Overview of the Special Issue “Selected Papers from the 2nd Atmospheric Chemistry and Physics at Mountain Sites Symposium”

Mountain sites provide a unique window on atmospheric chemistry and physics. These sites allow for continuous observations at high elevation, often in the free troposphere, where many important processes occur. Observations at these mountain sites allow for studies on long-range transport of pollution, cloud and precipitation processes, boundary layer ventilation and long-term observations of climate relevant gases and aerosols. However operating at mountain sites presents a unique set of challenges, and for this reason scientists doing research at these sites sought a forum to share knowledge on both the science and challenges of working at these sites.