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

Diurnal cycle of air pollution in the Kathmandu Valley, Nepal: 2. Modeling results The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. 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 D21308 1of20 D21308 PANDAY ET AL.: KATHMANDU VALLEY MODELING RESULTS D21308 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 2of20 D21308 PANDAY ET AL.: KATHMANDU VALLEY MODELING RESULTS D21308 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.

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