**Preface to "Air Quality Characterisation and Modelling"**

Ambient air pollution is the primary environmental health risk concern worldwide that causes seven million preventable deaths per year and the loss of healthy years of life; it threatens the sustaining of the environment through acidification and eutrophication. Almost all of the global population breathes air that exceeds the latest health-based guideline levels set by the World Health Organization, with low- and middle-income countries experiencing the highest exposure. Key sources of air pollution are road transport vehicles, domestic heating and industrial installations, and transboundary emissions. These sources produce the main ambient air pollutants of concern to which the population is exposed: particulate matter, ground-level ozone, nitrogen dioxide, carbon monoxide, and sulphur dioxide.

Air quality models constitute a complementary approach to monitoring and characterising air pollution. Spatial and temporal variability of air pollution is mainly investigated across urban areas or those hosting industrial activities, especially in developed countries, where the highest concentration of air pollutants is expected. Understanding these spatial and temporal variabilities is essential for both the implementation of air quality policies and the definition of effective measures to mitigate air pollution and its effects.

This Special Issue aims to showcase selected and original research articles concerning air quality characterisation and modelling. It includes 16 manuscripts covering a wide range of modelling topics, geographical scopes, and the characterisation of different fractions of air pollution.

Among modelling approaches are emission inventories, simulation scenarios, dispersion models, source contribution techniques, and, more particularly, cluster analysis and linear and non-linear regressions, to cite a few. In some manuscripts, air quality indexes or coefficients are used to characterise air quality; others, such as the thermal sensation index or a particular decomposition of the Gini coefficient, are not as frequently used. The geographical scope was diverse, both in urban agglomerations and rural areas, with air quality studies in several countries. Among the studied facets of air pollution were outdoor and indoor pollution, thermal comfort, or the quantification of suspendable road dust. Other included studies examined the role of administrations in air quality, evaluating the effectiveness of local emission reduction plans or carbon trading policies.

We hope the air quality characterisation and modelling community will find this special issue to be an informative and useful collection of articles and serve as an impetus to spur much more research on the topic.

#### Acknowledgements

This work was financially supported by (i) LA/P/0045/2020 (ALiCE) and UIDB/00511/2020- UIDP/00511/2020 (LEPABE), funded by national funds through FCT/MCTES (PIDDAC).

> **Jos ´e Carlos Magalh˜aes Pires and Alvaro G ´omez-Losada ´** *Editors*
