atmosphere Review Atmospheric Pollutant Dispersion over Complex Terrain: Challenges and Needs for Improving Air Quality Measurements and Modeling Lorenzo Giovannini 1,*, Enrico Ferrero 2 , Thomas Karl 3, Mathias W. Rotach 3, Chantal Staquet 4, Silvia Trini Castelli 5 and Dino Zardi 1,6 1 Department of Civil, Environmental and Mechanical Engineering, University of Trento, 38123 Trento, Italy; [email protected] 2 Department of Science and Technological Innovation, University of Eastern Piedmont, 15151 Alessandria, Italy; [email protected] 3 Department of Atmospheric and Cryospheric Sciences, University of Innsbruck, 6020 Innsbruck, Austria; [email protected] (T.K.); [email protected] (M.W.R.) 4 Université Grenoble Alpes, CNRS, Grenoble INP, LEGI, 38000 Grenoble, France; [email protected] 5 Institute of Atmospheric Sciences and Climate, National Research Council, 10133 Torino, Italy; [email protected] 6 C3A—Center Agriculture Food Environment, University of Trento, 38010 San Michele all’Adige, Italy * Correspondence: [email protected]; Tel.: +39-0461-28-2649 Received: 27 May 2020; Accepted: 16 June 2020; Published: 18 June 2020 Abstract: Pollutant dispersion processes over complex terrain are much more complicated than over flat areas, as they are affected by atmospheric interactions with the orography at different spatial scales. This paper reviews recent findings and progress in this field, focusing on both experimental and modeling perspectives. It highlights open questions and challenges to our capability for better understanding and representing atmospheric processes controlling the fate of pollutants over mountainous areas. In particular, attention is focused on new measurement techniques for the retrieval of spatially distributed turbulence information and air quality parameters, and on challenges for meteorological and dispersion models to reproduce fine-scale processes influenced by the orography. Finally, specific needs in this field are discussed, along with possible directions for future research efforts. Keywords: air pollutant; turbulence; dispersion; transport; diffusion; atmospheric boundary layer; mountain; complex terrain 1. Introduction Many mountains in the world are undergoing a process of increasing anthropization. Indeed, nowadays roughly 900 million people, i.e., 13% of the world’s population, live in mountainous regions [1,2]. An even higher number of persons are attracted to mountainous areas for tourism and recreational activities every year. For example, in the Alps, an overall demand of more than 464 million overnight stays per year is estimated [3]. Moreover, populations living in mountain regions increasingly concentrate in rapidly expanding urban areas; it is estimated that roughly 30% of the world’s mountain population lives nowadays in cities, and this percentage reaches 50% in developed countries [2]. Mountain valleys and passes are also often crossed by main transport routes connecting different countries. For example, the Alps are crossed by 4200 km of main roads, with 6 million vehicles and 1.24 1011 kg of freights being transported across the Alpine range every year [4]. As a consequence, × Atmosphere 2020, 11, 646; doi:10.3390/atmos11060646 www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 646 2 of 32 Atmosphere 2020, 11, x FOR PEER REVIEW 2 of 32 the1 anthropogenic.24 × 1011 kg of impactfreights onbeing mountainous transported areas across raises the Al concernspine range from every many year viewpoints. [4]. As a consequence, In particular, onethe of anthropogenic the most serious impact hazards on mountainous involves areas pollutant raises concerns emissions from and many their viewpoints. effects onIn particular, air quality. Majorone sourcesof the most of pollutants serious hazards in mountainous involves pollutant regions emissions are generally and their located effects inside on valleysair quality. and Major basins, wheresources the of main pollutants urban in areas mountainous and traffi regionsc routes are develop, generally and located most inside of the valleys population and basins, lives. where This is the main urban areas and traffic routes develop, and most of the population lives. This is shown, as shown, as an example, in Figure1, displaying annual emissions of NO x and PM10 in South Tyrol, an example, in Figure 1, displaying annual emissions of NOx and PM10 in South Tyrol, in the Italian in the Italian Alps [5]. Under unfavorable meteorological situations, pollutants may accumulate at the Alps [5]. Under unfavorable meteorological situations, pollutants may accumulate at the bottom of bottom of valleys and basins, implying that a large part of the population is potentially exposed to valleys and basins, implying that a large part of the population is potentially exposed to high high concentrations. In fact, in such conditions, atmospheric dispersion, i.e., the combined effect of concentrations. In fact, in such conditions, atmospheric dispersion, i.e., the combined effect of transport/advection by the mean wind and diffusion due to turbulent mixing, may be strongly inhibited. transport/advection by the mean wind and diffusion due to turbulent mixing, may be strongly Highinhibited. levelsof High pollutants levels of are pollutants harmful, are not harmful, only to not humans, only to wildlife humans, and wildlife livestock, and livestock, but may alsobut may cause damagealso cause to the damage environment, to the especiallyenvironment, vegetation especially and vegetation forest heritage and forest [6]. Furthermore, heritage [6]. Furthermore, pollutants may bepollutants transported may by be orographic transported flows by orographic towards mountainousflows towards areasmountainous from adjacent areas from regions, adjacent where regions, they arewhere emitted. they As are a consequence,emitted. As a consequence, significant concentrations significant concentrations of harmful of species, harmful especially species, especially secondary pollutants,secondary may pollutants, also be ma foundy also in be areas found where in areas they where were they not were locally not emittedlocally emitted [7–9]. [7–9]. FigureFigure 1. Annual1. Annual emissions emissions of of (a ()a) PM PM1010 andand ((bb)) NONOx inin South South Tyrol, Tyrol, in in the the Italian Italian Alps, Alps, during during 2013, 2013, alongalong with with height height contour contour lines. lines. Adapted Adapted from from [[5].5]. TheThe relevance relevance of theof the problem problem of pollutant of pollutant dispersion dispersion has beenhas abeen primary a primary driver driver for many for researchmany effortsresearch focused efforts on focused atmospheric on atmospheric processes processes over complex over complex terrain terrain in recent in recent decades. decades. In fact, In fact, the the need forneed better for understanding better understanding and suitable and suitable modeling modeling of theof the fate fate of of pollutants pollutants in mountainous mountainous regions regions stimulatedstimulated a a number number ofof projects,projects, which which involved involved both both experimental experimental activities, activities, such as such extensive as extensive field fieldcampaigns campaigns in incomplex complex environments, environments, and and the the ad advancementvancement of ofnew new modeling modeling capabilities capabilities over over complexcomplex terrain, terrain, encompassing encompassing both both meteorological meteorological andand air quality models. Among Among the the projects projects primarilyprimarily focusing focusing on theon studythe study of air of pollution air pollution dispersion dispersion over complex over complex terrain, itterrain, is worth it mentioningis worth Monitoringmentioning and Monitoring minimization and ofmini traffimizationc-induced of traffic-induced noise and air pollution noise and along air pollution major Alpine along transport major routesAlpine (ALPNAP) transport [routes10–12 ],(ALPNAP) Bolzano Tracer[10–12], Experiment Bolzano Tracer (BTEX) Experiment [13,14], (BTEX) Persistent [13,14], Cold-Air Persistent Pool Cold-Air Pool (PCAPS) [15], Transport of Air Pollutants over Complex Terrain (TRACT) [16], (PCAPS) [15], Transport of Air Pollutants over Complex Terrain (TRACT) [16], Mesoscale transport Mesoscale transport of atmospheric pollutants across the central Alps (TRANSALP) [17,18], Vertical of atmospheric pollutants across the central Alps (TRANSALP) [17,18], Vertical Ozone Transports in Ozone Transports in the Alps (VOTALP) [19], and Vertical Transport and Mixing (VTMX) [20]. Other the Alps (VOTALP) [19], and Vertical Transport and Mixing (VTMX) [20]. Other projects aiming at projects aiming at characterizing dispersion in complex terrain and directly or indirectly focusing on characterizing dispersion in complex terrain and directly or indirectly focusing on air pollution will be air pollution will be reviewed in this paper. Results from these projects have greatly contributed to reviewedthe advancement in this paper. of Resultsknowledge from in these this projectsfield. Ho havewever, greatly several contributed aspects, concerning to the advancement both our of knowledgeunderstanding in this and field. our However, ability of several observing aspects, and modeling concerning processes both our related understanding to pollutant and dispersion our ability of observingover complex and terrain, modeling still processesremain to relatedbe investigated. to pollutant In fact, dispersion