**Preface to "Urban Atmospheric Aerosols"**

Atmospheric fine particulate matter (PM2.5, diameter less than 2.5 μm) has profound effects on radiative climate forcing, atmospheric chemistry, air quality, visibility, and human health. PM2.5 can be directly emitted into the atmosphere (primary aerosols) or formed in the atmosphere (secondary aerosols) through gas-to-particle conversion processes of gaseous species. Furthermore, primary and secondary aerosols may also undergo chemical and physical transformations as they are subjected to transport, atmospheric aging, and removal from the atmosphere. The physical and chemical characterization of PM2.5, its source apportionment, and the assessment of the magnitude and distribution of its emissions are crucial for establishing effective fine air particle regulations and assessing the associated risks to human health.

Urban locations are a very special case as far as PM2.5 concentrations, compositions, sources, and health effects are concerned. Understanding the physical and chemical properties of urban PM2.5 (e.g., atmospheric concentration, size distribution, surface area, chemical composition, and water solubility) is essential for better predicting the tight connection between PM2.5 and its contribution to atmospheric chemistry and the health of both human populations and environmentally sensitive ecosystems. Therefore, it is highly beneficial to conduct studies on the physico-chemical characteristics and toxicological effects of urban PM2.5 in order to establish efficient control strategies. Furthermore, understanding how urban aerosols affect the air quality of indoor environments in urban buildings is also essential for assessing the potential health effects.

This book emerged from the Special Issue "Urban Atmospheric Aerosols: Sources, Analysis, and Effects", published in *Atmosphere*. The papers presented in this book highlight some important aspects concerning the chemical characteristics, optical properties, size distribution, sources, and potential health effects of urban air particles. These studies will be of interest to the atmospheric research community, including those interested in outdoor and indoor air quality, air particle toxicity, and atmospheric chemistry, as well as global climate modelers.

The Editors would like to thank the authors who generously contributed their time and knowledge to ensure the high quality of this work. The Editors also express their gratitude to the *Atmosphere* journal editors, reviewers, and the production team for their invaluable support and cooperation in the publication of the book. The Editors are also deeply grateful to FCT/MCTES for the financial support to CESAM (UIDP/50017/2020+UIDB/50017/2020), AMBIEnCE project (PTDC/CTA-AMB/28582/2017), and the Exploratory Research Project (IF/00798/2015/CP1302/CT0015). The Surface Ocean-Lower Atmosphere Study (SOLAS) is also acknowledged for endorsing the AMBIEnCE project. We sincerely hope that this work will stimulate further development to improve the current understanding of the sources, composition, fate, and impact of urban air particles.

> **Regina M. B. O. Duarte, Armando da Costa Duarte** *Editors*
