The simultaneous aerosol sampling at two heights in southern Spain may provide valuable information on
the vertical structure of the dust transport from North Africa to the Iberian Peninsula. It also allows the
characterization of the ambient air at two sites with distinct anthropogenic influence. This work presents
the results obtained from the first field campaign of the FRESA project (Impact of dust-laden African air
masses and of stratospheric air masses in the Iberian Peninsula. Role of the Atlas Mountains), performed in
the period July-November 2017 at El Albergue Universitario in Sierra Nevada (2550 m a.s.l.) and the city of
Granada (650 m a.s.l.). The two sites were instrumented with a low-volume sampler with PM10 inlet for
daily sampling and mass and chemical composition characterization, a high-volume sampler for total
suspended particles (TSP) for weekly sampling and radionuclide activity determination, and with a GRIMM
365 optical particle counter that provides continuously the aerosol size distribution.
In Sierra Nevada, daily PM10 concentrations ranged from 0 to 104 \ub5g m-3 depending mainly on the origin
and features of the air masses that reach this high-elevation site. Levels overpassed 50 \ub5g m-3 during 5 days
and were lower than 20 \ub5g m-3 for 90 days over a total of 124 sampling days. The impact of dust-laden
African airflows at this site is particularly intense as dust is transported in most of the cases within welldefined
low mid-tropospheric layers. The associated episodic concentrations exceeded 40 \ub5g m-3
. In turn,
clean subsiding airflows associated to the polar jet strongly reduced concentrations.
In Granada, daily PM10 concentrations are moderately high with values generally between 20 and 40 \ub5g m3
before the first snows fall over Sierra Nevada. Levels were over 50 \ub5g m-3 on 2 days and below 20 \ub5g m-3
on 32 days over a total of 103 sampling days. The highest concentrations do not reach the peaks found at
Sierra Nevada. The impact of anthropogenic aerosols of local origin, which are accumulated during highstability
conditions, and also of regional origin (both from the Mediterranean and from the SW Iberian
Peninsula) is significant. The contribution of African dust outbreaks superimposes to background ambient
air concentrations and in some events the increase is observed one day after the African dust impacts over
Sierra Nevada. After the first snowfall of the year in Sierra Nevada, there\u2019s a change in wind regime, PM10
levels drop and only accumulation periods or African dust events are able to increase concentrations at
levels similar to those detected before.
The diurnal pattern of PM10 and particle number concentrations (both of the fine and the coarse fractions)
at Granada presents the two typical peaks in the morning and evening; there\u2019s one peak at 16 Local Time -
LT in Sierra Nevada. Particle concentrations are smaller in the colder period than in the warmer one,
primarily due to the much higher concentrations in the coarse fraction (> 1 \ub5m) in summertime. However,
the fine fraction (submicronic) presents a stronger morning peak (centered about 10 LT) in the colder
period and also concentrations are slightly higher at about 20-21 LT. Trajectories (calculated 96-hour backward in time starting at 00, 06, 12 and 18 UTC with HYSPLIT using ERAInterim
data of 0.5 degree resolution) show that air flows are quite often decoupled at the two altitudes.
Dust-laden African flows reached Sierra Nevada on 33 days; of these, dust was advected poleward over the
Atlas near the Algerian-Moroccan border on 19 days and on the remaining 14 it followed a pathway over
the Atlantic close to the Moroccan coast. However, only on 3 days large-scale flows of African origin
reached Granada. In turn, airflows reaching Granada passed previously over the western Mediterranean
Sea on 32 days while only on 3 days Mediterranean flows reached Sierra Nevada. Moreover, the total
residence time over North Africa of the air parcels reaching Sierra Nevada amounts 9474 hours, and during
the corresponding days the total residence time over the Mediterranean of the parcels reaching Granada is
10,055 hours. In contrast, the residence time of the air masses arriving to Granada after residing over Africa
is only of 56 hours. This implies that during African dust outbreaks the air masses reaching the study area at
the lowest levels do not have African origin but have resided over the Mediterranean, and it is in
accordance with Cabello et al. (2017) for M\ue1laga and Orza et al. (submitted) for the whole Spanish
Mediterranean. Chemical analysis of the PM10 samples (in progress) will provide the first direct
experimental evidence of this fact.
The identification of episodes was done by analysing the fine and coarse fractions registered with the
Grimm OPCs at the two sites and by the aerosol optical depth (AOD) and Angstr\uf6m exponent (AE) from the
AERONET station of IISTA-CEAMA in Granada, in combination with back-trajectories. The low availability of
satellite information (Dust RGB product and MODIS aerosol properties) is a major limitation at the study
area.
A number of distinct episodes can be identified: strong African dust impact at Sierra Nevada while Granada
is heavily polluted by anthropogenic aerosols (1 August 2017); strong African dust impact at Sierra Nevada
which is observed the following day in Granada (15-16 August 2017); African dust outbreaks impacting
simultaneously both sites (e.g., 25 November 2017); episode of accumulation of pollutants at Granada
whilst very low concentrations are registered at Sierra Nevada (19-24 November 2017); impact of remote
fires at both sites (27 July and 9 September 2017) and fair air quality at both sites (e.g., 16 September
2017).
Differences in the onset and duration of the African dust episodes are found between the two sites as well
as between the surface and the columnar measurements
