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

Abstract

<p><strong>Abstract.</strong> 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. <br><br> 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. <br><br> 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.</p>

Air quality in the Kathmandu Valley: WRF and WRF-Chem simulations of meteorology and black carbon concentrations

“…This suggests that biases in the modeled dispersion (horizontal and vertical) alone are unlikely to be able to explain the large differences in modeled and observed black carbon levels. This, in turn, suggests that the top-down emissions determined by Mues et al (2017) Despite that offset that is apparently due to the emissions, the temporal correlation coefficient between daily data of the WRFchem_BC_D02 results and the Bode observations is relatively high (0.7) in February, while it is much lower (0.2) in May 2013. There are likely two factors that contribute to this difference.…”

“…In Mues et al (2017) a method is presented to estimate black carbon emission fluxes for the Kathmandu Valley from mixing layer height data, derived from ceilometer measurements, and black carbon concentrations measured during SusKat-ABC at 5 the Bode station (number 0017) located within the valley (Tab. 3 and Fig.…”

“…This confirms the strong need for an updated black carbon emission database for this region. However, it also became clear that a simple correction of the emission fluxes using the top-down method by Mues et al (2017) also has several limitations. One of these limitations is an over-representation of emissions which are relatively constant throughout the day (e.g., from brick kilns) while underrepresenting emissions which are mainly occurring during the daytime (e.g., traffic).…”

“…For all other grid cells the EDGAR HTAP emissions are used. For a more detailed description of the estimation of the black carbon emission fluxes we refer to Mues et al (2017).…”

Wintertime Boundary Layer Evolution and Air Pollution Potential Over the Kathmandu Valley, Nepal

“…Panday et al () investigated movement of pollutant clouds by plotting trajectories of nonbuoyant air parcels released at various times and locations in the Kathmandu Valley. Mues et al () discussed possible effects of seasonal and diurnal variations of mixing layer height (MLH) on black carbon (BC) concentrations at surface levels based on their observations. By using outputs of Weather Research and Forecasting (WRF)‐Chem model and observations of BC, Mues et al () further compared monthly variations of daily mean BC concentrations in February 2013 (winter) and May 2013 (premonsoon season) and pointed out the necessity of improvement in BC emission inventory.…”