Dec 20, 1993 - sures from 0.07 to 15 tort depending on the wavelength range of the measurement .... ical ionization mass spectrometric analysis shows that the.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. DI2, PAGES22,937-22,948,DECEMBER 20, 1993
Temperature
Dependence of the HN03
UV Absorption
Cross
Sections JAMESB. BURKHOLDER,RANAJIT K. TALUKDAR,A. R. RAVISHANKARA 1 AND SUSANSOLOMON Aeronoray Laboratory, NOAA, Boulder, Colorado,and Cooperative Institute for Research in Environmental Sciences, University o] Colorado, Boulder
The temperature dependence of the HNO3 absorption cross sections between 240 as•d 360 K over the wavelength range 195 to 350 nm has been measured using a diode array spectrometer. Absorption crosssectionswere determined using both (1) absolutepressurelneasm'ementsat 298 K and (2) a dual absorption cell arrangement in which the absorption spectrum at vaxious temperatures is measured relative to the room temperature absorption spectrum. The HNO3 absorption spectrum showed a temperature dependence which is weak at. short wavelengths but stronger at longer wavelengths wlfich are important for photolysis in the lower stratosphere. The 298 K absorption crosssections were found to be larger than the values cun'e,,tly recommended for atmospheric modeling (DeMore et al., 1992). Our absorption cross section data are critically compared with the previous lneasurenaentsof both room temperature anti t elnperattu'e-dependent absorption cross sections. Temperature-dependent absorption cross sections of HNO3 are recommended for use in atmospheric modeling. These temperatm'e dependent HNO3 absorption cross sections were used in a two-dimensional dynamical-photochemical model to delnonstrate the effects of the revised absorption cross sections on loss rate of HNO3 mxd the abtmdm•ce of NO_, in the stratosphere.
INTRODUCTION
Nitric acid is a reservoir of odd nitrogen speciesin the
accurate valuesare neededfor calculating the concentrations of HNO3 and NO.• in the stratosphere. The UV absorption spectrum of HNO3 is continuous with
atmosphere and is formed via the reaction of OH with NO_•,
OH + NO•_+ M
, HNO3 + M
(1)
or via hydrolysis of N•.O• in atmospheric sulfate aerosol or
on polar stratosphericcloud (PSC) particles,
N•.O• + H•.O (sulfateaerosol/PSC)
,2HNO3
(2)
a few very broad features. At wavelengthsgreater than 290 nm, where the lower stratospheric and tropospheric photon flux is largest, the absorption cross sections of HNO3 are relatively small.
Further, the absorption cross sections of
HNO3 are dependenton temperaturein this region[Rattigon et al., 1992a,b,c]. Therefore, to calculate the lifetimes of HNO3 and concentrations of NOx in the extratropical lower
HNO3 formed via (2) may remainin the condensedphase stratosphere, accurate values of the temperature dependence or be releasedinto the gas phase. The formation of HNOs in the troposphere may lead to irreversible removal of nitrogen oxide compounds from the atmosphere because ttNO3 •nay precipitate. HNO3 is also an important constituent
of tropospheric acid precipitation. The ga.sphase HNOa is photolyzed or reacts with OH. In either case, NO.c (-- NO + NO2) is releasedinto the atmosphere. In the stratosphere, HNO3 is a major temporary reservoir of odd nitrogen. Conversionof the reactive NO• speciesinto HNO3 greatly diminishesthe effectiveness of nitrogen compoundsin destroyingozoneor in bufferingozonedestruction by chlorinespeciesvia the formation of C1ONO2. Photolysis of HNO• and its reaction with OH releasethe sequestered odd nitrogen. To a first approximation, the bMa.nce between lossof HNO3 via photolysis or reaction with OH and its formarion via heterogeneousor homogeneousreactions deter-
•ninesthe fraction of nitrogenoxide NOy speciesthat can be in the more reactive NOx form. The rate of photolysisof HNOs dependson its UV absorptioncrosssectionsso that
Also at the Depaxtmentof Chemistryand Biochemistry,University of Colorado, Boulder.
Copyright 1993 by the American GeophysicalUnion. Paper nmnber 93JD02178.
0148-0227/93/93JD-02178505.00
of the HNOa absorption crosssectionsare needed. Owing to this te•nperature dependence, the rate of HNO3 photolysis will vary with altitude, latitude, and season. At low solar zenith angles and high altitudes, photolysis of HNO3 in the short-wavelength region, < 220 ran, is also important. The gas phase UV absorption crosssectionsof HNO3 have
been measuredseveralti•nes [Johnstonand Graham, 1973; Biattme, 1973/1974; Molina and Molina, 1981; Rattigan et al., 1992a,b,c]. Yet the agreement among these measurements in the atmospherically important wavelength regions
is not very good (300 < A < 360 nm) or the data are sparse (195 < ,k < 220 nm). Thereforeinterpretationof measured atmospheric abundances suffers, and calculations of the atmospheric concentrations of speciessuch as NO, NOe, and NOs are inaccurate. Further, the temperature dependence
of the UV absorptioncrosssectionshas been neglectedand adds to the inaccuracy. Rattigan et al. [1992a,b,c]have shown that the HNO3 absorption cross sections a.t wavelengths greater than 300 nm depend on temperature strongly enoughto changethe calculated atmosphericconcentrations of N Oz speciesby factors of 2 or more in some regions. In this work, we have reexamined the UV absorption spectrum of HNO3 both at room temperature and as a function of temperature between 240 and 360 K. The wavelength range covered is 195 to 360 nm. The present. measurements are critically compared with the previously reported HiSOs absorption cross sections at, and below room tern-
22,937
22,938
BURKHOLDER ET AL.: HNOa
perature. The discrepancies bet•veen the various data sets are discussed, and absorption cross sections for atmospheric model calculations are recommended. The impact of the revised HNO3 absorption cross sections is assessedby using a two-dimensional dynamical-photochemical model.
ABSORPTION CROSS SECTIONS q- 1 cm were used. The shorter path length cells •vere used for measurements below 220 nm, where HNO3 cross sections are large and the longer cells were used at all other wave-
It is difficult to measure the UV absorption cross sections of HNO3 and its temperature dependence at the at-
lengths. The relative optical path length of the absorption cell pairs was measuredusing the room temperature absorption spectrum of NO2. The path lengths of the absorption cell pairs agreed to within q- 0.5% by this method. The variable temperature absorptioncell was temperature regulated by flowing cooling or heating fluid through its jacket. The temperature over the length of the absorption cell was con-
toospherically important wavelengths (300 < • < 360 nm)
stant to q- 1 K. The absorption cells were connected such
EXPERIMENTAL
SECTION
because(1) the absorptioncrosssectionsare small, that is, that both cells could be filled from the same sample simulthey approach10-23 cln2; (2) NO2, which is unavoidably taneously to the same total pressure. All absorption spectra present in HNO3 samples, has significantly larger absorption cross sections than HNO3 at these wavelengths; and
were measured by filling the cells to a known pressure,which was measured using a 10-or 100-tort capacitance manome(3) the magnitudeof the changein absorptioncrosssection ter. Absorption measurementswere made usingHNOa preswith temperatureis relatively small. In this study, HNO3 suresfrom 0.07 to 15 tort dependingon the wavelengthrange absorption cross sections were measured using both (1) ab- of the measurement.
solutepresstiremeasurementsand (2) a dual absorptioncell arrangement in which the absorption spectrum at various temperatures is measured relative to the room temperature absorption spectrum. Two methods were used t.o minimize systematic errors in absolute concentration determinations and to accurately define the temperature dependence of the absorption spectrum. A diode array spectrometer was employed to accurately account for the presence of NO2 impurities. A description of the experimental apparatus and methods of data analysis used in these measurements fol10•VS.
The spectrograph was equipped with a 600 groovesper millimeter grating, a 1024-element diode array detector, and 150- to 400-tim-wide entrance slit widths. A 400-tim entrance slit corresponds to a spectral resolution of • 0.4 nm as
measuredby the full width at half maximum(FWHM) of the 253.7-nm Hg line. The wavelength of the spectrographwas calibrated using emissionlines from Hg, Zn, and Cd lamps and a 10-tim entrance slit. The wavelength calibration is accurate to q- 0.2 nm. To cover the 195- to 380-nm wavelength range, absorptionspectra were recordedin six overlapping
84-nm intervals with the overlap between adjacent intervals being between 5 and 20 nm. A Pyrex window was placed in front of the spectrographto suppressshort-wavelength scattered light inside the spectrographwhen operating at wavelengths greater than 295 nm. Spectra were measured in 500 coadded scans. Depending on the wavelengthrange of the measurement and lamp intensity, exposure times of 0.025 to 0.08 s were used. The total time required for lneaabsorbance,A, of • 5 x 10-•. Two setsof absorptioncells suring a spectrum in a given wavelength segment was less with geometric optical path lengths of 200 + 1 on and 36 than 2 min.
The apparatus is shown in Figure 1. It consists of two 30-W D2 ]amps, two absorption ceils, a 0.5-m spectrograph, and a diode array detector. One of the absorption cells is maintained at room temperature, nominally 298 K, and the other at the desired temperature •. The D• ]amps provided a spectrally stable output over the time scale of the measurements with fluctuations in intensities equivalent to an
GAS
COOLANT
PRESSURE
IN
WINDOW
OUT
DEUTERIUM IN
LAMPS
__•.% WINDOW
PRESSURE PUMP DIODE SPECTROGRAPH
I I
_/_ _/ MOVABLEMIRROR
ARRAY DETECTOR
Fig. 1. Schematic of the expe•mentM apparatus used to measure the absorption crosssectionsof HNO3 as a ftmc tion of t en•p eratm'e.
MIRROR
BUI•KHOLDEI• ET AL.' HNO3 ABSORPTION CROSS SECTIONS Absorption spectra were recorded using the following pro-
22,939
rates of HNO3 decomposition as well as the temperature
cedure.With both cellsevacuated,the light levelI(•)0 pass- dependence of the NO2 spectrum. At high NO2 concentraing through the first absorption ceil was measured. HNO3 tions and low temperatures, N204 would also be present. was added to the absorption cell. The attenuated light pass-
However, the observed NO2 concentrations are sufficiently
ing through the first cell, I(•), was recorded. The transfer low that significant amounts of N204 would not be formed. mirror was moved to monitor the attenuated light passing through the second cell. Both absorption cells were evacuated, and the unattenuated light passingthrough the second cell was recorded. This procedure enabled I and I0 to be measured in both absorption cells without altering the optical alignment between the measurementof I and I0 for each cell. This procedure was adopted because the transfer mirror could not be repositioned exactly; the light level changed by • 0.1% upon repositioning. However, independent absorption measurementsusingjust one of the absorption cells, while not moving the transfer mirror, showedthat the light. level returned to the prefilling value upon reevacuation of the absorption cell. Also, measurements made by reversing the order of the absorption cells showed no problems with the adopted procedure. The measured spectra from each fill were transferred to a computer for analysis. For each te•nperature and wavelength range, this procedure was repeated a minimum
of three
times.
Absorption spectra of HNO3 were calculated using
'2') - -lnLZo(,x) 2') 1
(3)
From these measurements, the ratios of the absorption cross sections at temperature T and 298 K,
ratio-- A()•,T) 9s)
x
T
(4)
werecalcnlated.A(,k,T) and A(,k,298)are the •neasuredabsorbancesat wavelength • and temperatures T and 298 K, respectively. Absorption measurements were made at 360, 337, 315, 298, 280, 260, and 240 K. Although the spectra recorded at temperatures greater than 298 K are not necessary for atmospheric photolysis calculations, they provide an extended data set from which an empirical tempera,ture dependence could be more accurately defined and extended with confidence to the lower atmospheric temperatures. NO•, present as an impurity in the HNO3 samples, has a weak diffuse absorption band over the wavelength range
of these measurementsIDeAlore et ,l., 1992]. If the NO• impurity in the HNO3 samplesis greater than 0.005%, its
To account for small amounts of N204 the NO2 reference spectra used for the spectral subtraction were measured at temperature and concentration levels comparable to those
observed in the HNO3 measurements. The NO2 absorption was subtracted using the structure in its spectrum. In addition to NO2, it is possible that our anhydrous
HNO3 contains N2Os [Crowlcy ½t al., 1993]. N•_Oswould be in equilibrium with NO3 and NO2 such that small concentrations of NO3 may also be present. We could not detect
NO3, [NO3]< 2 x 10TMmoleculecm-3, in our HNO3 sample via absorptionin the 662-nm region. Usingthis [NO3] limit, the NO2, NO3, N2Os equilibrium constant [Demote ½t al., 1992], and our measuredNO2 and HNO3 concentrations,the N2Os mole fraction in our HNO3 sample is < 0.002. Chemical ionization mass spectrometric analysis shows that the abundance of N2Os in anhydrous HNO3 is less than 0.1%. Further, some absorption spectrum measurements were car-
ried out using a HNO3 sample to which several drops of H20 were added. The measured HNO3 absorption spectra in the 300- to 350-nm region were identical to those measured using anhydrous HNO3. Therefore we believe that our HNO3 absorption cross section measurements are not affected by the presence of N2Os. The precision with which the temperature dependence of the HNO3 absorption spectrum is measured can be estimated by comparing spectra measured in the two absorp-
tion cells while both cells are at 298 K. In the wavelength regions where corrections due to NO2 are not required, the random error in the ratio was q- 5% in the wavelengthrange 200 to 220 nm and less than -4-1% in all other wavelength regions for absorption signals greater than 0.05. For absorbancesless than 0.05, the instability of the lamp over the time required for measuring a spectrum, equivalent to A =
5 x 10-4, becomesanothersourceof error. In the present. mea,surements, the HNO3 absorption was less than 0.05 at wavelengths longer than 315 rim. For wavelengths greater than 315 nm, the uncertainty in the ratio of spectra increased as the HNO3 absorption crosssection decreased; the uncertainty was + 5% at 330 rim. Another potential source of error at long wavelengths was the accuracy of subtract-
absorption relative to that of HNO3 becomes significant. at ing the contribution due to NO2. At wavelengthsgreater wavelengths greater than 310 nm. The NO• impurities in than 370 nm, where HNO3 has no measurable absorption, our HNO3 sampleswere typically greater than 9.005%, and the residual after subtraction was approximately the sa•ne the HNO3 samples slowly decomposedin the absorption cells as the fluctuations due to the D2 lamp instability. Thereto produce additional NO•. The measured NO• levels typi- fore the subtraction of NO• contribution does not lead to an cally doubled over a period of about 10 min in the absorp- error larger than the reproducibility of the measurements at. tion cell. Therefore the contribution of NO• had to be sub- these wavelengths. The maximum HNO3 pressure in the tracted from the measured absorption spectrum to obtain absorption cells used wa,s 15 torr. This pressure and our absorption (A = 5 x 10-4) correspond the HNO3 spectrum. The diode array spectrometer is well minimummeasurable suited for such spectral subtraction. It minimizes complica- to a measurablelimit of • 5 x 10-•4 cm2 for unit signalto tions caused by the increase in NO• concentration with time noise and establish • 350 nm as the long wavelength limit because the entire absorption spectrum is recorded simultaneously. Furthermore, the spectrum is recorded on a time
of our measurements.
scalefast relative to the HNO3 decomposition. Ratiolug the HNO• absorption spectra recorded in the two absorption
trated H•_SO• under vacuum. The HNO3 was collected as a
cells at different temperatures does not completely remove the NO• contribution. This is because of small differences in the NO• concentrations in the two cells due to different
Pure HNO3 was prepared by mixing KNO3 with concensolid in a trap at dry ice temperature. NO2 was prepared by reacting purified NO with excess02 which had been passed through •nolecular sieve at dry ice te•nperature. The NO• was collected in a dry ice cooled trap and trap-to-trap dis-
22,940
BURKHOLDER ET AL.' HNO3
ABSORPTION CROSS SECTIONS
tilled in an excess02 flow until a pure white solid reina.ined.
0.12 .
He (ultrahigh purity, 99.99%) wasusedto flush the absorp-
0.10
tion cells between spectruin Ineasurements.
.........•.............•..............:-.............•.............,...............{. ..............:-.....
RESULTS
During the course of this work, we measured the absolute absorption cross sections of HNO3 at. 298 K over the wavelength range of 195 to 350 nm. The absorption crosssections at other temperatures were measured relative to that at 298
i
•
•i
i
!
i
.. !
•% ....k/'-../\3:"
O.04 ........ •k":.............':..............:'........'•z":'"½V'"'•':;•'""a'" '*"'v::...........:.....
....:?, ....... "M"• .... i
i
K over the same wavelength range.
0.02 :::::::.,:j•::::.i .............. 'r ............. '• ............. i............. i.............. :'..... I" i '• i __:.'-- !
Room Temperature Abso•Ttion Cross Sections
o.oo ........... {............ 310
Figure 2 shows the absorption cross sections of HNO3
measuredin the presentwork at 298 K over the wavelength range 195 to 350 nm. This spectrum was constructedusing spectra recorded in six overlapping wavelength segments. The overlap between adjacent segmentswas between 5 and 20 nm. This construction of a composite was necessarybecauseof the nearly 7 orders of magnitude changein absorption
cross section
between
.....4........
320
330
340
350
360
9
!
i
i
:
370
380
100
uJ
uJ
195 and 350 nm and the conse-
::
quent requirement to use a large range of HNO3 concentrations in the absorption cell. The HNO3 concentrations used
were chosen to provide as large an absorption as possible but with a maximum absorbanceof 1. Over the rangeof absorbancesused, the Beer-Lambert law was strictly obeyed.
As discussedearlier, spectra measuredat wavelengths longer than 310 nm were corrected for absorption by NO2 which is present as an impurity in HNO3. Figure 3a shows a measured HNO3 absorption spectrum along with a reference spectrum of NO2, which was used for subtraction.
The residual absorption, the HNO:.,.absorption spectrum, obtained following subtraction is also shown. The NO2 impurity was typically ~ 100 parts per million volume(ppmv).
uJ
uJ
20 ........... i...........i ............. •.............. !..............
0 310
320
330
340
350
360
:
370
.i....
380
WAVELENGTH (nm)
Fig. 3. (a) Example of absorptionspectrummeastu'edusing the diode array spectrometer in the wavelength region where the NO2
impm'ity contributesto the measuredabsoll•tionspectrum(solid cm've). The UV absoll•tionspectrumof NO2 (dotted curve,multiplied by a factor of 5 for darity) that was subtractedfront the measm-edspectrum (solid curve) to obtain the HNO3 absorption spectrun•(dashedcurve). (b) The fraction of the measuredabsoq•tion spectrum shown in Figtire 3a attributed to NO2 (solid ctu-ve). For comparison,the fractional correctionapplied in the
10-17
Rattiganet al. [1992a,b,c]studyis alsoshown(solidcircles). ...... 4............ .......
4 ............ ,
• ...........
: ...........
; ...........
4-•:
• ...........
: ...........
• ..........
4....
,
,
10'•8
Figure 3a shoxvsthat the residual absorption goesto zero at than 370 nm indicating the absenceof interference by scattered light in these measurements. At wavelengthslonger than 360 nm, as mentioned earlier, the
wavelengths greater ================================================ ......
10'•g
..
,
. ......
HNO3 absorption crosssectionwaslessthan 5 x 10-2• cm2. Figure 3b showsthe fraction of the measured absorbanceat each wavelengththat is due to NOv.for the spectrumshown
'20 '2•
:
: ::::::::::::::::::::::::::::
:
in Figure 3a.
:.
============================================================== :::!
Temperature Dependence of Absorption Cross Sections
.
10'22
10'23
Figure 4 shows the ratio of the absorption spectrum at temperature T to that at 298 K. The data shown in Fig-
ure 4 are also given in Table 1 (data on disk are available from the authorson request). The measuredabsorbances
============================================================================
,:
':
200
220
..4 ...........
: ...........
i ...........
,
,
..:.
•
280
300
320
.:............
:........
:
I 0'24 240
260
340
WAVELENGTH (nm) Fig. 2. Absorption spectrum of HNO3 at 298 K measured in tiffs work (solid curve) and in previous studies: Rattigan et al.
at various pressures of HNO3 and cell temperatures were also analyzed to obtain the absolute cross sections in the same manner with which the room temperature cross sections were determined. The results obtained were essentially the same as those calculated
relative
to the
298 K value.
However, these values were less precise, and we prefer to quote the relative crosssectionsat temperatures other than [1992a,b,c](solidtriangles),Molina and Molina [1981](solidcircles), Johnstonand Graham[1973](opensquaJ'es), and Biaume 298 K. The HNO3 spectrum was measured at six temperatures between 240 and 360 K. As seen in Figure 4, the [1973/1974] (open circles).
BURKHOLDERET AL.: HNOa ABSORPTIONCROSSSECTIONS
22,941
the practice. Also, we note the differences in the calculated photolysis rates between using the recommended HNO3 ab-
2.0
sorption crosssectionsof DeMote et al. [1992] and those recommended
here.
Room Temperature Absorption Cross Sections
There are four previous measurements of the HNO3 absorption cross sections at 298 K •vith which we can compare
our data [Johnstonand Graham, 1973; Biaume, 1973/1974; Molina and Molina, 1981; Rattigan et al., 1992a,b,c]. These •neasurements were made using scanning monochrmnators
[Johnston and Graham, 1973; Molina and Molina, 1981], single wavelengthatomic resonancelamp sources[Biaume, 1973/1974], and a diode array spectrometer[Ratti9an et al., 1992ct,b,c]. Figure 2 comparesthe 298 K absorptioncross
0.0
i 200
220
240
260
280
300
320
340
WAVELENGTH (nm) Fig. 4. The ratio of the absmq•tion cross sections measm'ed at various temperatin'es to that measured at 298 K as a fimction of wavelength. The curves, from top to bottom, are for spectra measm'ed at 360, 337, 315, 280, 260, and 240 K.
sections of the present study with those from these previous measurements. Over the •vavelength range 220 to 310 nm, •vhere HNO3 cross sections are large, easy to measure, and basically free frmn NO2 interference, the agreement between the different studies is very good. Figures 6 and 7 show the same cross section data at wavelengths shorter than 220 nm and longer than 300 nm, respectively, on an expanded scale. In these atmospherically important regions, the agreement among all the data. sets is not as good.
The absorptioncrosssectionsof Okabe[1980], shownin
Figure 6, have beennormalizedto a valueof 1.63 x 10-•7 c•n2 at 184.9 n•n, which was reported by Biaume et al. HNO3 absorption spectrum has a relatively •veak temperature dependence over the •vavelength region 195 to 280 nm. At •vavelengthslonger than 280 nm, the spectrum showsan increased dependence on temperature. The absorption cross sections decreasewith decreasing temperature; for example at 340 nm the decreasewas 50% in going from 298 to 240
[1973/1974] and continned in several other studies [Wine et o1., 1981; Cormell ctndHoward, 1985]. The agremnent
among all tlm studies is excellent at •vavelengthsloungerthan ;205 nm. At wavelengths shorter than 205 nm, the absorption cross sections reported by Molina and Molina agree with those of Biautne but are systmnatically higher, by as much as 3(1•/•,at 195 nm, than our results. Our resttits, on the K. other hand, agree •vell with those of Johnston and Graham. To obtain a. simple analytical expression for the temperIn the absence of an identifiable reason for these discrepature dependence of the absorption cross section of HNO3, ancies, an average of the available cross sections has been a(X,T), the data were fit to computed for use in atmospheric models and is shmvn by the dotted curve in Figure 6. In At wavelengths greater than 300 nm the agreement bet•veen the various studies degrades •vith increasing waveThis is the same parameterization used by Ratti9an [1992a,b,c]. Although (5) has no real physicalbasis, this length as shmvn in Figure 7. We believe that the source parameteriza.tion does reproduce the observed temperature for this disagreement lies primarily in the relatively large dependencewell. B(X) is a parameter which describesthe contrilmtions to the measured cross sections by the NO2 temperature dependence of the absorption spectrum. The impurity in the previous studies and difficulties associated 298 K data (ratio: 1) were Msoincludedin the fit, whose in correcting for its presence. Considering the combined uncertainlies in the t•vo studies, our absorption cross sections results are shown in Figure 5. The fitted values of B(•) do not change significantly upon excluding •ny of the seven are in good agreement with those of Rattigan et al. but different. t.emperatures frmn the fit. The uncertainties in are significantly larger than those of Johnston a,d Graham
-
.98)
B(X), as shownin Figure 5, are directly proportionalto the uncer[ainties in the spectrum measurements as discussedin the experimental sec[ion. DISCUSSION
In titis section, xve critically compare our results with those from previous studies. From this comparison a recommended set of temperature-dependent HNO3 absorption cross sections for atmospheric model calculations is derived. Finally, the results from two-dimensional photochemical model calculations are presented which illustrate the effects of using the temperature-dependent HNO3 absorption cross seclions instead of just the 298 K values, as is currently
[1973],2Ioli,, o,d Molina [1981],and Biaume[1973/1974].
Like Rattigan et al., we employed a diode array spectrometer and had the best means for measuring and correcting for NO2 impurities. Unlike Rattigan et al., we also measured the absorption spectrum at longer •vavelengths•vhere NO2 absorbs strongly and HNO3 does not; hence our corrections for NO2 •vere likely more accurate.
Rattigan et al. [1992a,b,c] measuredHNO3 absorption crosssections at 298 K over the wavelength range 210 to aa5 nm using a 100-cm-long absorption cell. An NO2 impurity of ,-• 0.5% was present in their HNO3 samples. The large NO2
impurity (see Figure 3b) led to relatively large corrections to the measured spectrum at long wavelengths. Uncertainties in the absorption crosssections•verequoted as 2.5% for
22,942
BURKHOLDER ET AL.: HNO3 ABSORPTIONCROSSSECTIONS TABLE 1. HNO3 Absorption Cross Section Temperature Dependence
o'(T)/o'(T - 298 K) Wavelength, nm
T=
360K
T = 337K
T=
315K
T-
280K
T=
260K
T = 240K
196
1.185
1.047
1.026
1.186
1.151 1.148
1.132
198
1.129
1.047
1.028
0.961 0.965
200 202 204 206 208 210 212 214 216 218 220 222 224 226
1.190 1.195 1.201 1.208 1.213 1.225 1.234 1.238 1.232 1.227 1.217 1.201 1.184 1.1 70
1.147 1.147 1.146 1.145 1.150 1.152 1.154 1.151 1.147 1.142 1.136 1.127 1.117 1.110
1.125 1.122 1.118 1.112 1.107 1.104 1.099 1.091 1.083 1.082 1.078 1.073 1.068 1.065
1.045 1.044 1.041 1.040 1.034 1.028 1.021 1.015 1.009 1.001 1.002 0.996 0.992 0.994
1.028 1.027 1.025 1.024 1.018 1.010 1.002 0.992 0.986 0.979 0.977 0.979 0.979 0.981
0.965 0.965 0.963 0.963 0.959 0.955 0.949 0.942 0.939 0.931 0.930 0.927 0.924 0.928
228 230 232 234 236 238 240 242 244 246 248 250 252 254 256 258 260 262 264 266 268 270
1.161 1.158 1.157 1.162 1.162 1.160 1.155 1.146 1.135 1.122 1.115 1.101 1.098 1.093 1.089 1.090 1.089 1.094 1.096 1.095 1.104 1.107
1.103 1.101 1.098 1.097 1.093
0.991 0.989 0.982 0.982 0.980 0.977 0.972 0.971 0.973 0.974 0.980 0.979 0.984 0.987 0.987 0.989 0.988 0.990 0.990 0.986 0.989 0.987
0.979 0.975 0.967 0.966 0.957
0.927 0.923 0.916 0.914 0.905
1.090 1.085 1.078 1.072 1.065 1.063 1.055 1.055 1.054 1.052 1.054 1.05,t 1.058 1.060 1.060 1.066 1.067
1.061 1.059 1.056 1.057 1.050 1.047 1.040 1.036 1.033 1.029 1.030 1.026 1.028 1.028 1.027 1.029 1.029 1.032 1.033 1.031 1.035 1.035
0.954 0.950 0.951 0.954 0.957 0.967 0.967 0.974 0.978 0.978 0.981 0.979 0.982 O.980 0.975 0.979 0.976
0.903 0.900 0.902 0.908 0.914 0.927 0.927 0.937 0.939 0.941 0.943 0.941 0.945 0.943 0.939 0.943 0.938
272 274 276 278 280 282 284 286 288 290 292 294 296 298 300 302 304 306 308 310 312 314 316 318
1.108 1.116 1.120 1.123 1.131 1.137 1.142 1.153 1.159 1.167 1.179 1.189 1.201 1.211 1.225 1.243 1.264 1.279 1.309 1.332 1.364 1.399 1.423 1.484
1.068 1.073 1.075 1.078 1.082 1.085 1.089 1.094 1.098 1.103 1.110 1.116 1.123 1.127 1.132 1.142 1.150 1.158 1.1 70 1.183 1.196 1.212 1.224 1.250
1.035 1.037 1.038 1.037 1.039 1.040 1.040 1.042 1.043 1.044 1.045 1.047 1.048 1.046 1.047 1.048 1.050 1.051 1.055 1.059 1.058 1.053 1.054 1.060
0.984 0.985 0.982 0.979 0.979 0.976 0.972 0.972 0.968 0.962 0.960 0.958 0.952 0.949 0.942 0.939 0.932 0.923 0.917 0.912 0.901 0.885 0.873 0.868
0.971 0.971 0.967 0.962 0.961 0.956 0.950 0.947 0.941 0.933 0.929 0.920 0.908 0.901 0.891 O.890 0.882 O. 869 0.868 0.866 0.846 0.828 0.839 0.838
0.936 0.936 0.931 0.930 0.926 0.923 0.921 0.919 0.913 0.908 0.911 0.903 0.893 0.866 0.855 0.863 0.848 0.821 0.841 0.818 O. 784 0.751 O. 762 0.736
BURKHOLDERET AL.' HNO3 ABSORPTIONCROSSSECTIONS
22,943
TABLE 1. (continued)
a(T)/a(T
= 298 K)
Wavelength, nm
T -
360 K
T -
337 K
T -
T = 280 K
T -- 260 K
T -
240 K
320
1.524
1.266
1.047
0.853
0.823
0.685
322
1.520
1.249
1.023
0.840
0.801
0.640
324
1.615
1.299
1.033
0.808
0.823
0.640
326
1.675
1.297
1.031
0.809
0.850
0.605
328
1.731
1.312
0.996
0.779
0.832
0.573
330
1.808
1.344
0.981
0.724
0.758
0.491
332
1.761
1.279
0.919
0.707
0.822
0.420
334
1.879
1.310
0.961
0.725
0.859
0.471
336
2.013
1.305
0.896
0.667
0.884
0.427
338
1.805
1.238
0.861
0.621
0.829
0.366
340
1.845
1.142
0.799
0.629
0.780
0.301
342
1.977
1.156
0.820
0.643
0.809
0.316
344
1.866
1.104
0.713
0.615
0.989
0.390
346
2.221
1.237
0.732
0.577
0.869
0.341
348
1.746
0.783
0.695
0.596
0.573
0.262
350
1.359
0.6:30
0.745
0.411
0.864
0.247
xvavelengths lessthan 300 n•n, and they increasedto 95% at 320 nm. Also, Rattigan et al. observednon-zero baselines following the NO2 subtraction, •vhich suggestsinterference œrmnscattered light in their spectrograph. Scattered light. signals can significantly effect the HNO3 absorption cross sections at long wavelengths. Our measurements at long wavelengths are likely to be more accurate because we extended
315 K
our •ncasurc•nents
to 380 nm. This extension
ena, bled
sections of HNO3 obtained did not vary with the abundance of the NO2 impurity.
The crosssectionsof Johnstona•d Graham,[1973] agree xvcll with our results except for their single data point at 325 nm which
is a factor
of 5 s•naller
than
our value.
Their
stated uncertainty was less than + 5% at wavelengthsless than 320 nm and greater than 10% at 325 n•n. It should
us to accurately •neasurc the NO2 i•npurity and correct for its presence. The absorptions due to HNO3 above 360 nm,
be noted that this is the s•nallest absorption cross section reported by these authors and that there is a significant.increase in the curvature of the absorption spectrum between
after correcting for NO2 subtraction, were zero (i.e., A < 5
t.hcir last two reported data points (see Figure 7). The dra-
x 10-4); thisindicates the absence ofinterference frmnscat- matic change in the spectral shape between 320 and 325 nm tercd light and complete accounting for NO2. Also, the cross has not bccn o})scrvcd in any of the other measurements. 0.020 16-
/-
'\ ß
_/_•.......................\! .............
14-
0.015
i
o
12-
'-
10-
ß
ß ß
ß
0 0.010
-
0
8-
•
6-
0
0.005
4 -
-
2-
o
0.000
/ 17o
200
220
240
260
280
300
320
180
190
200
I 210
220
340
WAVELENGTH (nm)
WAVELENGTH (nm) Fig. 6. The absorption cross sections measured in the •vavelength
Fig. 5. Plot of B(h) as a function of wavelengthderived in this stucty(dotted curve) and Rattig•n et al. [1992 a,b,c] (solid cir-
range of 185 to 220 mn by Bia•tme [1973/1974] (open circles), Molina and Molina [1981] (solid circles), Johnstonand Graham
cles). The error bars showthe precisionof ore'valuesin the wavelength regions indicated. The B values derived by the combined
cm2 at 184.9 nm, (dashedcurve), and this study (solid cm've).
data set (seetext) are representedby the solid curve. Below 270 and above 320 nm, the solid ctu-ve represents our data only.
[1973](opensquares),Okabe[1980],normalizedto 1.63x 10-• The dotted curve was obtained by averaging the results of these studies and was used for our atmospheric model calctdations.
22,944
BURKHOLDER ET AL.' HNO3
This discrepancy may be due to uncertainties in the methods used by Johnstmn and Graham to correct, for NO2 interference.
The absorptioncrosssectionsof Molina and Moll'ha[1981] are lower than ours at •vavelengthslonger than 310 nm. The discrepancy increases •vith increasing wavelength to a fact,or of 2.5 at 330 nm. Molina and Molina used reagent grade
(70%) HNO3 and undoubtedlyInadecorrections for NO2 absorption. It is possible that they overcorrected their data, which
led to the
lower
values.
It is curious
that, the mea-
ABSORPTION CROSS SECTIONS figan et al., is not significantly different, 4- 10%, fi'om the values obtained in the present work. Therefore the recommended room temperature HNO• absorption cross sections at wavelengths greater than 205 nm given in Table 2 were taken from the present work. Temperature Dependence of the Abso•Ttion Cross Sections
Rattigon et al. [1992a,b,c]werethe first to report the temperature dependenceof the HNO3 absorptioncrosssections. They measured the cross sections between 220 and 335 mn
sure•nent. s of Molina and Molina [1981] and Johnsto.n, and in the temperature range 298 to 238 K, using a diode array Graham,[1973]are not in better agreement.Thesetwo stud-
spectrmneter. As noted for the room temperature measurements, they subtracted the contributions due to NO2 impurities. In their experiments, NO2 impurities were -.• 0.5% of HNO3 and contribute greatly to the measured absorbduces [1981] are currently recommendedby DeMote et ed. [1992] at longer wavelengths. The levels of impurities in our samfor atlnospheric modeling. ples were lower and likely better characterized than in the Bioume[1973/1974]measuredHNO3 UV absorptioncross studies of Rattigan et al. sections at discrete wavelengths from atomic line sources The temperature dependence measured in this study over the range 185 to 325 nm. He subtra.cted the contriagrees qualitatively with that reported by Rattigan et al.
ies used the same measureInent techniques with the possible exception of the magnitude of the NO2 corrections. The absorption cross sections reported by Molina and Molina
bution due to -- 0.045% NO2 impurity using a measured reference spectrum. Further, in the data analysis he assumed that HNO3 absorption crosssectionsbeyond 325 nm are negligible. However, if his absorption cross sections are corrected for the finite absorption cross section at 330 nm, as measured in this work, his results are in excellent agreement with ours. For example, his longest wavelength data
' [1992a,b,c]. There is a weak negative temperature dependence over the xvavelengthrange 220 to 280 nm. At longer wavelengths,the dependenceon temperature is larger and increasesmonotonically with increasing wavelength. The
magnitude of the temperature dependence measured here does not agree with the values reported by Rattigan et al., as shown in Figure 5. Their temperature dependence is point at 325.2nm xvillchangefrom 6.0 x 10-23 cm2 to 10.3 ahnost a factor of 2.5 larger than our values at all •vaveX 10 --23 cm 2. lengths. However, a closer examination of the absorption Based on this discussion, it is clear that the absorption cross section data of Rattigan eta]. at, 273, 263, 248, and cross sections of Molina and Molina at wavelengths greater 238 K (which was kindly supplied to us by the authors) than 310 nm are sraMlet than the data obtained from all othshowsgood to excellent agreement with our measurements ers. An average of all the other data, excluding the longest xvith the important exception of their 238 K data. The 238 wavelength data points of Johnston and Graham and RatK data set showsa significantly larger temperature dependence than observedin this work. Rattigan et M. opted to include this data set in their anMysis and derived their much larger temperature dependence. Excluding the 238 K data set and reanalyzing their data yields values of B(A) which are in excellent,agreement with our measurements, which were made over a larger temperature range. We })elievethat ...............::•;:'::::::: .......:;:':':-':-':.....i: ..... ';-': '--: there is a systematic error in the lowest-temperature data. set of Rattigan et al. Combining all but. their 238 K data with ours yields B values very close to those obtained from 3 ........................... ß--:ß ' -,,--our data alone. Figure 5b shows values derived from our data alone, the Rattigan et al. data, and the cmnbined data ,
set. .................. ...............
, ................... , .................
• ....... • .......
ß...... ,---4
, ..... ..........
We recommend
the values
obtained
from
the combined
data analysis for atmospheric modeling. The recommended
• .........
i4 ............... :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :-.i:_i.:._-. -..... values are listed in Table i .............. • ........... i• ........ •....
2.
ß
2 .................. •................... :....... tD......ß......•-'............................ Atmospheric Model Calculations The rate of HNO3 photolysis and its effects on NOd: abun-
10-23................. J................... i........... J ..................
• ...................
; ................
.... .....:_} ; .... .• .•
-
: ......... •.: ......... •...2:. ........
:.
,
6
-
dance in the stratosphere were examined using the twodimensionM dynamicM-photochemicM model of Garcia and
Solomon[Garcia and Solomon,1983; Solomonet al., 1985]. 300
310
320
330
340
350
WAVELENGTH (nm) Fig. 7. A comparison of absmt)tion crosssections of HN03 at 298 K as a function of wavelength measm'ed by Molina and Molina
The model includes a detailed treatment of oxygen, nitrogen, hydrogen, and chlorine chemistry. The photolysis schemeis a two-stream approach that includes multiple scattering and
utilizes the World MeteorologicalOrganizaiton[1986] rec-
[1981] (solidcircles),Biaume [1973/1974] (opencircles),Johnston mmnendations for solar fluxes and absorption crosssections and Graham [1973] (open squares),Ratti#an et al. [1992 a,b,c] for oxygen and ozone. Photochemical parameters and reac(solid triangles), and this study (dots). The corrected datum (seetext) of Biaurae [1973/1974] at, 325.5 nm is shownby a solid tion rate coefficientsrecommendedby DeMote ½tal. [1992] square.
were used.
BURKHOLDER ET AL.: HNO3 ABSORPTION CROSS SECTIONS
22,945
TABLE 2. Recommended HNO3 Absorption Cross Sections and Temperature Coefficients
Wavelength, nm
Cross Section, 10 -2ø Cnl2
B, 10 -3 Ition cross sections, measured in this work, as a function of altitude
for tlu'ee latitudes.
Note
that
the differences
are smaller
than that calculatedsolelydue to changesin the J values(Figure 9; also see text).
22,948
BURKHOLDER ET AL.: HNOs ABSORPTION CROSS SECTIONS
other factors, between these two loss processesdetermines
the NOx to HNOs ratio. Significant changesare obtained near 20-30 kin, particularly at some latitudes. At high altitudes, HNO3 concentrationsare much smaller than that of NO2, so that changesin the photolysis rate of the former are not important to determining the NO2 abundance. At altitudes near 20-30 kin, the calculated overall HNOs loss rates are not proportional to the J values becauseHNOs is also lost via reaction with OH:
OH + HNOs
, NOs + H20
(6)
Assuming an approximate steady state for NO2, we may write
DeMore, W. B., S. P. Sander,D. M. Golden, R. F. Hampson,M. J. Kurylo, C. J. Howard, A. R. Ravishankara,C. E. Kolb, and M. J. Molina, Chemical kinetics and photochemicaldata for use in stratospheric modeling, JPL Publication 92-20, NASA Jet Propulsion Laboratory, Pasadena, Calif., 1992. Garcia, R. R., and S. So.lomon,A numerical model of the zonally averaged dynamical and chemical staxtctureof the middle atmosphere, J. Geophys. Res., 88, 1379-1400, 1983.
Johnston,H., and R. Graham, Gas phaseultraviolet absorption spectrum of nitric acid vapor, J. Phys. Chem., 77, 62-63, 1973.
Molina, L. T., and M. J. Molina, UV absorptioncrosssectionsof HO2NO2 vapor, J. Photochem., 15, 97-108, 1981. Okabe, H., Photodissociation of nitric acid and water in the vacuum ultraviolet; vibrational and rotational distributions of OH
2S+, J. Chem.Phys.,7œ,664.2-6649,1980. Pyle, J. A., and A.M.
[NO•]/[HNO3] = (J + k•[OH])/k,[OH]
(7)
Expression (7) shows that the fractional change in NO2
Zavody, The derivation of hych-ogen-
containing radical concentrationsfrom satellite data sets, Q. J. R. Meteorol. $oc., 111,993-1012, 1985. Rattigan, O., E. R. Lutman, R. L. Jones,and R. A. Cox, Temperature dependentabsorptioncrosssectionsand atmospheric photolysisrates of nitric acid, Ber. Bunsen. Ges. Phys. Chem.,
when the temperature dependenceis consideredshould not 96, 399-404, 1992a. follow the changein the photo]ysisrate alone. Rather, the situation becomesquite complicated, not only becausethe Rattigan, O., E. Lutman, R. L. Jones,R. A. Cox, K. Clemitshaw, and J. Williams, Temperature dependentabsorptioncrosssecOH concentration is important but also because the OH tions of gaseousnitric acid and methyl nitrate, J. Photochem. concentration itself will change with the HNO3 photolysis Photobiol. A, 66, 313-326, 1992b. frequencychanges.Photo]ysismakesup about 60% of the Rattigan, O., E. Lutman, R. L. Jones,R. A. Cox, K. Clemitshaw, and J. Williasns, Corrigenum: Temperature-dependentabsorpmodel-calculated loss of HNOs at 40øN near 20 km but only
a few percent at 68øN, implying that the changesin calculated N Oo. when temperature-dependent cross sections are
tion cross-sectionsof gaseousnitric acid and methyl nitrate, J. Photochera. Photobiol. A, 69, 125-126, 1992c. Solomon,S., R. R. Garcia, and F. Stordal, Transport processes and ozone pertm'bations, J. Geophys. Res., 90, 12981-12989,
consideredwill be far larger at 40øN than at 68øN, in spite 1985. of colder telnperatures and the consequentlarge decreases Wine, P. H., A. R. Ravishan'kara,N.M. Kreutter, R. C. Shah, in crosssections at the higher latitudes. Acknowledgments. We thank R. A. Cox for providing raw HN O3 crosssection data as•d discussionsand D. Hanson for C IMS analysis of ore' HNO3 sample. This research was funded in part by NASA's upper atmospheric research program. REFERENCES
J. M. Nicovich, R. L. Thompson, and D. J. Wuebbles, Rate of reaction of OH with HNOs, J. Geophys.Res., 86, 1105-1112, 1981.
World MeteorologicalOrganization (WMO), Atmosphericozone 1985: Assessmentof our understanding of the processescontrolling its present distribution and change, WMO Rep. 16, 1095 pp., Global Ozone Research and Monitoring Project, Geneva, Switzerland 1986.
Biaume, F., Nitric acid vapotu' absorption cross section spectrum and its photodissociation in the stratosphere, J. Photochem.,
2, 139-149, 1973/1974. Connell, P.S., and C. J. Howas'd, Kinetics study of the reaction HO + HNO3, Int. J. Chera. Kinet., 17, 17-31, 1985. Crowley, J. N., J.P. Burrows, G. K. Moortgat, G. Poulet, and G. LeBras, Optical detection of NOs and NO2 in pm'e HNO3 vapor, the liquid phase decomposition of HNOs, Int. J. Chem. Kinet., œ5, 795-803, 1993.
J. B. Burkholder, R. K. Talnkdar, A. R. Ravishan'kin'a, and S.Solomon, NOAA, R/E/AL2, 325 Broadway, Boulder, CO 80303.
(ReceivedMay 24, 1993; revised July 28, 1993; accepted July 30, 1993.)
