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The stability of particulate nitrate in the Los Angeles atmosphere
The Science o f the Total Environment, 25 (1982) 263--275 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
263
THE STABILITY OF PARTICULATE NITRATE IN THE LOS ANGELES ATMOSPHERE
DANIEL GROSJEAN
Environmental Research & Technology, Inc., 2625 Townsgate Road, Westlake Village, CA 91361 (U.S.A.) (Received March 30th, 1982; accepted May 10th, 1982)
ABSTRACT Simultaneous measurements of atmosphere particulate nitrate (NO3) and its gas phase precursor nitric acid (HONO2) were conducted in Los Angeles during severe smog episodes, and the results were compared to those predicted on the basis of thermodynamic data for the HONO2 (g) + NH3 (g) ----NH4 NO3 (s) equilibrium. Over the wide range of conditions studied (four-hour averaged NO~ --- 0.5--44.3 pg m -3, HONO2 -- 1.4--36.0 pg m -3, T = 10--33°C, humidity = 16--99%, ozone up to 460 ppb) the solid equilibrium model was found to be applicable to only about one-third of the total number of data sets. In this case good agreement was found between measured nitrate values and those expected from comparison of measured and equilibrium nitric acid concentrations. Samples collected a~ humidities above the deliquescent point of ammonium nitrate at the sampling temperature, for which the solid NH4NO 3 equilibrium no longer applies and solution chemistry must be considered, accounted for two-thirds of the data sets. F o r this subset, good agreement was also found between experimental results and theoretical considerations.
INTRODUCTION: N I T R A T E STABILITY AND SAMPLING CONSIDERATIONS
The possibility of significant bias when measuring particulate atmospheric inorganic nitrate collected on filters has received increasing attention during the past recent years. Although reliable measurements of particulate nitrate and its atmospheric precursor, nitric acid are of critical importance in developing a better understanding of the transformations of oxides of nitrogen relevant to tropospheric chemistry including urban pollution and acid precipitation [1], existing nitrate data may contain large uncertainties reflecting two types of errors, a positive bias resulting from adsorption of nitric acid on alkaline filters and from reaction of nitric acid with particulate salts during sampling [2], and a negative bias resulting from displacement by stronger acids, e.g., HC1 [3] and H2SO4 [4] as well as from the thermal instability of nitrate salts such as ammonium nitrate [ 5]. Extensive laboratory and field studies [2] have shown that adsorption of nitric acid, which is highly substrate-dependent, is the major cause of positive 0048-9697/82/0000--0000/$02.75
©1982 Elsevier Scientific Publishing Company
264 bias (positive "artifact" nitrate). These studies have shown that, over a wide range of sampling conditions, adsorption of nitric acid is negligible on Teflon filters [6] which have thus recently become the substrate of choice for sampling of atmospheric nitrate [7, 8]. Reaction of nitric acid with deposited salts, e.g.,: HNO 3 (g) + NaC1 (s) --> HC1 (g) + NaNO 3
(1)
may also introduce a positive bias in nitrate measurements. Although the kinetics of reactions such as (1) have received little attention at the low reactant concentrations relevant to atmospheric conditions, reaction ( 1 ) a p pears rapid enough for atmospheric nitric acid to be quantitatively collected on NaCl-impregnated filters [9, 1 0 ] . The corresponding bias may be of concern when sampling marine or rural atmospheres, but is n o t significant in urban areas such as Los Angeles where particulate NaC1 concentrations derived from sea salt are low compared to nitrate levels [ 11 ]. Negative bias due to displacement of nitric acid from its salts by stronger acids, e.g.: H2SO 4 + N H 4 N O 3 --> NH4HSO 4 (s) + HNO s (g)
(2)
has received some attention [3, 4] and may be significant in the case of acidic aerosols in ammonia-deficient atmospheres. In the case of the Los Angeles aerosol, which is the object of this study, particulate sulfate has been shown to consist essentially of ammonium salts [ 1 1 ] , and the resulting low aerosol acidity makes the contribution of reaction (2) of minor importance. Of major importance, however, is the possibility of negative bias resulting from the strong and inverse temperature dependence of the stability of a m m o n i u m nitrate [5] which is the major c o m p o n e n t of nitrate aerosol in Los Angeles air [12, 13]. At ambient humidities lower than the deliquescent point of NH 4 NO3, the stability of NH4 NO3 in ambient air is represented by the simple equilibrium between solid NH 4 NO 3 and its gas phase precursors: N H 4 N O 3 (s) ~ NH 3 (g) + HONO 2 (g)
(3)
and thus depends only on the precursor concentrations and on temperature. The corresponding equilibrium constant is a strong inverse function of temperature [5, 14] : In Kc -
70.68 -- (24,090/T) -- 6.04 In (T/298)
(4)
where Kc -- (HONO2) (NH3) is in units of ppm 2 . At ambient humidities higher than the deliquescent point of NH4NO3, eqn (4) no longer applies and aerosol aqueous droplet chemistry must be considered. Tang [ 15] and Stelson and Seinfeld [16] have presented detailed theoretical considerations for the aqueous aerosol situation. Unfortunately, the temperature dependence of nitrate stability in aqueous aerosols is still unknown. Stelson et al. [5] have examined experimental data for nitrate aerosol in Los Angeles [11--13] and concluded that reported nitrate levels were
265 consistent with those predicted from the equilibrium between solid amm o n i u m nitrate and its precursors, i.e., equation (3). However, most literature data for particulate nitrate [11--13] and its gaseous precursors [17] include only daytime samples collected during the smog season {summer and early fall) when meteorological and air quality conditions are obviously more conducive to the formation of solid, rather than aqueous droplet, aerosol nitrate. In this study, simultaneous measurements of particulate nitrate and nitric acid were performed as part of an experimental investigation of the atmospheric transformations of oxides of nitrogen in Los Angeles air [8]. Daytime and nighttime samples were collected over a wide range of ambient temperature, humidity and air quality. The corresponding results are discussed here in terms of the applicability of solid NH 4 NO 3 equilibrium considerations to atmospheric particulate nitrate in Los Angeles, an area where high levels of nitrate, e.g., 30--40 pg m -3 , have been consistently measured and appear to be unique among urban areas in the United States and perhaps the world. As is discussed in detail in the following sections, it was found that the solid equilibrium model has limited application, i.e., does n o t apply to a b o u t t w o thirds of the total number of nitrate samples collected round-the-clock during the period studied. For the limited number of situations consistent with the solid N H 4 N O 3 equilibrium model, good agreement was found between experimental measurements and theoretical considerations. For the larger number of situations where aqueous chemistry must be considered, good agreement was also observed between theory and experimental data.
EXPERIMENTAL SECTION Particulate nitrate and nitric acid were collected using dual filter units m o u n t e d on a sequential filter sampler (SFS) operated at a sampling flow rate of 201 min-1. The dual filter unit consisted of an upstream Teflon filter (47 mm diameter, Millipore Corp. FALP 4700, 1 pm pore size} and a downstream nylon filter (47 mm diameter, Ghia Corp. No. M8PL, I pm pore size) m o u n t e d in series using 47 mm filter holders and adapters (Millipore Corp. No. 430400 and 470400, respectively}. Detailed SFS operations and on-site flow calibrations are given elsewhere [ 8]. After sampling, Teflon filters were placed in 47 mm diameter petrislides (Millipore Corp.) and nylon filters were immersed in pyrex tubes containing the buffer solution employed for ion chromatographic analysis [8, 18]. All samples were collected in Claremont, CA, a b o u t 45 km east of Los Angeles, on the Harvey Mudd College campus [8]. Samples were collected round the clock during smog episodes (ozone max up to 460 ppb) on September 22--23 (six consecutive four-hour samples}, September 25--27, O c t o b e r 1--3 (twelve four-hour samples} and October 7--9 (six four-hour samples followed by twelve two-hour samples}. Nitrate collected on Teflon and nylon filters are listed in Table 1 along with temperature and humidity
266 TABLE 1 P A R T I C U L A T E N I T R A T E , N I T R I C A C I D , T E M P E R A T U R E , H U M I D I T Y , A N D EQUIL I B R I U M N I T R I C A C I D C O N C E N T R A T I O N , C L A R E M O N T , CA, S E P T . - - O C T . 1980
Date (1980)
9/22 9/23
9/25
9/26
9]26--27
10/1
10]1--2 10/2
10/7
1017--8 10]8
Time (PDT)
N i t r a t e , gg m -3
T (°C)
Humidity
HONO2
b
%
Above deliquescenee a
pg m-3
Achieved c
Teflon filter
Nylon filter
16--20 20-24 0-4 4--8 8--12 12--16
5.9 6.1 29.3 44.3 11.5 1.9
4.0 1.4 1.6 2.6 21.0 16.0
23.4 14.8 12.2 15.8 28.0 32.6
76 98 99 95 64 46
+ ~-~ ~&d d
12.7 4.2 3.0 4.8 21.7 39
8--12 12--16 16--20 20-24 0-4 4--8 9--13 13--17 17--21 21--1 1--5 5--9
3.3 2.9 2.5 9.8 11.9 26.3 5.5 1.3 0.7 2.1 13.3 8.3
12.1 17.4 12.4 4.2 4.2 4.6 26.6 21.2 13.0 5.4 4.8 3.6
24.3 30.8 25.0 17.3 15.2 15.1 27.8 30.1 19.8 14.3 12.2 15.5
31 31 71 94 84 77 64 44 70 94 89 84
---
-÷ -b + -{-
14.0 32 15.7 5.5 4.4 4.3 21.5 29.2 8.1 3.9 3.0 4.5
10--14 14--18 18--22 22--2 2--6 6--10 10--14 14--18 18--22 22--2 2--6 6--10
0.9 1.3 1.9 20.3 19.5 6.1 1.3 0.7 1.1 33.1 17.3 14.3
3.6 12.0 11.2 8.2 6.2 13.4 36.0 22.4 8.4 13.8 9.4 16.4
33.4 33.0 21.4 17.9 15.3 19.7 30.7 28.9 19.5 15.6 13.4 16.3
16 38 81 76 59 44 32 42 88 88 75 72
--+ + -----~ -}+ -}-
44.5 41.0 10.0 6.5 4.4 8.0 31.5 26.0 7.6 4.6 3.5 5.0
10--14 14--17 18--22 22--02 2--6 6--10 10--12 12--14 14--16 16--18
5.1 2.1 2.5 28.3 8.3 10.1 3.9 1.9 1.1 0.5
34.2 25.4 14.8 14.0 9.0 15.8 30.0 19.8 24.9 21.3
31.9 28.7 18.7 15.1 13.9 15.4 22.6 26.8 27.9 24.2
32 46 73 50 47 47 35 39 48 72
---}------+
37 25.9 7.0 4.3 3.5 4.4 11.5 19.5 22.5 13.9
.~_d + + -{-
.~_d
m
m
d
d
d ÷ + + + + d
+ + + + _~_d + q+ + d d + + + + + +d +d +
267 T A B L E 1 (continued) Date (1980)
Nitrate, pg m -3
Time (PDT)
Teflon filter
T (°C)
Nylon filter
Humidity
HONO2
b
%
pg m : 3
Achieved c
Above deliquescence a
10/8
18--20 20--22 22--24
0.5 3.1 11.9
5.4 3.0 2.7
18.7 15.5 14.0
93 98 98
+ -}+
7.0 4.5 3.7
-d
10/9
0--2 2--4 4--6 6--8 8--10
21.7 8.8 6.5 11.3 >15
2.3 1.8 2.0 1.6 8.7
13.9 11.8 10.5 12.2 17.3
97 97 98 97 89
-~--~ + -}+
3.5 2.8 2.3 3.0 5.5
d d d d +
d
a A b o v e ( + ) or b e l o w (--) deliquescent p o i n t of N H 4 N O s at sampling temperature. b E q u i l i b r i u m Nitric acid c o n c e n t r a t i o n at sampling temperature, calculated f r o m equation [ 4 ] . c A c h i e v e d ( + ) or n o t (--), c o m p a r i n g measured and equilibrium nitric acid concentrations. d Close to deliquescent p o i n t or equilibrium value.
loo
il ii il
s oo
: ...... :::::!
ii
;.:::
.....
:
RH(%)
T( ° F)
.......
'2~:x:i
ii::::::i2i:? i
ii 30
i?i[!!i!!!iiiiiii
NO3-, Nylon Filters
(~g m-3)
................
zo ..... . . . . . . . :. . . ......... lo
ii
}
NO3-,Teflon Filters
:i
o i
...............: .............
o[i
("Q m-a)
(ppb)
200
lo0
:
150
!
,oo
:::
:::
::::::::
::
; ; 6
'~Sept.
12
18
25
I
I
::::
: :::
.:i!! : ! ! ! ! i ! A
:i;i;i:
::::r
I~ ~ : ~ / ' ~ 0 r
:::::
::. I 6
12
j
::J' :k:::::: 18
Sept. 2 6 ~ i ~
6
NO2(ppb) N O (ppb)
12
Sept.
8
27~
TIME, POT Fig. 1. Diurnal variations of particulate nitrate, nitric acid, temperature, humidity, oxides of nitrogen and ozone, Claremont, CA, Sept. 25--27, 1980.
268
data averaged over the sampling periods. Nitrate concentrations have been corrected for small contributions from filter blanks and field controls (passive exposure) as described elsewhere [8]. The fact that all gaseous nitric acid (that present in ambient air plus that possibly evolved from loss of NH4 NO3 on upstream Teflon filters) was collected on downstream nylon filters was demonstrated in a detailed study of nylon filter collection efficiency for atmospheric nitrate [ 18]. During the period studied, Teflon filter-collected nitrate and nylon filtercollected nitrate (a lower limit for particulate nitrate and an upper limit for nitric acid, respectively, if losses of NH4NO 3 by evaporation occur) exhibited substantial variations, 0.5 to 44.3 pg m -3 for particulate nitrate and 1.4 to 3 6 . 0 p g m -3 for nitric acid (four-hour averages). The total nitrate (sum of Teflon and nylon filter-collected nitrate irrespective of possible nitrate loss from the Teflon filter) ranged from 5 to 4 7 # g m -3 (four-hour averages). Diurnal variations of the pollutants and air quality parameters of interest are shown in Fig. 1 for a typical multiday smog episode, Sept. 25--27, 1980.
THERMODYNAMIC DATA
Two sets of temperature-dependent data are required for the analysis of our experimental results, namely the deliquescence relative humidity and the equilibrium constant for reaction [3]. Deliquescence humidities as a function of temperature were obtained from Dingemans [19] and from Stelson and Seinfeld [16]. The two equations are given and plotted in Fig. 2, and are seen to agree within ~ 3 % RH in the range of temperature of interest for this
70
~
65
"I" W
.~
6o
..J Ltl I1: W U.l
(A)
In DRH "
1.7037+723.7/T
( S t e l s o n et al)
(B)
In DRH "
1.2306+856.2/T
(Dingamens)
tLl
_1 m 121
2'o
2'5
4'0
TEMPERATURE, ° C
Fig. 2. A m m o n i u m nitrate d e l i q u e s c e n c e relative h u m i d i t y as a f u n c t i o n o f temperature.
269 40
/ A
(A) I n K c :
35 ~" ,~
3O
62"296-215101T'Kcinppm2
/riB
(Brandner) // // (B) In K c : - 7 0 . 6 8 - 2 4 0 9 0 / T - 6 . 0 4 In T/298, // K c in ppm 2 (Stelson et al) / / / C (C) In Kc : 8 4 . 6 - 2 4 2 2 0 1 T - 6 . 1 In T/298,
//" ' /
.=, z~
25
o
20
--% 15
5
10
15
20
25
30
35
40
TEMPERATURE, °C Fig. 3. Solid a m m o n i u m nitrate--gaseous precursors equilibrium c o n s t a n t as a f u n c t i o n of temperature.
study, 10--40°C. Variations of the solid ammonium nitrate--gaseous precursors equilibrium constant with temperature were calculated from the relations of Stelson et al. [5], Brandner et al. [14] and Stelson and Seinfeld [26], and are given and plotted in Fig. 3 for the range of temperature of interest to this study. Again, the agreement is reasonably good, even though the calculated equilibrium concentrations may differ by as much as "~4 ppb at higher temperatures, e.g., 35--40°C. For the following discussion, we have retained the median curve in Fig. 3, which corresponds to eqn (4) discussed earlier in this article.
DISCUSSION
From the thermodynamic data of Brandner [14] and other data compiled and discussed by Stelson et al. [5, 16] for the solid ammonium nitrate equilibrium, we have plotted in Fig. 4 the equimolar equilibrium nitric acid concentration required to satisfy eqn (4) over the range of ambient temperature relevant to this study. As is clearly seen from Fig. 4, formation o:t ammonium nitrate at the high daytime summer temperatures typically encountered in the Los Angeles area requires photochemical production of significant amounts of nitric acid. For example, if the ambient temperature is 35°C and the humidity is below the deliquescent point of NH4NO3 (i.e., less than 55% at 35°C), "~53pgm -3 of nitric acid will be required to satisfy
270
|quimolar equi|ibrium
?~tormic :q luCiat~:nn ~4n t rat i°n ]
co
60
I
E o~ :::I.
50
tO
40
,--
30
o
20
0
i
10 I
10
. . . .
I
. . . .
I
, , i l l
20
. . . .
I
. . . .
11
30
T, o C Fig. 4. Equimolar equilibrium nitric acid concentration as a function of temperature (solid line) and measured nitric acid (ordinate of the base of the rectangles) and particulate nitrate (height of the rectangles) for samples collected at humidities below the deliquescent point of NH4NO3 at the sampling temperature.
eqn (4), along with an equimolar a m o u n t of ammonia. On such a day, no particulate nitrate would be formed even though the nitric acid concentration may be as high as 5 0 p g m -3 ( ~ 2 0 ppb). For typical nitrogen dioxide and hydroxyl radical concentrations of 0.05--0.20 p p m and 1--3 10 -7 ppm, respectively, 5 to 6 0 p p b h r -1 of nitric acid can be produced by reaction of OH with NO 2 during a smog episode. It is obvious from the above estimates that daytime production of aerosol nitrate on a smoggy day may be limited by temperature rather than by the concentration of its precursor, nitric acid. Examination of data in Table 1 shows that, of the 48 simultaneous measurements of nitrate and nitric acid, only 18 were performed at ambient humidity below the deliquescent point of NH4 NO3 at the sampling temperature. Thus, the solid ammonium nitrate equilibrium model applies to only ~ o n e - t h i r d of the number of samples during the period studied. Since this study was c o n d u c t e d during the smog season with high daytime temperatures and low daytime humidities, it is apparent that the solid ammonium nitrate equilibrium model n o t only has limited application to aerosol nitrate formation and stability during the smog season, but will have very little applicability during the remainder of the year. For the subset of 18 samples collected at humidities below the deliques-
271 cent point of NH4NO 3 at the sampling temperatures, agreement between measured nitrate and that predicted from the solid a m m o n i u m nitrate equilibrium model is excellent. Table 2 summarizes the data for this subset of samples according to equimolar equilibrium nitric acid concentrations, and the corresponding results are plotted in Fig. 4. Measured nitric acid levels exceeded the equilibrium value in six cases for which high particulate nitrate concentrations were measured, in agreement with theoretical predictions. When equilibrium nitric acid concentrations were n o t achieved (five samples), or nearly achieved (within + 5pg m -3 , seven samples)the corresponding particulate nitrate levels were low, again in agreement with the theory. This agreement is remarkable considering the number of factors that may result in departure from ideal equilibrium conditions, e.g., presence of solid solutions [5], vapor pressure lowering effects (aerosol mixture and/or Kelvin effect for small particles), and reduction of the NH4 NO3 volatilization rate due to adsorption or mass transfer considerations. As expected from the steep temperature dependence shown in Figs. 3 and 4, the six high particulate nitrate cases corresponded to nighttime and early morning samples, while the low particulate nitrate cases were all daytime samples, even though appreciable levels of nitric acid (up to 37 pg m -3) were present in Claremont air during the photochemical smog episodes studied. The above analysis has also implications in terms of filter artifact. Since all low nitrate cases correspond to measured nitric acid concentrations that are too low to satisfy eqn (4), there is no need to invoke losses of NH4NO3 from the Teflon filters (negative artifact) to explain the low particulate TABLE 2 DISTRIBUTION OF PARTICULATE NITRATE ACCORDING TO EQUILIBRIUM NITRIC ACID CONCENTRATION FOR SAMPLES COLLECTED AT HUMIDITIES BELOW THE DELIQUESCENT POINT OF AMMONIUM NITRATE AT THE SAMPLING TEMPERATURE Equilibrium
Exceeded
Nearly achieved (within 5~gm -3)
Not achieved
6 3--11 9--30
7 14--37 12--37
5 30--44 4--20
High, 6--28 14--22 35--50 3 Nighttime, 3 Early morning
Low, ~5 24--32 31--48 7 Daytime
Low, ~3 30--33 16--46 5 Daytime
nitric acid
concentration
Number of samples Equil. HONO2, pg m-s HONO2, measured, pg m-3 Particulate nitrate, pg m-s Temperature, °C Humidity, % Diurnal distribution of samples
272
nitrate and high gaseous nitrate levels measured on the Teflon and nylon filter, respectively. Turning now our attention to the entire data set, it can be seen that 30 of the 48 samples were collected at humidities greater than that corresponding to NH 4 NO3 deliquescence at the sampling temperature. Stelson and Seinfeld [ 16] have recently applied theoretical considerations for non-ideal solutions to the effect of humidity on the aqueous ammonium nitrate--gaseous precursors equilibrium constant at 25°C. Their results are summarized in Fig. 5. As the relative humidity increases, the equilibrium gaseous precursor concentration product (HONO2)(NH3) dramatically decreases. In other words, gas phase nitric acid concentrations required to satisfy equilibrium conditions with solid particulate nitrate are upper limits for those required to satisfy equilibrium conditions with aqueous NH4 NO3. Although the analysis of Stelson and Seinfeld is limited to a single temperature, 25°C, it is interesting to examine our subset of data for humidities higher than the deliquesence point of NH+ NO3 at the sampling temperature in terms of the precursor concentration requirements given by Fig. 5. This can be accomplished by dividing our subset of data into two categories (Table 3): (a) The measured nitric acid concentration exceeds the value calculated for the solid NH4NO 3 equilibrium. If one assumes that these values are upper limits for the aqueous NH4 NO3 equilibrium not only at 25°C but also at any temperature in the range 10--35°C, one should observe high nitrate levels providing the atmosphere is not ammonia deficient. Of the fourteen data sets in this category, nine indeed exhibit high levels of particulate
+fooc
1---.
L25c o
[
,,
15°C
Deliquescence +",, \ Xtl Humidity \ \ xx fill AOUEOUS \i',xt/
E 0 -
~
so,,+ 0
30
.
40
50
1
60
~
70
80
90
100
RELATIVE HUMIDITY, %
Fig. 5. Effect of relative humidity on gas phase ammonia--nitric acid concentration product. The reference curve is that given by Stelson and Seinfeld [16] at 25°C.
273 TABLE 3 DISTRIBUTION OF PARTICULATE NITRATE IN SAMPLES COLLECTED AT HUMIDITIES ABOVE THE DELIQUESCENT POINT OF AMMONIUM NITRATE AT THE SAMPLING TEMPERATURE Solid NH4 NO3 equilibrium a
Aqueous NH4NO3 equilibrium b
Exceeded
Exceeded
Not achieved
14 3--21
15 b
1 b
5--27
1.4--21.0
4.0
Temperature, °C Humidity, %
13--24 64--94
10--28 64--99 c
23 76
Particulate nitrate p g m -3
9 high, 3--33 5 low, ~ 2 d
14 high (3--44) 1 low (0.5)
5.9
Diurnal distribution of samples:
High nitrate: 5 night, 4 early morning
10 night, 3 early morning, 2 late afternoon e
Late afternoon
Number of samples Equilibrium HONO2, pg m -3 Measured HONO2, pg m -3
Low nitrate: 4 late afternoon, 1 early evening a b c d e
From equation (4). See Fig. 5. With only two values below 84% RH. Probably due to lack of ammonia, see text. Includes the single low nitrate value of 0.5 pg m -3 .
n i t r a t e , while t h e r e m a i n i n g five e x h i b i t l o w n i t r a t e levels, p r o b a b l y d u e to a m b i e n t levels o f a m m o n i a b e i n g t o o l o w t o satisfy e q u i l i b r i u m c o n d i t i o n s . (b) T h e m e a s u r e d nitric acid c o n c e n t r a t i o n d o e s n o t e x c e e d t h e v a l u e calc u l a t e d f o r t h e solid e q u i l i b r i u m ( 1 6 d a t a sets). In this case, o n e has t o e s t i m a t e if a q u e o u s e q u i l i b r i u m c o n d i t i o n s are m e t b y e x t r a p o l a t i n g t h e c a l c u l a t i o n s o f S t e l s o n a n d Seinfeld [16] a t 2 5 ° C t o o t h e r t e m p e r a t u r e s . T h e s e e x t r a p o l a t e d curves are also s h o w n in Fig. 5 a n d were c o n s t r u c t e d a s s u m i n g the s h a p e o f t h e 2 5 ° C c u r v e in t h e region o f high h u m i d i t i e s is r e t a i n e d f o r all t e m p e r a t u r e s in t h e r a n g e o f interest, 1 0 - - 3 5 ° C . A t e a c h t e m p e r a t u r e , t h e c u r v e joins t h e h o r i z o n t a l line (solid e q u i l i b r i u m c o n s t a n t f r o m Fig. 3) at t h e d e l i q u e s c e n c e h u m i d i t y value f o r t h a t t e m p e r a t u r e ( f r o m Fig. 2). N e x t , the m e a s u r e d nitric acid c o n c e n t r a t i o n s are c o m p a r e d t o t h e e q u i l i b r i u m values e s t i m a t e d f r o m t h e e x t r a p o l a t e d curves. O f t h e 16 d a t a sets i n c l u d e d in this c a t e g o r y , 15 are seen t o e x c e e d t h e e x t r a p o l a t e d equil i b r i u m value, o f w h i c h 14 e x h i b i t high n i t r a t e levels as e x p e c t e d . T h u s ,
274 d e s p i t e the n u m b e r o f a s s u m p t i o n s m a d e a n d the l i m i t a t i o n s i n h e r e n t to t h e s e e s t i m a t e s , g o o d a g r e e m e n t is o b t a i n e d b e t w e e n m e a s u r e d n i t r a t e levels and theoretical considerations. A m o r e r i g o r o u s discussion o f the c o m p l e x b e h a v i o r a n d s t a b i l i t y o f a t m o s p h e r i c p a r t i c u l a t e n i t r a t e w o u l d have r e q u i r e d m e a s u r e m e n t s t h a t were b e y o n d t h e s c o p e o f this p r o j e c t , including as a m i n i m u m gas-phase a m m o n i a as well as a e r o s o l w a t e r c o n t e n t a n d p a r t i c u l a t e a m m o n i u m , sulfate, a n d acidity. S u c h a discussion m u s t also a w a i t t h e t e m p e r a t u r e d e p e n d e n c e o f n i t r a t e f o r m a t i o n a n d s t a b i l i t y in a q u e o u s a e r o s o l d r o p l e t s to be established. Nevertheless, e x p e r i m e n t a l nitric acid a n d n i t r a t e d a t a o b t a i n e d in this s t u d y are in g o o d q u a l i t a t i v e a g r e e m e n t w i t h t h e l i m i t e d t h e o r e t i c a l d a t a curr e n t l y available, u n d e r l i n e the l i m i t e d a p p l i c a b i l i t y o f the solid N H 4 N O 3 e q u i l i b r i u m m o d e l , a n d illustrate the s t r o n g t e m p e r a t u r e d e p e n d e n c e o f n i t r a t e aerosol s t a b i l i t y in L o s Angeles air.
ACKNOWLEDGEMENTS This w o r k was s u p p o r t e d in p a r t b y the C o o r d i n a t i n g R e s e a r c h Council, CAPA-19 Project Group.
REFERENCES 1 D. Grosjean (Editor), Nitrogenous Air Pollutants: Chemical and Biological Implications. Ann Arbor Science Pub., Ann Arbor, MI, 1979. 2 C.W. Spicer and P. Schumacher, Atmos. Environ., 11 (1977) 873--876. 3 B.R. Appel and Y. Tokiwa, Atmos. Environ., 15 (1981) 1087--1089. 4 A.B. Harker, L. W. Richards and W. E. Clark, Atmos. Environ., 11 (1977) 87--91. 5 A. W. Stelson, S. K. Friedlander and J. H. Seinfeld, Atmos. Environ., 13 (1979) 369. 6 R. K. Stevens (Editor), Current Methods to Measure Atmospheric Nitric Acid and Nitrate Artifacts. U.S. Environmental Protection Agency Report No. EPA-600/2-79051, Research Triangle Park, NC, 1979. 7 B.R. Appel, Y. Tokiwa and M. Haik, Atmos. Environ., 15 (1981) 283--289. 8 D. Grosjean, Distribution of Atmospheric Nitrogenous Pollutants at a Los Angeles area smog receptor site, Environ. Sci, Technol. (1982) submitted for publication. 9 T. Okita, S. Morimoto, M. Izawa and S. Konno, Atmos. Environ., 10 (1976) 1085-1089. 10 J. Forrest, R. L. Tanner, D. Spandau, T. D'Ottavio and L. Newman, 1980. Atmos. Environ. 14, 137--144. 11 G.M. Hidy, P. K. Mueller, D. Grosjean, B. R. Appel and J. J. Wesolowski (Editors), The Character and Origins of Smog Aerosols, Wiley New York, NY, 1979. 12 R.J. Gordon and R. J. Bryan, Environ. Sci. Technol., 7 (1973) 645. 13 D. Grosjean and S. K. Friedlander, J. Air Pollut. Control Assoc., 25 (1975) 1038. 14 J. D. Brandner, N. M. Junk, J. W. Lawrence and J. Robins, J. Chem. Engng. Data, 7 (1962) 227. 15 I.N. Tang, Atmos. Environ., 14 (1980) 819. 16 A.W. Stelson and J. H. Seinfeld, Atmos. Environ., 16 (1982) 993.
275
17
G. J. Doyle, E. C. Tuazon, R. A. Graham, T. M. Mischke, A. M. Winer and J. N. Pitts,Jr.,Environ. Sci. Technol., 13 (1979) 1416. 18 D. Grosjean, Quantitative collection of atmospheric nitrate on nylon filters,Anal. Lett. (1982) in press. 19 P. Dingemans, Rec. Trav. Chim. Pays Bas, 60 (1941) 317.
263
THE STABILITY OF PARTICULATE NITRATE IN THE LOS ANGELES ATMOSPHERE
DANIEL GROSJEAN
Environmental Research & Technology, Inc., 2625 Townsgate Road, Westlake Village, CA 91361 (U.S.A.) (Received March 30th, 1982; accepted May 10th, 1982)
ABSTRACT Simultaneous measurements of atmosphere particulate nitrate (NO3) and its gas phase precursor nitric acid (HONO2) were conducted in Los Angeles during severe smog episodes, and the results were compared to those predicted on the basis of thermodynamic data for the HONO2 (g) + NH3 (g) ----NH4 NO3 (s) equilibrium. Over the wide range of conditions studied (four-hour averaged NO~ --- 0.5--44.3 pg m -3, HONO2 -- 1.4--36.0 pg m -3, T = 10--33°C, humidity = 16--99%, ozone up to 460 ppb) the solid equilibrium model was found to be applicable to only about one-third of the total number of data sets. In this case good agreement was found between measured nitrate values and those expected from comparison of measured and equilibrium nitric acid concentrations. Samples collected a~ humidities above the deliquescent point of ammonium nitrate at the sampling temperature, for which the solid NH4NO 3 equilibrium no longer applies and solution chemistry must be considered, accounted for two-thirds of the data sets. F o r this subset, good agreement was also found between experimental results and theoretical considerations.
INTRODUCTION: N I T R A T E STABILITY AND SAMPLING CONSIDERATIONS
The possibility of significant bias when measuring particulate atmospheric inorganic nitrate collected on filters has received increasing attention during the past recent years. Although reliable measurements of particulate nitrate and its atmospheric precursor, nitric acid are of critical importance in developing a better understanding of the transformations of oxides of nitrogen relevant to tropospheric chemistry including urban pollution and acid precipitation [1], existing nitrate data may contain large uncertainties reflecting two types of errors, a positive bias resulting from adsorption of nitric acid on alkaline filters and from reaction of nitric acid with particulate salts during sampling [2], and a negative bias resulting from displacement by stronger acids, e.g., HC1 [3] and H2SO4 [4] as well as from the thermal instability of nitrate salts such as ammonium nitrate [ 5]. Extensive laboratory and field studies [2] have shown that adsorption of nitric acid, which is highly substrate-dependent, is the major cause of positive 0048-9697/82/0000--0000/$02.75
©1982 Elsevier Scientific Publishing Company
264 bias (positive "artifact" nitrate). These studies have shown that, over a wide range of sampling conditions, adsorption of nitric acid is negligible on Teflon filters [6] which have thus recently become the substrate of choice for sampling of atmospheric nitrate [7, 8]. Reaction of nitric acid with deposited salts, e.g.,: HNO 3 (g) + NaC1 (s) --> HC1 (g) + NaNO 3
(1)
may also introduce a positive bias in nitrate measurements. Although the kinetics of reactions such as (1) have received little attention at the low reactant concentrations relevant to atmospheric conditions, reaction ( 1 ) a p pears rapid enough for atmospheric nitric acid to be quantitatively collected on NaCl-impregnated filters [9, 1 0 ] . The corresponding bias may be of concern when sampling marine or rural atmospheres, but is n o t significant in urban areas such as Los Angeles where particulate NaC1 concentrations derived from sea salt are low compared to nitrate levels [ 11 ]. Negative bias due to displacement of nitric acid from its salts by stronger acids, e.g.: H2SO 4 + N H 4 N O 3 --> NH4HSO 4 (s) + HNO s (g)
(2)
has received some attention [3, 4] and may be significant in the case of acidic aerosols in ammonia-deficient atmospheres. In the case of the Los Angeles aerosol, which is the object of this study, particulate sulfate has been shown to consist essentially of ammonium salts [ 1 1 ] , and the resulting low aerosol acidity makes the contribution of reaction (2) of minor importance. Of major importance, however, is the possibility of negative bias resulting from the strong and inverse temperature dependence of the stability of a m m o n i u m nitrate [5] which is the major c o m p o n e n t of nitrate aerosol in Los Angeles air [12, 13]. At ambient humidities lower than the deliquescent point of NH 4 NO3, the stability of NH4 NO3 in ambient air is represented by the simple equilibrium between solid NH 4 NO 3 and its gas phase precursors: N H 4 N O 3 (s) ~ NH 3 (g) + HONO 2 (g)
(3)
and thus depends only on the precursor concentrations and on temperature. The corresponding equilibrium constant is a strong inverse function of temperature [5, 14] : In Kc -
70.68 -- (24,090/T) -- 6.04 In (T/298)
(4)
where Kc -- (HONO2) (NH3) is in units of ppm 2 . At ambient humidities higher than the deliquescent point of NH4NO3, eqn (4) no longer applies and aerosol aqueous droplet chemistry must be considered. Tang [ 15] and Stelson and Seinfeld [16] have presented detailed theoretical considerations for the aqueous aerosol situation. Unfortunately, the temperature dependence of nitrate stability in aqueous aerosols is still unknown. Stelson et al. [5] have examined experimental data for nitrate aerosol in Los Angeles [11--13] and concluded that reported nitrate levels were
265 consistent with those predicted from the equilibrium between solid amm o n i u m nitrate and its precursors, i.e., equation (3). However, most literature data for particulate nitrate [11--13] and its gaseous precursors [17] include only daytime samples collected during the smog season {summer and early fall) when meteorological and air quality conditions are obviously more conducive to the formation of solid, rather than aqueous droplet, aerosol nitrate. In this study, simultaneous measurements of particulate nitrate and nitric acid were performed as part of an experimental investigation of the atmospheric transformations of oxides of nitrogen in Los Angeles air [8]. Daytime and nighttime samples were collected over a wide range of ambient temperature, humidity and air quality. The corresponding results are discussed here in terms of the applicability of solid NH 4 NO 3 equilibrium considerations to atmospheric particulate nitrate in Los Angeles, an area where high levels of nitrate, e.g., 30--40 pg m -3 , have been consistently measured and appear to be unique among urban areas in the United States and perhaps the world. As is discussed in detail in the following sections, it was found that the solid equilibrium model has limited application, i.e., does n o t apply to a b o u t t w o thirds of the total number of nitrate samples collected round-the-clock during the period studied. For the limited number of situations consistent with the solid N H 4 N O 3 equilibrium model, good agreement was found between experimental measurements and theoretical considerations. For the larger number of situations where aqueous chemistry must be considered, good agreement was also observed between theory and experimental data.
EXPERIMENTAL SECTION Particulate nitrate and nitric acid were collected using dual filter units m o u n t e d on a sequential filter sampler (SFS) operated at a sampling flow rate of 201 min-1. The dual filter unit consisted of an upstream Teflon filter (47 mm diameter, Millipore Corp. FALP 4700, 1 pm pore size} and a downstream nylon filter (47 mm diameter, Ghia Corp. No. M8PL, I pm pore size) m o u n t e d in series using 47 mm filter holders and adapters (Millipore Corp. No. 430400 and 470400, respectively}. Detailed SFS operations and on-site flow calibrations are given elsewhere [ 8]. After sampling, Teflon filters were placed in 47 mm diameter petrislides (Millipore Corp.) and nylon filters were immersed in pyrex tubes containing the buffer solution employed for ion chromatographic analysis [8, 18]. All samples were collected in Claremont, CA, a b o u t 45 km east of Los Angeles, on the Harvey Mudd College campus [8]. Samples were collected round the clock during smog episodes (ozone max up to 460 ppb) on September 22--23 (six consecutive four-hour samples}, September 25--27, O c t o b e r 1--3 (twelve four-hour samples} and October 7--9 (six four-hour samples followed by twelve two-hour samples}. Nitrate collected on Teflon and nylon filters are listed in Table 1 along with temperature and humidity
266 TABLE 1 P A R T I C U L A T E N I T R A T E , N I T R I C A C I D , T E M P E R A T U R E , H U M I D I T Y , A N D EQUIL I B R I U M N I T R I C A C I D C O N C E N T R A T I O N , C L A R E M O N T , CA, S E P T . - - O C T . 1980
Date (1980)
9/22 9/23
9/25
9/26
9]26--27
10/1
10]1--2 10/2
10/7
1017--8 10]8
Time (PDT)
N i t r a t e , gg m -3
T (°C)
Humidity
HONO2
b
%
Above deliquescenee a
pg m-3
Achieved c
Teflon filter
Nylon filter
16--20 20-24 0-4 4--8 8--12 12--16
5.9 6.1 29.3 44.3 11.5 1.9
4.0 1.4 1.6 2.6 21.0 16.0
23.4 14.8 12.2 15.8 28.0 32.6
76 98 99 95 64 46
+ ~-~ ~&d d
12.7 4.2 3.0 4.8 21.7 39
8--12 12--16 16--20 20-24 0-4 4--8 9--13 13--17 17--21 21--1 1--5 5--9
3.3 2.9 2.5 9.8 11.9 26.3 5.5 1.3 0.7 2.1 13.3 8.3
12.1 17.4 12.4 4.2 4.2 4.6 26.6 21.2 13.0 5.4 4.8 3.6
24.3 30.8 25.0 17.3 15.2 15.1 27.8 30.1 19.8 14.3 12.2 15.5
31 31 71 94 84 77 64 44 70 94 89 84
---
-÷ -b + -{-
14.0 32 15.7 5.5 4.4 4.3 21.5 29.2 8.1 3.9 3.0 4.5
10--14 14--18 18--22 22--2 2--6 6--10 10--14 14--18 18--22 22--2 2--6 6--10
0.9 1.3 1.9 20.3 19.5 6.1 1.3 0.7 1.1 33.1 17.3 14.3
3.6 12.0 11.2 8.2 6.2 13.4 36.0 22.4 8.4 13.8 9.4 16.4
33.4 33.0 21.4 17.9 15.3 19.7 30.7 28.9 19.5 15.6 13.4 16.3
16 38 81 76 59 44 32 42 88 88 75 72
--+ + -----~ -}+ -}-
44.5 41.0 10.0 6.5 4.4 8.0 31.5 26.0 7.6 4.6 3.5 5.0
10--14 14--17 18--22 22--02 2--6 6--10 10--12 12--14 14--16 16--18
5.1 2.1 2.5 28.3 8.3 10.1 3.9 1.9 1.1 0.5
34.2 25.4 14.8 14.0 9.0 15.8 30.0 19.8 24.9 21.3
31.9 28.7 18.7 15.1 13.9 15.4 22.6 26.8 27.9 24.2
32 46 73 50 47 47 35 39 48 72
---}------+
37 25.9 7.0 4.3 3.5 4.4 11.5 19.5 22.5 13.9
.~_d + + -{-
.~_d
m
m
d
d
d ÷ + + + + d
+ + + + _~_d + q+ + d d + + + + + +d +d +
267 T A B L E 1 (continued) Date (1980)
Nitrate, pg m -3
Time (PDT)
Teflon filter
T (°C)
Nylon filter
Humidity
HONO2
b
%
pg m : 3
Achieved c
Above deliquescence a
10/8
18--20 20--22 22--24
0.5 3.1 11.9
5.4 3.0 2.7
18.7 15.5 14.0
93 98 98
+ -}+
7.0 4.5 3.7
-d
10/9
0--2 2--4 4--6 6--8 8--10
21.7 8.8 6.5 11.3 >15
2.3 1.8 2.0 1.6 8.7
13.9 11.8 10.5 12.2 17.3
97 97 98 97 89
-~--~ + -}+
3.5 2.8 2.3 3.0 5.5
d d d d +
d
a A b o v e ( + ) or b e l o w (--) deliquescent p o i n t of N H 4 N O s at sampling temperature. b E q u i l i b r i u m Nitric acid c o n c e n t r a t i o n at sampling temperature, calculated f r o m equation [ 4 ] . c A c h i e v e d ( + ) or n o t (--), c o m p a r i n g measured and equilibrium nitric acid concentrations. d Close to deliquescent p o i n t or equilibrium value.
loo
il ii il
s oo
: ...... :::::!
ii
;.:::
.....
:
RH(%)
T( ° F)
.......
'2~:x:i
ii::::::i2i:? i
ii 30
i?i[!!i!!!iiiiiii
NO3-, Nylon Filters
(~g m-3)
................
zo ..... . . . . . . . :. . . ......... lo
ii
}
NO3-,Teflon Filters
:i
o i
...............: .............
o[i
("Q m-a)
(ppb)
200
lo0
:
150
!
,oo
:::
:::
::::::::
::
; ; 6
'~Sept.
12
18
25
I
I
::::
: :::
.:i!! : ! ! ! ! i ! A
:i;i;i:
::::r
I~ ~ : ~ / ' ~ 0 r
:::::
::. I 6
12
j
::J' :k:::::: 18
Sept. 2 6 ~ i ~
6
NO2(ppb) N O (ppb)
12
Sept.
8
27~
TIME, POT Fig. 1. Diurnal variations of particulate nitrate, nitric acid, temperature, humidity, oxides of nitrogen and ozone, Claremont, CA, Sept. 25--27, 1980.
268
data averaged over the sampling periods. Nitrate concentrations have been corrected for small contributions from filter blanks and field controls (passive exposure) as described elsewhere [8]. The fact that all gaseous nitric acid (that present in ambient air plus that possibly evolved from loss of NH4 NO3 on upstream Teflon filters) was collected on downstream nylon filters was demonstrated in a detailed study of nylon filter collection efficiency for atmospheric nitrate [ 18]. During the period studied, Teflon filter-collected nitrate and nylon filtercollected nitrate (a lower limit for particulate nitrate and an upper limit for nitric acid, respectively, if losses of NH4NO 3 by evaporation occur) exhibited substantial variations, 0.5 to 44.3 pg m -3 for particulate nitrate and 1.4 to 3 6 . 0 p g m -3 for nitric acid (four-hour averages). The total nitrate (sum of Teflon and nylon filter-collected nitrate irrespective of possible nitrate loss from the Teflon filter) ranged from 5 to 4 7 # g m -3 (four-hour averages). Diurnal variations of the pollutants and air quality parameters of interest are shown in Fig. 1 for a typical multiday smog episode, Sept. 25--27, 1980.
THERMODYNAMIC DATA
Two sets of temperature-dependent data are required for the analysis of our experimental results, namely the deliquescence relative humidity and the equilibrium constant for reaction [3]. Deliquescence humidities as a function of temperature were obtained from Dingemans [19] and from Stelson and Seinfeld [16]. The two equations are given and plotted in Fig. 2, and are seen to agree within ~ 3 % RH in the range of temperature of interest for this
70
~
65
"I" W
.~
6o
..J Ltl I1: W U.l
(A)
In DRH "
1.7037+723.7/T
( S t e l s o n et al)
(B)
In DRH "
1.2306+856.2/T
(Dingamens)
tLl
_1 m 121
2'o
2'5
4'0
TEMPERATURE, ° C
Fig. 2. A m m o n i u m nitrate d e l i q u e s c e n c e relative h u m i d i t y as a f u n c t i o n o f temperature.
269 40
/ A
(A) I n K c :
35 ~" ,~
3O
62"296-215101T'Kcinppm2
/riB
(Brandner) // // (B) In K c : - 7 0 . 6 8 - 2 4 0 9 0 / T - 6 . 0 4 In T/298, // K c in ppm 2 (Stelson et al) / / / C (C) In Kc : 8 4 . 6 - 2 4 2 2 0 1 T - 6 . 1 In T/298,
//" ' /
.=, z~
25
o
20
--% 15
5
10
15
20
25
30
35
40
TEMPERATURE, °C Fig. 3. Solid a m m o n i u m nitrate--gaseous precursors equilibrium c o n s t a n t as a f u n c t i o n of temperature.
study, 10--40°C. Variations of the solid ammonium nitrate--gaseous precursors equilibrium constant with temperature were calculated from the relations of Stelson et al. [5], Brandner et al. [14] and Stelson and Seinfeld [26], and are given and plotted in Fig. 3 for the range of temperature of interest to this study. Again, the agreement is reasonably good, even though the calculated equilibrium concentrations may differ by as much as "~4 ppb at higher temperatures, e.g., 35--40°C. For the following discussion, we have retained the median curve in Fig. 3, which corresponds to eqn (4) discussed earlier in this article.
DISCUSSION
From the thermodynamic data of Brandner [14] and other data compiled and discussed by Stelson et al. [5, 16] for the solid ammonium nitrate equilibrium, we have plotted in Fig. 4 the equimolar equilibrium nitric acid concentration required to satisfy eqn (4) over the range of ambient temperature relevant to this study. As is clearly seen from Fig. 4, formation o:t ammonium nitrate at the high daytime summer temperatures typically encountered in the Los Angeles area requires photochemical production of significant amounts of nitric acid. For example, if the ambient temperature is 35°C and the humidity is below the deliquescent point of NH4NO3 (i.e., less than 55% at 35°C), "~53pgm -3 of nitric acid will be required to satisfy
270
|quimolar equi|ibrium
?~tormic :q luCiat~:nn ~4n t rat i°n ]
co
60
I
E o~ :::I.
50
tO
40
,--
30
o
20
0
i
10 I
10
. . . .
I
. . . .
I
, , i l l
20
. . . .
I
. . . .
11
30
T, o C Fig. 4. Equimolar equilibrium nitric acid concentration as a function of temperature (solid line) and measured nitric acid (ordinate of the base of the rectangles) and particulate nitrate (height of the rectangles) for samples collected at humidities below the deliquescent point of NH4NO3 at the sampling temperature.
eqn (4), along with an equimolar a m o u n t of ammonia. On such a day, no particulate nitrate would be formed even though the nitric acid concentration may be as high as 5 0 p g m -3 ( ~ 2 0 ppb). For typical nitrogen dioxide and hydroxyl radical concentrations of 0.05--0.20 p p m and 1--3 10 -7 ppm, respectively, 5 to 6 0 p p b h r -1 of nitric acid can be produced by reaction of OH with NO 2 during a smog episode. It is obvious from the above estimates that daytime production of aerosol nitrate on a smoggy day may be limited by temperature rather than by the concentration of its precursor, nitric acid. Examination of data in Table 1 shows that, of the 48 simultaneous measurements of nitrate and nitric acid, only 18 were performed at ambient humidity below the deliquescent point of NH4 NO3 at the sampling temperature. Thus, the solid ammonium nitrate equilibrium model applies to only ~ o n e - t h i r d of the number of samples during the period studied. Since this study was c o n d u c t e d during the smog season with high daytime temperatures and low daytime humidities, it is apparent that the solid ammonium nitrate equilibrium model n o t only has limited application to aerosol nitrate formation and stability during the smog season, but will have very little applicability during the remainder of the year. For the subset of 18 samples collected at humidities below the deliques-
271 cent point of NH4NO 3 at the sampling temperatures, agreement between measured nitrate and that predicted from the solid a m m o n i u m nitrate equilibrium model is excellent. Table 2 summarizes the data for this subset of samples according to equimolar equilibrium nitric acid concentrations, and the corresponding results are plotted in Fig. 4. Measured nitric acid levels exceeded the equilibrium value in six cases for which high particulate nitrate concentrations were measured, in agreement with theoretical predictions. When equilibrium nitric acid concentrations were n o t achieved (five samples), or nearly achieved (within + 5pg m -3 , seven samples)the corresponding particulate nitrate levels were low, again in agreement with the theory. This agreement is remarkable considering the number of factors that may result in departure from ideal equilibrium conditions, e.g., presence of solid solutions [5], vapor pressure lowering effects (aerosol mixture and/or Kelvin effect for small particles), and reduction of the NH4 NO3 volatilization rate due to adsorption or mass transfer considerations. As expected from the steep temperature dependence shown in Figs. 3 and 4, the six high particulate nitrate cases corresponded to nighttime and early morning samples, while the low particulate nitrate cases were all daytime samples, even though appreciable levels of nitric acid (up to 37 pg m -3) were present in Claremont air during the photochemical smog episodes studied. The above analysis has also implications in terms of filter artifact. Since all low nitrate cases correspond to measured nitric acid concentrations that are too low to satisfy eqn (4), there is no need to invoke losses of NH4NO3 from the Teflon filters (negative artifact) to explain the low particulate TABLE 2 DISTRIBUTION OF PARTICULATE NITRATE ACCORDING TO EQUILIBRIUM NITRIC ACID CONCENTRATION FOR SAMPLES COLLECTED AT HUMIDITIES BELOW THE DELIQUESCENT POINT OF AMMONIUM NITRATE AT THE SAMPLING TEMPERATURE Equilibrium
Exceeded
Nearly achieved (within 5~gm -3)
Not achieved
6 3--11 9--30
7 14--37 12--37
5 30--44 4--20
High, 6--28 14--22 35--50 3 Nighttime, 3 Early morning
Low, ~5 24--32 31--48 7 Daytime
Low, ~3 30--33 16--46 5 Daytime
nitric acid
concentration
Number of samples Equil. HONO2, pg m-s HONO2, measured, pg m-3 Particulate nitrate, pg m-s Temperature, °C Humidity, % Diurnal distribution of samples
272
nitrate and high gaseous nitrate levels measured on the Teflon and nylon filter, respectively. Turning now our attention to the entire data set, it can be seen that 30 of the 48 samples were collected at humidities greater than that corresponding to NH 4 NO3 deliquescence at the sampling temperature. Stelson and Seinfeld [ 16] have recently applied theoretical considerations for non-ideal solutions to the effect of humidity on the aqueous ammonium nitrate--gaseous precursors equilibrium constant at 25°C. Their results are summarized in Fig. 5. As the relative humidity increases, the equilibrium gaseous precursor concentration product (HONO2)(NH3) dramatically decreases. In other words, gas phase nitric acid concentrations required to satisfy equilibrium conditions with solid particulate nitrate are upper limits for those required to satisfy equilibrium conditions with aqueous NH4 NO3. Although the analysis of Stelson and Seinfeld is limited to a single temperature, 25°C, it is interesting to examine our subset of data for humidities higher than the deliquesence point of NH+ NO3 at the sampling temperature in terms of the precursor concentration requirements given by Fig. 5. This can be accomplished by dividing our subset of data into two categories (Table 3): (a) The measured nitric acid concentration exceeds the value calculated for the solid NH4NO 3 equilibrium. If one assumes that these values are upper limits for the aqueous NH4 NO3 equilibrium not only at 25°C but also at any temperature in the range 10--35°C, one should observe high nitrate levels providing the atmosphere is not ammonia deficient. Of the fourteen data sets in this category, nine indeed exhibit high levels of particulate
+fooc
1---.
L25c o
[
,,
15°C
Deliquescence +",, \ Xtl Humidity \ \ xx fill AOUEOUS \i',xt/
E 0 -
~
so,,+ 0
30
.
40
50
1
60
~
70
80
90
100
RELATIVE HUMIDITY, %
Fig. 5. Effect of relative humidity on gas phase ammonia--nitric acid concentration product. The reference curve is that given by Stelson and Seinfeld [16] at 25°C.
273 TABLE 3 DISTRIBUTION OF PARTICULATE NITRATE IN SAMPLES COLLECTED AT HUMIDITIES ABOVE THE DELIQUESCENT POINT OF AMMONIUM NITRATE AT THE SAMPLING TEMPERATURE Solid NH4 NO3 equilibrium a
Aqueous NH4NO3 equilibrium b
Exceeded
Exceeded
Not achieved
14 3--21
15 b
1 b
5--27
1.4--21.0
4.0
Temperature, °C Humidity, %
13--24 64--94
10--28 64--99 c
23 76
Particulate nitrate p g m -3
9 high, 3--33 5 low, ~ 2 d
14 high (3--44) 1 low (0.5)
5.9
Diurnal distribution of samples:
High nitrate: 5 night, 4 early morning
10 night, 3 early morning, 2 late afternoon e
Late afternoon
Number of samples Equilibrium HONO2, pg m -3 Measured HONO2, pg m -3
Low nitrate: 4 late afternoon, 1 early evening a b c d e
From equation (4). See Fig. 5. With only two values below 84% RH. Probably due to lack of ammonia, see text. Includes the single low nitrate value of 0.5 pg m -3 .
n i t r a t e , while t h e r e m a i n i n g five e x h i b i t l o w n i t r a t e levels, p r o b a b l y d u e to a m b i e n t levels o f a m m o n i a b e i n g t o o l o w t o satisfy e q u i l i b r i u m c o n d i t i o n s . (b) T h e m e a s u r e d nitric acid c o n c e n t r a t i o n d o e s n o t e x c e e d t h e v a l u e calc u l a t e d f o r t h e solid e q u i l i b r i u m ( 1 6 d a t a sets). In this case, o n e has t o e s t i m a t e if a q u e o u s e q u i l i b r i u m c o n d i t i o n s are m e t b y e x t r a p o l a t i n g t h e c a l c u l a t i o n s o f S t e l s o n a n d Seinfeld [16] a t 2 5 ° C t o o t h e r t e m p e r a t u r e s . T h e s e e x t r a p o l a t e d curves are also s h o w n in Fig. 5 a n d were c o n s t r u c t e d a s s u m i n g the s h a p e o f t h e 2 5 ° C c u r v e in t h e region o f high h u m i d i t i e s is r e t a i n e d f o r all t e m p e r a t u r e s in t h e r a n g e o f interest, 1 0 - - 3 5 ° C . A t e a c h t e m p e r a t u r e , t h e c u r v e joins t h e h o r i z o n t a l line (solid e q u i l i b r i u m c o n s t a n t f r o m Fig. 3) at t h e d e l i q u e s c e n c e h u m i d i t y value f o r t h a t t e m p e r a t u r e ( f r o m Fig. 2). N e x t , the m e a s u r e d nitric acid c o n c e n t r a t i o n s are c o m p a r e d t o t h e e q u i l i b r i u m values e s t i m a t e d f r o m t h e e x t r a p o l a t e d curves. O f t h e 16 d a t a sets i n c l u d e d in this c a t e g o r y , 15 are seen t o e x c e e d t h e e x t r a p o l a t e d equil i b r i u m value, o f w h i c h 14 e x h i b i t high n i t r a t e levels as e x p e c t e d . T h u s ,
274 d e s p i t e the n u m b e r o f a s s u m p t i o n s m a d e a n d the l i m i t a t i o n s i n h e r e n t to t h e s e e s t i m a t e s , g o o d a g r e e m e n t is o b t a i n e d b e t w e e n m e a s u r e d n i t r a t e levels and theoretical considerations. A m o r e r i g o r o u s discussion o f the c o m p l e x b e h a v i o r a n d s t a b i l i t y o f a t m o s p h e r i c p a r t i c u l a t e n i t r a t e w o u l d have r e q u i r e d m e a s u r e m e n t s t h a t were b e y o n d t h e s c o p e o f this p r o j e c t , including as a m i n i m u m gas-phase a m m o n i a as well as a e r o s o l w a t e r c o n t e n t a n d p a r t i c u l a t e a m m o n i u m , sulfate, a n d acidity. S u c h a discussion m u s t also a w a i t t h e t e m p e r a t u r e d e p e n d e n c e o f n i t r a t e f o r m a t i o n a n d s t a b i l i t y in a q u e o u s a e r o s o l d r o p l e t s to be established. Nevertheless, e x p e r i m e n t a l nitric acid a n d n i t r a t e d a t a o b t a i n e d in this s t u d y are in g o o d q u a l i t a t i v e a g r e e m e n t w i t h t h e l i m i t e d t h e o r e t i c a l d a t a curr e n t l y available, u n d e r l i n e the l i m i t e d a p p l i c a b i l i t y o f the solid N H 4 N O 3 e q u i l i b r i u m m o d e l , a n d illustrate the s t r o n g t e m p e r a t u r e d e p e n d e n c e o f n i t r a t e aerosol s t a b i l i t y in L o s Angeles air.
ACKNOWLEDGEMENTS This w o r k was s u p p o r t e d in p a r t b y the C o o r d i n a t i n g R e s e a r c h Council, CAPA-19 Project Group.
REFERENCES 1 D. Grosjean (Editor), Nitrogenous Air Pollutants: Chemical and Biological Implications. Ann Arbor Science Pub., Ann Arbor, MI, 1979. 2 C.W. Spicer and P. Schumacher, Atmos. Environ., 11 (1977) 873--876. 3 B.R. Appel and Y. Tokiwa, Atmos. Environ., 15 (1981) 1087--1089. 4 A.B. Harker, L. W. Richards and W. E. Clark, Atmos. Environ., 11 (1977) 87--91. 5 A. W. Stelson, S. K. Friedlander and J. H. Seinfeld, Atmos. Environ., 13 (1979) 369. 6 R. K. Stevens (Editor), Current Methods to Measure Atmospheric Nitric Acid and Nitrate Artifacts. U.S. Environmental Protection Agency Report No. EPA-600/2-79051, Research Triangle Park, NC, 1979. 7 B.R. Appel, Y. Tokiwa and M. Haik, Atmos. Environ., 15 (1981) 283--289. 8 D. Grosjean, Distribution of Atmospheric Nitrogenous Pollutants at a Los Angeles area smog receptor site, Environ. Sci, Technol. (1982) submitted for publication. 9 T. Okita, S. Morimoto, M. Izawa and S. Konno, Atmos. Environ., 10 (1976) 1085-1089. 10 J. Forrest, R. L. Tanner, D. Spandau, T. D'Ottavio and L. Newman, 1980. Atmos. Environ. 14, 137--144. 11 G.M. Hidy, P. K. Mueller, D. Grosjean, B. R. Appel and J. J. Wesolowski (Editors), The Character and Origins of Smog Aerosols, Wiley New York, NY, 1979. 12 R.J. Gordon and R. J. Bryan, Environ. Sci. Technol., 7 (1973) 645. 13 D. Grosjean and S. K. Friedlander, J. Air Pollut. Control Assoc., 25 (1975) 1038. 14 J. D. Brandner, N. M. Junk, J. W. Lawrence and J. Robins, J. Chem. Engng. Data, 7 (1962) 227. 15 I.N. Tang, Atmos. Environ., 14 (1980) 819. 16 A.W. Stelson and J. H. Seinfeld, Atmos. Environ., 16 (1982) 993.
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17
G. J. Doyle, E. C. Tuazon, R. A. Graham, T. M. Mischke, A. M. Winer and J. N. Pitts,Jr.,Environ. Sci. Technol., 13 (1979) 1416. 18 D. Grosjean, Quantitative collection of atmospheric nitrate on nylon filters,Anal. Lett. (1982) in press. 19 P. Dingemans, Rec. Trav. Chim. Pays Bas, 60 (1941) 317.