Labor de las religiosas de la Madre Laura, las Carmelitas Misioneras y las

The modelling of daily PM concentrations is relevant because of the daily average standards in the EU in 2005. The regional dispersion models EUROS and LOTOS are used to obtain insight into daily averaged PM concentrations, sources, sinks and precursors, their dynamics and the mutual influence of all these factors on ambient PM levels.

Annually as well as daily averaged PM concentrations for Europe are calculated using two regional chemical dispersion models, EUROS (Jacobs and van Pul, 1994; van Loon 1996; Matthijsen et al., 2002) and LOTOS (Builtjes, 1992). In both models, daily and annual averages are based on a summation of calculated hourly values.

a. EUROS

The EUROS model was developed at RIVM. It is a Eulerian air quality model used to simulate the dispersion and transport of components in the lower troposphere in order to evaluate possible policy measures for Long-Range Transboundary Air Pollution (LRTAP). The modelled area extends over a large part of Europe. The horizontal base

grid consists of 52 x 55 grid cells with a 0.55° x 0.55° lat/lon shifted pole projection (about 60 x 60 km2 in the Netherlands). Local uniform grid refinement is possible up to 4 levels, resulting in a maximum latitude-longitude resolution of 0.069° x 0.069° (about 7.5 x 7.5 km2 in the Netherlands). Transport is based on half-hourly updates from ECMWF meteorological fields for wind velocity components u and v,

temperature, relative humidity and the geopotential height. Horizontal transport is described by advection and diffusion, whereas vertical transport is treated using a well-mixed boundary layer concept.

The atmospheric vertical grid structure in the EUROS version used here consists of four layers: the surface layer (SL), the mixing layer (ML), the reservoir layer (RL) and the top layer (TL). The surface layer and the mixing layer together form the atmospheric boundary layer. The depths of the four layers are modelled to be uniform over the whole domain, but vary in time during the day due to the growth of the mixing layer – except for the surface layer whose depth is fixed at 50 m. The growth of the mixing layer during daylight hours is represented through a constant

climatological growth rate.

Primary emitted particles are distributed in the same four size classes as included in the CEPMEIP emission inventory (see the extensive information on the web site

http://www.mep.tno.nl/emissions/). Secondary sulphate and nitrate particles are

removed by dry and wet deposition (at present, scavenging of primary PM by rain is not applied). Secondary PM formation is described in a condensed ozone scheme, which includes four production reactions for particulate sulphate and nitrate: two gas- phase production reactions and two first order reactions representing formation in cloud water and on existing aerosols. The SO2 and NOx emissions are described with

monthly, weekly and daily variations. PM10 emissions have a daily emission profile.

The results for NO3 particles are not included here since validation showed that their

parameterisation is not yet adequate.

Sources, sinks and processes that are not, or not yet, included or for which data are currently too provisional are:

1. Natural PM (e.g. sea salt, wind-blown crustal material).

2. Re-suspension of soil dust due to anthropogenic activity (e.g. traffic). 3. Explicit description of ammonia (NH3) and ammonium (NH4+).

4. Formation of secondary organic aerosol (SOA).

5. Particle microphysics (interaction between particles, e.g. coagulation). 6. Explicit treatment of aqueous-phase and heterogeneous chemistry.

For nitrate aerosol, a simplified mechanism is used (no aerosol thermodynamics or ammonium variations) and there is a simple wet removal parameterisation in the current version of the EUROS model.

Future efforts will be dedicated to improving secondary inorganic aerosol (SIA) modelling: a more detailed mechanism for nitrate and the inclusion of ammonium. In addition, sea salt will be included and attention will also focus on validation of the (already implemented) improved description of vertical resolution and the vertical advection process.

b. LOTOS

The LOTOS model was developed at TNO. It is a three-dimensional transport chemistry model of intermediate complexity covering Europe. The idea behind the model is that it should contain all relevant processes (explicitly or in a parameterised form) in such a way that hour-by-hour calculations over periods of years are feasible. LOTOS was originally developed for ozone modelling. In the last few years the model has been extended to include aerosols. A general and detailed description of the model can be found in Builtjes (1992). Some relevant details on the model concept and the modelling of aerosols within LOTOS are given below.

The horizontal resolution of the model is 0.5º by 0.25º lon/lat (about 55 x 27 km2). In the vertical, the concept of dynamical layers is applied: the depths of the three layers depend on the height of the mixing layer (varying in time and space). The first layer represents the mixing layer, the other two are equally distributed over the rest of the vertical domain. The meteorological input for LOTOS is prepared by the Free University of Berlin. The emission database of relevant components was constructed by TNO for the base year 1995. For later years, the 1995 emissions are scaled, based on the national emission totals.

Sulphate is formed in the gaseous phase as well as in the liquid phase. The oxidation of sulphur dioxide by the OH radical is represented in the gas phase reaction mechanism CBM-IV. Another important oxidation pathway, in particular in winter, is the formation of sulphate in clouds. Due to insufficient data on clouds in the meteorological input, it is difficult to explicitly represent this process in a model. Therefore, it is represented with a first order reaction constant that varies with cloud cover and relative humidity, similar to the approach followed in EUROS (Matthijsen et al., 2002).

The equilibrium in the formation of ammonium nitrates, for example, is very sensitive to ambient conditions and has been calculated using a modified version of the MARS system, a module which is embedded in the MADE module (Ackermann et al., 1995; Ackermann et al., 1998). This module also calculates the size distribution of aerosols using a modal approach.

Sources, sinks and processes that are not, or not yet, included or for which data are currently too provisional are:

1. Natural PM sources (e.g. sea salt, wind-blown crustal material). 2. Re-suspension of soil dust due to anthropogenic activity (e.g. traffic). 3. Highly parameterised cloud chemistry (oxidation of SO2).

4. Formation of secondary organic aerosols (SOA).

5. Surface layer (the mixing layer is taken as the lowest layer).

The LOTOS module contains a simple wet removal parameterisation. Also, the distribution over time of NH3 emissions is very uncertain. Efforts will be dedicated to

implementing a more detailed mechanism for SO2 oxidation/cloud chemistry, the

inclusion of sea salt, black carbon (BS or EC) and improved NH3 emissions and

depositions. Finally, attention will also focus on improving the vertical resolution (e.g. inclusion of surface layer) and coupling with a grid-refined version.

It should be noted that model results for PM are provisional, because validation of the PM10 concentrations and sources and sinks is still ongoing. For this reason, the data

inferred from the simulations should be treated as qualitative information rather than quantitative. Results from comparative model exercises will therefore be used, since these are probably the most reliable.

The primary PM emissions in EUROS are based on emissions from subsections 2.5.1 and 2.5.2. Emissions for the formation of secondary aerosols (SO2, NOx, NH3) are

taken from the Fifth National Environmental Outlook (further referred to by its Dutch abbreviation MV5; RIVM, 2000). For LOTOS, slightly higher country totals were used for primary PM as a result of a different CEPMEIP version (e.g. primary PM emissions in the Netherlands are about 10% higher).

Before discussing the daily-average results, we first give an overview of the distributions of the annually averaged total PM concentrations for 1995 in the Netherlands from the models OPS, EUROS and LOTOS; Figures 2.25.a, b and c respectively. It should be remembered that within OPS and LOTOS total PM consists of primary PM10 and the secondary aerosols sulphate, nitrate and ammonium, while

for EUROS ammonium and nitrate are not incorporated, which complicates a quantitative comparison. In order to obtain a better comparison between the models, the EUROS results should be increased by annually averaged concentrations of ammonium and nitrate, i.e. in total approximately 8 µg/m3

(OPS estimate). The gradients of these components over the Netherlands are relatively small when compared with PM10. We corrected this difference visually by using a slightly

different classification (i.e. concentrations below 20 µg/m3

are shown at a higher resolution) to present the EUROS results in Figure 2.25.b.

The main features of urban and/or industrialised areas in the Netherlands (and in Belgium and Germany for EUROS) are resolved equally well by OPS and EUROS modelling despite their grid size differences (5 x 5 km2 and 15 x 15 km2 respectively). The use of different classifications illustrates nicely the qualitative agreement between the models. The large concentration gradients found near populated areas mainly result from local sources of primary PM10 emissions.