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Corresponding uncertainty

What the reports say


What this means in terms of ‘confidence’

The model has been adequately adjusted to take into account the influence of hourly, daily, and monthly variation

Aircraft engine emissions, particularly those of NOx, vary with ambient temperature, pressure and humidity.  

Many aspects of Heathrow operations and associated traffic are untypical of other developments re. temporal variation.

“Annual-average emission rate is not the only parameter of interest in air quality assessment, even when calculating annual-mean concentrations: the diurnal and seasonal variation in emissions is also important, given that the frequency of meteorological conditions leading to better (or worse) atmospheric dispersion varies with hour of day and month of year. QinetiQ found that, for the most sensitive type of engine, the hourly NOx emission rate at a given thrust varied over the year by up to ±50% from the value calculated assuming ISA conditions...  On an hour-by-hour basis, however, much larger changes are observed, with the ratio of ground level emissions (revised/ previous) in a given hour ranging from 0.84 to 1.14 at the 5th/95th percentile...”

The NOx prediction model includes assumptions about ground-level temperature and pressure.

“The movement data for future cases provide no information on day-of-the-week or monthly variation.  Based on current LHR profiles, it was judged adequate to assume that the distribution of movements of a given aircraft type by day of the week and month of the year is uniform for all future cases”.

Diurnal profiles of roads are representative ones by road type; they could be different in practice.

DfT Chapter 1 – Synthesis of Key Issues and Findings, Table 1.1

AEA Revised Emissions Methodology, 2.7, 2.12, E.13

AEA Energy & Environment, Emissions Methodology for Future LHR Scenarios, S.10

The NOx prediction model has included assumptions about ground-level temperature, pressure and humidity.  However it is quite possible that actual conditions will be different from those assumed.

Temporal variations in the use of different types of aircraft have not been modelled; and temporal change has only been roughly modelled for roads.  It is unclear whether and how this affects confidence in the model.

Car and lorry engines will become much cleaner as a result of

Vehicle engines are becoming much cleaner; the question is how much.  The model assumes that vehicles

Road vehicle fleet emissions changes for Euro VI come with a wide range.  The assessments have used the least optimistic end of this range.

Euro standards have not led to expected emission

Atkins (2007) Demonstrating confidence in the PSDH air quality work,

Vehicles’ actual NOx emissions can be much greater than their Euro standard; the full reduction in NOx due

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