Diurnal Cycle of Air Pollution in the Kathmandu Valley, Nepal: 2

Diurnal cycle of air pollution in the Kathmandu Valley, Nepal: 2. Modeling results

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Citation Panday, Arnico K., Ronald G. Prinn, and Christoph Schär. “Diurnal Cycle of Air Pollution in the Kathmandu Valley, Nepal: 2. Modeling Results.” Journal of Geophysical Research 114, no. D21 (2009). As Published http://dx.doi.org/10.1029/2008jd009808 Publisher American Geophysical Union

Version Final published version Accessed Sun Dec 09 04:08:33 EST 2018 Citable Link http://hdl.handle.net/1721.1/85645 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. Detailed Terms JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D21308, doi:10.1029/2008JD009808, 2009 Click Here for Full Article

Diurnal cycle of air pollution in the Kathmandu Valley, Nepal: 2. Modeling results Arnico K. Panday,1,2,3 Ronald G. Prinn,4 and Christoph Scha¨r5 Received 10 January 2008; revised 27 May 2009; accepted 10 June 2009; published 6 November 2009.

[1] After completing a 9-month field experiment studying air pollution and meteorology in the Kathmandu Valley, Nepal, we set up the mesoscale meteorological model MM5 to simulate the Kathmandu Valley’s meteorology with a horizontal resolution of up to 1 km. After testing the model against available data, we used it to address specific questions to understand the factors that control the observed diurnal cycle of air pollution in this urban basin in the Himalayas. We studied the dynamics of the basin’s nocturnal cold air pool, its dissipation in the morning, and the subsequent growth and decay of the mixed layer over the valley. During mornings, we found behavior common to large basins, with upslope flows and basin-center subsidence removing the nocturnal cold air pool. During afternoons the circulation in the Kathmandu Valley exhibited patterns common to plateaus, with cooler denser air originating over lower regions west of Kathmandu arriving through mountain passes and spreading across the basin floor, thereby reducing the mixed layer depth. We also examined the pathways of pollutant ventilation out of the valley. The bulk of the pollution ventilation takes place during the afternoon, when strong westerly winds blow in through the western passes of the valley, and the pollutants are rapidly carried out through passes on the east and south sides of the valley. In the evening, pollutants first accumulate near the surface, but then are lifted slightly when katabatic flows converge underneath. The elevated polluted layers are mixed back down in the morning, contributing to the morning pollution peak. Later in the morning a fraction of the valley’s pollutants travels up the slopes of the valley rim mountains before the westerly winds begin. Citation: Panday, A. K., R. G. Prinn, and C. Scha¨r (2009), Diurnal cycle of air pollution in the Kathmandu Valley, Nepal: 2. Modeling results, J. Geophys. Res., 114, D21308, doi:10.1029/2008JD009808.

1. Introduction 2800 masl. There are five passes of 1500–1500 masl on the west and east sides, and one winding river outlet (the [2] In this paper we present results of theoretical inves- Bagmati River) to the south. The valley is inhabited by tigations using a numerical model aimed at addressing questions about processes that control air quality in the close to 3 million people. Its vehicle fleet exceeds Kathmandu Valley, Nepal. These questions arose from the 300,000 motorcycles and a hundred thousand other vehicles. analysis of data from a field experiment that we conducted In recent years its growing air pollution problem has caught in 2004–2005 [Panday and Prinn, 2009]. The Kathmandu increasing attention, especially during winters. Valley is a bowl shaped basin in central Nepal, halfway [3] In urban mountain basins, an important effect of the between the Ganges Plains and the Tibetan plateau. It has a surrounding topography is to reduce the exchange of valley flat floor area of 340 km2, and it is located 1300 m above air with the free troposphere and thereby to concentrate sea level (masl), surrounding by mountains reaching 2000– pollutants near the basin bottom. Ventilation of basins is often dominated by complex valley wind systems that are driven by differential heating and cooling of surrounding 1Program in Atmospheric and Oceanic Sciences, Princeton University, mountain slopes and characterized by a strong diurnal cycle. Princeton, New Jersey, USA. At night, basins often fill with pools of cold air. In addition 2 Formerly at Center for Global Change Science, Massachusetts Institute to local radiative cooling, basin bottoms experience an of Technology, Cambridge, Massachusetts, USA. 3Now at Department of Environmental Sciences, University of Virginia, additional cooling component due to the arrival of drainage Charlottesville, Virginia, USA. flows from colder surrounding mountain slopes [Whiteman 4Center for Global Change Science, Massachusetts Institute of et al., 2004a]. Such cold air pools have been studied Technology, Cambridge, Massachusetts, USA. 5 extensively in the Columbia Basin [Whiteman et al., Institute for Atmospheric and Climate Science, Swiss Federal Institute 2001; Zhong et al., 2001] Danube Basin [Za¨ngl, 2005], of Technology, Zurich, Switzerland. Virginia [Allwine and Lamb, 1992], Colorado [Whiteman, Copyright 2009 by the American Geophysical Union. 1982; Kelly, 1988; Neff and King, 1989; Whiteman et al., 0148-0227/09/2008JD009808$09.00 1999], Bulgaria [Savov et al., 2002], Slovenia [Rakovec et

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Figure 1. Kathmandu Valley, viewed from the south in Google Earth (printed with permission). The high Himalayas and Tibetan Plateau are visible at the top edge of the image. Kathmandu city is labeled in red. Yellow labels show important measurement sites. al., 2002] and Japan [Kondo et al., 1989]. Cold pools in the maximum and minimum values for each 10-min time small basins and sinkholes in the Rockies and in Austria interval, in order to show the values below which CO never have been chosen for the extensive field observations dropped, and the values that were never exceeded. In addition, designed to understand their processes [Clements et al., Figure 2 shows the median for each 10-min time interval as a 2003; Whiteman et al., 2004a, 2004b]. Often the top of a measure of where 50% of the data may be located in a cold air pool is marked by a strong temperature jump possibly not normal distribution. The diurnal cycle shows a [Whiteman et al., 2001]. Within and immediately above distinct peak in the morning, and one in the evening. In part 1 the cold pools, there is a strong temperature inversion, which [Panday and Prinn, 2009] we have shown that the two peaks suppresses buoyant vertical transport of pollutants. The are not merely driven by the emissions but strongly affected daytime growth of a mixed layer over a basin thus requires by the diurnal circulation and mixing in the Kathmandu the dissipation of the basin’s cold air pool and its associated Valley. In particular, CO bag sampling on small neighboring temperature inversion. peaks indicated the nighttime lifting of pollutant layers, while [4] In the Kathmandu Valley, we conducted a field exper- ozone measurements on a tall peak showed the isolation of iment between September 2004 and October 2005. A com- these sites from Kathmandu Valley air at night. In the current panion paper [Panday and Prinn, 2009], and a dissertation paper (part 2) we will use a high-resolution numerical model [Panday, 2006], describe the results in detail. Here we to clarify the underlying dynamical mechanisms. summarize some salient aspects of the observational cam- [6] To set the scene for the simulations, Figures 3 and 4 paign and formulate the objectives of the current paper. The show meteorological observations on six representative, observational set up included a laboratory in Bouddha, east of consecutive days in February 2005. Most days were sunny, Kathmandu city, where we continuously measured carbon with low wind speeds at night, and strong westerly winds monoxide (CO), ozone, fine particulates (PM10) and weather during the afternoons. Figures 4a and 4b indicate that variables. In addition, during part of the time we had a mini saturation occurred on most mornings, and that on several sodar operating nearby, and we had four temperature loggers afternoons the dew point dropped, indicating the arrival of a on a TV tower, as well as additional temperature loggers on a different air mass. Figure 4c shows the temperature recorded valley rim peak, at the base of the valley rim, and on a small on Nagarkot peak (1972 masl), and at Kharipati (1373 masl), peak overlooking the lab site. These sites are shown on at the base of Nagarkot. While Kharipati was warmer than Figure 1. Nagarkot during the daytimes, it was colder during most [5] One of the key results discussed in the companion nights, indicating a temperature inversion. Figure 4d com- paper is the observation of two daily CO peaks in the pares the temperature at Kharipati with a valley-center Kathmandu Valley. Figure 2 shows the diurnal cycle of location. We see that in the evening, the temperature carbon monoxide measurements at the laboratory in Bouddha dropped more rapidly in Kharipati. This is consistent with in October 2004 and January 2005. We have chosen to show the formation of a nocturnal cold air pool in the valley

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Figure 2. Diurnal cycle of CO at the main laboratory in Bouddha during the months of (a) October 2004 and (b) January 2005, averaged over 10-min intervals. The centerline represents the monthly median value for each 10-min interval. Vertical bars show the highest and lowest 10-min averages recorded that month for each time interval. The x axis is in Nepal Standard Time, which is UTC + 5 h 45 min. through the arrival of katabatic winds from the valley rim mixed layer between the valley bottom and the altitude of slopes. Figure 5a shows the lapse rate computed from the Nagarkot. Figures 5b and 5c show the lapse rate calculated temperature difference between Nagarkot and Kharipati. We using temperatures at different levels on the TV tower. We see stable conditions at night, and a midday lapse rate near see stable conditions down to the lowest levels at night the 9.8 K/km dry adiabatic lapse rate, indicating a well- (consistent with a cold air pool in the valley), and strong

Figure 3. (a) Solar radiation, (b) wind speed, and (c) wind direction measured by the weather station on the Hyatt roof in Bouddha during a typical 6-day period in spring 2005. The period considered contains a weekend (26–27 February).

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Figure 4. (a) Relative humidity at the Hyatt roof in Bouddha during the same 6 days as in Figure 2, (b) temperature (black) and dew point (green) on the Hyatt roof during the same time, (c) temperature on Nagarkot Peak (red) and Kharipati at the base of Nagarkot (blue), and (d) temperature at Kharipati (blue) and at 5 m on the TV tower (black).

Figure 5. Lapse rates (K/km) estimate between different measurement sites: (a) between Kharipati and Nagarkot, (b) between the bottom two loggers on the TV tower, and (c) between the 5-m and the 48-m logger on the TV tower.

4of20 D21308 PANDAY ET AL.: KATHMANDU VALLEY MODELING RESULTS D21308 thermals at midday that shrink with height. Sodar data were by 75 grid squares. The outermost domain spanned much of not available in February, but sodar-derived mixed layer the Ganges Basin and a big part of the Tibetan Plateau, heights in May showed that the mixed layer peaked at while the innermost domain spanned approximately three midday at a depth of 400 to 800 m (see part 1). These values times the horizontal extent of the Kathmandu Valley, reach- are surprisingly low. It is possible that the sodar’s range ing into neighboring valleys. prevented us from seeing the full height of the mixed layer. [10] Land use and vegetation for the simulations were However, the fact that sodar data showed a shrinking mixed taken from the public USGS 25-category 30 arc-second layer height over the course of the afternoon suggests that database downloadable via the MM5 homepage (ftp://ftp. perhaps other processes were at play. ucar.edu/mesouser/MM5V3/TERRAIN_DATA/), while un- [7] The observational results are discussed in more detail derlying topography was taken from the USGS GTOPO-30 in part 1 [Panday and Prinn, 2009]. It explained the (global 30 arc-second) database and smoothed appropriately observed daily twin peak pattern of CO in terms of diurnal for each nest level. At the 1-km grid resolution of the variations in emissions and ventilation. The morning peak innermost domain, the model topography was able to capture decreased at the same time that the nocturnal cold air pool essential features of the Kathmandu Valley, including the had dissipated, and strong westerly winds removed pollu- surrounding mountains, passes, and relatively flat basin tants that had accumulated in the city over the course of the bottom. It did not capture small-scale features within the morning. The evening peak began when evening rush hour valley bottom, such as Chobhar hill, Chobhar Gorge, took place after rapid ventilation ceased. Parts of the Swayambhu hill, the Pullahari-Kapan ridge, Changu Narayan evening emissions were found to be lifted and recirculated hill or any of the river channels. MM5 is not designed for use the next morning, adding to the morning pollution peak. Our at resolutions less than 1 km without modifications to the observations left some important questions unanswered, code [Gohm et al., 2004]. however. They did not give us a complete picture of the [11] Our model runs were initialized with NCEP analyses dynamics of the nocturnal cold air pool and the daytime at 0600 UTC, corresponding to 1145 Nepal Standard Time. mixed layer, nor did they indicate the pathways of pollutant Initialization at midday rather than midnight was chosen ventilation out of the valley. In this paper we use numerical because mountain valleys often contain stably stratified air modeling to address these questions, using Pennsylvania masses at night, which are disconnected from overlying State University (PSU)/National Center for Atmospheric regional flows and poorly represented in the synoptic-scale Research (NCAR) mesoscale meteorological model (also analysis that initialize and drive the model. The model runs known as MM5). We will use observations to validate the were allowed to spin up for 36 h before the results were numerical model results, and then turn attention to two used. Experimental runs with various vertical sigma coor- specific scientific issues. These concern the dissipation of dinate configurations led to a final choice of 40 vertical the nocturnal cold-air pool and its replacement by a mixed layers extending from the surface to 100 mbar. The five layer in the Kathmandu valley, and the analysis of air-parcel bottommost layers were each 15 m thick (corresponding to trajectories to elucidate the fate of polluted valley air. 0.006s), while above them the layer thickness was gradu- [8] Section 2 describes the model set up. Section 3 ally increased. At the top of the model domain, boundary compares model results with observations. Section 4 dis- conditions were chosen to allow radiation of internal gravity cusses possible mechanisms for the dissipation of nocturnal waves [Klemp and Durran, 1983]. Simulation time steps cold air pools and mixed layer growth in mountain basins, were 60 s in the outermost domain with 27 km resolution, and shows modeling results for Kathmandu. Section 5 necessitating a 1 s time step in the innermost domain. The examines pathways of pollutant ventilation out of the valley lateral boundaries of the outermost domain used linear during different times of day. The study is concluded in interpolation between the 6-hourly time steps of the NCEP section 6. analysis, with a relaxation extending four grid cells into the domain. 2. Numerical Modeling [12] Cloud microphysics was parameterized using the SimpleIceScheme[Dudhia, 1989]. This uses explicit [9] We set up version 3.6.1 of the mesoscale meteoro- microphysics calculations to simulate cloud and rainwater logical model MM5 for use around the Kathmandu Valley. as well as cloud ice. MM5 was allowed to calculate cumulus Initial and boundary conditions were taken from global final clouds explicitly in the inner two domains (3 km and 1 km analyses (FNL), prepared every 6 h at 1° resolution by the resolution), while in the coarser resolution outer domains National Center for Environmental Prediction (NCEP), and cumulus clouds were parameterized using the updated Kain- available online through the National Center for Atmospheric Fritsch scheme [Kain, 2004]. The boundary layer was Research (NCAR) Data Support Section webpage (http:// parameterized using the MRF scheme [Hong and Pan, dss.ucar.edu/datasets/ds083.2/). The modeling domain was 1996], which is suitable for fine resolution runs while set up using a Lambert conformal projection, with four maintaining computational efficiency. Solar and terrestrial levels of two-way nesting, stepping down in resolution from radiation were calculated every 30 min, using a parameter- one degree NCEP FNL data at the side boundaries while ization that included long- and short-wave radiation inter- maintaining a 1:3 grid resolution ratio. The four nested acting with clear air, clouds, precipitation and the ground domains had horizontal grid resolutions of 27, 9, 3, and 1 km [Dudhia, 1989]. For the surface energy budget, a five-layer and were all centered at 27.7°N, 85.3°E, corresponding to a soil model was used [Dudhia, 1993], with soil moisture point in central Kathmandu City, close to the Dharahara initialized by the driving analysis. tower (Figure 6). The outer two domains had 50 by 50 [13] We carried out simulations for February and May horizontal grid squares, while the inner two domains had 75 2005, and used the MM5-customized interpolation and

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Figure 6. The four nested domains used in our MM5 simulations. Domain 4 (labeled) is centered on the Kathmandu Valley. plotting package RIP4 (ftp://ftp.ucar.edu/mesouser/MM5V3/ Such an error could derive from too high a soil moisture RIP4.TAR.gz) to generate plots and to calculate forward and content in the initial conditions. In addition, part of the bias backward trajectories. The trajectories were calculated at seen here might have explanations other than model error. 1-min intervals using horizontal and vertical vectors output First, it is plausible that during the daytime the temperature by MM5 every half hour. at the rooftop weather station was slightly contaminated by local infrared radiation from the tile roof underneath it, 3. Model Validation while the MM5 results represent an average over a grid cell of 1 km by 1 km. Second, the weather station was located [14] Next we look at MM5’s ability to reproduce the 22 m above ground, while MM5’s bottom layer center was Kathmandu Valley’s meteorology. To this end we use a 7.5 m above ground. Fog tends to develop from the ground simulation initialized on 8 February 2005, 0600 UTC (1145 up, so it is indeed possible that there was fog near the ground NST), and analyze a representative 2-day period. Figure 7 earlier in the morning than the time when 100% relative shows that MM5 accurately captured the nighttime temper- humidity was reached at the weather station. ature, but underestimated the daytime temperature at our [15] Figures 8a and 8b show that MM5 captured the Bouddha station. It also shows that MM5 accurately cap- diurnal temperature cycle at the TV tower and at Nagarkot tured the daytime relative humidity minimum, but that it Peak remarkably well. At Kharipati MM5 slightly under- overestimated relative humidity for most of the simulation, estimated the temperature during the day and overestimated including an overestimate of the duration of saturation. The it at night. Part of that may be due to topographic smoothing underestimate of the diurnal temperature cycle, in combi- by MM5, which lifted the location of Kharipati to a higher nation with the overestimate of the diurnal humidity cycle, elevation. Figure 9 shows the temperature in the lowest suggests that this bias is at least partly due to an incorrect model layer (between s = 1 and s = 0.994) over the representation of the Bowen ratio (overestimated evapotrans- Kathmandu Valley at dawn (Figure 9a) and in the afternoon piration at the expense of underestimated sensible heat flux). (Figure 9b). At dawn the entire basin bottom was filled with

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Figure 7. Intercomparison of measurements by the weather station on the Hyatt roof (blue) on 9 and 10 February 2005 with MM5 simulations for the same time period at the grid cell containing the Hyatt roof and main laboratory (red). (a) Air temperature and (b) relative humidity. a pool of cold air whose temperature was similar to that to the Kathmandu Valley’s confined basin shape, these river of the highest peaks on the valley rim. We also see that valleys allows drainage flows to travel unimpeded. In the surrounding river valleys (for example, in the upper left afternoon the Kathmandu Valley’s center was warmer than quadrant of the plots) did not contain cold pools. In contrast its edges, and the cold pool was gone.

Figure 8. Intercomparison of air temperature measurements by Hobo loggers (blue) at three locations on 9 and 10 February 2005 with MM5 simulations (red) for the same time period at grid cells containing the logger sites. (a) TV tower bottom (5 m above ground), (b) Nagarkot, and (c) Kharipati.

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Figure 9. (a) Air temperature in the lowest MM5 layer (color shading) at 0545 NST on 10 February 2005. The underlying topography is indicated by contour lines with a contour interval of 100 m. (b) Same as in Figure 9a but for air temperature in the lowest MM5 layer at 1445 on 10 February 2005. The black line indicates the location of the cross section in Figures 12, 14, 15, 16, and 17.

[16] Figure 10 shows the relative humidity in the surface tuations in the locations of the area reaching the 100% layer at dawn and shortly before noon on 10 February 2005 threshold, indicating that many of the areas shown in dark and compares it to photographs from Hattiban taken at the blue actually had relative humidity close to 100% as well. same times. At dawn a large part of the valley center had Indeed at dawn that morning we photographed a thin fog 100% relative humidity, while the rest of the valley floor layer covering much of the valley. Time-lapse photography had relative humidity exceeding 90%. In fact, during the showed that over the course of the morning the fog layer early morning hours the simulation showed frequent fluc- thickened, and then disappeared from the surface by late

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Figure 10. (left) Photographs of the valley from Hattiban Ridge and (right) MM5 output of relative humidity in the bottommost layer on 10 February 2005. The color scale goes from white to blue to indicate increasing relative humidity; red indicates where the threshold value of 100% is achieved. The glow in the early morning photograph results from a thin sheet of fog. morning. Consistent with this, the MM5 simulation shows Figure 3, observations of the lapse rate between Kharipati that by 1145 NST the relative humidity near the surface was and Nagarkot in Figure 5, and observations of mixed layer below 40% in the entire valley. height by the sodar shown in part 1. [17] Figure 11 shows wind vectors computed by MM5 in the lowest model layer before dawn and in the afternoon of 4. Dynamics of the Nocturnal Cold Air Pool and 10 February 2005. The modeled patterns fit with what we the Daytime Mixed Layer observed. At dawn the valley center had almost zero surface winds. At the edges of the valley, katabatic winds descended [19] Having verified MM5’s ability to reproduce observed from mountain ridges, while drainage flows traveled down meteorology in the Kathmandu Valley, we now proceed to both the Bagmati river valley (coordinates 20 km NS, 32 km address the first of our two questions: What mechanisms are EW) and the river valleys outside of the Kathmandu Valley. responsible for the dissipation of the nocturnal cold air pool We also see easterly winds crossing the passes on the western in the Kathmandu Valley, and how does the mixed layer edge of the Kathmandu Valley, such as at Bhimdhunga Pass behave during the day? We discuss four possible mecha- (42 km NS, 31 km EW). During the afternoon, strong winds nisms for cold-pool dissipation before examining model traveled up the river valleys, while the entire Kathmandu results. Valley bottom was swept by westerly/northwesterly winds [20] The first mechanism is often observed over flat entering through the western passes. Afternoon wind speeds plains, where inversion breakups are driven by surface on the order of 5–6 m/s over the valley were close to the heating [Stull, 1988; Young et al., 2000; Angevine et al., average afternoon wind speeds measured at the Bouddha 2001; Garcia et al., 2002]. A convective mixed layer grows station. We also see that the up-valley winds from the upward as the surface warms up after sunrise. Thermals Bagmati River valley were able to enter the southern parts generated by the heated bottom rise to increasingly greater of the valley, but were pushed eastward by the stronger heights, chipping away at the inversion above. As the mixed westerlies entering from the western passes. layer grows, it entrains air that was previously isolated from [18] Figure 12 shows west–east cross sections through the surface by the inversion. Entrainment of ozone-rich air the innermost domain of the MM5 simulation, with con- can cause a rapid rise in ozone observed at the surface in the tours of potential temperature as well as wind vectors. At morning. The mixed layer above a flat plain often grows night there was a strongly stratified cold air pool in the until the late afternoon, breaking through the uppermost valley (indeed its top corresponded to the height of the remnants of the inversion, and then entraining air from the valley’s passes). By 1145 NST the cold air pool was gone, less stable residual layer that had spent the night above the and the mixed layer extended up beyond the height of the inversion. The mixed layer has been observed to grow faster valley rim peaks while westerly winds travel through the over cities owing to enhanced convection driven by urban valley. This is consistent with our observation of winds in heat islands [Kallistratova and Coulter, 2004] and to be

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Figure 11. Surface wind vectors (yellow) from MM5 superimposed upon gray scale shaded topography (meters above sea level) at (a) 0545 NST and (b) 1615 NST on 10 February 2005. The Kathmandu Valley is the flat area at the center of the frame. For visual clarity, wind vectors have only been plotted for every second grid cell. Their length is proportional to the wind speed; a vector length of two grid cells corresponds to a speed of 10 m/s. affected by reduced soil moisture content [Santanello et al., riences very little shading by surrounding mountains, making 2005]. The Kathmandu Valley has a ratio of vertical relief this mechanism plausible. (altitude difference from the bottom to the rim) to horizontal [21] The second mechanism usually dominates in narrow expanse on the order of 1:30. The basin bottom thus expe- mountain valleys. Higher slopes receive morning sunlight

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Figure 12. West to east vertical cross-section A through the Kathmandu Valley on 10 February 2005, showing the Trisuli River (TR) low tributary valleys west of Kathmandu (WV), the western valley rim peaks (Nagarjun, NJ), the elevated flat basin of the Kathmandu Valley (KV), the eastern valley rim peaks (Nagarkot, NK), and the lower valleys to the east. Color shading and contours in the atmosphere indicate potential temperature; vectors are proportional to wind velocity in the plane of the plot. Vectors displayed in the bottom right of each plot correspond to maximum wind velocity (horizontal, vertical) in each plot: 20 m/s, 23.4dPa/s at 0245 NST; 15.2 m/s, 22.4 dPa/s at 0945 NST, 15.6 m/s, 29.1 dPa/s at 1045 NST, and 16.8 m/s, 37 dPa/s at 1145 NST. first. As the slopes warm up, upslope flows are generated upslope flows as in the second mechanism. But, rather than which carry air up from the valley bottom. The air departing generating a replacement flow up the river valley, the air from the valley bottom is replenished by the generation of mass leaving the basin bottom is replenished by subsidence a wind blowing up the river valley. In many places strong overhead. Subsidence heating over the basin center provides winds are found blowing up river valleys during the additional erosion of the inversion from the top. This daytime, while a shallow layer of air blows up sidewalls mechanism has been observed in basins ranging in size perpendicular to the valley axes [Atkinson, 1981; Whiteman, from 1 km [Whiteman et al., 2004b] to several hundred 1990; Pre´voˆt, 1994; Li and Atkinson, 1999; Pre´voˆt et al., kilometers across [Banta and Cotton, 1981; Kondo et al., 2000; Whiteman, 2000]. Up-valley winds driven by the 1989; Whiteman et al., 1999; Zhong et al., 2001]. If a basin warming of the Mustang Basin have been studied in detail is wide enough, then the upslope flows and inversion in the Kali Gandaki Valley in northern Nepal [Egger et al., erosion might begin first on the east-facing west side of 2000, 2002]. the basin [Kelly, 1988]. There have also been observations [22] A third mechanism is found in basins and wide of a hybrid mechanism that combines aspects of the first and valleys [Whiteman, 1982]. Here warming sidewalls generate the third, whereby the breakdown of the cold air pool

11 of 20 D21308 PANDAY ET AL.: KATHMANDU VALLEY MODELING RESULTS D21308 happens through a combination of thermals rising from a [25] By 1145 NST, there was a uniform mixed layer over warming surface, as well as subsidence descent over the the valley (the valley-center stratification was gone), but cold air pool to compensate for the fraction of the air mass there continued to be higher potential temperatures over the traveling up the slopes [Whiteman, 1982]. In complex peaks, driving upslope flows, which allowed air parcels valleys (and probably also valleys of intermediate size), a reaching the peaks to mix to higher elevations than ones combination of the second and third mechanism are also rising straight up from the basin floor. possible [Rotach and Zardi, 2007]. [26] We can conclude that in February in the Kathmandu [23] In the fourth mechanism the cold pool is dissipated Valley, the cold pool dissipation is more strongly influenced by turbulent erosion caused by vertical wind shear at the top by the third mechanism described above than by the second of the basin, leading to shear-generated turbulence and the mechanism; it appears that during the time studied, winds penetration of the upper-level wind downward through the up the Bagmati Valley played a smaller role in basin air cold air pool [Whiteman and Doran, 1993]. This mecha- replenishment during morning upslope flows compared to nism has been studied in Ljubljana, Slovenia, using MM5 the role played by subsidence over the basin center. mesoscale model simulations [Rakovec et al., 2002]. The [27] Further evidence for upslope flow driven mecha- study found that wind speeds aloft must exceed a certain nisms was found during time-lapse photography from threshold to begin eroding the cold air pool, and must Hattiban ridge (Figure 13). We found that the fog along increase over time to be able to completely remove the the edges of the basin dissipated rapidly, hours before the pool. In some basins, deep persistent temperature inversions fog over the center of the basin dissipated. At the time that can be removed rapidly when cold air advects overhead, the fog along the edges was dissipating, strong upslope reducing the inversion strength [Whiteman et al., 2001; winds were felt at the camera location. Za¨ngl, 2005]. The authors of a field study in the South [28] For comparison to our winter findings, Figure 14 Park Basin in Colorado [Banta and Cotton, 1981] describe shows a cross section at 0945 NST on 11 May 2005. We three wind regimes in the basin: downslope, upslope (both again see evidence of stronger vertical mixing over the of which take place in narrow valleys as well), and peaks in comparison to the basin center as well as upslope afternoon winds in the same the direction as the wind above flows; these were found, that day, to initiate the mixed layer the basin (something usually not found in narrow valleys). In growth around the basin perimeter as early as 0700 NST, the Kathmandu Valley, during the dry season, the prevailing with a deep mixed layer already existing by 0945 NST. In upper-level winds are westerlies. During afternoons the addition, though, we see in Figure 14 that the land surface surface winds within the valley are westerlies as well. over the basin bottom was heating as well, with an [24] Figure 12 shows the time evolution of potential unstable near-surface region of higher potential tempera- temperature and wind vectors over the Kathmandu Valley ture than in the air above. However, this was only seen on a west-to-east cross section during the morning of 10 briefly; by 1015 NST the air over the entire valley was February 2005. We see evidence for the dominance of the well mixed. The MM5 simulations suggest that in May a third mechanism described above in the episodes that we hybrid of the first and the third mechanisms controls the modeled: namely, that the inversion breaks up through the growth of the mixed layer. Shortly after dawn, upslope combined effect of upslope flows along the valley edge, and flows begin, at least on sunlit slopes, eroding the nocturnal subsidence over the valley center. The potential temperature stable stratification as described in the third mechanism. contours in Figure 12 show that on the morning of 10 However, as the Sun rises higher, the basin bottom gen- February 2005, a mixed layer characterized by constant erates thermals that let the mixed layer grow from below potential temperatures first grew at the valley edge, while (first mechanism). A likely reason that this mechanism was the valley center remained stably stratified. A zoomed-in seen in the May simulation but not in the February simu- view of the high-resolution model output confirms that the lation is the fact that in February the basin bottom was winds near the surface were indeed blowing upslope. The covered by fog, which reduced the surface heating. A delay downward dip in potential contours over the valley center in the breakup of a valley inversion in winter was also found seen at 0945 and 1045 NST suggests that wind vectors over in a lidar study in Bulgaria [Savov et al., 2002]. Note that the valley center had a downward component. However, to the surface thermals seen in the TV tower data in February confirm that subsidence took place, we carry out a back-of- (Figure 5) only appear after the morning fog has dissipated. the-envelope calculation. If there were no subsidence, then [29] In the episodes modeled, no evidence was found in the air leaving the valley would need to be balanced support of the fourth mechanism (turbulent erosion by by inflows through the Bagmati Valley. Assuming a valley winds flowing across the top of the cold air pool) playing circumference of 90 km, an average upslope flow of 3 m/s, a dominant role in the removal of the Kathmandu Valley’s and an upslope flow depth of 200 m, we get an outflow of nocturnal inversion. The mechanism would have required 5.4 Â 107 m3/s. For that to be balanced by inflows through us to find the cold pool remaining intact in the basin bottom, the roughly 2 km2 cross-sectional area of the Bagmati while its top was slowly eroded away by winds. Valley, we would need wind speeds of 27 m/s or 97 km/h! [30] The dissipation of the nocturnal cold air pool is not Such winds speeds were not found in our simulations, nor the only factor that affects the growth of the Kathmandu did local residents of the Bagmati Valley ever mention such Valley’s daytime mixed layer. If a basin sits at a higher high wind speeds. A more realistic upper bound on the elevation compared to regions outside the surrounding inflow through the Bagmati Valley of 10 m/s would only mountains, it can generate thermal circulations similar to provide 2 Â 107 m3/s, leaving 3.4 Â 107 m3/s for subsi- what is found on raised plateaus. Plateaus often serve as dence. Over a valley-bottom area of 340 km3 that would elevated heat sources [Egger, 1987; Molnar and Emanuel, give a subsidence rate of 0.1 m/s or about 360 m/h. 1999]. The daytime heating expansion of air columns above

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Figure 13. Kathmandu Valley from Hattiban on the morning of 8 December 2005 at (a) 0752 NST and (b) 0830 NST. a plateau and above nearby lowlands results in higher as is the case in the Kathmandu Valley, these passes are pressure and lower temperature above the lowlands than found to play crucial roles in the circulation. Detailed field above the plateau [Gaertner and Fernande´s, 1993]. Plateau and modeling studies of the gap flows taking places at the circulations have been studied in theory by several researchers, edges of plateaus and elevated basins have been carried out including the effects of valleys leading to the plateau [Egger, for the Tibetan Plateau [Egger et al., 2000; Za¨ngl et al., 1987; Za¨ngl et al., 2001], and the effect of plateau height 2001; Egger et al., 2002], the Bolivian Altiplano [Egger et and the existence of a mountain rim around the plateau al., 2005; Za¨ngl and Egger, 2005; Reuder and Egger, 2006], [Za¨ngl and Gonzales Chico, 2006]. If there are mountain and the Mexico City Basin [Doran and Zhong, 2000; de Foy passes connecting a basin to the low-lying outside regions, et al., 2006].

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Figure 14. Potential temperature and wind vectors on the same cross section as Figure 17, but at 0945 NST on 11 May 2005. Vectors displayed in the bottom right of the plot correspond to maximum wind velocity: 14.2 m/s horizontal, 99.3 dPa/s vertical.

[31] Figure 15 shows a continuation of the time series through the Kathmandu Valley bottom during the afternoon of MM5 cross sections from Figure 12. We see the arrival originated at a higher elevation than the valley. into the Kathmandu Valley through the western passes of lower potential temperature air originating above the lower 5. Ventilation Pathways neighboring valleys to the west. Starting from the valley’s western rim, this air spreads across the entire bottom of the [33] The timing and evolution of the mixed layer in the Kathmandu Valley in less than an hour. This creates a new Kathmandu Valley has an effect on when and how pollu- stratification that confines the mixing to much lower heights tants can be ventilated from the valley. Our second question than was possible moments earlier. We thus have an thus concerns the pathways of the pollutants when they explanation of sodar observations (see part 1), which show leave the valley. Before looking at model simulations, let us that the depth of the mixed layer decreased over the course consider some hypotheses of possible pollutant ventilation of the afternoon. pathways. [32] A modeling study of diurnal flows through passes [34] The first hypothesis is that the pollutants are venti- leading from the Pacific lowlands to the Bolivian Altiplano lated from the Kathmandu Valley because the daytime also found a density current of cooler air spreading from the mixed layer rises above the height of the surrounding pass across the Altiplano as a cold front [Za¨ngl and Egger, mountains, to altitudes where polluted air can flow over 2005]. The winds arriving into the Kathmandu Valley from the rim mountains. Such behavior has been observed and the west during the afternoon appear to be part of a larger modeled in Mexico City [Bossert, 1997; Fast and Zhong, plateau circulation. Earlier researchers calculated a temper- 1998]. During the TRACT experiment over the Black Forest ature difference of 2.5 K at noon time between air at the in southern Germany, it was found that the altitude of the valley bottom, and the cooler air at the same elevation top of the mixed layer is higher over mountains than over above lowlands 60 km to the southwest of Kathmandu surrounding flat land, and that the mixed layer top rises and [Regmi et al., 2003]. The Iberian Peninsula also experiences falls with the underlying topography [Kalthoff et al., 1998; a plateau circulation, with an afternoon heat low, and here Kossmann et al., 1998]. In other words, the top of the fully too a front of cooler air travels in from the plateau edge, formed mixed layer may be flat over the Kathmandu Valley pushing underneath the existing boundary layer [Gaertner center, and then rise as one approaches the valley rim and Fernande´s, 1993]. Similar gravity driven flows have mountains, reaching levels higher than the mountain tops. been found in idealized simulations [de Wekker et al., 1998] The sodar data indicated a mixed layer height approaching and in the lee of the Sierra Nevada [Zhong et al., 2008]. the height of the surrounding mountains even at the valley Researchers of the plateau circulation at Zugspitzplatt, center, while the temperature logger data indicated well Germany [Gantner et al., 2003], found air arriving on the mixed conditions at least up to the height of Nagarkot near plateau from higher elevations, marked by a drop in the dew the valley’s edge. Once mixed to heights above the sur- point. Indeed, our data showed an afternoon drop in dew rounding mountains, pollutants can easily be carried east- point in Kathmandu (Figure 4b). Following the isentropes ward by the strong westerly winds that blow parallel to the from left to right on Figure 15, we see that the air sweeping Himalayas during the dry season [Moore, 2004].

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Figure 15. Continuation of Figure 12, showing potential temperature and wind vectors during the afternoon of 10 February 2005. Vectors displayed in the bottom right of each plot correspond to maximum wind velocity (horizontal, vertical) in each plot: 17.7 m/s, 71.0 dPa/s at 1515 NST; 16.8 m/s, 56. dPa/s at 1545 NST; 15.7 m/s, 73.0 dPa/s at 1615 NST; 14.5 m/s, 90.5 dPa/s at 1645 NST.

[35] The second hypothesis is that the pollutants are mountain ridge into a basin have been studied before [Kimura removed from the valley by flows through mountain passes, and Kuwagata, 1993], as have flows through passes and with relatively clean air blowing in through the western mountain gaps in Athens [Asimakopoulos et al., 1992], valley rim passes and polluted air spilling out through the Mexico City [de Foy et al., 2006] and the Brenner Pass in eastern passes. A field study in the South Park basin of the Austrian Alps [Mayr et al., 2002]. In winter, Mexico Colorado [Banta and Cotton, 1981], found that an inversion City regularly experiences a low-level jet exceeding 10 m/s in the morning prevented upper-level westerly winds from entering the basin through a pass to the southeast [Doran penetrating to the surface. However, once the inversion was and Zhong, 2000]. Tracer transport modeling has shown gone the westerlies could reach down to the surface. An that the flow through this pass plays an important role in idealized modeling study of flows through valleys that flushing pollutants out of Mexico City [Bossert, 1997]. A terminated at the edge of a plateau [Egger, 1987] (similar 2003 field campaign studying the diurnal variations in to the valleys to the west of Kathmandu Valley that terminate circulation over the Bolivian Altiplano (an elevated plateau at its western passes) shows that the existence of such valleys surrounded by mountains) carried out vertical soundings at enhances the wind intensity above the plateau. Egger [1987] the in-flow passes that found inflow to start a few hours also shows that the up-valley wind intensity in these valleys after sunrise, at the time when the stable nighttime layer over is determined by the difference in elevation of the plateau the Altiplano had heated sufficiently to become neutrally heat source (1300 m in the Kathmandu Valley’s case), and buoyant [Egger et al., 2005]. the lowland heat source (500–600 m in the valleys west [36] The third hypothesis is that pollutants are vented out of Kathmandu). Tracers passing from a lowland area over a of the Kathmandu Valley by upslope flows at the valley rim

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Figure 16. Lagrangian trajectories released at 0745 NSTon 10 February 2005 on a line between 38 km EW 26 km NS and 38 km EW, 46 km NS, (top) shown on a contour map (masl) and (bottom) projected onto a west to east cross section. Green denotes times from 0745 to 1145 NST, red is from 1145 to 1545 NST, orange is from 1545 to 1945 NST, and brown is from 1945 to 2345 NST. mountains, which transport the pollutants into more rapid have been found to transport tracers out of Freiburg [Kalthoff flows aloft. Thermally driven upslope and up-valley winds et al., 2000]. Polluted air has been observed traveling up have been studied in many places and have been found to the Yahagi River valley in Japan [Kitada et al., 1986]. A be efficient ways of transporting air pollutants from lower modeling study of Santiago de Chile found pollutant export altitudes and venting them into fast-flowing upper-level above mountain peaks through the convergence of upslope winds. At the Jungfraujoch mountain observatory in Swit- flows [Schmitz, 2005]. Mountains near Phoenix, Arizona, zerland, pollutants are seen arriving from the valleys during provide sufficient ventilation to the city that there is no warm summer afternoons [Baltensperger et al., 1997]. multiple-day buildup of ozone from local emission sources Winds blowing up the valleys of the Black Forest mountains [Fast et al., 2000]. Up-valley flows in the Ticino area of

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Figure 17. Same as Figure 16, but for a release time of 1545 NST on 10 February 2005. southern Switzerland have been found to carry pollutants innermost domain (26 km NS, 38 km EW, to 46 km NS, upward from industrial northern Italy to the crest of the Alps 38 km EW). For visual clarity, release points are only shown [Pre´voˆt et al., 2000]. These flows can transport several times at 2-km intervals; these are a small selection of hundreds of the valley volume over the course of a day [Henne et al., trajectories that were examined to draw conclusions. 2004]. [38] Figure 16 shows trajectories with a release time of [37] We examined ventilation pathways from the 0745 NST. We see that, over the course of the morning, Kathmandu Valley using the results of Lagrangian model most of the trajectories traveled toward the nearest valley trajectories for 10 February 2005. The trajectories in rim slope, before traveling up the slopes to a height of 500– Figures 16–18 were released in the surface layer, along 1000 m above the basin bottom. Then, before 1145 NST a 20-km north-south line running through the center of the they started to travel eastward, crossing the Kathmandu

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passes during the night, consistent with the strong easterly winds observed at Bhimdhunga Pass during the night [Panday and Prinn, 2009]. It is likely that the drainage winds in the neighboring valleys were also responsible for driving the light easterly winds that were found along the top of the Kathmandu Valley’s cold pool at night and that also were observed in the nighttime sodar soundings.

6. Conclusion

[41] In our field observations of air pollution in the Kathmandu Valley during the dry season of 2004–2005, we noted that carbon monoxide, as a tracer of anthropogenic emissions, had a distinct diurnal pattern of morning and evening peaks. In this paper we have described simulations using the MM5 mesoscale meteorological model nested up to 1-km resolution, and verified its ability to capture the local meteorology in the valley as measured during our campaigns. We studied the dynamics of the dissipation of the nighttime cold-air pool and temperature inversion, and the subsequent evolution of the daytime mixed layer. We concluded that the cold pool dissipation was dominated by the mechanism of Figure 18. Trajectories released at 2345 NST on the same upslope flows along the valley rim mountains, accompanied line as in Figures 11 and 12, shown on a contour map by subsidence over the basin center. In addition, during fog- (masl). Black denotes time from 2345 to 0345 NST, blue is free days there was erosion of the inversion from below by from 0345 to 0745 NST, green is from 0745 to 1145 NST, the growth of thermals forming over a warming surface. The red is from 1145 to 1545 NST, orange is from 1545 NST to Kathmandu Valley’s mixed layer height peaked around noon, 1945 NST, and brown is from 1945 to 2345 NST. and then decreased again over the course of the afternoon. This pattern was explained using MM5 simulations that Valley at high elevation. A closer look at individual tracers showed gravity-driven flows penetrating into the basin bot- shows that upon arriving at the eastern valley rim, trajectories tom through the western mountain passes. These brought air which passed over passes or low ridge regions maintained that originated above lower-lying terrain to the west of the their altitude, whereas trajectories passing over the eastern valley that was cooler and denser than the air found over mountain peaks experienced a step-like gain in altitude that the Kathmandu Valley in the late morning. The Kathmandu was driven by the convergence of upslope flows over the Valley thus exhibited an afternoon circulation pattern sim- mountain tops east of the Kathmandu Valley. ilar to what had been found on plateaus elsewhere in the world. [39] Figure 17 shows the trajectories that were released along the north–south line on the valley floor at 1545 NST [42] Using forward trajectories released in the MM5 flow on 9 February 2005. Most trajectories converged to flow fields, we found that the dominant pathway of pollutant through the Sanga and Nala passes on the eastern valley ventilation depends upon the time of day. On the February rim, and a higher pass on the southern valley rim, above the day investigated, we found that during the morning, tracers town of Tika Bhairav. Most of the trajectories reached were advected out of the valley by transport up the slopes of outside of the valley within less than 2 h of their release. the valley rim mountains, and from there they were carried They no longer rose to the height of the valley rim peaks. east by the upper-level westerly winds. During the after- They could not rise as high during the afternoon because noons, tracers were found to be removed from the valley at the mixed layer had shrunk as a result of the arrival of a faster rate through the mountain passes, both to the east cooler air from neighboring valleys that formed a low- and to the south. During the night, some tracers near the altitude afternoon inversion over the Kathmandu Valley, as Bagmati River were advected out down the river valley, and we saw in section 4. near the western passes some tracers were able to escape from the Kathmandu Valley through drainage flows. [40] Finally, Figure 18 shows trajectories released at 2345 NST along the same 20 km north–south line. The [43] Our extensive new observations and numerical trajectories originating at the southern end of the Kathmandu modeling add significantly to the current understanding of Valley traveled down the Bagmati river valley. Most of the the valley’s atmospheric dynamics and air pollution mete- trajectories released within the central Kathmandu Valley orology. The Kathmandu Valley’s air quality is strongly lingered close to their points of origin. Trajectories released affected by the wind systems generated by the surrounding near the western valley-rim passes rose and crossed over the topography. Our study has several implications for air passes, and traveled down the western valleys until the quality policy in the Kathmandu Valley. It points to the fact morning, when upslope and up-valley winds carried them that the timing of emissions is key. If the evening rush hour back up. In Figure 11a we saw strong nocturnal drainage takes place before sunset, as it does in summer, its emissions winds in these western valleys before dawn. The strong are ventilated out of the valley before the cold pool sets in. drainage winds indicate a sufficient pressure gradient to If the rush hour takes place after sunset (as it does in force some Kathmandu Valley air up and over the western winter), it is responsible for much higher pollution levels

18 of 20 D21308 PANDAY ET AL.: KATHMANDU VALLEY MODELING RESULTS D21308 in the evening. As discussed in detail in part 1 [Panday and Egger, J., et al. (2005), Diurnal circulation of the Bolivian Altiplano. Part I: Observations, Mon. Weather Rev., 133, 911–924, doi:10.1175/ Prinn, 2009], these pollutants are lifted up, but recirculated MWR2894.1. in the morning, adding to the morning pollution. The Fast, J. D., and S. Zhong (1998), Meteorological factors associated with Kathmandu Valley also appears to be a rather unsuitable inhomogeneous ozone concentrations within the Mexico City basin, J. Geophys. Res., 103(D15), 18,927–18,946, doi:10.1029/98JD01725. place for combustions sources that operate 24 h a day, such Fast, J. D., J. C. Doran, W. J. Shaw, R. L. Coulter, and T. J. Martin (2000), as the brick kilns on the outskirts of the city. Emissions The evolution of the boundary layer and its effect on air chemistry in the that take place outside of the well ventilated midday and Phoenix area, J. Geophys. Res., 105(D18), 22,833–22,848, doi:10.1029/ afternoon hours contribute much more to air pollution in 2000JD900289. Gaertner, M. A., and C. Fernande´s (1993), A two-dimensional simulation of Kathmandu than would emissions that are restricted to well- the Iberian summer thermal low, Mon. Weather Rev., 121, 2740–2756, ventilated time periods. Designing an effective air pollution doi:10.1175/1520-0493(1993)121<2740:ATDSOT>2.0.CO;2. control strategy for the Kathmandu Valley thus requires Gantner, L., et al. (2003), The diurnal circulation of Zugspitzplatt: observations and modeling, Meteorol. Z., 12(2), 95 – 102, doi:10.1127/0941- careful consideration of the pollution diurnal cycle, and 2948/2003/0012-0095. time-dependent restrictions on polluting activities. Garcia, J. A., et al. (2002), A case study of the morning evolution of the convective boundary layer depth, J. Appl. Meteorol., 41, 1053–1059. Gohm, A., et al. (2004), South Foehn in the Wipp Valley on 24 October 44 Acknowledgments. The research described in this paper was [ ] 1999 (MAP IOP 10): Verification of high-resolution numerical simula- made possible through generous support from an MIT Presidential Fellow- tions with observations, Mon. Weather Rev., 132, 78–102, doi:10.1175/ ship, the Martin Fellowship for Sustainability, a pilot research project grant 1520-0493(2004)132<0078:SFITWV>2.0.CO;2. from the Alliance for Global Sustainability, the PAOC Houghton Fund, the Henne, S., et al. (2004), Quantification of topographic venting of boundary TEPCO Chair account, NSF grant ATM-0120468 to MIT, NASA grants layer air to the free troposphere, Atmos. Chem. Phys., 4, 497–509. NAG5-12099 and NAG5-12669 to MIT, and the federal and industrial Hong, S.-Y., and H.-L. 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