Table 3 Gases present in exhaust fumes, their likely worst-case concentrations and level of absorbance over a 1m pathlength.

Water, H₂O

6.6%

90%RH at 40 °C

1.8 × 10^–3

65

Carbon dioxide, CO₂

1%

Mixed exhaust/air, TRL data

4.1 × 10^–4

 < 6.2 × 10^–4

Carbon monoxide, CO

20 ppm

Worst-case 8 h exposure limit²⁹. UK consistently below 10 μg m^−3 30

–

–

Nitrogen dioxide, NO₂

0.5 ppm

Worst-case hourly average can be 700 μg m⁻³³¹, or 0.34 ppm

–

1.4 × 10^–3

Nitric oxide, NO

500 ppm (4000 ppm occasionally)

TRL Euro V and Euro VI truck data

–

3.9 × 10^–4 (0.0031 occasionally)

Ammonia, NH₃

100 ppm (900 ppm occasionally)

TRL Euro V and Euro VI truck data

3.6 × 10^–5

0.035 (0.32 occasionally)

Ozone, O₃

10 ppb

8-h running mean is below 8μg m^−3 30, corresponding to 2.3ppb

Sulfur dioxide, SO₂

2 ppb

Annual mean concentrations below 5 μg m^−3 30, corresponding to 1.75ppb

–

–

Methane, CH₄

100 ppm

Large outdoor natural gas leak, inferred from³²

1.3 × 10^–3

2.7 × 10^–4

Ethane, C₂H₆

10 ppm

As above: max 10% of ethane

2.9 × 10^–5

–

Propane, C₃H₈

3 ppm

As above, also present in calor gas

1.1 × 10^–5

–

Butane, C₄H₁₀

2 ppm

As above, also present in calor gas

9.3 × 10^–6

–

Pentane, C₅H₁₂

1 ppm

As above

6.4 × 10^–6

–

Octane, C₈H₁₈

100 ppm

Present in petrol, also indicative of other medium hydrocarbons

1.3 × 10^–3

1.4 × 10^–4

Methanol, CH₃OH

100 ppm

Common solvent, indicative hydrocarbon

2.6 × 10^–4

2.9 × 10^–4

Benzene, C₆H₁₂

2 ppb

UK meets the limit value of 5 μg m^−3 30, corresponding to 1.4 ppb

–

–