Analysis and evaluation of selected local-scale PM10 air pollution episodes in four European cities: Helsinki, London, Milan and Oslo

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Atmospheric Environment 39 (2005) 2759–2773

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Analysis and evaluation of selected local-scale PM10 airpollution episodes in four European cities: Helsinki, London,

Milan and Oslo

Jaakko Kukkonena,�, Mia Pohjolaa, Ranjeet S Sokhib, Lakhu Luhanab,Nutthida Kitwiroonb, Lia Fragkoub, Minna Rantamakia, Erik Bergec,Viel Ødegaardd, Leiv Havard Slørdale, Bruce Denbye, Sandro Finardif

aAir Quality Research, Finnish Meteorological Institute, Sahaajankatu 20 E, Helsinki, FinlandbUniversity of Hertfordshire, ASRG, Science and Technology Centre, Hatfield, UK

cKjeller Vindteknikk, P.O.Box 122, 2007 Kjeller, NorwaydNorwegian Meteorological Institute, Postboks 43, Oslo, NorwayeNorwegian Institute for Air Research, Postboks 100, Norway

fARIANET, via Gilino 9, 20128 Milano, Italy

Received 18 February 2004; received in revised form 8 September 2004; accepted 20 September 2004

Abstract

We have analysed in detail four selected episodes involving substantially high concentrations of PM10 that occurred in

Oslo on 4–10 January 2003, in Helsinki on 3–14 April 2002, in London on 18–27 February 2003 and in Milan on 14–19

December 1998. We have also utilised a more extensive dataset containing relevant information regarding 21 episodes

from seven cities in six countries. The four episodes analysed in detail were recently occurring cases that were at least

partly caused by various local emission sources. In particular, we have addressed the evolution of the measured

concentrations in terms of the measured, meteorologically pre-processed and predicted (using numerical weather

prediction models and a meso-scale meteorological model) meteorological variables. All the four episodes addressed were

associated with areas of high pressure (Oslo, Helsinki and London) or a high-pressure ridge (Milan). The best

meteorological prediction variables were found to be the temporal evolution of the temperature inversions and

atmospheric stability and, in some of the cases, wind speed. Strong ground-based or slightly elevated temperature

inversions prevailed in the course of the episodes in Oslo, Helsinki and Milan, and there was a slight ground-based

inversion also in London; their occurrence coinciding with the highest PM10 concentrations. The same result was also

obtained by considering an additional set of seven PM10 episodes from the larger dataset. The inversions in Oslo and

Milan were mainly caused by the advection of warmer air above a relatively colder surface, and that in Helsinki by

radiation cooling of snow-covered ground. It was also found that a low wind speed is not necessarily a good indicator of

episodes; this is the case, e.g., in the Po valley, due to the frequently occurring calm and low wind speed conditions there.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Episode; Urban; PM10; Inversion; Forecasting

e front matter r 2005 Elsevier Ltd. All rights reserved.

mosenv.2004.09.090

ing author. Tel.: +358 9 1929 5450; fax: +3589 1929 5403.

ess: [email protected] (J. Kukkonen).

ARTICLE IN PRESSJ. Kukkonen et al. / Atmospheric Environment 39 (2005) 2759–27732760

1. Introduction

European cities have major problems in coping with

the current, and especially the indicative stage-two air

quality limit values set in the Council Directive 1999/30/

EC for concentrations of PM10. The EU Directives

require practical measures to be taken, if air quality

limits are exceeded. The current and future situation for

PM10 concentrations was recently assessed in the

AutoOil-II programme (http://europa.eu.int/comm/en-

vironment/autooil/index.htm). The conclusion reached

was that by 2010, PM10 limit values are expected to be

exceeded in possibly half the European cities. There is,

therefore, an urgent need for action at European as well

as on national and local scales in order to reduce

particulate matter (PM) concentrations.

Under episodic conditions, concentrations of air

pollutants can considerably exceed national and inter-

national standards and limit values. Available informa-

tion concerning European peak pollution episodes in 13

countries has been reviewed within the COST 715

Action ‘‘Meteorology applied to Urban Air Pollution

Problems’’ (e.g., Fisher et al., 2001) by Kukkonen

(2001a). However, this report does not include a

systematic analysis of the meteorological conditions in

the course of such cases.

The causes of air pollution episodes include various

factors, e.g., emissions, local and synoptic-scale meteor-

ological conditions, topography and atmospheric che-

mical processes. The relative importance of such factors

is dependent on the geographical region, its surrounding

emission source areas and the related climatic character-

istics, as well as the season of the year (Piringer and

Kukkonen, 2002). For example, in many cities PM

episodes are experienced in winter and spring. Nitrogen

dioxide episodes can occur both in winter and in

summer, and ozone levels can be particularly high

during summer periods. Previously, the present authors

have analysed selected specific historic episodes in Oslo

(Berge et al., 2002), Helsinki (Pohjola et al., 2004) and

various Northern Italian cities (Finardi et al., 2002).

Berge et al. (2002) presented results from numerical

weather prediction (NWP) modelling in Northern

Europe during strong wintertime inversions. They

utilised the numerical results produced by two NWP

models (High-Resolution Limited Area Model (HIR-

LAM) and European Centre for Medium-range Weath-

er Forecasts (ECMWF)), combined with the non-

hydrostatic MM5 model. The numerical runs for Oslo

using 10, 3 and 1 km nesting revealed the necessity of

high resolution for resolving topographical features.

They also simulated an episode in Helsinki in December

1995 that involved an extremely strong ground-based

temperature inversion, and compared the predicted

results with those measured at the radio tower Kiven-

lahti.

At present, the formation and evolution of air

pollution episodes is only understood in general terms.

A more thorough analysis and understanding would be

needed, before it would be possible to forecast such

cases with sufficient reliability and accuracy. The present

paper aims to improve such an understanding. Actually,

present-day models for predicting or forecasting air

pollution episodes have serious deficiencies (e.g., Bakla-

nov et al., 2002). Major international efforts, such as

COST 715, the EU-funded FUMAPEX project (Inte-

grated Systems for Forecasting Urban Meteorology, Air

Pollution and Population Exposure, http://fumapex.d-

mi.dk/) and COST 728, have therefore been initiated in

order to reduce these deficiencies.

This study is partially based on an inventory of

episodes that has been compiled within COST 715 and

the FUMAPEX project (Valkama and Kukkonen,

2004). A total of 21 episodes from seven cities or

metropolitan areas in six countries were examined. The

cities include Castellon (Spain), Helsinki (Finland),

Turin (Italy), Bologna (Italy), Oslo (Norway), London

(UK) and Paris (France). These cities represent North-

ern, Western (Atlantic and continental) and Mediterra-

nean regions of Europe.

Based on the above-mentioned survey, an overview

of European air quality episodes is presented in

Tables 1a–b. The episodes can be broadly categorised

into two classes according to the scale of the main source

areas, i.e., those originating mainly from local emissions,

and those connected with regionally and long-range

transported (LRT) polluted air masses. The local-scale

episodes can be further classified as those caused

predominantly by either mobile or stationary sources.

The so-called ‘‘spring dust episodes’’ refer to those

caused by PM that is suspended from road and street

surfaces; these cases are especially characteristic of

northern parts of Europe.

The larger-scale episodes can be schematically classi-

fied as either those involving photochemical pollution

from both local, and regional and LRT sources, or those

caused by LRT air masses. Photochemical episodes are

especially prevalent in Southern European cities. These

may characteristically involve recirculation of air

masses, caused by meso-scale meteorological (MM)

effects (such as the land-sea breeze) and orographic

flows. Episodes can also be influenced by the interaction

of phenomena at different meteorological scales; e.g.,

the meso-scale forces can perturb the synoptic condi-

tions (e.g., Millan, 2002; Gangoiti et al., 2002). The

transport of dust from the Saharan desert, and from

Iraq, Kuwait, and Saudi Arabia has been addressed by

Grigoryan and Erdman (1996), and Draxler et al.

(2001), respectively.

The present paper is based on the above-mentioned

inventory of episodes, but undertakes a more detailed

evaluation of selected PM10 episodes. We have analysed

ARTIC

LEIN

PRES

S

Table 1a–b

Classification of European air quality episodes

1. Local-scale episodes

Characteristic pollutants Characteristic season of the year Characteristic meteorological

factors

Main emission source category European regions affected

1.1 Wintertime inversion-induced episodes

PM10, PM2.5 Winter half-year Inversion, stable stratification Local traffic All regions

NO2, CO Winter half-year Inversion, stable stratification,

low wind velocity

Local traffic All regions

1.2 Spring dust episodes

PM10 Spring, autumn Dry conditions, melting snow Suspended dust, mainly due to

local traffic

All regions, except for southern

Europe

1.3 Episodes caused in part or wholly by stationary emission sources

PM10, PM2.5, SO2 Winter, spring Inversion, stable stratification,

low wind velocity

Local stationary sources All regions

2. Regional and long-range transport episodes

Characteristic pollutants Characteristic season of the year Characteristic meteorological

factors

Main emission source category European regions affected

2.1 Photochemical pollution episodes

O3, PM10, PM2.5, other

photochemical species

Summer half-year High pressure, strong radiation,

recirculation

Various precursor emissions, on

a regional and urban scale

Especially in Southern Europe

2.2 Long-range and regional transport episodes

PM2.5 All seasons Stable stratification, low wind

speed

Primary and precursor

emissions, on LRT and regional

scales

All regions

PM10 Winter half-year Stable stratification, low wind

speed

Primary emissions, on LRT and

regional scales

All regions

NO2, CO, SO2 Winter half-year Stable stratification, low wind

speed

Primary or precursor emissions,

on LRT and regional scales

All regions

Local-scale episodes are presented in the upper table, while episodes that are caused mainly by regional and long-range transport appear in the lower table.

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ARTICLE IN PRESSJ. Kukkonen et al. / Atmospheric Environment 39 (2005) 2759–27732762

the episodes in four European cities in relation to

prevailing meteorological conditions, local emissions,

and regionally and LRT background concentrations.

We have primarily analysed PM10 concentrations

instead of those of PM2.5, due both to the better

availability of experimental data, and the fact that the

European Union has only issued PM air quality

directives for the PM10 fraction. However, the PM2.5concentrations are included in the analysis wherever

such measurements were available.

The literature on urban air pollution episodes is

scarce, especially that regarding their systematic char-

acterisation in terms of the prevailing meteorological

conditions. Furthermore, in previous literature usually

specific episodes have been analysed separately, or, at

best, a few episodes within one specific country. In the

present study, we have aimed at a structured and

homogeneous analysis in all the four cities. A particular

aim of this study is to gain a better insight of the

influence of various meteorological variables on the

evolution of high pollutant concentrations in European

cities.

2. Material and methods

The cities considered in this study are located in

geographic and climatic regions of Northern (Oslo and

Helsinki), Northwestern (London) and Southern (Mi-

lan) Europe. These areas represent maritime climate

(London and Oslo), a partly maritime-influenced and

partly continental climate (Helsinki), and a mainly

continental climate (Milan). Two of the cities are located

in or surrounded by mountainous terrain (Oslo and

Milan, respectively) and two cities are situated in fairly

flat areas (Helsinki and London). London and Milan are

amongst some of the largest cities in Europe (their

populations are 7.5 and 3.5 million, respectively), while

the metropolitan areas of Oslo and Helsinki are

relatively smaller conurbations (the populations of both

of these are approximately 1.0 million).

For the analysis we have selected recently occurring

(1998–2003) episodes that were predominantly caused

by various local emission sources. Study by Valkama

and Kukkonen (2004) has indicated that episodes having

similar levels of concentrations and belonging to the

same episode category (described above) tend to occur

regularly every year, or at least every other year. Hence,

the episodes considered in this paper may be considered

as being characteristic of each region.

Corresponding spring dust episodes have occurred in

Helsinki every year during the last few years, sometimes

several times in the same spring. Spring dust episodes

are the most common category of PM10 episodes in

Helsinki. In Oslo, both ‘‘wood-burning’’ and ‘‘road-

dust’’ episodes of varying strengths are to be expected

every winter or spring. The wood-burning episodes

characteristically occur in cold, calm and stable winter

conditions, in which the ground is covered with ice and

snow, and the traffic- induced suspension of road dust is

therefore of minor importance. In Milan, from one to

two corresponding PM10 episodes have occurred every

winter during recent years; these events are characterised

by simultaneous infringements of PM10 and NO2 limit

values, and by similar meteorological conditions (Fi-

nardi and Pellegrini, 2004).

Some published studies on UK episodes (Ryall et. al.,

2002; Malcolm et. al., 2000) have shown that they can

arise because of local emissions (such as traffic or

industrial emissions) under stagnant conditions as well

as because of contributions from long-range transport of

polluted air masses including Saharan dust storms.

However, these studies have considered the UK as whole

and have not focused on urban areas. An analysis of the

occurrence of elevated 24 hour mean concentrations of

PM10 in London (that is, concentrations greater than

50mgm�3 lasting for two or more days) for 2000–2003

has shown that five out of eight episodes occurred

during late winter or spring periods.

2.1. Topography of the cities and the main emission

sources

The city of Oslo is located at the northern end of the

Oslo fjord, surrounded by a large topographical pot

formation. The topographical features of the area tend

to worsen the dispersion conditions, capturing pollu-

tants emitted within the urban airshed. The most

important local sources of PM in Oslo are domestic

wood-burning in stoves that are used for wintertime

house heating, and vehicular traffic (Laupsa and

Slørdal, 2002). The influence of wood-burning PM

emissions is most dominant in the densely populated

central city area, where a large fraction of the flats are

equipped with strongly polluting old stoves.

The city of Helsinki and its surrounding regions are

situated in a fairly flat coastal area. The PM10concentrations in street level air are dominated by the

combustion, non-combustion and suspension emissions

originating from vehicular traffic (e.g., Kukkonen et al.,

2001b). The influence of small-scale local sources, such

as that of domestic wood burning, is negligible.

London is situated in relatively flat terrain with

shallow hills to the west and south. London is one of

the most congested cities in Europe; most of the CO and

more than half of NOx emissions result from road

transport (GLA, 2002). In central London, approxi-

mately 80% of road-transport-related PM10 originates

from diesel vehicles, such as taxis, buses and goods

vehicles (GLA, 2002).

The city of Milan and its surrounding urban area are

located in the central part of the Po river basin, in a flat

ARTICLE IN PRESSJ. Kukkonen et al. / Atmospheric Environment 39 (2005) 2759–2773 2763

area. The atmospheric circulation of the Po valley is

characterised by the strong modification of synoptic

flow due to the high mountains (Alps and Apennines)

that surround the valley on three sides. Calm conditions

(defined as conditions in which the hourly-averaged

wind speed at a height of 10m is lower than 1.0m s�1)

and light winds occur frequently. The most severe winter

episodes are commonly associated with elevated tem-

perature inversions. According to the regional emission

inventory, road traffic is mostly responsible for the PM10emissions in the Milan Province.

2.2. The selected air quality and meteorological

measuring stations

The locations of the air quality and meteorological

stations utilised in this study are presented in Figs. 1a–d.

In Oslo, the stations of Kirkeveien, Alna, Furuset,

Løren and Manglerud are in urban traffic environments,

while Iladalen and Birkenes are urban and rural

background stations, respectively. In Helsinki, the

stations of Toolo and Vallila are located in urban traffic

Fig. 1. (a–d) The location in the four cities (Oslo, Helsinki, London a

in this study.

environments, while Kallio 2 and Luukki are urban and

rural background stations, respectively. In London, the

station of Bloomsbury is in an urban centre, the station

of Marylebone Road is in an urban kerbside environ-

ment, the station of Bexley is in a suburb, the stations of

North Kensington and Brent are urban background

stations, and the stations of Harwell and Rochester are

rural background stations. In Milan, the station of

Zavattari is in an urban traffic environment, the stations

of Juvara and Limito are urban background stations.

The station of Limito is not located in Milan, but in a

smaller town in the Milan Province. The classifications

of the air quality stations have been made according to

EU directives, except in the case of London, where

national definitions are used.

The classifications of the meteorological stations used

in this study are as follows. In Oslo, Blindern and

Tryvann synoptic stations are in an urban and a rural

area, respectively; the station of Hovin is an urban

station. Helsinki–Vantaa is a synoptic station in a

suburban area, Helsinki–Isosaari is a synoptic station

located on a sea island and Kivenlahti is a World

nd Milan) of the air quality and meteorological stations utilised

ARTICLE IN PRESSJ. Kukkonen et al. / Atmospheric Environment 39 (2005) 2759–27732764

Meteorological Organisation (WMO) station that is on

a 327-m-high radio tower situated in a partly rural,

partly suburban environment. London Weather Center

is located in central London, Heathrow airport is a

synoptic station in a suburban area, and Herstmonceux

is a WMO station in a rural area. Milano Linate Airport

is a WMO station in a suburban area, and Juvara is an

urban background station.

2.3. Experimental methods for measuring PM10 and

PM2.5 concentrations

In Oslo, a Tapered Element Oscillating Micro-balance

(TEOM) was used at all of the stations where both PM10and PM2.5 are measured. At two stations, Alna and

Iladalen, the Eberline FH 62 IR (based on b-attenua-tion) was employed. The measurement height at all

stations was 3.5m. These instruments are maintained on

a three-monthly basis, and the flow rates are calibrated

on a six-monthly basis against the BIOS DC-2 primary

flow meter. The Partisol 2025 sequential air-sampling

instrument was utilised at the rural background station

of Birkenes. A national quality control system is now

implemented in Oslo based on criteria developed in

EUROAIRNET (Larssen et al., 1999) and EU direc-

tives.

In Helsinki, at the station of Toolo, the concentration

of PM10 was measured with a TEOM, and at the

stations of Vallila, Kallio 2 and Luukki, with Eberline

FH 62 I-R. The concentration of PM2.5 was measured at

the stations of Vallila and Kallio 2, with Eberline FH 62

I-R instruments. Sillanpaa et al. (2002) compared the

results of these continuously-operating monitors (Eber-

line and TEOM) with the reference gravimetric sampler

(Standard EN 12341, LVS-PM10). The sampling was

carried out in autumn for 7 weeks, and in winter and

spring for 8 weeks. The correction factors k (defined as:

reference sampler ¼ k�test instrument) for the concen-trations measured by the Eberline and TEOM varied in

autumn from 0.95 to 1.07, and in winter and spring from

0.84 to 0.86. These factors are near unity, probably

caused by the smaller semi-volatile proportion of aerosol

mass in Helsinki, compared with that in most other

European areas (Sillanpaa et al., 2002).

In London, the PM10 measurements are conducted

with TEOM instruments. The network in London

incorporates comprehensive QA/QC procedures (Ste-

venson, 2003). As part of the process, a six-monthly

audit is conducted; this involves flow checks, sample

inlet checks, as well as checks of site infrastructure and

location classification. The filters are changed when they

approach a mass loading of 80% or more.

In Milan, at the monitoring stations utilised here,

PM10 is measured with TEOM instruments. The QA/QC

of the air quality monitoring network of the Lombardia

Region is performed by the European Reference

Laboratory for Air Pollution. These procedures have

been evaluated in comparison with the EUROAIRNET

scheme for a complete QA/QC by the National

Environmental Protection Agency (Desiato et al., 2000).

2.4. Modelling methods

At present, there are no generally applicable theore-

tical schemes for the interpretation of the data measured

in episodic conditions, both due to the related extremely

stable atmospheric conditions and to the characteristics

of the urban meteorology. However, some improve-

ments have recently been achieved in the urbanisation of

NWP, MM and meteorological pre-processing (MPP)

models (e.g., Baklanov et al., 2002).

The synoptic-scale meteorological analyses are based

on the results computed by the national versions of the

NWP model HIRLAM in the case of Norway and

Finland, and on the ECMWF model in the case of UK

and Italy. The HIRLAM and ECMWF models are used

only for the synoptic analysis. The meso- and micro-

scale meteorological conditions are analysed using

locally measured data and sounding data for all the

cities. In addition, we have utilised the predictions of the

two MPP models (for Helsinki and Milan), and the

MM5 model (for London).

The MM5 model has been used to predict the

conditions in London, as additional meso- and micro-

scale data was especially needed in order to interpret

that episodic case. We have used five nesting levels in the

MM5 computations with the horizontal resolutions of

81, 27, 9, 3 and 1 km; the finest grid covers the entire

London area. We utilised the land cover information

from the US Geology Survey, and the reanalysis data of

the National Centers for Environment Prediction and

Atmospheric Research (NCEP and NCAR) as meteor-

ological input data. The physical schemes were as

follows: Simple Ice Explicit Moisture Scheme and

Multi-Layer Soil Surface Scheme (Dudhia, 1996),

MRF boundary layer scheme (Hong and Pan, 1996),

and Grell Convective Parameterisation (Grell et al.,

1994).

In Helsinki, the Monin–Obukhov length was eval-

uated using a MPP model, MPP-FMI, that has been

adapted for an urban environment (Karppinen et al.,

2000a); this model was originally based on the energy

budget method of van Ulden and Holtslag (1985). The

model utilises meteorological synoptic and sounding

observations, and its output consists of estimates of the

hourly time series of the relevant atmospheric turbulence

parameters and the boundary layer height. The compu-

tation is based on a combination of the data from the

stations at Helsinki–Vantaa and Helsinki–Isosaari. The

performance of the integrated modelling system contain-

ing the MPP-FMI model has been evaluated, e.g., by

Kousa et al. (2001).

ARTICLE IN PRESSJ. Kukkonen et al. / Atmospheric Environment 39 (2005) 2759–2773 2765

In Milan, the Monin–Obukhov length was evaluated

using a meteorological pre-processor, SURFPRO (Fi-

nardi et al., 1997 and Arianet, 2002); the model is based

on the energy budget method of van Ulden and Holtslag

(1985), and Paine (1988). The model utilises surface-

based data and vertical profiles of temperature, and

evaluates atmospheric turbulence scaling parameters

and the boundary layer height. The computation is

based on the data from the stations of Juvara and

Linate. The performance of an integrated modelling

system containing SURFPRO and a Lagrangian particle

dispersion model has been evaluated for urban air

quality applications by Calori et al. (2004).

3. Results

3.1. Concentration time series

The evolution of measured hourly pollutant concen-

trations that were considered relevant for an analysis of

the PM10 episodes is presented in Figs. 2a–d. In all

figures, the ticks on the horizontal axis indicate the

beginning of the day marked. The measured PM10concentration may also contain a substantial secondary

component, as well as a non-exhaust component, such

as suspended dust. The secondary photo-chemically-

formed aerosol can be especially important in southern

European cities, while suspended dust can play an

Oslo, 2003

0

50

100

150

200

250

300

3 4 5 6 7 8 9 10 11

Day of January

PM

10, µ

g/m

3

KirkeveienAlnaFurusetLøren ManglerudIladalen

Helsinki, 2002

0

50

100

150

200

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Day of April

PM

10 µg

/m3

TöölöPM10Kallio 2PM10Kallio 2PM2,5

(a)

(b)

(c

(d

Fig. 2. (a–d) The concentrations of PM10 in the course of the sel

important role especially in northern and central

Europe. A comparison of the concentrations measured

at various site categories makes it possible to evaluate

the importance of local emissions.

3.1.1. Periods of elevated PM10 concentrations

The periods of elevated concentrations are sum-

marised in Table 2. For the episodes in Oslo and

Helsinki, there were two separate periods of elevated

PM10 concentrations at the urban measurement stations.

In London, the elevated concentrations prevailed for

approximately 10 days, a more extended period than the

other episodes considered in this study. The character-

istically lower emissions during weekends (12–13 and

18–19 December, 1998; 6–7 and 13–14 April, 2002; 4–5

January, 2003; and 22–23 February, 2003) did not

substantially affect the concentrations in any of the cases

considered.

3.1.2. Evaluation of PM10 and PM2.5 concentrations at

various sites

The measured regional background PM10 levels

measured at a station southwest of Oslo (Birkenes) were

below 4mgm�3 during the whole time period considered

here. These levels are negligible compared with the

corresponding measured urban concentrations; the

episode, therefore, was not significantly influenced by

regional or LRT.

London, 2003

0

20

40

60

80

100

120

140

160

15 16 17 18 19 20 21 22 23 24 25 26 27 28

Day of February

PM

10, µ

g/m

3

BloomsburyMaryleboneRoadHarwell

Milan, 1998

0

100

200

300

400

10 11 12 13 14 15 16 17 18 19 20 21

Day of December

PM

10 µg

/m3

JuvaraZavattariLimito

)

)

ected episodes in the four cities. All hours are in local time.

ARTICLE IN PRESS

Table 2

A summary of the selected episodes and the main findings

City and episode

duration

Characteristic

synoptic

meteorological

conditions

Characteristic local

scale meteorological

factors

Type of temperature

inversion

Main pollutant

source category for

PM10

Brief classification

of the episode

Oslo, 4–5 and 7–10

January 2003

An area of high

pressure

Temperature

inversion, low wind

speed, stable

stratification

Ground-based,

advection, (max

15 1C)

Local wood

combustion and

local vehicular

emissions

Wintertime

inversion-induced

episode

Helsinki, 3 and 8–13

April 2002

An area of high

pressure

Temperature

inversion, low wind

speed, stable

stratification

Ground-based,

radiation, (max

8 1C/50m)

Suspended dust and

local vehicular

emissions

Spring dust episode

London, 18 to 27

February 2003

Varying high and

low pressure areas

Slight temperature

inversion, varying

wind speed

Ground-based (less

than 1 1C/50m)

Urban vehicular

emissions and

suspended dust, and

long-range

transport

Partly local, partly

long-range

transport episode

Milan, 14–19

December 1998

High pressure ridge Temperature

inversion, stable

stratification

Elevated, advection

(max 15 1C/1500m)

Vehicular emissions

and long-range

transport

Wintertime

inversion-induced

episode

J. Kukkonen et al. / Atmospheric Environment 39 (2005) 2759–27732766

At the urban stations of Oslo, the PM2.5 concentra-

tions (not shown here) were elevated during the same

periods as the PM10 concentrations. A detailed analysis

of the observed daily average values at the urban traffic

station of Kirkeveien shows that very high ratios of

PM2.5–PM10 prevailed during the period considered

(from 3 to 11 January 2003), varying between 0.83 and

0.94. The suspended road dust episodes are, however,

characterised by low PM2.5/PM10 ratios, commonly

ranging from 0.2 to 0.3 (Laupsa and Slørdal, 2002).

Both the high PM2.5/PM10 ratios and the occurrence of

the highest measured concentrations at the urban

background station (Iladalen) indicate that local traffic

emissions were not the most important pollution source.

The other substantial local source of PM10 in Oslo,

domestic wood-burning, is therefore probably the most

dominant source during the episode considered.

In Helsinki, the regional background PM10 levels (at

Luukki) were substantially lower (below 70 mgm�3) than

the corresponding highest measured urban traffic-site

concentrations (at Toolo and Vallila) during the whole

time period. The local sources were therefore mainly

responsible for the highest concentrations. The PM2.5concentrations at both the urban traffic-site (Vallila) and

the urban background site (Kallio 2) are clearly lower

than the corresponding concentrations of PM10; this is

an indication of the mainly local origin of the PM

pollution.

In London, we mainly consider the Bloomsbury

(urban centre), Marylebone Road (urban kerbside) and

Harwell (rural) sites, as the concentrations of both PM10and PM2.5 are monitored at these stations. The PM10

concentrations at these two urban sites are higher, on

the average by a factor of two, than those at the rural

site. A similar temporal variation of the PM10 concen-

trations was also observed for the same period at other

London sites including Brent, Bexley and North

Kensington. The PM2.5 concentrations (not shown here)

at the urban sites (Bloomsbury and Marylebone Road)

were comparable with those at the rural sites (Harwell

and Rochester). These results indicate that the local

contribution to the PM10 concentrations is approxi-

mately one half, while the PM2.5 concentrations are

probably mainly of LRT origin.

A charging scheme for reducing traffic congestion

came into effect in part of Central London on 17

February 2003. The stations of Bloomsbury and

Marylebone Road are inside and on the periphery of

the charging zone, respectively. However, as the

corresponding concentrations of PM10 at the other

urban stations in London showed similar temporal

evolutions, the influence of the charging scheme on

pollution does not seem to be significant compared to

the meteorological and other factors during the period

considered.

In Milan, the PM10 concentrations measured at the

urban background site (Juvara) are of the same order of

magnitude or are higher, compared with those at the

urban traffic station (Zavattari). The values measured at

both of these stations in Milan are only slightly higher

than those measured at the urban site of Limito that is

located in a smaller town in the Milan Province. The

measurements of PM10 are not conducted at rural

stations within the region. These results indicate that the

ARTICLE IN PRESSJ. Kukkonen et al. / Atmospheric Environment 39 (2005) 2759–2773 2767

elevated concentrations prevailed over a region, and did

not solely originate from urban sources in Milan.

3.2. Synoptic-scale meteorological conditions

The synoptic-scale meteorological analyses are based

on the results of the Norwegian and Finnish variants of

the HIRLAM model and the NWP model at ECMWF.

On 3 January 2003, an area of high pressure prevailed

over the Atlantic sea region; this high moved eastwards,

arriving in the sea area between the UK and Southern

Norway on 7 January. The measured ground tempera-

tures at the beginning of January in Oslo were low,

approximately �20 1C. On 6 January, relatively warmer

air masses were transported in at higher altitudes from

the northwest; this led to the formation of a strong

ground-based advection inversion that lasted from 7th

to 10th of January. The ground surface was covered

with snow or ice during the course of the episode.

During the episode in Helsinki, an area of high

pressure extended from northwestern Russia to Finland.

There was a fan of snow at the beginning of the episode

period (from 3rd to 4th of April 2002), and the sky was

overcast during the remaining period (until 13 April).

The ground surface was partly covered with snow. The

Oslo, 2003

-202468

101214161820

1 2 3 4 5 6 7 8 9 10 11

Day of January

Ob

serv

ed d

iffe

ren

ce in

Po

ten

tial

Tem

per

atu

re (

K)

0

1

2

3

4

5

6

7

8

9

Win

d s

pee

d. m

/s

Helsinki, 2002

0123456789

1 2 3 4 5 6 7 8 9 10 1112 13 14

Day of April

Win

d s

pee

d (

m/s

)

-0.2

-0.10

0.10.2

0.3

0.40.5

0.6

Inve

rse

Mo

nin

-O

bu

kho

v le

ng

th (

1/m

)

Tem

per

atu

re (

C).

win

d

(a) (c

(b) (d)

Fig. 3. (a–d) The temporal variation of the relevant local-scale meteo

LWC ¼ London Weather Center.

stable atmospheric stratification led to the formation of

a ground-based radiation inversion that lasted from 7 to

12 April.

At the beginning of the episode in London (17

February 2003), an area of high pressure extended over

most of the UK. This area gradually moved to north-

eastwards, and from 20 to 23 February a series of low

pressure areas arrived in the vicinity of the Southern

UK. On 24 February, the UK laid between a north-

easterly zone of high pressure and two zones of low

pressure.

The synoptic weather conditions during the episode in

Milan (from 14 to 19 December 1998) were charac-

terised by a high-pressure ridge. This ridge arrived in the

Mediterranean basin on 13 December and remained

quasi-stationary, centred over the western Mediterra-

nean for the whole of the episode duration (until 19

December).

3.3. Meso- and micro-scale meteorological conditions

We have analysed the temporal variation of the

concentrations in terms of the measured or pre-

processed meteorological variables that are relevant for

air quality. The hourly time-series of the most relevant

London, 2003

-4-2

02

468

1012

1416

17 18 19 20 21 22 23 24 25 26 27 28

Day of February

Tem

per

atu

re (

C)

0

2

4

6

8

10

12

Win

d s

pee

d (

m/s

)

Milan,1998

-2

0

2

4

6

8

10

12

10 11 12 13 14 15 16 17 18 19 20 21

Day of December

spee

d (

m/s

)

-0.1

-0.05

0

0.05

0.1

0.15

0.2

Inve

rse

Mo

nin

-Ob

ukh

ov

le

ng

th (

1/m

)

)

,,

rological variables in the four cities. All hours are in local time.

ARTICLE IN PRESSJ. Kukkonen et al. / Atmospheric Environment 39 (2005) 2759–27732768

meteorological variables are presented in Figs. 3a–d; the

vertical temperature profiles are presented separately in

Figs. 4a–d.

3.3.1. Meteorological analysis for Oslo

In Oslo, the heights of the meteorological measure-

ment stations differ substantially from each other. The

heights above sea level of the stations of Hovin, Blindern

and Tryvann are 95, 94 and 514m.

In the early stages of the episode, the wind direction

was predominantly easterly; while in the latter stages, it

was very variable. The wind speed was variable (from

calm to almost 8.0m s�1) in the early stages of the

episode, and mostly low or moderate (from calm to

Oslo,Blindern,

2003

0

200

400

600

800

1000

-20 -15 -10 -5 0 5

T (C)

Hei

gh

t (m

)

January 3 January 4 January 5

January 6 January 7 January 8

Helsinki, Kivenlahti, 2002

0

50

100

150

200

250

300

350

-10 -5 0 5 10 15

T(C)

Hei

gh

t (m

)

April 1 April 3 April 5

April 7 April 9 April 11

April 13

Hei

gh

t (m

)

(a)

(b) (d)

Hei

gh

t (m

)

(c)

Fig. 4. (a–d) The observed temporal evolution of the vertical profiles o

the profiles are for midnight (00.00UTC on the date in question), an

4.3m s�1) in the latter stages. The atmospheric pressure

was elevated, especially during the latter stages. The

ambient temperature at the station of Hovin varied from

�20 to +1 1C at a height of 2m.

The magnitude of the inversion can be estimated

based on simultaneous measurements at stations located

at different heights. The temperature differences be-

tween Hovin and Tryvann, and Blindern and Tryvann,

are presented in Fig. 3a. The results show that a

substantial inversion structure of up to 20 1C prevailed

over the city. According to these measurements, the

largest magnitudes of the inversions occurred during the

weekend from 4 to 5 January and from 6 to 10 of

January.

Milan, Linate, 1998

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

-20 -10 0 10 20

T (C)

Dec 13 Dec 15

Dec 17 Dec 19

London, Herstmonceux,2003

0

50

100

150

200

250

300

350

400

-4 -2 0 8 10 12

T (C)

17-Feb 18-Feb 19-Feb20-Feb 21-Feb 22-Feb23-Feb 24-Feb 25-Feb26-Feb 27-Feb 28-Feb

642

f temperature in the four cities. For Oslo, Helsinki and London,

d for Milan, noon (12.00UTC on that date).

ARTICLE IN PRESSJ. Kukkonen et al. / Atmospheric Environment 39 (2005) 2759–2773 2769

The evolution of the nocturnal vertical profiles

(measured at 00UTC) of the ambient temperature at

the station of Blindern is presented in Fig. 4a. The

results show that there were ground-based inversions on

all days from 6 to 10 January. The main reason for the

formation of the inversion was the advection of warm

air above the lowest atmospheric layers, in combination

with the intensive radiative cooling of the ground. The

occurrence of the inversions mostly coincides with the

very highest concentrations that occurred from 7 to 10

January.

3.3.2. Meteorological analysis for Helsinki

The ground surface temperatures at the station of

Helsinki–Vantaa varied from �4 to +15 1C. The wind

direction, speed, atmospheric pressure and Monin–O-

bukhov length values have been evaluated for the central

Helsinki area, using a MPP model. In the early stages of

the episode, during the period 1–6 December, the wind

speed varied from low (on 3 April) to high; during the

period from 7 to 10 April, the wind speed was low or

moderate. The wind direction was predominantly

between north and west. The highest PM10 concentra-

tions (during 3rd of April and from 8 to 13 April 2002)

show correlation with the occurrence of low and

moderate wind speeds.

Regarding PM10 episodes related to the suspension of

road and street material, the wind speed actually has two

opposite effects. A low wind speed is commonly related

to stable atmospheric stratification, favouring the

occurrence of high concentrations. On the other hand,

a strong wind tends to increase the suspension of road

and street materials in the air.

The evolution of the inverse Monin–Obukhov length

(L�1) is presented in Fig. 3b. The atmospheric pressure

started to increase on 1 April, and remained at an

elevated level until approximately 13th of April. The

atmospheric stability can be classified approximately as

stable or neutral during the whole period considered

(i.e., L40), except for short periods on 7 and 8 April.There were several periods of very stable atmospheric

conditions (classified as L�140.1m�1); these follow a

diurnal variation, in which the most stable conditions

commonly occur at night. L�1 values, as those of the

wind speed, correlate with the occurrence of the highest

PM10 concentrations.

The evolution of the nocturnal vertical temperature

profiles at the Kivenlahti mast is presented in Figs. 4b.

This mast represents the meteorological conditions in

the vicinity of Helsinki, although the mast is not located

in the urban area (its distance from the centre of

Helsinki is 15 km). The measured midnight temperature

profiles show that there was a moderate ground-based

temperature inversion on 3 April, and moderate or

strong ground-based inversions on all days from 7 to 12

April. The maximum ground-based inversion occurred

on 11 April, when the measured temperature increased

8 1C within the lowest 50m of the atmosphere. The

inversions were radiation inversions, caused by the

cooling of the ground surface by outgoing long-wave

radiation. The highest PM10 concentrations (during 3rd

of April and from 8 to 13 April 2002) coincided with the

occurrence of ground-based inversions.

3.3.3. Meteorological analysis for London

The atmospheric pressure, ambient temperature, and

wind speed and direction measured at the stations of

London Weather Center (LWC) and Heathrow and

vertical temperature profiles at the sounding station of

Herstmonceux were used for this analysis. The corre-

sponding meteorological parameters were also predicted

using the MM5 model. The temporal variation of all the

meteorological parameters considered (wind speed and

direction, pressure and temperature) measured at the

Heathrow station did not substantially differ from the

corresponding data measured at the LWC. As an

example, the wind speeds measured at these two stations

have been presented in Fig. 3c.

Wind speeds at a height of 10m varied substantially

during the period considered, from calm to almost

10m s�1; these values do not show any systematic

variation during the period addressed. The wind

direction was predominantly easterly. The atmospheric

pressure was fairly high during most of the period

considered. The ambient temperatures varied from �3

to +14 1C, and the daily average values show an

increasing trend during this period (Fig. 3c). There was

no rainfall throughout the period. Most of the time, the

predicted planetary boundary layer (PBL) height was

not especially low, although some values of less than

100m occurred during 20 and 21 February.

The measured nocturnal temperature profiles show

that there was a slight ground-based temperature

inversion continuously from 17 to 18, and from 20 to

24 February. However, the temperature only increased

from approximately 1 to 4 1C within the lowest 200m of

the atmosphere. In summary, an analysis of the

meteorological variables at Heathrow, LWC and

Herstmonceux did not reveal any exceptional condi-

tions, except for the ground-based inversions.

We have also predicted the structure of inversions at

midnight using the MM5 model. The model perfor-

mance regarding the vertical variation of temperature

was first evaluated against the available data, measured

at the station of Herstmonceux (Fig. 5a). The magni-

tudes of the inversions (i.e., the total vertical tempera-

ture difference in the inversion layer) are predicted fairly

well; however, the absolute temperature values are in

most cases slightly under-predicted by the model. The

model predictions for the LWC site in central London

show temperature inversions on all days from 17 to 22

February (Fig. 5b).

ARTICLE IN PRESS

London,Herstmonceux,

2003

0

250

500

750

1000

-4 -2 0 2 4 6 8 10

T (C)

Geo

po

ten

tial

Hei

gh

t (m

)

OBS Feb 21 MM5 Feb 21

OBS Feb 22 MM5 Feb 22

OBS Feb 23 MM5 Feb 23

London,London WeatherStation 2003

0

250

500

750

1000

-10 -8 -6 -4 -2 0 2 4 6

T (C)

Geo

po

ten

tial

Hei

gh

t (m

)

17 FEB 18 FEB

19 FEB 20 FEB

21 FEB 22 FEB

23 FEB

(a)

(b)

Fig. 5. The temperature profiles predicted by the MM5 model

at midnight (a) compared with the corresponding data

measured at the Herstmonceux station, and (b) for the LWC

site in central London.

J. Kukkonen et al. / Atmospheric Environment 39 (2005) 2759–27732770

3.3.4. Meteorological analysis for Milan

The wind direction, speed, atmospheric pressure,

temperature and humidity values were measured at the

urban station of Juvara, located in Milan. The

Monin–Obukhov length values have been evaluated

for the central Milan area, using the MPP model

SURFPRO, based on the data from the Juvara station.

From 12 to 17 December, the atmospheric pressure

remained at an elevated level. The wind speed was low

or it was calm during the whole period considered (Fig.

3d); light winds are one of the main prevailing features

of the Po valley climatology. Long-term statistics during

the period 1951–1991 for the Milano–Linate synoptic

station show a yearly frequency of calms (0–1 knots)

larger than 50 % (ENEL-AM, 1994). These conditions

are mainly due to the blocking effect of the high

mountains that surround the valley on three sides;

this orography tends to prevent synoptic flows from

reaching the lower atmospheric layers in the valley. The

city of Milan is at a substantial distance from the

mountains (approximately 40 km), and is therefore

usually only weakly influenced by a mountain breeze

circulation. The wind speed is therefore not a good

predictor variable in terms of peak pollution episodes

within this area.

Based on the predicted values of the Monin–Obukhov

length, the atmospheric stability can be classified as

stable or neutral during the whole period considered

(i.e., L40), except for short daytime periods (Fig. 3d).There was an extended period with very stable

stratification (L�140.1m�1) during the nights of 13 to

18 December. An intense slightly elevated temperature

inversion was formed on 13 December that reached its

maximum depth and magnitude (with a temperature

increase in height of about 15 1C in the lowest 1500m)

on 15 December and prevailed until 19 December (Fig.

4d). This period nearly coincided with the occurrence of

the highest concentrations from 14 to 19 December. The

inversions were caused by the advection of warm air

associated with the incoming high-pressure ridge. The

maximum temperature warming occurred between

heights of 1000 and 1500m above ground level, where

the temperature reached values of up to 15 1C. The

temperature of the surface layer did not change

substantially during the course of the episode.

4. Conclusions

This paper presents an analysis and evaluation of air

pollution episodes of PM10 concentrations in four

European cities. We have also utilised the episode

inventory within the FUMAPEX project. The discus-

sion is based on meteorological observations, accom-

panied by computations with NWP models and MPP

models.

ARTICLE IN PRESSJ. Kukkonen et al. / Atmospheric Environment 39 (2005) 2759–2773 2771

A summary of some of the key factors for the selected

episodes is presented in Table 3. Based on an examina-

tion of the concentrations of PM10 and PM2.5 measured

at various site categories, we concluded that the selected

episodes were caused in Oslo mainly by domestic wood

burning, partly also by local traffic, in Helsinki mainly

by suspended dust and local traffic emissions, in London

by both urban traffic and LRT, and in Milan partly by

local traffic, partly by LRT. All the episodes considered

were associated with the influence of areas of high

pressure (Oslo, Helsinki and London) or of a high-

pressure ridge (Milan). High atmospheric pressure is

commonly related to stable stratification; clearly, how-

ever, it does not necessarily lead to extremely stable

conditions or strong inversions near the ground level.

Strong ground-based or slightly elevated temperature

inversions prevailed in the course of the episodes in

Oslo, Helsinki and Milan, and there was also a slight

ground-based inversion present in London. Inversions

can be caused by various atmospheric processes, such as

subsidence, fronts, radiation and advection; radiation

and advection inversions are those that occur most

frequently in the course of air pollution episodes. In the

cases examined here, the inversions in Oslo and Milan

were mainly caused by advection, and that in Helsinki

was a radiation inversion. Clearly, the influence of

inversions on air quality crucially depends on their

detailed vertical structure and magnitude (elevation,

temperature gradient and the total temperature differ-

ence over the inversion layer) and the temporal

evolution (e.g., lasting only during the night or

continuously over several days and nights).

In order to find out whether temperature inversions

would also be concomitant with the highest concentra-

tions of PM10 in other corresponding episodic cases, we

investigated the episodes contained in the FUMAPEX

database. The result found was that, in all the

corresponding episodes included in the inventory, in

Oslo (three local wood combustion-originated cases and

two spring dust cases), Helsinki (two spring dust cases)

and Milan (two wintertime inversion-induced cases),

and London (two cases), this was indeed the case. The

evolution of the measured temperature inversions could

therefore be recommended—at least for some loca-

tions—as one predictor variable for forecasting the

likelihood of the formation of air pollution episodes in,

e.g., statistical and semi-empirical models.

The more pronounced maritime climatological con-

ditions in London tend to moderate the most extreme

local meteorological conditions, when compared with

the mountainous areas in Northern Italy and Southern

Norway, and the partly continental climate in Finland.

London also differs from the other cities considered here

because of its severe traffic congestion situation,

especially in the central parts of the city. For instance,

the average vehicle speed in central London has fallen

from 20 to about 16 kmh�1 over the last 20 years (Tfl,

2000). A peak pollution episode could, therefore, be

caused by high local emissions during relatively less

severe meteorological conditions, compared with smaller

cities that have a substantially better situation in terms

of traffic flow and the resulting emissions.

In the cases studied here, the best meteorological

predictors for the elevated concentrations of PM10 were

the temporal (hourly) evolutions of temperature inver-

sions, atmospheric stability and in some cases, wind

speed. However, in some cases a high wind speed can

also increase PM10 concentrations, due to an increased

suspension of particles from street and ground surfaces.

It was also discovered that a low wind speed is not

necessarily a good indicator of poor air quality or

pollution episodes in the case of all European regions. In

particular, in the Po valley, wind speed is a poor

indicator of peak pollution episodes, due to frequently

occurring calm and low wind speed conditions.

Acknowledgements

We wish to acknowledge the financial support of the

European Commission for the FUMAPEX project and

the Cluster of European Air Quality Research

(CLEAR). In addition, the support of the COST 715

Action and the Academy of Finland for the FORE-

CAST project is appreciated. The following are thanked

for their help in provision of air quality and meteor-

ological data: OPHA—the Municipality of Oslo, the

Helsinki Metropolitan Area Council, the Department

for Environment Food and Rural Affairs and the British

Atmospheric Data Centre, and the ARPA Lombardia

and Regione Lombardia.

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Further reading

Karppinen, A., Kukkonen, J., Elolahde, T., Konttinen, M.,

Koskentalo, T., Rantakrans, E., 2000b. A modelling system

for predicting urban air pollution, model description and

applications in the Helsinki metropolitan area. Atmospheric

Environmant 34–22, 3723–3733.

Karppinen, A., Joffre, S.M., Kukkonen, J., 2002. Evaluation of

meteorological data measured at a radio tower in the

Helsinki Metropolitan Area. In: Piringer, M., Kukkonen, J.

(Eds.), Proceedings of the Workshop 3 and 4 October 2001,

Toulouse, France. COST Action 715, EUR 20451, Eur-

opean Commission, Brussels, pp. 89–98.