Atmospheric Deposition of Nitrogen and Sulphur Compounds in the Czech Republic

Estimates of dry and wet deposition of nitrogen and sulphur compounds in the Czech Republic for the years 1994 and 1998 are presented. Deposition has been estimated from monitored and modeled concentrations in the atmosphere and in precipitation, where the most important acidifying compounds are sulphur diOxide, nitrogen Oxides, ammonia, and their reaction products. Measured atmospheric concentrations of SO2, NOx, NH3, and aerosol particles (SO4, NO3, and NH4), along with measured concentrations of SO4, NO3, and NH4 in precipitation, weighted by precipitation amounts, were interpolated with Kriging technique on a 10- x 10-km grid covering the whole Czech Republic. Wet deposition was derived from concentration values for SO4, NO3, and NH4 in precipitation and from precipitation amounts. Dry deposition was derived from concentrations of gaseous components and aerosol in the air, and from their deposition velocities. A multiple resistance model was used for calculation of SO2, NOx, and NH3 deposition velocities. Deposition velocities of particles were parameterized. It was estimated that the annual average deposition of SOx in the Czech Republic decreased from 1384 to 1027 mol H ha a between 1994 and 1998. The annual average NOy deposition was estimated to be 972 and 919 mol H a in 1994 and 1998, respectively. The annual average NHx deposition was estimated to be 887 mol H a and 779 mol H a in 1994 and 1998, respectively. It was estimated that the annual average of the total potential acid deposition decreased from 3243 to 2725 mol H a between 1994 and 1998. Sulphur compounds (SOx) contributed about 38%, Oxidized nitrogen species (NOy) 34%, and reduced nitrogen species (NHx) 28% to the total potential acid deposition in 1998. The wet deposition contributed 42% to the total potential acid deposition in 1998.


INTRODUCTION
The environment in the Czech Republic showed an increasing damage to forests due to acid deposition. For the assessment of potential effects of air pollution on ecosystems, it is essential to know the actual atmospheric deposition load. It is necessary to know where threshold deposition loads are exceeded and which compounds contribute most to the loads. In this way abatement measures on emission controls can be optimized. The working plan for the implementation of the UN ECE Convention on Longrange Transboundary Air Pollution (LRTAP) includes the production of maps of deposition loads, critical loads, and exceedances as a basis for developing potential abatement strategies for nitrogen and sulphur. The aim of this study is to estimate deposition fluxes of nitrogen and sulphur compounds on a 10-× 10-km scale in the Czech Republic for the period from 1994 to 1998. This study summarizes some important results from the Czech Programme on Acidification [1,2,3,4,5,6,7,8], which was carried out on behalf of the Ministry of the Environment of the Czech Republic from 1993 to 2000.

EXPERIMENTAL METHODS/PROCEDURES Acid Atmospheric Deposition
The following acidifying components were considered in this study: sulphur dioxide (SO 2 ), nitrogen oxides (NO, NO 2 ), nitric acid (HNO 3 ), ammonia (NH 3 ), sulphates (SO 4 2 ), nitrate (NO 3 ), and ammonium (NH 4 + ) in aerosol, in air, and in precipitation, respectively. One mole of SO 2 can form two equivalents of acid; one mole of NO x or NH 3 can form one equivalent of acid [9]. The maximum amount of acidifying components removed from the atmosphere by deposition, hereafter referred to as total potential acid deposition, is estimated by where SO x is the total of oxidized sulphur compounds (gaseous SO 2 and SO 4 2 particulates in air and precipitation), where NO y is the total deposition of oxidized nitrogen compounds (NO, NO 2 , HNO 3 , and NO 3 in air and precipitation), and where NH x is the total deposition of reduced nitrogen compounds (NH 3 and NH 4 + in air and precipitation).

Wet Deposition
For the purpose of this study, the concentrations of SO 4 2 , NO 3 , and NH 4 + in precipitation, collected by monitoring stations of the Czech Hydrometeorological Institute [10,11], the Water Management Research Institute T.G.M. [4,8], and the Czech Geological Survey [4,8], were available. Precipitation at most of these stations was collected by the bulk method (samplers are exposed continuously, and are therefore enriched with dry deposition). At a few stations of the Czech Hydrometeorological Institute, the samples were collected by the so-called wetonly method (an automatic collection of precipitations excluding the contribution of dry deposition). Correction factors were applied to correct the dry deposition in precipitation collected by the bulk method. The concentrations of SO 4 2 , NO 3 , and NH 4 + in precipitation collected by the bulk method were estimated to be higher than those in precipitation collected by the wet-only method. This was done on the basis of comparison of parallel measurements, performed both by the bulk and wet-only method, consideration of geographic location of monitoring stations (clear or polluted areas), frequency of precipitation collection, and the average of correction factors presented in the literature [5]. In a medium the difference for SO 4 2 was 26%, for NO 3 20%, and for NH 4 + 29%.
The total potential acid wet deposition of nitrogen and sulphur compounds estimated in this study is Total potential acid (wet deposition) = 2 SO 4 2 + NO 3 + NH 4 + (2) Annual average wet deposition of SO 4 2 , NO 3 , and NH 4 + for every 10-× 10-km grid on the territory of the Czech Republic in 1994 and 1998 was computed as a product of the annual average concentrations of SO 4 2 , NO 3 , and NH 4 + in precipitation and of the annual precipitation amounts.

Dry Deposition
Dry deposition fluxes were estimated from measured concentrations of gases and aerosol particles in air multiplied by the corresponding deposition velocities: where F is the deposition flux of the component to a unit area (e.g., m 2 , ha), V d is the deposition velocity of the component, and C(z) is the concentration of the component at a height z above surface.
The concentrations of SO 2 , nitrogen oxides (NO x ), and dust aerosol are routinely monitored and evaluated in the Czech Republic. For this study the databases of the Air Quality Information System of the Czech Republic [10,11] and the Ekotoxa Opava [4,8] were used. These two monitoring networks consist partly of manually controlled stations on a 24-h basis, using absorptionphotometric measuring methods (286 stations), partly of automatic monitoring stations operating continuously on a 30-minute time resolution (132 stations). Annual average concentrations of NO x and SO 2 computed from daily measurements were interpolated using Kriging technique on the 10-× 10-km resolution grid. Annual average concentrations of NH 3 , HNO 3 , SO 4 2 , NO 3 , and NH 4 + on the territory of the Czech Republic were not available for the years 1994 and 1998, therefore the data from the EMEP-LRTAP model [12] at 150-× 150-km resolution for 1993 had been used. Annual average concentrations of NH 3 in 1993 from the EMEP-LRTAP model were corrected according to a spatial distribution of the annual emissions of NH 3 on a 10-× 10-km grid for the Czech Republic territory for the years 1994 [4] and 1998 [8]. Annual emissions of NH 3 in 1994 and 1998 were computed by averages of the emission model on the basis of precise total emission balance of NH 3 from all sources (livestock, manures, artificial fertilizers, natural losses of soil, humans, wastes, forest soils, and industry) for the years 1994 and 1998 [4,5,8]. For the other components (HNO 3 and aerosol), the annual average concentration was extrapolated onto a 10-× 10-km grid.
Deposition velocity for gases was calculated using the resistance analogy [13,14]. Deposition velocity, V d , may be expressed by the inverse of the sum of three resistances: The three resistances represent three stages of transport: the aerodynamic resistance, R a , for the turbulent layer, the laminar layer resistance, R b , for the quasi-laminar layer, the surface or canopy resistance, R c , for the receptor itself. In this study the aerodynamic resistance, R a , is calculated from micrometeorological relations suggested by Voldner et al. and Hicks et al. [13,15], and the quasi-laminar layer resistance, R b , is calculated from micrometeorological relations suggested by Hicks et al. [15]. R a and R b may be assessed on the basis of known wind velocity and surface roughness. The annual average values of surface roughness, z 0 , for different surface types were derived from the literature [13,14]. Annual averages of the surface roughness, z 0 , were related to the corresponding surface characteristics on the territory of the Czech 9 ] 5 ] 5 5 Republic according to the geographical model of the land-use types at 1-× 1-km resolution [1]. The land-use classes used are coniferous forests, deciduous forests, cultivated land, grassland, urban areas, and water surfaces, e.g., lakes and rivers. Surface resistance was calculated using the following equation [14]: R c was expressed on the basis of known global radiation, surface temperature, relative humidity, land cover according to Eq. (5), using the results and assumption obtained from literature for computing and parameterization of the canopy stomatal resistance, R sto [16,17], the mesophyll resistance, R m [13,16], the canopy cuticle or external leaf resistance, R ext [17], the soil resistance, R soil [18], and the incanopy resistance, R inc [19], respectively. The resistance model of the deposition velocities calculation under conditions in the Czech Republic was applied as follows: the area of the Czech Republic was divided into 37 regions, and the annual average horizontal wind velocity, u z , for the years 1994 and 1998 at 37 meteorological stations was extrapolated for all these regions. The values R a and R b were calculated from the micrometeorological relations [13,15] by using the average value, z 0 , according to individual surface types and annual average values, u z , in the different regions.
Deposition velocity for particles was obtained by parametrization of friction velocity, u * , for low vegetation according to Wesely et al. [20]: and for forests according to Erisman et al. [21]: The friction velocity, u * , and the MoninObukhov length, L, were averaged annually.
Annual average deposition velocities, V d (z), of gases at a 10-m reference height for individual surfaces represented in areas surrounding the 37 meteorological stations on the Czech ter-ritory were calculated from Eq. (4) by averages of the values R a , R b , and R c . The annual average deposition velocities of SO 4 2 , NO 3 , and NH 4 + aerosol particles were calculated using Eqs. (6) and (7). The annual average areal weighted value of deposition velocity V vaz was calculated for all grid cells (10 × 10 km) in the region of interest by weighing values of deposition velocities, V d (z), relating to the particular surface in a 1-× 1-km grid cell. Each grid cell (1 × 1 km) was assigned the dominant surface type.
Because monitoring stations for concentration monitoring, C(z), of gaseous components and aerosol in the atmosphere are not spread evenly over the territory of the Czech Republic, the concentration data had to be interpolated to 10-× 10-km grids covering the total territory. Annual average areal weighted value of dry deposition, D vaz , was calculated from annual average concentration, C(z), annual average areal weighted deposition velocity, V vaz , and time, t, in a 10-× 10-km grid resolution.

Wet Deposition
The annual average total wet deposition of sulphur and nitrogen compounds (SO 4 2 , NO 3 , and NH 4 + ) was estimated in the Czech Republic for 1994 at 1229 mol H + ha 1 a 1 and for 1998 at 1143 mol H + ha 1 a 1 . The annual average precipitation amount, annual average wet deposition of sulphur and nitrogen compounds (SO 4 2 , NO 3 , and NH 4 + ), and total potential acid in 1994 and in 1998 in the Czech Republic are presented in Table 1.

Dry Deposition
The annual average total dry deposition of sulphur and nitrogen compounds (SO 2 , SO 4 2 , NO x , HNO 3 , NO 3 , NH 3 , and NH 4 + ) was estimated in the Czech Republic in 1994 at 2014 mol H + ha 1 a 1 and in 1998 at 1582 mol H + ha 1 a 1 . The annual average dry deposition fluxes of sulphur and nitrogen compounds and total potential acid in the Czech Republic in 1994 and in 1998 are presented in Table 2.

Total Deposition
The annual average total deposition of sulphur and nitrogen compounds on the territory of the Czech Republic has been computed as the sum of the annual averages of wet deposition and dry deposition. Spatial distribution of total deposition of SO x on a 10-× 10-km grid in 1994 and in 1998 is shown in Fig. 1 and Fig. 2, respectively. Spatial distribution of total deposition of NO y on a 10-× 10-km grid in 1994 and in 1998 is shown in Fig. 3 and Fig. 4, respectively. Spatial distribution of total deposition of NH x on a 10-× 10-km grid in 1994 and in 1998 is shown in Fig. 5 and Fig. 6, respectively. Spatial distribution of total deposition of potential acid on a 10-× 10-km grid in 1994 and in 1998 is shown in Fig. 7 and Fig. 8, respectively. Contribution  [12] have been used. In the period 1994 to 1998 a remarkable decrease of annual average of gaseous SO 2 deposition can be observed. (mol H + ha 1 a 1 ) of different compounds to the total potential acid deposition in the Czech Republic in 1994 and 1998 is shown in Fig. 9. Spatial distribution of the total deposition of SO x and NO y in 1994 and in 1998 shows the gradient over the Czech Republic, with the highest values in the northwest and the lowest in the south. This gradient cannot be observed for the total NH x deposition. Total deposition of SO x is influenced by the emissions from large industrial sources (the northwest part of the Czech Republic, Prague, and the Ostrava region). Total deposition of NO y is influenced by both the emissions from agglomeration areas of high combustion (industry, traffic and heating/fuel consumption) and, to lesser extent, by emissions from car traffic on important roads. The total deposition of NH x is especially influenced by NH 3 emission in agrarian areas with intensive animal husbandry (the north and east regions of the Czech Republic). The dry, wet, and total depositions of SO x , NO y , and NH x in 1994 and in 1998 are presented in Table 3. The total potential acid deposition in 1994 and 1998 is presented in Table 4. The emission of SO 2 , NO x , and NH 3 in 1994 and in 1998 for comparison purposes is presented in Table 5.
Decrease of total deposition of SO x from 1994 to 1998 was caused by abatement of SO 2 emission in the Czech Republic (Table 5) and, to a lesser extent, in the countries of middle and western Europe. Tendency of the total deposition of SO x to decrease is especially apparent in the most loaded regions of the northwest Czech Republic, Prague, and the Ostrava region ( Fig. 1 and Fig. 2). Trend in a decrease of dry deposition of SO x is much more obvious than that of wet deposition of SO 4 2 . Trend in a decrease of dry deposition of NO y is not too obvious between 1994 and 1998 (Table 3).

CONCLUSION
The estimates derived in this study show that the annual average deposition of SO x in the Czech Republic decreased from 1384 to 1027 mol H + ha 1 a 1 between 1994 and 1998. The annual average NO y deposition was estimated to be 972 and 919 mol H + ha 1 a 1 in 1994 and 1998, respectively. The annual average NH x deposition was estimated to be 887 mol H + ha 1 a 1 and 779 mol H + ha 1 a 1 in 1994 and 1998, respectively. The annual average deposition of total (potential) acid in the Czech Republic decreased from 3243 to 2725 mol H + ha 1 a 1 between 1994 and 1998. SO x contributed about 43%, oxidized nitrogen species (NO y ) 30%, and reduced nitrogen species (NH x ) 27% to total potential acid deposition in the Czech Republic in 1994. Wet deposition contributed 38% to the total potential acid deposition in 1994. SO x contributed about 38%, NO y 34%, and NH x 28% to total potential acid deposition in 1998. Wet deposition contributed 42% to the total potential acid deposition in 1998.