Thermal inversion analysis in the Andorra Central Valley and its relationship with pollutants and meteorological variables

The behaviour of pollutants in urbanized mountainous areas is complex due to the interactions between the atmosphere and the orography, particularly noticeable in winter due to frequent thermal inversions inside the valleys. This stable Mountain Boundary Layer, present during anticyclonic situations, can lead to critical air pollution episodes that are detrimental to human health and the environment. Another growing concern is the impact of climatic extremes such as increased heat waves and their impact on air quality. 

This study aims to characterize the thermal inversion episodes in a complex orography area like the Central Valley of Andorra (ACV), answering questions like: (i) Which are the frequency and duration of winter inversion episodes in the central Valley of Andorra? (ii) How these characteristics correlate with pollutant concentrations and meteorological variables? Complementary, we explore the limitations of the current observation strategy for monitoring thermal inversions in the valley using low-cost sensors of temperature: (iii) Could the use of a larger number of stations on the slopes (pseudo-profile) overcome these limitations?

Findings indicate an increasing frequency and duration of thermal inversion episodes over the last decades, as well as the heatwaves on mountain areas, predominantly influenced by synoptic high-pressure conditions as daily synoptic classification centred on the Pyrenees has shown. These episodes significantly impact NO2 concentrations, nearly doubling their average levels, while PM10 and O3 did not show a direct correlation. In contrast, during the warm months, exceedances of critical O3 thresholds have been increased in last decades. Furthermore, results show that the effectiveness of low-cost sensors is notably dependent on their placement, particularly in terms of altitude and orientation relative to solar radiation.