Diurnal cycle of air pollution in the Kathmandu Valley,
Nepal: Observations
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Panday, Arnico K., and Ronald G. Prinn. “Diurnal Cycle of Air
Pollution in the Kathmandu Valley, Nepal: Observations.” Journal
of Geophysical Research 114, no. D9 (2009).
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http://dx.doi.org/10.1029/2008jd009777
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D09305, doi:10.1029/2008JD009777, 2009
Diurnal cycle of air pollution in the Kathmandu Valley, Nepal:
Observations
Arnico K. Panday1,2 and Ronald G. Prinn2
Received 1 January 2008; revised 11 November 2008; accepted 28 January 2009; published 9 May 2009.
[1] During the dry season of 2004–2005 we carried out field measurements of air
pollution and meteorology in the Kathmandu Valley, Nepal, a bowl-shaped urban basin in
the Himalayan foothills of Nepal. We measured the trace gases carbon monoxide (CO) and
ozone (O3) and particulates (PM10), as well as meteorological variables. In our field
observations we noted a very regular pattern of morning and evening peaks in CO
and PM10 occurring daily in the valley bottom, interspersed with low values in the
afternoons and at night. This pattern occurred even on days with unusual timing of
emissions and was influenced by the timing of ventilation from the valley. Meteorological
variables showed great day-to-day similarity, with a strong westerly wind blowing through
the valley from late morning until dusk. We found that the air mass on nearby
mountaintops was disconnected from pollution within the valley during the night, but
received significant pollution during the morning, when up-slope flows began. At a pass
on the western edge of the valley we found a diurnal switch in wind direction, with an
inflow from late morning until late evening, and an outflow during the rest of the time. We
found that part of the morning peak in pollution was caused by recirculation of pollutants
emitted the night before, which spend the night in elevated layers over the valley.
Citation: Panday, A. K., and R. G. Prinn (2009), Diurnal cycle of air pollution in the Kathmandu Valley, Nepal: Observations,
J. Geophys. Res., 114, D09305, doi:10.1029/2008JD009777.
1. Introduction
[2] Cities that are located in basins surrounded by tall
mountains often face severe air pollution problems. The
diurnal cycle of air pollution in those cities is often strongly
influenced by the mountains. Mountains block horizontal
winds that would otherwise blow locally emitted emissions
away. Nighttime cooling of the mountain sides creates downslope flows of denser cold air; in wide basins this cold air
can pool, suppressing vertical mixing of pollutants. Daytime
heating of mountain slopes, on the other hand, creates vertical transport of pollutants up the slopes. Meanwhile horizontal flows through a basin are strongly affected by the
location and orientation of mountain passes and gaps, as well
as by the shape of the topography outside of the basin.
[3] Mountain-enclosed cities that are growing rapidly,
and that are located in developing countries, are particularly
vulnerable to air pollution problems. In recent years the
international scientific community has extensively studied
several such cities, including Mexico City [Bossert, 1997;
Fast and Zhong, 1998; de Foy et al., 2006; McKinley et al.,
2005; Molina et al., 2007; Molina and Molina, 2002; Raga
1
Atmospheric and Oceanic Sciences Program, Princeton University,
Princeton, New Jersey, USA.
2
Center for Global Change Science, Massachusetts Institute of
Technology, Cambridge, Massachusetts, USA.
Copyright 2009 by the American Geophysical Union.
0148-0227/09/2008JD009777
et al., 2001; Shirley et al., 2006; Whiteman et al., 2000b],
and Santiago de Chile [Rappenglück et al., 2000, 2005;
Romero et al., 1999; Schmitz, 2005]. Other urban mountain
basins in developing countries have seen much less
research. We present here results from a study designed to
understand the processes responsible for the accumulation
and ventilation of air pollution in the Kathmandu Valley, a
mountain basin in the Himalayan foothills of Nepal.
[4] We carried out field measurements in the Kathmandu
Valley during the 9-month dry season from September 2004
to June 2005. Our principal goal was to understand how
surface emissions and valley ventilation together shaped the
observed diurnal cycles in air pollutants. We focused on
continuous measurements of two primary pollutants (CO,
particulates) and one secondary pollutant (O3), as well as
several meteorological variables.
[5] CO is a particularly useful indicator of urban air
pollution. It shares many combustion sources with particulate matter and with other urban primary pollutants. For
example it is well correlated with NMHC from combustion
sources [Kuebler et al., 1996; Möllman-Coers et al., 2002].
It tends to show greater spatial homogeneity [Holmes et al.,
2005] than many shorter-lived pollutants whose concentration is strongly affected by local sources. In fact, compared
to most urban air pollutants, carbon monoxide has a long
atmospheric lifetime, from 1.1 to 2.4 months in the tropical
boundary layer, and from 2.2 to 3 months in the midlatitude
lower troposphere [Martin et al., 2002]. The long lifetime
has several implications. The gas has often been utilized as
a tracer of long distance and intercontinental pollution
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transport [Bertschi and Jaffe, 2005; Guttikunda et al., 2005;
Lobert and Harris, 2002; Stohl et al., 2002]. More importantly, it meant that, on the timescale of its emission and
ventilation from urban areas (hours to a day), CO could be
treated effectively as an inert tracer.
[6] We measured PM10 even though it shares many of the
same combustions sources with CO, because its shorter
lifetime could help us distinguish spikes in the data from
local pollution sources. In addition, the portability of the
PM10 instrument allowed it to be occasionally lent to local
university students for their class projects on pollution
exposure, in exchange for help with simultaneous bag
sampling of CO at multiple locations.
[7] Section 2 of this paper introduces the Kathmandu
Valley. Section 3 describes past field research in Kathmandu. Section 4 describes our field experiment, Section
5 presents results from the field observations. Section 6
discusses these results, distilling out key questions needing
further investigation. These questions are addressed through
theoretical investigations using a numerical model, which
are presented in a companion paper (A. J. K. Panday et al.,
Diurnal cycle of air pollution in the Kathmandu Valley,
Nepal: 2. Modeling results, submitted to Journal of Geophysical Research, 2009).
2. Kathmandu Valley
[8] The Kathmandu Valley is located in the midhills of
Nepal, halfway between the Ganges Plains and the Tibetan
Plateau, at a latitude of 27.7°N. It is shaped like a circular
bowl with a flat floor area of 340 km2, averaging 1300 m
above sea level (masl). Its total watershed area is 625 km2.
Surrounding the valley is a ring of mountains ranging from
2000 to 2800 m above sea level, with five mountain passes
dipping down to 1500 –1550 masl. There is no river inlet
into the valley, and only one narrow winding outlet (the
Bagmati River) at the basin floor level. Neighboring valleys
to the west, north, northeast, and south have substantially
lower elevations. The Kathmandu Valley’s climate is influenced by the South Asian summer monsoon. It receives up
to 90% of its annual rainfall during the three summer
months. During that time intense convection and heavy
rains keep the valley’s air relatively clean. Air pollution is a
concern largely during the remaining nine months’ dry
season. A more detailed description of the Kathmandu
Valley’s geography can be found in a doctoral dissertation
[Panday, 2006].
[9] Over the past quarter century the Kathmandu Valley’s
population has quadrupled to more than 2.5 million.
Between 1990 and 2003 the total vehicle fleet grew from
45,871 to 249,282 with the number of motorcycles growing
from 22,359 to 173,646 [Faiz et al., 2006]. Today the
number of motorcycles exceeds three hundred thousand, while
larger vehicles total more than a hundred thousand. Traffic
is the biggest source of CO within the Kathmandu Valley,
accounting for 60% of the total emissions [Kitada and
Regmi, 2003; Shrestha and Malla, 1996]. Rush hours are
responsible for distinct morning and evening peaks in traffic
and in congestion. Most of the valley’s population cooks
one large meal in the morning, and one in the evening,
mostly using fuel sources that produce CO (LPG, kerosene,
and firewood). The third major source of CO, namely
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industries (especially brick kilns), do not have emissions that
vary as distinctly over the course of the day.
3. Past Research in the Kathmandu Valley
[10] Air pollution in the Kathmandu Valley has become a
visible problem that catches much attention, with widely
publicized statistics showing a doubling in hospital admissions due to respiratory illnesses over a 5-year period, with
especially large numbers recorded in the winter months
[Clean Energy Nepal, 2005]. Less attention has been paid to
the physical processes responsible for the accumulation and
ventilation of air pollutants within the valley. Research
conducted prior to our study provided only partial glimpses
into what appeared to be a complex system of humanenvironmental interactions.
[11] Records kept at the valley-center airport show a large
decrease in visibility between the late 1960s and the mid1990s [Larssen et al., 1997]. The first published measurements of CO in Kathmandu found concentrations in the city
center averaging 2 ppm, and ranging from less than 1 ppm
to 2.5 ppm in winter 1982 – 1983 [Davidson et al., 1986].
These concentrations are not too different from what we
found in 2004 – 2005. In 1982 –1983, the vehicle fleet was
very small compared to today, but a much larger fraction of
households used biofuels.
[12] Several passive sampling campaigns have collected
NO2 and SO2 data around the valley over several weeks
each [Larssen et al., 1997; Leaders Nepal, 1999; Nepal
Environmental and Scientific Services, 1999; Regmi and
Kitada, 2003] quantifying the average pollution exposure
in different regions of the valley. They generally found
polluted conditions in the city and in the brick-kiln region
of the valley’s southeast. One study quantified 28 nonmethane hydrocarbons in 38 flask samples collected around
Kathmandu [Sharma et al., 2000] and found significant
weekday to weekend differences in species emitted by
automobiles. High-frequency measurements of atmospheric
turbidity over 21 days [Sapkota and Dhaubadel, 2002] and
of condensation nuclei count over 9 days [Hindman and
Upadhyay, 2002] provided some observations of the aerosol diurnal cycle. Both aerosol studies showed that the
Kathmandu valley generally experienced morning and evening peaks in pollution. Hindman and Upadhyay [2002] also
operated two automated weather stations in the valley for
nine days, finding a regular pattern of calm nights and strong
afternoon westerly winds. A team from MIT and Chalmers
University ran a DOAS instrument in Kathmandu in January
2003, finding morning and evening peaks in NOx [Yu et al.,
2008].
[13] As part of the Atmospheric Brown Clouds project
[Ramanathan et al., 2005], two nephelometers were
installed near Kathmandu in winter 2003. During their setup
phase, a high-resolution micropulse lidar was run from 9 to
17 February 2003 to measure the vertical aerosol extinction
profile over Kathmandu [Ramana et al., 2004]. The lidar
found a layer of local pollutants near the surface, as well as
an elevated layer of regional pollutants centered at 1.3 km
above the surface. The same project also measured aerosol
optical depth, and broadband global radiative fluxes
[Ramanathan and Ramana, 2005]. The authors found aerosol optical depths in Kathmandu in the range of 0.3 –0.5, and
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Figure 1. The Kathmandu Valley, viewed from the south in Google Earth. The high Himalaya and
Tibetan Plateau are visible at the top edge of the image. Kathmandu city is labeled in red. Yellow labels
show important measurement sites.
a diurnally averaged surface aerosol forcing of 25 Wm2.
They speculate that the layer of aerosols observed at 1.3 km
above the surface might have been lofted by orographic
flows. More recently published data of ozone and NOx
measurements at Pulchowk near Kathmandu between
November 2003 and October 2004 found the highest ozone
levels in the premonsoon period, and the lowest in winter
[Pudasainee et al., 2006]. The authors also found that ozone
was higher on weekends and days with general strikes than
on regular weekdays. An increase in ozone with a drop in
NOx emissions suggests a VOC limited regime [Sillman et
al., 1990].
[14] The only other routine air quality measurements in the
valley prior to our field experiment have been gravimetric
daily average PM10 measurements carried out by the Ministry
of Population and Environment at six locations since late
2002 (http://www.most.gov.np/pollution/pollution.php).
4. Experimental Setup
[15] From September 2004 until June 2005 we measured
carbon monoxide, PM10, and ozone, as well as meteorolog-
ical parameters with an automated weather station and an
acoustic sounder (SODAR) around the Kathmandu Valley.
The temperature profile of the boundary layer was recorded
by loggers installed at the top and bottom of a mountain, at
four different heights on a 72-m TV tower in the valley
center, near the top of another 50-m tower, and on a hill
overlooking our main laboratory site. Figure 1 and Table 1
show the locations of individual sites. Table 2 provides
details about the instrument deployment. Bag sampling
provided information about the spatial distribution of CO.
In this section we describe briefly the set up at each site.
[16] A main laboratory was set up in Kathmandu, to
house the continuous monitoring instruments and to serve
as a base of operations. Selection criteria for the main
laboratory included that it be downwind of Kathmandu city,
and that it have good security, generator back-up power
supply, and public transportation access. The site also had to
be surrounded by open space and thus be some distance
from local pollution sources. Available space at the Hyatt
Regency Hotel, which occupies 37 acres of gardens in
Bouddha, met all these criteria. The southern end of the
hotel building had a free-standing fire escape tower
Table 1. Locations of Our Instrument and Sampling Sites in and Around the Kathmandu Valley
Location
Description
Longitude
Latitude
Bouddha laboratory
Hyatt roof
Nagarkot
Kharipati
TV tower
Dharahara
SODAR
Hattiban
Pullahari
Bhimdhunga
Enclosure on Hyatt fire escape, Bouddha
Weather station above top gable
Pipe from roof of Hotel Viewpoint
Tree among fields
72-m-tall metal tower
City-center 52-m tower with park at base
Roof of CTEVT building
10-m view tower overlooking valley
Pole on seven-storey tall monastery roof
Grassy ridge, 50 m from pass
27°43.290N
27°43.270N
27°43.420N
27°42.450N
27°41.770N
27°42.010N
27°40.920N
27°37.730N
27°44.740N
27°43.580N
85°21.420E
85°21.420E
85°31.520E
85°28.220E
85°19.600E
85°18.700E
85°22.470E
85°16.410E
85°22.490E
85°13.830E
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Altitude
1350
1363
1972
1372
1300
1294
1321
1784
1476
1496
m
m
m
m
m (base)
m (base)
m
m
m
m
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PANDAY AND PRINN: KATHMANDU VALLEY FIELD OBSERVATIONS
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Table 2. Details of the Instrument Deployment at Each Site
Location
Instrument
Parameters Measured
Bouddha laboratory
Teledyne API 300EU
2B Technology model 202
TSI Dusttrak
Rainwise Portlog
CO
Ozone
PM10
Temperature
Humidity
Solar radiation
Pressure
Rainfall
Temperature
Ozone
Temperature
Temp. at 4 levels
Temperature
Wind profiles
Mixed layer height
Temperature
Temperature
Temperature
Humidity
Solar radiation
Pressure
Rainfall
Hyatt roof
Nagarkot
Kharipati
TV tower
Dharahara
SODAR
Onset Hobo H8 Pro
2B Technology model 202
Onset Hobo H8 Pro
4 Onset Hobo H8 Pro
Gemini Tinitag
Remtech PA-0
Hattiban
Pullahari
Bhimdhunga
Gemini Tinitag
Gemini Tinitag
Rainwise Portlog
connected to the main building via bridges. A metal-frameand-wood-panel enclosure was built on the fifth-floor landing of this tower. It housed our gas filtercorrelation CO
instrument (API 300EU, Teledyne Advanced Pollution
Instrumentation, San Diego, CA), a UV-absorption ozone
instrument (Model 202, 2B Technology, Boulder, CO) zero
air and calibration gas cylinders (Scott Marrin and BOC
Gases), computers, spare parts and tools, field sampling
equipment, and, after each bag sampling campaign, filled
sampling bags prior to their analysis. The optical PM10
instrument (Dusttrak, TSI Inc, Shoreview, Minnesota) sat
nearby in its own environmental enclosure. Our air sample
inlet funnel was suspended 1.8 m south of the edge of the
building, 12.5 m above ground, overlooking the hotel’s
grounds. The nearest road and parking lot were 60 m away.
The sampling inlet was located at a point which had direct
views of the valley-rim mountains to the northeast, east,
south, west, and northwest of the Kathmandu Valley, and
where it was fully exposed to ambient winds. Additional
details about the available instrument choices, instrument
measurement techniques, inlet and sample line plumbing, as
well as calibration procedures, are given by Panday [2006,
section 2.4].
[17] An automated weather station (Portlog, Rainwise
Inc., Bangor, Maine) was installed on the main roof of the
Hyatt building, approximately 20 m above the ground, on
its own tripod mast, at a corner of the north-south running
gable of the hotel’s roof. During several days in April, May
and June 2005, the weather station was alternatively
deployed at Bhimdhunga Pass, on the western entrance to
the valley for a few days at a time. There it was set up on a
grassy ridge at the water divide of the valley, 50 m south of
and 15 m higher than the road.
[18] Choosing a site for our sodar (acoustic sounder,
model Pa-0, Remtech S.A., Velizy Cedex, France) required
special considerations. A sodar works by emitting discrete
acoustic frequencies into the atmosphere, and measuring the
timing and frequency shift of the return signal to calculate
the mixed layer height and vertical wind profile. The sodar
antenna needed to have a clear sky view down to large
Time Period of Deployment
Sep 2004 to Jun 2005
Sept 2004 to Jun 2005
Sep 2004 to May 2005
Sep 2004 and Dec
2004 to Jun 2005
Jan 2005 to Jun 2005
Dec 2004, Jan and Apr 2005
Jan 2005 to Jun 2005
Jan 2005 to Jun 2005
Jan 2005 to Apr 2005
Sep 2005 and Apr
2005 to Jun 2005
Apr 2005 to May 2005
Feb 2005 to Jun 2005
3 times in April 2005
zenith angles to avoid beam reflection off nearby objects.
Ambient noise also had to be low, while the sodar’s loud
beeping sounds should not disturb nearby residents. After
evaluating 12 sites with a digital sound meter (Model 322,
Center Technology, Taipei, Taiwan), we installed the sodar
on the fourth-floor roof of the Council for Technical
Education and Vocational Training (CTEVT) building in
Sano Thimi, due east of the airport, and close to valley’s
geographic center.
[19] Generally sodars perform best during the day when a
convective boundary layer is topped by an inversion; they
have more difficulty detecting the nighttime boundary layer
height when an inversion extends down close to the ground
[Beyrich, 1997; Venkatesan et al., 1995]. We also needed
detailed measurements of the nighttime temperature inversions. Since tethersondes were incompatible with the daytime air traffic in the valley and helium was not readily
available, we chose 1-min measurements of temperature on
existing towers and on the valley rim mountains. This
approach has successfully been compared with tethersondes
by Whiteman et al. [2004]; inexpensive temperature loggers on basin sidewalls were found to provide good proxies
for temperature soundings in a basin’s free air during times
of low wind speeds and stable nighttime conditions. We
used seven Hobo H8 Pro temperature loggers (Onset Computers, Bourne, Massachusetts), with external thermistors in
an epoxy-potted cylinders installed inside radiation shields.
These sensors have been extensively tested [Whiteman et
al., 2000a], and have been used to study cold pools in the
Columbia basin over horizontal scales of tens of kilometers
[Whiteman et al., 2001] as well as in a tower study of
drainage flows in gullies [Mahrt et al., 2001]. In addition
we used two Gemini Tinytag loggers with temperature
shields (Gemini Data Loggers, Chichester, U.K.). After
several days of intercomparison at the laboratory in Kathmandu, one of the nine loggers was set aside for mobile
measurements on the roof of a GPS-equipped automobile.
Four loggers were installed 62, 48, 26, and 5 m above
ground on the tallest structure at the center of the valley: the
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Figure 2. Two weeks each of CO measurements at the Bouddha laboratory on the eastern edge of
Kathmandu City in (a) October 2004 and (b) January 2005. Weekends are marked in dark shading, while
the medium shading between 20 and 25 October marks the holidays associated with the Dashain festival.
72-m-tall Nepal Television (NTV) Metro transmission tower
inside the Singhadurbar government secretariat compound.
[20] Another logger was first installed 48 m above ground
on the Dharahara tower in central Kathmandu, and then
moved to Hattiban ridge (1785 masl) in early April 2005.
Other temperature loggers were installed at Nagarkot Peak’s
Hotel Viewpoint (1975 masl), on a tree at Kharipati (elevation 1375 masl) at the base of Nagarkot, and at Pullahari
Monastery (1500 masl), on a hill overlooking the Hyatt
hotel. Details of these sites are described by Panday [2006].
[21] We only had one real-time CO instrument in Kathmandu, yet were interested in understanding the spatial
heterogeneity of this trace gas, especially its concentrations
upwind of the city, and on mountaintops surrounding the
valley. Spatial heterogeneity of CO has previously been
studied in Santiago de Chile with the help of 30 investigators spread around the city, who simultaneously filled
flasks that were then analyzed at a central laboratory to
provide contour maps of CO around the city [Chen et al.,
1999]. To emulate this approach, we used six model 222–
2301 Grab Air bag sampling pumps (SKC Inc., Pennsylvania)
to fill a set of up to 190 5-L foil bags with polypropylene
fittings (SKC model 245–25). We collected at least hourly
samples at a number of locations around the Kathmandu
Valley, over the full 24-h diurnal cycle, and then brought the
samples to our main laboratory for analysis.
[22] For this task, we collaborated with the Central
Department of Environmental Science at Tribhuvan Uni-
versity. A typical bag sampling day saw teams of 2 – 3
students camping at five or six locations within the Kathmandu Valley and on surrounding passes and peaks, filling
bags every 30 –60 min at preset times while recording local
meteorological conditions. Successful bag sampling campaigns were carried out eight times between February and
May 2005. On several bag sampling days a digital camera
(Nikon D70, Nikon, Japan) was set up on the Hattiban view
tower for time-lapse photography of the valley fog and haze
layers to help inform our meteorological analysis. Studies of
the dynamics of cold air pools by photographing the heights
of fog and air pollution layers viewed from mountaintops
have been carried out extensively in the former Yugoslavia
in the 1970s [Rakovec et al., 2002].
5. Results
[23] Throughout the nine month study, we observed a
very regular diurnal pattern in the air quality and meteorology of the Kathmandu Valley. Figure 2 shows measurements of CO at the Bouddha laboratory during 2 weeks each
in October 2004 and January 2005. Several patterns stand
out immediately. First, every single day showed a morning
and an evening peak. The low values were similar on most
days (between 350 and 500 ppb), but the morning and
evening peak values showed greater variation. Second, peak
values were significantly higher in January than in October.
While they seldom reached 1.5 ppm in October, they
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Figure 3. Bag sampling results for CO on 9 – 11 February 2005. The yellow line shows CO
measurements at the Bouddha laboratory during the same time period. Bag sample data are displayed for
Nagarkot (red), Hattiban (blue), the base of Dharahara tower (black), the top of Dharahara tower (50 m
above ground, purple), and the sodar site at CTEVT (green).
regularly approached 3 ppm in January. Third, nighttime
and afternoon low values still far exceeded typical northern
hemispheric clean air values; at Nepal’s latitude, clean-air
tropospheric CO mixing ratios would be closer to 150 ppb
(http://agage.eas.gatech.edu/images/Data_figures/gcmd_
month/co_AmonS5.pdf). During our entire measurement
period, CO rarely dropped below 350 ppb, with low values
more frequently hovering around 500 ppb.
[24] Figures 3 – 8 show data from a 48-h period in
February when all instruments except the sodar functioned
reliably, and when we also had a 24-h period of bag
sampling, shown in Figure 3. Other locations on the valley
floor, Dharahara and CTEVT, also showed the twin peak
diurnal pattern found at the Bouddha laboratory. The
mountaintop locations, Nagarkot and Hattiban, meanwhile,
showed steady nighttime CO values around 450 ppb. At
night these mountaintop sites were far above the valley floor
boundary layer, so their CO measurements reflected regional
background levels similar to what was found arriving in
Kathmandu during the afternoon when westerly winds were
sweeping into the valley. During the daytime, the mountaintop sites experienced higher, fluctuating CO values, with
a very pronounced increase in the morning (presumably
when the first polluted air parcels from the city started
arriving). These patterns were repeated during subsequent
bag sampling campaigns at these same sites and at other
similar locations.
[25] Figure 4 shows simultaneous CO, O3, and PM10
measurements at the Bouddha laboratory during the same
48-h period. It is evident that CO and PM10 were very well
correlated (R = 0.88) and that they both showed the distinct
morning and evening peaks already seen for CO. Ozone had
very low values at night, typical of urban areas where it is
titrated out by abundant NO, and also lost owing to dry
deposition. In the late morning ozone rose rapidly to a
narrow peak, before dropping to 50% lower values for the
rest of the afternoon. Two processes can cause ground-level
ozone in urban areas to rise in the morning: photochemical
formation after sunrise, and downward mixing of ozone-rich
air from aloft displacing air depleted in ozone owing to
nighttime surface loss. The rapid growth from minimum to
maximum daily ozone levels suggests a strong contribution
by the second process; this is supported by the rapid
decrease in CO concentrations at the same time. The low
values of CO and PM10 found during the afternoons and late
at night imply the existence of processes that remove
pollutants from the near-surface air during those times.
[26] Our meteorological observations in the Kathmandu
valley help explain the observed diurnal cycle of pollutants.
Figure 5 shows wind and solar radiation data from the
rooftop weather station at the Bouddha laboratory. Nights
were characterized by low wind speeds and varying wind
directions. The highest wind speeds were reached during the
afternoon, when there were consistent westerlies. Mountain
basins often experience a sharp increase in wind speed at the
time when the nocturnal inversion has completely dissipated
[Banta and Cotton, 1981; Whiteman, 1982]. In the Bolivian
Altiplano, strong inflows through the western passes (facing
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Figure 4. (a) CO, (b) PM10, and (c) ozone measurements at the Bouddha laboratory from 1200 Nepal
Standard Time (NST) on 9 February 2004 through 1200 NST on 11 February 2004.
toward the Pacific lowlands) have been observed a few
hours after sunrise, coinciding with the transformation of a
stable nocturnal boundary layer over the Altiplano into a
convective boundary layer [Egger et al., 2005]. Figure 6
shows relative humidity, temperature, dew point, and barometric pressure at the same rooftop weather station. The
relative humidity data (Figures 6), shows saturated conditions during early morning hours, matching our observation
of fog that lasted several hours. In Figures 6b and 6c, we see
daytime fluctuations in dew point temperature, suggesting
arrival of different air masses. There is an afternoon dip in
barometric pressure.
Figure 5. (a) Wind direction, (b) wind speed (1-min average in black, maximum gusts in green), and
(c) solar radiation measured by the roof-top weather station at the Bouddha laboratory from 1200 NST on
9 February 2004 through 1200 NST on 11 February 2004.
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Figure 6. (a) Relative humidity, (b) temperature and dew point (DP) in °C, and (c) barometric pressure
at the weather station on the Hyatt roof during the same 48-h period.
[27] Figure 7 gives a visual overview of the Kathmandu
Valley’s morning fog as seen from Hattiban view tower on
the morning of 4 February, while collecting bag samples
(Figure 4). It illustrates the formation of a thin veil of fog
over the city before dawn, and its gradual thickening and
lifting.
[28] Figure 8a shows the temperature on Nagarkot Peak
(1975m), and at Kharipati (1375m), at the base of Nagarkot.
Nagarkot was cooler than Kharipati during the day, but
warmer at night. The lapse rate, computed from the temperature difference between the two sites (Figure 8b), shows
a temperature inversion at night, while from 1200 to 1500
Nepal Standard Time (NST) the lapse rate approached the
dry adiabatic lapse rate, indicating a vertically well mixed
atmosphere. Figure 8c shows temperature at the Bouddha
rooftop weather station (1363 m) and Pullahari monastery
(1476 m), while Figure 8d shows the lapse rate between the
two. It is evident that the 1363 – 1476 m altitude layer also
experienced a temperature inversion at night; in fact our TV
tower temperature measurements showed stable conditions
at night down to the level of 5 – 25 m above ground,
suggesting that vertical mixing was suppressed at the
elevations where most of the pollutant emissions took place.
During afternoons, the TV tower loggers showed rapid
fluctuations including unstable thermals (lapse rate exceeding 40 K/km), while the Kharipati and Nagarkot loggers
showed an observed lapse rate approaching the dry adiabatic lapse rate of 9.8 K/km, suggesting a mixed layer of at
least 600 m depth. An earlier modeling study had found the
Kathmandu Valley’s mixed layer height peaking at 700 –
900 m above the basin bottom [Regmi et al., 2003].
Additional comparisons between loggers at the same altitude (not shown), found that during the first 2 hours after
sunset, Kharipati’s temperature was 3°– 5° colder than the
temperature at valley center locations (Bouddha and TV
tower). This is consistent with the formation of a cold air
pool driven by the arrival of katabatic flows from surrounding slopes. Observations of smoke plumes on the valley rim
mountains (not displayed) also consistently indicated downslope flows after sunset.
[29] The nighttime vertical stratification within the Kathmandu Valley, and its isolation from air masses above, can
be further seen in ozone measurements made alternately at
the Bouddha laboratory and on Nagarkot Peak (Figure 9).
Ozone levels in Nagarkot did not go to zero at night. This is
consistent with ozone observations on remote mountaintops
in Arizona [Fast et al., 2000], China [Gao et al., 2005] and
Thailand [Pochanart et al., 2001]. At mountaintop locations
at night there is insufficient NO to titrate ozone. Often there
are additional inputs of ozone from the free troposphere, in
contrast to the valleys below, which are isolated at night by
stable air [Baltensperger et al., 1997].
[30] Unfortunately between November 2004 and April
2005 the sodar was at the manufacturer’s for repairs. Figure 10
shows sodar data for 11 and 12 May 2007. Other data
collected in May (not shown) indicates that, except for
warmer temperatures, lack of morning fog, and longer
daylight hours, these days in May were very similar to the
February days discussed above. While the sodar was capable of estimating the mixed layer height to an altitude
exceeding 1000 m, its measurement of wind speed and
direction was usually confined to the bottom few hundred
meters. Nevertheless, the sodar data shows several patterns
that augment what we observed with the weather station on
the Hyatt roof in Bouddha. The afternoon westerlies are
confined close to the surface. After 2200 NST much weaker
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Figure 7. View from Hattiban on 10 February 2005 looking toward the northwest (city is in the fog in
the right middle ground), and toward the east. All times in NST.
easterlies push underneath the westerlies, producing a rising
region of wind shear. These weak easterlies are seen throughout the night. In the mornings the wind direction changes
several times, with some distinctly visible southerlies.
[31] Figure 11 shows 48 h of sodar observations of the
mixed layer height above central Kathmandu Valley in May
2005. The sodar-derived mixed layer height was found to be
very regular from day to day, peaking between 700 and
1000 m on most days when we had observations, consistent
with the temperature logger findings of a mixed layer reaching at least the height of Nagarkot Peak. This is relatively
low compared to the height of more than 2 km that is
often found over the United States [Berman et al., 1999]
and India [Krishnan and Kuhnhikrishnan, 2004], and also
significantly lower than what is often found over Mexico
City [de Foy et al., 2006]. It is possible that the surprisingly
low sodar observations of afternoon mixed layer height in
the Kathmandu Valley were due to the limited range of the
Pa-0 sodar and its inability to capture the full height of the
mixed layer. An alternative explanation will be discussed
in a companion paper (Panday et al., submitted manuscript,
2009), where model results show the afternoon westerly
winds bringing cooler air into the Kathmandu Valley, creating
renewed stratification.
[32] Figures 12a and 12b show wind direction and wind
speed at Bhimdhunga Pass on the western valley rim in
April 2005. The wind switched back and forth between
daytime westerlies (from about 1000 to 2200 NST), and
nighttime easterlies. We hypothesize that the nighttime
easterlies at the pass began at the time when the valley’s
cold air pool filled up to the level of the pass. Our timelapse photography from Hattiban ridge indicated that the top
of the Kathmandu Valley’s winter fog layer matched the
height of the passes. The fog has also been observed
flowing out across the western passes in the morning and
dissipating while descending into the neighboring western
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Figure 8. (a) Temperature at Kharipati (black) and Nagarkot (green); (b) lapse rate between the
Kharipati and Nagarkot station altitudes; (c) temperature at the Bouddha laboratory weather station
(black) and at Pullahari monastery (green); and (d) lapse rate between the Bouddha and Pullahari station
altitudes.
valleys (Figure 13). The descending motion outside the
pass indicates that ‘‘overflow’’ of the Kathmandu Valley
cold pool across the passes occurs. In several studies
elsewhere, cold pools in basins have been found to fill
up to the elevation of the height of the lowest mountain
barrier [Allwine and Lamb, 1992], from where they can
overflow [Neff and King, 1989]. As we shall see later, the
Bagmati valley outflow, which is lower than the western
passes, has too small a cross section to allow the cold air pool
to drain off.
[33] Figure 12c shows CO measurements made simultaneously at the main laboratory in Bouddha, and (with bag
sampling) at Bhimdhunga Pass and Nagarkot Peak. Shortly
after midnight on 20 April, the main laboratory in the basin
Figure 9. Ozone measurements before and after the instrument was moved from Bouddha (blue) to
Nagarkot (black) in (a) December 2004, (b) January 2005, and (c) April 2005.
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Figure 10. Vertical profiles of (a and c) wind direction and (b and d) wind speed on 11 May 2005
(Figures 10a and 10b) and 12 May 2005 (Figures 10c and 10d) as measured by the sodar. Wind speeds
are coded in gray scale, with darker shades indicating stronger winds. Wind directions are coded in color,
with reddish colors indicating westerly and bluish colors indicating easterly flows. All times in NST.
bottom, Bhimdhunga Pass, and Nagarkot Peak all had
similar CO concentrations. However, CO concentrations at
Bhimdhunga Pass rose by almost 50% (to 1.8 ppm) from
0000 to 0400 NST while they remained low at the other two
sites. Given the lack of local sources, there is only one
plausible explanation for this rise in CO observed at the
pass, namely, that the easterly winds arriving at the pass
were bringing air that had been at the surface within the
Figure 11. The mixed layer height measured by the SODAR in Sano Thimi during a typical 48-h time
period in May 2005. The sodar’s lower detection limit was 50 m above ground.
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Figure 12. (a) Wind direction on Bhimdhunga Pass in April 2005. (b) Minute average wind speed
(black) and maximum gusts (green) on Bhimdhunga Pass during the same days. (c) CO measurements at
the laboratory in Bouddha, and CO measured with bag sampling on Bhimdhunga Pass and Nagarkot Peak
during the same days.
Figure 13. Air photo of fog at Bhimdhunga (BP) and Nagdhunga (NP) passes from the northwest. The
fog-covered Kathmandu Valley is in the left half of the middle ground. The Bagmati outflow valley (BV)
is in the upper right, and the fog-covered Ganges Plains are in the background.
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Figure 14. CO at the Bouddha laboratory on selected sunny days in January. The color codes are as
follows: blue, Tuesday, 11 January 2005; red, Wednesday, 12 January 2005; yellow, Thursday, 13
January 2005; and orange, Friday, 14 January 2005.
Kathmandu Valley and polluted by the city during the
evening before.
6. Discussion
[34] The results presented in the previous section indicate
that the Kathmandu Valley’s air quality and meteorology
show a lot of day-to-day similarity. There is a morning and
an evening peak in CO and PM10 every day, while ozone
decreases to zero in the valley bottom at night. Afternoons
are marked by strong westerly winds, while at night there
appears to be a pooling of cold air in the basin bottom up to
the level of the western passes. Here we consider several
questions in order to understand this pattern better.
[35] First, why is there such a regular twin peak pattern in
CO and PM10 in the valley bottom in Kathmandu? Morning
and evening peaks in urban CO have been observed in some
other cities as well, including in Bangkok [Zhang and
Oanh, 2002], Santiago de Chile [Rappenglück et al.,
2000, 2005; Schmitz, 2005], Buenos Aires [Bogo et al.,
2001], Athens [Gros et al., 2002], and at times in Hong
Kong [Wang and Kwok, 2003]. In southern California [Qin
et al., 2004] and in Brisbane [Morawska et al., 2002]
morning and evening CO peaks were found to be stronger
on weekdays than on weekends. Often the observed peaks
were found to be a direct result of local rush-hour traffic
emissions; in Buenos Aires and Santiago de Chile the peaks
correlated very well with traffic volumes.
[36] Figure 14 compares daily CO variations on four
consecutive days in January 2005. If the morning and
evening peaks were directly correlated with traffic or
cooking, they would be found at the same time each day.
Instead, on Tuesday and Wednesday, 11 and 12 January, the
morning peak dropped off sharply around 0900 NST, while
on Thursday and Friday, 13 and 14 January, the morning
peak reached a maximum shortly after 0900 NST and did
not drop off until 1000 NST. Office and school start and end
times did not vary substantially from one day to the next,
and neither did the rush-hour traffic patterns. We also see
that the evening CO peaks began around 1800 NST, even
though the heaviest evening rush-hour traffic took place just
after 1700 NST. It is also worth noting that the morning
peak often began shortly after 0600 NST (at dawn), even
though morning traffic and cooking activities remained light
until at least 0800 NST. It is thus clear that the twin peak
pattern was not just simply result of morning and evening
peaks in emission.
[37] Looking back at Figure 2a, we see additional evidence that the CO peaks were not simply caused by traffic
or cooking. Days 20 through 25 October 2004 were the
main days of the Dashain festival, when hundreds of
thousands of people left the Kathmandu Valley to return
to their ancestral villages, while offices remained closed.
People stayed home or visited relatives, and cooked the
morning meal later than usual. There were therefore no
morning or evening rush hours. Yet the morning and
evening peaks in CO are still visible on these days. The
existence of the peaks was thus not a result of diurnal
emissions patterns alone. We see, however, that the peak
values were lower on the holidays, indicating that emissions
volumes might have had an effect on the peak amplitude
observed among different days in October. However, because
there was no seasonal difference in traffic between a regular
weekday in October and a regular weekday in December,
emission volumes do not explain the difference between peak
amplitudes in these 2 months.
[38] The diurnal cycle of atmospheric CO concentrations
near the ground can be affected by factors other than local
CO emissions. For example, CO from biomass combustion
in rural Thailand was found to have nighttime highs and
daytime lows due to reduced nighttime ventilation caused
by inversions [Pochanart et al., 2003]. During a year of
street-side measurements in Buenos Aires, Argentina [Bogo
et al., 2001], days with no wind had higher nighttime values
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Figure 15. Close-up look at (a) ozone, (b) CO, (c) wind speed, and (d) wind direction at midday on 10
February 2005 at the Bouddha laboratory and rooftop weather station.
of CO, and higher morning peaks. Downwind of Bangkok,
Thailand, the heights of the morning CO peaks are strongly
dependent upon wind direction [Zhang and Oanh, 2002]. In
spring 2002, CO canister sampling upwind, within, and
downwind of Santiago de Chile [Rappenglück et al., 2005]
found distinct morning and evening peaks within the city,
with minimum values at noon, like we found in Kathmandu.
An earlier study in Santiago de Chile indicated that there
were no evening CO peaks during the summer, because
there was still strong ventilation at the time of rush hour
[Rappenglück et al., 2000]. It is likely that the winter
evening peaks in Kathmandu were taller because of an
earlier suppression of ventilation. The pattern observed in
Kathmandu appears to be strongly affected by the timing
and mechanism of ventilation.
[39] Returning to Figure 4, we see that the prominent
noontime spike in ozone on 10 February was accompanied
by concurrent small peaks in CO and PM10. Figure 15
zooms in to show that the increase in westerly wind speed at
1218 NST on 10 February apparently initiated a small peak
each in ozone and CO that was then followed by larger broader
peaks in both gases from 1230 to about 1252 NST. The
average wind speed from 1130 to 1217 NST was 2.0 m/s,
but from 1218 to 1252 NST it was 4.9 m/s. After 1252 NST
it was even faster. On the basis of the wind data and
Kathmandu Valley geography, we hypothesize that when
the stronger westerly winds began, they carried the polluted
air mass that had previously been stationary over the city
toward the east and past the instruments in Bouddha. The
initial peak marked the arrival of polluted air from the
nearby Chabahil Ring Road intersection. This was followed
by a trough indicating cleaner air from the residential area
immediately west of there, and then by a broad peak
marking the arrival of polluted air from the main city farther
west. The distance from the Bouddha site to the western
edge of the city is 10 km. At 4.9 m/s, it would take 34 min
to cross the city: that was almost exactly the duration of the
ozone and CO peaks. We thus have evidence that the
observed peaks in pollution corresponded to rapid transport
past our Bouddha sensors of the air mass that had been
almost stagnant over the city during the morning. The drop
in pollutant concentrations after 1252 NST happened once
the tail end of that polluted air mass had gone past the
measurement station. From that time onward, throughout
the afternoon, the sensors appeared to be measuring air that
had initially been upwind of the city (including outside of
the western passes), and that had then traversed the city
rapidly, picking up less CO and other ozone precursors over
the city, while having less time to form ozone in situ by the
time of its arrival at the Bouddha laboratory.
[40] The afternoon dip in CO and PM10 concentrations in
Kathmandu is clearly a result of westerly winds sweeping
through the valley. These winds bring cleaner air through
passes such as Bhimdhunga on the west, and sweep the
valleys’ pollutants out toward the east. But why do pollutant
levels drop at night? Figure 16 shows PM10 at the time of
the Shivaratri festival on 8 March, when hundreds of bonfires were lit at dusk. Unfortunately we don’t have reliable
CO data from those days. In the evening of 8 March, we see
much higher PM10 values than on adjacent evenings. By
2100 NST most of the fires had been extinguished. We see
PM10 values decrease during the night; however recordhigh PM10 values we measured again the next morning
despite the absence of fire emissions. The only plausible
explanation is that aerosols emitted by the fires the previous
night had returned to the surface observation site after
initially having been transported away. This recirculation
of pollutants also helps explain why we see the morning
peak in pollutants appearing shortly after dawn, even
though the peak in cooking and traffic emissions does not
happen until several hours later. But where do the pollutants
go during the nighttime?
[41] Let us consider the hypothesis that the city’s polluted
air mass might follow nighttime drainage flows down the
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Figure 16. PM10 at the Bouddha laboratory. After sunset on 8 March 2005, bonfires were lit around the
city on the occasion of Shivaratri festival. These fires were mostly extinguished before 2100 NST.
Bagmati River Valley, and then return to the city when
the wind direction in the Bagmati River Valley reversed in
the morning. This hypothesis can easily be rejected: drainage flows leaving the Kathmandu Valley down the Bagmati
River must flow through the very narrow Chobhar Gorge,
and simple dimensional analysis shows that unrealistic
supersonic wind speeds would be needed to drain the entire
polluted air mass of the valley through the Chobhar Gorge
outlet over the course of one night.
[42] What we did observe, and frequently photograph at
dawn, were elevated polluted layers over the Kathmandu
Valley. Nighttime elevated layers of pollutants have been
measured at the top of skyscrapers in Phoenix [Berkowitz et
al., 2006] and Milan [Rubino et al., 1998], and by sampling
air with a customized tethersonde in Japan [Sahashi et al.,
1996]. Elevated layers of aerosols have also been observed
over the Rhine Valley [Frioud et al., 2003]. In Figure 12,
we saw that at Bhimdhunga Pass, 200 m above the valley
floor, CO levels rose after midnight. That was after the
evening peak at the valley bottom had subsided, and two
hours after an easterly wind had started transporting Kathmandu Valley air out of the pass. Could this rise in CO at
the pass indicate the arrival of a lifted layer of CO from the
valley bottom?
[43] Figure 17 shows CO bag sampling results at 3
different elevations during the night of 4 – 5 April at the
Bouddha laboratory on the valley bottom, at Pullahari
monastery (150 m higher), and at Hattiban (600 m above
the valley bottom). During the night, shortly after the
evening CO peak subsided in Bouddha around 2300 NST,
CO started to rise in Pullahari, and after 0200 NST it started
to rise in Hattiban as well. Since Pullahari and Hattiban
have no local sources of CO, the observation indicates the
arrival of a polluted air mass. This could have been
advected horizontally from elsewhere, or it could be pollutants originating in the Kathmandu Valley that are lifted up.
Seeing in Figure 17 that CO levels started rising first at the
lower-elevation site in Pullahari, let us assume for the
moment that we are indeed seeing a nocturnal lifting of
pollutants originating in the Kathmandu Valley bottom.
What mechanism could lift these during the night, in the
presence of a stable cold air pool?
[44] Could pollutants from the valley bottom be lifted by
mechanical turbulence? Turbulence in stable nocturnal
boundary layers is generally not caused by surface heating,
but by shear in the nocturnal jet. A field study involving a
60-m-high instrumented tower near Wichita, Kansas, found
turbulence in the stable nocturnal boundary layer that was
created by wind shear and that was increasing with height
[Mahrt and Vickers, 2002]. Substantial vertical mixing
within stable layers is also possible owing to internal gravity
waves [Monti et al., 2002; Stull, 1988]. Turbulent transport,
like any diffusive process, only happens if there is a
gradient in the tracer being transported. For turbulence to
be able to transport nighttime pollutants away from the
surface, CO concentrations must decrease with altitude.
Although we did not have the capability to carry out vertical
soundings of CO, we did collect bag samples at several
locations near the top of the Kathmandu Valley’s nighttime
cold air pool. A look at Figures 12 and 17 shows that CO
concentrations at higher locations (Pullahari, Bhimdhunga,
Hattiban) exceeded CO concentrations at the valley bottom
(as measured at the Bouddha laboratory) for extended
periods. In fact concentrations rose at Pullahari, while they
continued to drop in Bouddha. Vertical transport appeared
to continue for several hours beyond the time when the
gradient was reversed. We can thus exclude turbulent transport as the primary agent of nighttime CO removal from the
valley bottom.
[45] A more likely explanation of the observed nighttime
lifting in pollutants as suggested by the observed low values
in the valley bottom and the rising values at higher
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Figure 17. Bag sampling CO data from 4– 5 April 2005 at Pullahari and Hattiban, along with CO
measurements at the Bouddha laboratory during the same time period. The horizontal axis spans from
0448 NST on 4 April through until 1912 NST on 5 April.
elevations is based upon the mechanism that forms the cold
air pool in the valley bottom in the first place. This is
illustrated in Figure 18. Radiative cooling of mountains
generates downslope (katabatic winds) which lead to a
pooling of cold air in the basin bottom, as seen in the
temperature data (Figure 8a). While doing our field experiments, we observed plumes of smoke on valley rim slopes
heading downhill during the evening, indicating such katabatic winds. Parcels of relatively clean cold air arriving in
the city from the valley rim mountains flow underneath the
less cold, less dense air parcels that are already there, lifting
the latter up along with the pollutants they contain. The air
parcels that were heavily polluted in the evening rise higher
and higher as colder cleaner airflows underneath that is less
polluted by the few emissions sources that remain on all
night. In the morning, the elevated pollutants mix down
again as the growing mixed layer entrains elevated layers of
polluted air. We see in Figure 17 that around 0500 NST in
Pullahari, and 0600 NST in Hattiban, concentrations of CO
dropped suddenly to similar levels found at the Bouddha
laboratory, where the CO was starting to rise to a morning
peak. This is consistent with the dilution that takes place
Figure 18. Idealized time sequence illustration of deduced circulation, showing the (top) nighttime
lifting and (bottom) morning downward mixing of pollutants. The cities of Kathmandu, Bhaktapur, and
Dhulikhel are marked by the letters K, B, and D, respectively. At the eastern and western ends of the
valley, both passes and peaks are shown.
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when elevated pollutant layers are mixed downward as they
are entrained into a growing mixed layer in the morning.
[46] To be viable, this explanation does need to pass one
check: If the katabatic winds were transporting cold air from
the valley-rim mountains to the basin bottom, they would
also be bringing in a fresh supply of ozone. Achieving the
observed near-zero ozone levels found at the Bouddha
laboratory at night would require an efficient ozone removal
mechanism along the way. Since NO emissions are most
likely small late at night (and hence the reaction of NO with
ozone is small), we would need ozone to be removed by dry
deposition before the cold air arrived at the Bouddha
measurement location. Even with the low nighttime wind
speeds, converging katabatic winds would arrive at the
Hyatt sensors from the valley rim base in, at most, a few
hours. The katabatic flows would be following the surface,
traveling underneath layers of air from which ozone would
already have been largely removed. Thus they would not
have any en-route replenishment. Existing literature [Lagzi
et al., 2004; Matsuda et al., 2005; Sorimachi et al., 2003;
Zhang et al., 2002] suggests that agricultural lands (as
found between the valley-rim slope and the Hyatt sensors)
have ozone dry-deposition velocities on the order of 0.2 cm/s,
or 7.2 m/hour. Given that katabatic flows are only a few
meters thick, this implies an ozone deposition lifetime of
less than 1 h, thus providing a reasonable mechanism for the
removal of ozone from air masses arriving at the Hyatt
measurement station at night. We conclude that the cold air
streaming down the valley rim mountains at night is the
most plausible explanation for the clean (low CO, PM10 and
ozone) ground-level air observed at the Kathmandu Valley
bottom between midnight and dawn, and the rising pollution
levels observed higher up. In the morning, once the mixed
layer starts to grow, the elevated polluted air is mixed down
again. This helps explain the early start of the morning
pollution peak.
7. Conclusion
[47] We carried out 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. During the afternoon, the valley was
ventilated by strong westerly winds entering through the
western passes. In the evening emissions in the valley
accumulated in the city and its environs. Late at night, the
city air quality improved again, as polluted surface layers
appeared to be lifted up owing to the convergence of cleaner
colder air into the city driven by katabatic flows on the
valley rim slopes. In the early morning, as the mixed layer
started to grow, the elevated pollutants were mixed back
down into the city.
[48] Analysis of our observations indicates that the twin
peak pattern existed owing to an interplay between the
timing of emissions and ventilation. The morning peak
decreased when the strong westerly winds removed pollutants that had accumulated in the city over the course of the
mornings. Our observations do not tell us the pathways of
pollutant ventilation out of the valley. They also do not give
us a complete picture of the dynamics of the valley’s daytime mixed layer, and nighttime cold air pool. These topics
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are investigated using the numerical model MM5 in a companion paper (Panday et al., submitted manuscript, 2009).
[49] Although local emissions within the valley play an
important role in creating pollution peaks, our CO measurements on mountaintops and Bhimdhunga Pass, as well as
the high minimum CO levels ever recorded at the Bouddha
laboratory make it clear that there is significant background
air pollution present that comes to the Kathmandu Valley
from elsewhere. Part of it may be from biofuel combustion
in rural Nepal, while part of it is likely to originate in the
Ganges Plains.
[50] Acknowledgments. The research described in this paper were
made possible through generous support from an MIT Presidential Fellowship, the Martin Fellowship for Sustainability, a pilot research project grant
from the Alliance for Global Sustainability, the PAOC Houghton Fund, the
TEPCO Chair account, NSF grant ATM-0120468 to MIT, NASA grants
NAG5-12099 and NAG5-12669 to MIT, and the federal and industrial
sponsors of the MIT Joint Program on the Science and Policy of Global
Change.
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