Stratospheric and Total NO2 Column Measurements with a Modified Canterbury Filter Photometer
G. T. A M A N A T I D I S , A. F. BAIS , A . G . K E L E S S I S , C S . Z E R E F O S , and 1. C. Z I O M A S Laboratory of Atmospheric Physics. University of Thessaloniki, 54006 Thessaloniki, Greece
(Received: 13 February 1989; revised; 25 July 1989)
Abstract. A programme of ground-based stratospheric and total NO 2 column measurements was instituted at the Laboratory of Atmospheric Physics (40.5 ° N, 22.9 ° E) in August 1985. We present here the results of the first two years of measurements with a modified Canterbury filter photometer, details of which are given in the text. The stratospheric NO 2 column, obtained at twilight during low local NO2 levels, shows the seasonal variation with monthly mean values of about 6 × 10 -15 molec. cm -2 in the summertime to about 2.2 × 10 -15 molec, cm -2 in the wintertime. These measurements compare well with measurements obtained with different instruments by other groups at similar latitudes (about 40 ° N) but in different places. Also, the asymmetry of the evening-to-morning strato- spheric NO2 over Thessaloniki was found to be on the average equal to 1.58. Total NO 2 column over Thessaloniki has a pronounced seasonal variation with amplitude of 0.68 matin, cm which can be explained partly from measured local NO2 sources which discharge in the mixing layer and partly from photolysis of the NO 2 reservoir species.
Key words. Stratospheric NO 2 column, total NO 2 column~
1. Introduction
Because of the importance of NO2 in controlling the ozone layer (Crutzen, 1970; Chameides et al., 1977), a large number of measurements of its vertical and latitudinal distribution have been made, while numerous models have been developed to describe its seasonal, diurnal, and latitudinal variability. According to these studies (WMO, 1986; Noxon, 1979; Noxon et al., 1979; Solomon et al., 1983; Johnston and McKenzie, 1989), the annual variability of stratospheric NO2 is evident at all latitudes with maximum values during summertime and minimum values during wintertime, with smaller amplitudes near the equator. The general aspect is that the NO2 column increases with increasing latitude in both hemispheres and up to 30 deg., while at higher latitudes, a sharp decrease of the NO2 column occurs during wintertime. Observations during sunrise and sunset, show that there is a significant morning-to-evening variation of the NO2 column, since the morning values almost double at the evening (McKenzie and Johnston, 1982; Kerr, 1988).
Although in the past decade or so the stratospheric NO2 column has been
436 G.T. AMANATIDIS ET AL.
extensively studied, few papers have studied ground-based total NO2 spectro- photometric measurements. Brewer et al. (1973) and Bloxam et al. (1975) have measured the total NO2 column using the well-known, from total ozone obser- vations, direct sun differential absorption spectrophotometry. More recently, some results from direct sun observations of the total NO2 column have been reported by Schroeder and Davies (1987), using a modified Brewer ozone spectrophotometer (Kerr, 1988). Other results of measurements of NO2 column abundances by different spectrometers, during the Globus NOx campaign, were recently reported by Zander et al., (1989).
The limited number of long-term measurements of the NO2 column provided the stimulus for this study. Thus, a programme of routine measurements of the stratospheric and total NO2 column was instituted at the Laboratory of Atmo- spheric Physics (40.5 ° N, 22.9 ° E) in August 1985 using a modified Canterbury filter photometer which was originally designed for total ozone measurements by W. A. Matthews (Matthews, 1974; Matthews et al., 1974). The major modifica- tion of the Canterbury instrument was the replacement of the UV interference filters with narrow-band (about 1 nm half-bandwidth) interference filters, centered at suitable wavelengths in the visible, where the NO2 absorption spectrum has a significant structure.
The photometer measurements were supplemented by measurements ob- tained with a commercial ground NO/NO2 analyser and by meteorological observations. In this work, we present the results on the variability of the total and stratospheric NO2 column from the first two years of observations.
2. Instrumentation
The Canterbury filter photometer, initially developed for total ozone observa- tions (Matthews et al., 1974) was modified to be used for NO2 stratospheric and total column measurements. For this purpose, the original ultraviolet inter- ference filters were replaced by a new set of filters which operate in the visible region of the spectrum (between 430 and 460 nm), where the NO2 absorption spectrum has a pronounced and significant structure (Johnston and Graham, 1976). Five wavelengths were selected to correspond to the maxima or minima of the NO 2 absorption spectrum. Thus, five interference filters were constructed by Barr Associates, Inc. during June 1985, with peak transmittance centered at 437.8, 439.3,442.0, 444.9, and 450.1 nm, respectively. The same wavelengths were also used for NO2 measurements by the AES group (Ridley et al., 1984). The half-bandwidth of the filters was chosen to be as narrow as possible (ranging from 0.66 to 1.10 nm), to minimize the dependence of the filters' transmission curve to the solar zenith angle, known as the Forbes effect (Basher, 1977; Vanier and Wardle, 1969). The effective NO2 absorption coefficient for each filter was calculated from data by Johnston and Graham (1976). These coefficients with the optical characteristics of the five interference filters are summarized in Table I.
STRATOSPHERIC AND TOTAL NO2 COLUMN MEASUREMENTS
Table 1. Optical characteristics of the new interference filters
In addition to the replacement of the filters, the blocking filter of the photom- eter was replaced by a broad-band attenuator, transmitting between 420 and 480 nm, to protect the photomultiplier and the filters from the undesired energy from shorter or longer wavelengths entering the photometer, since the incident variation may cause a shift of the peak to shorter wavelengths and decrease the transmittance of the filters (Title et al., 1974). An important disadvantage of interference filters is their extreme sensitivity to variations in humidity (Furman and Levina, 1971) and temperature (Title, 1974). To avoid such problems, the filters were mounted inside a temperature-controlled housing, which keeps the filters at a constant temperature of about 35 °C and essentially decreases the humidity in the filters' environment.
The stability of the optical characteristics of the interference filters reflects the overall stability and good performance of the instrument, since all the other parts are stable and wavelength independent (broad-band attenuator, photo- multiplier, lenses and windows). The stability of the filters was periodically checked by means of two laboratory spectrophotometers (namely a Perkin- Elmer 550 (VIS-UV and a Hitachi 150-20), thus obtaining in this way the transmission curves of the filters versus wavelength. During the period of the observations, no significant change of the peak wavelength or of the trans- mittance and the half-bandwidth of the filters was observed (Amanatidis, 1988).
Since frequent removal and transportation of the filters might damage them and since it is difficult to measure the filters under constant temperature and humidity, a new testing procedure has been used since October 1986. This method uses a standard light source which is mounted on the entrance window of the instrument, and this testig procedure can be followed more frequently than the spectrophotometer method. The intensity F of the source was measured for the five wavelengths as described in the following paragraph for direct Sun measurements. It is obvious that the stability of the quantity F throughout the period of study provides an independent test on the stability of the instrument. In addition, by using these measurements it is possible to correct the sun or sky measurements, if any changes of the instrument were observed. Almost one year of lamp measurements showed that the instrument is stable through time, since no trend of the quantity F has been observed and since the deviation o f F is less than +1% (Amanatidis et al., 1988).
438 G.T. A M A N A T I D I S ET AL.
3. Methods of Measurements
The method used to estimate the total NO2 column has been used in the past for total ozone measurements and, more recently, for spectrophotometric NO2 column measurements by the AES group (Brewer et al., 1973; Kerr, 1988; McElroy, 1985). It is based on the well-known equation of monochromatic light propagation through the atmosphere, taking into accout the absorption of various atmospheric constituents:
I~ = I~0 exp(-a~l.tX- a'~l.tY- b~m), (1)
where 2 is the wavelength of the monochromatic beam, I~ is the measured intensity at the ground, Ia0 is the intensity outside the atmosphere a~ and at are the absorption cross-sections (cmZ/molec.) for ozone and NO2, respectively, X and Y are the column amounts of ozone and NO2 in (molec./cm2), respec- tively, ~t (airmass) is the ratio of the slant path to the vertical path of the radia- tion through the absorber, fla is the Rayleigh scattering coefficient and m is the ratio of the slant path through the atmosphere to the vertical path.
The above equation for the five wavelengths can be combined to give:
F = F o - A ~ t X - A ~ ' l a Y - A f lm (2)
where
5
F = ~ w2 log(I2) 2=1
for the solar radiation intensity at the ground,
5
F0 = w2 log(60) 2=1
for the solar radiation intensity outside the earth's atmosphere,
5
Ao~-- ~ w20c~ 2 : 1
for the 03 absorption cross-section,
5
A=I
for the NO2 absorption cross-section,
5
A b = X w2/3 2=1
for Rayleigh scattering coefficient. The weighting coefficients wl to w5 were selected so that the effect of Rayleigh
scattering can be eliminated, while at the same time the effect of the weak ozone
STRATOSPHERIC AND TOTAL NO 2 COLUMN MEASUREMENTS 439
absorption in this region were also neglected. This means that the terms Ao~ and A/3 are almost zero. The final result is that the measured parameter F becomes independent on all but NO2 atmospheric constituents and it is also a linear function of the NO2 column:
F = F0 - A~'~t Y (3)
From Equation (3) it appears that if the NO2 amount Y is constant through- out a certain period of time, the measured quantity F is a linear function of ~t. By this method, it is possible to calculate the extraterrestrial constant F0, which is the intercept of the regression line of F versus ~t. The slope of this line is the NO2 column amount (Kerr, 1988).
An example for the determination of the extraterrestrial constant is given in Figure 1. In this case, the measurements of the quantify F were made during the sunset of 9 February 1986 on the top of Mount Hortiatis (1300 m high), near Thessaloniki and cover a wide range of the airmass (~t). From the regression line of F versus tl, it appears that the F0 value (intercept) is 0.295 _+ 0.002, while the NO2 column amount is about 0.55 matm cm (slope of the line).
Once the value ofF0 has been determined, the NO2 column can be estimated by Equation (3), rewritten as
y _ ( F - Fo) (4) Ao~'~t '
for direct sun measurements of the quantity F. Zenith sky measurements of the quantity F, for a range of solar zenith
angles between 85 and 96 degrees during twilight, can be used to determine the amount of stratospheric NO, , as described in the past by (Brewer et al., 1973; Noxon et al., 1979; Syed and Harrison, 1980; Solomon et al., 1987). The
U.
z <
0
0.32 l February 9, 1 9 8 7
/ 0 . 3 0
0 . 2 6 z , . ,
0 1 2 3 4 5 6 A I R M A S S
Fig. 1. An example of direct Sun observations with the modified Canterbury filter photometer, used to calculate the extraterrestrial constant of the instrument.
440 G.T. AMANATIDIS ET AL.
observed twilight absorption curves of F versus the solar zenith angle are compared to those calculated by a multiple scattering radiative transfer model (Kerr, 1988), to estimate the stratospheric NOz column abundance.
4. Observations and Results
Stratospheric NO2 column observations using the modified Canterbury filter photometer, were based on zenith sky measurements of the scattered solar radiation during sunrise and sunset (at zenith angles between 85 and 96 degrees) following the zenith sky absorption technique previously mentioned. Since our observing site is polluted by local NO2 sources (Zerefos et al., 1989), great care was taken in applying the zenith sky absorption technique, because, on several occasions, the absorption curves were distorted, thus making impossible the experimental estimation of the stratospheric NO2 column. For this reason, we were left with only a limited number of days in each month (on an average 4 days per month), for which the observed absorption curves could coincide with the model curves calculated by Kerr (1988).
Unfortunately, we cannot provide any comparison between the total and stratospheric NO2 columns, since these measurements are about 5 to 6 h apart. This is because our observational program includes direct sun total NO2 measurements near local noon and zenith sky stratospheric NO2 measurements at large zenith angles, when the absorption signal is poor for direct sun observa- tions. Another complication arises from the diurnal variation of total NO2 column, which is maximum near local noon and drops significantly in the early morning and evening hours (Bloxam et al., 1975; McElroy, 1985; Amanatidis, 1988).
The mean monthly evening stratospheric NO2 column from August 1985 to August 1987 are shown in Figure 2. The gaps in this figure are due to a lack of measurements during February and May 1986. By applying a harmonic analysis to the mean monthly values, it was found that the amplitude of the annual cycle is 1.53 x 10 -15 molec, cm 2. The annual maximum occurs on 25 July with a value of 5.5 × 10 -15 molec, cm 2, while the annual minimum occurs on 25 January with a value of 2.5 × 10 -15 molec, cm 2. The observed high summer values of stratospheric NO2 abundance, is believed to be caused by the photo- lysis of the NO2 reservoir species which occurs deeper in the atmosphere due to higher solar elevation. Such reservoir species, converted during winter, are HNO3, HNO4, C1ONO2 and N205. It is believed that the major winter reservoir species is N205 (Knight et al., 1982; Solomon and Garcia, 1983), although earlier investigators (Noxon et al., 1979; Evans et al., 1982) suggested that NO2 is depleted during winter due to heterogeneous conversion to HNO3. It is also believed that HNO4 end C1ONO2 have a negligible participation in winter- time NO2 conversion because of their low concentrations (Ridley et al., 1984; Solomon and Garcia, 1983).
STRATOSPHERIC AND TOTAL NO2 COLUMN MEASUREMENTS 441
8
tD
c,
.7
:D _J ,O
C4 O z
i
" / " + + ~-+ ' , . ': x .
0 I I I I I I I I I l
AUG OCT DEC FEB APR JUN AUG OCT DEC FEB APR JUN AUG
1985 1986 1987
Fig. 2. Mean monthly evening stratospheric NO_, column, from August 1985 to August 1987. Bars in this figure indicate the standard deviation of the measurements. Observations made at Thessaloniki (40 ° N).
Our measurements are compared in Figure 3 with measurements by others done at similar latitudes to that of Thessaloniki, namely in Colorado, U.S.A. (40 ° N) during 1974-1977 (Noxon, 1979) and in North Caucasus U.S.S.R. (43.7 ° N) during 1979-1984 (Elansky et al., 1984). The agreement is good and small differences are probably due to the zonal asymmetry and the interannual variability of stratospheric NO2.
The mean value of the evening/morning asymmetry ratio (i.e. the ratio of the stratospheric N O 2 column measured during sunset to that measured during sunrise) for the examined time period for Thessaloniki is 1.58+_0.12. This
I E O
Z :S
..J o o
8 Z
.. . . . . . . . . . . Elsnsky . ! i I . {1984) 43.7 N
. . . . . Noxon (1979) 40.0 N • "This work 40.5 N
/ . .. "~.. / ' . \ .
/" \ , /" .. \ ,
~ / . \ , ....... /.-- _~ ................
l I t I I I I I I I 1 ~ I
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN
Fig. 3. Comparison of monthly mean values of stratospheric NO2 measured at Thessaloniki (40 ° N), with measurements of others made at similar latitudes.
442 G.T. AMANATIDIS ET AL.
Table 11. Evening/morning asymmetry of stratospheric NO2 column
Reference Season Latitude [NO2 ]pro / [NO2 ]am
Noxon et al., (1979) summer 40.0 ° N 1.70 Ridley et al. (1984) winter 40.0° N 1.70 Elansky et al• (1985) all seasons 43.7 ° N 1.33 McKenzie and Johnston (1982) all seasons 45.0 ° S 1.54 Kerr (1988) all seasons 43.7 ° N 1.50 This work all seasons 40.5 ° N 1.58
evening/morning asymmetry is believed to be caused mainly by the photolysis, during daytime, of N205 and C1ONO2 which were formed during the night (WMO, 1985). The value of 1.58, when compared with the mean evening/ morning ratio measured by other scientists (seen in Table 1I), appears to be in very good agreement•
Figure 4 shows the daily mean values of the total NO2 column in matm cm, from measurements done around local noon, for the whole period of study (August 1985-August 1987)• From this figure, it appears that the NO2 total column has a significant seasonal variation, with maximum values occurring during the warm part of the year (April through September) and with minimum values during the colder part of the year (December and January). The mean values for these warm and cold periods are 2 .7_ 0.5 rnatm cm and 1.6 __. 0.4 matm cm, respectively. For a better determination of the characteristics of the annual variation, a Fourier analysis has been applied to the monthly mean values of the total NO2 column• The results of this analysis show that the amplitude of the annual variation is about 0.68 matm cm, the maximum
u I
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50 ~
0 Z
25 ~ O p -
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Fig. 4. Daily values of total N O 2 column at Thessaloniki, from measurements done near local noon from August 1985 to August 1987.
STRATOSPHERIC AND TOTAL NO2 COLUMN MEASUREMENTS 443
occurs during the middle of June and that the annual variation explains 65% of the total variance of the mean monthly values.
The annual variation of the total NO2 column over Thessaloniki is partly explained by the photolysis of the NO2 reservoir species and partly by the contribution from local NO2 sources. The later contribution, being the larger, can be roughly estimated from measurements of ground-level ambient-air NO2 and aerological observations of the mixing height. Under the incorrect assump- tion of constant NO2 mixing ratio from the ground up to the height of the mixing layer, we have calculated an 'effective' NO2 column which is obtained from the ground NO2 ambient air value and the mixing height.
The mixing height in this work was determined from early afternoon (12:000 GMT) radiosonde observations of temperature, humidity and wind velocity profiles. Only clearly defined mixing heights were used in this work, i.e. the mixing height was defined if and only if, at the same altitude, there occurred a sharp increase in temperature, sharp change in relative humidity, and distinct changes in both wind velocity and direction.
Figure 5(a) shows the monthly values of the calculated 'effective' NO2 column as compared with the monthly mean values of measured NO2 column amounts obtained with the Canterbury instrument. The correlation of the two curves is obviously high (r = 0.77) and emphasizes the significant contribution to the total NO2 column of about 1.5 matm cm from urban local pollution sources. For comparison, Figures 5(b) and (c) show, as monthly means, the data from which the 'effective' NO2 column was obtained. All data points entering the monthly means of Figures 5(a), (b), and (c) were taken from near-local measurements.
The above-mentioned correlation of the measured total NO: column to the calculated 'effective' NO2 column, confirms the capability of the modified Canterbury photometer to measure the total NO: column amounts.
5. Conclusions
The Canterbury ozone spectrophotometer has been modified to be suitable for NO2 column measurements. The results of two years' measurements proved the capability of the instrument to measure both stratospheric (on unpolluted days) and total NO2 column abundances.
The annual variation of the stratospheric NO2 column measured at Thessa- loniki (40.5 ° N), has an amplitude of 1.53 × 10 -15 molec, cm -2 and is com- parable to the annual variation of stratospheric NO2 at similar latitudes of the Northern Hemisphere. The maximum occurs on 25 July, corresponding to high photolysis of the NO2 reservoir species, while the minimum occurs in winter- time on 25 January.
The total column of NO 2 is highly influenced by local pollution sources discharging in the mixing layer. At least 1.5 matm cm of the NO2 column is
E O !
g
z :Z
8 (',4 O z
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(b)
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444 G.T. AMANATIDIS ET AL
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JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG
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,~ 1.0
0.5 I I ) L I I t i 1 1 i i
(c)
JUN JUt. AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUt. At,~
1986 1987
Fig. 5. (a) Monthly values of the calculated "effective' NO 2 column, below the mixing height, compared to the totaJ NO2 column measurements with the Canterbury instrument at common days. (b) Monthly values of the ground level NO 2 concentration and (c) the mixing height for the same days.
STRATOSPHERIC AND TOTAL NO, COLUMN MEASUREMENTS 445
contributed by local pollution. This resulted from a comparison of photometric column measurements with ground-level ambient-air NOz measurements, when the mixing height was known.
Acknowledgements
We would like to acknowledge the Research Center for Atmospheric Physics and Climatology of the Academy of Athens for providing the original Canter- bury Ozone photometer.
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