Alison Redington* and Derrick Ryall*

Dick Derwent**

* Met Office, Exeter, United Kingdom

** rdscientific, Newbury, United Kingdom

EMEP Workshop on PM Measurement and Modelling

New Orleans, April 2004

MODELLING PARTICULATE SULPHATE AND NITRATE IN NORTH WEST EUROPE

WITH A LAGRANGIAN DISPERSION MODEL

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Belfast Centre

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Southampton Centre

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Target air quality

PM10 TRENDS IN THE UNITED KINGDOM IN URBAN CENTRES

What are the levels, sources and characteristics of PM10 and PM2.5 in the UK ?

What are the trends in PM10 and PM2.5 ?

What are the extent of exceedances of air quality targets currently and in the future ?

Some form of modelling is required to answer these questions.

UK POLICY-MAKERS CONCERNS

Primary vs secondary and inorganic vs organic particulate matter

Particulate sulphate and nitrate is main focus of this study Role of long range transboundary transport and local formation Application of the UK Met Office NAME Lagrangian dispersion

model Aim is to give source attribution to particulate sulphate and

nitrate at 15 minutes time resolution and 15 km spatial resolution

MODELLING SUSPENDED PARTICULATES IN THE UNITED K INGDOM

NAME MODEL

UK Met Office’s operational dispersion model (1-1000’s kms) Lagrangian - pollutant modelled by large numbers of ‘parcels’

released into the ‘model’ atmosphere Model driven by meteorological fields from the Met Office’s

operational forecast model Particles are transported by local mean wind in 3-dimensions Diffusion by turbulence is represented by random walk

techniques, displacing particles in both the horizontal and vertical

LAGRANGIAN DISPERSION MODEL

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43°N

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emissions grid

Long-range transport and dispersion of an inert tracer

ReceptorSource

max O3

125 ppb11/08/03

FORMATION OF SECONDARY INORGANIC AEROSOLS

SO2 + OH = HOSO2

HOSO2 + O2 = HO2 + SO3

SO3 + H2O = H2SO4 = sulphate aerosol

SO2aq + H2O2 = H2SO4 = sulphate aerosol

SO2aq + O3 = H2SO4 = sulphate aerosol

NO2 + OH = HNO3 = nitrate aerosol

NO2 + O3 = NO3 + O2

NO2 + NO3 = N2O5 = nitrate aerosol

NH3aq + HNO3aq = NH4NO3aq = nitrate aerosol

Emit some new air parcels, each loaded up with SO2 and NOx

Move air parcels to new locations with 3-d turbulent wind fields Locate air parcels in Eulerian grid Calculate air concentrations in Eulerian grid Allow for chemical transformations and deposition Recalculate air parcel masses

This is the main time-stepping algorithm in a source-oriented Lagrangian dispersion model.

OPERATIONS IN A LAGRANGIAN DISPERSION MODEL

ANNUAL AVERAGE SO2 CONCENTRATIONS - 1996

Compares well with EMEP observations for 1996

ANNUAL AVERAGE PARTICULATE SULPHATE CONCENTRATIONS - 1996

Compares well with observations for 1996

ANNUAL AVERAGE NO2 CONCENTRATIONS - 1996

Compares well with rural observations for 1996

ANNUAL AVERAGE HNO3 CONCENTRATIONS - 1996

No observations for direct comparison

ANNAUL AVERAGE PARTICULATE NITRATE CONCENTRATIONS - 1996

No observations for direct comparison

ANNUAL AVERAGE SECONDARY INORGANIC AEROSOL - 1996

Tendency to overestimate rural PM10 observations

17 00/XXXX © Crown copyright

EMEP MONITORING SITE NETWORK

Site Correlation Bias, g SO4 m-3

NMSE % within a factor of 2

Strathvaich 0.36 -0.25 3.23 21.3 Yarner Wood 0.32 1.20 2.93 40.1 London 0.35 -1.94 1.30 45.8

STATISTICS FOR EVALUATION OF DAILY MEASURED AND MODELLED PARTICULATE SULPHATE FOR 1996

agreement is somewhat disappointing, over-prediction during wintertime, lack of background sulphate from North Atlantic

Site species Bias,

g m-3 NMSE % within

a factor of 2

Strathvaich HNO3 -0.14 4.27 33.3 Strathvaich NO3 0.17 1.12 54.5 Yarner Wood HNO3 -0.52 2.34 16.7 Yarner Wood NO3 0.97 0.92 36.4

STATISTICS FOR EVALUATION OF MONTHLY MEASURED AND MODELLED HNO3 AND NO3 FOR 1999-2000

nitrate aerosol is slightly over-predicted, nitric acid is under-predicted and shows poorer performance, however the data are inadequate

21 00/XXXX © Crown copyright

EMEP MONITORING SITE NETWORK

WINTERTIME AND SUMMERTIME MODEL PERFORMANCE

timing of peaks is excellent, but overestimation during winter

23 00/XXXX © Crown copyright

SOURCE ATTRIBUTION

Each air parcel emitted into the NAME model keeps a record of the location where it was emitted

It is straightforward to construct a map showing the origins of the particulate sulphate and nitrate found at any location in the model

The source allocation given for secondary pollutants refers to the origins of the primary pollutant precursors

Yarner Wood 2002

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MEAN DIURNAL VARIATIONS IN PARTICULATE NITRATE DURING 10 DAYS IN MAY 2003 AT A RURAL EMEP SITE AT

HARWELL OXFORDSHIRE UK

Preliminary data kindly provided by Steve Moorcroft, Casella Stanger

Model development is severely hampered by lack of good observations of the individual components of PM10 and PM2.5

These need to be of hourly time resolution and co-located with other air quality measurements

Artefact-free nitrate observations are particularly sparse in Europe and must distinguish ammonium nitrate from sodium and calcium nitrates

Cloud liquid water content, low cloud amount, precipitation amounts and boundary layer depths are difficult quantities to obtain from meteorological models with sufficient accuracy for secondary particle modelling

CONCLUSIONS

To the United Kingdom Department for Environment Food and Rural Affairs for their support through contract CPEA 7

To Alison Redington and Derrick Ryall United Kingdom Met Office for their patient work with the NAME model

To Steve Moorcroft, Casella Stanger for contributing his preliminary continuous observations of particulate nitrate

To members of the United Kingdom Air Quality Expert Group for their helpful discussions

To Environment Canada for their generous offer of help with travel costs

ACKNOWLEDGEMENTS