CONOCIÉNDOME A MÍ MISMO

It is evident that air pollution concentrations depend on the meteorological dispersion conditions. In particular, it is the upper tail of the concentration frequency distribution that shows relatively large sensitivity. In AOP II the inter-annual variability was estimated at about 6 % for SO2 concentration annual mean values and about 10 % for the 98-percentile values.

For the ShAIR scenario NO₂ calculations were made for 2010 emissions using actual meteorological conditions for the years 1991–95. The results show that conclusions on exceedance of the one-hour air quality guideline do not depend at all on the meteorological situation: estimated urban concentrations are so far below the threshold value that even under worst dispersion conditions exceedance is modelled only incidentally. As 18

exceedances per year are permitted for the one-hour guideline, there is no violation of the guideline modelled.

In selecting exceedance of the annual guideline as the evaluation criterion, it is shown that the number of non-compliance cities varies between 42 and 48 (corresponding range in population is 20.7 to 22.2 %). The model runs suggests that overall, 1991 is the year with the worst and 1995 the year with the best dispersion conditions. However, a more in-depth analysis shows that this conclusion is not valid at a city level.

A second example of the large variation introduced by different meteorological conditions can be illustrated with a calculation of SO₂. As can be seen from the results presented above, compliance with the daily mean concentration guideline will lead to the most stringent requirements on emission reduction. For the reference year 2010 we have made a first-order estimate for each city of the emission reduction which must be in compliance with the air quality guideline. In estimating the emission reduction, only the local urban contribution has been considered; the impact of emission reduction on neighbouring cities has not been considered. Therefore the reductions presented here are only indicative. Required emission reductions are calculated using meteorological data for the years 1991–95. In Figure 6.12 the cities are ranked according to the averaged required reduction; the minimum and maximum required reduction in this five-year period is shown as well. Three groups of cities can be identified. In the first group of about 240 cities, the guideline is met, even under the worst dispersion condition. In the third group (about 20 cities), the guideline is largely exceeded and a sharp reduction is required, irrespective of the meteorological conditions. In the second group (about 50 cities), the concentration levels are just above or below the guideline. Accidental meteorological conditions lead to relatively large variations in the required reductions.

In optimisation studies the required emission reduction may vary highly, depending on the selected meteorological year. It is therefore recommended that multiannual dispersion calculations are used in this type of study.

Table 6.2.

Number of cities that are (partly) not in compliance with the air quality guidelines and percentage of total modelled population living in the non-compliance cities. Results for reference situation and

assuming a spatial concentration distribution over the city. Top: guideline for NO2 annual mean;

bottom: guideline for SO2 daily mean

NO2 Number of cities Percentage of population exposed

Year Reference Variable Reference Variable

1990 124 200 59.7 43.5

2010 42 93 20.6 15.1

2020 30 79 12.4 8.2

SO2

Year Reference Variable Reference Variable

1990 135 173 51.7 36.0

2010 51 77 16.4 10.8

The effect of uncertainties in meteorological data on ozone production has been studied by comparing predicted differences in the ‘target output’ values for the cities of Athens and Stuttgart (Moussiopoulos et al., 2000). Emission and meteorological data for 1990 have been used in the reference calculation. In the sensitivity calculation wind velocity was changed by ± 10 % and wind direction by ± 45o_{. The sensitivity of the ‘target output’ values is shown in} Table 6.3, separated for the whole domain and the urban area, respectively. While a 10 % variation in the wind speed does not appear to have a noticeable effect on the ‘target output’ values in Stuttgart, differences emerge in the case of Athens. Although most of the air pollution episodes in the Greater Athens area are associated with the development of a sea breeze, the situation is probably even worse in the case of stagnant conditions (i.e. a critical balance between synoptic and mesoscale circulations). Consequently, an increase in wind speed leads to lower ‘target output’ values, whereas a decrease in wind speed leads to a higher ozone burden. In general, the variation of the wind direction has only a marginal effect on the ‘target output’ values in Stuttgart. Differences occur mostly due to the fact that different

Estimated reduction in urban SO2 emission required to meet the air quality guideline for the daily mean

calculated using emission conditions for 2010 and meteorological conditions for the five-year period 1991–95. Cities are ranked according to the required averaged reduction, the range in required reduction calculated for the individual meteorological years is plotted as well

Figure 6.12.

Influence of meteorological input data uncertainties on the uncertainty of various ‘target output’ values (TOV), characterising the ozone levels in Stuttgart and Athens (Max: maximum exceedance days, Ave: average exceedance days, 8hmax: maximum 8-hour ozone concentration, Cex: averaged concentration on days showing exceedance)

Table 6.3. Symbol ! " # $ % & ' Difference ±5% +5% to +20% +20% to +50% > +50% -5% to -20% -20% to -50% < -50%

Speed Dir Speed Dir

TOV Area considered Stuttgart -1 0% + 10% -4 5 ° +4 5 ° Athens -1 0% + 10% -4 5 ° +4 5 °

Max domain 71 days " ! ! ! 110 days " ! ! "

urban 52 days ! ! ! % 43 days " % ! %

Ave domain 60 days " ! ! ! 65 days " % ! !

urban 42 days ! ! % % 17 days " % # &

8hmax domain 274 µg/m3 _{! ! ! ! 438} µ_g/m3 _{! ! % !}

urban 222 µg/m3 _{! ! ! ! 236} µ_g/m3 _{" % # !}

Cex domain 154 µg/m3 _{! ! ! ! 140} µ_g/m3 _{! ! ! !}

urban 149 µg/m3 _{! ! ! ! 152} µ_g/m3 _{! % " !}

Variation in required SO2emission reduction (range (Ri-<R>); 24h AQO)

-0.3 -0.1 0.1 0.3 0.5 0.7 0.9 240 250 260 270 280 290 300 City number % r eduction

urban areas upwind of the city were taken into account, depending on the changes in the prevailing wind direction. The picture is different in Athens, since different days with sea breeze now occur due to changes in the wind direction and have their strongest impact in the ozone exceedance days averaged over the urban area.

The effect of uncertainties in initial and boundary concentrations on ozone production has been studied by comparing predicted differences in the ‘target output’ values. Initial and boundary concentrations were varied with ± 10 % for ozone and NO₂ and with ± 50 % for VOCs. The sensitivities of the ‘target output’ values are shown in Table 6.4 separated into the whole domain and the urban area, respectively. In general, the variation of the boundary concentrations has only a marginal effect on the maximum 8-hour mean ozone concentration and the averaged concentration on exceedance days. However, decreasing the ozone

boundary concentration by 10 % lowered the exceedance days by 10 % in Stuttgart and 20– 40 % in Athens, whereas an increase of 10 % led to considerably higher days of exceedance (more than 50 % in the case of Athens). The response to the change in NOx boundary concentrations is marginal in the case of Stuttgart and remains low (of the order of 10 %) in the case of Athens. Simulations using the VOC boundary concentrations varying by ±50 % show a similar pattern to the variations in boundary ozone.

6.7. Conclusions

The GEA-model tools for the assessment of urban air quality applied in the Auto-Oil II Programme are further extended for application to cities in central and eastern Europe. The model system is extended with procedures to estimate the number of excess deaths attributed to exposure to SO₂ and NO₂.

Under the assumption of the ShAIR emission scenario, urban air quality shows great improvement but violations of (proposed) air quality guidelines are still expected in 2020. Major problems with SO₂ exposure are found in eastern Europe; in a limited number of cities a deterioration in air quality is modelled for 2010–2020. The estimated number of excess deaths attributed to SO₂ exposure shows a sharp decrease between 1990 and 2010; from 2010 to 2020 a further decrease to about six excess deaths per 100 000 inhabitants is estimated. Compared with the calculations based on the emission scenarios developed for EEUTC and AOP II, the current results are generally higher.

Sensitivity calculations show that the modelled concentrations are sensitive to meteorological conditions. For example, using meteorological data for the period 1991–95, we found the inter-annual variability in SO2 concentration to be about 6 % for annual mean and 10 % for the 98-percentile. However, in considering the compliance to air quality guidelines, sensitivity depends strongly on the ratio between threshold value and current concentrations. For

Table 6.4. Similar to table 6.3, but shows boundary concentration input data

Symbol ! " # $ % & ' Difference ±5% +5% to +20% +20% to +50% > +50% -5% to -20% -20% to -50% < -50% O3 NOx VOC O3 NOx VOC TOV Area considered Stuttgart -10% +10% -10% +10% -50% +50% Athens -10% +10% -10% +10% -50% +50%

Max domain 71 days % " ! ! % " 110 days & # ! ! % "

urban 52 days % # ! ! & # 43 days & $ % " & "

Ave domain 60 days % " ! ! % " 65 days & $ % " & "

Urban 42 days % " ! ! & # 17 days % # ! ! % "

8hmax Domain 274 µg/m3 _{! ! ! ! % " 438} µ_g/m3 _{! ! ! ! ! !} Urban 222 µg/m3 _{! ! ! ! % " 236} µ_g/m3 _{! ! ! ! % "} Cex Domain 154 µg/m3 _{! ! ! ! ! ! 140} µ_g/m3 _{! ! ! ! ! !} Urban 149 µg/m3 _{! ! ! ! ! ! 152} µ_g/m3 _{! ! ! ! ! !}

example, conclusions on exceedance of the one-hour air quality guideline for NO₂ do not depend at all on the meteorological situation: estimated urban concentrations are so far below the threshold value that even under the worst dispersion conditions the guideline is realised. However, when making a first estimate of the required reduction in urban emissions needed to meet the 24-hour air quality guideline for SO2, the reduction may vary up to 50– 60 %, depending on the meteorological year selected.

7. Assessment of current approach