Advection pathways at the Mt. Cimone WMO-GAW station: Seasonality, trends, and influence on atmospheric composition
Introduction
There is a pressing need to improve understanding of processes contributing the seasonal variability of background/baseline (i.e. well-mixed tropospheric) atmospheric composition in the central north Mediterranean region, a hotspot of air pollution and climate change. In fact, due to the sunny, hot and dry weather typical of this region especially during summer, together with the convergence of long-range transport over the basin, air pollution in the form of reactive compounds is often higher than in most European inland regions (Dulac et al., 2016). In addition, climate change will significantly impact air quality with numerous two-way interactions not always well understood.
Air pollution in the Mediterranean basin is primarily in the form of particulate matter and ozone and nitrogen deposition (Ochoa-Hueso et al., 2017).
In this framework, clustering of backward trajectories has been used to study the influence of the origin and pathway of air masses on composition change (for a review see Fleming et al., 2012). The investigation of vertical motions in the atmosphere may take advantage of using 7Be and 210Pb radiotracers, because of their naturally contrasting origin: in fact, 7Be (half-life 53.3 days) is produced by cosmic ray spallation reactions with nitrogen and oxygen in the stratosphere (about 75%) and in the upper troposphere (Usoskin and Kovaltsov, 2008), while 210Pb (half-life 22 years) is a tracer of continental air masses (Balkanski et al., 1993), being emitted as decay product of 222Rn (half-life 3.8 days) deriving from crustal rocks and soils (Turekian et al., 1977). Once produced, both radionuclides attach to submicron-sized aerosol particles peaking in the accumulation mode (e.g., Gaffney et al., 2004). Thereafter, the main removal mechanisms of 7Be and 210Pb from the atmosphere are wet and dry scavenging of the carrier aerosol (Feely et al., 1989; Kulan et al., 2006). For this reason, simultaneous measurements of 7Be and 210Pb, and analysis of their ratio, can provide useful information about the vertical motion of air masses as well as on convective activity in the troposphere (e.g., Koch et al., 1996; Lee et al., 2007).
The use of air mass classification together with atmospheric radiotracers is not common, but has been the subject of some studies (e.g. Arimoto et al., 1999; Hernández et al., 2008; Dueñas et al., 2011; Lozano et al., 2012; Chambers et al., 2013, 2014, Chambers et al., 2016a, Chambers et al., 2016b; Grossi et al., 2016; Hernandez-Ceballos et al., 2016). However, most of the previous studies of this kind in the Mediterranean region focused on relatively short time series, and focused on understanding the variability of atmospheric radiotracers without a clear connection to other atmospheric compounds. Moreover, while the relation between natural radionuclides and teleconnection indices has been the subject of recent studies (Grossi et al., 2016; Sarvan et al., 2017), the variability in the occurrence of each trajectory group and the assessment of trends in association with large-scale atmospheric circulation indices, such as the North Atlantic Oscillation index (NAOi), is less common (Orza et al., 2013). Even less studied is the association in the occurrence of advection pathways with the remaining modes of atmospheric circulation over Europe, such as the Eastern Atlantic (EA) pattern, Eastern Atlantic/Western Russia (EA/WR), and the Scandinavian (SCA) pattern. Together with NAO, these indices represent the most important mid-latitude modes for the Mediterranean climate at the monthly time scale (Trigo et al., 2006).
In this context, long-term measurements at the high-elevation WMO-GAW baseline station of Mt. Cimone (Italy; 44°11′ N, 10°42’ E, 2165 m asl) are of paramount importance. In particular, they are useful for the identification of dominant advection pathways, assessing associations between pathways and atmospheric composition, and investigating links between flow pathways and circulation modes in the Mediterranean region on seasonal and interannual time scales. In addition, the occurrence of trends and the relationships of trends in atmospheric composition with those in advection pathways and teleconnection indices in the monthly time series are also explored, focusing not only on large-scale indices but considering two additional regional low-frequency atmospheric circulation pathways, namely the Mediterranean Oscillation (MO) and the Western Mediterranean Oscillation (WeMO). It should be emphasized here that, historically, the investigation of atmospheric circulation model influences (both large-scale and regional) typically focused on precipitation and temperature pathways. To date, there has been limited exploration of the relationship between these modes and advection pathways/atmospheric composition.
This work is organized as follows. We first describe the measurement techniques and the statistical methods used. We then present and discuss our results on: 1) the description of the main advection pathways found by the cluster analysis of back trajectories; 2) the analysis of the relationships between advection pathways and meteorological parameters/other atmospheric components; 3) the temporal analysis of the monthly time series, including trends; 4) the associations of air flow types with teleconnection indices and meteorological/atmospheric variables. We finally summarize our main conclusions.
Section snippets
Sampling site
Mt. Cimone, the highest peak of the Italian northern Apennines, hosts a global station of the Global Atmosphere Watch (GAW) programme of the World Meteorological Organization (WMO) constituted by a meteorological observatory by the Italian Air Force (active since 1941) and a research facility managed by the Institute of Atmospheric Sciences and Climate (ISAC) of the National Research Council of Italy (CNR), active since 1996. The site is located far away from large industrialized and urban
Characteristics of the main advection pathways
Fig. 1 shows the centroids (representative trajectories) of the 8 clusters obtained at 2200 m asl and the relative percentage frequency of each flow pattern over the whole 1998–2011 period, together with the mean height evolution over time and the monthly variation of the frequency of the air flow pathways reaching the receptor site.
Cluster names were chosen based on their region of provenance. Most of the trajectories correspond to westerly flows; in particular, westerly trajectories are
Summary and conclusions
This work focused on finding relationships between the advection pathways and atmospheric composition observed in a long time series of essential climate variables (ECVs) observed at the WMO-GAW station of Mt. Cimone (Italy). Advection pathways were identified by a cluster analysis of back trajectories starting at Mt. Cimone at three different heights; the cluster analysis identified 8 groups at the initiation height of 2200 m, approximately at the height of the station. The results reflect
CRediT authorship contribution statement
E. Brattich: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Validation, Visualization, Writing - original draft, Writing - review & editing. J.A.G. Orza: Conceptualization, Formal analysis, Software, Validation, Visualization, Investigation, Methodology, Writing - original draft, Writing - review & editing. P. Cristofanelli: Data curation, Resources, Project administration, Writing - original draft. P. Bonasoni: Data curation, Resources, Project
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments and Data
The authors would like to gratefully thank Dr. Scott Chambers from ANSTO and an anonymous reviewer for providing constructive comments during the review process, which overall contributed to improve the quality of the manuscript. CAMM Monte Cimone by Italian Air Force and ISAC-CNR are gratefully acknowledged for their precious technical support at the Mt. Cimone station and for the help in the collection of compositional datasets, and in particular for providing data of meteorological and
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