1 A Parameterization of Heterogeneous Hydrolysis of N 2 O 5 for 3-D Atmospheric Modelling : Improvement of Particulate Nitrate Prediction

Heterogeneous hydrolysis of N2O5 on the surface of deliquescent aerosol particles leads to HNO3 formation and acts as a major sink of NOx in the atmosphere during nighttime. The reaction constant of this heterogeneous hydrolysis is determined by temperature (T), relative humidity (RH), aerosol particle composition as well as the surface area concentration (S). However, its parameterization in previous 3-D modelling studies did not 20 comprehensively consider these parameters. In this investigation, we propose a sophisticated parameterization of the heterogeneous hydrolysis of N2O5 with respect to T, RH, aerosol particle compositions and S, based on laboratory experiments. This new parameterization was incorporated into a 3-D fully online coupled model: COSMO-MUSCAT. As case study, we used the data from the HOPE-Melpitz campaign (10-25 September 2013). Here, we investigated the improvement of nitrate prediction over the western and central Europe. The modelled 25 particulate nitrate mass concentrations ([NO3 ]) were validated by filter measurements over Germany (Neuglobsow, Schmücke, Zingst, and Melpitz). The modelled [NO3 ] were significantly overestimated for this period by a factor of 5-19, with the corrected NH3 emissions (reduced by 50%) and the original parameterization of N2O5 heterogeneous hydrolysis. The proposed new parameterization significantly reduces the overestimation of [NO3 ] by ~35%. Particularly, the overestimation factor was reduced to approximately 1.4 within our case 30 study period (September 12, 17-18 and 25, 2013), when [NO3 ] was dominated by local chemical formations. Furthermore, the organic coating effect on a suppression of the N2O5 reaction probability may have been also significantly overestimated in previous modelling studies, due to a strong overestimation of the N2O5 reaction probability on coatings. Based on the original parameterization, previous studies reported a decrease of modelled [NO3 ] up to 90%, where both secondary organic aerosol (SOA) and N2O5 were built-up over western and central 35 Europe. For this case study, the suppression of organic coating was negligible over western and central Europe, with influence on [NO3 ] less than 2% on average and 20% at the most significant moment. As for a significant impact of the organic coating effect, N2O5, SOA and NH3 need to be present when RH is high and T is low. However, those conditions were rarely fulfilled simultaneously over western and central Europe. Hence, the organic coating effect on reaction probability of N2O5 over Europe may not be as important as expected in 40 previous studies. Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-105, 2017 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 16 February 2017 c © Author(s) 2017. CC-BY 3.0 License.


Introduction
The budget of nitrogen oxides (NOx) is of fundamental importance for tropospheric chemistry (Ehhalt and Drummond, 1982).The most important removal path of nitrogen from the atmosphere is the formation of HNO 3 , which is transferred to particles or deposited eventually (Riemer et al., 2003).HNO 3 is mainly produced via the reaction of NO 2 and OH at daytime.At nighttime, the heterogeneous hydrolysis of N 2 O 5 on the surface of 5 deliquescent aerosol particles forming HNO 3 is a major sink of NOx (Jacob, 2000;Brown and Stutz, 2012;Platt et al., 1984;Brown et al., 2004).Given that NOx is the key precursor of ozone, chemical mechanisms controlling the budget of NOx also have an important impact on ozone and oxidizing capacity of the atmosphere on a global scale (Dentener and Crutzen, 1993;Evans and Jacob, 2005).).It has been measured for surfaces of different aqueous solutions by several techniques (Mozurkewich and Calvert, 1988;Van Doren et al., 1990;Fenter et al., 1996;Robinson et al., 1997;Behnke et al., 1997;Hu and Abbatt, 1997;Hallquist et al., 2000).They reported that typical values for 25 NO  are in the order of 10 -2 .Organic coating of the particles may reduce this reaction probability.Anttila et al. (2006) proposed a parameterization (Anttila06) that described the organic coating suppression effect on 25 NO  .

15
Based on laboratory experiments and Anttila06, Gaston et al. (2014) proposed that the suppression of 25 NO  by organic coating is dependent on a range of factors including the O:C ratio, the organic particle mass fraction and the relative humidity (RH).Bertram and Thornton (2009) developed a parameterization to describe the influence of chloride salts on 25 NO  as a function of RH.However, the influence of temperature was not considered.
Several studies have implemented the heterogeneous hydrolysis of N 2 O 5 in global and regional chemical transport models, in order to investigate its influences on atmospheric chemistry.However, the parameterizations 30 may not properly represent this process.Dentener and Crutzen (1993) investigated the importance of the heterogeneous hydrolysis of N 2 O 5 on a global scale, but assumed a constant value 25 NO  =0.1, which might be overestimated.Riemer et al. (2003) reported that the scheme of Chang et al. (1987) could only be representative of heavily polluted conditions or if cloud droplets are at presence, with a particle surface area concentration of 2700 µm 2 /cm 3 .Riemer et al. (2003)  However, the influence of temperature and particle compositions on 25 NO  was still not considered in Riemer et al. (2003).Later, by applying Anttila06 to Riemer03, Riemer et al. (2009) found that organic coatings could decrease particulate nitrate concentrations by up to 90% where both N 2 O 5 and secondary organic compounds were built-up.Evans and Jacob (2005) developed a parameterization scheme (EJ05) that has an extensive 5 description of aerosol composition to improve the GEOS-CHEM simulations.EJ05 included 25 NO  of dust (Bauer et al., 2004), sea salt (Sander et al., 2003), sulfate (Kane et al., 2001;Hallquist et al., 2003), elemental carbon (EC, Sander et al., 2003) and organic carbon (OC, Thornton et al., 2003), also took into account the dependence on RH.However,


of nitrate and the dependence on temperature were not carefully considered in EJ05.Archer-Nicholls et al. (2014) incorporated Bertram and Thornton (2009) into WRF-Chem.Lowe et al. 10 (2015) further took the organic coating effect into account by applying Anttila06 to Bertram and Thornton (2009).However,


with respect to EC, OC and dust was lacking in Bertram and Thornton (2009).As mentioned above, the reported influence of chloride on 25 NO  (Bertram and Thornton, 2009) may not be realized in northwestern Europe (Morgan et al., 2015).Chemistry Aerosol Transport, http://projects.tropos.de/cosmo_muscat,Wolke et al., 2004;Wolke et al., 2012), in order to investigate the impact of heterogeneous hydrolysis of N 2 O 5 on the particulate nitrate prediction.The 20 measurements of the HOPE campaign (HD(CP) 2 Observational Prototype Experiment, Macke et al., 2016) at Melpitz site (12.93 o E, 51.53 o N, 86 m a.s.l., a regional background observatory of central Europe) and other three stations of the German federal environmental agency (UBA) over Germany were used to validate the simulation results.

The model system COSMO-MUSCAT
The online-coupled chemical transport model COSMO-MUSCAT is qualified for process studies as well as the operation forecast of pollutants in local and regional areas (Heinold et al., 2011;Hinneburg et al., 2009;Stern et al., 2008;Renner and Wolke, 2010).Two nested domains with 50 vertical layers were used for this model study.
The outer domain covers the whole Europe, with a spatial grid resolution of 14×14 km.The inner domain (N2) 30 covers Germany, the Netherlands and near-by regions, with a spatial grid resolution of 7×7 km (Fig. 1).The simulation period was divided into overlapping short-term cycles.Each of these cycles consisted of a one-day spin-up for the meteorology followed by a two-day coupled run of meteorology and chemistry transport.The main features of the model system are described below.More details are given in Wolke et al. (2004Wolke et al. ( , 2012) ) and Baldauf et al. (2011).
An adequate modelling of dynamics requires an online coupling between the chemical transport model MUSCAT and the meteorological model COSMO.Here, the compressible non-hydrostatic flow in a moist atmosphere is described by the primitive hydro-thermodynamical equations (Steppeler et al., 2003;Doms et al., 2011a).The vertical diffusion is parameterized by a level 2.5 closure scheme based on a prognostic equation for turbulent kinetic energy (Doms et al., 2011b).Moist convection is parameterized according to Tiedtke (1989).A 5 two-stream formulation (Ritter and Geleyn, 1992) is applied for radiative transfer.Aerosol particles, clouds and tracers gases are considered as optically active constituents, modifying the radiative fluxes by absorption, scattering and emission.The meteorological forcing of COSMO is performed by reanalysis data of the German Weather Service DWD, which are derived from the global meteorological model GME (Majewski et al., 2002).
MUSCAT describes the transport, chemical and removal processes.The chemical mechanism RACM-MIM2 10 Karl et al., 2006;Stockwell et al., 1997) with 87 species and more than 200 reactions is applied to represent the gaseous chemistry.A simplified mass based approach (similar to EMEP model, Simpson et al., 2003) is used to represent the aerosol processes.The formation of secondary inorganic particulate matter is performed by reactions between ammonia and sulfuric or nitric acid, which are produced from the gas phase precursors SO 2 and NOx (Hinneburg et al., 2009).The applied particle/gas partitioning depends on temperature and humidity.

15
As in ISORROPIA (Nenes et al., 1998), the equilibrium is shifted towards the gas phase for dry and warm conditions.The implementation of this partitioning scheme is comparable to Galperin and Sofiev (1998) by using the equilibrium approach of Mozurkewich (1993).SORGAM (Schell et al., 2001) is coupled with the mass-based aerosol approach to predict the formation of secondary organic aerosol (SOA).Dry deposition is modelled by using the resistance approach described by Seinfeld and Pandis (2006), considering the atmospheric 20 turbulence state, the kinetic viscosity, and the gravitational settling of particles.The aerodynamic and quasilaminar layer resistances are taken from COSMO by analogy with the deposition of water vapour.The wet deposition is parameterized in dependence on the size resolved scavenging and collection efficiency (Simpson et al., 2003).
The European anthropogenic emission inventory and the temporal resolved emission factors are provided by 25 TNO for the AQMEII project (Pouliot et al., 2012;Wolke et al., 2012).The inventory includes the gaseous pollutants (CO, NOx, SO 2 , NH 3 methane and non-methane volatile organic compounds) and primary emitted particulate matters (PM 2.5 , PM 2.5-10 , organic carbon-OC and elemental carbon-EC) with a spatial resolution of 0.125 o × 0.0625 o (lon-lat, about 7×7 km).Note that EC and BC are usually interchangeable in modelling studies (Vignati et al., 2010;Chen et al., 2016a;Nordmann et al., 2014).The emission of NH 3 was reduced by 50%, 30 since over 90% of NH 3 emissions in Europe are contributed by agricultural sources (Hertel et al., 2011;Erisman et al., 2008;Reidy et al., 2008) and agriculture emissions of NH 3 are overestimated by ~50% or even more (Sintermann et al., 2012;Backes et al., 2016).Also, Chen et al. (2016b) adopted the same NH 3 emission inventory in the WRF-Chem model and reported that total NH 3 was overestimated by a factory of ~2 at Melpitz during the campaign period.The modelled dust emissions depend on surface wind friction velocities, surface 35 roughness, soil particle size distribution, and soil moisture (Heinold et al., 2011).Sea salt emissions are parameterized depending on salinity and wind speed (Long et al., 2011).Biogenic emissions depends on landuse and meteorology by the approach of Steinbrecher et al. ( 2009) and for "soil NO" by Williams et al. (1992) and Stohl et al. (1996).Saarikoski et al. (2007) scheme was applied to estimate the biomass burning emissions.
Furthermore, in order to investigate the influence of transport from the NH 3 source regions (the Netherlands and southern Germany) on particulate nitrate, the marker tracer (T-NH3) was emitted into the first layer of regions with high ammonia emissions (white bars in Fig. 1) with an emission rate of 2×10 -8 mol/m 2 /s.

A novel parameterization of heterogeneous hydrolysis of N 2 O 5
The reaction of N 2 O 5 with water vapour is very slow, therefore a considerable loss of N 2 O 5 is assumed to occur 10 on the surface of deliquescent aerosol particles (Platt et al., 1984) Riemer03, which adapted from Chang et al. (1987), was originally adopted in COSMO-MUSCAT to represent 15 the heterogeneous hydrolysis of N 2 O 5 .Reaction R1 is implemented into chemical transport models as a firstorder loss (Riemer et al., 2003).The reaction constant ( 25NO k ) is defined as:

 
(2) RH is the relative humidity in %, which was used as an indicator for the influence of hygroscopic growth on S,

25
NO k results in min -1 , and 'a=5' was originally adopted in Chang et al. (1987).According to Riemer et al. (2003), 'a=5' represents the surface area concentration of 2700 µm 2 /cm 3 , when RH is higher than 60%.However, this high surface area concentration can only be expected in highly polluted areas or if cloud droplets are present.
Therefore, we adopted 'a=17' in this study as suggested by Riemer et al. (2007).This produced a In Riemer03,

NO
 was considered as independent of temperature, RH and detailed particle compositions (only nitrate and sulfate were considered).Furthermore, S was set to a constant value (600 µm 2 /cm 3 ) without sufficient consideration of the influence of particle surface area concentration.Thus, some uncertainties were found, applying this scheme in 3-D chemical transport models.In this study, we proposed a sophisticated  2009), as given in equation ( 5).The influence of O:C ratio on the organic coating effect (Gaston et al., 2014) was not considered here, since the O:C ratio information is not available in COSMO-MUSCAT.Also the "high" O:C regime defined in Gaston et al. (2014) may not represent the northwestern Europe case compared with airborne measurements (Morgan et al., 2015).considering recently reported values of 11.9 m 2 /g and 10.2 m 2 /g from laboratory studies (Okuda, 2013) and measurements in Japanese urban regions (Hatoya et al., 2016).A value of 1.2 m 2 /g was used for coarse SA (Okuda, 2013).Note that the nitrate mass concentration in (4) is considered as 1. 4 where R is the universal gas constant, T is the temperature, is the radius of the particle, and shell l is the thickness of the organic coating shell.core  can be estimated from previous laboratory experiments (Table 1) of inorganic and primary organic compositions (Davis et al. 2008;Evans and Jacob, 2005, and references therein;).Davis et al. (2008) proposed an extended parameterization for N 2 O 5 hydrolysis on ammonium-sulfate-nitrate particles, with respect to RH and temperature.Evans and Jacob (2005) provided the parameterizations for N 2 O 5 hydrolysis on primary organic particles (Thornton et al., 2003), black carbon (Sander et al., 2003), sea salt (Sander et al., 2003) and dust (Bauer et al., 2004).The core of the aerosol particle was assumed to be homogeneously internally mixed.

Observations
The filter chemical composition measurements of the HOPE-Melpitz campaign (10-25 September 2013)  agreement with the measurement.

Improvement of the particulate nitrate prediction 10
In previous evaluation studies (Im et al., 2015;Wolke et al., 2012), the COSMO-MUSCAT model predicted particulate nitrate mass concentrations ([NO 3 -]) in a fair agreement with the measurements, with an overestimation in the range of 50% on long-term average.This is comparable with other models (Im et al., 2015).However, short periods with strong overestimations of [NO 3 -] were also observed in these previous studies.This seems to be the case for the HOPE-Melpitz campaign simulation, where COSMO-MUSCAT highly 15 overpredicted [NO 3 -] over Germany in this study (Fig. 3) as well as WRF-Chem in a previous study (Chen et al, 2016b).In order to evaluate the improvement of our new N 2 O 5 scheme and quantify the influence of NH 3 emission overestimation on the particulate nitrate prediction, three sensitivity simulations were conducted (Table 2).
In this HOPE-Melpitz campaign case, the particulate nitrate mass concentrations were overestimated by factors 20 of 23.7, 12, 6.5 and 6.3 for Neuglobsow, Schmücke, Zingst, and Melpitz, respectively (Fig. 3).The overestimation of NH 3 emission might contribute about 20-30% of the particulate nitrate overprediction, compared between Riemer03-FullNH3 and Riemer03 cases.Similar results were reported in Renner and Wolke et al. (2010).However, even with a 50% reduction of NH 3 emissions, the particulate nitrate was still highly overestimated over Germany with factors of about 19, 9, 4.5 and 5 for these four stations, respectively.Our new
Correspondingly, the overestimation factors of particulate nitrate were reduced to about 10.7, 6, 2.5 and 3 for the four stations, respectively.The N 2 O 5 was almost all consumed by the reaction at Melpitz in Riemer03 case, but not the NewN2O5 case (Fig. 3e).This indicated a significant decrease in the reaction constant of heterogeneous hydrolysis of N 2 O 5 by the new scheme.However, there must be other reasons that might explain the remained September 13-14 and 19-24 (blue shaded period in Fig. 3), as indicated by the high T-NH3 concentration (Fig. 3h) and the negligible local chemical formations (Fig. 3g).In contrast, the local chemical formations dominated the [NO 3 -] in September 12, 17-18 and 25 (red shaded period in Fig. 3).During the red shaded period, T-NH3 was almost zero (Fig. 3h) and the modelled wind speed was less than 4 m/s in average (Fig. 2c).A much

Comparison between NewN2O5 and Riemer03
The NewN2O5 case improved the particulate nitrate overestimation problem compared with Riemer03.
Meanwhile, the spatial distribution pattern of [NO 3 -] was similar between these two cases (Fig. 4).Here, we focus on the nighttime period of the HOPE-Melpitz campaign, since the N 2 O 5 heterogeneous reaction is much more significant during the night than in the daytime.The lowest [NO 3 -] was found over Poland and ocean cases, respectively, due to the high agriculture emission of NH 3 in this region.There was also a remarkably high 20 amount of particulate nitrate over southern Germany, about 8-10 µg/m 3 and 5-6.5 µg/m 3 in the Riemer03 and NewN2O5 cases, respectively.In general, the [NO 3 -] was reduced by ~35% over the entire N2 domain (Fig. 4).
The most significant reduction of [NO 3 -] is found over the Netherlands and southern Germany where the highest [NO 3 -] (reduced by about 3-4.5 µg/m 3 ) was found, followed by the near Melpitz region (reduced by about 2-3 µg/m 3 , Fig. 4c).This is caused by a significant reduction of during nighttime indicates considerable nitrate formation from the heterogeneous hydrolysis of N 2 O 5 , which was 30 reduced to a more reasonable value in our new scheme.However, this heterogeneous hydrolysis was slow over the regions where [NO 3 -] was low during nighttime, and did not have relevant contribution on the formation of particulate nitrate.Therefore, the improvement of particulate nitrate prediction by the new scheme was more significant over the high-[NO 3 -] regions than the low-[NO 3 -] regions.

Influence of organic coating on the N 2 O 5 heterogeneous hydrolysis 35
The secondary organic coating on particle surface may significantly decrease the reaction probability of In this study, we introduced a more sophisticated parameterization for heterogeneous hydrolysis of N 2 O 5 .The 5 influence of the organic coating suppression effect on particulate nitrate prediction would be re-evaluated, based on our new scheme.
The nighttime results were analysed, since much higher N 2 O 5 concentrations occur and its heterogeneous hydrolysis is more important than during daytime (Jacob, 2000).As shown in Fig. 5a and Fig. 5b, the influence of the organic coating effect was negligible over the domain N2 including the Netherlands and Germany.Even at 10 24 September 23:00 CET when changes were most significant, the organic coating reduced [NO 3 -] only by about 2-4 µg/m 3 (less than 10-20%) over the black-polygon marked and near-by regions (Fig. S3).Meanwhile, for nighttime averages during the campaign, the organic coating could only reduce [NO 3 -] by less than 0.1 µg/m 3 or 2% over the whole domain (Fig. 5).This is because much stricter conditions, as described following, are needed in NewN2O5 for a significant organic coating effect compared with Riemer03.In addition to the simultaneous 15 build-up of SOA and N 2 O 5 (Riemer et al., 2009), high NH 3 concentrations and  by organic coating.Therefore, a large impact should be expected in the regions with high RH and low temperature.As show in Fig. 5, the most significant organic coating effect (still less than 2% influence 20 on [NO 3 -]) could be found over the Netherlands and near-by regions (black polygon).Over this area, these five conditions were fulfilled to some extent: (1) temperature was 13.5-14.5o C; (2) RH was higher than 75%; (3) SOA concentration was ~1.6 µg/m 3 ; (4) N 2 O 5 concentration was about 0.4-0.6 µg/m 3 ; (5) NH 3 concentration was about 4-6 µg/m 3 (Fig. 1).There was almost no influence of organic coating over the other regions (Fig. 5a and Fig. 5b).These five conditions (not very high temperature; relatively high RH; built-up of SOA, N 2 O 5 and NH 3 )

25
could not be simultaneously fulfilled over the western and central Europe, therefore the organic coating effect was not very significant.

Conclusions
Generally, the COSMO-MUSCAT model predicted particulate nitrate in a reasonable range in long-term average.
The results were comparable with other models in previous studies.However during the HOPE-Melpitz ), by assuming constant values of 600 µm 2 /cm 3 and 0.1, respectively.Based on many previous laboratory experiments, the influences of temperature, RH, aerosol particle compositions and surface area concentration on the heterogeneous reaction constant of N 2 O 5 were considered in NewN2O5.The reaction 10 constant was reduced by a factor of more than 10 in NewN2O5.Correspondingly, the overestimation of particulate nitrate was reduced by ~35% for the whole period.Particularly, the NewN2O5 significantly improved particulate nitrate prediction, with a factor of ~1.4 compared with the filter measurements, when particulate nitrate was dominated by the local chemical formations at Melpitz (September 12,(17)(18).
Aiming to improve the representativeness of heterogeneous hydrolysis of N 2 O 5 in 3-D models, we propose a new considered.This new scheme was incorporated into the 3-D fully on-line coupled model COSMO-MUSCAT (Consortium for Small-scale Modelling and Multi-Scale Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2017-105,2017 Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 16 February 2017 c Author(s) 2017.CC-BY 3.0 License.The chemical inactive tracers (T1, T2 and T3) were added into RACM-MIM2 to investigate the chemical fluxes of the selected reactions.T1, T2 and T3 (representing R1, R2 and R3 respectively) were reset to zero every hour in the simulation in order to quantify the chemical fluxes from N 2 O 5 to nitrate avoiding the influence of transport.T1 represents the contribution of N 2 O 5 on chemical formation of nitric acid; T3-T2 represents local chemical net formation of particulate nitrate.

5
particle composition and water content (and so atmospheric RH).Reducing the RH, or adding organics or nitrate to the solutions, suppresses 25 NO  by an order of magnitude(Chang et al., 2011 and references therein).
scheme to represent 25 NO k .

10 parameterization
based on Riemer03 with the full consideration of temperature, RH, aerosol particle compositions and S. As shown in equation (1), 25 NO k is linearly related to S and 25 NO  .We adapted equation (2) with factors s f and 25 NO f  , which represent the impact of S and 25 NO  respectively, as shown in equation (3).s f can be estimated from the particle mass concentration, according to equation (4).25 NO f  can be estimated from the core-shell 15 model of aerosol particles considering the secondary organic coating effect according to Anttila et al. (2006) and Riemer et al. ( surface area for fine/coarse mode particles in m 2 /g, PM fine / PM coarse is the mass concentration of fine/coarse mode particles in µg/m 3 .A value 11 m 2 /g was used for fine SA ,

30 overestimations
in the simulated particulate nitrate mass concentrations.Possible reasons should be investigated in future studies, e.g., deposition process, long-range transport, formation of nitrogen-containing OC, neutralization process.The improvement of particulate nitrate prediction with the new scheme can be more clearly shown associated with the tracers (T1 in Fig.3f; T3-T2 in Fig.3g, and T-NH3 in Fig.3h) and the comparison with Melpitz 35 measurements (Fig. 3d), which have a higher temporal resolution.The overestimation of [NO 3 -] in September 10-11 (grey shaded period in Fig. 3) stemmed from the uncertainty of boundary conditions in the model.As shown in Fig. S1, an air mass with high [NO 3 -] was transported from the southwestern boundary area to Melpitz.Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2017-105,2017 Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 16 February 2017 c Author(s) 2017.CC-BY 3.0 License.The [NO 3 -] at Melpitz was dominated by the transport from the Netherlands and southern Germany on

5
stronger reduction on the overestimation of particulate nitrate occurred during the red shaded period (a factor of ~1.4 in average), which was dominated by the local chemical formations.This further confirmed the improvement of heterogeneous hydrolysis of N 2 O 5 by the new scheme.During September 15-16 (without shaded period in Fig.3), the contributions from both transport and local chemical formations of particulate nitrate were very limited (Fig.3f-h), resulting in very low [NO 3 -].

10
lower than 4 µg/m 3 and 3 µg/m 3 in Riemer03 and NewN2O5 cases, respectively.Moderate [NO 3 -] was found over central Europe (Melpitz and the surrounding region), about 6-8 µg/m 3 and 4-5.5 µg/m 3 in the Riemer03 and NewN2O5 cases, respectively.The highest [NO 3 -] occurs over the region of the Netherlands and near-by regions, about 9-12 µg/m 3 and 6-8 µg/m 3 in Riemer03 and NewN2O5 the new scheme compared to Riemer03, which assumed a constant value of 600 µm 2 /cm 3 and 0.1 for these two parameters, respectively.As shown in Fig. in average reduced to ~14% and ~23% of the Riemer03 case, respectively.a factor of more than 10.Therefore, the regions with high[NO 3  -] N 2 O 5 and influence the particulate nitrate concentration.As shown by Riemer et al. (2009), which coupled Anttila06 to Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2017-105,2017 Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 16 February 2017 c Author(s) 2017.CC-BY 3.0 License.Riemer03, organic coating could decrease [NO 3 -] by up to 90% where both N 2 O 5 and secondary organic compounds were built-up.The highest reduction over Europe was found over the Netherlands followed by western Germany (both covered by the domain N2) in their study.However as discussed above, Riemer03 may overestimate nitrate formation from the heterogeneous hydrolysis of N 2 O 5 , because of simplifying S and 25 NO  .
significant organic coating effect.High NH 3 concentrations are necessary for neutralizing the HNO 3 , which came from the heterogeneous hydrolysis of N 2 O 5 during the night.

30 campaign ( 10 -
25 September 2013), particulate nitrate was significantly overestimated by the COSMO-MUSCAT model over Germany, despite a good performance of meteorological simulation.This can be partly (~35%) attributed to the parameterization of heterogeneous hydrolysis of N 2 O 5 (Riemer03).A sophisticated parameterization of heterogeneous hydrolysis of N 2 O 5 (NewN2O5) was proposed in this study, aiming at improving the particulate nitrate prediction in atmospheric modelling.In order to quantify the improvement of the nitrate prediction by using NewN2O5, sensitivity studies were performed based on more realistic NH 3 emissions, which are reduced by 50%.This correction was based on previous studies that showed NH 3 emissions were overestimated by a factor of ~2.The overestimation of NH 3 emissions led to about 20-30% overprediction of particulate nitrate over Germany.The horizontal distribution patterns of particulate nitrate were in a good agreement between Riemer03 and NewN2O5 cases.Riemer03 5 overestimated particulate nitrate by a factor of 19, 9, 4.5 and 5 for Neuglobsow, Schmücke, Zingst, and Melpitz, respectively.This may be caused by an overestimation of the particle surface area concentration and the reaction probability of N 2 O 5 ( 25 NO 

25 NO
due to organic coating(Anttila et al., 2006)    15 influences the particulate nitrate prediction over western and central Europe.Based on NewN2O5, the simulation results with and without organic coating were analyzed.In contrast to previous studies (e.g.,Riemer et al. 2009), our results showed a negligible (less than 2% or 0.1 µg/m 3 ) influence of coating on particulate nitrate over the Netherlands and Germany.This is because considerable amounts of N 2 O 5 , SOA and NH 3 must be present at the same location for the organic coating to exert an impact.In addition, low RH and high temperature would lead to fulfilled simultaneously over western and central Europe; hence, the influence of the organic coating effect on particulate nitrate prediction was negligible in this study.This study suggests that temperature, RH, particle compositions and surface area concentration should be taken into account in the parameterization of the heterogeneous hydrolysis of N 2 O 5 .A sophisticated parameterization 25 is proposed in this investigation and should be included in model simulations to improve the representativeness of the N 2 O 5 hydrolysis of in the ambient atmosphere.The results also implicate that the organic coating effect on suppressing the heterogeneous hydrolysis of N 2 O 5 may not be as important as expected over Europe.Acknowledgements:The HOPE campaign was funded by the German Research Ministry under the project 30 number 01LK1212 C. We would like to thanks TNO (the Netherlands) and the AQMEII project (http://aqmeii.jrc.ec.europa.eu/aqmeii2.htm)provide the European anthropogenic emission inventory, and German federal environmental agency (UBA) provide the filter measurements of particle compositions.Furthermore, the JSC Jülich and the DWD Offenbach supported the work by providing computing time and meteorological data.We also would like to thanks Prof. Dr. Ulrich Pöschl, Prof. Dr. Hartmut Herrmann and Dr.35Andreas Tilgner for the discussion and help.Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2017-105,2017 Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 16 February 2017 c Author(s) 2017.CC-BY 3.0 License.Simpson, D., Fagerli, H., Jonson, J. E., Tsyro, S., and Wind, P.: Transboundary ACIDIFICATION, Eutrophication and Ground Level Ozone in Europe.PART I: Unified EMEP Model Description, EMEP/MSC-W: EMEP Report, Available from: http://emep.int/publ/reports/2003/emep_report_1_part1_2003.pdf,2003.Sintermann, J., Neftel, A., Ammann, C., Häni, C., Hensen, A., Loubet, B., and Flechard, C. R.: Are ammonia 5 emissions from field-applied slurry substantially over-estimated in European emission inventories?,

Figure 1 .
Figure 1.Results (domain N2) of NewN2O5 case of averaged NH 3 mass concentration during 10-25 September 2013.The added emissions of marker tracer (T-NH3) from NH 3 source regions (the Netherlands and south Germany) are marked by the white bars.The locations of the considered measurement stations are also marked.The region with the most significant organic coating effect is highlighted by the black polygon, and will be analysed together with Fig. 5.

Figure 3 .
Figure 3.Comparison of particulate nitrate mass concentration between filter measurements and modelled results: (a) Neuglobsow; (b) Schmuecke; (c) Zingst; (d) Melpitz.Modelled concentrations at Melpitz: (e) N 2 O 5 ; (f) marker species T1 for chemical reaction R1; (g) marker species for chemical formation of particulate nitrate (T3-T2); (h) the NH 3 marker tracer (T-NH3) for transport from the Netherlands and south Germany.The light-red colour bars indicate the results of Riemer03-FullNH3 case; the red colour bars indicate the results of Riemer03 case; and the blue colour bars indicate the results of NewN2O5 case.The shaded periods indicate the dominating processes for high concentrations of particulate nitrate: chemical formation (red), transport (blue), and boundary conditions (grey).

Figure 5 .
Figure 5. Horizontal distribution of averaged model results during nighttime in September 10-25, computed with the new N 2 O 5 scheme.(a) Difference of particulate nitrate mass concentration between model cases with and without considering organic coating effect; (b) difference of particulate nitrate mass concentration in percent between model cases with and without considering organic coating effect; (c) temperature; (d) RH; (e) SOA mass concentration; (f) N 2 O 5 mass concentration.The region with the most significant organic coating effect is highlighted by the black polygon.
and at three UBA stations (Neuglobsow, SchmückBrüggemann and Spindler, 1999;Poulain et al., 2011 validate the modelled results.The observations at the TROPOS research station Melpitz represent the regional background of central Europe(Spindler et al., 2012; Spindler et al., 2010;Brüggemann and Spindler, 1999;Poulain et al., 2011; measurements were ~1, except for an overestimation of wind speed with a factor of 1.44, possibly due to the vertical resolution of the model.Nevertheless, the temperature and RH, which are the most important parameters in this study for N 2 O 5 heterogeneous hydrolysis during nighttime, were in a good ), OC and EC from one filter.The filters were pre-heated before sampling for at least 24 hours at 105°C to minimize the blank values of OC.More details about filter measurement are given in(Spindler et al., 2013).Near-ground meteorological parameters (e.g.temperature, relative humidity, wind speed, wind direction) were simultaneously measured at Melpitz.More details about the HOPE campaign are given inMacke et al. (2016).30 Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2017-105,2017 Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 16 February 2017 c Author(s) 2017.CC-BY 3.0 License.meteorological

Table 1 .
Representation of reaction probability of aerosol particle core ( core  ) for N 2 O 5 hydrolysis.