In situ measurement of the solar radiance distribution within sea ice in Liaodong Bay, China

Cold Regions Science and Technology 71 (2012) 23–33

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In situ measurement of the solar radiance distribution within sea ice in Liaodong Bay, China Zhantang Xu b, Yuezhong Yang b,⁎, Zhaohua Sun b, Zhijun Li c, Wenxi Cao b, Haibin Ye a, b a b c

The graduate school of Chinese Academy of Sciences, Beijing, China State Key Laboratory of Oceanography in Tropics, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian, Liaoning, China

a r t i c l e

i n f o

Article history: Received 22 February 2011 Accepted 11 October 2011 Keywords: CDOM Albedo Transmittance Solar radiance distribution Liaodong Bay

a b s t r a c t Understanding the optical properties of polluted sea ice is important for analyzing remote sensing data, and studying energy balance. Liaodong Bay in China provides a kind of representative of “dirty” sea ice for there are many industrial and residential areas surrounding it. Optical properties of sea ice were observed. Albedos for different types of sea ice surface were obtained, and their peaks would tend to shift to longer wavelengths for the high concentrations of PM and CDOM in sea ice. Upwelling (Ku) and downwelling (Kd) irradiance extinction coefficients profiles were obtained in situ with a self-made instrument. At 586 nm, the maximum Ku reached 13.68 m − 1 in a layer close to ice bottom, while the maximum Kd reached 15.52 m− 1 in surface layer. Because understanding the energy distribution in sea ice is important to research mass budget in air–ice-sea, solar radiance distribution in sea ice was observed in a series of bored holes in situ. From 4 cm to ice bottom, the logarithms of radiance profiles almost decrease linearly with increasing the path length of light transfer. Because sea ice is a highly scattering medium and the effects of scattering become stronger with increasing depth, the dependence of scattering light on scattering angle (θ) becomes weaker with increasing depth. An optical model was brought forward to describe the solar radiance distribution at a random depth and θ. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction Sea ice is generally understood to be a key element in the Earth's climate system. Its physical structure is highly complex, consists of an usually unknown distribution of gas bubbles, irregularly shaped brine pockets, fractures, and embedded particles, which strongly scatter light (Maffione et al., 1998; Perovich and Govoni, 1991). In order to understand this physical structure in sea ice, it is important to obtain the irradiance extinction coefficient at different depths. Furthermore, heat and mass exchange within air–ice-sea system is important for the study of climate. Thus, it is necessary to know how solar radiance distributes within sea ice. Although the extinction coefficient of whole sea ice was studied frequently, there was still no report about the vertical extinction coefficient profiles observed in situ. Studies of light field within sea ice were carried out extensively (Pegau and Zaneveld, 2000), such as, according to the method of Hnoey (1979), Gilbert and Buntzen (1986) attempted to obtain a point spread function within sea ice by setting a Lambertian light source into the seawater beneath ice and recording the radiance distribution emerging above the ice with a camera. Maffione et al. (1998) lowered a collimated light source

⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Yang).

and an irradiance detector into two holes, respectively, to measure the beam spread function (BSF) over a full angular range of 0–180°, and found that the single scattering albedo was generally larger than 0.97 at visible spectrum. By measuring BSF in a series of distances between two bored holes, they also found that the irradiance light rapidly approached an asymptotic state. Pegau and Zaneveld (2000) determined portions of the radiance distribution by profiling a 3° field-of-view radiance head into a series of ice holes at a number of zenith and azimuth angles. The results show that the light field nearly reaches an asymptotic state within the first 0.25 m. Some of the inherent optical properties of sea ice were derived from the measurements as well. Light et al. (2008) lowered two kinds of probes sequentially into a bored hole to record downward and upward propagating radiation. The comparison of the measured and predicted values of irradiance shows that the effects on measurements caused by the bored hole and blind corner of detector itself are small. Ehn et al. (2008) used an optical detector which was similar to that used by Light et al. (2003) to observe downwelling irradiance profiles in a thick landfast sea ice. Then, the inherent optical properties of sea ice were obtained, by tying the input and output of radiative transfer simulations to the measurements, i.e., radiation profiles, albedo and transmittance. Although there are many reports about the measurements of light observed within sea ice, most of them are still observed in laboratory with an artificial light source. Although some experiments are

0165-232X/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.coldregions.2011.10.005

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conducted in situ with the solar source, the measurements are only refer to radiance distributed in vertical direction. In order to understand the energy distributing within sea ice, we should not only observe solar radiance distribution in vertical direction in situ, but also observe that in other directions with a number of different zenith and azimuth angles. Furthermore, environment pollution becomes more serious in recent years than ever before. As a result, the pollutants in sea ice derived from atmospheric fallout and ocean currents become more in the polar region. Understanding the optical properties of “dirty” sea ice is important for analyzing remote sensing data, and studying energy balance. For this study, we mainly concern three questions: first, how the solar irradiance is reflected by “dirty” sea ice; second, what about the extinction coefficient profiles of “dirty” sea ice; third, how the solar radiance distributes in both vertical and the other directions. In order to study the questions mentioned above, two conditions should be satisfied. One is that where has naturally occurred “dirty” sea ice, the other is that there should be a suitable optical instrument to observe these optical parameters. Liaodong Bay in China is surrounded by many industrial and residential areas and its mean depth is only 18 m, which are responsible for high concentrations of colored dissolved organic matter (CDOM) and particulate matter (PM) in sea ice. Even with visual inspection, some black ashes of incomplete combustion are found in ice surface due to residents heating, oilfield burning, and industrial burning. Thereby, Liaodong Bay provides an ideal field of natural “dirty” sea ice. For oceanic optics observation, there are a number of commercial optical instruments developed by many companies including Wet Labs, Satlantic, Ocean Optics and so on. Some of these optical instruments are immerged into seawater to observe the optical properties of seawater. However, sea ice is a kind of solid matter. It is impractical to set these instruments into sea ice to observe the optical properties of sea ice. Furthermore, low-temperature resistance of the instrument is an important requirement for the instrument would be used in low temperature environment. Thereby, in this article, we designed a hyperspectral radiation instrument for a specific application in observing the optical properties of sea ice. For this study, dominant sea ice in Liaodong Bay was selected. Spectral shapes of albedo was discussed by considering the concentration of PM and CDOM. Upwelling and downwelling irradiance extinction coefficient profiles were obtained using self-made upwelling and downwelling irradiance profilers. Additionally, in order to verify the applicability of the downwelling irradiance detector, we used it to observe sea ice transmittance, and compared the result with that observed using traditional method. Solar radiance distribution over a full angular range from θ = 0° to θ = 90° was observed in situ with self-made inclined profiler. An optical model was brought forward to describe the solar radiance distribution at a random depth and θ. 2. Material and methods The Bohai Sea is a semi-enclosed sea, which is surrounded by many industrial and residential areas. During periods with open water, wind and wave-induced currents are often sufficient to cause resuspension of sediments, thus increasing turbidity. Because Liaodong Bay is relatively well sheltered from the open sea, it becomes ice covered even during mild winters. The degrees of sea ice freezing are different each year, and the freezing period is often about three months ranging from Dec to Feb. The fieldwork was conducted in Liaodong Bay which lies in the northern part of Bohai Sea (Fig. 1), from 22 to 26 January 2010. During the observation, the sky was clear, and sea ice thickness around the observed sites ranged from 30 to 35 cm. Five sites were selected. With visual inspection, some black ashes of incomplete combustion were found in ice surface due to residents heating, oilfield burning, and industrial burning. Table 1 gives the sea ice condition for five sites. Sea ice at Site A was selected for detailedly analyzing

both the optical and physical properties, since that was the dominant type of sea ice distributed widely in Liaodong Bay. The other sites are chosen for observing the physical or optical properties. 2.1. Ice and seawater sampling Sites A and B were selected for observing the physical properties of sea ice. Surfaces at the two sites were smooth without any snow covered. Ice cores were sampled in situ. By taking the ice cores back to laboratory, salinity profiles were determined after ice melted. In order to determine the concentration of particle matter, melted seawater was filtered under low vacuum on to a 0.4-μm pre-weighed polycarbonate filter. The filter was rinsed with 25–50 ml of pure water to eliminate salts as much as possible, according to the method of Van der Linde (1998). Then, particulate concentration profiles were obtained, after drying the filters at 40 °C for 48 h and re-weighted them at room temperature. Particulate concentration in the underneath seawater was obtained using the same method as well. It is difficult to measure the CDOM concentration within sea ice quantitatively. In this study, the absorption coefficient magnitude of CDOM was used to describe relative CDOM concentration contained in sea ice. In laboratory, ice layers at different depths were cut from the ice cores, and melted at room temperature. The melted seawater was filtered by polycarbonate filters with pore size of 0.2 μm. Then, the filtrate was used to determine the absorption coefficients of CDOM, with a spectrophotometer (U-3010 spectrophotometer, Hitachi) following the procedure outlined by Bricaud et al. (1981). 2.2. Optical instrumentation and measurements Using a single optical channel to observe optical parameters at different places would introduce a large errors caused by the changes of solar incident irradiance. Therefore, a hyperspectral radiation instrument equipped with three spectrometers (Avaspec-2048FT-3-DT, AVANTES of Holand) was designed, which could record three optical channels simultaneously. The effective integration time is 3–3000 ms, which can be adjusted by itself intelligently, and the instrument could record the data automatically. A radiance and an irradiance detector with diameters of 3.1 cm were designed (Fig. 2). Radiance detector (Fig. 2.a) is a fiber-optic head protected with a capsule. Light can penetrate through a K9 optical glass (Optical Borosilicate Crown Glass) into the fiber-optic head. Radiance detector has a full field-of-view of 10°, which is assumed to sample irradiance from a specific angle. Irradiance detector (Fig. 2.b) is a polytetrafluorethylene cosine collector, which samples irradiance from all angles of a hemisphere (л sr) according to the cosine of the incident angle. As incident angle (σ) increasing, the response of irradiance detector meets a cosine law; such as E = E0cosσ, where E0 is solar irradiance at the direction of σ. By testing the response of irradiance detector in laboratory, the result shows that the response error of irradiance detector is less than ±0.05 when σ ≤ 70°, and the cosine characteristic curve of irradiance detector is given by Cao Wenxi et al. (2002). Optical detector calibrations were performed in optical laboratory using a Quartz halogen traceable source. In air the spectral scan limits of the instrument are 350–1000 nm, and in water the limits are 400–900 nm, with a wavelength resolution of 1.6 nm. The ratio of signal to noise for the instrument is 250:1. Optical error is within 0.5% over the spectral range 350–1000 nm. Furthermore, the instrument was equipped with four thermistors, an inclinometer, and a GPS. 2.2.1. Aldedos and transmittance (1) For measurements of albedos, an irradiance detector was set to look upward vertically to record downwelling irradiance ((Ed(λ,0−,t)), where 0 − and 0+ means the position just above and beneath ice surface, respectively. t is time. Using t can establish a link among different optical parameters observed at the same time), and another irradiance

Z. Xu et al. / Cold Regions Science and Technology 71 (2012) 23–33

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Fig. 1. Map of the study area in Liaodong Bay, China. Optical and physical properties were observed around the area marked by the star.

detector was set in an inverse direction being 80 cm up from the ice to record upwelling irradiance (Eu(λ,0−,t)) reflected by ice surface. (2) For measurements of transmittance using a traditional method, the irradiance detector which has been used to record upwelling irradiance (Eu(λ,0−,t)) in the measurements of albedo was mounted on a 1.25-m-long “arm” with a rotatable elbow joint, and the other detectors were kept in original position to observe Ed(λ,0−,t) continuatively. The arm was lowered through a 15-cm-diameter hole bored in ice to make the irradiance detector beneath ice bottom and adjusted to be parallel with it, so that the detector could look upward at a zenith angle of 0° to record the downwelling irradiance (Ed(λ,z+,t)), where z is ice depth. Then, the albedo and transmittance of sea ice could be obtained by α ðλ; t Þ ¼

Eu ðλ; 0− ; t Þ Ed ðλ; 0− ; t Þ

ð1Þ

T ðλ; t Þ ¼


Ed λ; zþ ; t : − Ed ðλ; 0 ; t Þ

ð2Þ

2.2.2. Solar radiance distribution Vertical profiles of irradiance within ice were observed using a downwelling and upwelling irradiance profiler as shown in Fig. 3. The ideas using to design downwelling and upwelling irradiance Table 1 Sea ice condition of the five sites during observation. Sites

Thickness Ice surface (cm)

Site A 30

Site B Site C

35 30.5

Site D 33

Site E

31

Surface is white and smooth, some black ashes of incomplete combustion are found in ice surface. This ice is the dominant type of sea ice distributed widely in Liaodong Bay. Surface is white and smooth, which contained few impurities. Surface appears white, and it is coarse for the diurnal melting and refreezing. The topography is higher than its surrounding ice. Substantial daytime surface melting and nighttime refreezing occurs, leaving the ice surface with a whitish high-scattering layer with substantial gas bubble inclusions. Surface is white with much PM. An intricate skeletal structure of fragile ice crystals permeated by void spaces formed on the surface layer, for the sufficiently melting and metamorphism.

profilers are referred to Ehn et al. (2008) and Light et al. (2008). The downwelling irradiance profiler consists of a radiance detector (Fig. 2.a) aiming at a disc-shaped diffuse reflecting Spectralon target, which has a diameter of 5.0 cm. The distance between the radiance detector and target is 5.1 cm. There are two support beams used to fix the diffusely reflecting Spectralon target upon the radiance detector. The downwelling irradiance profiler essentially records radiation incident on the diffuse reflectance standard from between zenith angles 14°and 90°. The upwelling irradiance profiler (Fig. 3) just consists of an irradiance detector (Fig. 2.b), which is used to observe irradiance from all angles of the upwelling hemisphere. In order to observe the upwelling and downwelling radiation profiles, a hole with a diameter of 5.5 cm was drilled. The holes used for measuring the upwelling and downwelling radiation profiles were not drilled all the way through the ice. During each depth measurement, a tube with a exterior diameter of 0.4 cm was lowered down till the hole bottom to pump the seawater out. Then, all of the measurements of radiation profiles were conducted in air. Solar incident irradiance (Es(λ,0 −,t)) was monitored simultaneously to take into account temporal changes in radiation field. First, downwelling irradiance above the surface (Fs(λ,0 −,t)) was observed, and the relative solar incident (Frs(λ,0 −,t)) was calculated as 

F rs ðλ; 0 ; t Þ ¼

F s ðλ; 0 ; t Þ Es ðλ; 0 ; t Þ

ð3Þ

Then, downwelling irradiance profiles (for F(λ,z,θ,t) at θ = 0°) were observed from ice surface to hole bottom with an interval of 2 cm. Relative downwelling irradiance (Fr(λ,z +,0,t)) can be written as F r ðλ; z; 0; t Þ ¼

F ðλ; z; 0; t Þ Es ðλ; 0 ; t Þ⋅F rs ðλ; 0 ; t Þ

ð4Þ

Spectral bulk transmittance T(λ,z,t) was given by T ðλ; z; t Þ ¼

F r ðλ; z; 0; t Þ : F r ðλ; 0 ; 0; t Þ

ð5Þ

Using upwelling irradiance profiler (Fig. 3), the upwelling radiance profiles (Fr(λ, z,180,t)) were observed in the same hole where has been used to observe the downwelling radiance profiles. Solar incident irradiance (Es(λ,0 −,t)) was also monitored simultaneously.

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Fig. 2. Fig. 2. Radiance (a), irradiance detector (b), instrument (c) and thermal cloth (d).

Fig. 3. Measurements of irradiance extinction coefficients and radiance distribution with profilers. In the vertical hole, vertical irradiance profiles could be observed with downwelling and upwelling irradiance profilers. Radiance profiles from θ = 0° to θ = 90°could be observed with an inclined profiler in inclined holes.

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Spectral irradiance extinction coefficient (K(λ,z)) was derived from the values of irradiance profiles at two depths, which can be given by K ðλ; zÞ ¼

lnðF ðλ; z; θ; t Þ=F ðλ; z þ Δz; θ; t ÞÞ : Δz

ð6Þ

Where K(λ,z) is a downwelling irradiance extinction coefficient when F is the relative downwelling irradiance, or K(λ,z) is a upwelling irradiance extinction coefficient when F is the relative upwelling irradiance. Inclined radiance profiles at a zenith angle of θ from 0° to 90° were observed by lowering an inclined profiler (Fig. 3) into a series of inclined holes. Inclined profiler consists of a radiance detector vertically mounted onto a hold pipe. A series of measurements would start within a vertical hole (θ = 90°). Upon completion of measurements within the vertical hole, a second hole was drilled at a given θ and radiance profiles were conducted. Subsequent holes were drilled at shallower angles to ensure the inclined profiler would observe in undisturbed ice. Actually, measuring the inclined profiles at all angles among 0° to 90° is impractical. It is also difficult to bore an incline hole at θ close to 0°. In this study, inclined profiles at θ equal to 30°, 45°, 60°, and 90° were chosen to observe. Light et al. (2008) used a similar downwelling profiler which has a angular response from 15° to 90° and a similar upwelling radiation profiler to observe vertical profiles, and found that the measurements were in agreement with the values predicted with a 2-D Monte Carlo radiative transfer model. Thereby, the measurements of relative radiance distribution at θ = 0° and θ = 90° were reasonable by using the downwelling and upwelling profiler. 3. Results and discussion 3.1. Physical properties Fig. 4 shows the salinity and particulate concentration profiles. Averaged ice salinity was 5.0‰ at Site A while that was only 3.1‰ at Site B, and the seawater salinities beneath Sites A and B were 30.5‰ and 31.3‰, respectively. The salinity ratio between sea ice and the underneath seawater was 1:6 at Site A, and was 1:10 at Site B. Owing to solar heating, the drainage of brine and melt-water would happen.

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The drainage phenomena would be seriously at the surface layer, which would influence salinity at the surface layer. However, we do not think the drainage would largely influence the salinity of whole sea ice (due to the thin of surface layer). We guess that the large difference in salinity ratio between sea ice and the underneath seawater would be mainly caused by ice formation rate. The faster the ice formation rate was, the shorter time for the salinity to escape from the freezing ice, and the more salinity would be captured in sea ice when ice formation. Thereby, we guess that the ice formation rate at Site A was faster than that at Site B, for the salinity ratio at Site A was 1:6 larger than that of 1:10 at Site B. Salinity profiles at Site B showed little change with depth compared with that at site A, then we guess that the environment of Site B during ice formation was steadier. There are many industrial and residential areas surrounding Liaodong Bay, which are responsible for a lot of suspended particulates in atmosphere and seawater. Thereby, PM was enriched in surface layer derived from atmospheric fallout (dust and soot). At shorter wavelengths, the contribution of particulate absorption to ice bulk absorption was considerable. More particulate in surface layer will give larger absorbance of shortwave irradiance, causing the more serious melting of sea ice and thus the more liquid drainage. Averaged particulate concentration in sea ice was more than that in seawater. The reason was that particulates from atmospheric fallout were captured into sea ice during ice formation. On the other hand, particulates are easy to fall down to sea bed with the gravity, which also caused lower particulate concentration in seawater. 3.2. Optical properties 3.2.1. Absorption of CDOM Fig. 5 shows the absorption coefficients of CDOM in sea ice and the underlying seawater at Sites A and B. The maximum absorption coefficient appeared in seawater which indicated that the maximum CDOM concentration was in seawater. During seawater freezing, dissolved matter is partly separated from the growing sea ice into the underlying water, which caused the CDOM concentration to be smaller in sea ice than that in seawater. This result was similar to that gotten by Belzile et al. (2002) who researched lake ice in Canada. At a wavelength of 440 nm, the ratio of absorption coefficient of CDOM between sea ice and the underneath seawater was almost 2:3 at Site A, and was

Fig. 4. Profiles of salinity (a) and particulate (b) concentration for sea ice and the underneath seawater at Site A and B. The sea ice was sampled nearby the position which has been used to observe the optical properties. Sea ice thickness is 30 and 35 cm at Sites A and B, respectively. Depth over the ice thickness is the underneath seawater.

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1:3 at Site B. It also indicated that the ice formation rate was faster at Site A. CDOM profiles were similar to salinity profiles. Comparing the absorption coefficients of CDOM changed with depth showed that the CDOM was enrichment in surface layer, and was the least upper the bottom layer. In general, the shape of COM absorption spectra shows an exponential decrease with increasing wavelength, aCDOM ðλÞ ¼ aCDOM ðλ0 Þ exp½−Sðλ−λ0 Þ:

ð7Þ

Where λ0 is a reference wavelength, commonly 380 nm, S is spectral slope parameter. S in sea ice and seawater were got by inputting the measurement parameters into this equation to produce the best possible fit to the predicted CDOM absorption spectra with an iterative method. Fig. 6 shows the trend of S changing with depth, and the trends are similar between Site A and B. At Site A, S in sea ice varied from 0.0139 to 0.023 nm − 1, S in seawater was 0.0197 which was similar to that in ice bottom. At Site B, S in sea ice varied from 0.0122 to 0.0195 nm − 1, S in seawater was 0.0174. The value of S in seawater was in the range of that in sea ice. Ehn et al. (2004) got S in Santala Bay, and found that S in sea ice varied from 0.0075 to 0.013 nm − 1, which was smaller than that in the seawater varied between 0.013 and 0.0176 m − 1. For a wide range of sea and lake waters, S was found to vary from 0.010 to 0.020 nm − 1 (HØjerslev and Aas, 2001;). S in Liaodong Bay is in this range. In general, oligotrophic waters containing mainly autochthonous DOM have higher S, while waters with significant terrestrial input are characterized by lower S (Green and Blough, 1994; Mäckivi and Arst, 1996). There are many industrial and residential areas surrounding Liaodong Bay. The reason why S is higher is not yet clear, and more work need to do in the future. 3.2.2. Albedos Albedos at five sites are plotted in Fig. 7. In order to show the albedo discrepancies between Liaodong Bay and Arctic, a mean albedo of multiyear bare sea ice was given, which was observed by Perovich et al. (2002) in the Arctic, from 7 July to 12 August. At shorter wavelengths, the albedos in Arctic are almost constant with increasing wavelength. However, the ice albedos in Liaodong Bay increase intensely from 350 to 600 nm. This is caused by the strong absorption character of PM and CDOM in surface layer. At the wavelengths from 350 to 500 nm, albedos increasing trend at Sites D was more intensely than that at Site E, therefore, it indicated that the total

absorption coefficients of PM and CDOM in surface layer at Site D was larger than that at Site E. At longer wavelengths of 700–1000 nm, the albedos in Liaodong Bay are decreasing more slowly than that in Arctic. There are many gas bubbles and brine pockets in ice surface layer in Liaodong Bay for its higher temperature, which scatters light more strongly than that in Arctic. While the absorption coefficients of PM and CDOM were generally negligible above 700 nm. Then, we consider PM and CDOM have no contribution to the bulk absorption of sea ice above 700 nm. Furthermore, there was small difference in the absorption of pure ice and brine between Liaodong Bay and Arctic. Thereby, we assumed the total absorption of PM, CDOM, pure ice and brine was similar at longer wavelengths between Liaodong Bay and Arctic. The similar absorption and the more scattering in Liaodong Bay compared with Arctic, which determined that the trends of albedo changing with wavelength decreased more slowly in Liaodong Bay than that in Arctic at longer wavelengths. The maximum albedo was at Site D reaching 0.46 at 600–700 nm. Because of the high topography at Site D, there were a lot of gas bubbles contained within surface layer for substantial drainage. Light is scattered strongly by gas bubbles within ice (Grenfell, 1983, 1991). Then, the high relief surface would give a large albedo owing solely physical structure. At Site E, the peak value of albedo was nearly 0.42 at about 600 nm, for that there were many fragile ice crystals permeated by void spaces formed upon the ice which also scattered light strongly. Albedo at Site B was the least with a peak value of 0.27 at about 600 nm. Ice surface at Site B was smooth, there were many impurity matter and a small number of gas bubbles in surface layer. Thereby, the strong absorption by impurity and small scattering by gas bubbles caused the least albedo at Site B. We have also observed the albedo in Liao Dong Bay, in January 2009. Most of the albedo peaks ranged from 0.30 to 0.47, except an ice surface that was similar like coarse-grained snow had an albedo peak of 0.85. Light (1995) studied the effects of included PM on the spectral albedo, and found that the peak of albedo would shift to longer wavelength with increasing PM concentration, by the absorption modulation of impurity and pure ice. PM and CDOM in Liaodong Bay is more than that in the polar regions, which causes the valley of bulk absorption coefficient shift to longer wavelengths. Then, the peaks of albedo would shift to longer wavelength (commonly, the peak value of albedo often appears at about 450 nm in the polar regions (Allison et al., 1993; Grenfell and Perovich, 2004; Perovich, 1994; Perovich et al., 2002)), at about 600 nm.

Fig. 5. Absorption coefficients of CDOM in sea ice and the underlying seawater at Sites A and B. The maximum absorption coefficient of CDOM appeared in seawater. By comparing the absorption coefficients of CDOM, the maximum absorption appeared at surface layer and the minimum appeared close to the bottom ice.

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Fig. 6. Spectral slope parameter S profiles at site A and B. It shows that S in seawater was in the range of that in sea ice. The trends of S changed with depth were similar between these two sites.

3.2.3. Irradiance extinction coefficients It is very difficult to calibrate the downwelling irradiance profiler due to its structure. Fortunately, we just only care the ratio of downwelling irradiance values between different depths for this study. Therefore, it is necessary to know the irradiance ratio precision for the irradiance profiler. In this study, the downwelling irradiance profiler and irradiance detector were used to obtain the transmittance of sea ice at the same position. Then, the precision of ratio values of the downwelling irradiance profiler could be gotten by comparing the transmittance result with that conducted by irradiance detector (the irradiance detector is calibrated in laboratory, and we assume the transmittance observed by irradiance detector is a standard value). Transmittances at Site A observed with downwelling irradiance profiler and irradiance detector were plotted in Fig. 8. By setting an irradiance detector beneath ice bottom to record solar irradiance penetrated from the ice, the transmittances was gotten combined with

solar incident irradiance observed upon ice. We named this method as traditional method. After observing the transmittance with traditional method, a hole penetrating completely through ice was bored at the same position. Downwelling irradiance profiler was lowered into the hole and the diffusely reflecting Spectralon plane of profiler was set parallel to the ice bottom to observe downwelling irradiance (Fr(λ,z,0,t)). Solar incident irradiance (Es(λ,t)) was monitored simultaneously as well. Then, transmittance was obtained with Eqs. (3), (4), and (5). From 350 to 755 nm, transmittance observed with traditional method was larger than that observed with the downwelling irradiance profiler. Observing solar radiance distribution in sea ice was disturbed by the shadow of instrument itself, when the downwelling irradiance profiler was lowered into the hole. The deeper the downwelling irradiance profiler was lowered, the longer metal piper which used to support the downwelling irradiance profiler would be within hole, therefore, the more self-shadow errors would be caused. Then, the Fr(λ,z,0,t) recorded with downwelling irradiance

Fig. 7. Albedos at five sites. A mean albedo of multiyear sea ice was given, which was observed by Perovich et al. (2002) in Arctic. There were distinct discrepancies in albedos at those sites. Albedos in Liaodong Bay were almost a half of that in Arctic. The wavelength at which the albedo peak occurring shifted to 600 nm by the absorption modulation of particulate, CDOM, and pure ice.

Fig. 8. Transmittances at Site A observed with two kinds of methods. From 350 to 755 nm, transmittance observed with traditional method is larger than that observed with downwelling irradiance profiler, then, begins to be smaller at the wavelength over 755 nm. Generally, spectrum trends are almost similar. The maximum relative error was 9% at 590 nm.

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profiler would be smaller than the value conducted in undisturbed ideal condition. In addition, Fr(λ,0,0,t) was a constant value. Calculated with Eq. (5), then, T(λ,z,t) would be smaller than transmittance observed with the traditional method. However, above 755 nm, transmittance observed with the downwelling irradiance profiler was larger than that conducted with traditional method. Even the ice flakes surrounding the removed to make sure a clear surface, there were still a number of ice flakes attached on the interior wall of the bored hole. We speculated the reason was that there was somewhat scattering caused by flake ice which caused the abovementioned phenomena above 755 nm. From 400 to 900 nm, the maximum relative error was 9% at 590 nm. Transmittances obtained with downwelling irradiance profiler and irradiance detector were also conducted at other sites, and maximum relative errors were within 9% at wavelength of 400 to 900 nm. Thereby, the ratio value error of downwelling irradiance profiler was assumed as 9%. We believed that the method using downwelling irradiance profiler to observe solar irradiance within sea ice is reasonable. Ku and Kd at Site A are shown in Figs. 9 and 10, respectively. Generally, the maximum extinction coefficients should appear in the surface and bottom layer. The most reason is that there are many impurity matter in surface and bottom layer absorbing light intensely. At Site A, the maximum Kd occurred at surface layer with a valley value of 15.52 at 586 nm. Actually, the maximum Ku appeared at the depth 20–22 cm instead of the surface layer, with a valley value of 13.68 m − 1 at 586 nm. In the most case, the spectral trends of Ku and Kd were similar at 400–850 nm. At 350–400 nm, Ku increased with increasing wavelength, then, it became to decrease with increasing wavelength at 850–900 nm. In the mean time, the spectral trends of Kd were opposite to Ku at 350–400 nm and 850–900 nm. Actually, upwelling irradiance consists of upwelling incident light and scattering light. There is no light source beneath ice. Then, upwelling incident light could be ignored. We assumed the obtained upwelling irradiance is just backward scattering light. Downwelling irradiance consists of downwelling incident light and forward scattering light.

Fig. 9. Ku changing with depth at Site A. The maximum value is at the depth of 20–22 cm, with a valley value of 13.68 m− 1 at 586 nm. At the depth of 18–20 cm, an unusual Ku occurs with about 0 m− 1 at 350–620 nm.

Fig. 10. Kd changing with depth at site A. The maximum value occurs at surface layer with a valley value of 15.52 at 586 nm. At the depth of 18–20 cm, an unusual value of Kd is about − 1 m− 1 at 350–790 nm.

Solar irradiance provides a downwelling incident light. Within sea ice, some of the downwelling incident light is absorbed and scattered by impurity, brine pocket, gas bubbles, and pure ice. The rest of downwelling incident light and the forward scattering light, then, are the irradiance that the downwelling irradiance profiler recorded. The different irradiance components recorded between by upwelling and downwelling irradiance profilers, would be the reason causes the spectral trends of Ku different from that of Kd at 350–400 nm and 850–900 nm. It is also the reason that the trend of Ku changed with depth is inconsistent with that of Kd. The spectral extinction coefficients of sea ice in Liaodong Bay appeared to be significantly larger than previously observed at other places (Grenfell and Maykut, 1977; Light et al., 2008; Untersteiner, 1961). This is likely attributable to the fact that the sea ice in Liaodong Bay is generally thin (30–35 cm). Light that would ordinarily suffer additional scattering and be reintroduced to a layer is instead scattered out of the top or bottom surfaces of domain. Additionally, we also consider that sea ice is dirty in Liaodong Bay would be the other reason. Irradiance extinction coefficients showed a broad minimum around 600 nm generally corresponding to the wavelength where the maximum albedo occurred at. At shorter wavelengths, the rates of Kd decreased with increasing wavelength were different, and it could be used to indicate the variations in concentration of components, such as particulate, algae and CDOM. At 22 to 24 cm, the spectral trend of Kd showed much different from that at the other depths. The high concentration of impurity would cause the high Kd at shorter wavelengths. In addition, the diffusely reflecting Spectralon plane of downwelling profiler may be a little deeper than the ice bottom during observation, which caused the recorded irradiance at the depth of 22–24 cm to be not only include the irradiance from sea ice, it also included irradiance from a thin layer of seawater. Then, at 22–24 cm, the mixed layer of sea ice and seawater may be the reason that caused the unusual spectrum of Kd. Negative K values were obtained for this study, for example, at the depth of 18–20 cm, Kd was −1 m − 1 at 350–790 nm, and Ku was

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about 0 m − 1 at 350–620 nm. Negative extinction coefficients were also observed by Pegau and Zaneveld (2000) and Ehn et al. (2008). We are not able to explain this phenomenon, but speculate that: Sea ice may be not strictly horizontally homogeneous. At the depth of 18–20 cm, the ice layer was horizontal variability, the size of ice crystals around the hole was much smaller than the other crystals at this layer, which caused the light more difficult to penetrate through this area. Or there was much more impurity at this area, which blocked the light transmitting as well. However, the light could easily penetrate through the ice at deeper depths around the hole. Less light would be recorded by the profiler at the depth of 18–20 cm comparing to further down depth in the sea ice, which caused the Negative K values. 3.2.4. Soar radiance distribution in sea ice 3.2.4.1. Observed radiance distribution in sea ice. Solar radiance distribution at θ = 0°, θ = 30°, θ = 45°, and θ = 90° was observed with the above-mentioned methods at various depths. Such as the normalized radiance (normalized radiance is a ratio of measurement radiance between depth of z and 0 cm) profile at θ = 0° was conducted with downwelling profiler, the reason was discussed in above. Normalized radiance profiles from 0° to 90° (0° θ ≦ 90°) were observed with the inclined radiance profiler. Some of the normalized radiance profiles

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were shown in Fig. 11. All the peaks of normalized radiance profile almost appeared at about 600 nm, which were similar to albedo spectrums. The wavelengths where radiance attenuated to 0 decreased with increasing θ. For example, radiance profiles attenuated to 0 above 1000 nm at θ = 0°, at about 950 nm when θ = 30°, and at about 900 nm when θ = 45°. Radiance became weaker with increasing θ. Especially at longer wavelengths, for the strong absorption of sea ice, as θ and depth increasing, radiance energy would turn to be so weak that the sensor of profiler could not sense. When θ = 30°, the peak value of radiance was 0.42 at depth of 1 cm, which decreased slowly to 0.39 at 3 cm, to 0.36 at 5 cm, then, it began to decrease sharply to 0.22 at 7 cm. Furthermore, When θ = 90°, the peak value of radiance was 0.15 at depth of 2.5 cm, then, decreased sharply to 0.06 at 7.5 cm. There were large discrepancies of physical properties between surface layer and the remainder ice, such as lower density and salinity, more gas bubbles and particulate in surface layer, which caused radiance distribution in surface layer significantly different from the rest of layers. Solar incident radiance distribution that was disturbed largely at the surface layer by the hole was also considerable. Fig. 12 shows the normalized radiance profiles changing with depths at 600 nm. From 4 cm to ice bottom, the logarithms of radiance profiles to 10 almost decrease linearly with increasing the path length of light transfer. Because sea ice is a highly scattering medium

Fig. 11. Radiance profiles at θ = 0°, θ = 30°, θ = 45°, and θ = 90° at different depths. All the peaks of radiance profile almost appear at about 600 nm, and the spectral shapes are similar to that of albedos.

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Z. Xu et al. / Cold Regions Science and Technology 71 (2012) 23–33

Fig. 12. Normalized radiance profiles at 600 nm changing with depths. From 4 cm to ice bottom, the logarithms of radiance profiles to 10 almost decrease linearly with increasing the path length of light transfer. The undulation normalized radiance occurs in the ice bottom at θ = 30°, which indicates that the physical properties of sea ice change sharply along the inclined holes at θ = 30°.

and the effects of scattering become stronger with increasing depth within the ice, the dependence of solar radiance on θ becomes weaker with increasing depth. Normalized radiance shows a large undulation close to the ice bottom at θ = 30°, while the normalized radiance decreases regularly at other values θ. The results show that sea ice is not homogeneous along the ice profile at θ = 30° near ice bottom. 3.2.4.2. Radiance distribution model. In order to describe solar radiance distribution in sea ice at a random depth and θ, a radiance distribution model should be established. Asymptotic radiative transfer theory predicts that the radiance distribution, once asymptotic, does not change its shape and decays in amplitude as exp(−kz), where k is the asymptotic attenuation coefficient (Maffione et al., 1998; Preisendorfer, 1959). Basing on the equations advanced by Schoonmaker et al. (1989) describing the forward radiance distribution in sea ice made in laboratory, improved mathematical expressions chosen to fit the observed data of forward radiance profile are shown mðzÞ

Lðθ; zÞ ¼ Lð0; zÞ cos

ðnðθÞÞ:

Fig. 13. Predicted and measurement values of the forward normalized radiance profiles at a wavelength of 600 nm. Predicted values are good agreement with the measurements of forward radiance profiles, except that close to the ice surface at θ = 90°, predicted values are little over estimate.

are not agreement with the measurements at 0–4 cm. Because the model is a single layer model, the predicted results of radiance profiles at the thin surface layer is not well. At depth of deeper than 4 cm, the predicted values are good agreement with the measurements. Predicted radiance distribution values changed with θ at different depths are shown in Fig. 14. From θ = 0° to θ = 60°, normalized radiance decreases sharply with increasing θ. For example, normalized radiance decreases from 0.17 to 0.02 at 25 cm, which decrease about 7 times. Then, the normalized radiance decreases slowly over θ = 60°. Especially at the θ from 90° to 180°, normalized radiance is almost consistent with increasing θ close to ice bottom. Because sea ice is a highly scattering medium and the effects of scattering becomes strong with increasing path length, then, the dependence of solar radiance on θ becomes weak with increasing depth. Comparing this result with the

ð8Þ

Where z is ice thickness, m(z) is a function of ice thickness, and n(θ) is a function of θ. The normalized downwelling radiance (L(0,z)) is given Lð0; zÞ ¼ Lð0; 0Þ expð−kd zÞ:

ð9Þ

By fitting the L(0,z) to the measurements of downwelling radiance profiles, normalized radiance L(0,0) and Kd are obtained, with the values of L(0,0) = 0.546 and Kd = 0.047 cm − 1, respectively. Furthermore, by fitting the L(θ,z) to the measurements of forward radiance profiles with Eqs. (8) and (10), m(z) and n(θ) are gotten. hðzÞ ¼ 30

z z max

ð10Þ hðzÞ−0:228

mðzÞ ¼ 0:2787hðzÞ

3

ð11Þ 2

nðθÞ ¼ 0:0001513θ −0:03659θ þ 2:948θ:

ð12Þ

Where zmax is sea ice thickness, h(z) is a function of z. Fig. 13 shows the solar radiance profiles at 600 nm measured in situ and predicted with this model. Obviously, the modeling values

Fig. 14. Predicted radiance values changing with θ at different depths. Normalized radiance decreases sharply at small θ, and is almost consistent with increasing θ from θ = 55° to θ = 90°. The change of normalized radiance with increasing θ becomes weak with increasing depth, for that the effects of scattering become strong with increasing pathlength.

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data obtained by Maffione et al. (1998) do reveal the conclusion of radiance distribution is similar.

4. Conclusions In recent years, environmental pollution has become more serious than ever before. As a result, pollutants derived from atmospheric fallout and ocean currents, and then captured in sea ice have been increasing. At this study, we designed a hyperspectral radiation instrument to observe the apparent optical properties of “dirty” sea ice in Liaodong Bay. Understanding the albedo of sea ice is important to analyze remote sensing data. We learned that the albedo ranged from 0.27 to 0.46. Since there was a high concentration of PM and CDOM contained within ice, the albedo peaks shifted to longer wavelengths, which appeared at wavelength of about 600 nm. Albedo increased more sharply with increasing wavelength at shorter wavelengths (350–600 nm) than that in the polar regions. Researching the irradiance extinction coefficients of sea ice is important to study energy balance. At 586 nm, the maximum Ku reached 13.68 m − 1 near ice bottom, and the maximum Kd was 15.52 m − 1 at surface layer. The variations in concentration of components in sea ice, such as particulates, algae, and CDOM could be monitored by comparing the decreasing rates of the irradiance extinction coefficients at shorter wavelengths. Researching the solar distribution within sea ice can help in researching the biological community within sea ice and the underneath seawater. From 4 cm to ice bottom, we learned that the logarithms of radiance profiles almost decrease linearly with increasing the path length of light transfer. Sea ice is a highly scattering medium and the effects of scattering become stronger with increasing pathlength, which cause the dependence of solar radiance on θ to become weak with increasing depth. As θ increases, the decreasing rate of normalized radiance becomes slow at an identical depth. According to the measurements of radiance distribution, an optical model was brought forward to describe the solar radiance distribution at a random depth and θ, which could provide some information about the photosynthetically active radiation for biological research. In the future, radiance distribution at large resolution of θ would be observed, which would be used to improve the optical model.

Acknowledgments This study was supported by a key Project of the National Natural Science Foundation of China (Grant No. 40876057), the Exploration Program of “863” Project from the Ministry of Science and Technology of China (Grant No. 2006AA09Z154), and the Norwegian Research Council project AMORA (NRC Project Number 193592/S30). We give our great appreciations to Professors from Finland for their kind help. We also express our appreciation to other colleagues in our group who supported this campaign, such as Chaoyu Yang, Guoqing Wang and so on.

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