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A reliable identification of desert dust (DD) episodesover north-central Spain is carried out based on the AErosol RObotic NETwork(AERONET) columnar aerosol sun photometer (aerosol optical depth, AOD, andÅngström exponent, α) and European Monitoring and EvaluationProgramme (EMEP) surface particulate-mass concentration (PMx, x = 10,2.5, and 2.5–10 µm) as the main core data. The impact of DD onbackground aerosol conditions is detectable by means of aerosol loadthresholds and complementary information provided by HYSPLIT (Hybrid SingleParticle Lagrangian Integrated Trajectory Model) air mass back trajectories,MODIS (Moderate Resolution Imaging Spectroradiometer) images, forecastaerosol models, and synoptic maps, which have been carefully reviewed by ahuman observer for each day included in the DD inventory. This identificationmethod allows the detection of low and moderate DD intrusions and also ofmixtures of mineral dust with other aerosol types by means of the analysis ofα. During the period studied (2003–2014), a total of 152 DD episodescomposed of 418 days are identified. Overall, this means ∼ 13 episodesand ∼ 35 days per year with DD intrusion, representing9.5 % days year−1. During the identified DD intrusions, 19 dailyexceedances over 50 µg m−3 are reported at the surface. Theoccurrence of DD event days during the year peaks in March and June, with amarked minimum in April and lowest occurrence in winter. A large interannualvariability is observed showing a statistically significant temporaldecreasing trend of ∼ 3 days year−1. The DD impact on the aerosolclimatology is addressed by evaluating the DD contribution in magnitude andpercent (in brackets) for AOD, PM10, PM2.5, and PM2.5 − 10,obtaining mean values of 0.015 (11.5 %), 1.3 µg m−3(11.8 %), 0.55 µg m−3 (8.5 %) and0.79 µg m−3 (16.1 %), respectively. Annual cycles of the DDcontribution for AOD and PM10 present two maxima – one in summer (0.03and 2.4 µg m−3 for AOD in June and PM10 in August) andanother in March (0.02 for AOD and 2.2 µg m−3 for PM10)– both displaying a similar evolution with exceptions in July and September.The seasonal cycle of the DD contribution to AOD does not follow the patternof the total AOD (close to a bell shape), whereas both PM10 cycles(total and DD contribution) are more similar to each other in shape, with anexception in September. The interannual evolution of the DD contribution toAOD and PM10 has evidenced a progressive decrease. This decline in thelevels of mineral dust aerosols can explain up to 30 % of the total aerosolload decrease observed in the study area during the period 2003–2014. Therelationship between columnar and surface DD contribution shows a correlationcoefficient of 0.81 for the interannual averages. Finally, synopticconditions during DD events are also analysed, observing that the northAfrican thermal low causes most of the events ( ∼  53 %). The resultspresented in this study highlight the relevance of the area studied since itcan be considered representative of the clean background in the westernMediterranean Basin where DD events have a high impact on aerosol loadlevels

atmospheric chemistry and physics

Inventory of African desert dust events in the north-central Iberian Peninsula in 2003–2014 based on sun-photometer–AERONET and particulate-mass–EMEP data