Optics: measuring and testing – By particle light scattering – With photocell detection
Reexamination Certificate
1999-12-22
2002-06-11
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By particle light scattering
With photocell detection
C356S339000, C356S340000, C356S342000, C356S441000, C356S442000, C250S574000, C250S575000
Reexamination Certificate
active
06404494
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a method and apparatus for monitoring aerosols or particulates in a fluid, and more specifically, to a method and apparatus for monitoring the lidar ratio for particulates or aerosols in the atmosphere.
BACKGROUND OF THE INVENTION
A recent National Research Council panel report summarizes six independent lines of evidence supporting the hypothesis that direct (i.e., clear-sky) climate forcing due to the scattering and absorption of sunlight by anthropogenic aerosols is a major factor in global climate change. Visibility is similarly known to depend on scattering and absorption of light by atmospheric aerosols. A variety of aerosol measurements (as well as theoretical models) contribute to this evidence, but notably lacking is a physically meaningful contribution from elastically scattering lidar. Nevertheless, the potential contribution of this technology is enormous, given its exquisite precision, vertical resolution, and the relative ease of data acquisition. This potential has yet to be exploited because of difficulties in quantitatively and accurately relating the elastically scattered lidar signal to the aerosol parameters relevant to climate forcing and visibility.
Analogous to radar but operating at shorter wavelengths, a lidar instrument transmits pulsed laser radiation and measures what is backscattered by gases, particles, or other objects in the atmosphere. The return time of the signal corresponds to distance from the transmitter such that range-dependent information is acquired. The intensity of the signal depends on two quantities: (1) how effectively the laser radiation is backscattered at a specific location in the atmosphere; and, (2) how effectively the laser radiation is extinguished by the intervening atmosphere. Interpreting the lidar signal depends on an ability to separate these two quantities—local 180° backscatter and optical depth over the entire range. It is this deconvolution of local backscatter and range-dependent optical depth, which is at the heart of the lidar retrieval challenge.
Following instrument calibration, a vertically pointing lidar provides a direct measurement of the quantity S(z), characterized by the following equation:
S
(
z
)
=
A
β
(
z
)
exp
[
-
2
∫
z
L
z
σ
e
(
z
′
)
ⅆ
z
]
=
A
β
(
z
)
exp
[
-
2
τ
(
z
L
,
z
)
]
(
1
)
where A is an instrumental calibration constant, &bgr;(z) is the 180° backscatter coefficient (m
−1
sr
−1
) from both molecules and aerosols at height z(m), &sgr;
e
is the extinction coefficient (m
−1
) from both molecules and aerosols at height z, and &tgr;(z
L
z) is the extinction optical depth between the lidar height, z
L
, and z. Equation 1 shows that the fundamental challenge of converting the lidar measurement, S(z), to a geophysically meaningful aerosol quantity is to disentangle &bgr; and &tgr;— or, equivalently, &bgr; and &sgr;
e
. Since molecular scattering can be predicted accurately from air density (i.e. temperature and pressure) information, this challenge reduces to disentangling particulate backscattering, &bgr;
p
, from particulate extinction, &sgr;
p
. Two types of technologically advanced lidar systems, Raman lidar and high spectral resolution lidar, are able to separate these terms by making auxiliary measurements of the return signal. These instruments are described briefly below.
For lidar systems that detect elastically scattered light only, the quantities &bgr;
p
and &sgr;
ep
can be disentangled if the ratio of the two parameters is known. This quantity is referred to as the lidar ratio, K,
K
(
sr
)
=
σ
ep
β
p
=
σ
sp
+
σ
ap
β
p
(
2
)
where &sgr;
sp
and &sgr;
ap
are the components of particulate extinction due to light scattering and light absorption, respectively.
Based on Mie calculations that incorporate the ranges of particle size distributions and refractive indices encountered in the troposphere, possible values of K span at least an order of magnitude, from approximately 10 to 100 (sr). The lower values correspond to coarse-particle aerosols like soil .dust and sea salt, while the higher values represent fine particles of smoke and products of gas-to-particle conversion. To explore the sensitivity of lidar-retrieved optical depth to uncertainties in K, we use data from the recent lidar demonstration Shuttle mission (LITE). Table 1 shows the effect on retrieved optical depth of allowing K to vary from 10 to 100. Data consists of two cases when aerosol layers were detected at night over Africa during the LITE mission. The columns labeled ∂log&tgr;
p
/∂logK indicate how a fractional uncertainty in lidar ratio would translate into a fractional uncertainty in optical depth. This sensitivity parameter is seen to vary between the two cases and to be a strong function of lidar ratio. For low K values, K and &tgr;
p
are nearly proportional. For the higher K values (which tend to be characteristic of pollution-derived particles in the sub-&mgr;m size range), the sensitivity is considerably higher—up to a factor of 4. Overall, the factor of ten range of possible lidar ratios translates into a factor of 10 to 40 uncertainty in retrieved optical depth. This range is too large to offer an adequate constraint on lidar retrievals for the problems of climate forcing or visibility.
TABLE 1
Case 1*
Case 2**
K
&tgr;
p
∂log&tgr;
p
/∂logK
&tgr;
p
∂log&tgr;
p
/∂logK
10
0.022
1.06
0.021
1.09
20
0.048
1.14
0.046
1.20
30
0.077
1.22
0.077
1.33
40
0.111
1.31
0.115
1.46
50
0.150
1.42
0.162
1.63
60
0.196
1.55
0.221
1.83
70
0.252
1.71
0.299
2.09
80
0.321
1.91
0.404
2.47
90
0.408
2.18
0.558
3.10
100
0.523
2.55
0.817
4.25
*Case 1: Average over 400 records beginning MET 009/01:09:02.60. Aerosol layer extends from 1388 m to 5013 m above sea level.
**Case 2: Average over 300 records beginning MET 009/01:10:32.60. Aerosol layer extends from 1532 m to 5832 m above sea level.
For lack of accurate knowledge of K, most aerosol measurements by elastically scattered lidar are reported as a “scattering ratio” — that is, the ratio of the calibrated signal to the expected signal for particle-free air. This term is useful for qualitative identification of aerosol layers, but not for input into radiative transfer models. The instrument described herein provides a relatively inexpensive method for accurate local measurement of &bgr;
p
. When combined with existing instrumentation for measuring &sgr;
ep
, this permits an empirical determination of K.
Being small and portable, the new device permits routine ground-based monitoring as well as airborne surveys of &bgr;
p
and K, which will, in turn, allow extensive lidar data sets on tropospheric aerosols to be applied in a quantitative fashion to the aerosol/climate and visibility problems.
The “backscattersonde” described by Rosen and Kjome In “Backscatersonde: a New Instrument for Atmospheric Aerosol Research,” Applied Optics, Vol. 30, pp. 1552-1561 (1991) offers a local measurement of &bgr;
p
. The backscattersonde is light and inexpensive, and thus well suited for balloon-borne measurements of atmospheric backscatter versus altitude; in contrast, the instrument described herein is currently both too large and too expensive for routine balloon deployment. The backscattersonde has been used to determine the lidar ratio by running it in parallel with a separate instrument that measures scattering and with assumptions about particle absorption.
The backscattersonde has an open sensing volume and a flash lamp light source, so it cannot be calibrated in the laboratory with gases or with particles of known concentration, size and refractive index, and it can only be used at night. The calibrations rely on measurements of air Rayleigh backscattering in the stratosphere in the winter Arctic polar vortex, where particle concentrations are believed to be insignificant. Previous or subsequ
Anderson Theodore L.
Charlson Robert J.
Masonis Sarah J.
Anderson Ronald M.
Font Frank G.
Nguyen Sang H.
University of Washington
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