Determination of atmospheric precipitable water vapour and turbidity parameters from diurnal infrared hygrometer and turbidimeter data

0020-0891/82/030149-07$03.00/0 CopyrightQ 1982PergamonPressLtd

rafraredPhys. Vol. 22 pp. 149 to 155.1982 Printedin GreatBritain.All rights reserved

DETERMINATION OF ATMOSPHERIC PRECIPITABLE WATER VAPOUR AND TURBIDITY PARAMETERS FROM DIURNAL INFRARED HYGROMETER AND TURBIDIMETER DATA A. BORGHESI, E. BUSSGLE~,

G. FALCICCHIA

Institute of Physics, University of Jxece, Lxxx, Italy

and A. MWAFRA Institute of Physics, University of Bari, Bari, Italy (Receiued

11 December 1981)

Alattract-In this paper, we present the preliminary results of a site analysis performed in Italy to identify places suitable for the installation of i.r. telescopes. The experimental arrangement consists of a near-i.r. photometer and a Volz turbidimeter mounted on an equatorial base controlled by a sun follower. Day-time atmospheric water vapour content and turbidity parameters have been measured. An empirical method which uses the dew-point temperature has been adopted to evaluate the nighttime atmospheric vapour content.

INTRODUCTION

Since last year our research group ha8 started an activity devoted to the analysis of several sites in Italy which could be possible candidates for the installation of i.r. telescopes. The goal is the evaluation of the atmospheric vertical vapour and particulate matter content with good accuracy. The adopted method follows the idea, firstly suggested by Fowle,“) of measuring the total atmospheric water vapour content using the features of the near-i.r. solar spectrum. It consists of the simultaneous comparison between the solar radiation intensity detected in two properly selected narrow spectral intervals. The first one falls within an atmospheric absorption band, while the other falls within an ‘atmospheric window’ which is assumed as reference. This procedure has already been successfully used by Tomasi and G~zzi,‘~’ and by Martin et al (‘) in the recent past. The particulate matter coitent of the atmosphere is evaluated by means of a Volz turbidimeter, operating on the same principle. We have also measured several meteorological parameters which allow the estimation of night-time water vapour atmospheric content.(4*s) In the present work we report the results obtained during the first three campaigns that we have performed respectively at the Physics Department of the University of Lecce (sea-level) during June and October 1980 and at Campo Imperatore Observatory (2200m) during the whole month of August of the same year.

EXPERIMENTAL

ARRANGEMENT

A block diagram of the experimental apparatus, which is fixed on an equatorial mount controlled by a sun follower, is reported in Fig. 1. The near-i.r. photometer (NIP) consists of two internally blackened parallel tubes, through which the solar radiation falls onto two silicon cells which have a spectral response between 0.4 and 1.2 ,um. Each detector is covered by a narrow band interference filter with halfwidth of 100A at center wavelengths & = 0.93 pm (absorption band) and n, = 1.03 pm (reference atmospheric window). 149

150

A. BORGHESIet a[.

Fig. 1. Experimental set-up.

The output signals Zl, and ZAPare recorded by the data acquisition system after amplification. Their ratio, obtained by means of an analog divider, is recorded at the same time. The Volz turbidimeter (mod. 294) has three channels, centered at AG= OSOpm, AR = 0.88 pm and il, = 0.94 pm, respectively, with a linear response within 2%. We use only the signals at I, and A,, which are transferred to the acquisition system in addition to some relevant meteorological data (atmospheric temperature, pressure and relative humidity). The NIP calibration has been performed by comparing the ratio W = il$Z1, with the total water vapour content measured simultaneously by means of radiosoundings~‘) The curve obtained for our inst~ment is reported in Fig. 2. Here X = m- Wis the total water vapour content, m is the relative optical air mass and Wthe vertical water vapour content measured in g*cmw2.

Water

vopour

X,

g,cm-‘!

Fig. 2. Calibration curve of the hygrometer.

Atmospheric water content and turbidity parameters PHYSICAL

PARAMETERS

AND

THEIR

151

RELATIONSHIPS

We remind here that the solar specific intensity Z, falling at the earth surface may be expressed by means of the B~~e~-~~~t equation ZL = Zor*exp(-m-5,)

(1)

where Zol is the specific intensity falling at the atmospheric top.@*‘) The parameter m is the relative optical air mass which can be calculated, for each observation, according to Bemporad’s approximation’879) as a function of z, the solar zenith angle. When the me~urements are performed at altitudes other than that of calibration, m must be scaled by multiplying for the factor P,,/P according to King and Parryoot and Tomasi and Guzzi.‘2’ P and PO are the actual pressure at the altitude of the observation and that at the calibration level, which is sea-level for our instrument, respectively. ri. in equation (1) is the total optical depth along the vertical local path and can be expressed as

i.e. as the sum of several contributions: (1) water vapour absorption, z,,&; (2) Rayleigh molecular diffusion, rR+?.;(3) particulate matter (aerosol) diffusion, T,,,.; ; (4) ozone absorption, ro+ Actually, the first three contributions are the most relevant within the wavelength range at which our hygrometer operates. In addition we note that at Al,= 0.93 pm and at 1, = 1.03 pm both the Rayleigh and aerosol diffusion may be assumed to be nearly equal because of the small difference between &, and I,. On the contrary, the water vapour absorption is much larger at &, than at 1,. Under these conditions, the following relationships hold:

By combining equations (1) and (3) we obtain I. a=?

*.

= K*exp(-m*r.W.lJ

(4)

where

Again, we remind that r w,?.,= C*.+- w where Cw,1,, is the atmospheric water vapour absorption W(g*cmm2) the precipitable water. Equation (4) becomes 8 = kt*exp(-C,.i..X)

(5) coefficient in g-r cm2 and

(6)

It is then possible to determine the actual amount of atmospheric precipitable water W by measuring the ratio .92once m is computed. This is obviously possible at day-time. On the other hand, if one investigates the possibility for a site to become suitable for i.r. astronomical observations, night-time

A. BORGHESIet al.

152

determination of the parameter W is also necessary in order to analixe the entire behaviour. Radiosoundings data unfortunately were not available for the places where we performed our campaigns. To overcome this problem we have followed a method posed by Tornas?*,‘) which is based on the empirical relationship suggested by tan:“‘) In W= a + b*T,

day have proRei(7)

Here Td is the dew-point temperature, a and b two empirical coefficients. The value of the slope coefficient b, determined by several authors,“‘-r3) is b = 0.061 (‘XI)-‘. The intercept coefficient a depends on seasonal and meteorological conditions of the atmosphere because it is bound to the absolute humidity vertical profile.“*) An empirical relationship has been found to hold(4,5*14) a = -0.73 - 0.97.ln B

(8)

where B = po/ Wand p. is the absolute humidity. B is generally slowly variable with time under stable atmospheric conditions. The adopted procedure, indicated as the ‘dew-point (DP) method’, is as follows:

(1) the dew-point Td time profile is determined from the late afternoon to the early hours of the next morning by measuring both the atmospheric temperature and the relative humidity. Psychrometric tables are used; (2) the coefficient a is calculated according to equation (8) by assuming a linear variation of B between the values experimentally determined in the late afternoon and in the early hours of the following morning. This approach is justified by the above mentioned behaviour of B; (3) finally the precipitable water vapour W is evaluated by means of equation (7). A typical result obtained following this procedure is reported in Fig. 3 for the couple of days 22nd/23rd and 26th/27th August 1980 at the Campo Imperatore Observatory. The 2.5

N

2.0

‘E 0 & d

I.5

; E 0 z 0 .t .P : a’

1.0

0.5

0

L

1

)6

12

L

16 Local

1

1

24

06

12

4

time

Fig. 3. Typical results of the night-time interpolation method used to determine the water vapour content. The dots represent the experimental day-time observations.

Atmospheric water content and turbidity parameters

153

dots represent the observations performed by the hygrometer while the broken and full lines correspond to the empirical values of the precipitable water W It is interesting to note, as it should be expected, that W decreases considerably at night-time. For what concerns the aerosol its optical thickness may be expressed as(‘.**t@ z&??,rl = B.A-a

(9)

where a and #Iare the so called turbidity parameters which depend upon the ~mensions, size distributions and chemical nature of the particulate matter. Two independent measurements are actually necessary to determine CIand #?.This is done by means of the Volz turbidimeter performing the observations respectively at R, and AR(see above). The values of r R,I and z~,,~ which appear in equation (2) are taken respectively from Kondratyev’*) and Guzzi et al.(‘) The water vapour absorption is finally determined by using the absorption coefficients at AG and JR while W is directly measured by means of the near-i.r. hygrometer. OBSERVATIONS

AND

RESULTS

In Figs 4a and 4b, we report the time profile determined respectively for U: a and /3 at Camp0 Imperatore on 22nd, 25th and 26th of August 1980 between 6.00 and 18.00hr, local time. The observations have been carried out every 30 min. These days were characterized by anticyclonic conditions and are representative examples of the whole set of measurements. For what concerns % we note a tendency to increase gradually in the morning hours up to peak values which occur between 11.00 and 13.00hr. Later on, W decreases with some fluctuations towards a minimum, which is attained at hight-time, usually around 22.00 and 23.00 hr (see Fig. 3). More complicated is the behaviour of the aerosol: p! appears essentially stable on the 22nd and 25th while it fluctuates considerably on August 26th. 19shows a general tendency to increase smoothly on the 25th with a peak at around 15.00hr; large ~uctuations are revealed on the 26th throu~out the day. An increase up to a m~imum around 11.00hr is seen on the 22nd followed by a smooth decrease until 16.00hr. The results obtained during our three campaigns are summarized in Table 1. The first column indicates the place where the measurements have been done while the period of observation is given in the second one; column three reports the percentage of anticyclonic days. The minimum, maximum and mean values for W Q and /I are finally given respectively in columns 4-12. The following conclusions can be drawn: Precipitable water, W (1) A seasonal variation seems to be present at sea-level. (2) The month of August at the Camp0 fmperatore Observatory does not seem to be very suitable for i.r. astronomical observations because of the not negligible amount of water present in the atmosphere. We note that the values of Ware quite larger than those measured at sea-level two months later. (3) Night-time precipitable water estimations, as it is to be expected, are lower than those at day-time but still high and comparable to those found at sea-level in October confirming the conclusions cited in Ref. (2). (1) A seasonal variation seems again to be present at sea-level, (2) The range of variability between minimum and maximum values both for a and j? at sea-level is considerably lower than that observed at the mountain site. (3) The aerosol optical depth haa always been found to be less than unity during the three campaigns.

A. E!ORGHESI et ai.

154 I

N

2.0

-

‘5 ir

s’ : E u

l.O-

3 .cCL : l?

Loco1

time

Fig. 4. (a) Precipitable water W

f

!1

!! !!

!!

=_=

06

IO

=

12 Local

I4

16

18

t

time

Fig. 4. (b) Turbidity parameters o(and B, measured on 22nd, 25th and 26th August 1980 at Camp0 Imperatore.

155

Atmospheric water content and turbidity parameters Tabfe 1. Synthetic scheme of the collected data

Site

Period of observation (1980)

Turbidity parameters

Precipitable water, W (g.cm-2)

max

min

mean

Anticyclonic days (%)

min

max

mean

(a)

(B)

(a)

(B)

(a)

(B)

40 21

0.6 0.6

2.3 1.5

1.5 1.1

OS 0.7

0.18 0.08

1.5 1.4

1.30 0.26

1.1 1.0

0.33 0.20

S-27 August ~~hr-ti~ (22.~3.~ hr)

7s

0.6

2.3

1.5

0. I

0.05

1.9

0.59

0.9

0.22

7-27 August

so

0.5

1.6

1.0

Day-time

Lecce Lecce Campo Imperatore Camp0 Imperatore Interpolation

(06.00-18.00 hr) 13-24 June 2-10 October

method (DP method).

In this case again the results obtained at Campo Imperatore appear to be comparable with those found at sea-level in October confirming the peculiarity of the atmospheric conditions found at the mountain site in August. All of the above aerosol results seem to be due to the local behaviour of both microclimate and pollution which are not easy to monitor, therefore requiring further specific investigations. CONCLUSIONS

Our preliminary work has clearly shown that the experimental apparatus that we have developed is suitable for performing hygrometric observations in sites which may be used to install i.r. telescopes. Water vapour content measurements are easily done during day-time; in addition, the empirical dew-point method allows extrapolation to estimate Wat night-time; so that the whole 24 hr period can be covered. On the other hand, it does not appear easy to interpret turbidity data since they seem to be correlated with very peculiar local conditions. Further effort in this direction will be devoted in the near future to overcome the present problems. Following these results, a continuous monitoring programme is planned for next year at Campo Imperatore and at other mountain sites in Sardinia and southern Italy. ~ck~ow~ed~e~~rs_The authors wish to express their thanks to Dr Tomasi, for useful suggestions and several stimulating discussions; to Professor P. Giannone, for the hospitality at the Astronomical Observatory in Campo Imperatore and to Mr G. Giuliani, of the same Observatory, for his assistance during the observations. Mr M. Vantaggiato of the Photographic Laboratory is also thanked for the photographic reproductions of the illustrations in this paper. REFERENCES 1. FOWLE F. E., Astrophys. J. 35, 149 (1912).

2. TOMASIC. & R. GUZZI, J. Phys. E scient. Instrum. 7, 647 (1974). 3. MARTINJ., J. GOMEZ-~NZALEZ & A. BARCIA,Infrared Phys. 21, 117 (1981). 4. TOMASIC,, Proc. Co& on Infrared Astronomy, Lecce, September 1977. &fern. Sot. astr. ital. 49, 119 (1978). 5. TOMASIC., Preprint Lab. FISBAT, CNR, Bologna (1980). 6. THEKAEKARAM. P. Sofur Energy, 14, 109 (1973). 7. Guzzr R., R. RIZZI & S. VINDIGNI,Proc. 2nd ~nte~tjo~~ So&w Form, Hamburg, Vol. II (1978). 8. KONDRATYEVK. YA, Radiation in the Atmosphere. Academic Press, New York (1969). 9. FALCICCHXA G., Thesis, Institute of Physics, University of Lecw (1981). 10. KING R. L, & H. D. PARRY,Humidity and Moisture, Vol. 2, pp. 450-457. Reinhold. New York (1965). 11. REITANC. H., J. a&. Meteorol. 2, 776 (1963). 12. SMIITHW. L., J. appl. Meteorol. 5, 726 (1966). 13. LOWRY D. A. & H. R. GLAHN, J. appl. Meteorol. 8, 762 (1969). 14. TOMA~IC., J. appl. Meteorol. 16, 237 (1977). 15. B~I-~ARID., A. DUVAL & G. SARRA,I.T.A.V., Ufficio Affari Generali (Sezione Tecnica), CNR (CENFAM)A.M.I. Servizio Metereologico (1962). 16. SCHUEPPW., Arch. Met. Geoph. Bisk. Bl, 257 (1949).