Analysis of aerosol vertical distribution and variability in Hong Kong

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2008jd009778, 2008 Analysis of aerosol vertical distribution and variability in Hong Kong Qianshan He, 1,2 Chengcai Li, 1,3 Jietai Mao, 1 Alexis Kai-Hon Lau, 4 and D. A. Chu 5,6 Received 1 January 2008; revised 12 March 2008; accepted 29 May 2008; published 25 July [1] Aerosol vertical distribution is an important piece of information to improve aerosol retrieval from satellite remote sensing. Aerosol extinction coefficient profile and its integral form, aerosol optical depth (AOD), as well as atmospheric boundary layer (ABL) height and haze layer height can be derived using lidar measurements. In this paper, we used micropulse lidar measurements acquired from May 2003 to June 2004 to illustrate seasonal variations of AOD and ABL height in Hong Kong. On average, about 64% of monthly mean aerosol optical depths were contributed by aerosols within the mixing layer (with a maximum (76%) in November and a minimum (55%) in September) revealing the existence of large abundance of aerosols above ABL due to regional transport. The characteristics of seasonal averaged aerosol profiles over Hong Kong in the study period are presented to illustrate seasonal phenomena of aerosol transport and associated meteorological conditions. The correlation between AOD and surface extinction coefficient, as found, is generally poor (r ) since elevated aerosol layers increase columnar aerosol abundance but not extinction at surface. The typical aerosol extinction profile in the ABL can be characterized by a low value near the surface and values increased with altitude reaching the top of ABL. When aerosol vertical profile is assumed, surface extinction coefficient can be derived from AOD using two algorithms, which are discussed in detail in this paper. Preliminary analysis showed that better estimates of the extinction coefficient at the ground level could be obtained using two-layer aerosol extinction profiles (r , slope 0.82, and intercept 0.15) than uniform profiles of extinction with height within the ABL (r , slope 0.27, and intercept 0.03). The improvement in correlation is promising on mapping satellite retrieved AOD to surface aerosol extinction coefficient for urban and regional environmental studies on air quality related issues. Citation: He, Q., C. Li, J. Mao, A. K.-H. Lau, and D. A. Chu (2008), Analysis of aerosol vertical distribution and variability in Hong Kong, J. Geophys. Res., 113,, doi: /2008jd Introduction [2] Tropospheric aerosol vertical distribution and optical properties are required for radiative transfer calculation and are of great importance to understanding aerosol effects on climate [Kaufman et al., 1997]. In the past, a great deal of aerosol vertical profiles have been observed during field experiments, such as TARFOX (1996), LACE (1998), ACE-1 (1995), ACE-2 (1997) and ACE-Asia [Russell et al., 1999; Ansmann et al., 2002; Bates et al., 1998; Raes et 1 Department of Atmospheric Sciences, School of Physics, Peking University, Beijing, China. 2 Also at Center for Satellite Remote Sensing and Measurement, Shanghai Meteorological Bureau, Shanghai, China. 3 Also at Institute for the Environment, Hong Kong University of Science and Technology, Kowloon, Hong Kong. 4 Institute for the Environment, Hong Kong University of Science and Technology, Kowloon, Hong Kong. 5 Joint Center for Earth Science and Technology, University of Maryland, Baltimore County, Baltimore, Maryland, USA. 6 Also at Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. Copyright 2008 by the American Geophysical Union /08/2008JD al., 2000; Huebert et al., 2003]. On a regional scale, the variations of aerosol vertical profile in major cities, such as Paris, Los Angeles and Tokyo have been extensively investigated [e.g., Menut et al., 2000; Lurmann et al., 1997; Sasano, 1996]. However, our knowledge of heightresolved aerosol properties over extended periods is still rather poor [Penner et al., 2001]. As a crucial step toward understanding of the complex roles of aerosol in the atmosphere, aerosol vertical distribution should be first quantified accurately. [3] A micropulse lidar (MPL) based on the elastic backscattering of the emitted low-power laser is a useful tool to provide both high vertical and temporal resolution distribution of aerosols [Spinhirne, 1993]. A number of literatures have reported on the applications of MPL to aerosol distribution studies [Chen et al., 2001; Parikh and Parikh, 2002; Campbell et al., 2003]. Optical and climate related characteristics of the lower atmosphere are also observed with MPL [Campbell et al., 2002; Welton et al., 2000, 2002; Welton and Campbell, 2002]. Aerosol vertical distribution and integral optical properties (such as aerosol optical depth, t or AOD) can be obtained with this technique simultaneously. 1of13

2 Figure 1. Time series of (top) diurnal (daytime) mean AOD at 523 nm, (middle) ABL heights, and (bottom) the total haze layer heights from MPL observations between 1 May 2003 and 30 June Black dots represent the diurnal mean values, and the error bars correspond to the standard deviations. [4] More importantly, the surface aerosol extinction coefficient is one of a few quantities of interest to determine aerosol effect on visibility [Elterman, 1970], which is directly linked to the issues of environmental pollution and public health of concern. The knowledge of the relationship between the aerosol optical depth (t) and aerosol surface extinction coefficient (s s ) allows a better estimation of the aerosol effects on radiative forcing and understanding of air pollution sources. Though Busen and Hanel [1987] showed aerosol extinction decreases exponentially with altitude, however, it would not be true in situations with transported aerosols over complicated terrains [Li and Lu, 1997; Qiu and Yang, 2000] such as in Hong Kong. According to Qiu and Yang [2000], t s dependence on s s is not only controlled by urban/industrial emissions but also has regional characteristics resulted from synoptic weather cycles. [5] Current radiative transfer calculations, for simplicity, often assume a uniform aerosol profile within the ABL, which is apparently inadequate for the cases with 36% of total aerosol extinction from above the ABL in Hong Kong. The calculations performed in this paper using a two-layer aerosol model with a lower layer of uniform aerosol extinction and an upper layer of aerosol extinction decreased exponentially with altitude show better estimates of the extinction coefficient at surface than those derived by using single layer model. These analyses further confirm the use of AOD combined with the ABL height and haze layer depth above the top of the ABL can provide more accurate estimate of extinction at the surface with known AOD. [6] This paper analyzes quantitatively aerosol vertical distribution in the lower troposphere by examining the aerosol extinction profiles derived from MPL measurements in Hong Kong. In section 2, we briefly describe the measurements and methodology used for deriving the aerosol integral and surface optical properties. In section 3, we discuss in detail the results and findings from the yearlong measurements. In addition, the lidar-derived aerosol vertical distributions are further discussed in terms of seasonal and diurnal variabilities in this section. In section 4 we summarize the results. 2. Measurements and Methodology [7] A MPL system has been operated by the Hong Kong University of Science and Technology (HKUST) since May 2003 at Yuen Long (22.44 N, E), which is a small town at the northwest of Hong Kong. Hong Kong is a highly populated city in the southeast of developing region in China known as the Pearl River Delta (PRD). The PRD is one of the most populated and also one of the fastest economic developing regions in China [Cao et al., 2003]. The weather in this region is influenced by the southerly or southeasterly east Asia monsoon (or South China Sea monsoon) in summer and northerly or northeasterly monsoon in winter. May and November are usually the transition months. This region owns higher annual mean relative 2of13

3 humidity (70%). In both summer and winter seasons, high AOD values were observed over the PRD with a seasonal mean 0.8 in the summer and 0.6 in the winter [Li et al., 2005a]. Aerosol emissions in this region include both natural (sea salt in summer and dust in spring) and anthropogenic sources (sulfate, black carbon, volatile organic compounds, etc). Therefore this study is of great importance to characterize transported and locally emitted pollutants to the region. [8] The MPL measurements used in this paper have a continuous coverage from 1 May 2003 to 30 June 2004, except for the period of system maintenance from 18 December 2003 to 13 February The MPL site is located on the rooftop of a building of Yuen Long, 28 m above ground level and 53 m above sea level. The bin time of the MPL receiver was set to 200 ns corresponding to a 30-m vertical resolution. The MPL pulse repetition rate was about 2500 Hz. The zone of incomplete afterpulse correction was approximately 130 m. More detail about the MPL can be found in [He et al., 2006a]. Although data were recorded every 15 s, to improve the signal-to-noise ratio for deriving aerosol extinction profiles, MPL signals were averaged over an interval of 60 min in which the shape of aerosol profiles have no sharp change in vertical variation. On the basis of the hourly lidar extinction profiles, only those performed during daytime under cloud-free conditions were chosen, for which there was either a well-mixed ABL or a homogenous haze layer above ABL resulting in the disappearance of the residual layer as far as possible. The cloud-contaminated lidar data were eliminated using experiential threshold cloud screened algorithm. [9] The description of the retrieval of aerosol parameters from MPL was given by He et al. [2006a] and therefore will only be briefly summarized here. The vertical profile of aerosol extinction is determined by a near end approach in solving the lidar equation as proposed by Fernald [1984]. The column-averaged aerosol extinction-to-backscatter ratios (EBR) are retrieved by constraining lidar derived aerosol optical depths (AOD) to those obtained by other measurements (e.g., surface Sun photometers). However, these retrieved EBR are discrete throughout the measurement period since the retrieval process requires both AODs (from lidar and other measurements) to be collocated in space and time. In order to investigate continuously the variation of aerosol optical properties and vertical distribution, the interpolation is performed within the retrieved EBR from 1 May 2003 to 30 June 2004, during which aerosol extinction profile of each individual hour can be obtained, assuming EBRs remain constant throughout the whole day. The error of the extinction coefficient is estimated within the range of 20 30% of the lidar signal [He et al., 2006a] from the uncertainty of EBR, lidar constant, and overlap correction as well as the effects from multiple scattering. To compute AOD, extinction of the lowermost 130 m is set equal to that of the surface. Then, the extinction coefficients are integrated up to 4 km. In most cases, this assumption can be made without incurring large errors because the boundary layer is usually well mixed during daytime hours when the measurements are taken. In cases when the ABL is not well mixed, additional errors in the optical depth determination are introduced by the extrapolation of the lowest extinction coefficient down to the surface. [10] The development of the ABL plays a key role in disturbing atmospheric constituents, especially in a highly polluted urban area. Thus, the atmospheric boundary layer height (h ABL ) is also derived from the lidar profiles using the algorithm proposed by He et al. [2006b], who regard the first significant negative gradient in the normalized rangecorrected lidar signal near the ground as the top of the ABL. The steep gradient in the normalized range of corrected lidar signal is shown as a result of the significant reduction in aerosol backscattering caused by the lower particle concentration and humidity above the ABL. Then, integrating the extinction coefficient from the ground to the ABL top yields the aerosol optical depth in ABL. [11] By assuming an exponential decay of aerosol extinction coefficient with altitude above the top of the ABL, the upper portion of aerosol optical depth can be derived. The scaling height of this exponential decay is considered to be the haze layer height (h HLH ) above the top of the ABL. The haze layer height is affected by the long-range transport of aerosol-rich air mass and the convective dispersion of aerosol in the free atmosphere, which are usually associated with large-scale weather systems. From the lidar observations, haze layer height can be obtained directly by finding the level where the aerosol extinction coefficient decreases to 1/e of that on the top of the ABL. In this study, total haze layer height (h THLH ) is defined as the sum of ABL height and the haze layer height (h THLH =h ABL +h HLH ). [12] In order to characterize the variation of aerosol vertical distribution, long-term lidar measurements of aerosol extinction coefficient were analyzed. The seasonal and annual mean aerosol extinction coefficients profiles at 523 nm were derived for the periods. 3. Results 3.1. Analysis Overview [13] A total of 188 daily mean observations of daytime were included in the analysis of the vertical characteristics of aerosol over this region. Figure 1 shows the time series of daily mean AOD, ABL and total haze layer heights (h THLH ) derived from MPL observations from 1 May 2003 to 30 June The error bars show the standard deviations of AOD for each day, which represent some information in regard to diurnal variability. Generally, the daily mean AOD varied between 0.02 and 1.44 at 523 nm, with a maximum value on day 340 and minimum on day 223 of The daily mean AOD values are shown to be relatively high around 0.5 with a slight variation from day 250 to 350 of 2003 in contrast to those before day 170 in 2003 and after day 90 in 2004 as characterized by more rapid diurnal change and day-to-day variation, indicating atmospheric turbidities dominated by multiple sources of pollution in spring. The atmospheric turbidities as characterized by AOD are shown in the range less than 0.5 between day 170 and 240 in 2003 because of the influence of southerly clean air mass originated from the ocean or/and frequent precipitation during the two months summer period allowing the removal of aerosols from the atmosphere over Hong Kong. [14] The derived ABL heights clearly show a semiannual cycle with higher values (1.2 km) in the summer and lower values (1.0 km) in the winter. The seasonal variation, however, is less obvious as found previously in Beijing 3of13

4 Figure 2. (a) HYSPLIT 96-h back trajectories of air mass analyzed at three different altitudes from 4 March The asterisk represents the lidar site at Hong Kong. (b) Lidar returns at 523 nm in 30-m vertical resolution steps as a function of height (kilometers) and time (hour), showing the presence of dust at 2 3 km altitude. (40 N, 116 E), 43 m above sea level and 183 km from the Bohai Sea, which has lower annual mean relative humidity (50%) and significant variation of solar radiation at surface in each season. The urban area of Beijing is located in the transition zone between mountainous area and the north China plain, so the geographical characteristic usually causes a larger temporal and spatial variation of aerosols [Li et al., 2005b]. According to the ABL height obtained from MPL measurements in Beijing by He et al. [2006a], the mean ABL height was above 2.0 km in May and approximately 1.0 km on average in November Hong Kong is more influenced by the subtropical climate characterized by nearly constant surface sensible heat fluxes throughout the year. Thus less variation of the ABL height is shown between seasons and throughout the year in Hong Kong. The diurnal variability of the ABL height appears to be relatively larger compared to that of haze layer height. The ABL daily means are found generally between 0.30 and 4of13

5 Figure 3. Monthly mean AOD and fractional AOD below ABL and below 1 km above surface based upon aerosol extinction profiles measured by MPL in Hong Kong from 1 May 2003 to 30 June km, with an annual average 1.01 km. The high standard deviations however reflect to a stronger diurnal variation as shown in Figure 1, which can be as large as 1.0 km. The relatively small standard deviations after day 90 of 2004 indicate small changes in ABL heights due to the lack of solar radiation at surface (because of cloud cover) that ABL is not well developed. [15] The haze layer height, on the other hand, show a more pronounced semiannual cycle with higher values in the summer than winter, except for exceptional low values induced by wet removal processes during the summer monsoon period. This semiannual cycle is associated not only with the stability of the total atmospheric column but also the origin of the upper level air. The reduced surface sensible heat fluxes in winter suppresses the lifting aerosol particles to the upper levels, whereas in spring dust layers originated from remote deserts are often flown over Hong Kong at altitudes between 3 and 5 km [Xuan et al., 2000]. Several dust events are observed by MPL over Hong Kong during the study period. On 29 February 2004, for example, a dust event was reported from routine meteorological observatories in the arid regions between northern China and Mongolia. A 4 day HYSPLIT [Draxler and Rolph, 2003] back trajectory analysis (Figure 2a) showed the dust air mass advecting southward and finally reached Hong Kong on 4 March when the aerosol EBR values was up to 35 observed with the combination of MPL and AOD measurement [He et al., 2006a] in association with AOD values of 1.0 from MODIS. The time-height sections of lidar normalized attenuated backscattering coefficients at 523 nm are shown in Figure 2b with a 2-D structure of a dust layer extending up to 3 km in altitude and another aerosol layer on top at km. This long-range transport process is a result of prevailing northeast winter monsoon that carries dust over the PRD. Therefore, it can be concluded that the desert-originated dust aerosol contributes significantly to elevated haze layer height observed over Hong Kong during the spring time. [16] The daily mean of h THLH (h ABL +h HLH ) varies from 0.68 to 3.9 km with an average 1.67 km. The small standard deviation of h HLH in most of the cases indicates a constant haze layer height lasting more than a day, which makes it possible to estimate the height representing the haze layer throughout the observing period Seasonal Characteristics of Aerosol Vertical Distribution [17] To study the seasonal variation of aerosol vertical distribution, the monthly mean AODs are computed and shown as histograms in Figure 3. In the same plot we also present the AOD values below the ABL height and below 1 km. The percentage fraction of AOD below the ABL height to the total AOD is shown on top of the bar of AOD below ABL. Averaging over the entire period of study, it is found that about 62% of aerosol optical depth is caused by particles below 1 km, while 64% is contributed by aerosols below the ABL height. As readily seen from Figure 3, AOD derived over this station shows a maximum in the May (0.71) and minimum in July (0.23) The trend of monotonic decrease in AOD from September to December 2003 is followed by a steadily increase till April In the periods of May June 2003 and March April 2004, the monthly mean AOD values are 0.71, 0.70, 0.57 and 0.62 respectively, which are times greater than the annual mean of These higher AOD values found in spring and autumn as opposed to those in the winter are due to stronger convection and surface heating with relatively weak wind and less precipitation. During the period of March June, Hong Kong is characterized by predominantly easterly winds with moderate wind speeds 5.3 m/s and continuously rising temperatures (>23 )[Louie et al., 2005]. As such, the condition of strong convection and surface heating causes particles of soil origin and gaseous and 5of13

6 Figure 4. Monthly variations of t (solid line with asterisk) and s s (solid line with circle) derived from the daily averages of MPL measurements in Hong Kong between May 2003 and June particle pollutions released at the surface by human activity to rise to upper levels of the mixed layer and stay suspended for a longer period of time. In addition, long-range transported dust aerosols are also responsible for high AOD values in spring. These high aerosol optical depth values are rather usual in Hong Kong as found by Li et al. [2003] who compared AOD measurements between the Sun photometer and MODIS from 2001 to 2002 and concluded the average AOD on the order of 0.4 in PRD with maximum often reaching [18] The contribution of aerosols below ABL to the total AOD is relatively larger in winter than that in spring and autumn. The highest value (76%) is found in November if we neglect one event in August for a small number of samples due to cloudy and rainy days. During the winter months, low ABL heights are caused by strong temperature inversion that trap aerosols within the ABL in contrast to more than 40% of aerosol loading above ABL in spring and autumn. For the maximum total AOD accompanied by the relatively smaller ABL contribution, it is understood that atmospheric turbidity can be significantly affected by aerosols originated from remote sources. [19] The minimum AOD in July is due to the most efficient aerosol removal processes: precipitation. The high precipitation rate over Hong Kong in summer tends to reduce the mean residence time of aerosols in the atmosphere, and, in turn, lowering mean AOD. In the event of continental aerosol transported over Hong Kong, this (precipitation) effect is expected to be even more evident. In fact, when the aerosol travel time is comparable to the mean time interval between two rainfall events, the advectionrelated aerosol load cannot be completely reestablished [e.g., Bergametti et al., 1989]. In addition, the winds blowing from the ocean bring clean air can also partially accounts for low AOD values. On the basis of the analysis of seasonality of AOD, it can be concluded that meteorological processes are evident in removing aerosols in lower troposphere and significant amounts of haze are left above the ABL height. [20] Figure 4 shows the monthly mean AOD (t) and surface aerosol extinction coefficients (s s ) obtained from the lidar observations over the study period. One can see that s s decreases from 0.34 km 1 in May to 0.08 km 1 in July 2003, and then increases from July to December with maximum values reaching 0.42 km 1, which is about a factor of 5 larger than that in July. It then decreased again until the end of the measurement period in June 2004 except for a slight surge in April The high monthly mean AOD values in May June 2003 and March April 2004 were , which also appears to be the case in September 0.6, and low monthly AOD values were in November and December around 0.4. Disregarding the variation of these two parameters in the strong summer monsoon months (July and August), the temporal evolution between t and s s appears to have a positive correlation (R0.93) in spring (from March to June) and a negative correlation (R 0.86) in the other seasons, especially in winter. Clearly, in winter the lower troposphere is very turbid in Hong Kong, which is consistent with lower ABL height in that season. The highest monthly mean s s found in winter is mainly due to elevated organic material, ammonium nitrate and vegetative burning emissions [Louie et al., 2005]. It is worth noting that surface aerosol mass concentration (PM 2.5 and PM 10 ) measured over Hong Kong show a similar trend with maximum in the winter season [Yu et al., 2004], attributed to low convective activity that keeps the aerosol particles near the surface and prevailing northerly winds which transport pollutants from northern continent. Strong radiative cooling over land in winter accompanied by subsidence and low precipitation suppress the dispersion and deposition of pollutants emitted from local sources. Conversely, in the summer months, wet removing effect by abundant precipitation, convection-driven mixing of atmospheric particulate to the higher levels, as well as the prevailing clean southerly winds from South China Sea translates into a lower s s measured at surface layer. These results affirm that knowing the vertical distribution of aerosols is a necessary step to correlate aerosol columnar quantities with aerosol extinction coefficients (or visibility) measured at surface [e.g., Chu et al., 2003; Wang and Christopher, 2003]. [21] A total of 4154 hourly aerosol extinction profiles at both daytime and nighttime from May 2003 to June 2004 were analyzed to derive the seasonally averaged profiles, among which 911 profiles fall in the domain of spring (March, April and May), 1637 in summer (June, July and August), 1183 in autumn (September, October and November), and 423 in winter (December, January and February). These seasonal mean extinction profiles are shown in Figures 5a 5d (thick solid lines), respectively, with corresponding standard deviations (thin solid lines). Overlaid gray area is the annual mean profile of the entire period studied. The annual mean extinction profile indicates that the majority (more than 90%) of the aerosol abundance is 6of13

7 Figure 5. Seasonal mean aerosol extinction profiles for (a) spring, (b) summer, (c) autumn, and (d) winter at 523 nm obtained from ground-based MPL over Hong Kong from May 2003 to June Standard deviation is shown as the horizontal error bars of the seasonal results. Annual mean extinction coefficient profile is shown in the background in gray color. confined to the first few kilometers above surface with slightly larger extinction values just below the top of ABL, where higher relative humidity values could contribute to enhancing the values. In spring and summer, the vertical extent of maximum aerosol extinction is found around 1.0 km, whereas in autumn the maximum extinction is only around 0.5 km above surface. The extinction value near surface is shown to be the largest in winter, which is quite different compared to other seasons as discussed above, and then decrease rapidly with altitude above 1 km. [22] The mean aerosol extinction profile obtained in spring shows that the majority of the aerosol loading is confined between surface and km but apparently having a second layer between 2 and 3 km in the free troposphere, as attributed most likely to dust aerosol. Dust phenomena are usually observed in spring at Hong Kong with a maximum frequency in March as shown in Figure 2. The larger values of extinction below 1.5 km are due to the presence of abundant anthropogenic aerosols. It is important to address the large standard deviation associated with extinction values around the top of ABL observed in spring and summer, for which the large variability of the aerosol vertical distribution is most likely the cause given the frequent occurrence of dust events in the spring and higher relative humidity below ABL top as accompanied by plenteous precipitation in the summer. [23] In summer the aerosol extinction values are the lowest of the year because of the combined effects of prevalent clean marine air mass and frequent wash out by Asian summer monsoon. In fact, aerosol extinction shows similar vertical distribution above 1.5 km in free troposphere as below ABL in all seasons except winter. In winter, the aerosol extinction coefficients are obviously lower above the top of ABL of about 1 km compared with that in the annual averaged profile. This can be explained by the lack of solar radiation at surface in winter season prohibits aerosol particles being lifted to higher altitude and the subsidence of winter high pressure system usually bringing fresh air down to the top of the ABL. [24] The extinction profile in autumn is very similar to that of summer above 1 km, showing no distinct aerosol layers in the free troposphere. However, large differences are found below 1 km where the extinction values are quite high (>0.3 km 1 ) in the first 1 km in autumn, while they are much higher in winter than in autumn by a factor of 1.3. [25] A quite different aerosol spatial distribution is found on the average profiles of s in winter, in comparison to the respective profiles in the other seasons. In winter the mean s profile shows smaller values (<0.2 km 1 ) above 1 km height and higher values (>0.5 km 1 ) below 1 km in comparison to those in the other seasons. As previously mentioned the meteorological conditions during winter are favorable to air pollution being deteriorated over the city 7of13

8 Figure 6. Diurnal (daytime) variation of the AOD (diamond), ABL height (circle) and surface extinction coefficient (cross) derived from lidar data averaged throughout the entire study period. The standard deviations of each parameter are shown in three subplots, respectively. and usually are accompanied by the formation of a stable air layer typically m above ground level. Therefore, the majority of the aerosol load is trapped inside the ABL, typically 1 km above the ground. In this season the mixing layer did not evolve to higher heights and thus an almost aerosol free atmosphere is found above m height. The highest s in the boundary layer during the winter season may be affected by the local urban pollution, which is mainly induced by local urban activities, like mobile traffic, industrial emissions and other urban sources Diurnal Characteristics of Aerosol Vertical Distribution [26] Figure 6 shows the variation of aerosol optical depth, ABL height, and s s, respectively, between 0800 and 1800 local solar time (LST). It is noted that at around 0800 LST ABL height is at about 615 m, and later in the day because of intense thermal convection by the solar heating at surface, ABL height continues to grow and reaches the maximum height 1200 m at 1500 LST. After 1500 LST, the ABL height begins to collapse because of the weakening surface heating and sets at 1000 m by 1800 LST. Aerosol optical depth measurements show a similar variation with ABL height. At soon after sunrise from 0800 to 1200 LST, aerosol optical depth increases rapidly with time from 0.38 to 0.52 and then decreases gradually in the afternoon hours. When the afternoon thermal convection continues weakening, larger aerosol particles quickly settled down to surface by gravity and thus result in a reduction of t. However, the surface aerosol extinction coefficient s s obtained from lidar generally shows a very different diurnal pattern that after a slight increase in the morning before 0900 LST (to 0.36 km 1 ) s s decreases continuously to 0.23 km 1 before sunset. The aerosol loading accumulated within the nocturnal boundary layer during the night is shown by the value of 0.35 km 1 at 0800 LST, which is linked to the daytime anthropogenic activities in the city and stable ABL in favor of the aerosol accumulation during the night maintained by steady advection and temperature inversion. [27] As shown in Figure 6, a decrease in s s is accompanied by an increase in AOD during the period of and LST, and vice versa during and LST. The anticorrelation between surface extinction coefficient and AOD is not surprising. In general, when surface extinction coefficient corresponding positively to aerosol loading near the surface is most likely influenced by local conditions, for example through mixing, and when surface extinction coefficient corresponding negatively to AOD is mainly influenced by advection with aloft aerosol layers. The increase in AOD accompanied by a decrease in s s is possible because of the stronger convection and turbulence processes in the daytime. While lifting aerosol particles to upper level (where the humidity is usually higher than lower level), hygroscopic process could increase aerosol sizes and consequently lead to an increase in AOD. On the other hand, convection and turbulence processes could also mix clear air from above ABL into the ABL and thus reduces the surface aerosol extinction coefficients, but the vertical mixing does not decrease AOD. [28] After 1600 LST, s s and AOD both appear to decrease with time. A possible explanation for the high correlation is the influence of coarse particles in the air mass, settling down to the ground to reduce both t and s s Surface Aerosol Extinction Coefficient [29] In order to establish the relationship between AOD and s s, we assume two models to describe aerosol vertical distributions as observed in Hong Kong (see Figure 7). The 8of13

9 Figure 7. Diagrams of aerosol vertical distribution: (a) model I (well-mixed aerosols confined below the ABL) and (b) model II (two layers of aerosols with well mixed in the ABL and exponentially decreased aerosol extinction with altitude above the ABL). model I (Figure 7a) considers aerosols are well mixed and confined in the ABL and the model II (Figure 7b) combines model I with an exponential decay function of aerosol extinction with altitude at the top of ABL. We describe the procedures mathematically as follows: [30] First, we assume a simple relationship between t and s s according to equation (1), s s ¼ t=h ABL where h ABL is the ABL height. For this assumption, aerosols are considered well mixed and confined in the ABL [Charlson et al., 1992; Intergovernmental Panel on Climate Change, 2001], which neglects contribution of aerosols from free troposphere and stratosphere. [31] In equation (2), we replace h ABL with h THLH s s ¼ t=h THLH ð1þ ð2þ where h THLH is the total haze layer height illustrated as h THLH ¼ h ABL þ h HLH where h HLH is the haze layer height (HLH): the scaling height of aerosol extinction decayed exponentially above the top of ABL. This relationship would be valid under the assumption that aerosol extinction is a constant within the ABL, and exponentially decayed with the height above the top of ABL with a scaling height h HLH as the haze layer depth. [32] The relationship between AOD (the integration of the aerosol extinction with height from MPL measurements) and surface aerosol extinction coefficient from the first valid bin of the aerosol extinction profile is illustrated in Figure 8a. One can see the large scattering of the data points and a poor correlation (r 2 = 0.42), which can be explained by 2 reasons: (1) A considerable amount of aerosols over the lidar site is attributed to advection rather than generated ð3þ Figure 8. Scatterplot and linear regression analysis of (a) t and s s derived from the lidar data, (b) measured and estimated s s from equation (1), and (c) measured and estimated s s from equation (2). Solid line represents the best fit of the two parameters given above, and dashed lines are shown as the 1:1 lines in Figures 8b and 8c. 9of13

10 Table 1. Monthly Statistics of Correlation Coefficient, ABL Height, Haze Layer Height, and AOD for the Period From 1 May 2003 to 30 June 2004 Month t Versus s s Correlation Coefficient Estimated s c s Versus s s Estimated s s d Versus s s ABL Height a (km) Average Standard Deviation Haze Layer Height b (km) Average Standard Deviation Average AOD Standard Deviation Number of Examples May Jun Jul Aug Sep Oct Nov Dec Feb Mar Apr May Jun Total a ABL height (equation (2)) derived from diurnal (daytime) averages of each month. b Haze layer height (equation (3)) derived from hourly averages of each month. c s s surface extinction coefficient estimated according to equation (1). d s s surface extinction coefficient estimated according to equation (2). from local emissions and (2) different ABL height development determines different aerosol transportation altitude and the proportion of aerosol load near the surface to the aerosol column. [33] Figure 8b presents the scatterplot of the surface s s from lidar extinction profile and estimated s s by equation (1). As constrained by the ABL height, a significant increase in correlation (r 2 = 0.65) is found between the measured and retrieved s s. The increase is attributed to the fact that the retrieval of s s accounts for the hour-to-hour variability of the ABL height. However, it also clearly shows the slope similar to that in Figure 8a. As a result, a large RMS error 0.39 is shown as attributed primarily to an underestimation of the aerosol column extinction within the haze layer (Figure 4) that substantial aerosol amount is above the ABL such as in spring and summer seasons. [34] Figure 8c presents the comparison of s s as in Figure 8b but obtained by using equation (2) instead of equation (1). For the case of retrieving surface extinction coefficient with a combination of the AOD and ABL height as well as haze layer depth, the correlation coefficient between the measured and retrieved surface extinction is increased to More importantly, the slope of the linear fit is also improved sharply to 0.82 with RMS error 0.10, which suggests that accounting for the aerosols above the ABL would reduce errors in estimating surface extinction coefficient. Table 1 shows the monthly statistics of correlation coefficient, ABL height, haze layer height, and AOD. The correlation coefficient between t and s s appears to be the highest in August 2003 (r 2 = 0.86) and lowest in February (r 2 = 0.01) The correlation coefficients of s s between measurements and estimates increase significantly for all months when the ABL heights are taken into account, especially for those months with low monthly AOD-s s correlation coefficients, such as in May (from r 2 = 0.07 to r 2 = 0.50) 2003, June (from r 2 = 0.28 to r 2 = 0.45) 2003 and February 2004 (from r 2 =0.01tor 2 = 0.48). The high correlations between the estimated s s derived by the ABL height constraint and measurements during the winter months are not surprising since the weak turbulent activities and strong temperature inversion can result in a slight contribution of the AOD above the top of the ABL. The correlation coefficients between measurements and estimates further increase with the consideration of the haze layer heights above the top of the ABL for almost all months, indicating that aerosol layers existing aloft are responsible for a significant fraction of the aerosol extinction. One exception shows that the decrease in the correlation coefficient with the consideration of the haze layer height in February 2004 can be interpreted by the complex vertical distribution of the haze layer induced by upper dust transportation observed frequently in spring, when the typical exponential decay of extinction above the top of the ABL may not agree with the real distribution. The various layers above the ABL height are often observed in the lidar backscatter profiles as seen in 29 February 2004 (Figure 2) show the complexities of the haze structure compared with those of other months of the study period. [35] The lidar vertical extinction profiles of 31 May at 0900 LST, 20 August at 1000 LST and 31 August 2003 at 1200 LST shown in Figure 9 are further studied for three typical atmospheric conditions. The symbol triangle on the x axis of each subplot in Figure 9 denotes s s estimated according to equation (2) when we know the true AOD value (here, the true values are integrated from the aerosol extinction coefficient profiles). Note that the vertical extinction profiles are averaged over a period of 60 min. For the case of 31 May 2003, the lidar captured a well mixed layer, in which aerosol-rich air mass is mixed up to 1600 m with a maximum extinction (0.52 km 1 ) near the top of ABL (1460 m). The estimated s s is in good agreement with measurement with a relative error <3%. It is interesting to note that for most of the observations aerosol extinction is low near the surface and then increases with altitude reaching a maximum below and approaching the top of the ABL, which is also defined as the aerosol mixing height [Baxter, 1991], above which, aerosol extinction decreases near exponentially with altitude. This kind of 10 of 13

11 Figure 9. Hourly mean aerosol extinction profiles (crossed) for 31 May at 0900 LST, 20 August at 1000 LST, and 31 August 2003 at 1200 LST in Hong Kong. Solid curve represent the integral (from ground) of extinction coefficients, i.e., AOD(z). Horizontal solid and dashed lines denote the ABL and haze layer height, respectively. Triangle symbol on the x axis denotes s s estimated according to equation (2). vertical distribution is attributed to (1) the presence of convectively driven surface-emitted aerosols, which tends to accumulate below the synoptic inversion layer; (2) advectively transported aerosols through long-range or regional transport mechanisms, which mixes (fumigation) as the growing boundary layer entrains the polluted air above. Such a layer has a significant and direct impact on the surface, bringing particles of different optical properties to the surface; and (3) increasing relative humidity (RH) with altitude. According to classic mixed layer theory, in mixed layer specific humidity is almost constant with altitude and temperature decreases linearly with altitude. Therefore, RH should be larger just below the top of the ABL, resulting in enhancing the aerosol scattering coefficient because of the hygroscopic growth effect, normally for RH greater than 70% [Hanel, 1976; Fitzgerald et al., 1982]. Kolev et al. [2000] compared the vertical distribution of aerosol extinction coefficient in a well mixed marine boundary layer with the extinction profiles determined following the analytical model proposed by Fitzgerald [1989]. Our results supported the assumption that in a well-developed mixing layer the change of the extinction coefficient with height is mainly due to the increase in the relative humidity. [36] For 20 August 2003, the extinction measured at ground level (0.11 km 1 ) is larger than the retrieved (0.07 km 1 ) given AOD 0.08 at 523 nm and an ABL height at 0.62 km. The accumulative extinction profile shows that the majority of the aerosol load (77%) is confined below the ABL. Above haze layer height 0.88 km very small aerosol extinctions are observed, which clearly indicates the free troposphere. In fact, aerosol extinction measured by lidar decrease monotonically with the increasing altitude below ABL, which is obviously different from the assumption of constant aerosol extinction within ABL. As a result, the retrieved surface extinction coefficient is much Table 2. Aerosol Parameters of Case Analysis Corresponding to the Results Shown in Figure 7 Day ABL Height (km) Haze Layer Height (km) AOD Measured s s (km 1 ) Estimated s s (km 1 ) Relative Error 31 May 2003, 0900 LST % 20 Aug 2003, 1000 LST % 31 Aug 2003, 1200 LST % 11 of 13

12 less than the measured one because of overestimated aerosol load in ABL with the proposed vertical distribution model II (equation (2)). [37] For 31 August 2003, a large extinction 0.21 was measured at noon (1200 LST) near the surface. During the 1-h averaging period, the ABL height is estimated at about 0.94 km. The distinct aerosol layer at 1.5 km with a maximum extinction coefficient of 0.09 km 1 can be attributed to the residual layer from the previous day [McKendry et al., 1997] or transported aerosol particles from remote areas. Lidar observations show up to 8% of the optical depth at 523 nm was contributed by the layer at above 1.5 km, which partially accounts for the large relative error (76%) in retrieving s s (Table 2). In addition, considerable variations of aerosol extinction are observed within ABL, suggesting the assumption of constant aerosol extinction with height within the ABL is unfit for retrieving s s with equation (2). 4. Conclusions [38] A micropulse lidar has been operated continuously in Hong Kong from 1 May 2003 to 30 June A total of 917 hourly range corrected lidar signals were used to derive aerosol extinction profiles. Three parameters (AOD, ABL height, and haze layer height) are obtained to characterize aerosol vertical distribution influenced by different synoptic systems in Hong Kong. The daily averaged AOD and ABL height that vary with season generally lie in the range of and km, respectively. Seasonal pattern of aerosol optical depth in the mixing layer and total aerosol optical depth are compared. It is found that on average 64% of the monthly mean aerosol optical depth is contributed by aerosols in the mixing layer with a maximum of 76% in November and a minimum of 55% in September. The lidar profiles revealed, for the first time, the large abundance of aerosols above ABL over Hong Kong. The characteristics of seasonal averaged aerosol profiles are analyzed and discussed. For the prevalent Asian dust outbreaks in spring, there exists an enhanced aerosol layer in free troposphere above ABL. In summer because of wet deposition and fresh oceanic southerly wind, lidar measured extinction profiles generally show the lowest extinction values at surface. In autumn, under stagnant atmospheric condition, the surface layer aerosols begin to accumulate, which reflect to the increase in extinction and in winter, due to the presence of temperature inversion, aerosol surface extinction reaches the maximum of the year. [39] The surface aerosol extinction coefficient, a parameter that can be directly related to environmental pollution and human health, is derived from lidar extinction profile. The significant differences in diurnal and monthly variations between the AOD and surface aerosol extinction coefficients, especially in the winter months, indicate the importance of understanding aerosol vertical distribution in order to better establish the relationship between these two parameters. The surface aerosol extinction coefficients estimated on the basis of AOD and ABL height from lidar backscatter profiles reveal somewhat better correlation (r 2 = 0.65) with measured ones than those with AOD only. An even higher correlation coefficient (r 2 = 0.78) with a slope 0.82 and intercept 0.03 between the measured and estimated surface extinction coefficients obtained using AOD, ABL height, and haze layer depth suggests that two-layer aerosol model could be very useful to provide continuous and accurate estimate of aerosol extinction coefficient near the surface for the urban pollution research. In the future, it is expected that by combining meteorological information with lidar measurements, more detailed analysis on aerosol distribution and evolution processes could be obtained. [40] Acknowledgments. The study is partially supported by the joint research grants from the National Natural Science Foundation of China (NSFC) and Research Grant Council (RGC) of Hong Kong (grant N_HKUST630/04) and grants from the NSFC (grants , , , and ), the National Basic Research Program (973 Program, grant G ), and the National High Technology Research and Development Program (863 Major Project, grant 2006AA06A303) of China. We would like to thank the three anonymous reviewers, whose useful comments have improved the paper. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY Web site ( used in this paper. References Ansmann, A., U. Wandinger, A. Wiedensohler, and U. 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