Modelling the black carbon dynamics over Almaty, Kazakhstan, during winter and summer seasons.

Due to the close proximity of large urban areas to mountainous environments, air pollution can pose a serious threat to sensitive ecosystems through rapid transport driven by advection and mountain–valley circulation. Almaty (Kazakhstan), frequently ranked among the most polluted cities globally, is situated at the foothills of the Ile Alatau (part of the northern Tien Shan mountains). The city’s urban area located about 15-35 km from the major glacial systems, that have experienced a substantial decrease over the past years.  In this study, we investigated the impact of locally emitted black carbon (BC) from Almaty on the surrounding mountain areas using the WRF-CHIMERE regional chemistry-transport model with three nested domains up to 1 km resolution for periods representative of winter and summer conditions (i.e. January and July of 2023, respectively).

Simulation results indicated that during winter, BC concentrations remained trapped over the Almaty basin, at the lower elevations north of the city, and along the main valleys, due to stable atmospheric conditions and limited vertical mixing. In contrast, in summer, despite lower anthropogenic emissions arising from the city, BC was found to reach the mountain tops more effectively (up to 4000 m a.s.l.), likely due to increased vertical mixing and enhanced mountain–valley circulation. The peak BC concentrations at the mountain stations occurred approximately 5 (in July) – 8 (in January) hours after the maximum values in the city, suggesting faster upslope transport from the city in summer than in winter.

Additionally, model runs with and without online exchange between meteorology and chemistry were conducted to quantify the effect of BC concentrations on the radiative fluxes. Estimates of BC direct radiative effect (DRE) confirmed that the presence of BC over Almaty decreases solar radiation at the bottom of the atmosphere (BOA, BC DRE_BOA up to -1.20 W m^-2) and enhances absorption within the atmosphere (BC DRE_ATM up to +1.33 W m^-2). Analysis of the potential temperature gradients in both months indicated, on average, no significant effect of BC concentrations on vertical atmospheric mixing, which in January can be attributed to strong temperature inversions over the region.

This research represents the first assessment of dynamics, transport and radiative effects of BC over the mountainous regions in Central Asia and highlights the need for further analysis extending to transitional periods (spring, autumn) when the temperature inversions are weaker or absent, but emissions rates remain high.