Identifying source regions of air masses sampled at the tropical high-altitude site of Chacaltaya using WRF-FLEXPART and cluster analysis
Authors: Aliaga Diego , Sinclair Victoria A. , Andrade Marcos , Artaxo Paulo , Carbone Samara , Kadantsev Evgeny , Laj Paolo , Wiedensohler Alfred , Krejci Radovan , Bianchi Federico 
Published: November 2021

Abstract. Observations of aerosol and trace gases in the remote troposphere are vital to
quantify background concentrations and identify long-term trends in atmospheric
composition on large spatial scales.
Measurements made at high altitude are often used to study free-tropospheric
air; however such high-altitude sites can be influenced by boundary layer air
masses.
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 clustering
approach, to identify source regions of air masses arriving at measurement
sites.
Our method is tailored to areas of complex terrain and to stations influenced
by 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 identify
where sampled air masses originate and to quantify the influence of the surface
and the free troposphere.
A key aspect of our method is that it is probabilistic, and for each observation
time, more than one air mass (cluster) can influence the station, and the
percentage influence of each air mass can be quantified.
This is in contrast to binary methods, which label each observation time as
influenced 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 are
concurrently influenced by both boundary layer and free-tropospheric air
masses.
A clear diurnal cycle is present, with very few air masses that have been in
contact 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 high
temporal frequency modulated by local meteorology driven by the diurnal cycle,
whereas the mid- and long-range clusters' (>200 km) variability occurs on
timescales governed by synoptic-scale dynamics.
To verify the reliability of our method, in situ sulfate observations from CHC
are combined with the SRR clusters to correctly identify the (pre-known) source
of the sulfate: the Sabancaya volcano located 400 km north-west from the
station.