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Increasingly emissions from ammonia (NH3) occur as a byproduct from NOx abatement measures in petrol and diesel vehicles (e.g. three-way catalytic converter, Selective Catalytic Reduction).

There is consequently an increasing requirement to consider emissions of NH3 when undertaking assessments which impact road traffic numbers close to sensitive ecological receptors or designated ecological sites.

As NH3 is not a pollutant that currently falls under the Local Air Quality Management (LAQM), there are currently no Defra-approved emission factors for ammonia emissions from vehicles (for example, via the Emission Factors Toolkit) or an indication how it should be assessed.

Practitioners have therefore taken it upon themselves to develop their own tools and methods to calculate ammonia concentrations arising from traffic.

There are currently three known approaches, each proposing different emission factors, with no consensus as to which is most appropriate.

Neither Natural England nor Defra can comment on the appropriateness of each method or endorse a particular approach, in part due to our lack of knowledge of roadside ammonia concentrations in practice, and how the models reflect these.

The National Ammonia Monitoring Network includes only a single roadside monitoring location.

There is therefore limited empirical evidence with regards to NH3 concentrations associated with emissions from road traffic sources.