ARTICLE IN PRESS

1352-2310/$ - se

doi:10.1016/j.at

�Correspondde Tenerife, Ca

E-mail addr

Atmospheric Environment 39 (2005) 6734–6746

www.elsevier.com/locate/atmosenv

Nucleation and growth of new particles in therural atmosphere of Northern Italy—relationship to

air quality monitoring

Sergio Rodrıgueza,b,�, Rita Van Dingenena, Jean-Philippe Putauda,Sebastiao Martins-Dos Santosa, Davide Rosellic

aEuropean Commission, Joint Research Centre, Institute for Environment and Sustainability, T.P. 290, 21020 Ispra (VA), ItalybInstitute of Earth Sciences ‘‘Jaume Almera’’, CSIC, Barcelona, Spain

cForschungszentrum Julich GmbH, Germany

Received 25 January 2005; received in revised form 30 June 2005; accepted 13 July 2005

Abstract

This study investigates the relationship between aerosols number size distribution on the one hand, and air quality in

terms of particulate matter (PM) mass concentrations (as usually monitored in the air quality networks) on the other

hand. For this purpose, time series of trace gases levels, submicron aerosol size distributions, both recorded at a rural

site in Northern Italy (ISPRA), and of trace gas levels and PM mass concentrations, recorded in the air quality network

operating in this region, have been compared and interpreted. Because of the regional nature of the PM pollution

events, the daily mean levels of the aerosol volume (V), surface area (S) and black carbon (BC) concentrations at

ISPRA rural site are well correlated with the daily mean levels of PM mass concentrations recorded at the other air

quality monitoring sites. At ISPRA, the submicron aerosol size distribution is strongly influenced by two main

competing processes: nucleation of new particles and condensation of gas-phase components onto pre-existing particles

(resulting in particles growth). These processes influence on the daily, seasonal and day-to-day variations of the

submicron aerosol features. Because increasing aerosol S concentrations favour condensation and hinder nucleation

(and vice versa) the ‘mean’ particle size DpN (mode of the dN/dlogD size distribution) increases with increasing PM

concentrations (e.g. 45 nm for V ¼ 4mm3 cm�3 and 110 nm for V ¼ 45mm3 cm�3). Owing to this, time series of aerosol

DpN and V , S, mass and BC concentrations are strongly anti-correlated with those of the smallest ultrafine particle

number concentration (N, 5–10 and 10–20 nm). Nucleation episodes occur under the clean air conditions prompted by

the North-Foehn meteorology. This anti-correlation between submicron aerosol mass and No20 nm concentrations

(prompted by the low contribution of the ultrafine particles to the aerosol mass) has important implications for a proper

air quality monitoring: the parameters classically used for the air quality assessing (e.g. PM2.5) are not suitable for

monitoring of this ultrafine PM pollution and consequently a specific monitoring of the ultrafine PM number

concentration should be performed. The significance of this specific ultrafine PM number concentration monitoring is

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

mosenv.2005.07.036

ing author. Current address: Izana Atmospheric Observatory, INM-CSIC, La Marina 20, 6a planta, 38071, Santa Cruz

nary Islands Spain.

ess: srodriguez@ija.csic.es (S. Rodrıguez).

ARTICLE IN PRESSS. Rodrıguez et al. / Atmospheric Environment 39 (2005) 6734–6746 6735

supported by facts already proven: a significant fraction of the current urban PM emissions occurs in the ultrafine PM

fraction and exposure to ultrafine PM is associated with adverse effects on human health.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Aerosols size distribution; Number concentration; Air quality; Nucleation; Po valley

Adriadic sea

Alps

Mediterraneansea

Study

area

0-100

100-200

200-500

500-800

800-1500

1500-3000>3000

m.a.s.l.

Fig. 1. Topographic map of Italy highlighting the location of

the study area.

1. Introduction

Studies performed during the last decade have

improved our knowledge on the processes affecting

levels and size distribution of atmospheric aerosol. The

combination of modelling and field measurements has

elucidated the main microphysical processes affecting

the ‘aerosol dynamics’, i.e. nucleation, condensation,

coagulation, wet and dry deposition (e.g. Raes et al.,

2000; Shi et al., 2001; Wehner et al., 2002). From the

point of view of air quality monitoring, most of the

experimental studies have been performed during short

campaigns, mainly focused on characterising road traffic

emissions. Although these studies have been significant,

important uncertainty on the relationship between the

parameters classically used for the air quality monitor-

ing (the mass concentration) and processes affecting the

aerosol dynamics still remain.

Epidemiological studies have observed consistent

associations between exposure to aerosols (or particu-

late matter (PM)) in ambient air and daily mortality

caused by cardiovascular and respiratory diseases

(Pope and Dockery, 1999). Owing to this, PM levels

in ambient air are regularly monitored in air quality

networks using the PM mass concentrations below a

cut-off size as reference metric (e.g.PM2.5 & PM10).

Epidemiological studies indicate that ultrafine particles

(o0.1 mm) and carbonaceous compounds (the main

ultrafine PM constituents) are significantly linked to

these adverse effects in human health (Wichmann et al.,

2000; Wyzga, 2002). Although these ultrafine particles

may be abundant in the air (number of particles/

volume) their contribution to the aerosol mass con-

centration is much lower than that of the particles

within the upper range of fine mode (0.1–1 mm). Forexample both 10 particles cm�3 of 1 mm and

107 particles cm�3 of 0.01 mm result in the same mass

concentration (5 mgm�3 for density 1 g cm�3).

In this study, we have investigated the relationship

between the submicron aerosol number size distribution

and the regional air quality in a rural site in Northern

Italy. We have combined the submicron aerosol size

distribution data recorded at this rural site with the data

produced by the air quality monitoring network operat-

ing in this area. We have studied the context in which the

PM events occur, how the submicron aerosol features

change when increasing PM levels from clean to polluted

conditions, the processes leading to these changes and

what the relationship between aerosol number, volume

and mass concentration is.

2. Methodology

2.1. The study area

The North of the Lombardy region (Northern Italy) is

located between the northern edge of the Po valley and

the Alpine foothills (Fig. 1). Due to the topography and

to its location in relation to the synoptic general

circulation, this region is characterised by low wind

speed conditions. Wind speed typically presents daily

mean values around 1m s�1. Moreover, inversion layers

near ground and fog episodes are frequent in the cold

season. These two factors hinder the horizontal and

vertical dispersion of the pollutants emitted in this

region, resulting in frequent exceedences of the EU 24h

limit value of 50mgPM10m�3 (see Van Dingenen et al.,

2004). These stagnant conditions are occasionally

interrupted by North-Foehn events, where relatively

ARTICLE IN PRESSS. Rodrıguez et al. / Atmospheric Environment 39 (2005) 6734–67466736

warm and dry downslope wind from the Alps flows over

the area. During its ascent at the northern Alpine slope,

the air mass gets supersaturated and looses its moisture,

thereby scavenging practically all particulate matter. At

the south slope, the air expands adiabatically during its

descent, and heats up, sometimes reaching 20 1C in

January when it reaches the valley. In such North-Foehn

conditions, extremely clean air prevails over the region.

During the cold season, relatively high temperature and

the highest O3 events are recorded during these North-

Foehn episodes; however, these O3 concentrations are

lower than those typically recorded in the warm season

(even at the same temperature; other details in Bauman

et al., 2001; Weber and Prevot, 2002).

2.2. Measurement sites

The main data set of this study was recorded at the

EMEP station located into the Joint Research Centre at

ISPRA (451480N, 81380E, 235m asl). We will refer to this

station as ISPRA. This measurement site is representa-

tive of the rural environment of Northern Lombardy.

The nearby cities located in our study area (into a radius

of 60 km) and the mobile sources along the roads and

motorways are the most important air pollutant sources.

Industrial emission around Milan area could also

contribute to background air pollution. In order to

place the measurements performed at ISPRA into the

context of the regional air pollution, data from seven air

quality monitoring stations (managed by ARPA—

Agenzia Regionale per la Protezione dell’Ambiente)

operating in the cities of Varese, Busto Arsizio and

Gallarate were also studied.

2.3. Data

At ISPRA the following measurements were per-

formed:

1.

Number concentration and dry size distribution

(5–800 nm particle diameter) by means of a con-

densation particle counter (CPC, TSI model 3010)

and a Vienna-type medium length differential mobi-

lity analyzer. The dry particle diameter was deter-

mined using a closed dry sheath air circuit (Jokinen

and Makela, 1997). The measurements were per-

formed from June 1999 to December 2000 (there are

some small interruptions in July 1999, and June and

September 2000).

2.

Black carbon (BC) levels were determined by

means of an aethalometer (Magee model AE-10,

incandescent lamp) from August 1999 to March

2000. Although this monitor actually determines

the equivalent black carbon, we will refer to this

as BC.

3.

Levels of O3, NO, NO2, SO2 and CO by means of

standard automatic methods.

The following data from the ARPA air quality

monitoring stations were used:

1.

Concentrations of O3, NO, NO2, SO2 and CO,

the mass concentration of total suspended particles

(TSP in 1999) and PM o10 mm (PM10 in 2000).

All data were obtained by means of standards

methods.

2.

Local meteorology (T, RH, wind, rain and total solar

radiation).

Moreover, surface pressure charts of the UK Meteor-

ological Office and satellite observations of the infrared

channel of Meteosat were used for assessing the day-to-

day changes in the meteorology.

2.4. Data treatment

The following parameters were calculated and stu-

died:

1.

N, V and S: number, volume and surface concentra-

tion of particles, respectively.

2.

NX–Y, VX–Y and SX–Y: number, volume and

surface concentrations of particles with size between

X and Y nm, respectively.

3.

DpN: diameter where the size distribution dN/dlogDp

reaches its maximum, i.e. the size distribution mode.

4.

The distance (in nm) from DpN for accounting to the

5, 10, 15,y% of N toward each side of DpN. For this

purpose, the dN/dlogD curve was integrated from

the central position (defined by DpN) toward each

extreme by steps containing 5% of total number

concentration. Thus, the diameters containing the 10,

20, y.and the 80% of the total number concentra-

tion, with respect to the DpN central position, were

determined. We will denote by ‘DDp X% N ¼ Y2Z0

the diameters Y and Z which contain the X% of the

total number concentration, being Y and Z at each

side of DpN. This is a measure of the ‘width or

dispersion’ of the aerosol diameter spectrum.

3. Results and discussion

3.1. Mean levels

A descriptive statistic of the aerosol parameters and

gases at ISPRA rural site is shown in Table 1. The mean

N5–800 concentration is �10,300 cm�3. The aerosol

number concentration reaches a maximum around the

diameter DpN ¼ 77 nm, with the 75% and 90% of the

ARTICLE IN PRESS

Table 1

Mean levels, standard deviation (stdev), median, maximum, minima and 98% percentile of several submicron aerosol parameters, and

levels of BC and gases pollutants recorded at ISPRA

Unit Mean Stdev Median Max Min %

1N5–800 cm�3 10283 3791 9615 20028 23021N5–10 cm�3 203 137 167 738 19 21N10–20 cm�3 789 349 726 2577 143 81N20–50 cm�3 2781 825 2742 6164 852 271N50–100 cm�3 3267 1504 3032 8068 564 321N100–200 cm�3 2518 1673 2041 7478 198 241N200–400 cm�3 666 539 499 2987 32 61N400–500 cm�3 43 48 28 371 1 o11N500–800 cm�3 9 11 5 92 0 o11DpN nm 77 26 77 157 201DDp 50%N nm 46–115 45–1151DDp 75%N nm 36–152 36–1531DDp 90%N nm 26–189 27–184

1S mm2 cm�3 461 316 385 1711 391V mm3 cm�3 17 14 14 82 1

2BC ngm�3 1610 1041 1425 5017 165

3SO2 ppbv 2.0 0.7 2.0 4.5 o0.13CO ppmv 0.6 0.4 0.4 2.7 0.13NO ppbv 8 12 1 63 03NO2 ppbv 13 8 11 37 23O3 ppbv 26 14 27 65 3

Percentage (%) with respect to the total number concentrations N1, 2, 3 denotes measurements performed between June 1999 and

December 2000, August 1999 and March 2000, and January and June 2000, respectively.

S. Rodrıguez et al. / Atmospheric Environment 39 (2005) 6734–6746 6737

particles in the ranges 35–150 and 25–182 nm around

DpN, respectively.

At ISPRA, the aerosol exhibits a lower number

concentrations and a higher mean diameter than those

typically recorded at urban sites. In urban environments,

the aerosol presents a predominant mode between 20

and 40 nm, and is typically characterised by a mean total

number concentration �20,000 cm�3 and even higher

(Ruuskanen et al., 2001; Van Dingenen et al., 2004;

Stanier et al., 2004a). These lower number concentration

and higher mean particle size at ISPRA are due to

coagulation and condensation processes during the

transport from the urban/road-traffic to the rural sites.

Table 1 also shows the mean levels of gases at ISPRA

rural site. A cross correlation analysis showed that (as

expected) levels of primary pollutants at ISPRA are

much lower than those recorded at the urban sites, in

such a way that, at ISPRA: (1) NO levels are 20% of

those recorded at the kerbsides, and 60% of those at the

urban background, (2) NO2 levels are 35–50% of those

recorded at the kerbsides, and 60–70% of those at

the urban background, (3) CO levels are 30–50% of

those recorded at the kerbsides, and 50–70% of those in

the urban background, (4) SO2 levels are 40–75% of

those recorded at the urban sites. Finally, because of the

O3 titration close to the NO sources, O3 levels at the

kerbside stations are 50–80% of those recorded at

ISPRA, whereas those recorded at the urban back-

ground sites are very close (95–100%) to those recorded

at ISPRA.

3.2. Daily and seasonal cycles

Figs. 2 and 3 show for each month the average

diurnal cycle of the particle number concentration in

several size ranges, as well as BC, NO2 and O3 levels at

ISPRA, i.e. each monthly bin has a 24 h sub-division

and shows the average hourly concentration during

that month. The N5–200 number concentration ex-

hibits daily cycles with higher levels during the morning

rush hours and the evening–night, and a seasonal

evolution with a maximum in autumn–winter. This

behaviour is similar to the one observed for PM10 and

TSP levels in the urban air quality monitoring stations

around ISPRA, and in the aerosol V and surface

concentrations at ISPRA (not shown graphically).

(Note that N5–800 and N5–200 are not distinguished

in Fig. 2A because they exhibited similar behaviour and

levels: N5–800 ¼ 1.07 �N5–200, r2 ¼ 0:99 for the hourlymean levels per month).

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20000

10000

5000

4000

0

4000

2000

8000

0

15000 6000

6000

600

2000

400

200

2000

0

800

1000

0

3000

0

40

60

120

100

80

0

1999 2000

DpN

, nm

N5-

10, c

m-3

cm-3

N5-

200

(= N

5-80

0), c

m-3

N10

-20,

cm

-3

N70

-100

N50

-70

N20

-50

N10

0-20

0, c

m-3

A S O N J J F M A M J J A

Fig. 2. Daily mean cycles per month of the particle number

concentration in several size ranges.

5000

315000

10000

20000

2

1

0

4

1000

0

3

2000

2

1

0

4

400

200

0

600

40

20

0

60

0.1·

NO

2, p

pbv

N5-

10, c

m-3

N5-

10,

3·N

10-2

0,cm

-33·

N50

-70,

cm-3

N5-

200,

O

J F M A M

N D J F M1999 2000

2000

O3,

ppb

vB

C,µ

g/m

-3

Fig. 3. Daily mean cycles per month of the particle number

concentration in several size ranges, and BC and O3 levels.

S. Rodrıguez et al. / Atmospheric Environment 39 (2005) 6734–67466738

Although N5–200 is the sum of its subsets, some of

these subgroups present significantly differenced daily

and seasonal cycles. N5–10 and N10–20 present daily

cycles similar to those of O3, with higher levels during

daylight than at night (Figs. 2C and 3C). This daily cycle

is mainly prompted by photochemically induced nuclea-

tion. These nucleation particles, with an initial size of a

few nanometers, rapidly grow by coagulation and

condensation processes in such a way that they will

contribute to N5–10 and N10–20 levels, and even to the

range 420 nm when growing.

N20–50 shows a daily cycle with higher levels during

daylight than at night and two maximum during the

morning and evening rush hours (Fig. 2B). This daily

cycle is attributed to: (1) a transfer of particles from the

o20 nm to the 20–50 nm ranges owing to coagula-

tion and condensation processes during daylight, and

(2) particle primary emissions in the nearby cities and

roads which results in N20–50, BC and NO2 peak

concentrations during the rush hours.

Levels of N50–70, N70–100, and N100–200 exhibit

daily cycles highly correlated with those of BC and NO2

levels, showing a marked maximum in the evening and

during the night (Figs. 2A, B and 3A). In some winter

months, there is a secondary maximum in the morning

rush hours. This maximum during the evening and night

is attributed to: (1) the concentration of particulates and

gases near the ground owing to the reduction of the

boundary layer height, (2) the contribution of particles

(with an initial size o50 nm) that have been growingalong the day by coagulation and condensation to enter

in the range 450 nm.The seasonal evolution of the number concentration

of particles o20 and 450 nm also exhibits significant

differences (Fig. 2). The autumn–winter maximum

exhibited by N50–70, N70–100 and mainly N100–200

is associated with an increase in the particle concentra-

tion during the night owing to the accumulation of

‘grown particles’ 450 nm, and mainly 4100 nm. BCand NO2 levels exhibit a similar seasonal cycle. In

contrast, the seasonal maximum observed in N5–10

levels is associated with an increase in the amplitude of

the daily cycle, no significant changes in the nocturnal

concentrations occur throughout the year. N10–20 and

N20–50 do not exhibit a significant seasonal evolution.

The discussion above highlights the anti-correlated

behaviour of the concentration of particles o10 and

ARTICLE IN PRESSS. Rodrıguez et al. / Atmospheric Environment 39 (2005) 6734–6746 6739

450 nm (e.g. daily and seasonal cycles; Figs. 2 and 3).

After the morning rush hours resulting in the N20–50

peak concentrations discussed above, the daylight

conditions (increase in the temperature, solar radiation

intensity, photochemistry and boundary layer height)

favour the dilution of pollutants, the photochemical

oxidation of secondary aerosols precursors and particle

nucleation, resulting in the N5–10 and N10–20 max-

imums during daylight. In contrast, the nocturnal

conditions favour the concentration of pollutants as

well as particles growth by coagulation (due to the

decrease in the mean free path) and condensation of

semi-volatile species onto the particle’s surface. Because

of these processes, the particle diameter DpN exhibits a

daily cycle with lower levels during daylight (Fig. 2D).

Moreover, the winter maximum in N5–200 (the classical

PM winter maxima) is associated with a strong increase

in the particle diameter DpN (Fig. 2D).

3.3. Episodes study

3.3.1. PM mass events

The day-to-day variations in the aerosol BC and V

concentrations at ISPRA have been used as representa-

tive of the day-to-day variations in the submicron

aerosol mass concentrations at this site (mass ¼

volume � density; e.g. density 1.5–1.7 g/cm3 for many

submicron aerosol species; Mc Murry et al., 2002). At

the urban air quality monitoring sites, TSP and PM10

levels were used as representative of the PM mass

concentrations at these sites.

At ISPRA, BC, V and S concentrations are highly

correlated (Fig. 4A and E). Moreover, these parameters

at ISPRA are also well correlated with the PM mass

300

90

60

30

00 100 200100

0

2

4

6

0 50

500

800 20 6040

200

400

600

800

00 500 1000 1500 2000

000

BC

,µg/

m3

ISPRA

ISPRA

ISPRA N10

-20,

cm

-3

N5-

10, c

m-3

1000

S,µm

2 /cm

3

S,µm2/cm3

ISPR

AV

, µm

3 /cm

3

V, µm3/cm3

V, µm3/cm3

y = 0.069xR2= 0.739

y = 25.19xR2= 0.944

y = 0.309xR2= 0.705

N5-

800,

cm

-3

TSP, µg/m3Urban (BUSTO ARSIZIO)

Fig. 4. Scatter plots of several submicron aerosol properties (24-h aver

(4B).

concentrations recorded at the urban sites owing to the

regional nature of the pollution events (Fig. 4B). The

time series of these parameters tend to exhibit a

concatenation of simultaneous ‘high’ and ‘low’ episodes.

For the sake of brevity we will not describe here episodes

in detail (the analysis we did is similar to that performed

by Rodrıguez et al., 2003). Fig. 5A and B shows some

examples of events occurring in September and October

1999, when the most important high PM mass events

happened on 9–17 and 23–26 September, and 2–3, 8–16

and 27–29 October 1999. Simultaneous high ‘urban PM

mass concentrations events’ and ‘rural BC and V

concentrations events at ISPRA’ are associated with

anticyclonic situations and/or weak pressure gradient

conditions. Under these conditions, the very low wind

speed (o1m s�1) and the vertical stability favour theaccumulation of pollutants in the boundary layer. In

contrast, low PM mass and V concentrations events

occurred during North-Foehn events.

3.3.2. N5–10 events

At ISPRA, N5–10 and N5–20 concentrations are

highly anti-correlated with those of S, V (Fig. 4F and

G), N5–800, BC, CO, NOx and SO2 (not shown for

gases). In contrast N5–10 concentrations are positively

correlated with those of O3. See in Fig. 5B how these

‘N5–10 and N5–20’ events typically occur under low PM

mass, V and S concentrations.

Meteorological analysis shows that these N5–10 and

N10–20 events occur during abrupt entries of ‘clean air’

in this part of the Po valley, mostly just after cold front

passages. These N5–10 events are associated with abrupt

drops in the relative humidity, high wind speeds, raise in

the total solar radiation intensity, and abrupt increases

20000

10000

0 0

40

160

80

120

800 20 6040

20000 500 1000 1500

3000

2000

1000

00 500 1000 1500 2000

6000

8000

4000

2000

0

ISPRA

N50

-100

, cm

-3

S,µm2/cm3 S,µm2/cm3

y = 446.45xR2= 0.661

V, µm3/cm3 V, µm3/cm3

800 20 6040

DpN

, nm

aged) at ISPRA and at the urban sites included in our study area

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4

2

1

0

120

4

100

60

140

40

20

0

80

40

30

20

0

50

10

1000

600

120

200

0

800

3

2

1

0

3

400

100

60

140

40

20

0

80

201 10201 10

4

1999September October

PM-mass events N5-10 events

BC at ISPRA

BC

, µg/

m3

V, µ

g/m

3

S/ 2

0, µ

m2/

m3

10·B

C, µ

g/m

3

BC

, µg/

m3

DpN

, nm

100·

N5-

800,

cm

-3

N5-

10, c

m-3

N10

-20

/ 3, c

m-3

TSP

, µg/

m3

Urban TSP

Fig. 5. Daily mean levels of BC at ISPRA-rural and TSP at several urban sites (5A) and other aerosol properties at ISPRA (5B–5C)

during September and October 1999. Arrows highlight the main PM-mass and N5–10 events.

S. Rodrıguez et al. / Atmospheric Environment 39 (2005) 6734–67466740

in ozone levels. These features can be noticed in the

events occurring from 15 November to 15 December

1999, shown as examples in Fig. 6. These are the

classical features of North-Foehn events in the Alps,

typically associated with high O3 concentrations from

autumn to spring (Davies and Schuepbach, 1994;

Schuepbach et al., 1999; Seibert et al., 2000; Weber

and Prevot, 2002). An example of these events is shown

in Fig. 7, where the arrows shown in Fig. 7A and B

highlight the entry of clean, dry and subsiding air at the

onset of the N5–10 episode occurring at the end of

September 1999 (highlighted in Fig. 5B). The strong new

particle formation starting at noon and its subsequent

growth is clearly observed in the size distribution

measurements (Fig. 7C).

Hourly averaged values of several aerosol parameters

and gases during N5–10 episodes are shown in the first

column of Fig. 8. These N5–10 & N5–20 episodes are

caused by a strong enhancement in the photochemically

induced nucleation rates that result in ‘‘nucleation

episodes’’. Notice that the daily evolution of N5–10

and N10–20 is highly correlated with the solar radiation

intensity and that it exhibits a much higher amplitude

than in the ‘mean daily cycles’ discussed above (Fig. 8A1

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0

10

0

20

2

1

0

3

60

10

80

40

0

15

5

%

515 20 25 101

10

0

15

5

1500

500

1000

Total Solar Radiation

Relative Humidity

Windspeed

Ozone

3·N5-10 N10-20

cm-3

100

m/s

ppbv

mW

/cm

2

December 1999November

Fig. 6. Daily mean levels of N5–10, N5–20, O3 and several

meteorological parameters highlighting the N5–10 episodes.

S. Rodrıguez et al. / Atmospheric Environment 39 (2005) 6734–6746 6741

and 2C). During these events, nucleation is strongly

favoured from the morning to the afternoon owing to

(Fig. 8B1 and E1): (1) the low available surface of pre-

existing particles (typically So200 mm2 cm�3), decreas-

ing the condensation rate of semi-volatile species onto

the particles, and (2) the low relative humidity (fre-

quently down to 50%) that hinder the gas-to-particle

transfer by wet chemical reactions (i.e. cloud droplet

chemistry). Sulphuric acid and products of the organic

volatile compounds oxidation have been identified as

key compounds during the nucleation events (Kulmala

et al., 1998, 2004; Weber et al., 1997; Yu et al., 1999).

Moreover, ammonia, ions and some aromatic acids may

also play a role (Korhonen et al., 1999; Turco et al.,

1998; Zhang et al., 2004).

Hourly mean levels of N5–10 during these nucleation

episodes are much lower at ISPRA rural site

(1000–3500 cm�3) than downwind of Pittsburgh urban

emissions (4000–20,000 cm�3; Stanier et al., 2004b),

where SO2 and NO levels are typically 10 and 1.7 times

higher than those at ISPRA during these nucleation

events, respectively. The cleaner air conditions at ISPRA

are also favoured by the North-Foehn conditions, when

the air masses subside from the mid-troposphere (Davies

and Schuepbach, 1994; Schuepbach et al., 1999).

Finally, these N5–10 events mainly occurred in

autumn (September–December) and spring (March–

May; Fig. 7D), being all them recorded during Foehn

events. This seasonal trend is similar to that observed at

other sites, where nucleation events are also observed

just after cold front passages (Birmili et al., 2001; Woo et

al., 2001; Stanier et al., 2004b).

3.3.3. Size distribution and air quality monitoring

At ISPRA, the submicron aerosol size distribution

experiences frequent and significant variations. The

diameter DpN is positively correlated with V, S,

N5–800, BC, NOx, SO2 and CO concentrations (Fig.

4C–E; not shown for gases). This indicates that the

particle size DpN increases with increasing PM pollution

conditions (measured as volume, mass or number

concentration). Fig. 5 illustrates how in 3–6 days

periods, DpN, V and BC levels at ISPRA experience

variations from 20–50 nm, o10mm3/cm3, o1mgm�3

during clean air conditions to 100–140 nm, 30–40mm3/cm3 and 2–4mgm�3 during pollution events, respec-

tively. Notice also how a decrease in the particle

diameter DpN is associated with the N5–10 and

N10–20 episodes described above.

The relationship between size distribution and PM

concentrations observed at ISPRA is mainly driven by

two competing processes which result in gas-to-particle

transfer of matter: homogeneous nucleation, and

condensation of gas-phase components onto pre-exist-

ing particles. Gas-to-particle conversion occurs when

the equilibrium vapour pressure of the gas phase is

exceeded. The equilibrium vapour pressure over the

surface of the molecular clusters which could initiate

the nucleation is much higher than that above the

surface of pre-existing particles (the equilibrium vapour

pressure over a sphere increases when the sphere’s

diameter decreases according to the Kelvin effect).

Thus, homogeneous nucleation is strongly favoured

during low PM events owing to the low available

aerosol surface. In these cases, the formation rate of the

ARTICLE IN PRESS

12

8

10

4

2

0

6

20151050

2

4

6

12

18

20

0

L

H

x1019

x992L

H

x1019

x992

cm-3

Time of Day

Dia

met

er, n

m

50

0

100

N5-

10 e

vent

s (d

ays)

September 28 1999

dN/dlogD–September 29 1999x103

Month

J F M A M J° J A S O N D

Fig. 7. Surface pressure (7A) and IR Meteosat image (7B) over Europe on 28 September 1999, hourly levels of dN/dlogD on

September 29 1999 (7C) and frequency of N5–10 episodes (7D; daily mean 4400 cm�3) during 1 year period.

S. Rodrıguez et al. / Atmospheric Environment 39 (2005) 6734–67466742

gas phase is much higher than its condensation rate

onto pre-existing particles, in such a way that nuclea-

tion occurs when the equilibrium vapour pressure is

exceeded. In contrast, during PM pollution conditions,

enough aerosol surface is available and the condensa-

tion rate compensates the formation rate of the gas

phase, hindering nucleation. Thus, nucleation is hin-

dered, whereas condensation is favoured when PM

levels increase. This is the origin of the ‘grown particles’

described above during the PM-mass pollution epi-

sodes (Fig. 5C). Fig. 4H illustrates how the number

concentration of particles 450 nm is highly correlated

with S. In contrast, N5–10 and N10–20 are anti-

correlated with S (Fig. 4F and G).

This competition between condensation and nuclea-

tion influences the aerosol size distribution in different

time scales, including the daily cycles, seasonal evolution

(Figs. 2 and 3) and day-to-day variations (Figs. 5 and

8—discussed below).

The daily cycle of the aerosol size distribution dN/

dlogD, the number concentration in the different size

intervals as well as V and S size distribution, display

significant differences from the ‘clean air–N5–10 epi-

sodes’, to the ‘mid-pollution’ and ‘high pollution’

episodes (Fig. 8 and Table 2) (episodes with low

N5–10 and V levels, mostly associated with rain, have

not been considered). As stated above, N5–10 episodes

are caused by nucleation starting in the morning and

followed by a particle growth until the evening and night

(Fig. 8A1–E1). Moreover, during these events, the total

number concentration does not decrease after the

morning rush hours because of the new particle’s

production during daylight. When the aerosol volume

and surface increase throughout the ‘mid’ to ‘high’

pollution events (Table 2 and Fig. 8A2–E2 and 8A3–E3)

it is observed that: (1) the amplitude of the N5–10 daily

cycle decreases because of the decrease in the nucleation

rates, (2) there is an increase in the mean values of DpN

and N5–800, BC and NO2 concentrations, and (3) the

effects of the morning rush hours are enhanced. Notice

that in all cases, high N5–10 levels during noon and

afternoon occur when the total aerosol surface de-

creases, and that particles exhibit a coarser size at night

because of the effects of coagulation and condensation

(Fig. 8E1–8E3).

We have quantified the relationship between the

particle diameter DpN, the aerosol volume, and aerosol

surface (Fig. 9). The relation between DpN and V is very

well expressed by means of the non-linear relationship

DpN ¼ a �Vb (r2 ¼ 0:977; a�30 and b�1/3; Fig. 9A).Owing to this trend, the slope of the relationship

between DpN vs. V is higher for low V concentrations.

ARTICLE IN PRESS

50

25

0

6

4

2

0

8

1000

25

30

0

10000

5000

10

20

125

7550

100

50

25

0

0

75100

30

10

6

4

2

0

8

0

1000

15000

10000

5000

40

50

50

20

20000

125

175

50

25

0

1500

0

110

70

3050

90

16

12

8

4

6

4

2

8

500

1000

100

0

100

50

0

200

100

200

nmnm

20

0 0

15000

0

0

10

0

15000

10000

5000

N5-800

N5-100

N5-50

N5-20

500

N5-10 & N5-20 events “ C lean Air”

V<10

cm-3

0.5·N10-20N5-10

Mid-pollution degree High-pollution degree10< V< 25 25< V<85

Tot

al S

olar

Rad

iati

on

0.5·N10-20N5-10

0.5·N10-20N5-10

N5-100

N5-800

N5-50

N5-20

N5-800

N5-100

N5-20

N5-50

−50%

of

N

cm-3

−50% of N

−50% of N

150

0

500

Dp

N, n

m

V

S/ 2

5

NO

210

·BC

150

103

·V5-

20dN

/dlo

gD

0 105 15 20

dN/dlogDdN/dlogDdN/dlogD

Time of day0 105 15 20

Time of day0 105 15 20

Time of day

0

Dia

met

er, n

m

0 4 6 12 18 20 0 4 6 12 18 20 0 4 6 12 18 20

max

Fig. 8. Daily mean evolution of several aerosol parameters and gases during Clean Air, Mid and High pollution conditions according

the volume (columns 2 and 3) and N5–10 concentrations (column 1: Vo10mm3/cm3 & N5–104400 cm�3). (7C: DpN and diameters

which comprises the 50% of the total number concentration). Units: volume (mm3 cm�3), Total Solar Radiation (mWcm�2), N (cm�3),

DpN (nm), NO2 (ppbv) and BC (mgm�3). Right side scale is (white to black): 0, 5, 7, 8, 10, and 12 (103) for E1 and 0, 7, 9, 11, 12, and 16

(103) for E2 and 0, 7, 9, 12, 14 and 18 (103) for E3.

S. Rodrıguez et al. / Atmospheric Environment 39 (2005) 6734–6746 6743

This indicates that the increase in the aerosol volume

(and therefore in the mass) when the atmosphere starts

to receive pollutants after a clean air episode, is

associated with an increase in the particle size DpN

(e.g. DpN increase from 40 to 100 nm when V increase

from 5 to 30 mm3 cm�3), whereas the increases in the

aerosol volume for relatively high V concentrations (e.g.

V430 mm3 cm�3) are associated with the accumulation

ARTICLE IN PRESS

Table 2

Mean values of the total number concentration (N, cm�3), the contribution of several size ranges to N (in %), volume (mm3 cm�3) and

surface (mm2 cm�3) of the aerosol, averaged in into three ranges (o10–N5–10 events, 15–25 and 25–85mm3 cm�3). BC in ngm�3, CO in

ppmv and other gases in ppbv

Size (nm) 5–10 10–20 20–50 50–100 100–200 200–400 400–500 500–800

Vo10 % % % % % % % %

N 8443 5 15 35 26 15 3 0 0

S 204 0 0 6 19 39 29 4 1

V 7 0 0 1 8 31 51 4 4

15oVo25N 11226 2 7 26 33 26 6 0 0

S 507 0 0 3 14 40 35 6 2

V 18 0 0 0 5 29 56 5 4

25oVo85N 14829 1 4 19 33 33 10 1 0

S 939 0 0 1 11 37 40 8 2

V 38 0 0 0 4 25 59 7 6

BC SO2 CO NO NO2 O3

Vo10 494 1.6 0.4 4.5 6.8 30.2

15oVo25 1690 2.3 0.7 16.2 18.6 13.3

25oVo85 2733 2.7 1.0 23.3 23.2 10.3

10 30 5020 7040 80600

0

70% of N 90% of N

500

300

250

Dia

met

er, n

m 200

150

100

50

0

300

250

200

150

100

50

y=30.45.x.0.333

r2=0.977

y=8.38.x.0.373

r2=0.981

DpN

V, µm3/cm3 S, µm3/cm3

250 750 1000 1250 1500 1750

Fig. 9. Averaged values of the particles diameter DpN and diameters comprising the 70% and 90% of N5–800 at each side of DpN for

different values of the aerosol volume (9A: averaged in 5mm3 cm�3 intervals) and aerosol surface (9B: averaged in 250mm2/cm3

intervals for S4250mm2 cm�3 and 25mm2 cm�3 intervals for So25mm2 cm�3). This analysis is based on 24-h means.

S. Rodrıguez et al. / Atmospheric Environment 39 (2005) 6734–67466744

of particles around the ‘accumulation mode’ (�100 nm

at ISPRA) in the size distribution dN/dlogD (Fig. 9A).

This relationship between the number concentration

of particles o20 nm and DpN with the aerosol volume

(Figs. 4E, G and 9A) has important implications for the

air quality monitoring strategies and health effects

assessment: a decrease in the aerosol volume (and

therefore in the aerosol mass) is associated with a

decrease in the size of the particles and an increase of

the number concentration of particles o20 nm. This

ARTICLE IN PRESSS. Rodrıguez et al. / Atmospheric Environment 39 (2005) 6734–6746 6745

decrease in the V and mass concentrations could happen

if, in the current framework, a reduction in the emission

rates of gaseous precursors of aerosols is performed in

order to try to improve the air quality. Because the

contribution of ultrafine particles to the submicron

aerosol mass and volume is very low (Table 2), the

parameters classically used for assessing the air quality

based on the mass concentrations (e.g. PM2.5 or PM1)

are not suitable for monitoring this type of aerosols

pollution, and consequently a specific monitoring of the

ultrafine particle number concentration should be

performed. Note in Table 2 how the particles o50 nmcontributes with 25–60% to the total number, whereas

their contribution to the aerosol volume is o1.5%.Observe also in Fig. 5 the previously discussed ‘high

N5–10 and N10–20’ and ‘low V and mass episodes’. The

fact that a significant fraction of the current urban PM

emissions occurs in the ultrafine fraction (Shi et al.,

2001; Wahlin et al., 2001) highlights the significance of

this specific ultrafine PM number concentration mon-

itoring.

4. Conclusions

In a rural environment in Northern Italy, the size

distribution of submicron aerosols and its relationship

with the aerosol volume and mass is strongly influenced

by two competing processes: homogeneous nucleation

(resulting in new o10 nm particles) and growth of

particles by condensation of gas-phase species onto pre-

existing particles. Because of the influence of these

processes, the aerosol size distribution experience

frequent and significant variations (including daily and

seasonal evolution, as well as important day-to-day

changes). Moreover, there is a strong link between

‘aerosol concentrations’ and ‘aerosol size distribution’:

the particle size increases with the aerosol volume and

mass concentrations, whereas the number concentration

of particles o20 nm is anti-correlated with the aerosol

volume and mass. This has important implications for

the strategies for the air quality monitoring: a reduction

in the aerosol volume or mass (e.g. a consequence of a

decrease in the emission rates of gaseous aerosols

precursors) would increase the number concentration

of the smallest ultrafine particles. Because the contribu-

tion of these particles to the aerosol volume and mass

concentrations is very low, the parameters classically

used for the air quality assessing (PM2.5 & PM1) are not

suitable for monitoring of this type of PM pollution, and

consequently a specific monitoring of the ultrafine PM

number concentration should be performed. The sig-

nificance of this specific ultrafine PM number concen-

tration monitoring is supported by two facts already

proven: (1) adverse effects on human health—epidemio-

logical studies have shown that ultrafine particles and

carbonaceous compounds (the main ultrafine PM

constituents), are associated with mortality in urban

areas, and (2) emissions—a significant fraction of the

current urban emissions of PM occurs in the ultrafine

fraction.

Acknowledgement

The air quality data from the Lombardy region were

provided by the Agenzia Regionale per la Protezione

dell’Ambiente (http://www.ambiente.regione.lombar-

dia.it). Surface pressure charts and infrared radiation

satellite observations maps have been provided by the

UK Meteorological Office and Meteosat, respectively.

This work has been developed at the JRC with a

postdoctoral grant awarded by the Ministry of Educa-

tion and Culture of Spain, and a collaboration agree-

ment between the JRC and CSIC.

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