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. Observations of aerosol and trace gases in the remote troposphere are vital toquantify background concentrations and identify long-term trends in atmosphericcomposition on large spatial scales.Measurements made at high altitude are often used to study free-troposphericair; however such high-altitude sites can be influenced by boundary layer airmasses.Thus, accurate information on air mass origin and transport pathways to high-altitude sites is required.Here we present a new method, based on the source–receptor relationship (SRR)obtained from backwards WRF-FLEXPART simulations and a k-means clusteringapproach, to identify source regions of air masses arriving at measurementsites.Our method is tailored to areas of complex terrain and to stations influencedby both local and long-range sources.We have applied this method to the Chacaltaya (CHC) GAW station (5240 m a.s.l.;16.35∘ S, 68.13∘ W) for the 6-month duration of the “Southern Hemisphere high-altitude experiment on particle nucleation and growth” (SALTENA) to identifywhere sampled air masses originate and to quantify the influence of the surfaceand the free troposphere.A key aspect of our method is that it is probabilistic, and for each observationtime, more than one air mass (cluster) can influence the station, and thepercentage influence of each air mass can be quantified.This is in contrast to binary methods, which label each observation time asinfluenced by either boundary layer or free-troposphere air masses.Air sampled at CHC is a mix of different provenance.We find that on average 9 % of the air, at any given observation time,has been in contact with the surface within 4 d prior to arriving at CHC.Furthermore, 24 % of the air has been located within the first 1.5 km above ground level (surface included).Consequently, 76 % of the air sampled at CHC originates from the free troposphere.However, pure free-tropospheric influences are rare, and often samples areconcurrently influenced by both boundary layer and free-tropospheric airmasses.A clear diurnal cycle is present, with very few air masses that have been incontact with the surface being detected at night.The 6-month analysis also shows that the most dominant air mass (cluster)originates in the Amazon and is responsible for 29 % of the sampled air.Furthermore, short-range clusters (origins within 100 km of CHC) have hightemporal frequency modulated by local meteorology driven by the diurnal cycle,whereas the mid- and long-range clusters' (&gt;200 km) variability occurs ontimescales governed by synoptic-scale dynamics.To verify the reliability of our method, in situ sulfate observations from CHCare combined with the SRR clusters to correctly identify the (pre-known) sourceof the sulfate: the Sabancaya volcano located 400 km north-west from thestation.

Identifying source regions of air masses sampled at the tropical high-altitude site of Chacaltaya using WRF-FLEXPART and cluster analysis