WRF and WRF-Chem v3.5.1 simulations of meteorology and black carbon concentrations in the Kathmandu Valley

Abstract

Abstract. An evaluation of the meteorology simulated using the Weather Research and Forecast (WRF) model for the region of south Asia and Nepal with a focus on the Kathmandu Valley is presented. A particular focus of the model evaluation is placed on meteorological parameters that are highly relevant to air quality such as wind speed and direction, boundary layer height and precipitation. The same model setup is then used for simulations with WRF including chemistry and aerosols (WRF-Chem). A WRF-Chem simulation has been performed using the state-of-the-art emission database, EDGAR HTAP v2.2, which is the Emission Database for Global Atmospheric Research of the Joint Research Centre (JRC) of the European Commission, in cooperation with the Task Force on Hemispheric Transport of Air Pollution (TF HTAP) organized by the United Nations Economic Commission for Europe, along with a sensitivity simulation using observation-based black carbon emission fluxes for the Kathmandu Valley. The WRF-Chem simulations are analyzed in comparison to black carbon measurements in the valley and to each other. The evaluation of the WRF simulation with a horizontal resolution of 3×3 km2 shows that the model is often able to capture important meteorological parameters inside the Kathmandu Valley and the results for most meteorological parameters are well within the range of biases found in other WRF studies especially in mountain areas. But the evaluation results also clearly highlight the difficulties of capturing meteorological parameters in such complex terrain and reproducing subgrid-scale processes with a horizontal resolution of 3×3 km2. The measured black carbon concentrations are typically systematically and strongly underestimated by WRF-Chem. A sensitivity study with improved emissions in the Kathmandu Valley shows significantly reduced biases but also underlines several limitations of such corrections. Further improvements of the model and of the emission data are needed before being able to use the model to robustly assess air pollution mitigation scenarios in the Kathmandu region.

Molecular characterization of organic aerosols in the Kathmandu Valley, Nepal: insights into primary and secondary sources

“…BC and O3 measurements in the two major SusKat-ABC sites (Paknajol and Bode) in the Valley show similar levels (Putero et al, 2015;Mahata et al, 2017b). Therefore, Bode site can be taken as a representative site for the Kathmandu Valley (Rupakheti et al, 2018). …”

“…These two airflows meet around central-eastern part of the Valley and move eastward towards Nagarkot-Sanga passes (Regmi et al, 2003;Panday and Prinn, 2009). Bode area receives these two air flows, and hence it is downwind of the Kathmandu city and Lalitpur or Patan city located in southwest, west and northwest direction during the 110 daytime, and Bhaktapur city located in east and southeast during nighttime (Rupakheti et al, 2018;Mahata et al, 2017a;Bhardwaj et al, 2017;Sarkar et al, 2016). In addition, it is situated in a residential area with urban buildings and houses scattered across agricultural fields with paddy, wheat, corn and vegetable farms.…”

“…Additionally, the valley is also influenced by local mountain valley circulation (Mues et al, 2018). …”

“…In this context, the international project of "A Sustainable Atmosphere for the Kathmandu Valley (SusKat)" was launched, aiming to comprehensively understand the causes of the severe air pollution in the region, and identifying appropriate solutions to reduce its impacts (Rupakheti et al, 2018). This paper presents analyses of samples collected as part of the SusKat field campaign.…”

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

“…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. However, in the above studies, spatial characteristics of boundary layer activity and diurnal wind development in the Kathmandu Valley have not been well explored.…”