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Investigation of the mixing layer height derived from ceilometer measurements in the Kathmandu Valley and implications for local air quality

Mues, Amdrea and Rupakheti, Maheswar and Münkel, Christoph and Lauer, Axel and Bozem, Heiko and Hoor, Peter and Butler, Tim and Lawrence, Mark G. (2017) Investigation of the mixing layer height derived from ceilometer measurements in the Kathmandu Valley and implications for local air quality. Atmospheric Chemistry and Physics (ACP), 17, pp. 8157-8176. Copernicus Publications. DOI: 10.5194/acp-17-8157-2017 ISSN 1680-7316

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Official URL: https://www.atmos-chem-phys.net/17/8157/2017/

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 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. 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.

Item URL in elib:https://elib.dlr.de/112445/
Document Type:Article
Title:Investigation of the mixing layer height derived from ceilometer measurements in the Kathmandu Valley and implications for local air quality
Authors:
AuthorsInstitution or Email of AuthorsAuthors ORCID iD
Mues, AmdreaIASS PotsdamUNSPECIFIED
Rupakheti, MaheswarIASS PotsdamUNSPECIFIED
Münkel, ChristophVaisala GmbH, Hamburg, GermanyUNSPECIFIED
Lauer, AxelDLR, IPAhttps://orcid.org/0000-0002-9270-1044
Bozem, HeikoUniv. MainzUNSPECIFIED
Hoor, PeterUniv. MainzUNSPECIFIED
Butler, TimIASS PotsdamUNSPECIFIED
Lawrence, Mark G.IASS PotsdamUNSPECIFIED
Date:4 July 2017
Journal or Publication Title:Atmospheric Chemistry and Physics (ACP)
Refereed publication:Yes
Open Access:Yes
Gold Open Access:Yes
In SCOPUS:Yes
In ISI Web of Science:Yes
Volume:17
DOI :10.5194/acp-17-8157-2017
Page Range:pp. 8157-8176
Publisher:Copernicus Publications
ISSN:1680-7316
Status:Published
Keywords:boundary layer height, Kathmandu, SusKat, black carbon, measurements
HGF - Research field:Aeronautics, Space and Transport
HGF - Program:Space
HGF - Program Themes:Earth Observation
DLR - Research area:Raumfahrt
DLR - Program:R EO - Erdbeobachtung
DLR - Research theme (Project):R - Vorhaben Atmosphären- und Klimaforschung
Location: Oberpfaffenhofen
Institutes and Institutions:Institute of Atmospheric Physics > Earth System Modelling
Deposited By: Lauer, Axel
Deposited On:06 Jul 2017 14:25
Last Modified:02 May 2019 13:59

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