Investigation of the mixing layer height derived from ceilometer measurements in the Kathmandu Valley and implications for local air quality
In this study 1 year of ceilometer measurements taken in the Kathmandu Valley, Nepal, in the framework of the SusKat project (A Sustainable Atmosphere for the Kathmandu Valley) were analysed to investigate the diurnal variation of the mixing layer height (MLH) and its dependency on the meteorolog...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-07-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/17/8157/2017/acp-17-8157-2017.pdf |
| Summary: | In this study 1 year of ceilometer measurements taken in the Kathmandu
Valley, Nepal, in the framework of the SusKat project (A Sustainable
Atmosphere for the Kathmandu Valley) were analysed to investigate the diurnal
variation of the mixing layer height (MLH) and its dependency on the meteorological
conditions. In addition, the impact of the MLH on the
temporal variation and the magnitude of the measured black carbon
concentrations are analysed for each season. Based on the assumption that
black carbon aerosols are vertically well mixed within the mixing layer and
the finding that the mixing layer varies only little during night time and
morning hours, black carbon emission fluxes are estimated for these hours and
per month. Even though this method is relatively simple, it can give an
observationally based first estimate of the black carbon emissions in this
region, especially illuminating the seasonal cycle of the emission fluxes.
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The monthly minimum median MLH values typically range between 150 and 200 m during night and early morning hours, the monthly maximum median
values between 625 m in July and 1460 m in March. Seasonal
differences are not only found in the absolute MLHs, but also
in the duration of the typical daytime maximum ranging between 2 and 3 h in
January and 6–7 h in May. During the monsoon season a diurnal cycle has
been observed with the smallest amplitude (typically between 400 and 500 m), with the lowest daytime mixing height of all seasons (maximum
monthly median values typically between 600 and 800 m), and also the
highest night-time and early morning mixing height of all seasons (minimum
monthly median values typically between 200 and 220 m). These
characteristics can mainly be explained with the frequently present clouds
and the associated reduction in incoming solar radiation and outgoing
longwave radiation.
<br><br>
In general, the black carbon concentrations show a clear anticorrelation with
MLH measurements, although this relation is less pronounced
in the monsoon season. The daily evolution of the black carbon diurnal cycle
differs between the seasons, partly due to the different meteorological
conditions including the MLH. Other important reasons are the
different main emission sources and their diurnal variations in the
individual seasons. The estimation of the black carbon emission flux for the
morning hours show a clear seasonal cycle with maximum values in December to
April. Compared to the emission flux values provided by different emission
databases for this region, the estimated values here are considerably higher.
Several possible sources of uncertainty are considered, and even the absolute
lower bound of the emissions based on our methodology is higher than in most
emissions datasets, providing strong evidence that the black carbon emissions
for this region have likely been underestimated in modelling studies thus
far. |
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| ISSN: | 1680-7316 1680-7324 |