Air Pollution Modelling, Monitoring and Management 707 · 2014-05-12 · Air Pollution Modelling,...

Urban Pollution: Milan 1987-1995

L. Barbieri,* L. Sesana/G. Tebaldi," M. Valentin!,* P. Bacci"

Institute of Physics, Milan University, G. Celoria 16-Milan, Italy

E-Mail: facchini@imiucca. csi. unimi. it

Province Protection Unit, K Juvara 22 - Milan, Italy

*Enel Cram, Monfalcone 15 -Milan, Italy

Abstract

The city of Milan and its surroundings are inhabited by more than two million people. The

city is located in the centre of the Po valley.

The city is heavily polluted by vehicles and surrounding industries all the year round, as well

as by the combustion of heating fuels in winter.

The Institute of Physics has been running a measurement station for ten years now, situated in

the central Brera area at a height of 29 metres from the ground.

The station monitors the hourly levels of the main pollutants: SOj, NO, NO%, 0%, CO.

This paper reports their yearly trends from 1987 to 1995. Such trends include a steady

decrease in SO% levels, whereas the other pollutants turn out to have remained at constant

levels.

Comparison with the measurements performed in stations on the ground gives interestingresults.

Transactions on Ecology and the Environment vol 15, © 1997 WIT Press, www.witpress.com, ISSN 1743-3541

706 Air Pollution Modelling, Monitoring and Management

It shows an decrease in the levels of some pollutants, such as NO and CO, with height,

whereas the levels of other pollutants, such as SO2, emitted at chimney stack height, are

higher.

Higher ozone levels are also detected at the Brera station than on the ground.

Introduction

The Institute of Physics has been running a station for air quality monitoring

since 1987; the station is housed in the tower of Brera palace in Milan at a

height of 29 metres from the ground.

Continuous monitoring is performed in the station of the concentrations of the

main pollutants present in the city air, namely, SOz, nitric oxides and ozone and

carbon monoxide concentration, which has also been monitored for some years.

Analysis of the data gave the trends over the years in the levels of such

pollutants. It is also possible to compare their respective concentrations and

trends with those collected at the city stations operating at ground level.

The results of the above mentioned investigations are now reported and

discussed.

Sulphur dioxide

Fig. 1 shows the average monthly values collected year by year; the values are

at their highest in winter, decreasing to a large extent in the summer months.

Winter levels have undergone an increasing reduction; with that for 1994 having

a value equal to a sixth of the values observed in 1989.

Most SO: present in the Milan air is released by combustion processes in

heating plants; with smaller amount coming from the industrial estates located in

the town surroundings.

The increasing reduction in SO: levels in winter is due to the lower sulphur

content in fuels and to the increased use of methane gas.


Air Pollution Modelling, Monitoring and Management 707

months

Figure 1 Sulphur dioxide: Average monthly values.

Milan Brera - June 87 / June 95.

Fig. 2 shows the comparison between the average yearly trend of SO] detected

at the Brera tower and the levels detected at the ground. It turns out that winter

SO: levels are higher in the Brera station; the station is closer to stacks, that are

the main source of SO:

The summer reduction in SO: levels is due mainly to decreased emissions, but

also to the increase in chemical processes.

The ultimate fate of this pollutant is, as known, the conversion of SO: into SOg,

a compound that combines with aerosols thus giving rise to sulphates.



months

Figure 2 Sulphur dioxide: average year.

Milan Brera - Milan ground station.

hours

Figure 3 Sulphur dioxide (Brera): average 1989 winter day.



The average winter day trend shows a maximum in the morning and in the

evening hours (Fig.3); on rainy days SO: levels are lower.

NO and NO:

The average monthly trend of nitrogen monoxide is reported in Fig. 4. There is

no general decrease in NO levels over the years; a slight decrease occurs up to

1994, but the 1995 values are the highest in the nine years of measurements.

As known, the biggest source of this pollutant is car traffic, and in winter

combustion in urban heating plants.

Winter levels are enhanced, owing to the higher stability of the urban

atmosphere, and summer levels are lowered.

Fig. 5 shows the comparison between values obtained in the tower and at

ground level.

NO turns out to be higher at ground level than up the Brera tower.

NOz shows the same yearly trend as its precursor, NO (Fig. 6).

It is interesting to note the high values ofNOz levels in summer.



t

Li, L1 ! I I I

t s• 1 1 1 ! !

months

Figure 4 Nitrogen monoxide: average monthly values.

Milan Brera - June 87 / December 95.

sep octmonths

Figure 5 Nitrogen monoxide: average year.

Milan Brera - Milan ground station.



i i ! ! ! I ! ! 1 1 1 ! 1 I I 1 1months

Figure 6 Nitrogen dioxide: average monthly values.

Milan Brera - June 87 / December 95.

The average NCVNO ratio in the upper station ranges between 0.4 and 0.8 in

winter (Fig. 7a), whereas it reaches 2-6 in summer when the oxidizing effect of

the atmosphere is higher (Fig.7b).

At the ground such ratios range from 0.3-0.6 in winter to 1-2 in summer.

In winter the NCVNO ratio was of the same order at the ground station as in

the elevated station.

In the hours of strongest sunlight, in summer, the strong oxidizing effect of the

atmosphere makes the NCVNO ratio lower at the ground station than at the

elevated station. The deviation between the NO^NO ratios at the two stations

is greatest in the evening hours, when ground level NO emission is at its highest.



hours

a)

hours

b)

Figure 7 NO^NO ratio (height and ground):

average 1994 season day.

a) Winter b) Summer.



Ozone

The average monthly ozone levels are shown in Fig. 8, which reports the trends

for the years 1987-1995. Particularly high levels were observed in 1987, 1991,

1994.

5 2 2 2 S 3 Si & i & i i i

Figure 8 Ozone: average monthly values.

Milan Brera - June 87 / June 95.

As is known, urban ozone arises for the most part from photochemical reactions

that involve nitrogen oxides and volatile organic compounds^' The maximum

ozone levels are therefore present in summer.

When thermal vertical turbulence takes place ozone diffuses toward the mixing

layer and mixes with the ozone present in the height.

It is interesting to point out that the average monthly ozone levels in the Brera

station are almost double those measured at ground level. The analysis of the

average 1994 day shows the difference is particularly high during daytime when

photochemical reactions are active, whereas it is almost insignificant overnight

(Figs.9a and 9b).



2 ^ 2 2hours

a)

b)

Figure 9 Ozone: average 1994 day.

a) Milan Brer a b) Milan ground station.



Carbon Monoxide

Carbon monoxide has a stable trend over the years, with the highest values in

winter and the lowest values in summer (Fig. 10).

2•g

monthsI

Figure 10 Carbon monoxide: average monthly values.

Milan Brera - December 90 / December 95.

The average daily carbon monoxide level at the ground changes consequent to

heavy traffic: at the elevated station the morning peak is less sharp, while there

is no evening peak and accumulation is gradual and constant starting from the

morning.

Conclusions

The station in the Brera tower at the height of 29 metres from the ground

enabled interesting observations on the pollutants in urban air and is different

from ground stations since it gives average values unaffected by street and

building topography.



From the analysis of the data collected during the ten years of monitoring it can

be concluded that:

1) SO:: levels decrease until 1995. As mentioned, such falls are due both to the

use of low-sulphur content fuels and to increasing exploitation of methane for

urban heating.

2) NO, NOi, CO: these pollutants, which mainly arise from car traffic, show

almost constant values that are greater at the ground than higher up.

3) Ozone: this has higher values in Brera tower than at the ground. Its average

monthly values during the monitoring time ranges between 40 and 50 ppb in

summer.

It would be interesting to make a comparison between ozone levels in towns

and the higher levels detected in agricultural and mountain areas in Lombards

and to discuss the processes of ozone development and dissolution in terms of

the transport and distribution of ozone and its precursors ' .

References

1. Trainer, M. Photo-oxidants, Problems and Solutions, Proceedings of

Eurotrac Symposium, 33-37, Ed. Academic Publishing Den Haag, The

Netherlands, 1992.

2. Beck, J. , Grennfelt, P. Distribution of Ozone over Europe, Proceedings of

Eurotrac Symposium, 43-58, Ed. Academic Publishing Den Haag, The

Netherlands, 1992.

3. Zanella, G. , Facchini, U. Regione Lombardia: analisi dei dati per il periodo

1994/1996, Proceedings of "Inquinamento da ozono nella valle padana",

Milano, Italy, 1997.

4. Facchini, U. , Sesana, L. A research by radon measurements in Milan and in

the Prealps, Proceedings of "Air Pollution V'\ Bologna, Italy, 1997.


Air Pollution Modelling, Monitoring and Management 707 · 2014-05-12 · Air Pollution Modelling,...

Documents

Transcript of Air Pollution Modelling, Monitoring and Management 707 · 2014-05-12 · Air Pollution Modelling,...