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Volume 17, issue 13 | Copyright

Special issue: Atmospheric pollution in the Himalayan foothills: The SusKat-ABC...

Atmos. Chem. Phys., 17, 8157-8176, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 04 Jul 2017

Research article | 04 Jul 2017

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 G. Lawrence1 Andrea Mues et al.
  • 1Institute for Advanced Sustainability Studies (IASS), 14467 Potsdam, Germany
  • 2Vaisala GmbH, 22607 Hamburg, 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 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.

The monthly minimum median MLH values typically range between 150 and 200m during night and early morning hours, the monthly maximum median values between 625m in July and 1460m 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 3h in January and 6–7h in May. During the monsoon season a diurnal cycle has been observed with the smallest amplitude (typically between 400 and 500m), with the lowest daytime mixing height of all seasons (maximum monthly median values typically between 600 and 800m), and also the highest night-time and early morning mixing height of all seasons (minimum monthly median values typically between 200 and 220m). 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 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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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...