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Long-range transport of gaseous and particulate oxidized nitrogen compounds
Atmospheric En~ironmenf
Vol. 18. No. 9. pp. 1731-1735,
PrintedinGreatBritain.
OOOk6981/84 E3.00 + 0.00 Pergamon Press Ltd.
1984
LONG-RANGE TRANSPORT OF GASEOUS AND PARTICULATE OXIDIZED NITROGEN COMPOUNDS MARTIN FERM*, ULLA SAMUELSSON, AKE
SJ~DIN
and PERINGE GRENNFELT
Swedish Environmental Research Institute, P.O. Box 5207, S-402 24 Gothenburg, Sweden Abstract-Measurements of NO,, PAN, HNO, and particulate NO; as 24-h mean concentration values have been carried out at a clean air station. The station is situated on the Swedish west coast 40 km south of Gothenburg. Measurements from November 1981 to October 1982 are presented. NO2 was measured with the Saltzman method after removal of ozone and PAN with a gas chromatographic method. HN09 was sampled in a denuder and particulate NO; was sampled on an impregnated filter behind the denuder. The denuder and filter were then analysed by ion chromatography. Episodes of high concentrations occurred several times during the year. On these occasions the air was transported over a long range from the continent. 72-h back trajectories are available for this station at 6-h intervals during the whole period of measurements. Monthly mean concentrations of these four species, together with data on SO2 and particulate sulphate are presented. Analysis of the data as a function of the direction of the back trajectory and correlations between nitrogen and sulphate compounds are also given.
INTRODUCTION In Scandinavia, the NO; concentration in precipitation is about twice as high today as 20 years ago (Persson, 1982). This change is assumed to be caused
by a corresponding increase in the atmospheric concentrations of oxidized nitrogen compounds transported from long range. Very little is, however, known about the relative abundance of different forms. Those very few investigations published so far indicate that long-range transport of oxidized nitrogen compounds occurs essentially as NOz, PAN, HN03 and particle-borne NO; (Nielsen et al., 1981; Spicer, 1978; Brosset, 1978). In all cases the investigations cover either a very short time or include only one or two of the compounds of interest. In this paper we will present data from a l-y (November 1981-October 1982) monitoring of the atmospheric concentrations of different oxidized N compounds at a remote monitoring station on the Swedish west coast (Riirvik 57” 25’N, 11” WE, see Fig. 1). Sampling was performed 3 m above ground. Nitrogen oxides are mainly emitted as NO. In the atmosphere a variety of oxidizing and other reaction products are formed from NO. Of these we believe that NOz, PAN, HN03 and particle-borne NO; occur in the highest concentrations. They were also involved in the monitoring programme. All measurements presented in this article are 24-h mean values starting and ending at 6.00 GMT.
METHODS Nitrogen dioxide
NOz was determined with the Saltzman technique (Saltzman, 1954). In this method ozone has a negative
interference. This was overcome by mounting a sodium thiosulphate impregnated filter in front of the bubbler. At a flow of0.2 Pmin-’ this filter reduced the ozone concentration by more than 98 %. Less than 5 % of the NO2 was lost in this filter. Samples were refrigerated and analyzed within one week. This procedure kept the absorbance decrease less than 2% per week. In wet chemical methods with alkaline solution PAN has a positive interference due to hydrolysation to NO; Laboratory tests showed no interference with the acid Saltzman solution. Results were calculated using a ‘Saltzman-factor’ of 0.72. The detection limit is estimated to be 10 nmole m-3. Peroxyacetyl
nitrate
PAN, CH,C(0)02NOI was sampled and analyzed immediately once an hour by the GC-EC technique described by Stephens and Price (1973). Air was injected from a continuously ventilated sampling valve (volume 1 ml) on a 1 m packed column with an internal diameter of 2 mm, containing 5 y0 carbowax 400 on Chromosorb W 10&l 20 mesh. N2 was used as carrier gas with a flow of 35 mlmin-‘. Oven temperature was 25”C, and an electron capture detector (63 Ni) was used. The instrument was calibrated with a heptane solution containing PAN which was injected in a Tedla@ bag together with a known volume of air. The PAN solution was produced and calibrated with a procedure described by Nielsen et al. (1982). The PAN air mixture was also analyzed on achemiluminescent NO, instrument which was calibrated with a NO, permeation tube. There was no difference between the calculated PAN concentration in the Tedlar ‘@ bag and the concentration obtained with the chemiluminescent instrument as NO,. The concentration of PAN was determined once an hour essentially. The detection limit was estimated to be 6 nmole mm3. Nitric acid and particulate
nitrate
HNO, and particulate NO; were determined with a combined denuder and impregnated filter technique (Ferm, 1982). HNO, was first trapped in a hollow tube internally coated with sodium carbonate. The denuder will absorb almost all acid gases, for instance SOZ, HNOZ, HF and HCI. Particles diffuse slower to the coated walls and therefore pass through the tube (denuder) to a filter holder containing a sodium carbonate impregnated filter. The sodium carbonate 1731
MARTIN FERM et al.
1732
b lb0
IO00
2000
km
Fig. 1. Map of wind sectors with Rorvik at center.
prevents NO; from leaving the filter as HNO+ Laboratory experiments have revealed that PAN does not interfere because it is converted to NO; when absorbed in the denuder. In the measurements presented here there is no correlation between HNOs and PAN. A high concentration of PAN often corresponded to a low HN03 concentration and vice versa. After sampling the filter and denuders were leached in water and analysed by ion chromatography for Cl-, NO; and SO:-. The detection limit for HNO, and NO; was estimated from the variation in the NO; blank values for the denuder and filter, respectively. The detection limit for HNO, is estimated to be 0.5 nmole me3 and for particulate NO;, 3 nmole rnm3. Parallel tests showed that the sulphate amount per unit volume of air in the denuder agreed very well with simultaneous values for SO2 (r = 0.83), obtained with the HZO, method. Moreover, the SO:- concentration on the coated filter was in good agreement (I = 0.89) with measurements of S with X-ray fluorescence on filters exposed directly to the ambient air. Sometimes high concentrations of CI- were found in the denuder, probably originating from sea salt particles. Very high concentrations of NO; and relatively high concentrations of SO:- were found on these days. Days with extremely high concentrations of Cl- (> 70 nmole m-‘) in the denuder were therefore excluded. The coating of the denuder can be dissolved during rain or periods with very high humidity. The denuders were therefore heated a few degrees centigrade and protected from rain. The
coating could still be destroyed by fog which frequently appeared during the morning hours. The comparison of SOi- in the denuder with ambient SO2 concentration was used to decide whether the sampling was successful or not. Usually, there was a good agreement between the two methods, but sometimes there was no agreement at all and those values were excluded. Approximately 30% of the samples were lost due to sea spray, fog and accidents during sampling. The mean concentration of SO2 + particulate SO:- was 2.4% higher for the days when the HNOJ and NO; sampling were successful than the mean SO2 + SOiconcentration for the whole year. Therefore, we assume that the presented mean values for HNO, and NO; are representative. By leaching different parts of the denuder separately the apparent diffusion coefficients for nitric acid and SO2 were determined. For SO* the diffusion coefficient was in good agreement with the experimentally obtained value but for HNOJ a lower value was found. The diffusion coefficient decreased with increasing relative humidity of the air. This had earlier been observed by Braman (1983, pers. comm.) who explained the lower diffusion coefficient with hydration of HNOg thus giving a higher molecular weight. If this is the case, the nitric acid concentration will be a little low and the particulate nitrate value a little high, but the sum of them will not be affected. Soot, SO2 and particulate sulphate The black component (soot) of airborne particles has been identified as carbon (Rosen et al., 1978). It can be measured
1733
Long-range transport of gaseous and particulate oxidized nitrogen compounds using optical methods. Here it was determined by a simple reflectance method which is used continuously at clean air stations in Sweden since 1967 (OECD, 1964). Ambient air is filtered through a Whatman 40 filter. The light reflectance is measured and the soot concentration is calculated from a table and expressed as pg particles m- 3. The table is arranged from empirical measurements of gravimetritally determined particle mass and reflectance. Particulate sulphate was determined from X-ray fluorescence measurements of sulphur on the soot filters. SOI was determined in the same sampling train. After filtration the air was sucked through a wash bottle containing a slightly acidified H,O,! solution. The SO:- amount in this solution corresponds to the sampled SOr amount and was determined with an ion chromatograph. WIND VECTOR
DIAGRAMS
850 mb air mass back-trajectories
with positions given every 4 h during the 72-h track were calculated by the Norwegian Meteorological Institute. Four trajectories every 24 h corresponding to an arrival time at Rorvik of 6, 12, 18 and 24 h GMT, respectively, were calculated for the whole period of measurements. The origin of the air mass during a 24-h measurement was classified into eight regions corresponding to equal size sectors of a circle with RBrvik in the center as shown in Fig. 1. When the air mass was transported within one sector for 72 h it was allocated to that sector. Two exceptions from this were, however, made. If the calculated trajectory went into other sectors within the last 100 km from Rorvik, this deviation was ignored. Sometimes, but not very often, the trajectory started outside the sector in question at about 2000 km from Rorvik. This part was also ignored. If at least 3 or 4 daily trajectories fulfilled these conditions and the 4th one was within the adjacent sector, the wind during this day was allocated to the most frequent sector in question. Trajectories fullfilled this classification for 183 days during the whole year’s measurements. Mean concentrations of different compounds for different trajectories together with the number of measurements of NO, are shown in Figs 3 and 4. It is almost the same for all species except for NO; and HN03 as mentioned earlier. RESULTS
The monthly variations of the average concentrations are presented in Fig. 2. Measurements of PAN started in March 1982 and ended in March 1983. No comparison of simultaneous data during the winter can therefore be made. Data from PAN measurements during the winter are not yet completely evaluated but the monthly mean concentrations seem to be somewhat lower than during the summer. One episode with high concentrations around 100 nmole mm3 occurred during the winter 1983. Wind vector diagrams showing the average concentrations of NO*, PAN, HNO, +NO; and SO, + SO:- are presented for the winter season in Fig. 3 and for the summer season in Fig. 4. The pattern with a
5 +
_ Nrn s -
90 60 30 0
“N
DIJ 1981
F
M
A
M
J
J
A
S
0
1982
Fig. 2. Monthly mean concentrations of the different species. The shaded areas denotes particle phase and the unshaded gas phase. All numbers are in nmole mm3.
maximum in the south or south-west direction is usually obtained for long-range transported pollutants at Riirvik. Episodes of high concentrations with a duration of several days occurred on six occasions during the monitoring period. On almost all these occasions the wind came from the southwest and did not change direction for a couple of days with a steady concentration increase as a result. The episode was interrupted when the wind direction changed. Mean and maximum concentrations for the NO, are given in Table 1. Daily values for soot and SO2 +particulate SOiwere compared for those occasions when simultaneous data for all NO,, SO, and SO:- were available. The relations are presented as linear regression lines in Fig. 5. Corresponding plots for the NO, are presented in Fig. 6. The positive intersection points between the regression lines for the NO, and the ordinate are high in comparison with the mean values which are indicated by circles. They are, however, not so high in comparison with the maximum values which strongly affect the equations for the lines. Both the fact that all lines have a positive intersection with the ordinate together with the occasionally low correlation coefficients r (shown in Fig. 6) indicates that NO, and sulphur compounds may, at least partly, originate
MARTIN FERM et al. Nor
SO,+
NO;+
HNO3
1
250
1000 1
so:-+
$02
SO$(nmol/m3)
Fig. 5. Linear regression lines for the plot of soot vs SO2 + particulate SO:-. The circles indicate the mean concentration.
500N02+ NO;+ HNOs
400 -
r=069
3cONO2 r=0.60
Fig. 3. Wind vector diagrams showing mean concentrations in nmole m-s for the winter season (November 1981-April 1982). The shaded areas denotes particle phase and the unshaded gas phase.
- zoo "E \
;
x,
0
loo
200
500
600
0
100
200 300 400 500 SOe + SO;- ( nmot /m3)
600
300
400
NO; 100
NOi+
HNOl
Fig. 6. Linear regression lines for the plot of different nitrogen oxides versus SO2 + particulate SO:-. The upper diagram represents the period November 1981-April1982and the lower May 19822October 1982. The circles indicate the mean concentrations.
Fig. 4. Wind vector diagrams showing mean concentrations in nmole m-s for the summer season (April 1982-October 1982). The shaded areas denotes particle phase and the unshaded gas phase.
from different sources. The particulate NO; concentration obtained with the combined denuder technique is of the same order of magnitude as the SOiconcentration. Previous measurements on unimpregnated Teflon@ filters directly exposed to air have given NO; concentrations one order of magnitude less than the SOi- concentration. This can be explained by the fact that particulate NO;, to a large extent in the form
Long-range transport of gaseous and particulate oxidized nitrogen compounds
1735
Table 1. Mean and maximum concentrations in nmole m- 3 of different oxides of nitrogen at Rorvik November 1981-April 1982 April 1982-0ctober 1982 mean max mean max
Compound
NO1
236
PAN HNO, Particle-borne NO;
32 54
1280 140 332
of NH,NO,, will dissociate to HNO, and NH,, when exposed to a pressure drop on the filter. HNO, is retained relatively weakly on Teflon @ filters and will partly evaporate. NH, will evaporate to higher extent and the evaporated amount is proportional to the NO; amount and dependent on the pressure drop. The original HNO, is to a not negligible extent adsorbed on the sampling equipment in front of the filter. The sampling artifact for HNO, and NO; has earlier been discussed (Ferm, 1982).
REFERENCES
Brosset C. (1978) Water-soluble sulphur compounds in aerosols. Atmospheric Environment 12, 2538. Ferm M. (1982) Method for determination of gaseous nitric acid and particulate nitrate in the atmosphere. EMEP Expert meeting on chemical matters, Geneva, 10-12 March.
76 27 18 31
267 92 86 199
Mean of all measurements 143 25 25 42
Nielsen T., Samuelsson U., Grennfelt P. and Thomsen E. L. (1981) Peroxyacetylnitrate in long-range transported polluted air. Nature Lond. 293, 553-555. Nielsen T., Hansen A. M. and Thomsen E. L. (1982) A convenient method for preparation of pure standards of peroxyacetylnitrate for atmospheric analyses. Atmospheric Environment 16, 2447-2450.
OECD (1964) Methods of measuring air pollution, p. 15. Organisation for Economic Co-operation and Development. Persson G. (1982) Acidification today and tomorrow, p. 18. Swedish Ministry of Agriculture Environment ‘82 Committee. Rosen H., Hansen A. D. A., Grundel L. and Novakov T. (1978) Identification of the optically absorbing component in urban aerosols. Appl. Opt. 17, 3859-3861. Saltzman B. E. (1954) Calorimetric microdetermination of nitrogen dioxide in the atmosphere. Analyt. Chem. 26, 19491955. Spicer C. W., Joseph D. W. and Ward G. F. (1978) Investigations of nitrogen oxides within the plume of an isolated city. Battelle (CAPA-9-77) Columbus, Ohio. Stephens E. R. and Price M. A. (1973) Analysis of an important air pollutant peroxyacetyl nitrate. J. Chem. Ed. so, 351-354.
Vol. 18. No. 9. pp. 1731-1735,
PrintedinGreatBritain.
OOOk6981/84 E3.00 + 0.00 Pergamon Press Ltd.
1984
LONG-RANGE TRANSPORT OF GASEOUS AND PARTICULATE OXIDIZED NITROGEN COMPOUNDS MARTIN FERM*, ULLA SAMUELSSON, AKE
SJ~DIN
and PERINGE GRENNFELT
Swedish Environmental Research Institute, P.O. Box 5207, S-402 24 Gothenburg, Sweden Abstract-Measurements of NO,, PAN, HNO, and particulate NO; as 24-h mean concentration values have been carried out at a clean air station. The station is situated on the Swedish west coast 40 km south of Gothenburg. Measurements from November 1981 to October 1982 are presented. NO2 was measured with the Saltzman method after removal of ozone and PAN with a gas chromatographic method. HN09 was sampled in a denuder and particulate NO; was sampled on an impregnated filter behind the denuder. The denuder and filter were then analysed by ion chromatography. Episodes of high concentrations occurred several times during the year. On these occasions the air was transported over a long range from the continent. 72-h back trajectories are available for this station at 6-h intervals during the whole period of measurements. Monthly mean concentrations of these four species, together with data on SO2 and particulate sulphate are presented. Analysis of the data as a function of the direction of the back trajectory and correlations between nitrogen and sulphate compounds are also given.
INTRODUCTION In Scandinavia, the NO; concentration in precipitation is about twice as high today as 20 years ago (Persson, 1982). This change is assumed to be caused
by a corresponding increase in the atmospheric concentrations of oxidized nitrogen compounds transported from long range. Very little is, however, known about the relative abundance of different forms. Those very few investigations published so far indicate that long-range transport of oxidized nitrogen compounds occurs essentially as NOz, PAN, HN03 and particle-borne NO; (Nielsen et al., 1981; Spicer, 1978; Brosset, 1978). In all cases the investigations cover either a very short time or include only one or two of the compounds of interest. In this paper we will present data from a l-y (November 1981-October 1982) monitoring of the atmospheric concentrations of different oxidized N compounds at a remote monitoring station on the Swedish west coast (Riirvik 57” 25’N, 11” WE, see Fig. 1). Sampling was performed 3 m above ground. Nitrogen oxides are mainly emitted as NO. In the atmosphere a variety of oxidizing and other reaction products are formed from NO. Of these we believe that NOz, PAN, HN03 and particle-borne NO; occur in the highest concentrations. They were also involved in the monitoring programme. All measurements presented in this article are 24-h mean values starting and ending at 6.00 GMT.
METHODS Nitrogen dioxide
NOz was determined with the Saltzman technique (Saltzman, 1954). In this method ozone has a negative
interference. This was overcome by mounting a sodium thiosulphate impregnated filter in front of the bubbler. At a flow of0.2 Pmin-’ this filter reduced the ozone concentration by more than 98 %. Less than 5 % of the NO2 was lost in this filter. Samples were refrigerated and analyzed within one week. This procedure kept the absorbance decrease less than 2% per week. In wet chemical methods with alkaline solution PAN has a positive interference due to hydrolysation to NO; Laboratory tests showed no interference with the acid Saltzman solution. Results were calculated using a ‘Saltzman-factor’ of 0.72. The detection limit is estimated to be 10 nmole m-3. Peroxyacetyl
nitrate
PAN, CH,C(0)02NOI was sampled and analyzed immediately once an hour by the GC-EC technique described by Stephens and Price (1973). Air was injected from a continuously ventilated sampling valve (volume 1 ml) on a 1 m packed column with an internal diameter of 2 mm, containing 5 y0 carbowax 400 on Chromosorb W 10&l 20 mesh. N2 was used as carrier gas with a flow of 35 mlmin-‘. Oven temperature was 25”C, and an electron capture detector (63 Ni) was used. The instrument was calibrated with a heptane solution containing PAN which was injected in a Tedla@ bag together with a known volume of air. The PAN solution was produced and calibrated with a procedure described by Nielsen et al. (1982). The PAN air mixture was also analyzed on achemiluminescent NO, instrument which was calibrated with a NO, permeation tube. There was no difference between the calculated PAN concentration in the Tedlar ‘@ bag and the concentration obtained with the chemiluminescent instrument as NO,. The concentration of PAN was determined once an hour essentially. The detection limit was estimated to be 6 nmole mm3. Nitric acid and particulate
nitrate
HNO, and particulate NO; were determined with a combined denuder and impregnated filter technique (Ferm, 1982). HNO, was first trapped in a hollow tube internally coated with sodium carbonate. The denuder will absorb almost all acid gases, for instance SOZ, HNOZ, HF and HCI. Particles diffuse slower to the coated walls and therefore pass through the tube (denuder) to a filter holder containing a sodium carbonate impregnated filter. The sodium carbonate 1731
MARTIN FERM et al.
1732
b lb0
IO00
2000
km
Fig. 1. Map of wind sectors with Rorvik at center.
prevents NO; from leaving the filter as HNO+ Laboratory experiments have revealed that PAN does not interfere because it is converted to NO; when absorbed in the denuder. In the measurements presented here there is no correlation between HNOs and PAN. A high concentration of PAN often corresponded to a low HN03 concentration and vice versa. After sampling the filter and denuders were leached in water and analysed by ion chromatography for Cl-, NO; and SO:-. The detection limit for HNO, and NO; was estimated from the variation in the NO; blank values for the denuder and filter, respectively. The detection limit for HNO, is estimated to be 0.5 nmole me3 and for particulate NO;, 3 nmole rnm3. Parallel tests showed that the sulphate amount per unit volume of air in the denuder agreed very well with simultaneous values for SO2 (r = 0.83), obtained with the HZO, method. Moreover, the SO:- concentration on the coated filter was in good agreement (I = 0.89) with measurements of S with X-ray fluorescence on filters exposed directly to the ambient air. Sometimes high concentrations of CI- were found in the denuder, probably originating from sea salt particles. Very high concentrations of NO; and relatively high concentrations of SO:- were found on these days. Days with extremely high concentrations of Cl- (> 70 nmole m-‘) in the denuder were therefore excluded. The coating of the denuder can be dissolved during rain or periods with very high humidity. The denuders were therefore heated a few degrees centigrade and protected from rain. The
coating could still be destroyed by fog which frequently appeared during the morning hours. The comparison of SOi- in the denuder with ambient SO2 concentration was used to decide whether the sampling was successful or not. Usually, there was a good agreement between the two methods, but sometimes there was no agreement at all and those values were excluded. Approximately 30% of the samples were lost due to sea spray, fog and accidents during sampling. The mean concentration of SO2 + particulate SO:- was 2.4% higher for the days when the HNOJ and NO; sampling were successful than the mean SO2 + SOiconcentration for the whole year. Therefore, we assume that the presented mean values for HNO, and NO; are representative. By leaching different parts of the denuder separately the apparent diffusion coefficients for nitric acid and SO2 were determined. For SO* the diffusion coefficient was in good agreement with the experimentally obtained value but for HNOJ a lower value was found. The diffusion coefficient decreased with increasing relative humidity of the air. This had earlier been observed by Braman (1983, pers. comm.) who explained the lower diffusion coefficient with hydration of HNOg thus giving a higher molecular weight. If this is the case, the nitric acid concentration will be a little low and the particulate nitrate value a little high, but the sum of them will not be affected. Soot, SO2 and particulate sulphate The black component (soot) of airborne particles has been identified as carbon (Rosen et al., 1978). It can be measured
1733
Long-range transport of gaseous and particulate oxidized nitrogen compounds using optical methods. Here it was determined by a simple reflectance method which is used continuously at clean air stations in Sweden since 1967 (OECD, 1964). Ambient air is filtered through a Whatman 40 filter. The light reflectance is measured and the soot concentration is calculated from a table and expressed as pg particles m- 3. The table is arranged from empirical measurements of gravimetritally determined particle mass and reflectance. Particulate sulphate was determined from X-ray fluorescence measurements of sulphur on the soot filters. SOI was determined in the same sampling train. After filtration the air was sucked through a wash bottle containing a slightly acidified H,O,! solution. The SO:- amount in this solution corresponds to the sampled SOr amount and was determined with an ion chromatograph. WIND VECTOR
DIAGRAMS
850 mb air mass back-trajectories
with positions given every 4 h during the 72-h track were calculated by the Norwegian Meteorological Institute. Four trajectories every 24 h corresponding to an arrival time at Rorvik of 6, 12, 18 and 24 h GMT, respectively, were calculated for the whole period of measurements. The origin of the air mass during a 24-h measurement was classified into eight regions corresponding to equal size sectors of a circle with RBrvik in the center as shown in Fig. 1. When the air mass was transported within one sector for 72 h it was allocated to that sector. Two exceptions from this were, however, made. If the calculated trajectory went into other sectors within the last 100 km from Rorvik, this deviation was ignored. Sometimes, but not very often, the trajectory started outside the sector in question at about 2000 km from Rorvik. This part was also ignored. If at least 3 or 4 daily trajectories fulfilled these conditions and the 4th one was within the adjacent sector, the wind during this day was allocated to the most frequent sector in question. Trajectories fullfilled this classification for 183 days during the whole year’s measurements. Mean concentrations of different compounds for different trajectories together with the number of measurements of NO, are shown in Figs 3 and 4. It is almost the same for all species except for NO; and HN03 as mentioned earlier. RESULTS
The monthly variations of the average concentrations are presented in Fig. 2. Measurements of PAN started in March 1982 and ended in March 1983. No comparison of simultaneous data during the winter can therefore be made. Data from PAN measurements during the winter are not yet completely evaluated but the monthly mean concentrations seem to be somewhat lower than during the summer. One episode with high concentrations around 100 nmole mm3 occurred during the winter 1983. Wind vector diagrams showing the average concentrations of NO*, PAN, HNO, +NO; and SO, + SO:- are presented for the winter season in Fig. 3 and for the summer season in Fig. 4. The pattern with a
5 +
_ Nrn s -
90 60 30 0
“N
DIJ 1981
F
M
A
M
J
J
A
S
0
1982
Fig. 2. Monthly mean concentrations of the different species. The shaded areas denotes particle phase and the unshaded gas phase. All numbers are in nmole mm3.
maximum in the south or south-west direction is usually obtained for long-range transported pollutants at Riirvik. Episodes of high concentrations with a duration of several days occurred on six occasions during the monitoring period. On almost all these occasions the wind came from the southwest and did not change direction for a couple of days with a steady concentration increase as a result. The episode was interrupted when the wind direction changed. Mean and maximum concentrations for the NO, are given in Table 1. Daily values for soot and SO2 +particulate SOiwere compared for those occasions when simultaneous data for all NO,, SO, and SO:- were available. The relations are presented as linear regression lines in Fig. 5. Corresponding plots for the NO, are presented in Fig. 6. The positive intersection points between the regression lines for the NO, and the ordinate are high in comparison with the mean values which are indicated by circles. They are, however, not so high in comparison with the maximum values which strongly affect the equations for the lines. Both the fact that all lines have a positive intersection with the ordinate together with the occasionally low correlation coefficients r (shown in Fig. 6) indicates that NO, and sulphur compounds may, at least partly, originate
MARTIN FERM et al. Nor
SO,+
NO;+
HNO3
1
250
1000 1
so:-+
$02
SO$(nmol/m3)
Fig. 5. Linear regression lines for the plot of soot vs SO2 + particulate SO:-. The circles indicate the mean concentration.
500N02+ NO;+ HNOs
400 -
r=069
3cONO2 r=0.60
Fig. 3. Wind vector diagrams showing mean concentrations in nmole m-s for the winter season (November 1981-April 1982). The shaded areas denotes particle phase and the unshaded gas phase.
- zoo "E \
;
x,
0
loo
200
500
600
0
100
200 300 400 500 SOe + SO;- ( nmot /m3)
600
300
400
NO; 100
NOi+
HNOl
Fig. 6. Linear regression lines for the plot of different nitrogen oxides versus SO2 + particulate SO:-. The upper diagram represents the period November 1981-April1982and the lower May 19822October 1982. The circles indicate the mean concentrations.
Fig. 4. Wind vector diagrams showing mean concentrations in nmole m-s for the summer season (April 1982-October 1982). The shaded areas denotes particle phase and the unshaded gas phase.
from different sources. The particulate NO; concentration obtained with the combined denuder technique is of the same order of magnitude as the SOiconcentration. Previous measurements on unimpregnated Teflon@ filters directly exposed to air have given NO; concentrations one order of magnitude less than the SOi- concentration. This can be explained by the fact that particulate NO;, to a large extent in the form
Long-range transport of gaseous and particulate oxidized nitrogen compounds
1735
Table 1. Mean and maximum concentrations in nmole m- 3 of different oxides of nitrogen at Rorvik November 1981-April 1982 April 1982-0ctober 1982 mean max mean max
Compound
NO1
236
PAN HNO, Particle-borne NO;
32 54
1280 140 332
of NH,NO,, will dissociate to HNO, and NH,, when exposed to a pressure drop on the filter. HNO, is retained relatively weakly on Teflon @ filters and will partly evaporate. NH, will evaporate to higher extent and the evaporated amount is proportional to the NO; amount and dependent on the pressure drop. The original HNO, is to a not negligible extent adsorbed on the sampling equipment in front of the filter. The sampling artifact for HNO, and NO; has earlier been discussed (Ferm, 1982).
REFERENCES
Brosset C. (1978) Water-soluble sulphur compounds in aerosols. Atmospheric Environment 12, 2538. Ferm M. (1982) Method for determination of gaseous nitric acid and particulate nitrate in the atmosphere. EMEP Expert meeting on chemical matters, Geneva, 10-12 March.
76 27 18 31
267 92 86 199
Mean of all measurements 143 25 25 42
Nielsen T., Samuelsson U., Grennfelt P. and Thomsen E. L. (1981) Peroxyacetylnitrate in long-range transported polluted air. Nature Lond. 293, 553-555. Nielsen T., Hansen A. M. and Thomsen E. L. (1982) A convenient method for preparation of pure standards of peroxyacetylnitrate for atmospheric analyses. Atmospheric Environment 16, 2447-2450.
OECD (1964) Methods of measuring air pollution, p. 15. Organisation for Economic Co-operation and Development. Persson G. (1982) Acidification today and tomorrow, p. 18. Swedish Ministry of Agriculture Environment ‘82 Committee. Rosen H., Hansen A. D. A., Grundel L. and Novakov T. (1978) Identification of the optically absorbing component in urban aerosols. Appl. Opt. 17, 3859-3861. Saltzman B. E. (1954) Calorimetric microdetermination of nitrogen dioxide in the atmosphere. Analyt. Chem. 26, 19491955. Spicer C. W., Joseph D. W. and Ward G. F. (1978) Investigations of nitrogen oxides within the plume of an isolated city. Battelle (CAPA-9-77) Columbus, Ohio. Stephens E. R. and Price M. A. (1973) Analysis of an important air pollutant peroxyacetyl nitrate. J. Chem. Ed. so, 351-354.