Nordic intercomparison for measurement of major atmospheric nitrogen species

J. Aerosol Sci. Vol. 30, No. 2, pp. 247—263, 1999( 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0021-8502/98 $19.00#0.00PII: S0021-8502(98)00039-1

NORDIC INTERCOMPARISON FOR MEASUREMENT OFMAJOR ATMOSPHERIC NITROGEN SPECIES

Tuomo A. Pakkanen,*s Risto E. Hillamo,* Minna Aurela,* Helle Vibeke Andersen,tLone 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, Finlandt 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 Hagvag 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 ammoniaand particulate nitrate and ammonium concentrations was conducted using various types of filterpacks and denuder systems. Some of the filter packs used are recommended by the EuropeanMonitoring and Evaluation Programme (EMEP) and are widely used in the Nordic Countries andelsewhere in Europe. In addition, nitrogen dioxide was measured using iodide-impregnated sinteredglass filters and the differential optical absorption spectrometer method. Particulate nitrate andammonium concentrations were measured with two different size segregating samplers: a Bernerlow-pressure impactor and a virtual impactor. The weather conditions were most of the time cooland humid. The agreement between different measurements was good for most species, but poor forgaseous ammonia. The virtual impactor collected on average 17% more nitrate than the denudersystems, 24% more than filter packs and 37% more than the Berner low-pressure impactor. Thenitrate discrepancy is believed to be due to inefficient coarse particle transport into the denudersystems, 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 ofPM

2.5may be lost if the nitrate evaporation cannot be accounted for. The denuder systems did not

collect particles larger than about 5.6 km equivalent aerodynamic diameter efficiently. Interferenceswere observed on the nitric acid measurements determined by a NaOH coated denuder. NaClcoated denuders seemed to operate well in this study since interferences were not observed. ( 1998Elsevier Science Ltd. All rights reserved

1 . INTRODUCTION

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 thedenuder systems (DS) are suitable for determination of ammonia (Ferm, 1979), ammonium,nitric acid and nitrate although some interferences may occur. Depending for instance onthe sampling site, weather conditions and the acidity of sampled aerosol the filter packmethods (FP) sometimes can suffer from serious problems leading to erroneous results(Andersen and Hovmand, 1994; Andersen and Hilbert, 1993). Earlier studies suggest furtherthat the size-segregation samplers, virtual impactor (VI) (Loo and Cork, 1988) and Bernerlow-pressure impactor (BLPI) (Berner and Lurzer, 1980), can be used as accurate methodsfor determination of nitrate. Wall et al. (1988) reported that volatile nitrate losses in a BLPIwere 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 determina-tion of nitrate.

In this work, NO2

was monitored by the differential optical absorption spectrometer(DOAS) and collected by reducing NO

2to NO~

2on impregnated filters. Two different

sAuthor to whom correspondence should be addressed.ttPresent address: CONAMA-RM, McIver 283, Piso 7, Santiago, Chile.

247

formulas for the reduction was used. In the older version KI and NaAsO2

were used toreduce NO

2and ethylene glycol to keep the filter humid. NaAsO

2also kept the pH high. To

avoid the handling of arsenite a new formula was designed. In this NaI was used to reduceNO

2to NO~

2and Na

2CO

3to keep the pH high (Ferm and Sjodin, 1992). NaI is more

deliquescent than KI and keeps the filter humid. To get a high efficiency at a reasonableflow, and a leak proof filter holder that was simple to handle, a glass bulb with a sinteredglass filter was constructed.

The concentrations measured in this work by each individual instrument were presentedin detail in the report to Nordic Council of Ministers: Nordic HNO

3/NO~

3and NH

3/NH`

4gas/particle intercomparison in Helsinki, 11—22 May, 1992 (Pakkanen et al., 1994). Insteadof the individual results the present paper compares the different types of measurementinstruments used: the DS, FP, VI, BLPI, impregnated glass sinters (IGS) and the DOASmethod. In addition, the differences between nitrate measurements and interferences on thenitric acid measurements are discussed. The mass of particulates was not measured but onbasis of later measurements can be estimated to have been on average about 10 kgm~3 forPM

2.5and 15 kgm~3 for PM

15.

2 . EXPERIMENTAL

2.1. Field site of the intercomparison study

The comparative field study was conducted in Helsinki, 11—22 May 1992. The samplingsite was on the roof of the Finnish Meteorological Institute, Air Quality Departmentbuilding, seven km to the east from the centre of Helsinki. The roof is about 20m aboveground level. Car traffic is obviously the most important local pollution source with onemajor road 350 m to the northwest and another road with moderate traffic adjacent to theFMI 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: theNational Environmental Research Institute (DMU) from Denmark, two groups from theFinnish Meteorological Institute (FMI) from Finland, the Norwegian Institute for AirResearch (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 correspondingsampling flow rates are listed in Table 1. Nitrogen dioxide concentrations were comparedfor two KI-impregnated and two NaI-impregnated glass sinters (Ferm and Sjodin, 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 acidconcentrations were measured using six DSs and two FPs. Nitrate measurements werecarried out with six DSs, two FPs, the VI and the BLPI. In addition, one FP was used tomeasure the sum of ammonia and ammonium (total ammonium) and another FP was usedto measure the sum of nitric acid and nitrate (total nitrate). These FPs measuring totalammonium and total nitrate were similar to those recommended by EMEP. The VI used inthis work was made of stainless steel. The BLPI and the VI were equipped with stainlesssteel aerosol inlets (University of Minnesota type inlets; Liu and Pui, 1981). The VI samplesparticles in two size fractions: particle aerodynamic diameter EAD(2.5 km and2.5(EAD (15 km. The BLPI collects particles in ten size fractions in the size range of0.03(EAD(15 km. Coarse particle substrates of the BLPI were greased to minimizeparticle 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 denudersystems and one filter pack collected for five 12 h and eight 24 h periods whilst the Berner

248 T. A. Pakkanen et al.

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

Code Inlet (cut-off ) Sampling line Flow rate

DMU.FP — TF—NaF/IF—KOH/IF—oxalic acid/IF 44.36DMU.DS1 — NaOH/D—NaOH/IF 1—2*DMU.DS2 — NaCl/D—NaOH/IF 1—2*DMU.DS3 — Na

2CO

3/D—Na

2CO

3/IF 1—2*

DMU.DS4 — Oxalic acid/D—oxalic acid/IF 1—2*FMI.FP1s — CF—NaOH/IF 18.5t

FMI.FP2s — Oxalic acid/IF 20.8t

FMI.ADS Cyclone (3.5 km) Na2CO

3/D—Na

2CO

3/D—citric acid/D—TF—NF 29.14

FMI.BLPI UM inlet (15 km) 11-stage Berner low-pressure impactor 25.0t

FMI.VI UM inlet (15 km) Virtual impactor; fine: TF—NF, coarse: PF 16.7t

FMI.NaI — NaI/IGS 0.51IVL.DS1 — NaCl/D—NaCl/IF 1.81IVL.DS2 — Oxalic acid/D—oxalic acid/IF 2.29IVL.KI — KI/IGS 0.43IVL.NaI — NaI/IGS 0.42NILU.FP — TF-KOH/IF—oxalic acid/IF 16.6*NILU.KI — KI/IGS 0.57ITML.ADS — NaCl/D—Na

2CO

3/D—Na

2CO

3/D—Na

2CO

3/IF 2.78*

ITML.DOAS — Differential optical absorption spectrometer

Symbols :IGS impregnated glass sinterD denuderDS denuder systemADS annular denuder systemFP filter packUM University of Minnesota inletNaI sodiumiodideTF Teflon filterCF cellulose filterIF impregnated filterNF nylon filterPF polycarbonate filterKI 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 24h and four 48 h periods. The denuder systems ofthe DMU and ITML were used to collect more 12h samples than the other instruments,but some of these 12 h periods were combined to 24h periods to match those of the otherinstruments.

2.4. Chemical analysis

After collecting the samples were dissolved and stored mostly in dark and cool. In orderto check the influence of variable sample transportation to and storage (because of possibleammonium losses storing needs to be made in dark as shown by Ferm, 1993) at eachparticipating laboratory, reference samples with known concentrations were prepared atthe FMI by dilution of appropriate commercial standard solutions. These reference sampleswere 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 forammonium were also employed.

2.5. ¼eather conditions

The temperature and the relative humidity, recorded at the sampling site, together withother weather data registered in a synoptic station near the FMI main building, 7 km from

Nordic intercomparison of major atmospheric nitrogen species 249

Table 2. Summary of weather condition

Temp (°C) Temp (°C) RH (%) RH (%) Rain (mm) Wind (m s~1) CloudinessMay range average range average average average average*

11n 2—7 5 75—95 89 2 5.312 4—10 7 50—94 75 8.5 3.4 5.813 4—10 7 60—95 75 0.4 4.4 3.614 6—16 11 30—80 50 2.3 1.615 7—12 9 60—85 75 3.4 1.016 8—13 10 60—90 75 0.6 5.2 4.417 6—15 11 40—75 55 4.2 1.618 10—20 14 35—60 47 4.0 4.419d 10—16 13 40—80 60 1.6 2.719n 8—17 11 30—90 70 2.6 1.720d 17—20 19 20—45 27 4.0 1.020n 8—17 11 45—75 60 0.1 1.4 3.721 8—18 12 50—85 75 2.1 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 andsunny with the temperatures usually between 5 and 15°C and the relative humidities usuallyhigher than 50%.

3. RESULTS AND DISCUSSION

Results deviating by more than twice the standard deviation were excluded as outliers.Some instruments occasionally suffered from identifiable problems and in such cases evenresults deviating by about the standard deviation were discarded. The outliers were nottaken into account while calculating the average values. The number of outliers is shown inTable 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 differencesobserved on the particle measurements, since the chemical nature of fine and coarseparticles is usually different for most constituents. In this work Berner low-pressureimpactors 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. Overallthe size distributions observed in this work showed characteristics similar to those observedin California (John et al., 1990).

Nitrate exhibited one or two fine particle modes and two overlapping coarse particlemodes (Pakkanen et al., 1996). Coarse nitrate is usually present in the form of non-volatilenitrate compounds (Yoshizumi and Hoshi, 1985). The VI and BLPI data agreed well forcoarse particle nitrate concentrations but it should be noted that about half of the fineparticle nitrate evaporated from the BLPI (see Fig. 5) which is further discussed in theSection 3.7.

Figure 1 indicates that ammonium and sulphate usually exhibited two fine particle modesand two coarse particle modes. About 95% of ammonium was contained in the fine particlesize range. The ammonium and sulphate size distributions were very similar for particlesbelow 1 km EAD. The observed ammonium mole concentration was about twice that ofsulphate. Given the low nitrate concentrations observed, it would appear that sulphate andammonium were mostly present as (NH

4)2SO

4and that the fine aerosol therefore was only

slightly acidic or neutral (Seinfeld, 1986). As discussed above, approximately half of the fineparticle nitrate evaporated from the BLPI (see Fig. 5) which implies that also some fineparticle ammonium likely evaporated (NH

4NO

3and NH

4Cl are the major volatile


Table 3. DOAS/average IGS concentration ratios for nitrogen dioxide and ave.FP/ave.DS, VI/ave.DS andBLPI/ave.DS concentration ratios for particulate ammonium and nitrate. In addition the ave.FP/ave.DS concen-tration ratios are shown for gaseous ammonia and nitric acid and for the sum of ammonia#ammonium ("totalammonium) and for the sum of nitric acid#nitrate ("total nitrate). The number of valid comparisons is indicated

in parantheses after each ratio value

Measurement Total TotalConc. ratio time (h) NO

2NH

3NH`

4NH`

4HNO

3NO~

3NO~

3

DOAS/average IGS 12 1.61 (4)24 1.64 (7)

Average FP/average DS 12 1.06 (5) 0.90 (4) 1.23 (5)* 0.94 (5) 0.95 (5) 0.93 (5)24 1.11 (8) 0.93 (7) 0.94 (8) 0.84 (7) 0.99 (8) 0.97 (8)

VI/average DS 12 1.04 (4) 1.33 (5)24 1.08 (7) 1.20 (6)

BLPI/average DS 24 1.02 (2) 0.85 (2)48 0.99 (4) 0.86 (4)

Total number of outliers (%) 5% 24% 13% 14% 1% 10% 4%

*Only one FP measured 12 h periods; this FP showed the highest FP concentrations for total ammonium for the24h periods.

ammonium salts). However, this ammonium volatilization had only a small influence on theammonium concentrations measured by the BLPI, since ammonium was mostly present asnon-volatile ammonium sulphate.

3.2. Nitrogen dioxide

Figure 2 and Table 3 show the results from the comparison between the averageconcentrations obtained by the four impregnated glass sinter (IGS) measurements and theDOAS method. The IGS measurements were consistent (Pakkanen et al., 1994) thoughindicating lower NO

2concentrations than the DOAS method. A possible explanation for

the disagreement observed may be an inhomogeneous nitrogen dioxide concentration alongthe 440m long DOAS measurement beam: the DOAS light source was situated only 40maway from a road with high traffic density, whereas the DOAS receiver together with theIGS samplers were about 350m from that road.

3.3. Ammonia

Figure 3A and Table 3 indicate that the ammonia concentrations determined by the twoFPs and the three DSs showed reasonable agreement: the average FP to average DS ratioswere 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 averageDS values. The likely reasons for the measurement errors are discussed in detail byPakkanen et al. (1994). In fact, for several sampling periods the individual measurementresults showed such a great variation that the estimation of the actual concentration in theatmosphere studied was difficult. The probable reason for the slightly high ratios could havebeen the evaporation of particulate ammonium nitrate and/or ammonium chloride fromthe FP Teflon pre-filter and subsequent collection of ammonia on the FP impregnated filterwhich 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 ratiosfor ammonia were higher (with the exception of the first sample) when the nitrate concentra-tions 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 BLPIammonium values to the average DS ammonium values, respectively. The corresponding


Fig. 1. Size distributions of ammonium, nitrate and sulphate during the intercomparison. Thenitrate mass below 2 km EAD is underestimated because of evaporation. The six figures refer toBLPI samples 1—6, respectively. The data inversion code MICRON (Wolfenbarger and Seinfeld,

1990, 1991) was used in the evaluation of the size distributions.


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

average ammonium ratios between the methods for 12 and 24h (and also 48 h for BLPI)measurements are indicated in Table 3. The average denuder values agreed well with theBLPI, VI and average FP values. The slightly larger FP-DS difference is probably due toevaporation of ammonium nitrate and/or ammonium chloride from the teflon pre-filter ofthe filter packs.

3.5. ¹otal ammonium

One of the three filter packs measuring total ammonium was similar to that recommen-ded by EMEP. Figure 3E and Table 3 indicate that the FPs and the DSs agreed well. Theaverage filter pack to average denuder system ratio for the 24 h measurements variedbetween 0.88 and 1.24 with an average of 0.99. The highest ratios, 1.10 and 1.24, occurredduring the four 12 h sampling periods of 19 and 20 May combined here as 24 hmeasurements.

3.6. Nitric acid

The ratios of average FP values to average DS values were usually below one for the 24 hmeasurements, except the outlier ratio of 1.57 (Fig. 4A and Table 3). The measurementsindicated 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 nitratefrom the FP prefilters may have occured. However, it is strange that this evaporationseemed to be much different for the other samples. Table 3 shows that the mean of averagefilter pack to average denuder system ratios for the 24 h measurements was 0.84 (the value of1.57 excluded), indicating that the denuder systems collected slightly more nitric acid. Thisis primarily because of the lower values obtained by the KOH-impregnated filter of theNILU filter pack (NILU.FP). The 12 h measurements exhibited slightly better FP-DSagreement but four of the five comparisons are based on only one FP value, that of theDMU.FP. In general, the three different denuder methods of DMU agreed well and thisdata is used for detailed discussion of interferences on the measurement of nitric acid (seeSection 3.9).


Fig. 3. Comparison of average denuder system (DS) values with average filter pack (FP), virtualimpactor (VI) and Berner impactor (BLPI) values. (A) FP-DS comparison for ammonia; (B) FP-DScomparison for ammonium; (C) VI-DS comparison for ammonium; (D) BLPI-DS comparisonfor 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 averagethe agreement was relatively good for the various methods, with the VI showing the highestand the BLPI the lowest concentrations. The mean values of the FP/DS ratios for nitratewere 0.95 and 0.99 for the 12 and 24h measurements, respectively. The VI/DS ratios werebetween 1.04 and 1.66 (one high ratio of 2.25 excluded) and the BLPI/DS ratios showedvalues ranging from 0.74 to 0.94. On average, the VI gave nitrate concentrations that were17, 24 and 37% higher than those of the denuders (two high percentages excluded;


Fig. 4. Comparison of average denuder system (DS) values with average filter pack (FP), virtualimpactor (VI) and Berner impactor (BLPI) values. (A) FP-DS comparison for nitric acid; (B) FP-DScomparison 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 highpercentage excluded), respectively. Three possible reasons were considered to be responsiblefor the differences observed: (i) a large fraction of fine particle nitrate evaporated from theBLPI and a smaller amount from the FPs (ii) the denuder systems collected coarse particlesinefficiently and, possibly iii) the VI may have collected a small amount of gaseous nitricacid indistinguishable from nitrate.

3.7.1. Nitrate evaporation in the B¸PI. Figure 5 shows the fine and coarse particle nitrateconcentrations as measured by the VI and the BLPI. The two instruments showed good


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

agreement for Na` and SO2~4

, which suggests that there were no problems with thesampling 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. Thesmall difference in the particle size cut-off of the VI and BLPI has practically no influenceon the conclusions drawn from this comparison.

Using this data, Kerminen et al. (1997) calculated theoretically that at the end of theBLPI sampling periods the atmospheric conditions favored the gaseous form of ammoniumnitrate except for samples 1 and 6. Thus, for samples 2, 3, 4 and 5, the ammonium nitrateaccumulated 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 fineparticle mode for nitrate. On average, the evaporative loss of nitrate from the BLPI wasabout 50% for fine particles and about 27% for nitrate below 15 km EAD. These values areclearly higher than those of about 10—20% reported by Wall et al. (1988) and Zhang andMcMurry (1992) for atmospheric conditions in California. The higher mass concentrationsof atmospheric particles and shorter sampling periods in the Californian studies lead tothicker particle deposits and shorter time available for evaporation, which both inhibitevaporation from BLPI. Further the longer sampling periods in the Helsinki study allow forgreater temperature and humidity changes. Nitrate evaporation may also be enhanced if theaerosol 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: onaverage about 30% of nitrate was lost from the VI Teflon filters and recollected on the VInylon filters (see Figs 5 and 6). In 1996—1997, simultaneous 24 h VI samples were collected atthree sites in the Helsinki area using the same VI design as in the 1992 study. Concerningnitrate evaporation characteristics, the 1996—1997 measurements were highly consistent forthe parallel samples at all the three sites, but the sample to sample differences were large. Onaverage, about 50% of nitrate evaporated from the Teflon filters (and were recollected onthe nylon filters) which means that when calculated as ammonium nitrate, about 9% ofPM

2.5was lost.

3.7.2. Collection efficiency of the denuder systems. The denuder systems had low flowrates (except FMI.ADS, see Table 1) and collected aerosol faced down which usually resultsin poor transport efficiencies for coarse particles. Thus, it is likely that the denuder systems


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 withUniversity of Minnesota—type inlets which are designed for the sampling of particles up to15 km EAD (Liu and Pui, 1981). The VI results indicate that the atmospheric concentra-tions of fine and coarse particle nitrate were similar, which underlines the importance ofcoarse particle sampling efficiency. Using the BLPI size distributions it was calculated thatthe denuder systems did not efficiently collect particles larger than about 5.6 km EAD. Thisestimation of 5.6 km can be compared to theoretical calculations of Ferm (1986): theaverage wind speed during the intercomparison was 3.5 m s~1 which leads to the average50% cut-off of about 7.2 km for the IVL denuders when the particle density is estimated tobe 1 g cm~3. Considering that the density of sea-salt and soil particles is about 2 g cm~3, theagreement between the two methods is good. The FPs measured more nitrate than the DSsin seven cases out of 11 which gives some support for the estimation that the 50% cut-offsize 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 thanfor 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, itcan be expected that nitric acid, being a highly condensable gas, condensed efficiently on theinner stainless-steel surfaces of the VI used in this study. Figure 6 indicates the measuredatmospheric nitric acid concentrations and the amounts of nitrate found on the nylon filtersof the VI and FMI.ADS (see also Table 1). The sodium carbonate denuders of theFMI.ADS removed practically all nitric acid from the collected air stream. Since Fig. 6indicates 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 besmall or negligible.

3.8. ¹otal nitrate

Table 3 and Fig. 4E show the summed concentrations of nitric acid and nitrate ("totalnitrate) as kg Nm~3. The overall agreement was good for the 24 h measurements: the ratiosof the average filter pack values to the average denuder system values ranged from 0.88 to1.06 with an average of 0.97. Also the results obtained with individual sampling instrumentsagreed well (Pakkanen et al., 1994). One of the filter packs used was identical to thatrecommended by EMEP.


Table 5. Statistical data from the comparison between NaOH, Na2CO

3and NaCl coated denuders. The inlet

NO~3

content in the Na2CO

3-coated denuders is not included. ‘‘Total’’ refers to the sum of filter and denuder tube

content

Slope Int.Mean (kg N m~3) sign.* sign.*

Number Corr. diff. diff.NaOH Na

2CO

3NaCl of obs. coeff. Slope from 1 Intercept from 0

Total NO~3

0.37 — 0.35 12 0.947 0.99 No 0.02 No0.37 0.38 — 12 0.927 0.84 No 0.05 No— 0.37 0.35 13 0.937 1.19 No !0.04 No

Particle NO~3

0.18 — 0.19 12 0.929 1.13 No !0.04 No0.18 0.19 — 12 0.928 0.94 No !0.00 No— 0.19 0.18 13 0.964 1.18 No !0.03 No

Gaseous HNO3

0.20 — 0.16 13 0.935 1.34 Yes !0.02 No0.20 0.19 — 13 0.859 1.43 Yes !0.07 No— 0.19 0.16 13 0.900 0.96 No 0.02 No

Total NO~2

0.14 — 0.16 13 0.948 1.05 No !0.03 No0.14 0.11 — 13 0.926 0.99 No 0.04 Yes— 0.11 0.16 13 0.934 1.06 No !0.07 Yes

Total NO~3#NO~

20.50 — 0.51 12 0.962 1.06 No !0.03 No0.50 0.48 — 12 0.966 1.01 No 0.03 No— 0.49 0.51 13 0.963 1.08 No !0.06 No

*At the 95% confidence level.

Table 4. Type of denuder coating and filter impregnation for sampling of HNO3/NO~

3

Coating tube Coating inlet Impregnation filter

NaOH 1% (w/v) in EtOH — NaOH 1% (w/v) in EtOHNaCl 0.5% (w/v) in H

2O/MeOH 1:9 — NaOH 1% (w/v) in EtOH

NaCl 0.5% (w/v) in H2O/MeOH 1:9 H

3PO

41% (v/v) H

3PO

4NaOH 1% (w/v) in EtOH

(85%) in MeOHNa

2CO

31% (w/v) in H

2O/MeOH 1:1 H

3PO

41% (v/v) H

3PO

4Na

2CO

31% (w/v) in

(85%) in MeOH H2O/MeOH 1 :1

3.9. Discussion about interferences in collection of nitric acid

3.9.1. Different coatings for HNO3/NO~

3sampling. The denuder is generally considered

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 leadto potential overestimation of the HNO

3concentration. Evaporation of HNO

3from

particles passing the denuder tube has also been discussed, although this seems to bea 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 andwithout 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. Datawas obtained by NaOH, Na

2CO

3and NaCl coated denuders. The inlet of the Na

2CO

3coated denuder was coated with H

3PO

4. The results have been analyzed statistically by

principal component analysis and are presented in Table 5. Further, the Na2CO

3coated

denuder was followed by a Na2CO

3-impregnated filter, while the other denuders were

followed by a NaOH-impregnated filter. This makes direct comparison of the total amountsof nitrate difficult.

3.9.2. Overestimation of HNO3due to interfering gases. Figure 7 shows the comparison of

total NO~3, particulate NO~

3and gaseous HNO

3for the different coatings. For the total

amount and for the particulate NO~3

no significant differences were seen between the


Fig. 7. HNO3

and NO~3

from denuders with different coatings.

different coatings (95% confidence level, see Table 5). For the gaseous HNO3, the NaOH

coated denuder measured significantly higher values than both the Na2CO

3and the

NaCl-coated denuders (95% confidence level). There was no difference between Na2CO

3and NaCl-coated denuders in the HNO

3determination, though due to the H

3PO

4-coated

inlet of the Na2CO

3-coated denuder, which reduces both the absorbing area for HNO

3and

the potential contribution of deposited NO~3

containing particles, these two denuder typesare not directly comparable here.

The difference in HNO3

determination between NaOH and NaCl-coated denuders isexpected to arise from interfering gases, since particle deposition must be assumed to beequal for the two denuders, independent of the coating. In the NaOH or Na

2CO

3coated

denuder NO2

and peroxyacetyl nitrate (PAN) might be potential interfering gases, but intests carried out no or very little interference was observed (Ferm, 1986). Koutrakis et al.(1988) found an artifact formation of NO~

3and NO~

2, representing about 5—10% of the total

amount of these species, in a Na2CO

3coated annular denuder. From laboratory experi-

ments for the same type of denuder as used here, Ferm and Sjodin (1985) found that NO2

alone did not constitute an interference of any major importance, but NO and NO2

together might form HNO2

during humid conditions. One could expect an error in theHNO

3determination, if the interfering NO~

2oxidizes to NO~

3either in the denuder or in

the denuder extract. Febo et al. (1986) suggested that during sampling in photochemicalsmog episodes NO~

2might be oxidized to NO~

3, probably by O

3. Perrino et al. (1990) also

found evidence for the oxidation of sampled NO~2

. Dasch et al. (1989) reported that evenadding H

2O

2to the denuder extracts did not oxidize NO~

2to NO~

3. When sampling by


Fig. 8. NO~2

on denuder tube and filter.

Na2CO

3-coated annular denuders, Appel et al. (1990) found a low percentage retention of

NO2, PAN and other possible pollutants.

The extracts from both gaseous and particulate determinations of NO~3

were analyzedalso for NO~

2. Figure 8 shows the total (tube#filter), tube and filter content of NO~

2


separately. The NO~2

might originate from HNO2

and/or some interfering gases. TheNaCl-coated denuder does not sample NO~

2(Fig. 8) and therefore the possibility of errors

in HNO3

determination due to oxidized NO~2

in the tube or extract can be excluded.Table 5 presents the comparison of NO~

2sampled by different types of denuder coatings

followed by filters with different impregnations. For the total amount of NO~2

, a very goodcorrelation was found, and no significant differences were observed between the NaOH andNaCl coatings, both followed by NaOH-impregnated filters. If the NO~

2originates from

interfering gases this interference therefore seems independent of the coating used. Thedifference in HNO

3determination between NaOH and NaCl coatings could originate from

oxidized NO~2, especially since there is no difference in total NO~

3plus NO~

2. Figure

8 shows the difference between the total amount of NO~2

in the NaCl and NaOH-coateddenuders versus the difference in HNO

3determination between the NaOH and NaCl-

coated denuders. If a few outliers are excluded, there might be some evidence for a correla-tion indicating that sampled NO~

2on the NaOH coating is oxidized to NO~

3and

consequently interpreted as HNO3

even though the differences are insignificant. Thedifference in HNO

3determination by the NaCl and NaOH-coated denuders does not

correlate to the amount of NO~2

in the NaOH tube. However, the oxidized amountprobably depends on the O

3level amongst other factors. The sampling periods for which

the largest difference in HNO3

concentrations between the NaOH and NaCl coatings wereobserved had either high O

3levels or a high NO~

2content in the NaOH tube (Pakkanen et

al., 1994). The difference in HNO3

determination between the coatings might therefore bea consequence of a number of parameters acting differently during the various climate andpollution climate conditions and as such are very difficult to identify.

4 . SUMMARY AND CONCLUSIONS

Nitrate size distributions, biased by evaporation of fine particle nitrate, exhibited one ortwo fine particle modes and two overlapping coarse particle modes. According to the virtualimpactor results, about 25—40% of nitrate was in the coarse particle mode. Ammonium hadtwo fine particle and two coarse particle modes. The Berner low-pressure impactor samplesindicated that only 1.5—4.8% of ammonium was contained in coarse particles and in severalcases the virtual impactor showed even lower percentages.

The overall agreement between denuder systems and filter packs was on average withinabout 10% for all the measured components: ammonia, ammonium, total ammonium,nitric acid, nitrate and total nitrate. However, in the calculation of the above averages someindividual measurements were excluded as outliers, especially for ammonia. For am-monium, the virtual impactor and the Berner low-pressure impactor data agreed well withthe denuder systems and the filter packs.

The virtual impactor collected on average 37% more particulate nitrate than the Bernerlow-pressure impactor (BLPI), indicating evaporation of about 50% of fine particle nitrate(&27% of total nitrate) from the BLPI. Considering that nitrate evaporates as ammoniumnitrate a considerable fraction of PM

2.5may be lost in BLPI measurements. Compared to

the average denuder system values, the virtual impactor gave on average 17% higher nitrateconcentrations. Most of this difference can be explained by inefficient transport of coarseparticle nitrate into the denuder systems. Further the virtual impactor collected on average24% more nitrate than the filter packs: this difference was considered to be due to theslightly lower sampling efficiencies for coarse particles and the possible evaporative nitratelosses in the filter packs. Using the BLPI size distributions, it was computed that particleslarger 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, duringperiods of high nitric acid and low nitrate concentrations, the virtual impactor may havecollected a reasonable percentage of nitric acid indistinguishable from nitrate. Moreexperiments are needed to verify the denuding efficiences of the stainless steel VI and thestainless-steel inlet for nitric acid.


The NaOH coating of the denuder for HNO3/NO~

3determination gave an overestima-

tion of about 30% compared to the NaCl coating. This overestimation was probably due tooxidized NO~

2, originating from HNO

2, NO

2, NO, PAN or other interfering gases. There

were also differences between the NaOH and Na2CO

3coated denuders.

Acknowledgements—The Nordic Council of Ministers is acknowledged for their financial support which made thisintercomparison possible. Also the Maj and Tor Nessling foundation and the Academy of Finland are acknow-ledged for their financial support. We thank all the participants of the Nordic intercomparison study for theircontribution and Mr. Ari Halm, Mr. Jukka Kiiski, Mr. Mauri Hypponen, and Mrs. Mirva Vuori from the FMI fortheir help in setting up the instruments and the measurement platforms.

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