Field intercomparison exercise on nitric acid and nitrate measurement (Rome, 1988): A critical approach to the evaluation of the results

The Science of the Total Environment, 133 (1993) 39-71 Elsevier Science Publishers B.V., Amsterdam

39

Field intercomparison exercise on nitric acid and nitrate measurement (Rome, 1988): A critical approach to the evaluation of the results A. Febo, C. Perrino and I. Allegrini Istituto Inquinamento Atmosferico - C N.R., Area della Ricerca di Roma, Via Salaria Km 29300, C P. 16- 00016, Monterotondo Stazione, Roma, Italy

(Received December 17th, 1991; accepted February 7th, 1992)

ABSTRACT A 5-day intercomparison of measurement techniques for nitric acid and particulate nitrate was carried out at the Area della Ricerca di Roma (Montelibretti, Rome, Italy) during September 18-24, 1988. Sixteen groups from eleven European countries participated in the experiment, intercomparing the performances of several denuder and filter pack systems. In order to obtain a better characterization of the performance of each technique, in addition to HNO3 and NO3- field sampling, the protocol included HNO3 determinations from a pure source and temporal self-consistency tests. Many ancillary measurements were also undertaken, which proved to be of help in the interpretation of the data. The evaluation of the results is not performed through a simple linear regression of the data, that is by assessing agreement or disagreement between pairs of methods, but by comparing the results obtained in both the additional tests and the field samplings with the predicted deposition pattern. Once the reliability of each single technique has been evaluated in the light of the potential interferiqg mechanisms, a comparison between the results yielded by groups using the same technique and, finally, different techniques is carried out. The application of these criteria to the data set gathered during the intercomparison shows that the diffusion techniques yield the most reliable results, while teflon-nylon filter packs do not allow a correct discrimination between nitric acid and nitrate, particularly in the presence of high ammonium nitrate concentrations, Filter packs using a cellulose prefilter are only able to measure total nitrate. Key words: intercomparison; diffusion denuder; filter pack; nitric acid; nitrate; deposition pattern; sampling artifacts

INTRODUCTION

Starting in 1975, the accurate measurement of nitric acid air concentration has been drawing the attention of scientists involved in environmental chemistry, since the determination of this species constitutes an important step in the comprehension of both photochemical pollution and acid deposi0048-9697/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved

40

A. FEBO El" AL

tion phenomena. As many different techniques were developed, ranging from infrared spectroscopy and chemiluminescence methods to filtration technique and denuder systems, the need for comparing the results obtained by using these techniques in the same ambient conditions became more important. This led to a number of intercomparison exercises carried out during the last decade (Appel et al., 1981; Spicer et al., 1982; Anlauf et al., 1985; Mulawa and Cadle, 1985; Fox et al., 1988; Hering et al., 1988, Anlauf et al., 1991). These intercomparison studies, in general, were designed only in the light of a statistical comparison of the data obtained with the various te.chniques. Such a design can only give information about the extent of the discrepancies among the tested methods, but little or no information is given about the mechanisms which lie behind the observed differences. In principle, the sole purpose of an intercomparison should be the evaluation of the performance of different techniques in terms of sensitivity, precision, reproducibility, time resolution, simplicity and cost effectiveness. A field validation of the methods in terms of accuracy and selectivity is assumed to have been already carried out during the development phase of each technique. However, the discrepancies observed in previous intercomparisons could not be attributed solely to experimental variability, suggesting that the field performances of the tested methods still have to be evaluated and understood and that interfering mechanisms still have to be identified (Spicer et al., 1982; Anlauf et al., 1985; Mulawa and Cadle~ 1985; Hering et al., 1988). This state-of-the-art suggested the planning of a study design which could be of help in a field evaluation of the methods and which would give information about the specific interfering mechanisms which might cause disagreement among the tested techniques. This design should include field tests of each individual method, the results from which would have to be carefully evaluated before any comparison between the data could be carried out. The Intercomparison Exercise on Nitric Acid, held in Montelibretti (Rome, Italy) during September 18-24, 1988 and jointly organized by the C.N.R. and the Commission of the European Communities was designed on this basis. We report here the criteria we used to plan the study design, some results obtained during the intercomparison and the method used to evaluate the data. Conclusions are drawn about the causes of deviations between the results yielded by the different sampling set-ups and about the performances of the different techniques forming part of the study.

Study design Before comparing the data gathered in a field intercomparison exercise, an

NITRIC ACID AND NITRATE MEASUREMENT

41

evaluation of the behaviour of each sampling technique must be carried out. For time-integrated techniques, this can be clone on the basis of two fundamental tests: (i) check of collection of HNO3 generated from a pure source; (ii) check of the self-consistency of the method. The first test is not trivial since the real sampling line which is used in field samplings does not necessarily exhibit the same behaviour with respect to HNO3 as the single collection stage (whose collection efficiency is generally well known). This could lead to confusion between the performance of the method and that of the sampling configuration actually employed during the intercomparison. For example, in the Nitrogen Species Methods Comparison Study, carried out during 1985 in the Los Angeles Basin (Hering et al., 1988), one of the three sampling set-ups which employed annular denuders suffered from a high retention of HNO3 on the inlet and manifold surfaces. Unfortunately, the data yielded by this system were included in the calculation of the average value for the annular denuder method, resulting in a low value which had nothing to do with the accuracy of the technique. Self-consistency is a necessary condition for the soundness of a method; that is, changes in the sampling parameters (flow rate, sampling duration, dimensions of the device) should not cause any disagreement between the results (obviously, the operative conditions of the "system mus~ be respected). Lack of self-consistency is an indicator of previously ~Jnidentified problems: for example, in the case of HNO3 determination the occurrence of an artifact due to nitrite to nitrate oxidation could be highlighted by a temporal self-consistency test. Temporal self-consistency can be easily verified for all time-integrated methods. In addition, some ancillary field measurements can be useful for obtaining a detailed characterization of the air composition during the study and may subsequently help in identifying the possible interfering species and in clarifying the interference mechanisms.

Sampling schedule A compromise between the time resolution of the less sensitive methods taking part in the intercomparison and the need for gaining a sufficient number of data points during the 5-day exercise led to the establishment of a four-per-day schedule, with three 4-h samplings during the day (08:00 to 12:00; 12:00 to 16:00; 16:00 to 20:00) and one 12-h sampling during the night (20:00 to 8:00). In addition, the schedule included two 24-h measurements, which started at 20:00 on September 20th and 21st. These samplings, performed parallel to the main sampling line, were specially included for the evaluation of the self-consistency of the method. The 24-h period started in the evening in order to enhance the possible artifacts due (i) to nitrous acid

42

A. FEBO ET AL.

collection (mostly during the night) with subsequent nitrite to nitrate oxidation (during the following day) and (ii) to dissociation of ammonium nitrate. As it was decided to focus on the evaluation of the accuracy of the methods, replicate measurements were not included in the sampling schedule. The data yielded by replicate measurements, in fact allow an easy statistical evaluation of the precision of each sampling technique, but do not give information about he reliability of the method. Two nitric acid determinations from a constant nitric acid permeation source, having a permeation rate of 1.43 ± 0.05 #g min -~, were scheduled for all the participating groups. The permeation source was realized by a low pressure permeation device similar to that proposed by Scarano et al. (1979) for generating HCI. Ancillary measurements performed throughout the whole study included HONO, SO2, NH3, peroxyacetyl nitrate (PAN) H202, HCHO, NO2, SO42-, NH4 +, ozone, total suspended particles (TSP) and routine meteorological parameters. All the samples collected during the intercomparison have been analyzed by the same technical staff and by using the same ion chromatographs in order to reduce the analytical variability. The overall reproducibility of the methods participating in the intercomparison was below a few percent (see also Tables 2 and 3). Sixteen groups from eleven European countries participated in the intercomparison and many different techniques were employed (see Table 1). Unfortunately, only a few participants followed the schedule rigorously; however, among these, each one of the main categories of techniques was represented: the teflon-nylon filter pack, the denuder technique and the cellulose-impregnated filter pack. A detailed description of the sampling site, meteorological conditions during the study, methods used for ancillary measurements aad HNO3-NO3determinations, methods for ion chromatographic analyses, analytical results of all the measurements and additional information regarding the intercomparison are reported in the Air Pollution Research Report 22, published by the Commission of European Communities (1989). METHOD

For each method in an intercomparison, two fundamental pieces of information, which can be gained from laboratory experiments, are needed: (i) the predicted deposition pattern on each stage of the sampling line, which, in general, is due to either the species of interest or the possible positive and negative artifacts; (ii) the connection between the amount of analyte on the collecting medium and the atmospheric concentration of the species to be

43

NITRIC ACID AND NITRATE MEASUREMENT

TABLE 1 Participants in the field intercomparison exercise on nitric acid and nitrate measurement (Rome, 1988) Institution (country)

Group-code

University of Essex (UK) A SKC/CEN (Belgium) BI SKC/CEN (Belgium) B2 Warren Spring Laboratory (UK) C National Env. Res. Inst. (Denmark)D Swedish Env. Res. Inst. (Sweden) F l Swedish Env. Res. Inst. (Sweden) F2 Swedish Env. Res. Inst. (Sweden) F3 Umweltbundesamt (Germany) Ol Umbeltbundesamt (Germany) G2 Umbeltbundesamt (Germany) G3 Norwegian Inst. Air Res. (Norway) H l Norwegian Inst. Air Res. (Norway) H2 Norwegian Inst. Air Res. (Norway) H3 E.C.N (The Netherlands) Kl E.C.N. (The Netherlands) K2 E.C.N. (The Netherlands) K3 C.N.R. (Italy) IB C.N.R. (Italy) IC Finish Met. Inst. (Finland) L C.E.G.B. (UK) M Universitade de Aveiro (Portugal) P University of Stockolm (Sweden) R Univ. College Dublin (Ireland) S Max Plank Inst. Chemie (Germany)W

Sampling set-up a Teflon f - 2 nylon f Na2CO3 den - cell. f - Na2CO2 den Na2CO3 den - cell. f Teflon f-nylon f Cell. f - NaF f Na2CO3 d e n - Na2CO3 f Na2CO3gly d e n - Na2CO3gly f NaCl den - NaC! f NaF thermodenuder NaF den NaOH f Na2CO3 den - Na2CO3 f Na2CO3 f Teflon f - K O H f NaF den - Teflon f - NaF f MgSO4 thermodenuder Wet denuder Cell. f - 2 KOH f 2 NaCl d e n - Na2CO3 d e n - nylon f Cell. - NaOH f NaF den Teflon f - 2 nylon f NaF - 2 Na2CO3 den - teflon f - nylon f 2 Na2CO3 d e n - nylon f Berner impactor- teflon f-nylon f

at-, filter; den, denuder; cell., cellulose.

determined. Thus, for each stage of the sampling line and for each species to be measured, it is necessary to know:(i) the collection efficiency for the species and the stability of the collected compound during the sampling period; (ii) the other species which are co-collected or may interact with the collecting medium; (iii) the nature of possible links between the co-collected compounds ~nd the species of interest (for example: chemical transformation of the species of interest; change in the reactivity of the coating, etc.; see Febo et al.. !989; Perrino et al., 1990). As a consequence of these studies, a mathematical relationship, describing the link between the ion amount deter-

44

A. FEBO ET AL

TABLE 2 Results of the pure source test Group-code

Inlet

Sampling set-up

Mass flow rate

(/tg min- i) D IB IB A A C C P W W B2 FI F3 G2 .q2 HI HI IC IC GI GI KI K2 K3 R G3 a G3 a

Y N N N N Y Y N Y Y N N N N N Not used Not used N N Y Y Y Y Y Y N N

Cell.-imp. fp Cell.-imp. fp Cell.-imp. fp Teflon-nylon fp Teflon-nylon fp Teflon-nylon fp Teflon-nylon fp Teflon-nylon fp Teflon-nylon fp Teflon-nylon fp Na2CO 3 denuder Na2CO 3 denuder NaC! denuder NaF denuder NaF denuder NaeCO3 denuder Na2CO3 denuder NaC! denuder NaC! denuder Thermodenuder Thermodenuder NaF denuder Thermodenuder Wet denuder NaF denuder NaOH-imp. filter NaOH-imp. filter

b.d.l. 0.42 0.43 1.23 1.25 0.78 0.78 0.82 0.01 0.05 1.47 1.32 1.27 1.09 1.26 1.36 1.40 1.40 1.42 0.75 0.75 1.10 0.90 0.87 0.40 0.5! 0.32

b.d.l., below detection limit; imp., impregnated. HNO~ permeation rate: 1.43 4-0.05 /~g min -I. aThis set-up was intended for determining total nitrate.

mined on the sampling line and the amount of the species of interest originally present in the sampled air volume, can be formulated. During the Intercomparison on Nitric Acid and Nitrate Measurement, filter packs and denuder systems in several configurations were used for the selective determination of HNO3 and NO3-. In the following sections we shall discuss the mathematical relationships which describe the predicted distribution of nitrate ion in these two cases.

45

NITRIC ACID AND NITRATE MEASUREMENT

TABLE 3 Results of the temporal self-consistence test [HNO3]

[NO3-]

S/V

D/V

~

[SO42-]

24 h AVG P 24 h AVG P

3.46 2.51 1.38 4.78 3.15 1.52

1.97 2.91 0.68 3.05 4.96 0.61

5.43 5.42 i.00 7.83 8.11 0,96

I.,49 -0.,40 -3.72 !.73 -!o81 -0.96

!.76 0.86 2.05 1.57 0.63 2.49

8.57 8.00 1.07 12.61 ! 1.74 1.07

20th 20th 20th 21st 21st 21st

24 h AVG P 24 h AVG P

2.46 2.14 1.15 3.66 2.46 1.49

!.50 2.33 0.64 2.37 4.27 0.55

3.05 4.47 0.88 6.03 6.73 0.89

0.96 -0~ 19 -0.81 !~29 -1.81 -0.71

1.64 0.92 1.78 i.54 0.58 2.65

7.96 7.10 1.12

IB

20th 20th 20th 21st 21st 21st

24 h AVG P 24 h AVG P

0.15 0.76 0.18 0.26 0.74 0.35

4,91 5.13 0.96 7.16 7.09 i.01

5.06 5.89 0.86 7.a2 7.83 0.95

-4.76 -4.37 ~.~9 -6.90 -6.35 !.09

0.03 0. ! 5 0.20 0.04 0.10 0.40

8.64 8.54 1.01 12,g0 12.34 1.04

IC

20th 20th 20th 21st 21st 21st

24 h AVG P 24 h AVG P

1.93 2.04 0.95 2.02 2.10 0.96

3.51 3,56 0,9~ 5.'~2 5.50 0.98

5.44 5.60 0.97 7.44 7.60 0.98

-1.58 -I.52 ! .04 -3.40 -3.40 !.00

0.55 0.57 0.96 0.37 0,38 0.97

7.7] 8.29 0.93 13.23 13.03 1.01

21st 21st 21st

24 h AVG P

2.06 i,63 !.26

5.39 5,20 1.04

7.45 6.83 1,09

-3.33 -3.57 0.93

0.38 0.3 ! !.22

11.49 11.45 1,00

20th 20th 20th 21st 21st 21st

24 h AVG P 24 h AVG P

1.79 1.94 0.92 !.91 2.02 0.94

Group

Date (Sept.)

A

20th 20th 20th 21st 21st 21st

M

AVG, average of the four-per-day determinations; P = 24 h/AVG. Concentrations are expressed in ~,g" m -3.

46

^. ,,=~_ao ET AL.

Distribution pattern on filter packs A filter pack for the determination of HNO3 and NO3- usually consists of an inlet and two or more filters set in series: the first filter is assumed to collect particulate nitrate, while the second or the following ones are assumed to collect gaseous nitric acid. For the sake of simplicity, in the following discussion we shall assume that the collection efficiency is constant during the sampling period for each stage of the system. This hypothesis should be verified in advance, as the collection efficiency can be assumed to be independent of the sampling time only as far as the collected amount remains below the operative capacity of the system (Febo et al., 1989). For this device, nitrate deposition pattern on the first two filters can be described by the following relationship:

(1)

M! = A p E~N (1 - ~) + A g E ~ (1 - eEl) + I:Qi M 2 = A g E~

e E ~ / 3 ( l - E~) + o / A p E~N/3

(1

-

E~) + EQ2

(2)

where superscripts p and g refer to particulate nitrate and gaseous nitric acid, respectively; subscripts 1, 2 and IN refer to the first, second filter and inlet, respectively; M is the amount collected (as nitrate ion); E is the penetration efficiency; A is the atmospheric amount entering the system (as nitrate ion); Q is the nitrate amount yielded by interferent species; ot is a loss factor due to evolution and/or displacement of HNOa from particulate NO3-; B is the nitric acid transmission factor between the first and the second filter; e is the nitric acid transmission factor through the particulate matter collected on the first filter. The first addend in Eqn (1) is the amount of particulate nitrate which is collected on the first filter (supposed to have a penetration efficiency E~P --- 0) decreased by the fraction removed during the sampling (e.g. dissociation of NH4NO3 and/or displacement of HNO3 due to H2SO4); the second addend is the amount of nitric acid retained by the first filter and/or by the particles collected on it; the third addend is the contribution of possible interferent compounds which yield nitrate ion directly or through chemical reactions. The first addend of Eqn (2) is the amount of nitric acid actually collected by the second filter (thus decreased by the amount possibly removed by the inlet, by the particles collected on the first filter, by the first filter itself and by the walls of the filter holder); the second addend is the nitric acid amount evolved from the first filter and not removed by the filter holder walls; the third addend is the contribution of interferent species, as in the case of Eqn (1). For each real sampling line, it is thus r~ecessaD• to know the values assumed by the parameters in Eqns (1) and (2). A filter pack system can be optimiz-

NITRIC ACID AND NITRATE MEASUREMENT

47

ed by an appropriate choice of filter media, so as to obtain: E~ -- 1; E~

=

O; I:Q~ ~. o; ~Q~ -- o

and of the geometry and construction materials of the transfer lines, so as to obtain:

EPN-- l; EYN-- l;

I

(3)

The parameters a and ~, on the other hand, cannot be optimized in any way, as they only depend on the thermodynamic conditions, the compounds present in the atmosphere and the sampling duration. Since the values assumed by these parameters are unpredictable, a considerable uncertainty may inherently affect the possibility of determining the atmospheric concentrations of HNO3 and N O : from the values M2 and MI with any filter pack method. Filter pack configurations used during the intercomparison can be grouped into two categories: configurations having a teflon filter first and those having a cellulose filter. Previous knowledge allows one to state that for the first group (teflon-nylon and tefion-Na2CO3 impregnated filters): El g = 1; E2g = 0 and I:Qm = 0 (Appel et al., 1979; Appel et al., 1980), while I:Q2 ~ 0, which might lead to positive artifacts (e.g. collection of" nitrous acid and subsequent oxidation to N O : , direct and indirect interference of NO2, etc.; Perrino et al., 1988). As discussed above, the values of EmNp, E~sg and/3 depend on the particular characteristics of the transfer lines and inlet; thus two systems making use of the same collecting media may even yield very different results (it is therefore not particularly sensible to group them in a statistical evaluation of the data). Configurations making use of a cellulose acetate filter followed by an impregnated filter result in: El g ~ 1 (Spicer and Schumacher, 1979); thus a discrimination between HNO3 and particulate N O : becomes impossible. It therefore seems clear from the literature studies mentioned above that the teflon-nylon configuration is 'a priori' the most reliable among the filter pack configurations used in the intercomparison, even if biased by the following possible effects: transfer of NO3- as HNO3 from the teflon to the nylon (which can be a relevant effect), retention of HNO3 on the particulate matter collected on the tello.n filter, interference of other gaseous species on the nylon filter (the configuration teflon-Na2CO3 suffers from the interference of HONO more than the teflon-nylon set-up; (Perrino et al., 1990)). For an optimum configuration of a teflon-nylon filter pack (T-Nfp), after substitution of the appropriate values and rearrangement, Eqns (1) and (2)

48

A. FEBO ET AL

reduce to the following: M! = A p -

[c¢ A p -

Ag

(1 - ~)]

(4)

g 2 =

[c¢ A p -

Ag

(1 - ~)] + ~Q2

(5)

Ag +

These relationships ~:larify the link between the measured amounts M~ and M2 and the true values A p and A g and point out how the determination of A P and A g from the values M~ and M2 suffers from artifacts. The term in square brackets (indicated as 6 in the following) consists of both the artifact due to HNO3 retention on the collected particles and the artifact due to HNO3 evolution from NO3-, artifacts which are of opposite sign; the term has a value > 0 or _< 0 depending on the phenomenon which prevails. In addition, it affects the values M~ and M2 in opposite directions. If we now define the parameters S, R and D as the sum, ratio and difference, respectively, between the values M2 and M~, we obtain, from Eqns (4) and (5), the following relationships: S = Ml + M2 = Ag + R=

M2 = Ml

(6)

A p + ~Q2

As + I:Q2 + 6 AP -

D = M2 - M! = A s

-

Ap +

(7) 2~

+ ~Q2

(8)

The choice of these three parameters is particularly appropriate: S does not contain the terms 6, thus its value coincides with the true value A g + A P apart from the possible small amounts due to the interference of other gaseous species (ZQ:). On the other hand, R contains the term 6 and, in addition, the difference between R and the true value A g/A p is strongly dependent on its value. The same holds for the parameter D. Thus, these three parameters constitute a particularly useful tool for the evaluation of the data obtained by teflon-nylon filter packs during the intcrcomparison. For example, if two teflon-nylon systems yield different values of S~ V, where V is the sampled volume, we must infer that at least one of the two configurations does not meet the optimum requirements, that is to say, one or more conditions reported in Eqn (3) are not satisfied. Distribution pattern on a~nuder systems

A denuder system is usually made up of one or more cylindrical or annular tubes, whose walls are coated with an appropriate chemical for the selective

NITRIC ACID AND NITRATE MEASUREMENT

49

collection of gaseous species and a filter pack for the collection of particulate matter. The study of the deposition pattern on denuders is much more complex than in the case of filter packs. A detailed theoretical description of the general problem has been reported by Febo et al. (1989). "[he simplest configuration of a denuder system consists of one denuder followed by one filter. For this system, if the collection efficiency can be assumed to be constant during the whole sampling period, the deposition pattern can be described by the following relationships: M~=A 8E~(I-E~.)+A

pEps(I-EFN )+~Q,

M2 = AP EYN E~ (l - c~E~) + A 8 E ~ E~ ~ (l - E~) + ~Q2

(9) (10)

where the optimum requirements, generally verified by all denuder systems, are the following:

E~---0;E~=

I

A good choice of the denuder coating material and filter medium allows the amounts I:Q~ and I~Q2 to be minimized (e.g. a denuder coated with NaCI and a filter impregnated with NaCI) and a value of E2g = 0 to be obtained; if the geometry and materials of the inlet and transfer lines have also been optimized, the determination of M~ and M2 allows a calculation of the values of Ag and A p with a good approximation (the small imprecision is due to particulate deposition on the denuder walls). Three different coatings (NaCI, NaF and Na2CO3) and different configurations of denuder systems were used during the intercomparison. It is known from the literature that positive artifacts may occur on Na2CO3coated tubes, due to HONO and NO2 retention (Febo et al., 1986; Perrino et al., 1990). This may be one reason for different results between different denuder systems; different sampling train inlets and different fluid dynamic conditions at the denuder inlet may be another reason (Appel et al., 1988; De Santis et al., 1988). As in the case of filter pack systems, the parameters S, R and D constitute useful trois for the evaluation of denuder performances during the intercomparison. In the following sections we shall report the procedure used for gaining information about the reliability of the techniques used in the intercomparison and for explaining the possible differences between the results; on the basis of the predicted deposition pattern we shall carry out: (i) an evaluation of the results obtained from the HNO3 pure source test (PST) and from the temporal self-consistency test (TSCT) and (ii) a comparison between the results yielded by similar methods and finally by different methods.

50

A. FEBO El" AL.

Evaluation of the PST results The availability of a HNO3 source, free from any other gaseous species and from particulate matter and having a known and constant mass flow rate, allows a first and basic check of the reliability of each method to be performed, by making use of the predicted deposition pattern. Under these conditions, Eqns (1) and (2), which hold for filter packs, reduce as follows: M~ = A8 E~N (1 - E~)

M2 = A g E ~ E~/3 (1 - E~) If E~g and E28 are known, the determination of the values M2 obtained in this test provides information about the value of the product E~Ng./3, that is to say about HNO3 transmission through the inlet, transfer lines and filter holder. Obviously, a value M~ ~ 0 indicates that HNO3 collection efficiency on the first filter was not negligible and that this configuration would be unsuitable for the determination of HNO3. By applying the same procedure to denuder systems, it appears that also in this case, the PST allows an evaluation of the performance of the different inlets with respect to HNO3 transmission to be made. In conclusion, the PST identifies which systems are affected by a systematic error due to HNO3 adsorption on the inlet and transfer lines, a bias not due to the sampling method itself but to the particular sampling line used. This bias will impair all the measurements carried out with these systems during the intercomparisGn, yielding values of S < (,4 g + A P) in all field trials. It is worth noting that the determination of HNOa transmission during the PST does not allow a quantitative estimate of the transmission factor under field conditions to be made. In fact, among the causes for HNO3 loss, an important role can be played by absorption (Appel et al., 1988; De Santis et al., 1988), a reversible process which depends on the absorbed amount. Thus, according to this hypothesis, the shorter the sampling time, the lower will be the transmission factor.

Evaluation of the TSCT results Once the systems have been checked for yielding correct results from the PST, the TSCT represents a useful tool for evaluating the reliability of the methods in real situations, i.e. in the presence of interfering gaseous and/or particulate compounds and under various atmospheric conditions. It should be stressed that a good result from the TSCT is a prerequisite for considering

NITRIC ACID AND NITRATE MEASUREMENT

51

a method as being reliable, but it is unfortunately not fool-proof. Some artifacts which may even seriously bias the determination cannot be revealed through a self-consistency analysis; e.g. a significant deposition of particulate nitrate on the denuder walls, caused by a poor design of the denuder inlet geometry. If the sampling period t is divided into n time intervals At, the temporal self-consistency will be satisfied if the mass amount collected at the time t equals the sum of the mass amounts collected during the n periods At (assuming the collection efficiency is constant with time). To discuss the inf,~rmation yielded by the TSCT it is thus necessary to perform a time study of the mass increase on the collecting media.

Filter packs From Eqns (4) and (5), which refer to the nitrate amount collected at the end of the sampling on the two stages of a T-Nfp, we can infer that the evaluation of the self-consistency of the method reduces to the evaluation of the self-consistency of the term 6. It is thus necessary to study the temporal behaviour of this term, which includes a number of possible different processes taking place during the sampling, such as the evaporation of ammonium nitrate, the displacement of nitrate from the first filter, caused, for example, by H2SO4, the interaction of HNO3 with the particulate matter collected on the first filter etc. The physico-mathematical description in differential terms of these phenomena is complex; for example, HNO3 evolution from NH4NO3 is described by a function fl which depends on the temperature and relative humidity, on the NH4NO3 amount retained on the filter, on the size and chemical composition of the collected particles, etc. The functions describing the other two processes exhibit the same complexity. All three phenomena are a function of the collected amount of particulate matter; thus the occurrence of only one of the three is sufficient to prevent the temporal self-consistency of the measurements. For our purposes it is thus sufficient to describe in differential terms the case, for example, ~.~~at HNO3 evolution from NH4NO3 is the only quantitatively important process. In this hypothesis, the mass deposition on the two collecting media of a T-Nfp is described by the following equations: dMl = [CP(t) + C~,(t)] F - fl dt

(11)

dM2 = Cg(t) F + fl dt

(i2)

52

A. F E B O El" ~L.

where C is the molar concentration at the inlet, subscripts a and b refer to NH4NO3 and to the remaining particulate nitrate amount, respectively, F is the sampling flow rate and f/is the molar mass flow rate of nitric acid which evolves from the first filter; the term corresponding to int6rferent species on the nylon filter has been neglected as it is generally very low. Adding and integrating Eqns (11) and (12) it clearly results that temporal self-consistency is always verified for the sum S. Since the term fl, which is contained in the right-hand side, is a complex function of the nitrate mass amount collected on the filter, ~ m p ~ a l self-consistency cannot be verified for either M~ or M2, or tb.~ difference D and the ratio R. In addition, with M~ --- M~a + M~b and with M,b self-consistent .dMib = C g F ) , from Eqn (11) we get: dt

. ~

=

C~ (t) F -

9

(14)

If we assume that the size and the chemical composition of the particulate matter remains approximately constant during the sampling, it is possible to express fl as a Taylor series with respect to Mma; limiting ourselves to the first term, we obtain;

fl = 17 (t) Mla where ,1 (t) is the instantaneous HNO3 evolution factor, and its timedependence contains the dependence from the temperature, relative humidity, etc, Under these conditions, the integral solution of Eqn (14) is the following: Mta(0 = exp [ - | ~(t) dt] .[K + | CP (t) F exp [| ~ (t) dt 1]

(15)

where K is the integrating constant. Recalling that M~ = Mla + Mlb, it results that the following relationships are always verified: Mi (0 + t) < M! (0 + tl) 4- Mi (tl + t) <

(0 + t)

(16)

M2(0+t) > M2(0+tl)+Me(tl +t) > Ag(0+t)

(17)

Ap

whenever t I < t. Thus, the nitrate mass amount measured during the period (0 + t) is always lower than the sum of the mass amounts collected during

NITRIC ACID AND NITRATE MEASUREMENT

53

the sub-periods At and the fraction lost is much higher the longer the time t. Likewise, Eqn (17) shows that the overestimation of HNO3 concentration is higher the longer is the sampling time. It follows that the differences between measured and true values of R and D are strongly dependent on the duration of t. If we assume that both ~ and Cap are constant during the sampling time, we can rewrite the solution of Eqn (14) as follows;

Mla (t) = exp ( - 7/t) [ K +

COaF~ exp (7/t) ]

where the value of K can be determined if the value Mia at the time tO is known. If it is zero (clean filter), we have:

M]a (t) = C$-I TF/

1 - exp ( - ~ t) ] = CO F t

l-exp(-~t) ~/t

(19)

The link between the integral expression (4), which in this case reduces to M I ~-/1 p ( l - o/) and the solution (Eqn (19)) can be worked out as follows. Remembering that CP F t = A P and writing: 1 - exp ( - ~ t) ~t

= 1 - or' (t)

(20)

we get: g l a + Mlb = A p (1 - o~) = APa (I - or') A ~

If r identifies the molar fraction of NH4NO3 with respect to atmospheric particulate nitrate amount, we get: Ap(1-rtx')=A

p(I-t~)

and thus: tx = r ol'

(21)

Equation (20) shows that c~' is a monotonic increasing function of the product ~.t, with c~' (0)= 0 and ix' (oo)= 1. To express the inequality (Eqn 16) as a function of the time duration and

54

A. FEBO El" AL

of ~ values, we solve Eqn (18) with respect to the time intervals (0 + tl) and (tl + t):

Mla (O + t) = Mla (O + tl) exp [ - rl (t - tl) ] + CP F (t - tl) 1 - exp [7 (t - tl)] 7/(t - tl)

= Mta (O + tl) exp [ - ~ (t - tl) ] + Mta (tl

+

t)

(22)

Similarly: M2(0+t)=Ms(0+tl)+M2(tl II-exp

+t)+Mi~(0+

[-~/(t-tl)]l

tl)

(23)

Equation (22) shows that the total amount of ammonium nitrate collected during the time t can be considered as due to the sum of two terms: one is the contribution M~a(tl - t) relative to the second period; the other one is a fraction of the NH4NO3 mass amount co!~ected by the system during the first period, which is lower, the higher is the product ~(t - tl) (for example, if the dissociative process becomes relevant during the second part of the sampling and this condition holds for a long time, it may even result that exp[-~(t- tl)] --. 0 and that Mla(t) coincides with the mass collected during the period (tl ÷ t)!). Thus: Ml (0 + t) < MI (0 + tl) + Ml (tl - t) = A p (0 + t) [I - O~ (0 + tl)] + (tl + t). [1 - ct (tl + t)] < A p (0 + tl) + A p (tl .-" t) = A p (t)

In short, we have: Mt (t) < ~Mti < ~A~ = A p (t) Ms (t) >

> gag,= Ag (t)

S(t) ~- ~Si = A p (t) + A s (t)

R(t) >

gM2 ~Mli

>

A g (t) A p (t)

D(t) > ~Di = A s (t) - A p (t) where subscript i indicates the sampling period.

A p

NITRIC ACID AND NITRATE MEASUREMENT

55

Denuder systems For the simplest denuder system, if the inlet is insensitive ~o the species which have to be determined (EINg = EINp -- 1) and E2g = 0, the mass deposition on the denuder and on the downstream filter is described by Eqns (24) and (25) below. Recalling that the determination can be affected by both direct and indirect (nitrite to nitrate oxidation) interferents (Febo et al., 1986; Febo et al., 1989), we shall define ~, as the amount of direct interferents collected in the unit time, B as the total nitrite amount present on the filter at the time t and ~ as the instantaneous conversion factor to nitrate; the nitrate amount yielded in the time unit by indirect interferents will therefore b e ~ • B. dMi dt

dt

= C g (t) F (1 - E~) + C p (t) F (1 - E~) + "~1 4" ~1 BI

(24)

= C p (t) F E~ + C g (t) F Ef fl (1 - E~) + ~'2 4" ~2 B2

(25)

If the collection efficiencies are c~=,nstant with the time, the term B, which depends on the collected amount and thus on the sampling duration, is the only one which might prevent the fulfilment of the temporal self-consistency. The term B, in fact, can be formally described by a relationship similar to Eqn (15) which we demonstrated not to be additive. If the requirements of the TSCT are not fulfilled, we can thus deduce that indirect interferents have appreciably affected the accuracy of the measurement (e.g. denuders coated with Na2CO3 (Perrino et al., !990)). However, the use of selective coating layers allows good self-consistency of the results to be obtained. On the other hand, in the case of the simple sampling line made of one denuder followed by one filter, a good result at the TSCT does not rule out the possibility of undesired phenomena such as particle deposition on the denuder walls, which causes overestimation of HNO3 and underestimation of NO3- concentration. This artifact could be highlighted only by using a more complex sampling set-up, as, for example, by inserting a back-up denuder (absolute differential technique, Febo et al., 1989).

Data evaluation In conclusion, the PST allows the identification of which systems suffer from inaccuracies due to the inlets and transfer lines. The data yielded by these groups should be discussed in this context and not mixed up with the remaining data set, as sometimes happened in previous exer:ises (Hering et

56

A. FEBO ET AL.

al., 1988). Similarly, the TSCT shows which systems suffer from artifacts due to phenomena depending on the collected amount (e.g. HNO3 evolution, NO2- to NO3- oxidation, HNO3 adsorption on particulate matter, etc.). Also in this case, a simple statistical treatment of the whole data set would hide the reasons for the discrepancies. It is worth noting that in field intercomparison studies the true values of A g and A P are often unknown. In many previous intercomparisons a reference value was identified by calculating the mean value or the median ofall the results (Spicer et al., 1982; Fox et al., 1988). In the light of the above discussion, this method is clearly inappropriate. If the results yielded by the biased methods (identified by the PST) are discussed, a sub-set of data can be identified, enabling further studies to be carried out in order to identify a good reference value for S. We can assume, in fact, that the remaining data (yielded by both denuder and filter pack systems) satisfy the temporal self.consistency for total nitrate (i.e. for S) and that the results converge towards a value very close to the true value. A statistical analysis of S values can now be useful for identifying the possible outlyers, due, for example, to a different cut-off size of the inlet and/or to a positive bias caused by direct interferents. An estimate of the true values of ~ " and C'~', instead, can be carried out on the basis of the data yielded by the systems which satisfy the temporal self-consistency for HNO3 and NO3-. These data have to be discussed in the light of *.he meteorological conditions, the results of the ancillary measurements and, whenever possible, of the analytical results yielded by the different stages of the sampling line. A data analysis carried out as a function of the time of the day, for example, is particularly interesting: as HNO3 and NO3- are secondary pollutants, which exhibit a strong diurnal pattern, we can study the relationships between the determined concentrations and the parameters which describe the atmospheric conditions (these relationships are expressed in the deposition pattern by parameters ~ and ~,). Once the best estimates of ~" and C'F are obtained, as a final step of this analysis, we can substitute C-gV and C-PV for A g and A P in the deposition patterns and verify the proposed explanations for the results yielded by other methods. In addition, we can obviously evaluate the extent of the disagreement between the whole concentration data set and these estimated values. RESULTS AND DISCUSSION

The intercomparison was held at a site placed 30 km NE from Rome. Up to the end of the first day (Monday 19th) the wind blew from N-NE; during the following days the area was characterized by a high pressure situation, which persisted until the end of the week; from Tuesday on, the wind speed

NITRIC ACID AND NITRATE MEASUREMENT

57

was always below 5 m s -j (variable direction). The plot of the temperature and relative humidity is reported in Fig. I. It appears that from Tuesday 20th to Friday 23rd the climatic conditions were constant; this allows an interesting interpretation of the results, as reported below. PST

The results of the PST are reported in Table 2. A first observation concerns the data of groups D and IB, which used a cellulose prefilter for collecting particulate NO3-:HNO3 mass flow rate (calculated from the analysis of the collecting stage for HNO3, i.e. the impregnated downstream filter) appears to be largely underestimated; in addition, more than 50% and 99% of the nitric acid was recovered on the prefilter of groups IB and D, respectively. These findings offer experimental evidence that E~g ~ 1 for these configurations; thus these set-ups are not able to discriminate HNO3 from NO3-. The results yielded by these groups during the whole intercompar•son, therefore, will be only considered in terms of the sum S~V of HNO3 and NO3- concentrations. The results yielded by both T-Nfp and denuder systems show considerable scattering. Since the nitric acid source exhibited a high stability, frequently checked during the whole period of the PST (standard deviation (SD) ± 3%), this data scattering has to be attributed to HNO3 losses at the

100"

--

00.

IO0

Temperoture

--- Rel. H u m i d i t y

;

70-

60-

i'1

I

,,..,,,.:'"

OO'

:,

-

OO

.-o =

t I

,,,,,'" • i

.,

O0

j".

i,°~,

,,,¢

•

!

=

~

:

.

.

~

;-:

.

70

#

60

# I

iI

t('C)

50-

"

S"

5o

, i

•

40

I

'

t

o t

;

•

#

r.h. Z

•

40

o

3O

30

20

20.

10

10. 0

an

O

16

Sept 19

0

8

i i

e

i

16

Sept 20

0

i

i

18

i

16

Sept 21

|

0

wI

O

i

16

Sept 22

i

0

ii

i

iii

B

i

ii

16

ii

0

Sept 23

Fig. 1. Temperature and relative humidity temporal trend during the intercomparlson.

58

A. FEBO El" AL.

inlets and transfer lines, i.e. to values of giN g and/or ~ ~ 1. It is thus clear that even systems which make use of the same sampling techniques may yield very different results. Noticeable discrepancies with the theoretical value of HNO3 mass flow rate are shown in many cases by T-Nfp. Especially, group W, which made use of a nine-stage cascade impactor placed upstream of the filter pack, exhibited almost negligible HNO3 recovery on the nylon filter. It follows that all the field results for HNO3 yielded by this set-up were also affected by a very high ,legative bias and were therefore totally unreliable. The data gathered by groups W during the field samplings confirm this hypothesis (Air Pollution Research Report 22, 1989); for this reason, they cannot be included in the following discussion. Among the results of T-Nfp, the data of group A are the closest to the theoretical value. The data in Table 2 show that all the denuder systems which did not use inlets yielded good results at the PST. On the contrary, the data of groups GI, KI, K2, K3 and R, which made use of various types of inlets, are appreciably lower (E~Ng ,~ 1). Clearly, denuders are also affected by the problem of HNO3 transmission through the various stages of the sampling set-up (that is, Eqns (9) and (10) do not contain the term 8). TSCT Table 3 gives the results of the groups which followed the schedule during at least one of the two trials. As well as the data of the 24-h samplings (24 h) and those obtained by averaging the results of ~:he4-h and 12-h sampling of the same day (AVG), we report the values of P, which is calculated as the ratio between 24-h and AVG values; this parameter expresses the selfconsistency of the determination. In addition to HNO3 and NO3- concentration, the value of P is also reported for the sum S, the difference D, the ratio R and for SO42- concentration. The latter constitutes a further check: it is known that the artifacts due to interconversion between the gaseous and the particulate phases are much less relevant for this species than for HNO3/NO3-. The good self-consistency exhibited for SO42- by all the methods- P(SO42- ) -- 1 - and the small scattering of the concentration values indicate a good accuracy of the analytical determinations and a correct execution of all the samplings. Filter packs The results in Table 3 show that cellulose-impregnated filter packs yield values of S~V close to the values yielded by the other systems, while HNO3 and NO3- concentration values are, respectively, largely underestimated and

59

NITRIC ACID AND NITRATE MEASUREMENT

overestimated. This indicates, again, ttNO3 retention on the first filter (El g ~[ 1), in agreement with the results of the PST and the predicted deposition pattern (Eqns (1) and (2)). For T-Nfp, the data of Table 3 show that only P(S) = 1, while P(HNO3) > 1, P(NOF) < 1, P(D) ~ 1 (negative values!) and P(R) ~ I. The data of groups A and C are qualitatively very similar; from a quantitative point of view, however, the values of S/V yielded by group C are lower than the values or all the other groups, which are in a close range. This behaviour can be explained in terms of losses at the inlet and transfer lines (E~, ~ s , /3 ~ 1), as also suggested by the results yielded by group C at the PST. For this reason, further study of the behaviour of T-Nfp will be c~.rried out only on the data yielded by group A, which proved to use the best configuration. Anyway, for both groups A and C and for both trials, NO3- values from the 24-h samplings are clearly lower than the values determined from th~e shortterm samplings; the opposite behaviour is exhibited by HNO3. The only explanation for this result is in terms of interconversion of HNO3 between the particulate and the gaseous phase, in agreement with the predicted deposition pattern (Eqns (4) and (5)). This first observation already shows that this technique is not able to discriminate HNO3 from NOF. The comparison between HNO3 and NO3- concentrations, S/V and D/V yielded by group A during the 24-h and the short-term samplings is reported in Fig. 2: the interconversion between HNO3 and NO3- is evident. The analytical results of these trials are reported in Table 4. If we COld,pare HNO3- amounts collected during the period 20:00-08:00, we can observe that the amount of NO3- sampled during a nighttime period (e.g. 26.8/zg on

1°!I rJ,g/m.3

[] Avg m24h

_iIJ,h, HN03 NO3-

S/V

Sept 20

.R. D/V

HN03 NO3-

S/V

D/V

Sept 21

Fig. 2. Temporal self-consistence test: comparison between the results yielded by a teflonnylon filter pack (group A) in the 24-h and the short-term samplings.

60

A. FEBO El" AL.

TABLE 4 Analytical results of group A at the TSCT Date (Sept. 1988)

Time

Teflon filter (.ug)

1st Nylon filter (/zg)

2rid Nylon filter (/zg)

Sampled volume (m 3)

20th 21st 21st

20:00-08:00 08:00-12:00 12:00-20:00

14.43 5.77 6.53

2.40 5.44 13.84

0.76 0.67 <0.2

4.600 1.520 3.070

26.73

21.68

1.43

9.190

Sum 20th

20:00-20:00

17.61

28.72

2.29

8.950

21st 22nd 22nd 22rid

20:00-08:00 26.80 08:00-12:00 I 1.97 12:00-16:00 2.99 16:00-20:00 4.56

8.00 5.36 9.12 5.92

0.45 < 0.2 0.20 0.35

4.730 ! .520 1.540 1.550

Sum

46.32

28.40

1.00

9.340

28.16

42.24

1.84

9.230

21st

20:00-20:00

Wednesday 21st) almost equal the amounts collected during the 24-h sampling (28.2 #g). Thus, in the case of Wednesday 21st, only about 1.4/zg would be expected to be collected during daytime (sampled volumes are comparable); the three determinations carried out during these periods, instead, yielded a total of 19.5/zg (!). Repeating this procedure for HNO3, we obtain a predicted amount of 34.2/~g during daytime, while only 20.4 #g were actually collected. These results can be easily explained if we assume that the artifact expressed by 8 is mainly due to evolution of HNO3 from NO3-. Under these conditions, HNO3 retention on particulate matter is assumed to be negligible. In fact, this retention depends on the amount 7 of particulate matter collected on the teflon filter and on the HNO3 concentration; thus, it would be more important during the daytime part of the 24-h sampling, in contrast to the data of Table 4. We can therefore suppose that > 0. This is also confirmed by the observation that D/V(24-h) ~> D/V(AVG). We conclude that the relationship between the mass amount M~ collected during the 24-h sampling and the amounts collected during the short-term samplings can be described by a generalization of Eqn (21). The hypothesis that NO3- to HNO3 conversion is mainly due to HNO3

61

NITRIC ACID AND NITRATE MEASUREMENT

evolution from NH4NO3 is supported by some of the ancillary measurements. A first confirmation is given by the trend of the temperature and relative humidity (Fig. 1), which strongly increased and decreased, respectively, during the first hours of the morning. Furthermore, ancillary measurements of evolved HNO3 carried out by using a T-Nfp preceded by denuders capable of removing gaseous HNO3 and NH3, yielded the data shown in Fig. 3. The phenomenon proves to be relevant and reaches its maximum extent during the period 08:00-12:00. From the above discussion, without making use of any comparison with the data yielded by other techniques, it has been possible to state that the T-Nfp were not able to discriminate between HNO3 and NO5-. In addition, we can estimate the minimum value of the relative error (RE) which occurred during the 24-h determination of HNO3 without knowing or estimating the true value of HNO3 air concentration. In fact it results (Eqn (23)) that HNO3(24-h) > HNO3(AVG) > true HNO3 and then: RE >

M2(24-h)- M2(AVG) M2(AVG)

In the case of group A, this error results in being greater than 40% and 52% for the samplings of September 20th and 21st, respectively. Denuder systems The results yielded by the three denuder systems (groups F, IC and M) reported in Table 3 show that the values of P are close to unity for all the

Vg/m3

|

08-12

12-16

16-20

Sept 20

--

20-00

•

|

081-12

12-16

16-20

Sept 21

20-OB

~.

Fig. 3. Temporal trend of nitric acid evolved from particulate ammonium nitrate. NH4NO~ is collected on a teflon filter, evolved HNO3 is recovered on a back,,up nylon filter; the set-up is preceded by denuders capable of removing atmospheric NH3 and HNO~.

62

A. FEBO ET AL.

parameters; therefore, as expected, the self-consistency of this technique proves to be good. For a further discussion we shall focus on the results of group IC, which constitute the most complete data set. The sampling set-up used by group IC comprised of two NaCl-coated annular denuders- connected by a mixing chamber- for HNO3 determination, a cyclone having a cut-off size of about 2.5 ~m at the operative flow rate of 15 I min-~ a Na2CO3 + glycerolcoated annular denuder for HNO2 removal (HNO2 is a possible interferent on the nylon filter; Perrino et al., 1988) and a nylon filter for collecting NO3". No inlet was used and the cyclone was inserted downstream of the NaCI denuders in order to avoid the adsorption of HNO3 on any surface prior to its collection (De Santis et al., 1988). NaCI was chosen because of its high selectivity. The only known interference on NaCI denuders is due to impact and turbulent deposition of particulate nitrate, but the second NaCI denuder allows this positive bias to be compensated for (see next section). Besides, this denuder enables the behaviour of the whole set-up in the operative conditions to be checked. The analytical results yielded by the group IC at the TSCT are reported in Table 5. The data show that the self-consistency is very good for all the stages except for, as expected, the Na2CO3-glycerol denuder. Some indirect interferents, in fact, are collected on this coating layer (HNO2, PAN, NOx); these compounds yield NO2- which can be converted to NO3- by atmospheric oxidants. This phenomenon is particularly prevalent in the 24-h sampling (which started in the evening), since the oxidative processes, which mainly occur during daytime, can act on the considerable nitrite amount collected during the night. These results confirm that Na2CO3-coated denuders are unsuitable for HNO3determinations (Perrino et al., 1990). Besides, we can observe that the artifact nitrate amount collected on the Na2CO3 denuder during the 24-h samplings is a remarkable fraction (> 30°/6) of the amount collected on the first NaCI denuder: it follows that any system relying on Na2CO3 denuders for determining HNO3 would suffer, under these conditions, from a strong positive bias. On the basis of the TSCT results we can assert that no bias other than particulate nitrate deposition on the denuder walls (discussed later) can affect denuder systems. We can conclude anyway that for set-up IC: measured HNO3 > true HNO3;measured NO3- < true NO3-; measured S --- true S. Recalling the conclusions drawn for T-Nfp and c~,mparing the data of groups A and IC we observe that: H N O ~ (24-h) ~, HNO3rN (AVG) IB, HNO #° (24-F, = HNO #D (AVG) > true HNOa

63

NITRIC ACID AND NITRATE MEASUREMENT

TABLE 5

Analytical results of group IC at the TSCT Date Time

1st NaCI denuder ~g)

2nd NaCI Cyclone denuder 0tg) (~g)

2.30 5.87 15.18 20.48

0.70 0.50 0.56 0.77

10.96 4.12 3.22 ~.74

43.83

2.53

20:00-20:00

42.89

2i st 20:00-08:00 22nd 08:00-12:00 22nd 12:00-16:00 22nd 16:00-20:00

Sum

(Sept.

I988) 20th 21th 21st 21st

20:00-08:00 08:00-12:00 12:00-16:00 16:00-20:00

Sum 20th

21 st

20:00-20:00

Na2CO3 denuder (~g)

Nylon filter (~tg)

Sampled volume (m3)

1.75 1.52 1.50 1.12

23.36 15.87 4.17 4.29

10.820 3.397 3.333 3.359

23.04

5.89

47.69

20.909

2.58

23.50

15.32

47.94

21.490

! 1.02 6.27 14.67 12.96

0.99 0.45 0.61 0.83

19.99 7.64 5.91 4.84

2.55 1.31 1.44 1.02

37.63 22.95 5.12 5.20

10.680 3.300 3.348 3.333

44.92

2.88

38.38

6.32

70.90

20.661

45.26

3.73

39.50

15.83

69.37

21.230

where superscripts TN and AD identify Teflon-nylon filter packs and annular denuders, respectively. The above relationship shows that HNO3 determinations carried out by means of a T-Nfp suffer from positive artifacts to a much greater extent than the described annular denuder set-up. Thus, a better estimate of the minimum value of the error associated with the 24-h filter pack determination can be obtained by using the denuder results, as follows:

RE >

M ~ (24-h) - M AD (24-h) M~ ° (24-h)

By using the AD results of group IC, the RE value for group A is calculated to be greater than 79% and 137%, for the two trials. By using the same procedure, the errors associated with the average of the short time determinations can also be estimated and are > 25% and > :).-/o, : - , , o , respectively.

64

A. FEBO El" AL.

Evaluation of particulate nitrate interference on denuders For set-up IC, the deposition pattern on the two NaCI denuders and the cyclone plus the nylon filter (identified by subscripts 1, 2 and 3, respectively) is given by the following relationships, which hold when a mixing chamber is inserted between the two denuders: M~ = A 8 (I - Eg) + A p (l - EI~)

(26)

M2 = Ag Eg (I - E s) + AP E~ (I - E~)

(27)

M3

(28)

=

Ap E/'

(in this configuration and under these operative conditions the term relating to interferents in Eqn (28) is of very little relevance and has been neglected). If the measurements were performed by using only one denuder, an ¢,~timate A g" of the value of A g would be obtained and would include the interference of particulate matter as follows: ~'i

A p (I - E p) _- As * > Ag

= As +

(29)

If two denuders in series are used - absolute differential technique (Febo et al., 1989)- a better estimate A s.. of A g is obtained. This is given by: As**=

MI+M2 =Ag+AP (t - Eg) 2

(1 - E ~ ) - ( l - E ~ ) + (1 - E ~ ) .

(I - Eg) 2

(1 - E ~ )

(30)

Thus, from Eqns (29) and (30): A s > Ag** > Ag Especially, if E! p = E2 p, the relative error RE in the determination of A g through A g'" will be:

RE -

AP I Ag

I-EP 12 1 - Eg

(31)

NITRIC ACID AND NITRATE MEASUREMENT

65

Equation (31) shows that if [1 - E g] --- 1 even a value [1 - EP] = 0.1 would lead to an interference of only 1% of A P. In the case of set-up IC (flow rate 15 1 min-', Eg -- 0.01) a close examination of the analytical data collected during the intercomparison leads to an estimated value of about 0.96 for E2p, calculated from Eqns (27) and (28). If it is assumed that El p = E2p, this would lead to an interference of only 0.15% of A P. By using this set-up, a good estimate A P**of A P is also obtained; A P'" is calculated by adding twice the value M2 which constitutes a good estimate of particulate interference on the two NaCI denuders (the first one exhibits a collection efficiency for HNO3 close to unity~, to value M3. It follows that: M3 < A p** < A p.

General discussion of the data The above discussion shows that general conclusions can already be drawn from the evaluation of the PST and TSCT results alone. In the following, we shall discuss the whole data set in order to verify these findings. The method generally used in the literature for the evaluation of a data set gathered during an intercomparison exercise is based on the simple correlation between the results yielded by different groups and/or techniques. Unfortunately, the information given by a simple correlation study is relevant only in certain cases: for example, if we want to assess the reproducibility of measurements carried out by using identical sampling techniques and setups, without, however, assessing the reliability of the method and the accuracy of the results. A correlation study can also be useful when we can count on a sound reference technique. In this case, the study of the correlation between the 'true' values and the results yielded by another method can give information about its reliability and possibly, the reasons for the deviations. In the general case of whichever two techniques devoted to the measurement of the same species, a reasonable correlation between the data is obviously to be expected; beyond this, the simple correlation proves to be inadequate for assessing the accuracy of the results and the performance of each one of the two methods. For example, a correlation study between the data yielded by set-ups A and IC, whose different behaviour has been extensively described above, yields a correlation coefficient r = 0.905 and r = 0.969 for HNO3 and NO3respectively (Fig. 4). By this analysis, the differences between the two techniques and the relating causes, are hidden, making the identification of the more reliable method impossible. However, a more detailed correlation study would highlight that the two data sets do not belong to the same population. In fact, considering the sum and the difference between HNO3 and NO3- concentrations, we obtain the regression plots of Fig. 5. It clearly

66

A. FEBOET AL

HNO 3 BB

(~g/)

°,,"

D ann°

..*°

,..'°

10-

Group A Filter ~ k

NO 3 10

12-

..-"

...""

moil

6 .°°"

6 4

°.°'"

• am

4 2

°°"

0!' 0

: 2

2

i 4

i 6

Q 8

~ ---.4 10 12

0

•

0

I

I

I

I

I

2

4

6

B

10

Group IC Denuder

(~g/m3) Fig. 4. Correlation between the data yielded by groups A and IC. HNO3 determination: y = 0.635 + 1.080x; r = 0.905. NO;" determination: y = -0.247 + 0.858x; r = 0.969.

results that the correlation for S~ V (r = 0.976) is higher than the correlation of each addend and the regression line is close to the optimum y = x; conversely, the correlation for D / V is lower (r = 0.886) and the intercept and slope are far from the values of 0 and 1, respectively. This analysis shows that actually the two techniques measure different amounts and that at least for one of the two methods the values of HNO3 and NO3- concentration are not independent from each other. This is in agreement with the abovereported interpretation of the results of the TSCT.

sly

D/V

16

•

.o"

12 Group A Filter Pock B, (Fg/m3) •..2

0!'" 0

4

I

8

I

12

18

-B

0

B

Group IC Denuder

Fig, 5. Correlation between the data yielded by groups A and IC. Sum of HNO3 and NO3-: y = -0.704 + I. 107x; r = 0.976. Difference between H N O 3 and NO3-: y = 1.399 + 0.886x; r = 0.886.

NITRIC ACID AND NITRATE MEASUREMENT

67

A more interesting analysis of the data collected during the intercomparison can be carried out by grouping them according to the sampling period, taking advantage of the constancy of the meteorological conditions which became established during the study. The temporal trend of HNO3 and NO3- concentration throughout the whole exercise is shown in Fig. 6, (groups A and IC). HNO3 concentration data yielded by group A are shown to be almost always higher than those of group IC, while NO3results are predominantly lower. If we group the data according to the sampling period and plot the difference between the average concentrations

HN03

,2t lO

I~IA

[]

8

IC

Conc

(pg/m3) 6, °

2-

o a

b

c

d

a

b

c

,:I

,~

b

c

d

a

b

c

d

a

b

c

NO3-

Conc

(Fg/m3)

'°:t

~A [] IC

4

2

o

a

b

c

d

a

b

c

d

a

w.

b

c

d

b

u

d

a

b

c

Fig. 6. Temporaltrend of HNO3and NO3-concentration as measured by groups A aad IC. Period a, 08:00-12:00; period b, 12:00-16:00; period c, 16:00-20:00; period d, 20:00-08:00.

A. FEBO ET AL.

68

yielded by the two groups, we obtain the graph of Fig. 7a, which shows noticeable differences for the two compounds, having opposite sign and exhibiting an interesting temporal behaviour. In Fig. 7b,c we report the differences between the values of S/V and D/V yielded by the two groups. It is clear that S~ V values obtained with the two techniques are very similar, while D/V values are appreciably different and follow the trend of dissociated nitrate, whose independent determination is reported in Fig. 7d. These results are in good agreement with the predicted deposition patterns. Recalling the values of S and D for filter packs (Eqns (6) and (8)) and those for denuders, which can be worked out by substituting E g = 0 in Eqns (25)-(27), we get the following relationships: s'IN

sAD

V

V

=

D~

D AD

V

V

~QTN = 0

(32)

= 2 ~ + ~ Q 2 TN+2Ap [ I - E ~ . E ~ - 2 E ~ ] A D

(33)

which hold if the two systems have the same particle cut-off size. From Eqn (32) it clearly results that the differences for S / V were expected to be negligible. Besides, the above discussion indicates that the last two addends of Eqn Fg/m 3

I~1 HN03 b

m NO3-

C[ 08--12

12--16

3"r

!,5--20

20-OB

c

08-12

: It///.,-//A I I 17)7)'j7~ . 12-16 16-20 20-08

3.

d

1.

0B-12

12-16

16-20

20-08

0

!

I

08-12

12-16

: 16-20

" 20-OB

Fig. 7. Grouping of the data collected during the whole exercise according to the sampling pel;od. Differences between the resu!t~ of groups A and IC: (a) HNO3 and NO3- concentrations; (b) S/V; (c) D/V; (d) trend of evolved ttNO3.

NITRIC ACID AND NITRATE MEASUREMENT

69

(33) are low; thus, an agreement between the experimental results of Fig. 7c and the trend of the parameter 6 (see Eqn (4) and (5)) was expected; in addition, the good agreement between the values of Fig. 7c and those of evolved NO3- indicates that under the atmospheric conditions of the intercomparison, the phenomenon of HNO3 retention on the collected particles was of minor extent. A similar data evaluation could have been also carried out for the remaining groups if the measurement protocol had been rigorously followed. Anyway, the analysis of the whole data set (Air Pollution Research Report 22, 1989) indicates that the same behaviour as seen for groups A and IC is exhibited by the other groups employing the same technique and that the discrepancies can easily be explained on the basis of the results yielded at the PST and TSCT (if available). CONCLUSIONS

On the basis of the work carried out for planning the sampling schedule of the experiment and for evaluating the results, we can draw the following conclusions. (i) Interesting information can be drawn from a field intercomparison of different sampling techniques when the study design includes some tests able to highlight both the performance of the method in the presence of a pure source and its self-consistency with respect to significant parameters (e.g. sampling time, flow rate, collecting media, etc.). It is also necessary, for each technique, to have as detailed a knowledge as possible of the predicted deposition pattern. (ii) The method for evaluating the results must take advantage of the data yielded by both the tests and the field samplings, which must be compared with the predicted deposition pattern. Only afterwards, can the results yielded by groups making use of the same technique and, finally, by groups employing different techniques be compared. (iii) The most reliable accumulation method for the determination of HNO3 and NO3" proved to be the diffusion denuder technique, since the teflon-nylon filter pack method was clearly shown to be biased by NO3--HNO3 interconversion processes. A reliable diffusion denuder technique, however, must satisfy a number of criteria, including the selectivity of the coating layer, the inactivity of the inlet with respect to the gaseous species and the sensitivity. From this last point of view, high flow rate annular denuders are at present exhibiting the best performances. ACKNOWLEDGEMENTS

The Field Intercomparison Exercise on Nitric Acid and Nitrate Measurement was jointly organized by C.N.R. (Consiglio Nazionale delle Ricerche),

70

A. FEBO ET AL

Rome, Italy and the Commission of the European Communities, BruxeUes, Belgium, within the framework of COST project 611 on 'Physico-Chemical Behaviour of Atmospheric Pollutants', Working Party l (Identification and Analysis of Pollutants). The financial support of C.N.R. and C.E.E. is gratefully acknowledged. The authors extend their appreciation to Mr. M. Giusto for assistance in the statistical evaluation of the results and for his va!uabl¢ wor~ on data handling. REFERENCES Air Pollution Research Report 22, 1989. Field Intercomparison Exercise on Nitric Acid and Nitrate Measurement - Methods and Data. I. AUegrini, A. Febo, C. Perrino (Eds.), Rome, Italy. Anlauf, K.G., P. Fellin, A.H. Wiebe, H.I. Schiff, G.I. Mackay, R.S. Braman and R. Gilbert, 1985. A comparison of three methods for measurement of atmospheric nitric acid and aerosol nitrate and ammonium. Atmos. Environ., 19: 325-333. Anlauf, K.G., H.A. Wiebe, E.C. Tuazon, A.M. Winer, G.I. Mackay, H.I. Schiff, T.G. Ellestad and K,T. Knapp, 1991. Intercomparison of atmospheric nitric acid measurements at elevated ambient concentrations. Atmos. Environ., 25A: 393-399. Appel, B.R., S.M. Wall, Y. Tokiwa and M. H a i l 1979. Interference effects in sampling particulate~nitrate in ambient air. Atmos. Environ., 13: 319-325. Appel, B.R., S.M. Wall, Y. Tokiwa and M. Haik, 1980. Simultaneous nitric acid, particulate nitrate and acidity measurements in ambient air. Atmos. Environ., 15: 283-289. Appel, B.R., V. Povard and E.L. Kothny, 1988. Loss of nitric acid within inlet devices intended to exclude coarse particles during atmospheric sampling. Atmos. Environ., 22: 2535-2540. De Santis, F., A. Febo and C. Perrino, 1988. Negative interference of Teflon sampling devices in the determination of nitric acid and particulate nitrate. Sci. Total Environ., 76: 93-99. Febo, A,, F. De Santis and C. Perrino, 1986. Measurement of atmospheric nitrous and nitric acid by means of annular denuders. Proceedings of Physico-chemical Behaviour of Atmospheric Pollutants, IV European Symposium, Stresa 23-25, September 1986. G. Angeletti and G. Restelli (Eds.), Riedel, Dordrecht, pp. 121-125. Febo, A,, F. De Santis, C. Perrino and M. Giusto, 1989. Evaluation of laboratory and field performance of denuder tubes: a theoretical approach. Atmos. Environ., 23:1517-1530. Forrest, J., D.J. Spandau, R.L. Tanner and L. Newman, 1982. Determination of atmospheric nitrate and nitric acid employing a diffusion denuder with a filter pack. Atmos. Environ., 16: 1473-1485. Fox, D.L. et al., 1988. Intercomparison of nitric acid diffusion denuder methods with tunable diode laser ~bsorption spectroscopy. Atmos. Environ., 22: 575-585. Hering, S.V. et al., 1988. The nitric acid shootout: field comparison of measurement methods. Atmos. Environ., 17: 2605-2610. Mulawa, P.A. and S.H. Cadle, 1985. A comparison of nitric acid and particulate nitrate measurements by the penetration and denuder difference methods. Atmos. Environ., 19: 1317-1324. Perrino, C., F. De Santis and A. Febo, 1988. Uptake of nitrous acid and nitrogen oxides by nylon surfaces: imolications for nitric acid measurements. Atmos. Environ., 22: 1925-1930.

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Perrino, C., F. De Santis and A. Febo, 1990. Criteria for the choice of a denuder sampling technique devoted to the measurement of atmospheric nitrous and nitric acids. Atmos. Environ., 24A: 617-626. Scarano, E., C. Calcagno and L. Cignoli, 1979. A reliable source of very small amounts of hydrogen chloride for analytical purposes. Anal. Chim. Acta, 110: 95-106. Spicer, C.W. and P.M. Schumacher, 1979. Particulate nitrate: laboratory and field studies of major sampling interferences. Atmos. Environ., 13: 543-552. Spicer, C.W., J.E. Howes, T.A. Bishop and L.H. Arnold, 1982. Nitric acid measurement methods: an intercomparison. Atmos. Environ., 16: 1487-1500. Sickles, J.E. II, C. Perrino, I. Allegrini, A. Febo, M. Possanzini and R.J. Paur, 1988. Sampling and analysis of ambient air near Los Angeles using an annular denuder system. Atmos. Environ., 22: 1619-1625.