New evidence shows that ammonia emissions from road vehicles are underestimated in the UK national inventory. Remote sensing was used to measure ammonia emissions from over 230,000 vehicles on road under real driving conditions. Estimates of total ammonia emissions from gasoline passenger cars in the UK are around 2.6 times higher than reported in the national inventory, with emissions in urban areas underestimated by a factor of 17. Remote sensing data provides insight into factors affecting ammonia emissions like vehicle model year, mileage, manufacturer, and driving conditions like cold starts. More accurate vehicle emission estimates are needed to understand impacts on air quality and public health.

1. New evidence on ammonia emissions from road vehicles
Naomi Farren
Wolfson Atmospheric Chemistry Laboratories, University of York, YO10 5DD
naomi.farren@york.ac.uk
27th September 2022
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2. Ammonia - a priority air pollutant
Ammonia (NH3) is a gaseous air pollutant that reacts rapidly with
nitric acid (HNO3) and sulphuric acid (H2SO4) to form fine airborne
particles.
Multiple harmful health effects arise when these particles are inhaled.
Deposition of excess nitrogen from atmospheric NH3 contributes to
exceedance of critical loads for eutrophication and reduction of plant
biodiversity.
Regulations
By 2030, CLRTAP and NECR require the UK to reduce NH3 emissions by 16%
compared to baseline emissions in 2005. The 2020 requirement to reduce emissions by
8% since 2005 was not met. Emissions were exceeded by 1.6 thousand tonnes of NH3.
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3. Ammonia emissions from road vehicles
Gasoline vehicles are equipped with three-way catalysts used to minimise CO,
hydrocarbon and NOx emissions. NH3 emissions are an unintended by-product
when excess reduction of engine-out NO occurs.
Selective catalytic reduction systems in diesel vehicles use NH3 (from urea) to
reduce NOx emissions. ‘Ammonia slip’ can occur if the amount of urea injected
exceeds the NOx emitted.
Agricultural emissions account for approximately 87% of total UK NH3 emissions
whereas vehicular NH3 are estimated to account for less than 2%.
Unlike agricultural emissions, NH3 emissions from road vehicles tend to be
co-emitted with NOx. They likely have a more effective pathway to particle
formation in urban environments compared to NH3 from farming activities in
rural, low NOx regions.
Priority research area
A better understanding of NH3 emissions from road vehicles in urban areas is important
due to their disproportionate significance for air quality and public health.
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4. Vehicle emission remote sensing
Remote sensing used to measure NH3 emissions from road vehicles under real
driving conditions.
UV and IR absorption spectroscopy provides a measure of the amount of
pollutants (NO, NO2, CO, HC and NH3) present in individual exhaust plumes.
Emissions are expressed as ratios to CO2, which can be converted to fuel-specific
emission factors (g pollutant per kg fuel).
Graphic by Jack Davison
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5. Vehicle emission remote sensing
A camera photographs each vehicle registration plate, which is used to obtain
vehicle technical information for each measurement, including fuel type, Euro
class, vehicle manufacturer, registration date, mileage and so on.
Over 230,000 passenger car measurements at 37 sites between 2017 and 2020.
Measurements are representative of the UK fleet and cover a vast range of
driving conditions.
Graphic by Jack Davison
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6. Estimating total vehicular NH3 emissions
The UK National Atmospheric Emissions Inventory (NAEI) estimate
of total NH3 emissions from road transport was 4.4 kt in 2018,
accounting for 1.6% of total NH3 emissions.
This is based on a limited number of measurements conducted mainly
under laboratory conditions.
Here we explore whether remote sensing measurements can be used
to improve the estimate of total UK vehicular NH3 emissions.
We focus on gasoline passenger car emissions of NH3 since the
remote sensing measurements suggest that diesel passenger cars make
a minor contribution to total passenger car NH3 emissions.
Two main approaches are used to compile emission inventories -
top-down vs. bottom-up.
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7. Calculating UK NH3 emissions from gasoline passenger cars
Top-down
Total UK annual gasoline sales
data, a quantity known to high
accuracy
Estimate total CO2 emissions
for gasoline passenger cars
Multiply by average NH3/CO2
ratio from remote sensing
measurements
Bottom-up
Derive distance-based emission
factors (g/km) for a range of
vehicle models
Combine with UK mileage data
to derive total NH3 emissions
Fleet statistics from remote
sensing measurements used to
partition mileage data
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Distance-based emission factors from vehicle emission remote sensing
measurements, J. Davison et al., Science of the Total Environment, 2020, 739, 139688.

8. Total UK NH3 estimates
Top-down estimate of 8.1 kt of NH3 emitted from gasoline passenger
cars in 2018.
Bottom-up estimate of 7.8 kt kt of NH3.
35.3 Mt of CO2 from the bottom-up approach is in excellent
agreement with the top-down estimate of 34.8 Mt, which is derived
from total UK gasoline fuel sales and is a quantity known to high
accuracy.
A key advantage of the bottom-up approach is that emissions can be
calculated at subnational scales e.g. urban areas. Vehicular NH3
emissions attributed to urban areas are found to contribute to 52%
of total UK vehicular emissions.
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9. Comparison with the UK national inventory
Estimated that total NH3 emissions from gasoline passenger cars are
a factor of 2.6 times higher than is reported in the NAEI (7.8 kt
year-1 compared with 3.0 kt year-1 for 2018).
The remote sensing data suggests that 52% of the total NH3
emissions from gasoline passenger cars can be attributed to urban
areas, compared to 9% estimated by the NAEI, suggesting emissions
of NH3 from road vehicles are underestimated by a factor of 17 in
urban areas.
The road transport contribution to total UK NH3 is still small, but
NH3 emissions from road vehicles are co-emitted with NOx. This
could have important atmospheric chemistry implications due to the
potentially more efficient route to particle formation.
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10. Distance-specific CO2 and NH3 emissions
Distance-specific emission factors (g/km) for passenger cars, grouped by fuel type,
Euro class and driving condition.
Potential use for air quality modelling and nitrogen deposition studies, and
national inventory development in other countries.
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11. Gasoline and gasoline hybrid cars
Average fuel-specific NH3 emissions by vehicle model year for gasoline and
gasoline hybrid passenger cars.
Based on over 200,000 measurements across a wide range of driving conditions.
We can use remote sensing data to adjust hybrid emission factors to account for
the fact that a proportion of the vehicle km is driven in battery mode.
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12. Accounting for hybrid battery use
Fraction of invalid CO2
remote sensing
measurements gives unique
insight into hybrid battery
use and can be used to
adjust the measured NH3
emission factors.
Model the relationship
between vehicle specific
power and percentage of
time spent using batteries.
Battery use for hybrid
electrics and PHEVs
predicted to be 29% and
42% respectively under
urban driving conditions.
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13. Emissions by vehicle mileage
Records of vehicle mileage at
the last MOT test can be
matched to emission
measurements.
Direct measure of vehicle
usage.
Emissions of NH3 increase as
vehicle mileage increases, close
to linearly, up to around
100,000 km.
Useful for the development of
emission factors and national
inventories.
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14. Cold start effects
A potentially important factor affecting NH3 emissions is the effect of cold-starts.
Initial investigation shows higher NH3 emissions when the engine is likely to be
cold, by a factor of 2–3 for Euro 5 and Euro 6 gasoline passenger cars.
The majority of car journeys begin in urban areas, which could be important due
to a more effective pathway to particle formation.
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15. Emissions by vehicle manufacturer
Remote sensing measurements allow us to investigate the effect of vehicle
manufacturer on emissions.
Emissions of NH3 for Euro 5 gasoline and gasoline hybrid passenger cars by
manufacturer group.
More than a factor of 3 difference across manufacturer groups. Manufacturer
composition of the fleet plays an important role and will vary significantly in
different countries.
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16. Concluding remarks
Total UK gasoline passenger car NH3 emissions are underestimated by
a factor of 2.6 compared to the NAEI. In urban areas, emissions are
predicted to be underestimated by a factor of 17.
Remote sensing measurements provide insight into the various factors
affecting vehicular NH3 emissions. Further research is needed to fully
understand these effects.
The underestimation of NH3 emissions from road vehicles likely
applies to many other European countries that use COPERT emission
factors as a basis for national inventory development.
Detailed regional modelling is needed to consider the implications for
nitrogen deposition and particle formation, particularly in urban areas.
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17. Acknowledgements
Ricardo Energy and Environment: thanks to the remote sensing field team for
collecting data using the Opus instrument (Ben Fowler, Tom Green, Les Phelps,
Sam Copsey, Paraic Marry, Sion Carpenter, Susannah Telfer) and to Rebecca Rose
and Tim Murrells for the emission inventory work.
University of York: thanks to Adam Vaughan, Stuart Young, Will Drysdale for
the collection of remote sensing data and to Jack Davison and David Carslaw for
the data analysis work.
University of Denver: thanks to Gary Bishop for the use of the remote sensing
instrument.
Publications
Underestimated Ammonia Emissions from Road Vehicles, Farren et al.,
Environ. Sci. Technol., 2020, 54, 24, 15689–15697.
Characterisation of ammonia emissions from gasoline and gasoline hybrid
passenger cars, Farren et al., Atmospheric Environment: X, 2021, 11, 100117.
naomi.farren@york.ac.uk
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18. Speed-emission curves
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