Simulation of nitrous acid formation taking into account heterogeneous pathways: application to the Milan metropolitan area

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1 Environmental Modelling & Software 15 (2000) Simulation of nitrous acid formation taking into account heterogeneous pathways: application to the Milan metropolitan area N. Moussiopoulos a,*, S. Papalexiou a, G. Lammel b, T. Arvanitis a a Laboratory of Heat Transfer and Environmental Engineering, Aristotle University, Thessaloniki, Greece b Max Planck Institute for Meteorology, Hamburg, Germany Abstract Nitrous acid (HONO) is of particular importance in atmospheric chemistry; its photolysis significantly enhances photooxidation processes early in the morning or during wintertime, through the rapid production of OH radicals in the presence of long wavelength radiation, and in urban areas with low O 3 levels. Previous studies reveal that neither direct HONO emissions from combustion sources nor homogeneous gas-phase reactions are sufficient to explain the observed HONO concentrations. Therefore, the role of heterogeneous production pathways is analysed in this work by means of the multilayer photochemical model MUSE, where a parameterization of heterogeneous NO y chemistry has been implemented. MUSE was then applied to simulate pollutant dispersion and transformation over the Milan metropolitan area on 9 March 1994, a day in the 1994 Milan field measurement campaign characterized by high pollutant concentrations. Simulation results for the concentrations of O 3 and NO x are in good agreement with the observations. During most of the time, the predicted HONO concentration levels are distinctly lower than the measured levels. The results are discussed and perspectives for further work are provided Elsevier Science Ltd. All rights reserved. Keywords: Nitrous acid; Heterogeneous formation; Photochemical modeling 1. Introduction Nitrous acid (HONO) has been observed to build up in urban atmospheres during the night-time hours. The first unambiguous identification of HONO in the troposphere was made in 1979 above Julich in Germany (Perner and Platt, 1979). Since then, HONO has been observed in urban, rural and remote locations (Platt and Perner, 1980; Harris et al., 1982; Sjödin and Ferm, 1985; Biermann et al., 1988; Lammel and Perner, 1988; Appel et al., 1990; Andres-Hernández et al., 1996; Harrison et al., 1996). The importance of HONO in tropospheric chemistry has long been recognized. Its significance lies especially in its ability to undergo rapid photolysis which gives rise to a pulse of OH radical early in the morning, before other sources (O 3 and HCHO photolysis) become dominant: HONO hψ (λ 400 nm) OH NO (N. Moussio- * Corresponding author. address: moussio@vergina.eng.auth.gr poulos). Therefore, in the NO x -loaded atmosphere it is regarded as one of the main initiators of the photochemical chain mechanism which leads to the decay of anthropogenic and biogenic emissions and the build-up of photooxidants (Harris et al., 1982; Finlayson-Pitts and Pitts, 1986). Although the removal processes for HONO are well known, its major formation pathways in urban atmospheres are highly uncertain. HONO source mechanisms discussed in the literature can be grouped into three main pathways: direct emissions from combustion sources, homogeneous gas-phase reactions, and heterogeneous phase reactions. Direct emissions have been quantified as % of the NO x emissions (Febo, 1994; Kirchstetter et al., 1996), so they cannot justify the high HONO concentration levels observed in the urban atmosphere. Studies in reaction chambers of homogeneous phase reactions leading to HONO production resulted in the exclusion of a quantitative significance of this formation pathway for atmospheric HONO (Kaiser and Wu, 1977; Svensson et al., 1987). At the same time, the hypothesis of a predomi /00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S (00)

2 630 N. Moussiopoulos et al. / Environmental Modelling & Software 15 (2000) nance of heterogeneous reactions in HONO formation is reinforced by recent experimental findings (Pitts et al., 1984; Svensson et al., 1987; Jenkin et al., 1988; Febo and Perrino, 1991; Kleffmann et al., 1998; see Lammel and Cape, 1996, for a review). In the present work, the role of heterogeneous pathways leading to the formation of HONO is analysed by means of the multilayer photochemical model MUSE in which a state-of-the-art parameterization of heterogeneous NO y chemistry has been implemented. This improved model was applied to simulate pollutant dispersion and transformation over the Milan metropolitan area on 9 March 1994, a day in the 1994 Milan field measurement campaign (Febo et al., 1996) which was characterized by high pollutant concentrations. Simulation results for the concentrations of O 3, NO x and HONO are discussed in the context of observed patterns. 2. The model system used for the simulations Wind flow simulations were performed with the aid of the non-hydrostatic mesoscale model MEMO, whereas pollutant dispersion and transformation simulations were performed with the multilayer model MUSE. Both models are constituents of the European Zooming Model which was developed initially for the refined modelling of transport and chemical transformation of pollutants in selected European regions in the frame of the EURO- TRAC subproject EUMAC (EZM was formerly called the EUMAC Zooming Model). MEMO is a physically complete non-hydrostatic mesoscale model which allows multiple nested grid simulations to be performed (Moussiopoulos et al., 1993; Kunz and Moussiopoulos, 1995). Within MEMO, the conservation equations for mass, momentum and scalar quantities as potential temperature, turbulent kinetic energy and specific humidity are solved. The model uses the anelastic approximation to filter sound waves. In the present paper the model version using the Boussinesq approximation is applied. The governing equations are solved in terrain-influenced coordinates. Non-equidistant grid spacing is allowed in all directions. The numerical solution is based on second-order discretization applied on a staggered grid. MUSE is a multilayer Eulerian photochemical dispersion model for reactive species in the local-to-regional scale (Sahm et al., 1997). Its conceptual basis is the division of the boundary layer into individual layers; the thickness of each layer is allowed to vary in the course of the day in order to adequately simulate the dynamics of the atmospheric boundary layer. In this study five vertical layers have been used. Chemical transformations can be treated in the model by any suitable chemical reaction mechanism. In the present application, it was used in conjunction with the EMEP mechanism (Simpson, 1995). For the needs of the present work, MUSE had to be extended by a suitable parameterization of heterogeneous NO y chemistry. The parameterization predicts an HONO formation rate based on a review of the knowledge accumulated mainly from laboratory studies (Svensson et al., 1987; Kleffmann et al., 1998; and others). It differentiates between various ground and aerosol surfaces. The influences of ambient humidity on aerosol (continental aerosol, sea salt, acidic aerosol, carbonaceous aerosol) and ground (vegetation, soils, urban surfaces, water) surface area and on water availability for surface reactions (differentiation of three wetness types) are considered. The kinetics of the process obeys first order in NO 2 concentration and either pseudo-zeroth order (for low surface coverage with H 2 O) or first order in water vapour concentration (for water molecules available on the surface in multilayer adsorption). Reactivities of laboratory surfaces were taken as representative for those ambient surfaces with no known particular reactivity which would be specific for their chemical properties. The parameterization predicts typical NO x - into-hono conversion rates of the order of 0.3 3% h 1 with highest values reached in urban areas and under conditions of high humidity and little mixing. It is capable of predicting the HONO source term in the nearground urban atmosphere with good agreement (Lammel, 2000a,b). 3. Case specification The simulations of pollutant dispersion and transformation were performed on a km 2 domain around the Milan agglomeration at a resolution of 3 km (Fig. 1). The simulated period was 9 March Fig. 1. Simulation domain and locations of the measuring stations (JUV, Juvara; LIG, Liguria; SEN, Senato; VER, Verziere; MAR, Marche; VIT, St Vittore). The Milan urban area is located at the centre of the domain and has an approximate diameter of 20 km (corresponding to seven grid cells).

3 N. Moussiopoulos et al. / Environmental Modelling & Software 15 (2000) All relevant meteorological data (three-dimensional wind fields, potential temperature, turbulent kinetic energy, surface roughness, Monin Obukhov length and friction velocity) were derived for pre-defined times from simulation results of the 3-D non-hydrostatic mesoscale model MEMO. Between these individual input times a temporal interpolation was performed. Photolysis rates were computed as functions of the solar zenith angle. As the available information was insufficient to describe the spatial and temporal variation of the cloud cover in the Milan region, a cloudless sky was assumed in the simulations. The pattern of the vertical temperature profile at 0 and 12 LST that was used as input to the simulations constituted an indirect indication that supported this assumption. Base case simulations were performed using the EMEP chemical reaction mechanism (Derwent, 1993; Simpson, 1995) and they were based on an emissions inventory comprising emissions from traffic and industry (large point sources were accounted for separately), as well as biogenic emissions. In particular, the latter comprised intense NO x ground emissions to account for recent findings according to which considerable NO x emissions may be of natural origin, leading to particularly high contributions in intensively managed agricultural areas (Stohl et al., 1996; Umweltbundesamt, 1998). In reality, NO x emissions from soils are subject to many influences (soil type and humidity, land use, fertilization rate, etc.) and therefore highly variable. As a first approach, spatially and temporally homogeneous natural emissions distributed over the whole domain, of the same order of magnitude as the anthropogenic emissions, have been assumed. As direct emission of HONO is thought to account for only a small fraction of HONO abundances, these sources are not considered in the base case simulations. Initial concentration fields were produced by performing a 24-h start-up run. Background concentrations consistent with results of the large-scale model EMEP for the area of Milan were used as lateral boundary conditions. In particular, the values of 50, 20 and 500 ppbv were used with regard to O 3,NO 2 and CO, respectively. For HONO, a boundary concentration value of 0.5 ppbv was assumed. All boundary conditions were assumed not to vary with space and time. As respective data were not available, the parameterization of heterogeneous NO y chemistry assumes aerosol surfaces which were scaled using an archetypal size distribution (for urban aerosol; Jaenicke, 1988) based on total suspended matter concentrations (Febo and Perrino, personal communication) and, for the ground surfaces, a surface-to-area ratio based on the situation in the city of Hamburg, Germany. In order to investigate the impact of several input parameters on the predicted HONO levels, several additional simulations were carried out. In particular, (a) the lowest nocturnal mixing height level allowed in the simulations was set to 40 m instead of 70 m considered in the base case calculations, (b) a boundary concentration of 1 ppbv was used for HONO, (c) direct HONO emissions of the order of 0.5% of NO x emissions were assumed and (d) no special assumption as regards NO x enhanced natural emissions was made. 4. Model results and discussion 4.1. Ozone With regard to O 3, no important formation is taking place at this time of year as shown in Fig. 2. Highest values are computed at the eastern part of the domain and are due to the ozone blown in from the boundaries. Simulations and observations at two measurement sites of the Milan permanent monitoring network are in good agreement with regard to the maximum values, which do not exceed 25 ppbv, as can be seen in Fig. 3, whereas with regard to the time at which the maximum occurs, the model predicts it almost 2 hours in advance. Several reasons could be at the origin of this discrepancy: underestimation of the mixing height in the morning hours, unsuccessful representation of the driving patterns in the diurnal variation of the emissions considered, or cloud presence which delayed ozone formation in combination with insufficiencies in the photolysis rates calculations, which are computed only as functions of the solar zenith angle. Further model improvements are consequently required to account for this inadequacy. In the course of the afternoon, at around 15 LST, a sharp and short drop of the ozone levels, not resolved by the model, was observed at both measurement sites, being most probably Fig. 2. Diurnal variation of the wind speed and the ground-level ozone concentration predicted with MUSE at four different times on 9 March 1994.

4 632 N. Moussiopoulos et al. / Environmental Modelling & Software 15 (2000) Fig. 3. Simulated diurnal variation of the surface-level ozone concentration on 9 March 1994, compared to observations at two measuring stations in the area of Milan. associated either with air masses rich in NO passing through the area, due to a change in wind direction, or with cloud appearance. Model simulations assuming that no heterogeneous HONO formation occurs have shown that the impact of the heterogeneous HONO formation on the ozone distribution, for the case investigated here, is negligible within the modelling domain. As a consequence of the characteristic times of OH-initiated decay of hydrocarbons, significant effects are expected on the time-scale of many hours to days Nitrogen dioxide Fig. 4 shows the calculated diurnal variation of the surface level NO 2 pattern in the Milan area on 9 March During the night, the NO 2 concentration pattern is spatially homogeneous at around 70 ppbv. Early in the morning, as the emissions are intensified, a higher concentration cell is developing in the core of the domain above the city and holding until the evening hours, reaching maxima at around 10 and 20 LST. As no important photochemical activity is taking place (see also Fig. 2), the NO 2 diurnal variation reflects that of the emissions. This behaviour is also shown in Fig. 5, where the simulated NO 2 concentration is compared to observations at six measurement sites, five belonging to the permanent monitoring network of Milan and the campaign station St Vittore (see Fig. 1). In general, the NO 2 diurnal pattern is very well reproduced by the model. The underestimation in the afternoon at some measurement sites (most pronounced at Senato site) reflects the fact that predicted maximum ozone levels are in advance, but also the sharp drop in ozone levels at around 15 LST which could not be resolved by the model Nitrous acid Simulated HONO concentration fields are shown in Fig. 6 at six different times. It is apparent that HONO accumulates during the night-time and in particular in the plume of the source area. HONO production during both the day and the night is a consequence of the heterogeneous sources while the homogeneous source reaction, which is a combination of NO with the hydroxyl radical (only during the daytime), does not contribute significantly. The predicted HONO formation averaged over the whole area (Fig. 6) equals ca. 0.3 ppbv h 1 or as a relative rate (in combination with the NO 2 values, Fig. 4) ca. 0.5% h 1. As a consequence of the interaction of chemistry and transport, the maximum HONO levels are computed downwind of the city in the western part of the domain. The urban plume is not very pronounced which is a consequence of the biogenic emissions distribution (uniform in time and space). In the morning, HONO is rapidly photolysed and kept at very low levels until late in the afternoon when the photolysis process ceases and the early morning HONO levels are re-established. These predictions reflect reasonable values as they are in agreement with HONO observations in the wider area (Febo, 1994; Andres-Hernández et al., 1996; Staffelbach et al., 1997) and at other urban sites in Europe (Platt and Perner, 1980; Sjödin and Ferm, 1985; Lammel and Perner, 1988; Harrison et al., 1996). In Fig. 7, the simulated diurnal variation of HONO concentration is shown at two locations: at a Milan centre site, St Vittore, and a site downwind of the urban area. During the night, HONO levels are of the order of 2 ppbv at the urban site, while they reach 2.5 ppbv downwind. In the daytime, they drop almost to zero as photolysis largely dominates HONO equilibrium. After sunset, the pollutant s concentration starts increasing again. HONO observational data, horizontally integrating over 0.4 km and vertically integrating between 10 and 30 m above ground, i.e. within the urban canopy, are available from one site, St Vittore (see Fig. 7). The measurements were made by means of a differential optical absorption system (DOAS) and were validated by intercomparison with a second measurement technique (annular denuders) at the site, which showed good agreement (Febo et al., 1996). During the stability episode 7 12 March 94, night-time maxima between 7 and 10 ppbv

5 N. Moussiopoulos et al. / Environmental Modelling & Software 15 (2000) Fig. 4. Diurnal variation of the wind speed and the ground-level NO 2 concentration predicted with MUSE at six different times on 9 March HONO were observed and daytime minima between 2 and 3 ppbv but not less than that were reported (Fig. 7). These values are astonishingly high considering the effectiveness of HONO photolysis (about 10 3 s 1 on a sunny day; Bongartz et al., 1991). The reason for such high daytime levels of HONO is not totally clear and we can only speculate that it must be related to the pronounced local stability at the measurement site and/or to near-ground haze (smog) capable of reducing the photolysis significantly and/or faulty measurements. Furthermore, although primary emission sources had not dominated HONO at the site (Febo et al., 1996), these may nevertheless have contributed to some extent. Predicted and observed values are in qualitative but not quantitative agreement as absolute predicted levels are significantly below those observed during the day and most of the night. In order to obtain a more complete picture regarding the impact of various parameters on HONO levels, in Fig. 8, predicted HONO levels corresponding to specific test cases, described at the end of Section 3, are shown. In particular, lowering the nocturnal mixing height from 70 to 40 m has as a result an enhancement of HONO maximum levels by 30%. Moreover, the increase in the HONO concentration at the lateral boundaries by 0.5 ppbv results in a similar increase in the HONO levels at the two test sites. Assuming HONO primary emissions of the order of 0.5% of NO x emissions leads, on one hand, to an increase in HONO maximum levels of about 23% at the city site and, on the other hand, to a modification of the HONO diurnal concentration pattern according to the emission diurnal pattern. Finally, when no special assumption with regard to NO x enhanced natural emissions is made, which corresponds to almost halving the overall NO x emissions

6 634 N. Moussiopoulos et al. / Environmental Modelling & Software 15 (2000) Fig. 5. Simulated diurnal variation of the surface-level NO 2 concentration on 9 March 1994, compared to observations at six measuring stations in the area of Milan. (this reduction is, however, much less pronounced in the urban area), the effect is a reduction of the order of 20% on HONO ambient concentration. In view of the above, in the following, several possible reasons for the discrepancy between predicted and observed HONO levels are discussed Heterogeneous chemistry This discrepancy is not caused by an underestimation of the HONO source reactions, because the apparent HONO formation after sunset would then be expected to be much steeper than observed: in fact, the HONO concentration increase triggered by the sunset was not observable on 9 March 94 when the HONO level remained at 2 3 ppbv until around midnight (Fig. 7a). The observable (apparent) relative formation rate during the hours after sunset on 9 March 1994 was 0.2% h 1 while the predicted rate is 0.6% h 1 (Fig. 7 in combination with Fig. 4). The parameterization of HONO formation is, however, preliminary in the sense that the availability of kinetic data is still insufficient (Lammel, 2000a). Thus, any parameterization has, to some extent, to assume that many important surface types (aerosols containing potentially catalytic transition metal ions, building materials, vegetation surfaces) behave like laboratory surfaces (chamber walls). We, therefore, cannot exclude that a future, more sophisticated parameterization would lead to higher predictions Local effects Subsequently, around 00:30 h on 10 March, HONO mixing ratios jumped to 7 ppbv at the site obviously in conjuncture with a decrease in wind velocity (from 1 2 to 0.5 m s 1 ) and mixing height (from ca. 300 to ca. 50 m). The increase in near-ground HONO of more than 4 ppbv within the hour after midnight was thus dominated by the dynamics on small spatial scales presumably much too small to be resolved by the model. Microscale simulations could give an answer to this question. In general, such local effects can give rise to considerable apparent disagreement: model results are averaged over the volume of one grid cell (9 km 2 of area and 20 m of height for the lowest grid cell) whereas the

7 N. Moussiopoulos et al. / Environmental Modelling & Software 15 (2000) Fig. 6. Diurnal variation of the wind speed and the ground-level HONO concentration predicted with MUSE at six different times on 9 March Fig. 7. Simulated (line) and observed (diamonds) diurnal variation of the surface-level HONO concentration on 9 March 1994 at a downtown site, St Vittore, and downwind of Milan (distance ca. 45 km from St Vittore). Observed data refer to the right axis.

8 636 N. Moussiopoulos et al. / Environmental Modelling & Software 15 (2000) Fig. 8. Diurnal variation of HONO concentrations at two sites corresponding to (a) the base case, (b) nocturnal mixing height of the order of 40 m, (c) boundary HONO concentration values of 1 ppbv, (d) direct HONO emissions of about 0.5% of NO x emissions and (e) no special assumptions on NO x natural emissions. observations discussed here are integrating over a short (0.4 km), almost horizontal line within the urban canopy. Observational data at several locations would be needed and especially from sites downwind of the city for an adequate evaluation of the performance of the model chemistry situation Advection The value of 0.5 ppbv that was used at the boundaries as background HONO concentration might well be too low: in particular, for the night-time hours with some delay after sunset one can expect that plumes considerably enriched in HONO and originating in upwind source areas of highly polluted Northern Italy reach the Milan area. For instance, if the NO 2 advected into the domain (20 ppbv) originated in the Verona area (located some 150 km upwind), given the horizontal winds and the predicted NO x -into-hono conversion rate (ca. 0.5% h 1 ), we expect advection of increasing HONO levels, reaching ppbv towards the end of the night. As the additional simulations have shown, higher HONO boundary concentrations would also result in higher HONO levels inside the domain, as the HONO produced is simply added to the background level. 5. Conclusions The role of the heterogeneous pathways leading to HONO formation was analysed by means of the multilayer photochemical model MUSE which was extended to include a parameterization of heterogeneous NO y chemistry leading to HONO formation. Simulations of pollutant dispersion and transformation over the Milan metropolitan area for 9 March 1994 were found to reproduce successfully the overall levels as well as the diurnal pattern of O 3 and NO 2. As a result of the parameterization of NO y chemistry on aerosol and ground surfaces, the diurnal variation of HONO is qualitatively well described by the model indicating the appropriateness of the heterogeneous production paths used. The predicted levels and relative formation rates are in the expected ranges (lower ppbv range and 0.1 1% h 1, respectively). The predicted levels for a citycentre ground site are, however, distinctly lower than those observed during most of the time. Even though we feel that the representativity of this site is in question, further causes might well have contributed to this discrepancy: small-scale dynamic effects; possible underestimation of HONO advected into the model domain and/or of local HONO primary emission; and possible underestimation of the NO y reactivity at ambient surfaces Future work The distribution of nitrous acid and its impact on photochemistry should be investigated in more detail. Given a case which is better specified by field measurements (data from receptor sites and above the canopies), we suggest the following model improvements: nesting of a microscale model into the domain to resolve the dynamics within the urban canopy; investigation of the impact of NO x and VOC emissions on HONO levels (to address different photochemical regimes characterized by the NO x -to-voc ratio); a more detailed sensitivity study on the effect of the biogenic emissions and more detailed representation of these (heterogeneous in time and space); time variation of the upwind HONO boundary conditions to simulate the combined effect of NO x emission pattern (in time) and night-time build-up on the HONO distribution; and

9 N. Moussiopoulos et al. / Environmental Modelling & Software 15 (2000) a more advanced scheme for calculating photolysis rates, particularly taking into account cloud cover. Although the model evaluation possibilities with respect to HONO were very limited in the case study presented here, the performance of any parameterization describing heterogeneous processes (and therefore of HONO predictability) was, nevertheless, clearly limited by the data characterizing the surfaces. Respective future improvements would imply a more detailed representation of the ground (e.g. surface-to-area ratio of the urban canopy) and aerosol surfaces (based on size distribution and chemical composition data). Acknowledgements This work was supported by the DGXII of the European Commission as part of the Environment and Climate Project Formation of Nitrous Acid in the Atmosphere. The provision of emission data for the Milan area by Professor G. Finzi and Dr C. Silibello is gratefully acknowledged. 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