Breaking the isothermal assumption in CFD air quality modeling: Solar irradiance effects on the wind velocity-concentration relationship

Abstract

This study examines how far solar irradiations modify the wind velocity–concentration relationship commonly used in isothermal computational fluid dynamics (CFD) modeling of urban air quality. The main aim is to evaluate the validity under non-isothermal conditions of this widely used relationship and to provide new insights into the influence of solar-induced thermal effects on urban pollutant dispersion. While this relationship enables long-term concentration estimates through extrapolation from a limited set of simulations—thus offering strong operational advantages—its validity under non-isothermal conditions remains untested. Yet, recent regulatory changes and empirical evidence increasingly highlight the limitations of the isothermal assumption, especially in capturing short-term pollutant dynamics influenced by solar-driven thermal effects. To address this gap, a systematic CFD analysis of pollutant dispersion within an idealized 5 × 5 urban building array was conducted. This array was exposed to varying inlet wind velocities and solar irradiance levels, under fixed solar position and thermal boundary conditions. Results reveal that thermally induced flow structures can significantly modify pollutant dispersion patterns, particularly under low wind and high irradiance conditions. However, as mechanical forcing increases, flow fields and resulting pollutant concentration distributions tend to converge, reducing the impact of thermal perturbations. A comparative analysis of simulated pollutant fields and those recalculated using the isothermal wind velocity–concentration relationship shows that the reliability of this approach depends on the balance between thermal and mechanical forcing. Under favorable conditions—i.e., high wind, low solar irradiance—using this relationship remains robust. Conversely, under solar-dominated scenarios, it introduces significant errors.