Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2016-1002
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
06 Jan 2017
Review status
A revision of this discussion paper was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.
Investigation of the mixing layer height derived from ceilometer measurements in the Kathmandu Valley and implications for local air quality
Andrea Mues1, Maheswar Rupakheti1, Christoph Münkel2, Axel Lauer3, Heiko Bozem4, Peter Hoor4, Tim Butler1, and Mark Lawrence1 1Institute for Advanced Sustainability Studies (IASS), Potsdam, 14467, Germany
2Vaisala GmbH, Hamburg, 22607, Germany
3Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
4Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, Mainz, Germany
Abstract. In this study one 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 analyzed to investigate the diurnal variation of the mixing layer height and its dependency on the meteorological conditions. In addition, the impact of the mixing layer height 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.

In all seasons the diurnal cycle of the mixing layer height is typically characterized by low heights during the night and maximum values during in the afternoon. Seasonal differences are found in the absolute mixing layer height values and the duration of the typical daytime maximum. During the monsoon season a diurnal cycle has been observed with the smallest amplitude, with the lowest daytime mixing height of all seasons, and also the highest nighttime and early morning mixing height of all seasons. These characteristics can mainly be explained with the frequently present clouds and the associated reduction in incoming solar radiation and outgoing longwave radiation.

In general, the black carbon concentrations show a clear anticorrelation with mixing layer height 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 mixing layer height. 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 here estimated values 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.


Citation: Mues, A., Rupakheti, M., Münkel, C., Lauer, A., Bozem, H., Hoor, P., Butler, T., and Lawrence, M.: Investigation of the mixing layer height derived from ceilometer measurements in the Kathmandu Valley and implications for local air quality, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-1002, in review, 2017.
Andrea Mues et al.
Andrea Mues et al.
Andrea Mues et al.

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Short summary
Ceilometer measurements taken in the Kathmandu Valley, Nepal, were used to study the temporal and spatial evolution of the mixing layer height in the valley. This provides important information on the vertical structure of the atmosphere and can thus also help to understand the mixing of air pollutants (e.g. black carbon) in the valley. The seasonal and diurnal cycles of the mixing layer were found to be highly dependent on meteorology and mainly anticorrelated to black carbon concentrations.
Ceilometer measurements taken in the Kathmandu Valley, Nepal, were used to study the temporal...
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