El desierto, la sierra, la selva tropical en el Vicariato Apostólico de la

The contribution of a regional highway to PM10 levels was estimated in a manner

similar to the traffic contribution in urban areas by comparing the PM measurements at three NAQMN sites: Breukelen (#641), a street station situated right next to the A2 highway from Utrecht to Amsterdam, and the regional background stations in

Westmaas (#437) and Wageningen (#724).

The daily variations are now considered in more detail for the street station

Breukelen; see Figure 2.49. The average PM10 concentration for weekdays (Monday–

Monday–Friday are 33.7 µg/m3

(± 1.4); for Saturdays and Sundays the average regional background concentration of PM10 is 32.7 µg/m3 (± 1.4).

Figure 2.49 Average weekday PM10 concentration in µg/m3 in 2000 for the street

station Breukelen (Highway A2) and average of the regional stations Westmaas and Wageningen.

The CAR model was used to find out whether the observed PM10 contribution made

by the highway could be explained by modelling. It should be noted that due to the uncertainties of both the measurements and the model, only a very general

comparison is possible. Input for the model was the numbers of passing vehicles per 24 hours, the percentage of trucks and buses, distances between road and monitoring station, type of landscape, average speed of passing cars, and regional background concentration levels. Estimated traffic densities in the year 2000 on the A2 near Breukelen are presented in Table 2.24.

Table 2.24 Traffic densities in the year 2000 as measured on the A2 near Breukelen (Source: Directorate-General of Public Works and Water Management, AVV

Transport Research Center); absolute number of vehicles averaged per 24 hours, and the relative contributions of the vehicle classes Cars, Light Duty Vehicles (LDV) and Trucks.

Mon Tues Wed Thurs Fri Sat Sun

Total vehicles 71,486 74,215 74,467 75,837 76,643 55,532 56,053

Cars % 86 86 86 87 87 93 95

LDV % 8 8 8 8 8 6 4

Trucks % 6 6 6 5 5 1 1

The background concentration is estimated as the average from two regional PM10

and NOx monitoring stations geographically nearest to the highway: Westmaas and

Wageningen. The traffic contribution of PM10 and of NOx at Breukelen in 2000 is

estimated as the local concentration levels at the highway minus the background concentration. The results of the simulation using CAR and the measured

concentrations are shown in Table 2.25. The contribution of NOx concentrations at PM10 and daily variation

0 5 10 15 20 25 30 35 40 45

Mon Tue Wed Thu Fri Sat Sun

Breukelen was calculated as a validation of the model assumptions, as the emission factors of NOx are reasonably well-known, unlike the PM emissions.

Table 2.25 Measured traffic contribution (concentration at Breukelen minus

background concentration) and contribution of NOx and PM10 in 2000 calculated by

the model CAR, by day of the week at Breukelen.

NOx (µg/m3) Measured NOx (µg/m3) Modelled PM10 (µg/m3) Measured PM10 (µg/m3) Modelled Monday 109 120 3.3 1.5 Tuesday 105 124 4.3 1.5 Wednesday 102 125 4.2 1.6 Thursday 95 120 3.7 1.5 Friday 118 121 4.4 1.5 Saturday 78 65 2.9 0.9 Sunday 67 66 2.7 1.0 Mon–Fri avg. 106 122 4.0 1.5 Sat–Sun avg. 72 66 2.8 1.0

Table 2.25 shows the contribution of NOx to have been estimated reasonably well. For

weekdays, the CAR model calculated 116% of the measured contribution; on weekend days 92%. The ratio of NOx concentrations in Breukelen on weekdays to

that on weekends was also modelled adequately by CAR: the ratio is 0.69 (measured) versus 0.54 (modelled). This result also indicates that the quality of the

parameterisation of dispersion in the CAR model is satisfactory and that it could be used in principle to calculate the particulate contribution made by highway traffic as well.

For PM10, the ratio of weekday traffic contribution to weekend contribution is 0.70 for

the measurements and 0.67 for the CAR model. This is a very fair result and shows that the relative results for PM10 are satisfactory. However, the absolute values differ.

In absolute terms, the CAR model presents only about 40% of the measured PM10

contribution in Breukelen. This rather poor result can be attributed to a number of factors. A plausible cause of the difference between measured and modelled PM10 in

Table 2.25 is re-suspended crustal material. The CAR model uses only primary PM10

emissions. The definition of traffic emissions does not take into account the

contribution of re-suspended crustal material caused by vehicle-induced turbulence. According to Visser et al. (2001), this re-suspension is considerable. The annual average contribution of road dust re-suspended by passing traffic at a major road in Amsterdam amounted to 2–3.5 µg/m3

,while the local tailpipe contribution of EC + OC was 2.1 µg/m3. This indicates that the indirect PM10 contribution made by re-

suspension is probably of the same order as that of the direct traffic contribution. If this re-suspension of crustal material by traffic at the highway by Breukelen had been of the same order as that in Amsterdam, it would account for nearly all of the mass in the CAR model currently still missing. This considerable contribution of re-suspended material was also found by the studies conducted in Rotterdam by Spoelstra and Keuken (2002).

The previously presented OPS model combined with national and European emission inventories results in annually averaged PM10 concentrations that seem only partially

to match measured results from the National Air Quality Monitoring Network. Recent work by Visser et al. (2001) indicated that the previous differences between models and measurements could be adequately explained when the non-included natural sources, anthropogenic crustal material and the contribution of the northern hemisphere, were taken into account. This is elaborated in subsection 2.6.1.

Figure 2.50 Measured and modelled concentrations of NO3 aerosol. Modelled NO3

does not include gaseous HNO3..

Figure 2.51 Measured and modelled concentrations of SO4 aerosol

SO4 conc. measured and modelled

0.00 2.00 4.00 6.00 8.00 10.00 12.00 1980 1985 1990 1995 2000 2005 S O $ c onc . [ ug/ m 3 ] measurement model

NO3 conc measured and modelled

0.00 1.00 2.00 3.00 4.00 5.00 1994 1995 1996 1997 1998 1999 2000 2001 NO 3 c o n c . [ u g /m 3 ] measurement model

Figure 2.52 Measured and modelled concentrations of NH4 aerosol.

The minor differences between the modelled SIA levels in Figures 2.50, 2.51 and 2.52 and the measurements in the NAQMN indicate that the model is capable of

reproducing concentration trends and also the absolute level of SIA. Ammonium aerosol is somewhat underestimated due to the previously reported discrepancy between NH3 measurements and emissions (Van Jaarsveld et al., 2000). The

difference is estimated at 0.4 µg/m3

ammonium and this is used as a correction in the 1995 and 2010 results. The volatilisation of ammonium nitrate from filters probably also plays a role. The filter method used underestimates the ‘true’ concentrations of ambient ammonium nitrate, as has been explained in 2.2.1.1.

The performance of state-of-the-art modelling of ammonium nitrate in aerosol is treated in Metzger’s PhD thesis (2001). Using thermodynamic descriptions of the dissociation of ammonium nitrate and the behaviour of aerosol/water mixtures, acceptable agreement is obtained with measurements at ECN, for example, in winter. In summer, though, the model predicts nitrate concentrations which are on average only 25% of the measured values. The obvious conclusion is that these models are able to predict wintertime nitrate levels adequately, but for summers produce results that are too low, especially at high temperatures (under Dutch circumstances). Validations of modelled and measured levels of SIA indicate that the models do a good job as they produce similar results, and that the trends in time (over periods of decades) can also be modelled adequately.