: "https://sylio.co/glossary/nox-sox-co"
last_updated: "2026-06-28T02:29:34.533Z"
---

**NOx** (nitrogen oxides — NO and NO₂), **SOx** (sulphur oxides — primarily SO₂ with smaller SO₃), and **CO** (carbon monoxide) are the principal regulated gaseous emissions from combustion plants, alongside [particulate matter](/glossary/particulate-matter). All three are measured continuously by [CEMS](/glossary/cems) and permit-limited under most jurisdictions' emission codes.

## Sources and controls

<table>
<thead>
  <tr>
    <th>
      Pollutant
    </th>
    
    <th>
      Formation
    </th>
    
    <th>
      Control
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      <strong>
        NOx
      </strong>
    </td>
    
    <td>
      Thermal NOx (high flame temperature) + fuel NOx
    </td>
    
    <td>
      Combustion control + <a href="/glossary/selective-catalytic-reduction">
        SCR
      </a>
      
       / <a href="/glossary/selective-non-catalytic-reduction">
        SNCR
      </a>
    </td>
  </tr>
  
  <tr>
    <td>
      <strong>
        SO₂
      </strong>
    </td>
    
    <td>
      Fuel sulphur oxidation
    </td>
    
    <td>
      Fuel selection + FGD (wet scrubber, dry sorbent injection)
    </td>
  </tr>
  
  <tr>
    <td>
      <strong>
        SO₃
      </strong>
    </td>
    
    <td>
      Catalytic oxidation of SO₂ over V₂O₅ in SCR
    </td>
    
    <td>
      Catalyst formulation + temperature control
    </td>
  </tr>
  
  <tr>
    <td>
      <strong>
        CO
      </strong>
    </td>
    
    <td>
      Incomplete combustion
    </td>
    
    <td>
      Combustion control (excess air, residence time, temperature)
    </td>
  </tr>
</tbody>
</table>

## How cleaning affects gaseous emissions

[Sonic horns](/glossary/sonic-horn) do not directly capture gaseous pollutants but support gaseous-emission control indirectly:

- **NOx** — clean [SCR catalysts](/glossary/selective-catalytic-reduction) achieve their rated NOx reduction; fouled catalysts under-perform
- **SO₃ and downstream ABS** — keeping the cold end clean reduces [ammonium-bisulphate](/glossary/ammonium-bisulphate) accumulation
- **CO** — preserved boiler performance maintains stable combustion

## Related terms

- [Selective Catalytic Reduction (SCR)](/glossary/selective-catalytic-reduction)
- [CEMS](/glossary/cems)
- [Opacity](/glossary/opacity)
- [Industrial Emissions Directive (IED)](/glossary/industrial-emissions-directive)