Abstract. Residents of the Kathmandu Valley experience severe particulate
and gaseous air pollution throughout most of the year, even during much of
the rainy season. The knowledge base for understanding the air pollution in
the Kathmandu Valley was previously very limited but is improving rapidly
due to several field measurement studies conducted in the last few years.
Thus far, most analyses of observations in the Kathmandu Valley have been
limited to short periods of time at single locations. This study extends the
past studies by examining the spatial and temporal characteristics of two
important gaseous air pollutants (CO and O3) based on simultaneous
observations over a longer period at five locations within the valley and on
its rim, including a supersite (at Bode in the valley center, 1345 m above
sea level) and four satellite sites: Paknajol (1380 m a.s.l.) in the Kathmandu
city center; Bhimdhunga (1522 m a.s.l.), a mountain pass on the valley's
western rim; Nagarkot (1901 m a.s.l.), another mountain pass on the eastern
rim; and Naikhandi (1233 m a.s.l.), near the valley's only river outlet. CO and
O3 mixing ratios were monitored from January to July 2013, along with
other gases and aerosol particles by instruments deployed at the Bode
supersite during the international air pollution measurement campaign
SusKat-ABC (Sustainable Atmosphere for the Kathmandu Valley – endorsed by
the Atmospheric Brown Clouds program of UNEP). The monitoring of O3 at
Bode, Paknajol and Nagarkot as well as the CO monitoring at Bode were
extended until March 2014 to investigate their variability over a complete
annual cycle. Higher CO mixing ratios were found at Bode than at the outskirt
sites (Bhimdhunga, Naikhandi and Nagarkot), and all sites except Nagarkot
showed distinct diurnal cycles of CO mixing ratio, with morning peaks and
daytime lows. Seasonally, CO was higher during premonsoon (March–May) season
and winter (December–February) season than during monsoon season
(June–September) and postmonsoon (October–November) season. This is
primarily due to the emissions from brick industries, which are only
operational during this period (January–April), as well as increased domestic
heating during winter, and regional forest fires and agro-residue burning
during the premonsoon season. It was lower during the monsoon due to
rainfall, which reduces open burning activities within the valley and in the
surrounding regions and thus reduces sources of CO. The meteorology of the
valley also played a key role in determining the CO mixing ratios. The wind
is calm and easterly in the shallow mixing layer, with a mixing layer height
(MLH) of about 250 m, during the night and early morning. The MLH slowly
increases after sunrise and decreases in the afternoon. As a
result, the westerly wind becomes active and reduces the mixing ratio during
the daytime. Furthermore, there was evidence of an increase in the O3
mixing ratios in the Kathmandu Valley as a result of emissions in the
Indo-Gangetic Plain (IGP) region, particularly from biomass burning
including agro-residue burning. A top-down estimate of the CO emission flux
was made by using the CO mixing ratio and mixing layer height measured at
Bode. The estimated annual CO flux at Bode was 4.9 µg m−2 s−1,
which is 2–14 times higher than that in widely used emission
inventory databases (EDGAR HTAP, REAS and INTEX-B). This difference in CO
flux between Bode and other emission databases likely arises from large
uncertainties in both the top-down and bottom-up approaches to estimating the
emission flux. The O3 mixing ratio was found to be highest during the
premonsoon season at all sites, while the timing of the seasonal minimum
varied across the sites. The daily maximum 8 h average O3 exceeded
the WHO recommended guideline of 50 ppb on more days at the hilltop station
of Nagarkot (159 out of 357 days) than at the urban valley bottom sites of Paknajol
(132 out of 354 days) and Bode (102 out of 353 days), presumably due to the influence of
free-tropospheric air at the high-altitude site (as also indicated by Putero
et al., 2015, for the Paknajol site in the Kathmandu Valley) as well as to
titration of O3 by fresh NOx emissions near the urban sites. More than
78 % of the exceedance days were during the premonsoon period at all
sites. The high O3 mixing ratio observed during the premonsoon period
is of a concern for human health and ecosystems, including agroecosystems in
the Kathmandu Valley and surrounding regions.
Supplementary material to "Investigation of the mixing layer height derived from ceilometer measurements in the Kathmandu Valley and implications for local air quality"