Black Carbon Instead of Particle Mass Concentration as an Indicator for the Traffic Related Particles in the Brussels Capital Region

doi:10.4236/jep.2011.25060

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Journal of Environmental Protection, 2011, 2, 525-532

doi:10.4236/jep.2011.25060 Published Online July 2011 (http://www.scirp.org/journal/jep)

Black Carbon Instead of Particle Mass

Concentration as an Indicator for the Traffic

Related Particles in the Brussels Capital Region

Peter Vanderstraeten1, Michael Forton1, Olivier Brasseur1, Zvi Y. Offer2

1Brussels Institute for the Management of the Environment, Laboratory for Environmental Research, Brussels, Belgium; 2The Jacob

Blaustein Institutes for Desert Research, Ben-Gurion University, Sede Boqer Campus, Israel.

Email: {pvd, mfo}@ibgebim.be, [email protected], [email protected]

Received February 20th, 2011; revised April 1st, 2011; accepted May 4th, 2011.

ABSTRACT

The Brussels Capital Region has difficulties in meeting the stringent EU daily limit va lue for PM10 in all its measuring

sites. Postponing the attainment of the deadline was not granted by the EU Commission, mainly due to insufficient

judged measures to reduce road traffic emissions. However, a thorough analysis of the data makes clear that neither

the particle mass concentration (PM10 and PM2.5) nor the particle number concentration a re specific metrics for eva-

luating the pa rticle pollution originated by traffic. In fa ct, increased formation o f secondary aerosol, together with ad-

verse meteorological conditions and the (re) suspension of the coarser fraction are by far the three main explanations

for the numerous PM10 exceeding values. From our experience, amongst the particles measured, only the results for

Black Carbon (BC), mainly present in the lower submicron range, a re reflective of the direct influence of lo cal traffic.

Measured at two traffic sites a long with PM mass and number concentra tions, the data for Black Carbon show a strik-

ing correlation with nitrogen monoxid e, a parameter strongly related with the proximity of the lo cal traffic. The corre-

lation factor between Black Carbon data and NO or NOX is much higher than between Black Carbon and the PM mass

or number concentration. Therefore the assessment of traffic related particles should consider Black Carbon rather

than PM10 or PM2.5.

Keywords: Black Carbon, PM10, PM2.5, Particle Mass Concentration, Particle Number Concentration

1. Introduction and Methods

The air quality in the Brussels urban region has been

measured since 1968. The telemetric network consists of

11 measuring sites, covering a variety of distinct urban

environments and measuring all pollutants imposed by

the EU regulations (2008/50/CE). Nitrogen oxides are

measured in all eleven measuring sites, particle mass

concentration PM10 and PM2.5 are measured in 6 and 5

of these locations respectively, by the use of continuous

TEOM 1400Ab analyzers (tapered element oscillating

microbalance) equipped with FDMS 8500 modules (fil-

ter dynamics measurement system).

Other measurements concern ozone, carbon monoxide,

sulfur dioxide, carbon dioxide, benzene with a selection

of twelve volatile organic components (VOC), benzo a

pyrene together with ten other polyaromatic hydrocar-

bons (PHA) and finally heavy metals such as lead, arse-

nic, nickel, cadmium and mercury vapor. Different

measurements started between 1968 and 1990, and sys-

tems were improved during the upgrade of the network to

its present config uration (1994-2002).

Since 2008 the particle number concentration is meas-

ured along with the particle mass concentration in two

traffic oriented sites. The number of particles is regis-

tered for 31 different size classes, with diameters ranging

from 0.25 µm to 32 µm, by means of Grimm laser light

scattering spectrometers, model 365. One of the two sites,

Woluwe, is located along a highway leading the traffic

coming from the East towards the centre of Brussels, its

immediate surrounding having a relatively open structure.

The second one, at Molenbeek, along the economic and

commercial axis of the Region, is situated mu ch closer to

the Brussels urban centre. At these same sites monitoring

Black Carbon was started, in July 2009 and July 2010

respectively, using an aethalometer, model AE22-ER from

Magee Scientific. At the Molenbeek site, Black Smoke

measurements according to the older OECD method [1]

Black Carbon Instead of Particle Mass Concentration as an Indicator for the Traffic Related Particles

526 in the Brussels Capital Region

are still available.

The air quality directive 2008/5 0/CE foresees two limit

values for PM10 in ambient air. The 40 µg/m3 limit value

for the annual average concentration is respected in all

six measuring sites. The Brussels Capital Region fails to

meet the second limit value: 50 µg/m3 as daily average

PM10 concentration may not be exceeded on more than

35 days per year. Since the year 2005 this second limit

value has been systematically violated in two of the six

PM10 measuring sites and occasionally in some of the

other sites. Postponing of the attainment of the deadline

to the year 2011 was no t granted by the EU commission.

The proposed emission reduction measures were judged

as insufficient to enable compliance, in due time, with

the limit value. In its refusal the Commission mentioned

especially the lack of low emission zones and measures

such as road pricing.

2. Results and Discussion

2.1. Non Compliance with PM10 Daily Limit

Value

A thorough analysis of the observed data shows that a

drastic emission reduction (70% to 80%) will be needed

if compliance is to be assured solely by local measures

[2]. In fact road traffic represents about 75% of the total

PM emissions registered by the Brussels emission in-

ventory. However, a comparison of PM mass concentra-

tion levels inside and outside Brussels seems to indicate

that, depending on site location and specific conditions,

particles emitted directly by the local traffic represent

only about 10% to 20% of the total measured PM10 mass

concentration.

Furthermore the dimensionless normalized average

weekly concentration pattern (Figure 1) shows a de-

crease, during the weekend, of only about 10 to 15 % for

the PM10 and PM2.5 concentration, compared to a con-

centration loss of 40 to 60% for NO and 20 to 30% for

NO2, both traffic related pollutants. Background concen-

tration for NO is generally very low and the presence of

elevated levels of NO is practically entirely of local traf-

fic origin. The background concentration for NO2 is sub-

stantial and represents at least 50% of the concentration

level measured near the urban centre. The PM10 back-

ground level represents about 65% of the inner city con-

centration. At this stage, even a permanent weekend

emission regime would not permit to guarantee compli-

ance with the PM10 daily limit value.

In the Brussels Capital Region at least 3 different phe-

nomena may lead, separately or by combination, to ele-

vated PM10 concentrations and to an increased risk of

exceeding the 50 µg/m3 daily limit value [2]. Adverse

meteorological conditions, due to a combination of tem-

perature inversion and low wind speed resulting in poor

dispersion, are a common factor leading to high concen-

tration levels. Under these conditions, mainly occurring

during winter time, between late November and early

March, high concentration levels are detected at all sites,

inducing simultaneous PM10 exceeding values at several

locations. For instance, in February 2008 and January

2009, between 8 and 12 exceeding days were counted at

Figure 1. Normalized weekly concentration pattern for NO, NO2, PM10 and PM2.5, Results averaged over 5 stations and 3

consecutive calendar years 2007-2009; Normalization at 1 by dividing the average concentration of each day of the week by

the average concentration for all working days.

Black Carbon Instead of Particle Mass Concentration as an Indicator for the Traffic Related Particles 527

in the Brussels Capital Region

the different measuring sites in Brussels.

A second phenomenon, very important and still unde-

restimated is the formation of secondary aerosol, mainly

during the period March-April and to a lesser extent dur-

ing September-October. The spreading of manure on a

large scale, in the surrounding regions, before and after

the agricultural season, releases a massive source of am-

monia. At conditions of moderate temperature and high

relative humidity, together with the presence of NO2, a

stable secondary aerosol is formed with ammonium ni-

trate as a main component. During the month of April

2007 and again in April 2009 this formation caused be-

tween 8 and 16 exceeding days at the different measuring

sites in Brussels. Due to the same phenomenon the 2006

car free Sunday achieved the second highest daily value

of that year [3] and the car free Su nd ay of 2009 proved to

be one of the other surprising exceeding days. Numerous

studies, performed at different locations in Europe as

well as in the USA, emphasize the importance of secon-

dary aerosol and the inorganic and/or organic compo-

nents involved. This has also been reported more recently

by several research teams [4-8].

The third phenomenon, (re)suspension of the coarser

fraction (particles between 2.5 and 10 µm) is linked with

the advection of dry air, mainly coming from the large

eastern sector. Under these conditions and in the pres-

ence of a local source, particles of the coarser fraction are

suspended by the wind and/or by the turbulences created

by the traffic [2]. Due to the presence of local sources,

this phenomenon is more frequently encountered at the

two measuring sites where the limit value is systemati-

cally exceeded.

2.2. Car Free Sunday 2009—Black Carbon,

Particle Mass and Number Concentration

On Sunday the 20 th September 2009, for the eighth time,

a car free Sunday was organized by the Brussels authori-

ties. From 9:00 till 19:00 h local time (7:00 till 17:00 h

UT), all private road traffic was banned over the entire

Brussels Capital Region. Only public transport, emer-

gency services, taxis and a limited number of exceptions

were allowed and the speed limit was set at 30 km/hr.

During the morning the wind was mainly coming from

the south, just before noon suddenly it changed and

started blowing from north-northwest till late in the eve-

ning, importing polluted air masses with secondary

aerosol already formed over the western part of Belgium.

Meteorological conditions could be considered as rela-

tively unfavorable to pollution dispersion during the ear-

ly morning, but rather favorable in the late morning and

the afternoon. The boundary layer computed from

ECMWF fields, increased from 60 m in the morning to

950 m in the afternoon.

In the middle of the car free period a sharp and sudden

concentration increase of PM10 and PM2.5 mass con-

centrations was observed in all Brussels measuring sites,

with maximum h alf hourly PM10 concen trations ranging

between 100 an d 120 µg /m3 (>90% as PM2.5), leading to

average PM10 concentrations exceeding the 50 µg/m3

daily limit value. At the same time particle number con-

centrations increased for all size ranges between 0.25 and

2.5 µm, with a maximum of ±1.200 .000 p articles per liter

of air for the total number of particles above 0.25 µm

(Figure 2), reaching one of the highest half hourly values

(99.8the percentile) of the whole year 2009. However,

such an increase was not observed for the coarser frac-

tion, particles above 2.5 µm, mainly from mineral origin,

put into (re)suspension by the wind or by the turbulences

created by road traffic.

The graph in Figure 3 represents, for the period from

Friday the 18th till Tuesday 22 September 2009, the dy-

namic evolution of the half hourly values for Black Car-

bon, the PM10 mass concentration and NO at the Wolu-

we site. The data for Black Carbon refer to the concentra-

tion scale on the left (0 - 21 µg/m3) and those for PM10

and NO to the scale on the right (0 - 140 µg/m3). The

traffic ban hours during the car free Sunday are indicated

by the short horizontal line in the middle. During the

traffic ban hours the PM10 concentration peaks, while

the Black Carbon level continues to decrease, quite op-

posed to PM10. Clearly, the evolution of Black Carbon

does not always follow that for PM10 but rather that of

the traffic related NO, as can be seen from the results on

Saturday the 19th and during the morning and evening

rush hours of Monday 21st Septembe r.

2.3. Correlation between Black Carbon, NO,

NOX, PM Mass and Number Concentration

The observations made during the 2009 car free Sunday

stand also for longer periods. This is well illustrated by

the evolution of the half hourly data for February 2010.

The graph in Figure 4 represents the data for Black Car-

bon and PM10 and the graph in Figure 5 those for BC

and NO. Data for BC refer to the left scale (0 - 14 µg/m3)

and those for PM10 and NO respectively to the scales on

the right (0 - 140 µg/m3).

On average and for most of the time, the BC and

PM10 concentrations follow nearly the same pattern

(Figure 4), meaning that there is roughly a factor 10,

corresponding to the scaling factor, between the concen-

tration levels. However, although many peak values do

coincide, there are several discrepancies during other

peak periods: high peak values for PM10 and not for BC

or vice versa.

Black Carbon Instead of Particle Mass Concentration as an Indicator for the Traffic Related Particles

528 in the Brussels Capital Region

Figure 2. Particle Number concentration—Number of Particles > 0.25 µm on an average Sunday, an average working day

and the car free Sunday 20 September 2009.

Figure 3. Black Carbon, PM10 and NO—Concentration Evolution between Friday 18 and Tuesday 22 September 2009 with

the Car free Sunday 20 Se ptember.

Black Carbon Instead of Particle Mass Concentration as an Indicator for the Traffic Related Particles 529

in the Brussels Capital Region

Figure 4. Black Carbon and PM10—Woluwe site—February 2010, Evolution half hourly concentration values.

Figure 5. Black Carbon and NO—Woluwe site—February 2010, Evolution half hourly concentration values.

A very clear example is illustrated by the situation

between February 6th - 9th, indicated in the graph between

two vertical dashed lines. The presence of high peak

values, both for PM10 and PM2.5, but without an associ-

ated Black Carbon peak, gives a strong indication for

pollution by particles that are not from local origin. This

may be the result of the formation of secondary aerosol

and/or an importation of particles from a distant origin.

When however no high PM2.5 values would be associ-

ated with a PM10 peak, then the (re)suspension of the

coarser particles (2.5 - 10 µm), would be a more likely

explanation.

Black Carbon Instead of Particle Mass Concentration as an Indicator for the Traffic Related Particles

530 in the Brussels Capital Region

For this observed PM10 and PM2.5 peak, measures

taken to reduce local Brussels traffic emissions will not

enable to lower the PM10 concentration levels signifi-

cantly and so the measures could be interpreted as sym-

bolic or sensitizing, although they always do limit the

presence of the more harmful carbon containing particles.

However, measures to reduce local traffic emissions

clearly would surpass the sensitizing stage if a Black

Carbon peak should coincide with such a PM10 and

PM2.5 peak.

Black Carbon and NO peak values do coincide more

often, both showing a regular diurnal pattern (Figure 5).

This is also true for the days corresponding to the re-

ported PM10 peak. A closer examination reveals that the

baseline for NO, compared to the height of the peak val-

ues, is generally lower than the baseline for BC. At sev-

eral occasions the NO levels fall down towards the de-

tection limit and therefore the observed ratio between the

average concentrations for BC and NO could signifi-

cantly differ from the ratio between the scales (~10) used

for presentation.

Correlation factors were computed between BC and

pollution data for NO, NO2, NOX, CO, P M10 and PM2.5

mass concentration and the particle number concentra-

tion (PNC) for all particles above 0.25 µm, above 0.65

µm or comprised between 0.25 and 0.65 µm. For the

Woluwe site the studied period concerns the whole cal-

endar year 2010, for the Molenbeek site data are avail-

able for the six month during period “July-December

2010”. Correlation factors were computed for the detailed

half hourly data as well as for the average daily values.

The computed results are represented in Tables 1 and 2.

At the Woluwe site, seen over the whole year 2010,

the correlation between the detailed Black Carbon data

and the data for NO or NOX (Rcorr ~0.80 to 0.85) is

much better than between the BC data and those for the

PM10 mass concentration (Rcorr ~0.50). Poor correlation

Table 1. Woluwe site—Correlation Factors between Black

Carbon data and the concentration values of other pollut-

ants. Calendar Year 2010.

WOLUWE site Correlation

Factor to BC Half Hourly

Data Daily Averages

NO 0.8030 0.8653

NO2 0.7014 0.7238

NOX 0.8551 0.8974

CO 0.5935 0.6821

PM10 0.5048 0.6453

PNC > 0.25 µm 0.4137 0.5577

PNC > 0.65 µm 0.2287 0.3294

0.25 µm < PNC < 0.65 µm 0.4146 0.5593

Table 2. Molenbeek site—Correlation Factors between

Black Carbon data and the concentration values of other

pollutants. Period “July-December 2010”.

MOLENBEEK site

Correlation Factor to BC Half Hourly

Data Daily Averages

NO 0.7962 0.8840

NO2 0.7813 0.8719

NOX 0.8485 0.9166

CO 0.7587 0.7981

PM10 0.5991 0.7196

PM2.5 0.5281 0.6609

PNC > 0.25 µm 0.4781 0.6133

PNC > 0.65 µm 0.2935 0.3972

0.25 µm < PNC < 0.65 µm 0.4779 0.6135

Black Smoke (OECD 1964) 0.9637

factors are obtained between BC and the particle number

concentration, even for the smaller size ranges, particles

between 0.25 and 0.65 µm (Rcorr ~0.40). The moderate

correlation factor between BC and PM10 and PM2.5

(below) confirms the poor correlation between EC (ele-

mentary carbon) and PM10 or PM2.5 reported by Sil-

lanpää et al. [7] in a study concerning 6 Euro pean cities.

For the daily average data the correlation factors with

NO and NOX remain high (Rcorr ~0.86 to 0.89), while

for PM10 a slightly better correlation is obtained (Rcorr

~0.64) as compar ed to the half hour ly data. It is uncertain

to what part the lower correlation factor for the half

hourly data could be explained by distinct properties of

the analyzing devices. Detection systems having larger

time constants show much smoother variations for the

shorter integrating periods than fast responding systems,

eventually resulting in a lower correlation for the half

hourly values than for the daily averages.

The results obtained at the Molenbeek site co nfirm the

findings from Woluwe. Regarding the six month period

“July-December 2010”, the correlation between the de-

tailed Black Carbon data and th e data for NO and NOX is

also better (Rcorr ~0.80 to 0.85) than between BC and

PM10 or PM2.5 (Rcorr ~0.52 to 0.60). Lower correlation

factors are computed between BC and PNC, with Rcorr

~0.47 for the smaller size ranges. As in the case of the

Woluwe site, the correlation factors with the NO and

NOX daily data remain high and those for the PM10 or

PM2.5 daily values (Rcorr ~0.66 to 0.72) are also higher

than for the hal f hourly data.

The good correlation with NO an d NOX at both traffic

sites, especially for the half hourly data, seems to indi-

cate that Black Carbon is strongly associated with local

traffic emissions and that the link is tighter than for the

PM10 or PM2.5 mass concentration. The percentage of

Black Carbon Instead of Particle Mass Concentration as an Indicator for the Traffic Related Particles 531

in the Brussels Capital Region

PM10 mass concentration explained by Black Carbon is

estimated, first by the ratio of the average concentration

values and secondly, by the slope of the orthogonal re-

gression line through the origin: Y[BC] = b * X[PM10].

Both these factors are computed for the half hourly data

as well as for the daily average concentrations and are

represented in Table 3. Estimated this way, the Black

Carbon concentration represents about 8% to 9% of the

PM10 mass concentration at the Woluwe site and about

12% to 13% at the Molenbeek site. Since BC is well

correlated with traffic specific parameters NO and NOX

and since it concerns mainly submicron carbon contain-

ing particles, one can state that the tailpipe emissions

from road traffic do not represent more than 8% to 13%

of the PM10 mass concentration at the considered traffic

oriented sites of the Brussels Capital Reg ion.

For the Molenbeek site, it is estimated in a similar way

that the BC concentration represents about 15% to 16%

of the PM2.5 mass concent ration (Table 4).

At the Molenbeek site a striking correlation (Rcorr =

0.9637) is observed between Black Carbon and Black

Smoke daily data. The ratio of about 0.20 between both

data sets [BC~0.2 * Black Smoke] is comparable to the

results reported for the Netherlands [9] and the UK [10].

3. Conclusions

Results for Black Carbon, PM10 and PM2.5 mass con-

centration, NO and NOX and the particle number concen-

tration were analyzed in detail for two traffic oriented

Table 3. Ratio of the Average Concentration values—Black

Carbon over PM10 Slope (b) of the Orthogonal Regression

Line Y[BC] = b * X[PM10].

Site

(Period) Data Ratio AVG

Conc

BC/PM10

Slope Regression

Line

BC = b * PM10

Woluwe

(1 year) Half Hourly 0.0912 0.0781

Woluwe

(1 year) Daily Average 0.0916 0.0860

Molenbeek

(6 months) Half Hourly 0.1283 0.1122

Molenbeek

(6 months) Daily Average 0.1287 0.1192

Table 4. Ratio of the Average Concentration values—Black

Carbon over PM2.5 Slope (b) of the Orthogonal Regression

Line Y[BC] = b * X[PM2.5].

Site

(Period) Data Ratio AVG

Conc

BC/PM2.5

Slope Regression

Line

BC = b * PM2.5

Molenbeek

(6 months) Half Hourly 0.1568 0. 1 3 39

Molenbeek

(6 months) Daily Average 0.1578 0.1484

sites in the Brussels Capital Region: the Woluwe site

situated along a highway and the Molenbeek site, located

much closer to the urban centre and situated along the

commercial and economic axis of the Region. The stud-

ied period concerns the calendar year 2010 for the Wo-

luwe site and a six month period “July-December 2010”

for the Molenbeek site.

There is a much better correlation between BC and NO

or NOX data than between BC and PM10 or PM2.5 mass

concentration. The correlation factor between BC and

NO reaches about 0.80 for both half hourly and inte-

grated daily data. Between BC and PM10 the correlation

factor yields only about 0.50 to 0.60 for the half hourly

data and about 0.65 to 0.71 for the daily data.

This concludes to a better correlation between the

short term peak values of BC and NO than between the

peak values of BC and PM10 or PM2.5. Since NO is still

one the best indicators for the vicinity of road traffic, it

can be stated that BC is more directly linked with the

local traffic than the PM10 or PM2.5 mass concentration.

Therefore Black Carbon rather than PM10 or PM2.5

should be considered in the assessment of the traffic re-

lated particles and in the evaluation of traffic emission

reductions.

At the Woluwe site Black Carbon represents only

about 8% of the total measured PM10 mass concentra-

tion. At the Molenbeek site, on average, BC represents

about 12% of the PM10 and 16% of the PM2.5 mass

concentration.

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