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Case study of Raman lidar measurements of Asian dust events in 2000 and 2001 at Nagoya and Tsukuba, Japan
Atmospheric Environment 36 (2002) 5479–5489
Case study of Raman lidar measurements of Asian dust events in 2000 and 2001 at Nagoya and Tsukuba, Japan Tetsu Sakaia,*,1, Takashi Shibatab, Yasunobu Iwasakab, Tomohiro Nagaic, Masahisa Nakazatoc, Takatsugu Matsumurac, Akinori Ichikic, Yoon-Suk Kimd, Koichi Tamurad, Dmitry Troshkind, Saipul Hamdid a
b
Japan Science and Technology Corporation, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan Graduate School of Environmental Studies, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan c Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan d Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan Received 7 December 2001; received in revised form 18 April 2002; accepted 1 May 2002
Abstract Vertical distributions of aerosol optical properties and relative humidity were measured with a Raman lidar at Nagoya and Tsukuba, Japan, during the Asian dust events in 2000 and 2001. The data obtained on 23 April 2001 showed a maximum of aerosol backscattering ratio ðRÞ of 3.5–4.0 at 532 nm at an altitude of 5 km over both measurement sites. Around that height the aerosol depolarization ratios ðda Þ; which indicate the aerosol nonsphericity, were higher than 20% and the relative humidities (RH) were lower than 30%. The aerosol optical thickness between 4 and 7 km was 0:1870:02 and the average aerosol extinction-to-backscatter ratio (lidar ratio) was 4675 sr at Tsukuba. This aerosol layer was present for over 6 h and finally showed the highest da value of about 33% and the lidar ratio of 1073 sr at the uppermost region, where the RH was almost saturated with respect to ice. The data obtained on 4 May 2000 at Nagoya showed relatively high R values ðB2:5Þ below 5 km: The values of da were higher than 15% between 3.5 and 8 km in which the maximum RH was about 60%. The values of da and RH showed a weak negative correlation below 3:5 km; where the RH varied between 30% and 70%. In the 5–8 km region, the da values were correlated negatively with the wavelength exponent of the aerosol backscattering coefficient ðaÞ; whereas they showed the lowest value ðB7%Þ at smaller a values ðo0Þ in the 2–5 km region. We hypothesized that these relations of da to a and RH are due to the mixing state of mineral dust, sea-salt, and sulfate-containing particles that are major aerosol constituents in the free troposphere during the Asian dust period and to the dependence of their shape and size on RH. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Aerosols; Vertical distribution; Particle nonsphericity; Particle size; Relative humidity
1. Introduction *Corresponding author. E-mail address: [email protected] (T. Sakai). 1 Present affiliation: Japan Society for the Promotion Science, Meteorological Satellite and Observation System Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan.
Mineral dust particles originated from the arid and semi-arid land of the Asian continent play many important roles in the climate system and biogeochemical cycle in the Asia-Pacific region. They affect the radiation balance by scattering and absorbing solar and terrestrial radiation (Nakajima et al., 1989), modify the
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cloud properties by acting as cloud condensation nuclei and ice forming nuclei (Isono et al., 1959), provide media for various heterogeneous reactions (Dentener et al., 1996), and serve as carriers for the chemical species (Zhuang et al., 1992). The dust storms generally occur in spring (March–May) when large amounts of mineral dust are uplifted into the free troposphere and transported over ranges of thousands of kilometers. The influences of the mineral dust particles on the tropospheric chemistry in East Asia have been recently evaluated by using chemical transport models (e.g., Zhang and Carmichael, 1999; Phadnis and Carmichael, 2000). However, large uncertainties remain because of the lack of the observational data that give the boundary conditions of the models and validate the model results. In particular, there is little information about the vertical distribution of the aerosol properties in the free troposphere due to the technical difficulties and analytical limitations. Liao and Seinfeld (1998) and Quijano et al. (2000) have shown that the vertical distribution of the aerosols is one of the most important parameters to be measured in order to evaluate their radiative effect. In addition, there are few observational data on the relation between the aerosol properties and ambient relative humidity (RH) in the free troposphere (e.g., Gasso! et al., 2000; Browell et al., 2001; Clarke et al., 2001). Because the aerosol shape, size, and chemical composition critically depend on RH (e.g., H.anel, 1976), it is necessary to measure in situ optical properties of the aerosols simultaneously with RH. Zhang et al. (1994) reported that RH is a key parameter to evaluate the chemical reaction rates of gases on the surfaces of dust particles.
Raman lidars have been developed to measure simultaneously the vertical profiles of RH and the aerosol optical properties in a noninvading way (Melfi, 1972; Whiteman et al., 1992; Ansmann et al., 1992a; Shibata et al., 1996). This paper reports on the Raman lidar measurements of the vertical distribution of free tropospheric aerosols and RH performed over Nagoya ð35:11N; 137:01EÞ and Tsukuba ð36:11N; 140:11EÞ; Japan, during the Asian dust events in 2000 and 2001. In Section 2 we describe the Raman lidar systems and the derived atmospheric parameters. In Section 3 we show two case studies of the measurements, focusing on the relation between the aerosol optical properties and RH. In the first case, we compare the results obtained at the two sites to study the horizontal variation of the aerosol properties. In Section 4 we propose a possible mixing state of aerosols that accounts for the observed relation between the aerosol optical properties and RH values, while the summary of this study is given in Section 5.
2. Raman lidar measurements of aerosol and humidity The Raman lidar measures the vertical distribution of aerosol backscattering, extinction, depolarization ratio, and water vapor mixing ratio by detecting the Raman backscattering of atmospheric water vapor and nitrogen (or oxygen) and the Mie/Rayleigh backscattering by atmospheric gases and aerosol particles. Table 1 shows the characteristics of the lidar systems used at Nagoya and Tsukuba in this study. Details of the system at Nagoya are given by Shibata et al. (1996). Both lidar
Table 1 Specifications of Raman lidar at Nagoya and Tsukuba
Transmitter Laser type Wavelength (nm) Energy/pulse (mJ) Repetition rate (Hz) Beam divergence (mrad) Receiver Telescope type Diameter (m) Field of view (mrad) Detector Signal detection Range resolution (m) Measured parameter Aerosol backscatter ratio and coefficient Aerosol extinction coefficient Depolarization ratio at 532 nm Water vapor mixing ratio a
Not used in this study.
Nagoya ð35:11N; 137:01EÞ
Tsukuba ð36:11N; 140:11EÞ
Nd:YAG 355 532 1064 200 50 430 10 0.2 (after collimator)
Nd:YAG 532 800 (maximum) 30 0.125 (after collimator)
Cassegrain 1.0 0.2–5.0 PMT Photon counting 30
Nasmyth and Cassegrain 1.0 0.35 0.1–2.0 1.5 PMT PMT Photon counting 6
532 and 1064 nm 355 nma J J
532 nm 532 nm J J
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systems transmit Nd:YAG laser pulses vertically into the atmosphere and the backscattered light from atmospheric gases and aerosol particles is collected with a telescope of 1 m diameter. The light is separated into five or six spectral components using interference filters, dichroic mirrors and polarizers, and the components are simultaneously detected by photomultiplier tubes (PMTs). The output pulses of the PMTs are filtered by signal discriminators and registered with photon counters. Finally, the data are stored on a hard disk of a personal computer. The temporal resolution of the lidar data obtained is 3:3 min at Tsukuba and 5 min at Nagoya. The vertical range resolution is 6 and 30 m at Tsukuba and Nagoya, respectively. To improve the signal to noise ratio, we smoothed the lidar data to obtain a 96 m range resolution for the data of Tsukuba (except for 600 m for the aerosol extinction coefficient) and 330 m for those of Nagoya. The measurements were carried out at nighttime under thick-cloud free condition. The horizontal distance between the two measurement sites is approximately 300 km: The atmospheric parameters obtainable with the Raman lidar are listed in Table 1. The backscatter lidar signals can be expressed as Pl ðzÞ ¼ K
OðzÞ b ðzÞ TlL ðz0 ; zÞ Tl ðz0 ; zÞ; z2 l
ð1Þ
where Pl ðzÞ is the backscatter signal at the wavelength l returned from the altitude zð¼ ct=2; where c is the speed of light and t is the elapsed time after laser emission), K is a constant that accounts for the detection efficiency of the channel, OðzÞ is the beam overlap factor, bl ðzÞ is the volume backscattering coefficient of the scattering material at l; Tl ðz0 ; zÞ is the atmospheric transmission at l between the lidar at altitudes z0 and z and is given by Rz ½sm;l ðz0 Þþsa;l ðz0 Þ dz0 ; ð2Þ Tl ðz0 ; zÞ ¼ e z0 where sm;l ðzÞ and sa;l ðzÞ are the volume extinction coefficients of air molecules and aerosol particles, respectively, and lL is the laser wavelength. The backscattering ratio R; aerosol backscattering coefficient ba ; and extinction coefficient sa are calculated from the Raman nitrogen (or oxygen) backscatter signal and the Mie/Rayleigh signals (e.g., Ansmann et al., 1992a; Whiteman et al., 1992). The value R is defined as R ¼ 1 þ ba =bm ; where bm is the molecular volume backscattering coefficient. The procedures to calculate R and ba are described in the appendix. The values of R 1 and ba are proportional to the aerosol mixing ratio and the concentration, respectively, if the aerosol size distribution and the backscattering efficiency remain constant. The total ðaerosol þ moleculeÞ depolarization ratio d is obtained by taking the ratio of the linear polarization components of the backscatter signals with respect to
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the polarization plane of the emitted laser to those of the total backscatter signals at the wavelength of 532 nm: The aerosol depolarization ratio da is calculated from the d and R at 532 nm by the equation da ðzÞ ¼
dðzÞRðzÞ dm ; RðzÞ 1
ð3Þ
where dm is the molecular depolarization ratio for this lidar system (dm B0:49%; calculated by H. Adachi, personal correspondence). We employed the correction procedure described by Adachi et al. (2001) to eliminate the depolarization components produced by the system. This value can be considered as an indicator of the particles’ nonsphericity (Bohren and Huffman, 1983); da is zero for homogenous spherical particles and da substantially deviates from zero for nonspherical particles. It must be noted the value of da measured with the lidar is averaged temporally and vertically so that if several kinds of particles with different da values coexist in the air, the value indicates the average weighted by the particle’s backscattering cross section. The wavelength exponent of the aerosol backscattering coefficient a is calculated from ba at 532 and 1064 nm assuming the relation ba pla (available only for Nagoya). The value a is approximately negatively correlated with the aerosol size; the value decreases as the size increases and vice versa. The water vapor mixing ratio is obtained using the ratio of the Raman water vapor to the Raman nitrogen (or oxygen) backscatter signals (Whiteman et al., 1992; Ansmann et al., 1992a). The calibration constant to calculate the water vapor mixing ratio is obtained by comparing with coincident radiosonde data. The relative humidity RH is calculated from the observed water vapor mixing ratio combined with the temperature and pressure data obtained with the radiosonde launched at the nearest meteorological observatory from the measurement sites (Tateno or Hamamatsu).
3. Vertical distributions of aerosols and relative humidity during the Asian dust events We have frequently measured the enhancements of aerosol backscattering in the free troposphere during the Asian dust period in 2000 and 2001. The aerosol enhanced layers usually showed high aerosol depolarization ratios ð> 20%Þ; indicating the predominance of nonspherical particles in those regions. Our previous results (Kwon et al., 1997; Sakai et al., 2000) have shown that the RH values in those aerosol layers were usually low ðo30%Þ; probably because they originate mainly from the arid regions in the Asian continent. However, we have sometimes measured moderately high RH ðB60%Þ at those high depolarizing regions, possibly as a result of mixing with moist air caused by a strong front
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activity. This case is of interest because we have little information about the optical properties of the dust aerosol in the moist air. In this section we show two measurement cases of the aerosol optical properties and RH obtained at the dust events in 2000 and 2001; one is the low RH case obtained on 23 April 2001 at Nagoya and Tsukuba, and the other is the moderately high RH case obtained on 4 May 2000 at Nagoya. 3.1. Case of 23 April 2001 Fig. 1 shows the vertical distribution of the aerosol optical properties (R; Sa and da ) at 532 nm and RH measured with the Raman lidar at Nagoya (averaged over the time period 19:27–20:27 JST) and Tsukuba (averaged over the time period 19:40–20:21 JST) on 23 April 2001. That night we found a peak of R of the order of 3.5–4.0 at an altitude of 5 km at both measurement sites. The values of da were as high as 20% between 2 and 8 km at Nagoya and between 4 and 7 km at Tsukuba, indicating the predominance of nonspherical particles in these regions. Because the RH values were mostly below 30% in these regions, presence of ice crystal was improbable for this period. The optical thickness ðta Þ of this aerosol layer derived from the Raman nitrogen signal was 0:1870:02 and the average extinction-to-backscatter ratio (lidar ratio, Sa ) was 4675 sr between 4 and 7 km at Tsukuba. Fig. 2 shows the 5-day isentropic backward air-mass trajectories arriving at the lidar sites on 23 April 2001.
Detail of the calculation is given in Sakai et al. (2000) except that the initial positions were located 25 points within 0:51 0:51 latitude–longitude grid around the lidar sites. The air parcels arrived at the lidar sites had been passed similar pathways over the Asian continent due to the prevailing westerly flow. They had passed over the Gobi desert on 21 April when the meteorological observatories at Umnugobi in Mongolia (TsogtOvoo, 44:41N; 105:31E; 1298 m a.s.l.; Saikhan-Ovoo, 45:51N; 103:91E; 1316 m a.s.l.) had reported severe dust storms, suggesting that mineral dust particles had been uplifted into the free troposphere around this arid area. Since the trajectories showed small spatial dispersion during the transportation, it is expected that the dust particles were transported directly from the source areas to the measurement sites without large disturbance or mixing with the other air masses. For comparison of the optical properties of the dust layers, Muller . et al. (2001) observed the optical properties of an elevated dust layer originated from the arid and semi-arid region in Arabian Peninsula using a sixwavelength lidar over Maldives. They measured the peak value of ba B2:4 M m1 sr1 (which corresponds to approximately RB3:5) at an altitude of 3:8 km and obtained the Sa values of 34–52 sr at the altitude range of 3.5–5:2 km: Powell et al. (2000) and Welton et al. (2000) measured the aerosol profiles of Saharan dust layers using a micro-pulse lidar over Canary Islands. They observed a peak of extinction coefficient at an altitude of approximately 3 km and reported
Fig. 1. Vertical profiles of aerosol backscattering ratio ðRÞ and lidar ratio ðSa Þ at 532 nm (left panels), aerosol depolarization ratio ðda Þ and relative humidity (RH) (right panels) obtained at Nagoya between 19:27 and 20:27 JST (a) and Tsukuba between 19:40 and 20:21 JST (b) on 23 April 2001. The thin-dotted line in the right panel of Fig. 1b shows humidity profile obtained with radiosonde on 20:30 JST at Tsukuba. Arrows in the right panels show the height of which backward trajectories are shown in Fig. 2. The horizontal distance between the two sites is approximately 300 km:
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can expect that the dust layers observed at around 5 km (Fig. 1) were horizontally continuous between the two sites. It is also to be noted that the uppermost part of this aerosol layer showed the highest d values (B33%; da was nearly equal to this value) with the highest R values (> 50; not shown) at about 8 km on 1:00 JST (Fig. 3a). This peak was probably due to the presence of ice crystals because the RH was greater than 70% (Fig. 3b) that was almost saturated with respect to ice ð> 90%Þ in the region. The value of Sa at this region was 1073 sr; which is close to that of cirrus clouds (5–15 sr) measured by Ansmann et al. (1992b) using Raman lidar at 308 nm and that of cirrostratus model containing hexagonal ice crystals (11 sr at 550 nm) calculated by Takano and Liou (1989) using ray-tracing technique. Since the Asian mineral particles effectively act as ice forming nuclei (Isono et al., 1959), the ice crystals may have formed on the mineral particles at this region. Murayama et al. (2001) observed cirrus clouds at the top of a thin dust layer above 9 km using a polarization lidar over Tokyo. 3.2. Case on 4 May 2000
Fig. 2. Horizontal and vertical cross section of isentropic backward trajectories arriving at the altitude of 5 km at Tsukuba (solid line with pluses) and Nagoya (dotted line with circles) on 23 April 2001. The symbols are plotted at 24-h intervals. The lowermost lines in the vertical cross section show the land elevations. Triangles in the upper panel show the location where severe dust storms were reported on 21 April 2001.
Sa ¼ 3575 sr and ta ¼ 0:17170:02 between 1.5 and 4:6 km; or Sa ¼ 3779 sr between 2.5 and 5 km and ta ¼ 0:161B0:21770:01: Although the source regions of the dust layers were different from our case, the values of Sa reported by them were close to or slightly smaller than those obtained at 4–7 km in our measurements. More observational data are necessary to obtain appropriate value and the variability of Sa for mineral dust layers in the free troposphere. Fig. 3 shows the temporal and height cross section of d and RH (with respect to water) obtained with the Raman lidar at Tsukuba from 19:40–2:25 JST on 23–24 April 2001. The temporal resolution is 3:3 min: The high depolarizing layer at the altitude range of 4–7 km was persistent for over 6 hours (Fig. 3a; we measured this layer until 4:45 JST when we terminated the measurement). Based on the wind speed of 23:5 m s1 from west at an altitude of 5 km obtained from a radiosonde measurement, this aerosol layer was expected to spread east and west for more than 500 km over Tsukuba, which is approximately 300 km east of Nagoya. Thus we
Fig. 4 shows the vertical profile of R; a; da ; and RH obtained on 4 May 2000 over Nagoya. Unfortunately, we have no data over Tsukuba on that day because we started the measurement there in October 2000. We measured moderately high RH values ðB70%Þ with relatively high aerosol backscattering ratio ðRB2:5Þ below the altitude of 5 km: The aerosol depolarization ratios showed high values ð> 15%Þ between 3.5 and 8 km and the lower value ðo10%Þ below 3:5 km: It should be noted that the maximum RH values were as high as 60% at the bottom of the high da region between 3.5 and 4:5 km; indicating that nonspherical particles were still predominant in such moderately high RH region. Fig. 5 shows the backward air-mass trajectories arriving at the altitude of 4 km at the lidar site. We found that most of the trajectories passed over the Asian continent and they terminated over the arid region in Mongolia ðB451N; B1101EÞ on 2 May because they intersected a vertically unstable layer. The dust storms were reported in Mongolia (Zamyn-Uud, 43:71N; 111:91E; 962 m a.s.l.; Tsogt-Ovoo, 44:41N; 105:31E; 1298 m a.s.l.) on 2 May, suggesting that these dust storms were the main source of the aerosols measured over Nagoya. Fig. 5 also shows that the trajectories had dispersed horizontally over the ocean in the northwest of Japan and some of them had been carried over the northern ocean, implying that the continental air mass had mixed with the maritime air mass at around the area. This dispersion of the trajectories was caused by a strong cold front activity that had developed over the Sea of Japan on 2–3 May. This front activity might also have promoted the vertical transport of moist air from
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23 April 2001 Total Depolarization Ratio at 532 nm 10
δ(%) 30 27 24 21 18 15 12 9 6 3 0
9 8 Altitude (km)
7 6 5 4 3 2 1 0 20:00
21:00
22:00
(a)
23:00
0:00
1:00
2:00
Time (JST)
23 April 2001 Relative Humidity with respect to Water 10
RH(%)
9
100 90 80 70 60 50 40 30 20 10 0
8 Altitude (km)
7 6 5 4 3 2 1 0 20:00
(b)
21:00
22:00
23:00
0:00
1:00
2:00
Time (JST)
Fig. 3. Temporal and vertical cross section of total depolarization ratio at 532 nm (a) and relative humidity with respect to water (b) obtained with the Raman lidar at Tsukuba from 19:40 to 2:25 JST on 23–24 April 2001.
the lower troposphere to the middle troposphere (lower panel in Fig. 5). Below the altitude of 3:5 km the values of da showed a weak negative correlation with RH (right panel in Fig. 4); da values varied between 15% and 7% where the RH varied between 30% and 70%, indicating that the aerosols were more spherical in the moist air than in the dry air at that lower altitude region. Possible reasons for this negative correlation between da and RH are: (a) the nonspherical particles (e.g., solid crystals) changed their shape to spherical (e.g., droplets) by uptaking water vapor at the higher RH, and/or (b) the proportion of nonspherical particles to the other low-depolarizing particles (droplets or small particles whose radii were less than the laser wavelength of 532 nm) decreased with increasing RH. To investigate the relation between the aerosol nonsphericity and the aerosol size, Fig. 6 shows the scatter plot of da as a function of a: We found that the
values of da decreased approximately linearly from 24% to 10% with increasing a from 0.5 to 2 for the data between altitudes of 5 and 8 km where the RH was less than 30%, suggesting that the aerosol nonsphericity increased with increase in the aerosol size in that region. The data between altitudes of 2 and 5 km; where the RH values were between 30% and 70%, showed the lower a values ðo0:5Þ than those at 5–8 km; suggesting that the aerosol sizes were larger than those in the 5–8 km region. They also showed that the da values were the lowest ðB7%Þ at the smaller a values ðo0Þ; suggesting that the aerosol nonsphericity was the lowest (i.e. spherical) for the larger aerosol sizes in that region. For comparison, vertical variation of aerosol optical properties and RH has been measured by Muller . et al. (2001) over Maldives. During the period of southwest monsoon, they observed the wavelength exponent of the aerosol extinction coefficient k of 0.37, 0.52, and 0.97 in the altitude ranges of 1.0–2.35, 3.6–4.2, and 4.2–5:2 km;
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30 Altitude 5-8 km 2-5 km
δa (%)
20
10
0 -1 Fig. 4. Vertical profile of aerosol backscattering ratio ðRÞ; wavelength exponent of aerosol backscattering coefficient ðaÞ (left panel), aerosol depolarization ratio ðda Þ and relative humidity (RH) (right panel) obtained at Nagoya on 4 May 2000. The arrow in the right panel shows the height of which back trajectories are shown in Fig. 5.
Fig. 5. Horizontal and vertical cross section of isentropic backward trajectories arriving at the altitude of 4 km at Nagoya (25 points around 35:11N; 137:01E at intervals of 0:51) on 4 May 2000. Symbols are plotted at 24-h intervals. The lowermost lines in the vertical cross section show the land elevations. Triangles in the upper panel show the location where dust storms were reported on 2 May 2000.
0
1 α
2
3
Fig. 6. Scatter diagram of aerosol depolarization ratio ðda Þ as a function of wavelength exponent of aerosol backscattering coefficient ðaÞ measured on 4 May 2000 at Nagoya. Squares and circles show the data between altitudes of 5 and 8 km and between 2 and 5 km; respectively.
respectively, where the RH derived from the Raman measurement decreased from 70% at 1:0 km to B20% at 2:7 km and again increased to 45% in the upper regions (the middle and upper altitude regions correspond to the dust layer refered in Section 3.1). The vertical variations of k and RH are similar to those of a and RH in our results, suggesting that the aerosol size was larger in the high RH region of the lower altitude and decreased with increasing height. Although they did not report da data, they derived the aerosol physical properties from the multiple wavelength measurements; for example, the mean aerosol effective radii were 0.3, 0.27, and 0:18 mm and the complex refractive indices were 1:6 þ 0:02i; 1:53 þ 0:005i; and 1:44 þ 0:004i for the three altitude regions. Based on the aerosol properties and RH derived from the lidar measurement and the backward trajectory analysis, they concluded that a mixture of clean-marine and soil-derived particles was predominant in the lower altitude region and mineral dusts were predominant in the elevated layer. Since it is difficult to conclude what kind of aerosol constituents was present only from our lidar data, we discuss possible aerosol constituents and their mixing state that account for the relation between da and a observed on 4 May at Nagoya in the next section.
4. Discussion Tropospheric aerosol particles usually consist of several chemical components and are mixed both
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Nonspherical
A Nonspherical particles
δa
(e.g., mineral dust; sea-salt crystal)
2-5 km (High RH)
5-8 km (Low RH) External mixing
B
C Spherical and large particles
Spherical
(e.g., sea-salt droplet; mineral dust coated with solution)
Large
Spherical and/or small particles (e.g., sulfate; small mineral dust)
α
Small
Fig. 7. Schematic diagram of relation between aerosol depolarization ratio ðda Þ and wavelength exponent of aerosol backscattering coefficient ðaÞ for three aerosol types (see text for detail).
externally and internally. In an attempt to account for the relation between da and a obtained on 4 May (Fig. 6), we suppose a simple model of aerosol mixture (Fig. 7) based on the aircraft measurements made by several researchers in the free troposphere over around Japan (Iwasaka et al., 1988; Pueschel et al., 1994; Ikegami et al., 1993; Sakai et al., 2001). The aircraft measurements have shown that the major aerosol constituents in the free troposphere were sulfatecontaining particles in the fine mode (approximately less than 0:5 mm in radius) and mineral dust and sea-salt particles in the coarse mode (approximately larger than 0:5 mm in radius). They also found that some of the mineral dust particles were coated with solution (e.g., sulfuric acid) in their electron micrographs (Iwasaka et al., 1988; Ikegami et al., 1993; Sakai et al., 2001). The values of a and da for these aerosols obtained from laboratory experiments and field measurements or inferred from theoretical calculations are as follows: da B2% (laboratory experiment by Sassen et al. (1989), measured at scattering angle X1781 at a wavelength of 632:9 nm) and a > 1:0 (calculation using Mie theory for the particle size distribution measured with optical particle counters by Sakai et al. (2001) for the sulfatecontaining particles; da E20% (laboratory experiment by Cooper et al. (1974)) and aB0:5 (estimation from the theoretical calculation using discrete dipole approximation method by Murayama et al. (2001)) for crystalline solid; da ¼ 5:6–11.1% (laboratory experiment by Cooper et al. (1974)) and ao0:8 (calculation using Mie theory for the particle size distribution measured by Gras (1991)) for solution droplets of sea-salt; da E27% or 30% (laboratory experiments by Volten et al. (2001) and Kuik et al. (1991), measured at scattering angle of 1731 or 1751 at 632:9 nm) and a ¼ 0:6B 0:1 (estimation from the observational data using semi-empirical model by Nakajima et al. (1989)) for the mineral dust particles.
It must be noted that the values of da listed above should be considered as a representative for those particles because the values depend on the particle size as well as the shape and are not generally monotonic function of size (e.g., Asano and Sato, 1980; Mischenko and Hovenier, 1995); for example, the value of oblate spheroid with aspect ratio 1.7 with refractive index of 1:55 þ 0:005i increases from 0% to 31% for the equal-volume sphere radius from 0 to 0:4 mm and decreases to 21% at 1:0 mm (calculated by T-matrix code of Mischenko et al. (1996)). The values of da and a of the mineral particles coated with solution were not well known, but we can expect that both the values are lower than those of pure (not coated with solution) minerals because they are larger and more spherical than the pure minerals by being coated by aqueous solution. Based on these results, we suppose the following mixing state of the aerosols that explain the relation between da and a observed on 4 May. Fig. 7 shows the schematic diagram of the relation between da and a for the mixture of those particles. At altitudes between 5 and 8 km where the RH was less than 30%, the nonspherical mineral particles and/or sea-salt crystals in the coarse mode (Particles ‘A’ in Fig. 7) and the sulfate-containing particles in the fine mode (Particles ‘B’) were predominant and their proportion varied with height. Another possibility is that the mineral particles that were smaller than the laser wavelength of 0:532 mm were predominant and its effective radius varied with height. At altitudes between 2 and 5 km where the RH was between 30% and 70%, the mineral particles and/or sea-salts crystals (Particles ‘A’) and the spherical particles (Particles ‘C’) that were larger than the mineral particles and/or sea-salts crystals were predominant and their proportion varied with height. One possibility of these large spherical particles is the mineral particles coated with aqueous solution as mentioned before. The other possibility is sea-salt droplets in metastable state because they can exist as liquid at the RH values between the crystallization and the deliquescence point (between 46% and 75%; Tang, 1996) due to the hysteresis. Field measurements by Rood et al. (1989) have reported the ubiquitous presence of metastable droplets in the surface atmosphere. These sea-salt droplets might be mixed internally with mineral dust because Niimura et al. (1998) found such mixed particles frequently in the surface atmosphere at Nagasaki in Japan during the Asian dust events. However, our model is just conceptual and qualitative. Further field studies (e.g., coincident measurements with lidar and in situ sampling) and laboratory experiments are essential to clarify the relation of the aerosol optical properties with RH and with the aerosol mixing state. We notice also that it might be important to measure the mixing state of the Asian dust particles to
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assess their effect on the atmospheric processes because they can modify cloud microphysical processes by acting as cloud condensation nuclei more effectively than the other major aerosols (e.g., submicrometer sulfates) if they are coated with water-soluble substances (Takeda and Kuba, 1982; Wurzler et al., 2000) and their radiative effects critically depend on whether they are mixed externally or internally (e.g., Ackerman and Toon, 1981).
Acknowledgements
5. Conclusion
Appendix
We have presented two case studies of the Raman lidar measurements of the aerosol optical properties and relative humidity in the free troposphere during the Asian dust events in 2000 and 2001 at Nagoya and Tsukuba, Japan. The data obtained on 23 April 2001 showed a maximum of aerosol backscattering ratio R of 3.5–4.0 at an altitude of 5 km at Nagoya and Tsukuba. Around that height the aerosol depolarization ratios da were larger than 20% and relative humidities RH were lower than 30%. The aerosol optical thickness between 4 and 7 km was 0:1870:02 and the average lidar ratio Sa was 4675 sr at Tsukuba. This aerosol layer was persistent for over 6 h and showed the highest da value of B33% and the Sa of 1073 sr at the uppermost height region, where the RH was almost saturated with respect to ice. The data obtained on 4 May 2000 at Nagoya showed the R of about 2.5 below an altitude of 5 km: The values of da were higher than 15% between 3.5 and 8 km where the maximum RH was B60%: The values of da and RH showed a weak negative correlation below 3:5 km; where the RH varied between 30% and 70%. The da values decreased almost linearly from 24% to 10% with increasing the wavelength exponent of the aerosol backscattering coefficient a ranging from 0.5 to 2 in the 5–8 km region, whereas they showed the lowest value about 7% at the smaller a values ðo0Þ in the 2– 5 km region. We hypothesized that these relations of da to RH and a were due to the mixing state of mineral dust, sea-salt, and sulfate-containing particles that are the major aerosol constituents collected in the free troposphere during the Asian dust period and to the dependence of their shape and size on RH. However, coincident measurements with lidar and insitu sampling are necessary to clarify the relation between the aerosol optical parameters obtainable with the lidar and the aerosol physicochemical properties such like size, shape, chemical composition, and mixing state. In addition, coordinated multi-site observations with lidar will provide valuable information about the spatial distribution of aerosol optical properties and their temporal evolution during the transportation.
To derive R and ba at the wavelengths of 532 and 1064 nm; we used the Raman oxygen signal ð375 nmÞ to estimate the aerosol transmission at those wavelengths for the data of Nagoya (Sakai, 2000); the aerosol extinction coefficient sa at 355 nm is derived from the Raman oxygen signal (e.g., Ansmann et al., 1990) and the value is converted to sa at 532 and 1064 nm by assuming the relation sa plk : The value of k; an unknown parameter, is assumed to be 1.0 with random uncertainty of 1 for a (wavelength exponent of ba ÞX1 which suggests the abundance of particles with radii smaller than laser wavelengths or k ¼ 070:5 for ao1 which suggests the predominance of large particles. This relation between k and a was obtained by calculations for aerosol models using Mie theory and T-matrix code (Mischenko et al., 1996). The procedure to derive R and sa at 532 nm for the data of Tsukuba is similar to that described by Ansmann et al. (1990, 1992a) except that we used k that varied with da based on the calculations for the particle size distributions obtained from the aircraft measurements (Sakai et al., 2001) to reduce the uncertainty associated with the unknown parameter of k; the value linearly decreased from 1.5 to 0 with increasing da from 0 to X25% with random uncertainty of 0.5 assuming the external mixture of small non-depolarizing ðda ¼ 0Þ particles (e.g., submicrometer sulfates) and large depolarizing ðda B25%Þ particles (e.g., mineral dust; sea-salt crystal). In addition, we used k ¼ 0 for RX5 regardless of da in the free troposphere ð> 2 kmÞ because these high R values are mostly due to cloud particles (large water droplets or ice crystals) that have nearly no wavelength dependence in sa : The calculations of these procedures are made in an iterative manner since a and da are a function of ba and R: However, for the height regions where the sa was not derived from the Raman signal because of incomplete overlap of the laser beam with the telescope’s field of view (approximately below 2.5–3 km) or where the measurement uncertainty in sa exceeds 100%, we employed the equation sa ¼ Sa ba ; where we assumed Sa ¼ 50730 or 40732 sr at 532 and 1064 nm; respectively.
We thank M. Nagatani and H. Nakada of Nagoya University for technical assistance of the lidar at Nagoya. This research was supported in part by the Ministry of Education, Culture, Sports, Scientific and Technology through Grant-in-Aid for Scientific Research on Priority Areas (A), No. 10144104, led by Y.I. of authors.
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Case study of Raman lidar measurements of Asian dust events in 2000 and 2001 at Nagoya and Tsukuba, Japan Tetsu Sakaia,*,1, Takashi Shibatab, Yasunobu Iwasakab, Tomohiro Nagaic, Masahisa Nakazatoc, Takatsugu Matsumurac, Akinori Ichikic, Yoon-Suk Kimd, Koichi Tamurad, Dmitry Troshkind, Saipul Hamdid a
b
Japan Science and Technology Corporation, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan Graduate School of Environmental Studies, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan c Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan d Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan Received 7 December 2001; received in revised form 18 April 2002; accepted 1 May 2002
Abstract Vertical distributions of aerosol optical properties and relative humidity were measured with a Raman lidar at Nagoya and Tsukuba, Japan, during the Asian dust events in 2000 and 2001. The data obtained on 23 April 2001 showed a maximum of aerosol backscattering ratio ðRÞ of 3.5–4.0 at 532 nm at an altitude of 5 km over both measurement sites. Around that height the aerosol depolarization ratios ðda Þ; which indicate the aerosol nonsphericity, were higher than 20% and the relative humidities (RH) were lower than 30%. The aerosol optical thickness between 4 and 7 km was 0:1870:02 and the average aerosol extinction-to-backscatter ratio (lidar ratio) was 4675 sr at Tsukuba. This aerosol layer was present for over 6 h and finally showed the highest da value of about 33% and the lidar ratio of 1073 sr at the uppermost region, where the RH was almost saturated with respect to ice. The data obtained on 4 May 2000 at Nagoya showed relatively high R values ðB2:5Þ below 5 km: The values of da were higher than 15% between 3.5 and 8 km in which the maximum RH was about 60%. The values of da and RH showed a weak negative correlation below 3:5 km; where the RH varied between 30% and 70%. In the 5–8 km region, the da values were correlated negatively with the wavelength exponent of the aerosol backscattering coefficient ðaÞ; whereas they showed the lowest value ðB7%Þ at smaller a values ðo0Þ in the 2–5 km region. We hypothesized that these relations of da to a and RH are due to the mixing state of mineral dust, sea-salt, and sulfate-containing particles that are major aerosol constituents in the free troposphere during the Asian dust period and to the dependence of their shape and size on RH. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Aerosols; Vertical distribution; Particle nonsphericity; Particle size; Relative humidity
1. Introduction *Corresponding author. E-mail address: [email protected] (T. Sakai). 1 Present affiliation: Japan Society for the Promotion Science, Meteorological Satellite and Observation System Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan.
Mineral dust particles originated from the arid and semi-arid land of the Asian continent play many important roles in the climate system and biogeochemical cycle in the Asia-Pacific region. They affect the radiation balance by scattering and absorbing solar and terrestrial radiation (Nakajima et al., 1989), modify the
1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 6 6 4 - 7
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cloud properties by acting as cloud condensation nuclei and ice forming nuclei (Isono et al., 1959), provide media for various heterogeneous reactions (Dentener et al., 1996), and serve as carriers for the chemical species (Zhuang et al., 1992). The dust storms generally occur in spring (March–May) when large amounts of mineral dust are uplifted into the free troposphere and transported over ranges of thousands of kilometers. The influences of the mineral dust particles on the tropospheric chemistry in East Asia have been recently evaluated by using chemical transport models (e.g., Zhang and Carmichael, 1999; Phadnis and Carmichael, 2000). However, large uncertainties remain because of the lack of the observational data that give the boundary conditions of the models and validate the model results. In particular, there is little information about the vertical distribution of the aerosol properties in the free troposphere due to the technical difficulties and analytical limitations. Liao and Seinfeld (1998) and Quijano et al. (2000) have shown that the vertical distribution of the aerosols is one of the most important parameters to be measured in order to evaluate their radiative effect. In addition, there are few observational data on the relation between the aerosol properties and ambient relative humidity (RH) in the free troposphere (e.g., Gasso! et al., 2000; Browell et al., 2001; Clarke et al., 2001). Because the aerosol shape, size, and chemical composition critically depend on RH (e.g., H.anel, 1976), it is necessary to measure in situ optical properties of the aerosols simultaneously with RH. Zhang et al. (1994) reported that RH is a key parameter to evaluate the chemical reaction rates of gases on the surfaces of dust particles.
Raman lidars have been developed to measure simultaneously the vertical profiles of RH and the aerosol optical properties in a noninvading way (Melfi, 1972; Whiteman et al., 1992; Ansmann et al., 1992a; Shibata et al., 1996). This paper reports on the Raman lidar measurements of the vertical distribution of free tropospheric aerosols and RH performed over Nagoya ð35:11N; 137:01EÞ and Tsukuba ð36:11N; 140:11EÞ; Japan, during the Asian dust events in 2000 and 2001. In Section 2 we describe the Raman lidar systems and the derived atmospheric parameters. In Section 3 we show two case studies of the measurements, focusing on the relation between the aerosol optical properties and RH. In the first case, we compare the results obtained at the two sites to study the horizontal variation of the aerosol properties. In Section 4 we propose a possible mixing state of aerosols that accounts for the observed relation between the aerosol optical properties and RH values, while the summary of this study is given in Section 5.
2. Raman lidar measurements of aerosol and humidity The Raman lidar measures the vertical distribution of aerosol backscattering, extinction, depolarization ratio, and water vapor mixing ratio by detecting the Raman backscattering of atmospheric water vapor and nitrogen (or oxygen) and the Mie/Rayleigh backscattering by atmospheric gases and aerosol particles. Table 1 shows the characteristics of the lidar systems used at Nagoya and Tsukuba in this study. Details of the system at Nagoya are given by Shibata et al. (1996). Both lidar
Table 1 Specifications of Raman lidar at Nagoya and Tsukuba
Transmitter Laser type Wavelength (nm) Energy/pulse (mJ) Repetition rate (Hz) Beam divergence (mrad) Receiver Telescope type Diameter (m) Field of view (mrad) Detector Signal detection Range resolution (m) Measured parameter Aerosol backscatter ratio and coefficient Aerosol extinction coefficient Depolarization ratio at 532 nm Water vapor mixing ratio a
Not used in this study.
Nagoya ð35:11N; 137:01EÞ
Tsukuba ð36:11N; 140:11EÞ
Nd:YAG 355 532 1064 200 50 430 10 0.2 (after collimator)
Nd:YAG 532 800 (maximum) 30 0.125 (after collimator)
Cassegrain 1.0 0.2–5.0 PMT Photon counting 30
Nasmyth and Cassegrain 1.0 0.35 0.1–2.0 1.5 PMT PMT Photon counting 6
532 and 1064 nm 355 nma J J
532 nm 532 nm J J
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systems transmit Nd:YAG laser pulses vertically into the atmosphere and the backscattered light from atmospheric gases and aerosol particles is collected with a telescope of 1 m diameter. The light is separated into five or six spectral components using interference filters, dichroic mirrors and polarizers, and the components are simultaneously detected by photomultiplier tubes (PMTs). The output pulses of the PMTs are filtered by signal discriminators and registered with photon counters. Finally, the data are stored on a hard disk of a personal computer. The temporal resolution of the lidar data obtained is 3:3 min at Tsukuba and 5 min at Nagoya. The vertical range resolution is 6 and 30 m at Tsukuba and Nagoya, respectively. To improve the signal to noise ratio, we smoothed the lidar data to obtain a 96 m range resolution for the data of Tsukuba (except for 600 m for the aerosol extinction coefficient) and 330 m for those of Nagoya. The measurements were carried out at nighttime under thick-cloud free condition. The horizontal distance between the two measurement sites is approximately 300 km: The atmospheric parameters obtainable with the Raman lidar are listed in Table 1. The backscatter lidar signals can be expressed as Pl ðzÞ ¼ K
OðzÞ b ðzÞ TlL ðz0 ; zÞ Tl ðz0 ; zÞ; z2 l
ð1Þ
where Pl ðzÞ is the backscatter signal at the wavelength l returned from the altitude zð¼ ct=2; where c is the speed of light and t is the elapsed time after laser emission), K is a constant that accounts for the detection efficiency of the channel, OðzÞ is the beam overlap factor, bl ðzÞ is the volume backscattering coefficient of the scattering material at l; Tl ðz0 ; zÞ is the atmospheric transmission at l between the lidar at altitudes z0 and z and is given by Rz ½sm;l ðz0 Þþsa;l ðz0 Þ dz0 ; ð2Þ Tl ðz0 ; zÞ ¼ e z0 where sm;l ðzÞ and sa;l ðzÞ are the volume extinction coefficients of air molecules and aerosol particles, respectively, and lL is the laser wavelength. The backscattering ratio R; aerosol backscattering coefficient ba ; and extinction coefficient sa are calculated from the Raman nitrogen (or oxygen) backscatter signal and the Mie/Rayleigh signals (e.g., Ansmann et al., 1992a; Whiteman et al., 1992). The value R is defined as R ¼ 1 þ ba =bm ; where bm is the molecular volume backscattering coefficient. The procedures to calculate R and ba are described in the appendix. The values of R 1 and ba are proportional to the aerosol mixing ratio and the concentration, respectively, if the aerosol size distribution and the backscattering efficiency remain constant. The total ðaerosol þ moleculeÞ depolarization ratio d is obtained by taking the ratio of the linear polarization components of the backscatter signals with respect to
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the polarization plane of the emitted laser to those of the total backscatter signals at the wavelength of 532 nm: The aerosol depolarization ratio da is calculated from the d and R at 532 nm by the equation da ðzÞ ¼
dðzÞRðzÞ dm ; RðzÞ 1
ð3Þ
where dm is the molecular depolarization ratio for this lidar system (dm B0:49%; calculated by H. Adachi, personal correspondence). We employed the correction procedure described by Adachi et al. (2001) to eliminate the depolarization components produced by the system. This value can be considered as an indicator of the particles’ nonsphericity (Bohren and Huffman, 1983); da is zero for homogenous spherical particles and da substantially deviates from zero for nonspherical particles. It must be noted the value of da measured with the lidar is averaged temporally and vertically so that if several kinds of particles with different da values coexist in the air, the value indicates the average weighted by the particle’s backscattering cross section. The wavelength exponent of the aerosol backscattering coefficient a is calculated from ba at 532 and 1064 nm assuming the relation ba pla (available only for Nagoya). The value a is approximately negatively correlated with the aerosol size; the value decreases as the size increases and vice versa. The water vapor mixing ratio is obtained using the ratio of the Raman water vapor to the Raman nitrogen (or oxygen) backscatter signals (Whiteman et al., 1992; Ansmann et al., 1992a). The calibration constant to calculate the water vapor mixing ratio is obtained by comparing with coincident radiosonde data. The relative humidity RH is calculated from the observed water vapor mixing ratio combined with the temperature and pressure data obtained with the radiosonde launched at the nearest meteorological observatory from the measurement sites (Tateno or Hamamatsu).
3. Vertical distributions of aerosols and relative humidity during the Asian dust events We have frequently measured the enhancements of aerosol backscattering in the free troposphere during the Asian dust period in 2000 and 2001. The aerosol enhanced layers usually showed high aerosol depolarization ratios ð> 20%Þ; indicating the predominance of nonspherical particles in those regions. Our previous results (Kwon et al., 1997; Sakai et al., 2000) have shown that the RH values in those aerosol layers were usually low ðo30%Þ; probably because they originate mainly from the arid regions in the Asian continent. However, we have sometimes measured moderately high RH ðB60%Þ at those high depolarizing regions, possibly as a result of mixing with moist air caused by a strong front
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activity. This case is of interest because we have little information about the optical properties of the dust aerosol in the moist air. In this section we show two measurement cases of the aerosol optical properties and RH obtained at the dust events in 2000 and 2001; one is the low RH case obtained on 23 April 2001 at Nagoya and Tsukuba, and the other is the moderately high RH case obtained on 4 May 2000 at Nagoya. 3.1. Case of 23 April 2001 Fig. 1 shows the vertical distribution of the aerosol optical properties (R; Sa and da ) at 532 nm and RH measured with the Raman lidar at Nagoya (averaged over the time period 19:27–20:27 JST) and Tsukuba (averaged over the time period 19:40–20:21 JST) on 23 April 2001. That night we found a peak of R of the order of 3.5–4.0 at an altitude of 5 km at both measurement sites. The values of da were as high as 20% between 2 and 8 km at Nagoya and between 4 and 7 km at Tsukuba, indicating the predominance of nonspherical particles in these regions. Because the RH values were mostly below 30% in these regions, presence of ice crystal was improbable for this period. The optical thickness ðta Þ of this aerosol layer derived from the Raman nitrogen signal was 0:1870:02 and the average extinction-to-backscatter ratio (lidar ratio, Sa ) was 4675 sr between 4 and 7 km at Tsukuba. Fig. 2 shows the 5-day isentropic backward air-mass trajectories arriving at the lidar sites on 23 April 2001.
Detail of the calculation is given in Sakai et al. (2000) except that the initial positions were located 25 points within 0:51 0:51 latitude–longitude grid around the lidar sites. The air parcels arrived at the lidar sites had been passed similar pathways over the Asian continent due to the prevailing westerly flow. They had passed over the Gobi desert on 21 April when the meteorological observatories at Umnugobi in Mongolia (TsogtOvoo, 44:41N; 105:31E; 1298 m a.s.l.; Saikhan-Ovoo, 45:51N; 103:91E; 1316 m a.s.l.) had reported severe dust storms, suggesting that mineral dust particles had been uplifted into the free troposphere around this arid area. Since the trajectories showed small spatial dispersion during the transportation, it is expected that the dust particles were transported directly from the source areas to the measurement sites without large disturbance or mixing with the other air masses. For comparison of the optical properties of the dust layers, Muller . et al. (2001) observed the optical properties of an elevated dust layer originated from the arid and semi-arid region in Arabian Peninsula using a sixwavelength lidar over Maldives. They measured the peak value of ba B2:4 M m1 sr1 (which corresponds to approximately RB3:5) at an altitude of 3:8 km and obtained the Sa values of 34–52 sr at the altitude range of 3.5–5:2 km: Powell et al. (2000) and Welton et al. (2000) measured the aerosol profiles of Saharan dust layers using a micro-pulse lidar over Canary Islands. They observed a peak of extinction coefficient at an altitude of approximately 3 km and reported
Fig. 1. Vertical profiles of aerosol backscattering ratio ðRÞ and lidar ratio ðSa Þ at 532 nm (left panels), aerosol depolarization ratio ðda Þ and relative humidity (RH) (right panels) obtained at Nagoya between 19:27 and 20:27 JST (a) and Tsukuba between 19:40 and 20:21 JST (b) on 23 April 2001. The thin-dotted line in the right panel of Fig. 1b shows humidity profile obtained with radiosonde on 20:30 JST at Tsukuba. Arrows in the right panels show the height of which backward trajectories are shown in Fig. 2. The horizontal distance between the two sites is approximately 300 km:
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can expect that the dust layers observed at around 5 km (Fig. 1) were horizontally continuous between the two sites. It is also to be noted that the uppermost part of this aerosol layer showed the highest d values (B33%; da was nearly equal to this value) with the highest R values (> 50; not shown) at about 8 km on 1:00 JST (Fig. 3a). This peak was probably due to the presence of ice crystals because the RH was greater than 70% (Fig. 3b) that was almost saturated with respect to ice ð> 90%Þ in the region. The value of Sa at this region was 1073 sr; which is close to that of cirrus clouds (5–15 sr) measured by Ansmann et al. (1992b) using Raman lidar at 308 nm and that of cirrostratus model containing hexagonal ice crystals (11 sr at 550 nm) calculated by Takano and Liou (1989) using ray-tracing technique. Since the Asian mineral particles effectively act as ice forming nuclei (Isono et al., 1959), the ice crystals may have formed on the mineral particles at this region. Murayama et al. (2001) observed cirrus clouds at the top of a thin dust layer above 9 km using a polarization lidar over Tokyo. 3.2. Case on 4 May 2000
Fig. 2. Horizontal and vertical cross section of isentropic backward trajectories arriving at the altitude of 5 km at Tsukuba (solid line with pluses) and Nagoya (dotted line with circles) on 23 April 2001. The symbols are plotted at 24-h intervals. The lowermost lines in the vertical cross section show the land elevations. Triangles in the upper panel show the location where severe dust storms were reported on 21 April 2001.
Sa ¼ 3575 sr and ta ¼ 0:17170:02 between 1.5 and 4:6 km; or Sa ¼ 3779 sr between 2.5 and 5 km and ta ¼ 0:161B0:21770:01: Although the source regions of the dust layers were different from our case, the values of Sa reported by them were close to or slightly smaller than those obtained at 4–7 km in our measurements. More observational data are necessary to obtain appropriate value and the variability of Sa for mineral dust layers in the free troposphere. Fig. 3 shows the temporal and height cross section of d and RH (with respect to water) obtained with the Raman lidar at Tsukuba from 19:40–2:25 JST on 23–24 April 2001. The temporal resolution is 3:3 min: The high depolarizing layer at the altitude range of 4–7 km was persistent for over 6 hours (Fig. 3a; we measured this layer until 4:45 JST when we terminated the measurement). Based on the wind speed of 23:5 m s1 from west at an altitude of 5 km obtained from a radiosonde measurement, this aerosol layer was expected to spread east and west for more than 500 km over Tsukuba, which is approximately 300 km east of Nagoya. Thus we
Fig. 4 shows the vertical profile of R; a; da ; and RH obtained on 4 May 2000 over Nagoya. Unfortunately, we have no data over Tsukuba on that day because we started the measurement there in October 2000. We measured moderately high RH values ðB70%Þ with relatively high aerosol backscattering ratio ðRB2:5Þ below the altitude of 5 km: The aerosol depolarization ratios showed high values ð> 15%Þ between 3.5 and 8 km and the lower value ðo10%Þ below 3:5 km: It should be noted that the maximum RH values were as high as 60% at the bottom of the high da region between 3.5 and 4:5 km; indicating that nonspherical particles were still predominant in such moderately high RH region. Fig. 5 shows the backward air-mass trajectories arriving at the altitude of 4 km at the lidar site. We found that most of the trajectories passed over the Asian continent and they terminated over the arid region in Mongolia ðB451N; B1101EÞ on 2 May because they intersected a vertically unstable layer. The dust storms were reported in Mongolia (Zamyn-Uud, 43:71N; 111:91E; 962 m a.s.l.; Tsogt-Ovoo, 44:41N; 105:31E; 1298 m a.s.l.) on 2 May, suggesting that these dust storms were the main source of the aerosols measured over Nagoya. Fig. 5 also shows that the trajectories had dispersed horizontally over the ocean in the northwest of Japan and some of them had been carried over the northern ocean, implying that the continental air mass had mixed with the maritime air mass at around the area. This dispersion of the trajectories was caused by a strong cold front activity that had developed over the Sea of Japan on 2–3 May. This front activity might also have promoted the vertical transport of moist air from
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23 April 2001 Total Depolarization Ratio at 532 nm 10
δ(%) 30 27 24 21 18 15 12 9 6 3 0
9 8 Altitude (km)
7 6 5 4 3 2 1 0 20:00
21:00
22:00
(a)
23:00
0:00
1:00
2:00
Time (JST)
23 April 2001 Relative Humidity with respect to Water 10
RH(%)
9
100 90 80 70 60 50 40 30 20 10 0
8 Altitude (km)
7 6 5 4 3 2 1 0 20:00
(b)
21:00
22:00
23:00
0:00
1:00
2:00
Time (JST)
Fig. 3. Temporal and vertical cross section of total depolarization ratio at 532 nm (a) and relative humidity with respect to water (b) obtained with the Raman lidar at Tsukuba from 19:40 to 2:25 JST on 23–24 April 2001.
the lower troposphere to the middle troposphere (lower panel in Fig. 5). Below the altitude of 3:5 km the values of da showed a weak negative correlation with RH (right panel in Fig. 4); da values varied between 15% and 7% where the RH varied between 30% and 70%, indicating that the aerosols were more spherical in the moist air than in the dry air at that lower altitude region. Possible reasons for this negative correlation between da and RH are: (a) the nonspherical particles (e.g., solid crystals) changed their shape to spherical (e.g., droplets) by uptaking water vapor at the higher RH, and/or (b) the proportion of nonspherical particles to the other low-depolarizing particles (droplets or small particles whose radii were less than the laser wavelength of 532 nm) decreased with increasing RH. To investigate the relation between the aerosol nonsphericity and the aerosol size, Fig. 6 shows the scatter plot of da as a function of a: We found that the
values of da decreased approximately linearly from 24% to 10% with increasing a from 0.5 to 2 for the data between altitudes of 5 and 8 km where the RH was less than 30%, suggesting that the aerosol nonsphericity increased with increase in the aerosol size in that region. The data between altitudes of 2 and 5 km; where the RH values were between 30% and 70%, showed the lower a values ðo0:5Þ than those at 5–8 km; suggesting that the aerosol sizes were larger than those in the 5–8 km region. They also showed that the da values were the lowest ðB7%Þ at the smaller a values ðo0Þ; suggesting that the aerosol nonsphericity was the lowest (i.e. spherical) for the larger aerosol sizes in that region. For comparison, vertical variation of aerosol optical properties and RH has been measured by Muller . et al. (2001) over Maldives. During the period of southwest monsoon, they observed the wavelength exponent of the aerosol extinction coefficient k of 0.37, 0.52, and 0.97 in the altitude ranges of 1.0–2.35, 3.6–4.2, and 4.2–5:2 km;
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30 Altitude 5-8 km 2-5 km
δa (%)
20
10
0 -1 Fig. 4. Vertical profile of aerosol backscattering ratio ðRÞ; wavelength exponent of aerosol backscattering coefficient ðaÞ (left panel), aerosol depolarization ratio ðda Þ and relative humidity (RH) (right panel) obtained at Nagoya on 4 May 2000. The arrow in the right panel shows the height of which back trajectories are shown in Fig. 5.
Fig. 5. Horizontal and vertical cross section of isentropic backward trajectories arriving at the altitude of 4 km at Nagoya (25 points around 35:11N; 137:01E at intervals of 0:51) on 4 May 2000. Symbols are plotted at 24-h intervals. The lowermost lines in the vertical cross section show the land elevations. Triangles in the upper panel show the location where dust storms were reported on 2 May 2000.
0
1 α
2
3
Fig. 6. Scatter diagram of aerosol depolarization ratio ðda Þ as a function of wavelength exponent of aerosol backscattering coefficient ðaÞ measured on 4 May 2000 at Nagoya. Squares and circles show the data between altitudes of 5 and 8 km and between 2 and 5 km; respectively.
respectively, where the RH derived from the Raman measurement decreased from 70% at 1:0 km to B20% at 2:7 km and again increased to 45% in the upper regions (the middle and upper altitude regions correspond to the dust layer refered in Section 3.1). The vertical variations of k and RH are similar to those of a and RH in our results, suggesting that the aerosol size was larger in the high RH region of the lower altitude and decreased with increasing height. Although they did not report da data, they derived the aerosol physical properties from the multiple wavelength measurements; for example, the mean aerosol effective radii were 0.3, 0.27, and 0:18 mm and the complex refractive indices were 1:6 þ 0:02i; 1:53 þ 0:005i; and 1:44 þ 0:004i for the three altitude regions. Based on the aerosol properties and RH derived from the lidar measurement and the backward trajectory analysis, they concluded that a mixture of clean-marine and soil-derived particles was predominant in the lower altitude region and mineral dusts were predominant in the elevated layer. Since it is difficult to conclude what kind of aerosol constituents was present only from our lidar data, we discuss possible aerosol constituents and their mixing state that account for the relation between da and a observed on 4 May at Nagoya in the next section.
4. Discussion Tropospheric aerosol particles usually consist of several chemical components and are mixed both
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Nonspherical
A Nonspherical particles
δa
(e.g., mineral dust; sea-salt crystal)
2-5 km (High RH)
5-8 km (Low RH) External mixing
B
C Spherical and large particles
Spherical
(e.g., sea-salt droplet; mineral dust coated with solution)
Large
Spherical and/or small particles (e.g., sulfate; small mineral dust)
α
Small
Fig. 7. Schematic diagram of relation between aerosol depolarization ratio ðda Þ and wavelength exponent of aerosol backscattering coefficient ðaÞ for three aerosol types (see text for detail).
externally and internally. In an attempt to account for the relation between da and a obtained on 4 May (Fig. 6), we suppose a simple model of aerosol mixture (Fig. 7) based on the aircraft measurements made by several researchers in the free troposphere over around Japan (Iwasaka et al., 1988; Pueschel et al., 1994; Ikegami et al., 1993; Sakai et al., 2001). The aircraft measurements have shown that the major aerosol constituents in the free troposphere were sulfatecontaining particles in the fine mode (approximately less than 0:5 mm in radius) and mineral dust and sea-salt particles in the coarse mode (approximately larger than 0:5 mm in radius). They also found that some of the mineral dust particles were coated with solution (e.g., sulfuric acid) in their electron micrographs (Iwasaka et al., 1988; Ikegami et al., 1993; Sakai et al., 2001). The values of a and da for these aerosols obtained from laboratory experiments and field measurements or inferred from theoretical calculations are as follows: da B2% (laboratory experiment by Sassen et al. (1989), measured at scattering angle X1781 at a wavelength of 632:9 nm) and a > 1:0 (calculation using Mie theory for the particle size distribution measured with optical particle counters by Sakai et al. (2001) for the sulfatecontaining particles; da E20% (laboratory experiment by Cooper et al. (1974)) and aB0:5 (estimation from the theoretical calculation using discrete dipole approximation method by Murayama et al. (2001)) for crystalline solid; da ¼ 5:6–11.1% (laboratory experiment by Cooper et al. (1974)) and ao0:8 (calculation using Mie theory for the particle size distribution measured by Gras (1991)) for solution droplets of sea-salt; da E27% or 30% (laboratory experiments by Volten et al. (2001) and Kuik et al. (1991), measured at scattering angle of 1731 or 1751 at 632:9 nm) and a ¼ 0:6B 0:1 (estimation from the observational data using semi-empirical model by Nakajima et al. (1989)) for the mineral dust particles.
It must be noted that the values of da listed above should be considered as a representative for those particles because the values depend on the particle size as well as the shape and are not generally monotonic function of size (e.g., Asano and Sato, 1980; Mischenko and Hovenier, 1995); for example, the value of oblate spheroid with aspect ratio 1.7 with refractive index of 1:55 þ 0:005i increases from 0% to 31% for the equal-volume sphere radius from 0 to 0:4 mm and decreases to 21% at 1:0 mm (calculated by T-matrix code of Mischenko et al. (1996)). The values of da and a of the mineral particles coated with solution were not well known, but we can expect that both the values are lower than those of pure (not coated with solution) minerals because they are larger and more spherical than the pure minerals by being coated by aqueous solution. Based on these results, we suppose the following mixing state of the aerosols that explain the relation between da and a observed on 4 May. Fig. 7 shows the schematic diagram of the relation between da and a for the mixture of those particles. At altitudes between 5 and 8 km where the RH was less than 30%, the nonspherical mineral particles and/or sea-salt crystals in the coarse mode (Particles ‘A’ in Fig. 7) and the sulfate-containing particles in the fine mode (Particles ‘B’) were predominant and their proportion varied with height. Another possibility is that the mineral particles that were smaller than the laser wavelength of 0:532 mm were predominant and its effective radius varied with height. At altitudes between 2 and 5 km where the RH was between 30% and 70%, the mineral particles and/or sea-salts crystals (Particles ‘A’) and the spherical particles (Particles ‘C’) that were larger than the mineral particles and/or sea-salts crystals were predominant and their proportion varied with height. One possibility of these large spherical particles is the mineral particles coated with aqueous solution as mentioned before. The other possibility is sea-salt droplets in metastable state because they can exist as liquid at the RH values between the crystallization and the deliquescence point (between 46% and 75%; Tang, 1996) due to the hysteresis. Field measurements by Rood et al. (1989) have reported the ubiquitous presence of metastable droplets in the surface atmosphere. These sea-salt droplets might be mixed internally with mineral dust because Niimura et al. (1998) found such mixed particles frequently in the surface atmosphere at Nagasaki in Japan during the Asian dust events. However, our model is just conceptual and qualitative. Further field studies (e.g., coincident measurements with lidar and in situ sampling) and laboratory experiments are essential to clarify the relation of the aerosol optical properties with RH and with the aerosol mixing state. We notice also that it might be important to measure the mixing state of the Asian dust particles to
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assess their effect on the atmospheric processes because they can modify cloud microphysical processes by acting as cloud condensation nuclei more effectively than the other major aerosols (e.g., submicrometer sulfates) if they are coated with water-soluble substances (Takeda and Kuba, 1982; Wurzler et al., 2000) and their radiative effects critically depend on whether they are mixed externally or internally (e.g., Ackerman and Toon, 1981).
Acknowledgements
5. Conclusion
Appendix
We have presented two case studies of the Raman lidar measurements of the aerosol optical properties and relative humidity in the free troposphere during the Asian dust events in 2000 and 2001 at Nagoya and Tsukuba, Japan. The data obtained on 23 April 2001 showed a maximum of aerosol backscattering ratio R of 3.5–4.0 at an altitude of 5 km at Nagoya and Tsukuba. Around that height the aerosol depolarization ratios da were larger than 20% and relative humidities RH were lower than 30%. The aerosol optical thickness between 4 and 7 km was 0:1870:02 and the average lidar ratio Sa was 4675 sr at Tsukuba. This aerosol layer was persistent for over 6 h and showed the highest da value of B33% and the Sa of 1073 sr at the uppermost height region, where the RH was almost saturated with respect to ice. The data obtained on 4 May 2000 at Nagoya showed the R of about 2.5 below an altitude of 5 km: The values of da were higher than 15% between 3.5 and 8 km where the maximum RH was B60%: The values of da and RH showed a weak negative correlation below 3:5 km; where the RH varied between 30% and 70%. The da values decreased almost linearly from 24% to 10% with increasing the wavelength exponent of the aerosol backscattering coefficient a ranging from 0.5 to 2 in the 5–8 km region, whereas they showed the lowest value about 7% at the smaller a values ðo0Þ in the 2– 5 km region. We hypothesized that these relations of da to RH and a were due to the mixing state of mineral dust, sea-salt, and sulfate-containing particles that are the major aerosol constituents collected in the free troposphere during the Asian dust period and to the dependence of their shape and size on RH. However, coincident measurements with lidar and insitu sampling are necessary to clarify the relation between the aerosol optical parameters obtainable with the lidar and the aerosol physicochemical properties such like size, shape, chemical composition, and mixing state. In addition, coordinated multi-site observations with lidar will provide valuable information about the spatial distribution of aerosol optical properties and their temporal evolution during the transportation.
To derive R and ba at the wavelengths of 532 and 1064 nm; we used the Raman oxygen signal ð375 nmÞ to estimate the aerosol transmission at those wavelengths for the data of Nagoya (Sakai, 2000); the aerosol extinction coefficient sa at 355 nm is derived from the Raman oxygen signal (e.g., Ansmann et al., 1990) and the value is converted to sa at 532 and 1064 nm by assuming the relation sa plk : The value of k; an unknown parameter, is assumed to be 1.0 with random uncertainty of 1 for a (wavelength exponent of ba ÞX1 which suggests the abundance of particles with radii smaller than laser wavelengths or k ¼ 070:5 for ao1 which suggests the predominance of large particles. This relation between k and a was obtained by calculations for aerosol models using Mie theory and T-matrix code (Mischenko et al., 1996). The procedure to derive R and sa at 532 nm for the data of Tsukuba is similar to that described by Ansmann et al. (1990, 1992a) except that we used k that varied with da based on the calculations for the particle size distributions obtained from the aircraft measurements (Sakai et al., 2001) to reduce the uncertainty associated with the unknown parameter of k; the value linearly decreased from 1.5 to 0 with increasing da from 0 to X25% with random uncertainty of 0.5 assuming the external mixture of small non-depolarizing ðda ¼ 0Þ particles (e.g., submicrometer sulfates) and large depolarizing ðda B25%Þ particles (e.g., mineral dust; sea-salt crystal). In addition, we used k ¼ 0 for RX5 regardless of da in the free troposphere ð> 2 kmÞ because these high R values are mostly due to cloud particles (large water droplets or ice crystals) that have nearly no wavelength dependence in sa : The calculations of these procedures are made in an iterative manner since a and da are a function of ba and R: However, for the height regions where the sa was not derived from the Raman signal because of incomplete overlap of the laser beam with the telescope’s field of view (approximately below 2.5–3 km) or where the measurement uncertainty in sa exceeds 100%, we employed the equation sa ¼ Sa ba ; where we assumed Sa ¼ 50730 or 40732 sr at 532 and 1064 nm; respectively.
We thank M. Nagatani and H. Nakada of Nagoya University for technical assistance of the lidar at Nagoya. This research was supported in part by the Ministry of Education, Culture, Sports, Scientific and Technology through Grant-in-Aid for Scientific Research on Priority Areas (A), No. 10144104, led by Y.I. of authors.
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