GEOPHYSICAL RESEARCH LETTERS, VOL. 18, NO. 12, PAGES 2261-2263, DECEMBER 1991
THE BLISS MEASUREMENTS OF NO2: SOME NEW INSIGHTS
D.J. Lary and J.A. Pyle
Department of Chemistry, Cambridge Urdversity, UK.
C.R. Webster and R.D. May
Jet Propulsion Laboratory, California Institute of Technology
Abstract - The diurnal variation of NO 2 measured near
30 km by the BLISS in-situ laser spectrometer is
compared with calculations from a photochemical model
that includes a detailed escription f multiple scattering.
Introduction
In a recent paper, the stratospheric chemistry ofNO 2
was studied by combining high resolution tunable diode
laser measurements from the balloon-borne laser in-situ
spectrometer (BLISS) with a numerical model [Webster
et al., 1990]. When the model was constrained with the
measured 03 and temperature, the main features of the
observed iurnal variation of NO 2 were well reproduced
by the model. In general, the study constituted a very
successful confirmation of our present understanding of
this subset of stratospheric hemistry. However, there
were some differences in detail between the model and
observations (Figure 1). In particular, the one daytime
measurement of NO 2 could not be reproduced unless an
unrealistically large value of the ground albedo was used,
and there was a very rapid, but very brief, decline in the
measured NO 2 immediately after the major sunset
increase which the model did not reproduce. Finally some
minor differences between the modelled and observed
nighttime decays were evident with the model slightly
overestimating the values immediately prior to dawn.
This study reconsiders the detailed aspects of the model
data intercomparison. We have used a numerical model
with similar chemical kinetics but a much more sophisti-
cated treatment of scattering than that used in the earlier
study. The radiative transfer model is a new implementa-
tion of the scheme described by Meier et al. (1982). It has
been extended after Anderson (1983) to describe correctly
the radiation field for solar zenith angles greater than 75 ø .
The radiation into any volume element of the model
atmosphere has four contributions: The direct solar flux,
the diffuse flux incident from all directions, the ground
reflection of the direct solar flux and the ground reflection
of the diffuse flux. In this paper clear sky conditions have
been assumed. Excellent validation of the scheme against
various data sets has been achieved [Lary and Pyle, in
press, 1991a,b].
Copyright 1991 by the American Geophysical Union.
Paper number 91 GL02732
0094-8534/91/91GL-02732503.00
Results
In addition to several other species Webster et al. (1990);
May and Webster (1989), made simultaneous in-situ
measurements of NO2, 03, temperature, and pressure
from the balloon based BLISS instrument launched at
Palestine, Texas (32øN, 96øW), on 13 September, 1988.
These measurements allowed a critical test of the
photochemical model to be made, with very small
differences between theory and data as discussed above.
This new study has reproduced some of the finer
BLISS measurement details (Figure 2) with a
model/measurement agreement of < 10%, which is
comparable to the measurement uncertainties of 8% for
sunset and nighttime NO2, and 10% for the daytime NO 2
measurement. The model used to produce Figure 2 has a
ground albedo of 0.25, a constant emperature of 233 K,
a constant ozone concentration of 8.5 ppmv, and a total
reactive nitrogen content of 15.24 ppbv. The model
pressure is kept at 11.0 mb, except for one hour after
sunset, when it is raised to 11.6 mb in accordance with the
observations of Webster et al. (1990).
Firstly, as pointed out by Webster et al. (1990), the
single day time measurement of NO 2 could only be expla-
ined using their scattering scheme based on Kurzeja
(1976), if a ground albedo of 0.5 was used. This is an
unrealistic value for desert like conditions. When the
detailed treatment of multiple scattering is used at all
wavelengths, then the single day time measurement of
NO 2 can be explained to within the measurement
uncertainty by a ground albedo of 0.25 (Figure 2).
Secondly, the peak in NO 2 concentration which
occurred just after sunset can be reproduced if the slight,
but brief, descent of the balloon which occurred then is
included in the model. This descent was accompanied by
an increase in the pressure, and hence, in the total
number density. Note that in their model calculations
Webster et al. (1990) used a constant value for the total
number density of 3.6x1047 molecules cm '3. The observed
peak would be less obvious if the concentration of NO 2
was expressed as a volume mixing ratio.
Finally, in the Webster et al. (1990) study the modelled
NO 2 prior to sunrise slightly overestimated the measured
values, although the measured and modelled nighttime
decay certainly agreed within the experimental error.
Webster et al. (1990), discussed the sensitMty of the decay
to temperature. By using a temperature of 233 K, instead
of 231 K as used in the earlier study, even better agree-
ment is found, especially immediately before dawn.
2261

2262 Lary et al.: New insights into the BLISS measurements of NO 2
2.50x10 
ß . BLISS DATA
. .......... MOOEL NO2
I 'X.// \,
o
........
(pro) LOCAL TIME
O6 06 10 12
Fig. 1. A comparison f the diurnal variation ofNO 2 close to 11 mb measured byBLISS (squares) and
the model of ,,,,Webster e al. (1990). The measurement da a is tabulated inWebster et al. (1990).
The ballon descended slightly for a short
period just af er sunset, ,
6 9 12 15 8 21 24
4
[½ebster et cL 19;90. Sunset Lry !990, Model. 
3 3
1
0 3 6
._ Suryise
ø 2
233 K
A-0.25
Palestine, Texas, 32øN.
September 13
1
9 12 15 18 2! 24
Local Time (Hours)
Note that the model has accurately reproduced the rapid
change in NO2 at both sunrise and sunset,
Fig. 2. A comparison of the diurnal variation f NO 2 close to !1 mb measured by Webster ta.1. (1990) (squares) and the model used in this study (line).

Lary et al.' New insights into the BLISS measurements of NO 2 2263
Conclusions
The data presented by Webster et al. (1990), have been
compared with a new numerical model including an
improved radiative transfer scheme. We now find even
better agracement between the data and the model, both
at sunset and during the day. We confirm the conclusions
of the earlier study that the high resolution in-situ
measurements of NO 2 have enabled a very successful
confirmation ofour understanding of the diurnal chemistry
of NO2.
_Acknowledgements. D Lary thanks SERC for a
studentship and R. Toumi for many helpful discussions.
References
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Di'. Lary'and S.A. Pyle, Department of Chemistry,
Cambridge University, Lensfield Road, Cambridge, CB2
1EW, UK.
R.D. May and C.R. Webster, Jet Propulsion
Laboratory, California Institute of Technology, USA.
CReceived August 14, 1991;
accepted September 28, 1991)