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Size distribution and chemical composition of atmospheric particulate nitrate in the Nagoya area
Atmospheric
Environment
Vol. Il. pp. 671-675.
Pergamon
Press 1977. Printed
in Great
Britain.
SIZE DISTRIBUTION AND CHEMICAL COMPOSITION OF ATMOSPHERIC PARTICULATE NITRATE IN THE NAGOYA AREA SATOSHIKADDWAKI Aichi Environmental Research Center, 74, Nagare, Tsuji-machi, Kita-ku, Nagoya 462, Japan Abstract-The size distribution and chemical composition of atmospheric nitrate particles have been investigated. A variety of size distributions of nitrate was measured in Nagoya, Japan. Air samples were collected and size-fractionated in cascade impactors. Size-classified particles were extracted with water and alcohol. The concentrations of nitrate were measured by specific ion electrode and absorption spectrophotometry. The chemical composition of nitrate was identified by paper chromatography. The size distributions of nitrate were bimodal. One modal dia. was co. 0.4-0.6~m, the other ca. 3-5 pm. The shapes of the size distribution curves changed periodically with the season. From the analysis by paper chromatography, it was shown that the submicron nitrate was NH.+NO, and the coarse nitrate was NaNO,. A strong relationship between size distributions of nitrate and meteorological conditions was found. The mechanisms of nitrate formation in the atmosphere are also discussed.
INTRODUCTION It is important to measure both the size distributions and chemical compositions of particles in ambient air, in order to understand the sources, behaviors and mechanisms of formation of particles in the atmosphere. Nitrate is a principal component associated with secondary aerosols, and generally acknowledged to be produced in the atmosphere by the oxidation of NO*. The size distribution and chemical composition of nitrate have been studied by several groups of workers (Lee and Patterson, 1969; Ltmdgren, 1970; Cunningham et al., 1974) however, these are not clearly established Whitby and coworkers (Whitby et al., 1972a; Whitby et al., 1972b; Whitby et al., 1974; Hidy et al., 1975) have reported that the size distribution of atmospheric aerosols is usually bimodal with mode occurring below 1 pm and another in the S-15 m range. In the previous paper of this series (Kadowaki, 1976), the author represented that the size distributions of nitrate were bimodal like those of total aerosols, and influenced by meteorological conditions significantly. However, the chemical compositions of the bimodal nitrate, mechanisms of formation and chemical origin were not known. The present investigation was undertaken in order to make clear the chemical compositions of the bimoda1 nitrate, and the relationship between size distributions of nitrate and meteorological conditions. Measurements were made in Nagoya from October 1973 to January 1976. Particles were collected and fractionated in an Andersen cascade impactor (Andersen, 1966). The water extracts were analyzed for nitrate. Size distribution curves of nitrate were calculated and analyzed with respect to the influence * 1 cfm = 1 ft3 rnin-’ = 28.3 1 min-‘. 671 A.E.11/x-*
of meteorological conditions. The bimodal nitrate compositions were identified by the paper chromatographic method (O’Brien et al., 1975a; O’Brien et al., 1975b) of the alcohol extracts. From the results of the paper chromatography, the mechanisms of nitrate formation in the atmosphere were discussed.
EXPERIMENTAL Sampling procedure
Air samples were collected at the Aichi Environmental Research Center in the Nagoya area indicated on the map of Fig. 1. Atmospheric particles were fractionated in the eight stages of the Andersen cascade impactor (2COOINC Model 21-000) operated at 1 cfm,* the collecting surfaces were stainless steel plates. A type A Gelman glass fiber filter, 85 mm dia., was used as the backup filter. The size range is shown in Table 1. In order to collect enough particles for chemical analysis, sampling times were 4-9 days, according to air pollution and weather conditions. Analysis of nitrate
Atmospheric particles collected on each impactor plate were scraped with a “policeman” and transferred respectively to 200 cm’ beakers with 150cm3 of distilled water. The suspended solutions were heated slightly and boiled for 2 min, and filtered after cooling. The filtrates were respectively concentrated to 25 cm3 by i.r. lamps. The backup filter was cut into pieces and treated in the same way as the impactor plates. The filtrate was concentrated to 50cm3. The water extracts were analyzed for nitrate. Nitrate was determined by the 2,4-xylenol method (Jutze and Foster, 1967) or the specific ion electrode (ORION Research INC Model 92-07).
672
SATOSHI
KAWWAKI
20 t
7~
t
f’
IO -
/’ NAGOYA City
, I +
E x
5.0 -
0" 2.0 0 P
1.0r
0, NUURA
dustrial
Seosldt Area
G 0.5 t a" 0.2 0.1
m.m.d.
I 5
I IO
I IllIll 20 30
Cumulative
50
mass
70 80
% less
0.90pm I 90
than
I
95
D,
Fig. I. Map of the Nagoya area showing sampling site. Fig. 2. Typical cumulative particle size distribution nitrate in Nagoya. Size distribution curve was calculated as in the previous paper (Kadowaki, 1974). Pqw
chrornutoqraphic
identification
of bimoa’al nitrate
Coarse and submicron nitrates were recovered from impactor plates of stage 1, 2, 3 and stage 6, 7 and 150 cm3 of alcohol (95% ethanol, 5% propanol) respectively, using an ultrasonic cleaner. The solutions of the nitrates in alcohol were refluxed for 2 hr, and filtered after cooling. The filtrates were respectively reduced in volume to 3-5 cm3 by i.r. lamps and transferred to vials. Whatman No. 41 paper was cut into 20cm square sheets and soaked in 2N acetic acid for a few days, and then washed with distilled water. The paper was dried and then soaked in acetone-ethanol (1: 1) for one week, and thoroughly dried. Solutions of 5-10~1 of the extracts were spotted in points from 5 cm the end of the paper. The paper was coiled and placed in a glass jar for elution. The solvent was alcohol-water-ammonia in the proportions 190--10-2. The solvent front was allowed to move 10 cm past the spot. The chromatogram was dried at room temperature.
of
Bromcresol purple (40 mg in 100 cm3 of formaldehyde-ethanol (1:5) adjusted to pH 10 with NaOH) was employed for spray reagent. In the solvent system used. inorganic nitrates apparently underwent a metathetical reaction with aqueous ammonia: sodium nitrate gave two spots (blue and yellow); ammonium nitrate gave one spot (yellow). RESULTS
AND DISCUSSION
Figure 2 shows a typical example of cumulative size distribution of nitrate in urban air in Nagoya. and the histogram and size distribution curve are shown in Fig. 3. The calculation process to represent Fig. 3 is shown in Table 3. As shown in Fig. 3, the shape of the size distribution curve reveals that the nitrates comprise two populations (A’ and B’). These populations are both in agreement with a log-normal distribution approximately. The size of the nitrates
Table I. Effective cutoff diameter (e.c.d.) Stage No.
50% e.c.d. (pm)
Afog, D,*
0 1 2 3 4 5 6 7 Backup filter
11 (<30) 7.0 4.7 3.3 2.1 1.1 0.65 0.43 (0.08)
1.00 0.451 0.398 0.350 0.45 1 0.646 0.525 0.412 1.68
* Alog, D,, = log, D,. _ 1 - log, D,,, D,, : Value of 50% e.c.d. of No. n Stage.
6 I.0 .+ ;: ; f 0.5 ;r :: 0.08
0.430.65
I.1
2.1 3.34.77.0
II
30
Fig. 3. Histogram and size distribution curve of nitrate in Nagoya (1622 May 1974, Cont. 5.4 pg/m3).
Atmospheric particulate nitrate in the Nagoya area
673
Table 2. Results of concentration and m.m.d. of nitrate and meteorological condition Sampling period
Cont. (H/m31
Nitrate (run)
Temp. 0
Ave. Rel. Humidity (%I
Ave. Wind Velocity (m/s)
Main Wind Direction
Nov. 2428173 Nov. 30-Dec. 3/73 Dec. 1520/73 Feb. 18-25174 Mar. 2-7174 Apr. 38174 Apr. 22-30174 May 1621174 June 10-15174 July 2331174 Aug. P-15174 Sept. 612174 Sept. 17-24174 Oct. 1417/74 June 12-18175 July 12-19/75 Aug. 8-14175 Aug. 25-30175 Sept. 23-29175 Oct. 23-28/75
5.7 4.6 1.7 3.7 4.2 3.5 3.9 5.4 3.1 1.3 2.3 1.6 5.6 3.1 2.8 1.2 2.8 3.2 5.2 5.2
0.65 0.62 0.60 0.92 0.66 1.2 1.5 1.6 0.80 3.3 3.7 1.6 0.70 0.52 0.84 0.91 2.4 3.2 0.56 0.62
9.8 8.4 7.0 8.7 11.6 16.7 18.4 23.5 24.4 29.1 31.1 27.6 23.0 19.2 26.9 30.9 29.8 30.4 25.1 14.6
66 69 72 68 73 71 64 75 82 83 80 78 76 69 79 76 76 79 85 74
2.8 2.0 2.5 3.0 2.3 2.5 2.7 2.3 2.1 1.9 1.6 2.8 2.3 2.0 1.5 1.7 3.4 1.3 1.6 1.6
NW NNW NW WNW NW SE, NW WNW S S, NW SE S SSE, NW NW NW S SE S, E SE NW N
Nitrate MMD of
Sample No. in Fig. 4 1 2 3 4 5 6 I 8 9 10 11 12 13 14 15 16 17 18 19 20
Table 3. Determination
Ave.
Cont. of nitrate Am @g/m31
Alog D,
Am/Alog D, @g/m?
:
17 35 126 90
0.095 0.197 0.708 0.505
1.00 0.451 0.350 0.398
0.10 0.44 2.02 1.27
4
105
0.590
0.451
1.31
; 7 Backup filter
115 81 139
0.646 0.455 0.781
0.525 0.646 0.412
0.70 1.23 1.90
251
1.41
1.68
0.84
959
5.4
0
1
Total
129 134 74 96 99 80 92 131 72 75 71 52 115 94 68 57 66 78 87 114
of histogram and size distribution curve of nitrate
Weight of nitrate AM @g)
Stage
Total Aerosol Cont. &/m”J
-
-
Sampling period: May 16-22, 1974; Air volume samples: 178 m3.
belonging to A’ were mainly 0.4-0.6~, and those to B’ were mainly 3-5 q. The atmospheric nitrates
invariably consisted of two separate populations, giving a bimodal distribution throughout the measurements, though the shape of the size distribution curve changed remarkably. Figure 4 shows twenty size distribution curves of nitrate in Nagoya from November 1973 to October 1975. Table 2 summarizes the concentrations and mass median diameters (m.m.d.‘s) of the nitrate in Fig. 4, and the meteorological conditions of sampling periods. The results shown in Fig. 4 and Table 2 indicate that the shapes of the nitrate size distribution curves change periodically with the meteorological conditions of the season, and are divided into the following three patterns. In the autumn and winter period, the shapes of nitrate size distribution curves represented the first pattern, in which submicron nitrates belonging to A’ population were predominant: Sample No. 1, 2, 13, I9 and 20 in Fig. 4. On the other hand, in the summer
period, the shapes of nitrate size distribution curves represented the second pattern in which coarse nitrates belonging to B’ population were predominant: Sample No. 11, 17 and 18 in Fig. 4. The third pattern in the spring period was intermediate of the two patterns presented above: Sample No. 6, 7, 8 and 15 in Fig. 4. It is clear that the above periodic change of nitrate size distribution arises from meteorological factors, since the quantity of gaseous nitrogen oxide compounds, the material of nitrate formation in air, is almost constant throughout the year in the Nagoya area. The meteorological factors in the Nagoya area are characterized by the results summarized in Table 2. The resulting m.m.d. values of the nitrate are useful for comparison of nitrate particle size. even though the distribution comprises two populations as seen in Fig. 3. The nitrate m.m.d values were predominantly influenced by the wind direction among the
SATOSHIKADCIWAKI a single yellow (acidic) spot. On the other hand, sodium nitrate gave two spots: a blue (basic) spot due to sodium hydroxide and a yellow (acidic) spot due to ammonium nitrate. Figure Sb shows a paper chromatogram of the bimodal nitrates: (3) the coarse nitrates belonging to B’ population; (4) the submicron nitrates belonging to A’ population; (5) un-fractionated nitrates. The coarse nitrates gave two spots. blue and yellow, which were in good agreement with those of sodium and nitrate respectively. The nitrates belonging to B’ population were identified as sodium nitrate. The submicron nitrates gave three yellow spots. One of them was identical as ammonium nitrate, and it was assumed that the other two spots were dicarboxylic acid and monocarboxylic acid from the results of O’Brien et al. (1975b). It is generally believed that nitric acid, the key intermediary in nitrate formation, can be pfoduced in the atmosphere through the following reactions: N0,+03--‘N03+02
(1)
NOz + NO, + N,O, Fig. 4. Size distribution curves of atmospheric Nagoya during 1973-1975.
nitrate in
meteorological factors in Table 2. The first pattern occurred under the condition of the wind blowing from the northwest, and the second pattern occurred under the condition of the wind blowing from the south. In the Nagoya area shown in Fig. 1, the seasonal wind blows alternately from the northwest and the south throughout the year, and sea-salt aerosols are transported by the south wind. Therefore, it is assumed that the coarse nitrates belonging to B’ population are carried from the sea by the south wind, or formed in the atmosphere through the reaction between gaseous species and particles from ocean spray, and that the rate of formation of the submicron nitrates belonging to A’ population decreases with an increase in the amount of particles from ocean spray. It is very important to make clear the chemical compositions of the bimodal nitrate as these contribute to an understanding of the mechanism of formation and the behaviour of nitrate in the atmosphere. As shown in Fig. 3, the nitrates collected on Stage 6 and 7 were submicron particles belonging to A’ population, and the nitrates collected on Stage 1, 2 and 3 were coarse particles belonging to B’ population. These samples were extracted in alcohol and analyzed by paper chromatography. Sulfates and nitrates comprise the bulk of the salts in urban particles. Nitrates are fairly soluble in alcohol while sulfates are almost insoluble. Therefore, the major inorganic portion of the alcohol extract would be nitrates. Figure 5a shows a paper chromatogram of the reference compounds sprayed with the acid-base indicator to make the spots visible: (1) ammonium nitrate: (2) sodium nitrate. Ammonium nitrate gave
Nz05
+ HzO--t2
(2)
HNO,
(3)
or NO,(+M)+
bH+HNO,(+M).
(4)
Nitrates are formed by means of reaction between gaseous or solid species with nitric acid. The prevalent form of nitrate in urban air is ammonium nitrate resulting reaction with ammonia: NH, + HNO, + NH,NO,.
(5)
Nitrates are also often found in coastal cities. These nitrates will be formed as a result from reactions between gaseous and particles from ocean spray: NaCl + HNO, -
NaNO,
+ HCl.
0’
(6)
Yellowspot
-------
leference ompounds
Fig. 5. Paper chromatographic identification of bimodal nitrate compositions. a. (1) ammonium nitrate; (2) sodium nitrate. b. (3) nitrate of B’ population (Stage 1-3, particle dia. 3.3-11 pm); (4) nitrate of A’ population (Stage 6-7. particle dia. 0.43-l .l 0); (5) un-fractionated nitrate.
Atmospheric particulate nitrate in the Nagoya area
675
and sodium nitrate respectively by paper chromatography. The results by paper chromatography were completely consistent with the size distributions obtained and the mechanisms responsible for nitrate formation in the atmosphere. Acknowle~e~ents-me author takes this opportunity to express his thanks to Dr. K. Yoshimoto for his continuing interest and encouragement, and to Mr. K. Satou for meteorological data.
REFERENCES 0. I
0.5
Particle
I
5
dia.,
IO
pm
Fig. 6. Idealized mass/size distribution and chemical composition for atmospheric nitrate.
The nitrate belonging to A’ population was identified as ammonium nitrate by paper chromatography. Furthermore. the size distribution of atmospheric ammonium is in good agreement with that of A ~pulation, and very little a~onium exists in the size range greater than 2 pm (Kadowaki, 1976). The nitrate belonging to B’ population was identified as sodium nitrate by paper chromatography, and had 3-10~ diameter. In maritime areas, airborne sea-
salt aerosols fall in the size range l-10 pm (Fennelly, 1975) so that the sodium nitrate formed by means of reaction (6) must have 1-10~ diameter. Furthermore, the mount of the nitrate belon~ng to B’ population increased when sea-salt aerosols were transported by the south wind. It is, hence, concluded that the chemical compositions of the nitrates belonging to A’ and B’ ~puIations are a~o~urn nitrate and sodium nitrate, respectively. An idealized bimodal mass distribution of atmospheric nitrates with a fractionation of chemical compositions is shown in Fig. 6.
SUMMARY
Size distributions and chemical imposition were described for the Nagoya area nitrates. The size distributions were bimodal (A’ and B’ populations shown in Fig. 3). The modal diameter of A’ was cu. 0.4-0.6m11; that of B’ was cu. 3-5,~m. The shapes of nitrate size distribution curves changed periodically with the meteorological conditions of the season. The chemical compositions of the bimodal nitrates, A’ and B’ populations. were identified as a~onium nitrate
Andersen A. A. (1969) A sampler for respiratory health hazard assessment. Am. Ind. Hyg. Ass. J. 27, i60-165. Cunnin~am P. T., Johonson S. A. and Yang R. T. (1974) Variations in chemistry of airborne particulate material with particle size and time. Environ. Sci. Technol. 8, 13l135. Fennelly P. F. (1975) Primary and secondary part&mates as ~llu~ts. J. Air Pollut. Control Ass. 25, 697-704. Hidy G. M., Appel B. R., Charlson R. J., Clark W. E., Friedlander S. K., Hutchison D. H., Smith T. B., Suder J., Wesolowski J. J. and Whitby K. T. (1975) Summary of the California aerosol characterization experiment. J. Air Poliut. Control Ass. 25, 1106-l 114. Jutze G. A. and Foster K. E. (1967) Recommended standard method for atmospheric sampling of fine particulate matter by filter media-High-volume sampler. J. Air Pollut. Control Ass. 17, 17-25. Kadowaki S. (1974) Deter~nation of size distribution aerosols in urban air by the Andersen sampler. Japan Analyst 23, 490-495. Kadowaki S. (1976) Size distribution of atmospheric total aerosols, sulfate, ammonium and nitrate particulates in the Nagoya area. Atmospheric Environment 10, 39-43. Lee R. E. and Patterson R. K. (1969) Size determination of atmospheric phosphate, nitrate, chloride and ammonium particulate in several urban areas. Atmospheric Environment 3, 249-255.
Lundgren D. A. (1970) Atmosphe~c aerosol composition and concentration as a function of particle size and of time. J. Air Pollut. Control Ass. 20, 603-608. O’Brien R. J., Holmes J. R. and Bockian A. H. (1975a) Formation of photochemical aerosol from hydrocarbons, chemical reactivity and products. ~~v~ro~. Sci. Technol. 9, 568-576. O’Brien R. J., Crabtree J. H., Holmes J. R., Hoggan M. C. and Bockian A. H. (1975b) Formation of photochemical aerosol from hydrocarbons, atmospheric analysis. Environ. Sci. Tech&. 9, 577-582. Whitby K. T., Husar R. B. and Liu B. Y. H. (1972a) The aerosol size distibution of Los Angeles smog. J. Colloid Interface Sci. 39, 177-204. Whitby K. T., Liu B. Y. H., Husar R. B. and Barsic N. J. (1972b) The Minnesota aerosol-~alyzing system used in the Los Angeles smog project. J. Colioid Interface sci. 39, 136-164. Whitby K. T., Charlson R. E., Wilson W. E. and Stevens R. K. (1974) The size of suspended particle matter in air. Science 183, 1098-1099.
Environment
Vol. Il. pp. 671-675.
Pergamon
Press 1977. Printed
in Great
Britain.
SIZE DISTRIBUTION AND CHEMICAL COMPOSITION OF ATMOSPHERIC PARTICULATE NITRATE IN THE NAGOYA AREA SATOSHIKADDWAKI Aichi Environmental Research Center, 74, Nagare, Tsuji-machi, Kita-ku, Nagoya 462, Japan Abstract-The size distribution and chemical composition of atmospheric nitrate particles have been investigated. A variety of size distributions of nitrate was measured in Nagoya, Japan. Air samples were collected and size-fractionated in cascade impactors. Size-classified particles were extracted with water and alcohol. The concentrations of nitrate were measured by specific ion electrode and absorption spectrophotometry. The chemical composition of nitrate was identified by paper chromatography. The size distributions of nitrate were bimodal. One modal dia. was co. 0.4-0.6~m, the other ca. 3-5 pm. The shapes of the size distribution curves changed periodically with the season. From the analysis by paper chromatography, it was shown that the submicron nitrate was NH.+NO, and the coarse nitrate was NaNO,. A strong relationship between size distributions of nitrate and meteorological conditions was found. The mechanisms of nitrate formation in the atmosphere are also discussed.
INTRODUCTION It is important to measure both the size distributions and chemical compositions of particles in ambient air, in order to understand the sources, behaviors and mechanisms of formation of particles in the atmosphere. Nitrate is a principal component associated with secondary aerosols, and generally acknowledged to be produced in the atmosphere by the oxidation of NO*. The size distribution and chemical composition of nitrate have been studied by several groups of workers (Lee and Patterson, 1969; Ltmdgren, 1970; Cunningham et al., 1974) however, these are not clearly established Whitby and coworkers (Whitby et al., 1972a; Whitby et al., 1972b; Whitby et al., 1974; Hidy et al., 1975) have reported that the size distribution of atmospheric aerosols is usually bimodal with mode occurring below 1 pm and another in the S-15 m range. In the previous paper of this series (Kadowaki, 1976), the author represented that the size distributions of nitrate were bimodal like those of total aerosols, and influenced by meteorological conditions significantly. However, the chemical compositions of the bimodal nitrate, mechanisms of formation and chemical origin were not known. The present investigation was undertaken in order to make clear the chemical compositions of the bimoda1 nitrate, and the relationship between size distributions of nitrate and meteorological conditions. Measurements were made in Nagoya from October 1973 to January 1976. Particles were collected and fractionated in an Andersen cascade impactor (Andersen, 1966). The water extracts were analyzed for nitrate. Size distribution curves of nitrate were calculated and analyzed with respect to the influence * 1 cfm = 1 ft3 rnin-’ = 28.3 1 min-‘. 671 A.E.11/x-*
of meteorological conditions. The bimodal nitrate compositions were identified by the paper chromatographic method (O’Brien et al., 1975a; O’Brien et al., 1975b) of the alcohol extracts. From the results of the paper chromatography, the mechanisms of nitrate formation in the atmosphere were discussed.
EXPERIMENTAL Sampling procedure
Air samples were collected at the Aichi Environmental Research Center in the Nagoya area indicated on the map of Fig. 1. Atmospheric particles were fractionated in the eight stages of the Andersen cascade impactor (2COOINC Model 21-000) operated at 1 cfm,* the collecting surfaces were stainless steel plates. A type A Gelman glass fiber filter, 85 mm dia., was used as the backup filter. The size range is shown in Table 1. In order to collect enough particles for chemical analysis, sampling times were 4-9 days, according to air pollution and weather conditions. Analysis of nitrate
Atmospheric particles collected on each impactor plate were scraped with a “policeman” and transferred respectively to 200 cm’ beakers with 150cm3 of distilled water. The suspended solutions were heated slightly and boiled for 2 min, and filtered after cooling. The filtrates were respectively concentrated to 25 cm3 by i.r. lamps. The backup filter was cut into pieces and treated in the same way as the impactor plates. The filtrate was concentrated to 50cm3. The water extracts were analyzed for nitrate. Nitrate was determined by the 2,4-xylenol method (Jutze and Foster, 1967) or the specific ion electrode (ORION Research INC Model 92-07).
672
SATOSHI
KAWWAKI
20 t
7~
t
f’
IO -
/’ NAGOYA City
, I +
E x
5.0 -
0" 2.0 0 P
1.0r
0, NUURA
dustrial
Seosldt Area
G 0.5 t a" 0.2 0.1
m.m.d.
I 5
I IO
I IllIll 20 30
Cumulative
50
mass
70 80
% less
0.90pm I 90
than
I
95
D,
Fig. I. Map of the Nagoya area showing sampling site. Fig. 2. Typical cumulative particle size distribution nitrate in Nagoya. Size distribution curve was calculated as in the previous paper (Kadowaki, 1974). Pqw
chrornutoqraphic
identification
of bimoa’al nitrate
Coarse and submicron nitrates were recovered from impactor plates of stage 1, 2, 3 and stage 6, 7 and 150 cm3 of alcohol (95% ethanol, 5% propanol) respectively, using an ultrasonic cleaner. The solutions of the nitrates in alcohol were refluxed for 2 hr, and filtered after cooling. The filtrates were respectively reduced in volume to 3-5 cm3 by i.r. lamps and transferred to vials. Whatman No. 41 paper was cut into 20cm square sheets and soaked in 2N acetic acid for a few days, and then washed with distilled water. The paper was dried and then soaked in acetone-ethanol (1: 1) for one week, and thoroughly dried. Solutions of 5-10~1 of the extracts were spotted in points from 5 cm the end of the paper. The paper was coiled and placed in a glass jar for elution. The solvent was alcohol-water-ammonia in the proportions 190--10-2. The solvent front was allowed to move 10 cm past the spot. The chromatogram was dried at room temperature.
of
Bromcresol purple (40 mg in 100 cm3 of formaldehyde-ethanol (1:5) adjusted to pH 10 with NaOH) was employed for spray reagent. In the solvent system used. inorganic nitrates apparently underwent a metathetical reaction with aqueous ammonia: sodium nitrate gave two spots (blue and yellow); ammonium nitrate gave one spot (yellow). RESULTS
AND DISCUSSION
Figure 2 shows a typical example of cumulative size distribution of nitrate in urban air in Nagoya. and the histogram and size distribution curve are shown in Fig. 3. The calculation process to represent Fig. 3 is shown in Table 3. As shown in Fig. 3, the shape of the size distribution curve reveals that the nitrates comprise two populations (A’ and B’). These populations are both in agreement with a log-normal distribution approximately. The size of the nitrates
Table I. Effective cutoff diameter (e.c.d.) Stage No.
50% e.c.d. (pm)
Afog, D,*
0 1 2 3 4 5 6 7 Backup filter
11 (<30) 7.0 4.7 3.3 2.1 1.1 0.65 0.43 (0.08)
1.00 0.451 0.398 0.350 0.45 1 0.646 0.525 0.412 1.68
* Alog, D,, = log, D,. _ 1 - log, D,,, D,, : Value of 50% e.c.d. of No. n Stage.
6 I.0 .+ ;: ; f 0.5 ;r :: 0.08
0.430.65
I.1
2.1 3.34.77.0
II
30
Fig. 3. Histogram and size distribution curve of nitrate in Nagoya (1622 May 1974, Cont. 5.4 pg/m3).
Atmospheric particulate nitrate in the Nagoya area
673
Table 2. Results of concentration and m.m.d. of nitrate and meteorological condition Sampling period
Cont. (H/m31
Nitrate (run)
Temp. 0
Ave. Rel. Humidity (%I
Ave. Wind Velocity (m/s)
Main Wind Direction
Nov. 2428173 Nov. 30-Dec. 3/73 Dec. 1520/73 Feb. 18-25174 Mar. 2-7174 Apr. 38174 Apr. 22-30174 May 1621174 June 10-15174 July 2331174 Aug. P-15174 Sept. 612174 Sept. 17-24174 Oct. 1417/74 June 12-18175 July 12-19/75 Aug. 8-14175 Aug. 25-30175 Sept. 23-29175 Oct. 23-28/75
5.7 4.6 1.7 3.7 4.2 3.5 3.9 5.4 3.1 1.3 2.3 1.6 5.6 3.1 2.8 1.2 2.8 3.2 5.2 5.2
0.65 0.62 0.60 0.92 0.66 1.2 1.5 1.6 0.80 3.3 3.7 1.6 0.70 0.52 0.84 0.91 2.4 3.2 0.56 0.62
9.8 8.4 7.0 8.7 11.6 16.7 18.4 23.5 24.4 29.1 31.1 27.6 23.0 19.2 26.9 30.9 29.8 30.4 25.1 14.6
66 69 72 68 73 71 64 75 82 83 80 78 76 69 79 76 76 79 85 74
2.8 2.0 2.5 3.0 2.3 2.5 2.7 2.3 2.1 1.9 1.6 2.8 2.3 2.0 1.5 1.7 3.4 1.3 1.6 1.6
NW NNW NW WNW NW SE, NW WNW S S, NW SE S SSE, NW NW NW S SE S, E SE NW N
Nitrate MMD of
Sample No. in Fig. 4 1 2 3 4 5 6 I 8 9 10 11 12 13 14 15 16 17 18 19 20
Table 3. Determination
Ave.
Cont. of nitrate Am @g/m31
Alog D,
Am/Alog D, @g/m?
:
17 35 126 90
0.095 0.197 0.708 0.505
1.00 0.451 0.350 0.398
0.10 0.44 2.02 1.27
4
105
0.590
0.451
1.31
; 7 Backup filter
115 81 139
0.646 0.455 0.781
0.525 0.646 0.412
0.70 1.23 1.90
251
1.41
1.68
0.84
959
5.4
0
1
Total
129 134 74 96 99 80 92 131 72 75 71 52 115 94 68 57 66 78 87 114
of histogram and size distribution curve of nitrate
Weight of nitrate AM @g)
Stage
Total Aerosol Cont. &/m”J
-
-
Sampling period: May 16-22, 1974; Air volume samples: 178 m3.
belonging to A’ were mainly 0.4-0.6~, and those to B’ were mainly 3-5 q. The atmospheric nitrates
invariably consisted of two separate populations, giving a bimodal distribution throughout the measurements, though the shape of the size distribution curve changed remarkably. Figure 4 shows twenty size distribution curves of nitrate in Nagoya from November 1973 to October 1975. Table 2 summarizes the concentrations and mass median diameters (m.m.d.‘s) of the nitrate in Fig. 4, and the meteorological conditions of sampling periods. The results shown in Fig. 4 and Table 2 indicate that the shapes of the nitrate size distribution curves change periodically with the meteorological conditions of the season, and are divided into the following three patterns. In the autumn and winter period, the shapes of nitrate size distribution curves represented the first pattern, in which submicron nitrates belonging to A’ population were predominant: Sample No. 1, 2, 13, I9 and 20 in Fig. 4. On the other hand, in the summer
period, the shapes of nitrate size distribution curves represented the second pattern in which coarse nitrates belonging to B’ population were predominant: Sample No. 11, 17 and 18 in Fig. 4. The third pattern in the spring period was intermediate of the two patterns presented above: Sample No. 6, 7, 8 and 15 in Fig. 4. It is clear that the above periodic change of nitrate size distribution arises from meteorological factors, since the quantity of gaseous nitrogen oxide compounds, the material of nitrate formation in air, is almost constant throughout the year in the Nagoya area. The meteorological factors in the Nagoya area are characterized by the results summarized in Table 2. The resulting m.m.d. values of the nitrate are useful for comparison of nitrate particle size. even though the distribution comprises two populations as seen in Fig. 3. The nitrate m.m.d values were predominantly influenced by the wind direction among the
SATOSHIKADCIWAKI a single yellow (acidic) spot. On the other hand, sodium nitrate gave two spots: a blue (basic) spot due to sodium hydroxide and a yellow (acidic) spot due to ammonium nitrate. Figure Sb shows a paper chromatogram of the bimodal nitrates: (3) the coarse nitrates belonging to B’ population; (4) the submicron nitrates belonging to A’ population; (5) un-fractionated nitrates. The coarse nitrates gave two spots. blue and yellow, which were in good agreement with those of sodium and nitrate respectively. The nitrates belonging to B’ population were identified as sodium nitrate. The submicron nitrates gave three yellow spots. One of them was identical as ammonium nitrate, and it was assumed that the other two spots were dicarboxylic acid and monocarboxylic acid from the results of O’Brien et al. (1975b). It is generally believed that nitric acid, the key intermediary in nitrate formation, can be pfoduced in the atmosphere through the following reactions: N0,+03--‘N03+02
(1)
NOz + NO, + N,O, Fig. 4. Size distribution curves of atmospheric Nagoya during 1973-1975.
nitrate in
meteorological factors in Table 2. The first pattern occurred under the condition of the wind blowing from the northwest, and the second pattern occurred under the condition of the wind blowing from the south. In the Nagoya area shown in Fig. 1, the seasonal wind blows alternately from the northwest and the south throughout the year, and sea-salt aerosols are transported by the south wind. Therefore, it is assumed that the coarse nitrates belonging to B’ population are carried from the sea by the south wind, or formed in the atmosphere through the reaction between gaseous species and particles from ocean spray, and that the rate of formation of the submicron nitrates belonging to A’ population decreases with an increase in the amount of particles from ocean spray. It is very important to make clear the chemical compositions of the bimodal nitrate as these contribute to an understanding of the mechanism of formation and the behaviour of nitrate in the atmosphere. As shown in Fig. 3, the nitrates collected on Stage 6 and 7 were submicron particles belonging to A’ population, and the nitrates collected on Stage 1, 2 and 3 were coarse particles belonging to B’ population. These samples were extracted in alcohol and analyzed by paper chromatography. Sulfates and nitrates comprise the bulk of the salts in urban particles. Nitrates are fairly soluble in alcohol while sulfates are almost insoluble. Therefore, the major inorganic portion of the alcohol extract would be nitrates. Figure 5a shows a paper chromatogram of the reference compounds sprayed with the acid-base indicator to make the spots visible: (1) ammonium nitrate: (2) sodium nitrate. Ammonium nitrate gave
Nz05
+ HzO--t2
(2)
HNO,
(3)
or NO,(+M)+
bH+HNO,(+M).
(4)
Nitrates are formed by means of reaction between gaseous or solid species with nitric acid. The prevalent form of nitrate in urban air is ammonium nitrate resulting reaction with ammonia: NH, + HNO, + NH,NO,.
(5)
Nitrates are also often found in coastal cities. These nitrates will be formed as a result from reactions between gaseous and particles from ocean spray: NaCl + HNO, -
NaNO,
+ HCl.
0’
(6)
Yellowspot
-------
leference ompounds
Fig. 5. Paper chromatographic identification of bimodal nitrate compositions. a. (1) ammonium nitrate; (2) sodium nitrate. b. (3) nitrate of B’ population (Stage 1-3, particle dia. 3.3-11 pm); (4) nitrate of A’ population (Stage 6-7. particle dia. 0.43-l .l 0); (5) un-fractionated nitrate.
Atmospheric particulate nitrate in the Nagoya area
675
and sodium nitrate respectively by paper chromatography. The results by paper chromatography were completely consistent with the size distributions obtained and the mechanisms responsible for nitrate formation in the atmosphere. Acknowle~e~ents-me author takes this opportunity to express his thanks to Dr. K. Yoshimoto for his continuing interest and encouragement, and to Mr. K. Satou for meteorological data.
REFERENCES 0. I
0.5
Particle
I
5
dia.,
IO
pm
Fig. 6. Idealized mass/size distribution and chemical composition for atmospheric nitrate.
The nitrate belonging to A’ population was identified as ammonium nitrate by paper chromatography. Furthermore. the size distribution of atmospheric ammonium is in good agreement with that of A ~pulation, and very little a~onium exists in the size range greater than 2 pm (Kadowaki, 1976). The nitrate belonging to B’ population was identified as sodium nitrate by paper chromatography, and had 3-10~ diameter. In maritime areas, airborne sea-
salt aerosols fall in the size range l-10 pm (Fennelly, 1975) so that the sodium nitrate formed by means of reaction (6) must have 1-10~ diameter. Furthermore, the mount of the nitrate belon~ng to B’ population increased when sea-salt aerosols were transported by the south wind. It is, hence, concluded that the chemical compositions of the nitrates belonging to A’ and B’ ~puIations are a~o~urn nitrate and sodium nitrate, respectively. An idealized bimodal mass distribution of atmospheric nitrates with a fractionation of chemical compositions is shown in Fig. 6.
SUMMARY
Size distributions and chemical imposition were described for the Nagoya area nitrates. The size distributions were bimodal (A’ and B’ populations shown in Fig. 3). The modal diameter of A’ was cu. 0.4-0.6m11; that of B’ was cu. 3-5,~m. The shapes of nitrate size distribution curves changed periodically with the meteorological conditions of the season. The chemical compositions of the bimodal nitrates, A’ and B’ populations. were identified as a~onium nitrate
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