Nordic intercomparison for measurement of major atmospheric nitrogen species

PII: S0021-8502(98)00039-1

J. Aerosol Sci. Vol. 30, No. 2, pp. 247—263, 1999 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0021-8502/98 $19.00#0.00

NORDIC INTERCOMPARISON FOR MEASUREMENT OF MAJOR ATMOSPHERIC NITROGEN SPECIES Tuomo A. Pakkanen,*s Risto E. Hillamo,* Minna Aurela,* Helle Vibeke Andersen,t Lone Grundahl,t Martin Ferm,° Karin Persson,° Vuokko Karlsson,± Anni Reissell,± Oddvar R+yset,E Inga Fl+isand,E Pedro Oyola**,tt and Tadeusz Ganko** * Finnish Meteorological Institute, Aerosol Research Group, Sahaajankatu 20E, FIN-00810 Helsinki, Finland t National Environmental Research Institute, Frederiksborgvej 399, DK-4000 Roskilde, Denmark ° Swedish Environmental Research Institute, P.O. Box 47086, S-402 58 Gothenburg, Sweden ± Finnish Meteorological Institute, Laboratory Group, Sahaajankatu 20E, FIN-00810 Helsinki, Finland E Norwegian Institute for Air Research, Box 100, N-2007 Kjeller, Norway ** Institute of Applied Environmental Research at the University of Stockholm Frescati Hagva¨g 16 B, S-106 91 Stockholm, Sweden (First received 12 November 1997; and in final form 9 March 1998) Abstract—A comparative study of measurements of atmospheric gaseous nitric acid and ammonia and particulate nitrate and ammonium concentrations was conducted using various types of filter packs and denuder systems. Some of the filter packs used are recommended by the European Monitoring and Evaluation Programme (EMEP) and are widely used in the Nordic Countries and elsewhere in Europe. In addition, nitrogen dioxide was measured using iodide-impregnated sintered glass filters and the differential optical absorption spectrometer method. Particulate nitrate and ammonium concentrations were measured with two different size segregating samplers: a Berner low-pressure impactor and a virtual impactor. The weather conditions were most of the time cool and humid. The agreement between different measurements was good for most species, but poor for gaseous ammonia. The virtual impactor collected on average 17% more nitrate than the denuder systems, 24% more than filter packs and 37% more than the Berner low-pressure impactor. The nitrate discrepancy is believed to be due to inefficient coarse particle transport into the denuder systems, fine particle nitrate evaporation in the filter packs and Berner low-pressure impactor and, to a lesser extent, possible collection of nitric acid by the virtual impactor. A substantial fraction of PM may be lost if the nitrate evaporation cannot be accounted for. The denuder systems did not 2.5 particles larger than about 5.6 km equivalent aerodynamic diameter efficiently. Interferences collect were observed on the nitric acid measurements determined by a NaOH coated denuder. NaCl coated denuders seemed to operate well in this study since interferences were not observed. ( 1998 Elsevier Science Ltd. All rights reserved

1 . I NT RO D UC T IO N

Previous comparisons (Pio, 1992; Benner et al., 1991; Harrison and Kitto, 1990; Dasch et al., 1989; Ferm et al., 1988; Eatough et al., 1988; Hering et al., 1988) have shown that the denuder systems (DS) are suitable for determination of ammonia (Ferm, 1979), ammonium, nitric acid and nitrate although some interferences may occur. Depending for instance on the sampling site, weather conditions and the acidity of sampled aerosol the filter pack methods (FP) sometimes can suffer from serious problems leading to erroneous results (Andersen and Hovmand, 1994; Andersen and Hilbert, 1993). Earlier studies suggest further that the size-segregation samplers, virtual impactor (VI) (Loo and Cork, 1988) and Berner low-pressure impactor (BLPI) (Berner and Lu¨rzer, 1980), can be used as accurate methods for determination of nitrate. Wall et al. (1988) reported that volatile nitrate losses in a BLPI were less than 10% and that interference from nitric acid was not detected; John et al. (1988) observed that the inner oxidized aluminum surfaces of a VI efficiently denuded nitric acid. Thus the VI was an interference and artifact free instrument when used for the determination of nitrate. In this work, NO was monitored by the differential optical absorption spectrometer 2 (DOAS) and collected by reducing NO to NO~ on impregnated filters. Two different 2 2 sAuthor to whom correspondence should be addressed. ttPresent address: CONAMA-RM, McIver 283, Piso 7, Santiago, Chile. 247

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formulas for the reduction was used. In the older version KI and NaAsO were used to 2 reduce NO and ethylene glycol to keep the filter humid. NaAsO also kept the pH high. To 2 2 avoid the handling of arsenite a new formula was designed. In this NaI was used to reduce NO to NO~ and Na CO to keep the pH high (Ferm and Sjo¨din, 1992). NaI is more 2 2 3 2 deliquescent than KI and keeps the filter humid. To get a high efficiency at a reasonable flow, and a leak proof filter holder that was simple to handle, a glass bulb with a sintered glass filter was constructed. The concentrations measured in this work by each individual instrument were presented in detail in the report to Nordic Council of Ministers: Nordic HNO /NO~ and NH /NH` 4 3 3 3 gas/particle intercomparison in Helsinki, 11—22 May, 1992 (Pakkanen et al., 1994). Instead of the individual results the present paper compares the different types of measurement instruments used: the DS, FP, VI, BLPI, impregnated glass sinters (IGS) and the DOAS method. In addition, the differences between nitrate measurements and interferences on the nitric acid measurements are discussed. The mass of particulates was not measured but on basis of later measurements can be estimated to have been on average about 10 kg m~3 for PM and 15 kg m~3 for PM . 2.5 15 2 . EXP ER I ME N TA L

2.1. Field site of the intercomparison study The comparative field study was conducted in Helsinki, 11—22 May 1992. The sampling site was on the roof of the Finnish Meteorological Institute, Air Quality Department building, seven km to the east from the centre of Helsinki. The roof is about 20 m above ground level. Car traffic is obviously the most important local pollution source with one major road 350 m to the northwest and another road with moderate traffic adjacent to the FMI building. The sea coast is about 2 km south of the sampling site. 2.2. Participants and sampling methods Five institutes from four Nordic countries participated in the intercomparison: the National Environmental Research Institute (DMU) from Denmark, two groups from the Finnish Meteorological Institute (FMI) from Finland, the Norwegian Institute for Air Research (NILU) from Norway, and the Swedish Environmental Research Institute (IVL) and the Institute of Applied Environmental Research at the University of Stockholm (ITML), from Sweden. A summary of the sampling methods and the corresponding sampling flow rates are listed in Table 1. Nitrogen dioxide concentrations were compared for two KI-impregnated and two NaI-impregnated glass sinters (Ferm and Sjo¨din, 1992) and for the DOAS instrument. Three DSs and two FPs were used to measure ammonia. Ammonium was determined by three DSs, two FPs, a VI and a BLPI. Nitric acid concentrations were measured using six DSs and two FPs. Nitrate measurements were carried out with six DSs, two FPs, the VI and the BLPI. In addition, one FP was used to measure the sum of ammonia and ammonium (total ammonium) and another FP was used to measure the sum of nitric acid and nitrate (total nitrate). These FPs measuring total ammonium and total nitrate were similar to those recommended by EMEP. The VI used in this work was made of stainless steel. The BLPI and the VI were equipped with stainless steel aerosol inlets (University of Minnesota type inlets; Liu and Pui, 1981). The VI samples particles in two size fractions: particle aerodynamic diameter EAD(2.5 km and 2.5(EAD (15 km. The BLPI collects particles in ten size fractions in the size range of 0.03(EAD(15 km. Coarse particle substrates of the BLPI were greased to minimize particle bounce-off (Hillamo and Kauppinen, 1991). 2.3. Sampling schedule Sampling was started either 9 a.m. or 9 p.m. The virtual impactor, most of the denuder systems and one filter pack collected for five 12 h and eight 24 h periods whilst the Berner

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249

Table 1. Sampling methods and the volume flow rates as measured by laminar flow elements during the comparison. Flow rates are as l min~1 at 101.3 kPa and 24°C Code

Inlet (cut-off )

Sampling line

Flow rate

DMU.FP DMU.DS1 DMU.DS2 DMU.DS3 DMU.DS4 FMI.FP1s FMI.FP2s FMI.ADS FMI.BLPI FMI.VI FMI.NaI IVL.DS1 IVL.DS2 IVL.KI IVL.NaI NILU.FP NILU.KI ITML.ADS ITML.DOAS

— — — — — — — Cyclone (3.5 km) UM inlet (15 km) UM inlet (15 km) — — — — — — — — —

TF—NaF/IF—KOH/IF—oxalic acid/IF NaOH/D—NaOH/IF NaCl/D—NaOH/IF Na CO /D—Na CO /IF 2 3 2 3 Oxalic acid/D—oxalic acid/IF CF—NaOH/IF Oxalic acid/IF Na CO /D—Na CO /D—citric acid/D—TF—NF 2 3 2 3 11-stage Berner low-pressure impactor Virtual impactor; fine: TF—NF, coarse: PF NaI/IGS NaCl/D—NaCl/IF Oxalic acid/D—oxalic acid/IF KI/IGS NaI/IGS TF-KOH/IF—oxalic acid/IF KI/IGS NaCl/D—Na CO /D—Na CO /D—Na CO /IF 2 3 2 3 2 3 Differential optical absorption spectrometer

44.36 1—2* 1—2* 1—2* 1—2* 18.5t 20.8t 29.14 25.0t 16.7t 0.51 1.81 2.29 0.43 0.42 16.6* 0.57 2.78*

Symbols : IGS impregnated glass sinter D denuder DS denuder system ADS annular denuder system FP filter pack UM University of Minnesota inlet NaI sodiumiodide TF Teflon filter CF cellulose filter IF impregnated filter NF nylon filter PF polycarbonate filter KI potassiumiodide * Flow rates indicated show an average or range of two separate sampling lines. s Similar to those recommended by EMEP. t Flow rate not checked with laminar flow elements.

low-pressure impactor collected for two 24 h and four 48 h periods. The denuder systems of the DMU and ITML were used to collect more 12 h samples than the other instruments, but some of these 12 h periods were combined to 24 h periods to match those of the other instruments. 2.4. Chemical analysis After collecting the samples were dissolved and stored mostly in dark and cool. In order to check the influence of variable sample transportation to and storage (because of possible ammonium losses storing needs to be made in dark as shown by Ferm, 1993) at each participating laboratory, reference samples with known concentrations were prepared at the FMI by dilution of appropriate commercial standard solutions. These reference samples were then analysed at each laboratory and the results exhibited only minor differences (Pakkanen et al., 1994). The components were usually analyzed by ion chromatography (IC), but other techniques such as the spectrophotometric indophenol blue method for ammonium were also employed. 2.5. ¼eather conditions The temperature and the relative humidity, recorded at the sampling site, together with other weather data registered in a synoptic station near the FMI main building, 7 km from

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250

Table 2. Summary of weather condition May

Temp (°C) range

Temp (°C) average

RH (%) range

RH (%) average

11n 12 13 14 15 16 17 18 19d 19n 20d 20n 21

2—7 4—10 4—10 6—16 7—12 8—13 6—15 10—20 10—16 8—17 17—20 8—17 8—18

5 7 7 11 9 10 11 14 13 11 19 11 12

75—95 50—94 60—95 30—80 60—85 60—90 40—75 35—60 40—80 30—90 20—45 45—75 50—85

89 75 75 50 75 75 55 47 60 70 27 60 75

Rain (mm) average 8.5 0.4 0.6

0.1

Wind (m s~1) average

Cloudiness average*

2 3.4 4.4 2.3 3.4 5.2 4.2 4.0 1.6 2.6 4.0 1.4 2.1

5.3 5.8 3.6 1.6 1.0 4.4 1.6 4.4 2.7 1.7 1.0 3.7 4.6

d: day; n: night. *Scale for cloudiness: 0"clear sky % 8"cloudy sky.

the sampling site, are summarized in Table 2. Generally, the weather was rather windy and sunny with the temperatures usually between 5 and 15°C and the relative humidities usually higher than 50%. 3 . RES UL TS AN D D IS CU SSI O N

Results deviating by more than twice the standard deviation were excluded as outliers. Some instruments occasionally suffered from identifiable problems and in such cases even results deviating by about the standard deviation were discarded. The outliers were not taken into account while calculating the average values. The number of outliers is shown in Table 3 as a percentage from total number of individual measurements. 3.1. Size distributions of nitrate and ammonium Mass size distributions are useful in the estimation of possible reasons to the differences observed on the particle measurements, since the chemical nature of fine and coarse particles is usually different for most constituents. In this work Berner low-pressure impactors were used to determine size distributions of the particulate nitrogen compounds, ammonium and nitrate. The data inversion code MICRON (Wolfenbarger and Seinfeld, 1990, 1991) was used in the evaluation of the size distributions, presented in Fig. 1. Overall the size distributions observed in this work showed characteristics similar to those observed in California (John et al., 1990). Nitrate exhibited one or two fine particle modes and two overlapping coarse particle modes (Pakkanen et al., 1996). Coarse nitrate is usually present in the form of non-volatile nitrate compounds (Yoshizumi and Hoshi, 1985). The VI and BLPI data agreed well for coarse particle nitrate concentrations but it should be noted that about half of the fine particle nitrate evaporated from the BLPI (see Fig. 5) which is further discussed in the Section 3.7. Figure 1 indicates that ammonium and sulphate usually exhibited two fine particle modes and two coarse particle modes. About 95% of ammonium was contained in the fine particle size range. The ammonium and sulphate size distributions were very similar for particles below 1 km EAD. The observed ammonium mole concentration was about twice that of sulphate. Given the low nitrate concentrations observed, it would appear that sulphate and ammonium were mostly present as (NH ) SO and that the fine aerosol therefore was only 42 4 slightly acidic or neutral (Seinfeld, 1986). As discussed above, approximately half of the fine particle nitrate evaporated from the BLPI (see Fig. 5) which implies that also some fine particle ammonium likely evaporated (NH NO and NH Cl are the major volatile 4 3 4

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251

Table 3. DOAS/average IGS concentration ratios for nitrogen dioxide and ave.FP/ave.DS, VI/ave.DS and BLPI/ave.DS concentration ratios for particulate ammonium and nitrate. In addition the ave.FP/ave.DS concentration ratios are shown for gaseous ammonia and nitric acid and for the sum of ammonia#ammonium ("total ammonium) and for the sum of nitric acid#nitrate ("total nitrate). The number of valid comparisons is indicated in parantheses after each ratio value Conc. ratio

Measurement time (h) NO 2

DOAS/average IGS

12 24 Average FP/average DS 12 24 VI/average DS 12 24 BLPI/average DS 24 48

1.61 (4) 1.64 (7)

Total number of outliers (%)

5%

NH 3

NH` 4

1.06 (5) 1.11 (8)

0.90 0.93 1.04 1.08 1.02 0.99

24%

13%

(4) (7) (4) (7) (2) (4)

Total NH` 4

HNO 3

NO~ 3

1.23 (5)* 0.94 (5) 0.94 (8) 0.84 (7)

0.95 0.99 1.33 1.20 0.85 0.86

14%

10%

1%

(5) (8) (5) (6) (2) (4)

Total NO~ 3

0.93 (5) 0.97 (8)

4%

* Only one FP measured 12 h periods; this FP showed the highest FP concentrations for total ammonium for the 24 h periods.

ammonium salts). However, this ammonium volatilization had only a small influence on the ammonium concentrations measured by the BLPI, since ammonium was mostly present as non-volatile ammonium sulphate. 3.2. Nitrogen dioxide Figure 2 and Table 3 show the results from the comparison between the average concentrations obtained by the four impregnated glass sinter (IGS) measurements and the DOAS method. The IGS measurements were consistent (Pakkanen et al., 1994) though indicating lower NO concentrations than the DOAS method. A possible explanation for 2 the disagreement observed may be an inhomogeneous nitrogen dioxide concentration along the 440 m long DOAS measurement beam: the DOAS light source was situated only 40 m away from a road with high traffic density, whereas the DOAS receiver together with the IGS samplers were about 350 m from that road. 3.3. Ammonia Figure 3A and Table 3 indicate that the ammonia concentrations determined by the two FPs and the three DSs showed reasonable agreement: the average FP to average DS ratios were 1.06 and 1.11 for the 12 and 24 h measurements, respectively. It should be noted, however, that several outliers were excluded while calculating the average FP and average DS values. The likely reasons for the measurement errors are discussed in detail by Pakkanen et al. (1994). In fact, for several sampling periods the individual measurement results showed such a great variation that the estimation of the actual concentration in the atmosphere studied was difficult. The probable reason for the slightly high ratios could have been the evaporation of particulate ammonium nitrate and/or ammonium chloride from the FP Teflon pre-filter and subsequent collection of ammonia on the FP impregnated filter which is a common artifact in FPs (see Table 1 for the configuration of the two FPs: DMU.FP and NILU.FP). A special feature in our measurements was that the FP/DS ratios for ammonia were higher (with the exception of the first sample) when the nitrate concentrations were high. This same feature has been observed also elsewere (Ferm, 1986a). 3.4. Ammonium Figures 3B, C and D show comparisons of measured average FP, VI and BLPI ammonium values to the average DS ammonium values, respectively. The corresponding

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Fig. 1. Size distributions of ammonium, nitrate and sulphate during the intercomparison. The nitrate mass below 2 km EAD is underestimated because of evaporation. The six figures refer to BLPI samples 1—6, respectively. The data inversion code MICRON (Wolfenbarger and Seinfeld, 1990, 1991) was used in the evaluation of the size distributions.

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253

Fig. 2. Comparison of DOAS and average impregnated glass sinter (IGS) concentration values for nitrogen dioxide.

average ammonium ratios between the methods for 12 and 24 h (and also 48 h for BLPI) measurements are indicated in Table 3. The average denuder values agreed well with the BLPI, VI and average FP values. The slightly larger FP-DS difference is probably due to evaporation of ammonium nitrate and/or ammonium chloride from the teflon pre-filter of the filter packs. 3.5. ¹otal ammonium One of the three filter packs measuring total ammonium was similar to that recommended by EMEP. Figure 3E and Table 3 indicate that the FPs and the DSs agreed well. The average filter pack to average denuder system ratio for the 24 h measurements varied between 0.88 and 1.24 with an average of 0.99. The highest ratios, 1.10 and 1.24, occurred during the four 12 h sampling periods of 19 and 20 May combined here as 24 h measurements. 3.6. Nitric acid The ratios of average FP values to average DS values were usually below one for the 24 h measurements, except the outlier ratio of 1.57 (Fig. 4A and Table 3). The measurements indicated that for this outlier there was a significant difference between the two techniques. The reason for the difference observed is unclear although evaporation of particulate nitrate from the FP prefilters may have occured. However, it is strange that this evaporation seemed to be much different for the other samples. Table 3 shows that the mean of average filter pack to average denuder system ratios for the 24 h measurements was 0.84 (the value of 1.57 excluded), indicating that the denuder systems collected slightly more nitric acid. This is primarily because of the lower values obtained by the KOH-impregnated filter of the NILU filter pack (NILU.FP). The 12 h measurements exhibited slightly better FP-DS agreement but four of the five comparisons are based on only one FP value, that of the DMU.FP. In general, the three different denuder methods of DMU agreed well and this data is used for detailed discussion of interferences on the measurement of nitric acid (see Section 3.9).

T. A. Pakkanen et al.

254

Fig. 3. Comparison of average denuder system (DS) values with average filter pack (FP), virtual impactor (VI) and Berner impactor (BLPI) values. (A) FP-DS comparison for ammonia; (B) FP-DS comparison for ammonium; (C) VI-DS comparison for ammonium; (D) BLPI-DS comparison for ammonium; (E) FP-DS comparison for total ammonium (sum of ammonia and ammonium). Note: The points in parentheses are not included in the calculations of Table 3.

3.7. Nitrate The results for particulate nitrate are presented in Figs 4B—D and in Table 3. On average the agreement was relatively good for the various methods, with the VI showing the highest and the BLPI the lowest concentrations. The mean values of the FP/DS ratios for nitrate were 0.95 and 0.99 for the 12 and 24 h measurements, respectively. The VI/DS ratios were between 1.04 and 1.66 (one high ratio of 2.25 excluded) and the BLPI/DS ratios showed values ranging from 0.74 to 0.94. On average, the VI gave nitrate concentrations that were 17, 24 and 37% higher than those of the denuders (two high percentages excluded;

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255

Fig. 4. Comparison of average denuder system (DS) values with average filter pack (FP), virtual impactor (VI) and Berner impactor (BLPI) values. (A) FP-DS comparison for nitric acid; (B) FP-DS comparison for nitrate; (C) VI-DS comparison for nitrate; (D) BLPI-DS comparison for nitrate (E) FP-DS comparison for total nitrate (sum of nitrate and nitric acid). Note: The points in parentheses are not included in the calculations of Table 3.

FMI.ADS excluded because the cyclone removes particles larger than 3.5 km of EAD), those of the filter packs (two high percentages excluded) and those of the BLPI (one high percentage excluded), respectively. Three possible reasons were considered to be responsible for the differences observed: (i) a large fraction of fine particle nitrate evaporated from the BLPI and a smaller amount from the FPs (ii) the denuder systems collected coarse particles inefficiently and, possibly iii) the VI may have collected a small amount of gaseous nitric acid indistinguishable from nitrate. 3.7.1. Nitrate evaporation in the B¸PI. Figure 5 shows the fine and coarse particle nitrate concentrations as measured by the VI and the BLPI. The two instruments showed good

256

T. A. Pakkanen et al.

agreement for Na` and SO2~, which suggests that there were no problems with the 4 sampling efficiencies. Figure 5 indicates a good agreement for the coarse particle nitrate (VI: 2.5(EAD(15 km; BLPI 2.0(EAD(15 km), but about half of the fine particle (VI: EAD(2.5 km; BLPI: 0.03(EAD(2.0 km) nitrate evaporated from the BLPI. The small difference in the particle size cut-off of the VI and BLPI has practically no influence on the conclusions drawn from this comparison. Using this data, Kerminen et al. (1997) calculated theoretically that at the end of the BLPI sampling periods the atmospheric conditions favored the gaseous form of ammonium nitrate except for samples 1 and 6. Thus, for samples 2, 3, 4 and 5, the ammonium nitrate accumulated during sampling may have partly evaporated by the end of the sampling. Figure 1 supports these calculations because only samples 1 and 6 show a pronounced fine particle mode for nitrate. On average, the evaporative loss of nitrate from the BLPI was about 50% for fine particles and about 27% for nitrate below 15 km EAD. These values are clearly higher than those of about 10—20% reported by Wall et al. (1988) and Zhang and McMurry (1992) for atmospheric conditions in California. The higher mass concentrations of atmospheric particles and shorter sampling periods in the Californian studies lead to thicker particle deposits and shorter time available for evaporation, which both inhibit evaporation from BLPI. Further the longer sampling periods in the Helsinki study allow for greater temperature and humidity changes. Nitrate evaporation may also be enhanced if the aerosol is acidic. Ion balances for individual impactor stages presented by Pakkanen (1996) suggest, however, that fine particles left in the impactor were only slightly acidic or neutral. For comparison, the nitrate loss from the parallel VI measurements was also calculated: on average about 30% of nitrate was lost from the VI Teflon filters and recollected on the VI nylon filters (see Figs 5 and 6). In 1996—1997, simultaneous 24 h VI samples were collected at three sites in the Helsinki area using the same VI design as in the 1992 study. Concerning nitrate evaporation characteristics, the 1996—1997 measurements were highly consistent for the parallel samples at all the three sites, but the sample to sample differences were large. On average, about 50% of nitrate evaporated from the Teflon filters (and were recollected on the nylon filters) which means that when calculated as ammonium nitrate, about 9% of PM was lost. 2.5 3.7.2. Collection efficiency of the denuder systems. The denuder systems had low flow rates (except FMI.ADS, see Table 1) and collected aerosol faced down which usually results in poor transport efficiencies for coarse particles. Thus, it is likely that the denuder systems

Fig. 5. Fine and coarse particle nitrate as measured by the VI and the BLPI (N, kg m~3).

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257

Fig. 6. Nitrate in the nylon filters and atmospheric nitric acid as N, kg m~3

did not collect the largest nitrate particles. The VI and the BLPI were equipped with University of Minnesota—type inlets which are designed for the sampling of particles up to 15 km EAD (Liu and Pui, 1981). The VI results indicate that the atmospheric concentrations of fine and coarse particle nitrate were similar, which underlines the importance of coarse particle sampling efficiency. Using the BLPI size distributions it was calculated that the denuder systems did not efficiently collect particles larger than about 5.6 km EAD. This estimation of 5.6 km can be compared to theoretical calculations of Ferm (1986): the average wind speed during the intercomparison was 3.5 m s~1 which leads to the average 50% cut-off of about 7.2 km for the IVL denuders when the particle density is estimated to be 1 g cm~3. Considering that the density of sea-salt and soil particles is about 2 g cm~3, the agreement between the two methods is good. The FPs measured more nitrate than the DSs in seven cases out of 11 which gives some support for the estimation that the 50% cut-off size of the FP samplers was slightly higher than that of the DSs. Also Harrison and Kitto (1990) observed slightly better particulate sampling efficiencies for their FP instrument than for their DS sampler. 3.7.3. Possible collection of nitric acid by the »Is. John et al. (1988) reported 100% condensation of nitric acid on oxidised aluminum surfaces of a VI instrument. Similarly, it can be expected that nitric acid, being a highly condensable gas, condensed efficiently on the inner stainless-steel surfaces of the VI used in this study. Figure 6 indicates the measured atmospheric nitric acid concentrations and the amounts of nitrate found on the nylon filters of the VI and FMI.ADS (see also Table 1). The sodium carbonate denuders of the FMI.ADS removed practically all nitric acid from the collected air stream. Since Fig. 6 indicates that the amount of nitrate on the nylon filter is similar for both FMI.ADS and VI, the amount of gaseous atmospheric nitric acid collected by the VI nylon filters has to be small or negligible.

3.8. ¹otal nitrate Table 3 and Fig. 4E show the summed concentrations of nitric acid and nitrate ("total nitrate) as kg N m~3. The overall agreement was good for the 24 h measurements: the ratios of the average filter pack values to the average denuder system values ranged from 0.88 to 1.06 with an average of 0.97. Also the results obtained with individual sampling instruments agreed well (Pakkanen et al., 1994). One of the filter packs used was identical to that recommended by EMEP.

T. A. Pakkanen et al.

258

3.9. Discussion about interferences in collection of nitric acid 3.9.1. Different coatings for HNO /NO~ sampling. The denuder is generally considered 3 3 to give the best separation of gases and particles, although some problems might occur. Interfering gases and/or particle deposition in the denuder tubes are factors which may lead to potential overestimation of the HNO concentration. Evaporation of HNO from 3 3 particles passing the denuder tube has also been discussed, although this seems to be a minor problem (Appel and Tokiwa, 1981; Larson and Taylor, 1983; Eatough et al., 1985; Harrison et al., 1990). A set of identical denuder tubes, with three different types of coatings and with and without an acid coated inlet, were run by the DMU during the intercomparison (Table 4). The type of coating of the denuder tube has an influence on potential interferences. Data was obtained by NaOH, Na CO and NaCl coated denuders. The inlet of the Na CO 2 3 2 3 coated denuder was coated with H PO . The results have been analyzed statistically by 3 4 principal component analysis and are presented in Table 5. Further, the Na CO coated 2 3 denuder was followed by a Na CO -impregnated filter, while the other denuders were 2 3 followed by a NaOH-impregnated filter. This makes direct comparison of the total amounts of nitrate difficult. 3.9.2. Overestimation of HNO due to interfering gases. Figure 7 shows the comparison of 3 total NO~, particulate NO~ and gaseous HNO for the different coatings. For the total 3 3 3 amount and for the particulate NO~ no significant differences were seen between the 3 Table 4. Type of denuder coating and filter impregnation for sampling of HNO /NO~ 3 3 Coating tube

Coating inlet

Impregnation filter

NaOH NaCl NaCl

— — H PO 3 4

NaOH NaOH NaOH

1% (w/v) in EtOH 0.5% (w/v) in H O/MeOH 1 : 9 2 0.5% (w/v) in H O/MeOH 1 : 9 2

Na CO 1% (w/v) in H O/MeOH 1 : 1 2 3 2

H PO 3 4

1% (v/v) (85%) in 1% (v/v) (85%) in

H PO 3 4 MeOH H PO 3 4 MeOH

1% (w/v) in EtOH 1% (w/v) in EtOH 1% (w/v) in EtOH

Na CO 1% (w/v) in 2 3 H O/MeOH 1 : 1 2

Table 5. Statistical data from the comparison between NaOH, Na CO and NaCl coated denuders. The inlet 2 3 NO~ content in the Na CO -coated denuders is not included. ‘‘ Total’’ refers to the sum of filter and denuder tube 3 2 3 content

Number Corr. of obs. coeff.

Slope

Slope sign.* diff. from 1 Intercept

0.35 — 0.35

12 12 13

0.947 0.927 0.937

0.99 0.84 1.19

No No No

0.02 0.05 !0.04

No No No

— 0.19 0.19

0.19 — 0.18

12 12 13

0.929 0.928 0.964

1.13 0.94 1.18

No No No

!0.04 !0.00 !0.03

No No No

0.20 0.20 —

— 0.19 0.19

0.16 — 0.16

13 13 13

0.935 0.859 0.900

1.34 1.43 0.96

Yes Yes No

!0.02 !0.07 0.02

No No No

Total NO~ 2

0.14 0.14 —

— 0.11 0.11

0.16 — 0.16

13 13 13

0.948 0.926 0.934

1.05 0.99 1.06

No No No

!0.03 0.04 !0.07

No Yes Yes

Total NO~#NO~ 3 2

0.50 0.50 —

— 0.48 0.49

0.51 — 0.51

12 12 13

0.962 0.966 0.963

1.06 1.01 1.08

No No No

!0.03 0.03 !0.06

No No No

Mean (kg N m~3) NaOH

Na CO NaCl 2 3

Total NO~ 3

0.37 0.37 —

— 0.38 0.37

Particle NO~ 3

0.18 0.18 —

Gaseous HNO 3

* At the 95% confidence level.

Int. sign.* diff. from 0

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259

Fig. 7. HNO and NO~ from denuders with different coatings. 3 3

different coatings (95% confidence level, see Table 5). For the gaseous HNO , the NaOH 3 coated denuder measured significantly higher values than both the Na CO and the 2 3 NaCl-coated denuders (95% confidence level). There was no difference between Na CO 2 3 and NaCl-coated denuders in the HNO determination, though due to the H PO -coated 3 3 4 inlet of the Na CO -coated denuder, which reduces both the absorbing area for HNO and 2 3 3 the potential contribution of deposited NO~ containing particles, these two denuder types 3 are not directly comparable here. The difference in HNO determination between NaOH and NaCl-coated denuders is 3 expected to arise from interfering gases, since particle deposition must be assumed to be equal for the two denuders, independent of the coating. In the NaOH or Na CO coated 2 3 denuder NO and peroxyacetyl nitrate (PAN) might be potential interfering gases, but in 2 tests carried out no or very little interference was observed (Ferm, 1986). Koutrakis et al. (1988) found an artifact formation of NO~ and NO~, representing about 5—10% of the total 2 3 amount of these species, in a Na CO coated annular denuder. From laboratory experi2 3 ments for the same type of denuder as used here, Ferm and Sjo¨din (1985) found that NO 2 alone did not constitute an interference of any major importance, but NO and NO 2 together might form HNO during humid conditions. One could expect an error in the 2 HNO determination, if the interfering NO~ oxidizes to NO~ either in the denuder or in 3 2 3 the denuder extract. Febo et al. (1986) suggested that during sampling in photochemical smog episodes NO~ might be oxidized to NO~, probably by O . Perrino et al. (1990) also 3 3 2 found evidence for the oxidation of sampled NO~. Dasch et al. (1989) reported that even 2 adding H O to the denuder extracts did not oxidize NO~ to NO~. When sampling by 3 2 2 2

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Fig. 8. NO~ on denuder tube and filter. 2

Na CO -coated annular denuders, Appel et al. (1990) found a low percentage retention of 2 3 NO , PAN and other possible pollutants. 2 The extracts from both gaseous and particulate determinations of NO~ were analyzed 3 also for NO~. Figure 8 shows the total (tube#filter), tube and filter content of NO~ 2 2

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separately. The NO~ might originate from HNO and/or some interfering gases. The 2 2 NaCl-coated denuder does not sample NO~ (Fig. 8) and therefore the possibility of errors 2 in HNO determination due to oxidized NO~ in the tube or extract can be excluded. 2 3 Table 5 presents the comparison of NO~ sampled by different types of denuder coatings 2 followed by filters with different impregnations. For the total amount of NO~, a very good 2 correlation was found, and no significant differences were observed between the NaOH and NaCl coatings, both followed by NaOH-impregnated filters. If the NO~ originates from 2 interfering gases this interference therefore seems independent of the coating used. The difference in HNO determination between NaOH and NaCl coatings could originate from 3 oxidized NO~, especially since there is no difference in total NO~ plus NO~. Figure 2 3 2 8 shows the difference between the total amount of NO~ in the NaCl and NaOH-coated 2 denuders versus the difference in HNO determination between the NaOH and NaCl3 coated denuders. If a few outliers are excluded, there might be some evidence for a correlation indicating that sampled NO~ on the NaOH coating is oxidized to NO~ and 3 2 consequently interpreted as HNO even though the differences are insignificant. The 3 difference in HNO determination by the NaCl and NaOH-coated denuders does not 3 correlate to the amount of NO~ in the NaOH tube. However, the oxidized amount 2 probably depends on the O level amongst other factors. The sampling periods for which 3 the largest difference in HNO concentrations between the NaOH and NaCl coatings were 3 observed had either high O levels or a high NO~ content in the NaOH tube (Pakkanen et 2 3 al., 1994). The difference in HNO determination between the coatings might therefore be 3 a consequence of a number of parameters acting differently during the various climate and pollution climate conditions and as such are very difficult to identify.

4 . SU M MAR Y AN D CO NC LU SI O N S

Nitrate size distributions, biased by evaporation of fine particle nitrate, exhibited one or two fine particle modes and two overlapping coarse particle modes. According to the virtual impactor results, about 25—40% of nitrate was in the coarse particle mode. Ammonium had two fine particle and two coarse particle modes. The Berner low-pressure impactor samples indicated that only 1.5—4.8% of ammonium was contained in coarse particles and in several cases the virtual impactor showed even lower percentages. The overall agreement between denuder systems and filter packs was on average within about 10% for all the measured components: ammonia, ammonium, total ammonium, nitric acid, nitrate and total nitrate. However, in the calculation of the above averages some individual measurements were excluded as outliers, especially for ammonia. For ammonium, the virtual impactor and the Berner low-pressure impactor data agreed well with the denuder systems and the filter packs. The virtual impactor collected on average 37% more particulate nitrate than the Berner low-pressure impactor (BLPI), indicating evaporation of about 50% of fine particle nitrate (&27% of total nitrate) from the BLPI. Considering that nitrate evaporates as ammonium nitrate a considerable fraction of PM may be lost in BLPI measurements. Compared to 2.5 the average denuder system values, the virtual impactor gave on average 17% higher nitrate concentrations. Most of this difference can be explained by inefficient transport of coarse particle nitrate into the denuder systems. Further the virtual impactor collected on average 24% more nitrate than the filter packs: this difference was considered to be due to the slightly lower sampling efficiencies for coarse particles and the possible evaporative nitrate losses in the filter packs. Using the BLPI size distributions, it was computed that particles larger than about 5.6 km EAD were not efficiently sampled by the denuder systems used. While calculating the above percentages some high values were excluded because, during periods of high nitric acid and low nitrate concentrations, the virtual impactor may have collected a reasonable percentage of nitric acid indistinguishable from nitrate. More experiments are needed to verify the denuding efficiences of the stainless steel VI and the stainless-steel inlet for nitric acid.

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The NaOH coating of the denuder for HNO /NO~ determination gave an overestima3 3 tion of about 30% compared to the NaCl coating. This overestimation was probably due to oxidized NO~, originating from HNO , NO , NO, PAN or other interfering gases. There 2 2 2 were also differences between the NaOH and Na CO coated denuders. 2 3 Acknowledgements—The Nordic Council of Ministers is acknowledged for their financial support which made this intercomparison possible. Also the Maj and Tor Nessling foundation and the Academy of Finland are acknowledged for their financial support. We thank all the participants of the Nordic intercomparison study for their contribution and Mr. Ari Halm, Mr. Jukka Kiiski, Mr. Mauri Hyppo¨nen, and Mrs. Mirva Vuori from the FMI for their help in setting up the instruments and the measurement platforms.

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