Vertical distribution of aerosol optical properties in the Po Valley during the 2012 summer campaigns.

Studying the vertical distribution of aerosol particles physical and chemical properties in the troposphere is essential to understand the relative importance of local emissions processes vs. long-range transport on column-integrated aerosol properties (e.g. the aerosol optical depth (AOD), affecting regional climate) as well as on the aerosol burden and its impacts on air quality at the ground. The main objective of this paper is to investigate the transport of desert dust in the middle troposphere and its intrusion into the planetary boundary layer (PBL) over the Po Valley (Italy), a region considered one of the major European pollution hot-spots for the frequency of particulate matter (PM) limit values exceedances. Events of mineral aerosol uplift from local (soil) sources and phenomena of hygroscopic growth at the ground are also investigated, possibly affecting the PM concentration in the region as well. During the PEGASOS 2012 field campaign, an integrated observing-modeling system was set up based on near-surface measurements (particle concentration and chemistry), vertical profiling (backscatter coefficient profiles from LiDAR and radiosoundings) and Lagrangian air masses transport simulations by FLEXPART model. Measurements were taken at the San Pietro Capofiume supersite (44°39' N, 11°37'0 E, 11 m a.s.l.), located in a rural area relatively close to some major urban and industrial emissive areas in the Po Valley. Mt. Cimone (44°12' N, 10°42' E, 2165 m a.s.l.) WMO/GAW station observations are also included in the study to characterize regional-scale variability. Results show that, in the Po Valley, aerosol is detected mainly below 2000 m a.s.l. with a prevalent occurrence of non-depolarizing particles (>50 % throughout the campaign) and a vertical distribution modulated by the PBL daily evolution. Two intense events of mineral dust transport from Northern Africa (19-21 June and 29 June to 2 July) are observed, with layers advected mainly above 2000 m height, but lately sinking and mixing in the PBL. As a consequence, a non-negligible occurrence of mineral dust is observed close to the ground (~7 % of occurrence during a 1-month campaign). The observations unambiguously show Saharan dust layers intruding the Po Valley summertime mixing layer and directly affecting the aerosol concentrations near the surface. Finally, LiDAR observations indicate also strong variability in aerosol on shorter timescales (hourly) highlighting: a) events of hygroscopic growth of anthropogenic aerosol, visible in shallow layers of low depolarization near the ground. Such events are identified during early morning hours at high relative humidity (RH) conditions (RH > 80 %). The process is observed concurrently with high PM1 nitrate concentration (up to 15 μg cm^-3), hence mainly explicable by deliquescence of fine anthropogenic particles, and during mineral dust intrusion episodes, when water condensation on dust particles could instead represent the dominant contribution; b) frequent events (mean diurnal occurrence of ~22 % during the whole campaign) of rapid uplift of mineral depolarizing particles in afternoon-evening hours up to 2000 m a.s.l. height. The origin of such particles cannot be directly related to long-range transport events, being instead likely linked to processes of soil particles resuspension from agricultural lands. [ABSTRACT FROM AUTHOR]