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In this study, we present and analyze the first continuous timeseries of relevant aerosol precursor vapors from the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. These precursor vapors include sulfuric acid (SA), methanesulfonic acid (MSA), and iodic acid (IA). We use FLEXPART simulations, inverse modeling, sulfur dioxide (SO2) mixing ratios, and chlorophyll-a (chl-a) observations to interpret the seasonal variability of the vapor concentrations and identify dominant sources. Our results show that both natural and anthropogenic sources are relevant for the concentrations of SA in the Arctic, but anthropogenic sources associated with Arctic haze are the most prevalent. MSA concentrations are an order of magnitude higher during polar day than during polar night due to seasonal changes in biological activity. Peak MSA concentrations were observed in May, which corresponds with the timing of the annual peak in chl-a concentrations north of 75°N. IA concentrations exhibit two distinct peaks during the year: a dominant peak in spring and a secondary peak in autumn, suggesting that seasonal IA concentrations depend on both solar radiation and sea ice conditions. In general, the seasonal cycles of SA, MSA, and IA in the central Arctic Ocean are related to sea ice conditions, and we expect that changes in the Arctic environment will affect the concentrations of these vapors in the future. The magnitude of these changes and the subsequent influence on aerosol processes remains uncertain, highlighting the need for continued observations of these precursor vapors in the Arctic. [ABSTRACT FROM AUTHOR]

The Arctic environment is rapidly changing due to accelerated warming in the region. The warming trend is driving a decline in sea ice extent, which thereby enhances feedback loops in the surface energy budget in the Arctic. Arctic aerosols play an important role in the radiative balance, and hence the climate response, in the region; yet direct observations of aerosols over the Arctic Ocean are limited. In this study, we investigate the annual cycle in the aerosol particle number size distribution (PNSD), particle number concentration (PNC), and black carbon (BC) mass concentration in the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. This is the first continuous, year-long dataset of aerosol PNSD ever collected over the sea ice in the central Arctic Ocean. We use a k-means cluster analysis, FLEXPART simulations, and inverse modeling to evaluate seasonal patterns and the influence of different source regions on the Arctic aerosol population. Furthermore, we compare the aerosol observations to land-based sites across the Arctic, using both long-term measurements and observations during the year of the MOSAiC expedition (2019 - 2020), to investigate interannual variability and to give context to the aerosol characteristics from within the central Arctic. Our analysis identifies that, overall, the central Arctic exhibits typical seasonal patterns of aerosols, including anthropogenic influence from Arctic Haze in winter and secondary aerosol processes in summer. The seasonal pattern corresponds with the global radiation, surface air temperature, and the timing of sea ice melting/freezing, which drives changes in transport patterns and secondary aerosol processes. In winter, the Norilsk region in Russia/Siberia was the dominant source of Arctic Haze signal in the PNSD and BC observations, which contributed to higher accumulation mode PNC and BC mass concentration in the central Arctic than at land-based observatories. We also show that the wintertime Arctic Oscillation (AO) phenomenon, which was reported to achieve a record-breaking positive phase during January - March 2020, explains the unusual timing and magnitude of Arctic Haze across the Arctic region compared to longer-term observations. In summer, the PNC of nucleation and Aitken mode aerosol is enhanced, but concentrations were notably lower in the central Arctic over the ice pack than at land-based sites further south. The analysis presented herein provides a current snapshot of Arctic aerosol processes in an environment that is characterized by rapid changes, which will be crucial for improving climate model predictions, understanding linkages between different environmental processes, and investigating the impacts of climate change in future Arctic aerosol studies. [ABSTRACT FROM AUTHOR]

The Asian Monsoon Anticyclone (AMA) represents the wettest region in the lower stratosphere (LS) and is the key contributor to the global annual maximum in LS water vapour. While the AMA wet pool is linked with persistent convection in the region and horizontal confinement of the anticyclone, there remain ambiguities regarding the role of tropopause-overshooting convection in maintaining the regional LS water vapour maximum. This study tackles this issue using a unique set of observations from onboard the high-altitude M55-Geophysica aircraft deployed in Nepal in Summer 2017 within the EU StratoClim project. We use a combination of airborne measurements (water vapour, ice water, water isotopes, cloud backscatter) together with ensemble trajectory modeling coupled with satellite observations to characterize the processes controlling water vapour and clouds in the confined lower stratosphere (CLS) of AMA. Our analysis puts in evidence the dual role of overshooting convection, which may lead to hydration or dehydration depending on the synoptic-scale tropopause temperatures in AMA. We show that all of the observed CLS water vapour enhancements are traceable to convective events within AMA and furthermore bear an isotopic signature of the overshooting process. A surprising result is that the plumes of moist air with mixing ratios nearly twice the background level can persist for weeks whilst recirculating within the anticyclone, without being subject to irreversible dehydration through ice settling. Our findings highlight the importance of convection and recirculation within AMA for the transport of water into the stratosphere. [ABSTRACT FROM AUTHOR]

During the monsoon season of the year 2017 the airborne StratoClim mission took place in Kathmandu, Nepal with eight mission flights of the M-55 Geophysica in the upper troposphere/lower stratosphere (UT/LS) of the Asian Monsoon Anticyclone (AMA) over northern India, Nepal and Bangladesh. More than hundred events of New Particle Formation (NPF) were observed. In total, more than two hours of flight time were spent under NPF conditions as indicated by the abundant presence of ultrafine aerosols, i.e. with particle diameters dp smaller than 15 nm, which were in-situ detected by means of condensation nuclei counting techniques. Mixing ratios of ultrafine particles (nuf) of up to ~ 50000 mg-1 were measured at heights of 15-16 km (θ ≈ 370 K). NPF was most frequently observed at ~ 12-16 km altitude (θ ≈ 355-380 K) and mainly below the tropopause, but nuf remained elevated (~ 300-2000 mg-1) up to altitudes of ~ 17.5 km (θ ≈ 400 K) while under NPF conditions the fraction (f) of submicrometre-sized non-volatile particle residues (dp > 10 nm) remained below 50%. At ~ 12-14 km (θ ≈ 355-365 K) the minimum of f (< 15%) was found, and underneath the median f generally remains below 25%. The persistence of particles at ultrafine sizes is limited to a few hours, mainly due to coagulation, as demonstrated by a numerical simulation. Thus, NPF is detectable only for a limited period of time and the frequency of NPF events observed during StratoClim 2017 underlines the importance of the UT/LS within the AMA as a source region for aerosols. The effective in-situ production of aerosol in the tropopause region and subsequent coagulation and/or condensation likely contribute to the formation and maintenance of the Asian Tropopause Aerosol Layer (ATAL). The observed abundance of NPF-produced ultrafine particles within the AMA is not unambiguously attributable to (a) specific source regions in the boundary layer (according to backward trajectory analyses), or (b) the direct supply with precursor material by convective updraught (from correlations of NPF with carbon monoxide), or (c) the recent release of NPF-capable material from the convective outflow (according to air mass transport times in the TTL). Temperature anomalies of more than one Kelvin, as observed with a wavelength of ~ 90 km during a level flight over several hours, could be associated with the NPF process as a possible cause for the increasing supersaturation of the NPF precursor system. The frequency of NPF observed during StratoClim 2017 exceeds all previous NPF detections with COPAS at TTL levels over Brazil, Northern Australia, or West Africa. The observed NPF abundance and productivity of fresh aerosols during StratoClim 2017 indicates that NPF is capable of directly affecting the extent and persistence of the ATAL. [ABSTRACT FROM AUTHOR]

A polarization diversity elastic backscatter lidar has been deployed in the equatorial island of Palau in February and March 2016, in the framework of the EU Stratoclim project. The system operated unattended in the Atmospheric Observatory of Palau Island from 15 February to 25 March 2016, during nighttime. Each lidar profile extends from the ground to 30 km height. Here the dataset is presented and discussed in terms of the temperature structure of the UTLS, obtained from co-located radiosondings. During the campaign, several high altitude clouds were observed, peaking approximately 3 km below the Cold Point Tropopause (CPT) located above 17 km. Their occurrence was associated with cold anomalies in the Upper Troposphere (UT). Conversely, when warm UT anomalies occurred, the presence of cirrus was restricted to a 5 km thick layer centered 5 km below the CPT. Thin and subvisible cirrus were frequently detected close to the CPT. The particle depolarization ratios of these cirrus was generally lower than the values detected in the UT clouds. CPT cirrus occurrence showed a correlation with cold anomalies likely triggered by stratospheric wave activity penetrating the UT. The back trajectories study revealed a thermal and convective history compatible with the convective outflow formation for most of the cirrus clouds, suggesting that the majority of air masses related to the clouds had encountered convection in the past and had reached the minimum temperature during its transport in less than 48 hours before the observation. A subset of SVC, with low depolarization and high lidar ratio and with no sign of significative recent uplifting, may be originated in situ. [ABSTRACT FROM AUTHOR]

In July 2017 three research flights circumnavigating the megacity of London were conducted as a part of the STANCO training school for students and early career researchers organised by EUFAR (European Facility for Airborne Research). Measurements were made from the UK's Facility for Airborne Atmospheric Measurements (FAAM) BAe-146-301 atmospheric research aircraft with the aim to sample, characterise and quantify the impact of megacity outflow pollution on air quality in the surrounding region. Conditions were extremely favourable for airborne measurements, and all three flights were able to observe clear pollution events along the flight path. A small change in wind direction provided sufficiently different air mass origins over the 2 d such that a distinct pollution plume from London, attributable marine emissions and a double-peaked dispersed area of pollution resulting from a combination of local and transported emissions were measured. We were able to analyse the effect of London emissions on air quality in the wider region and the extent to which local sources contribute to pollution events. The background air upwind of London was relatively clean during both days; concentrations of CO were 88-95 ppbv, total (measured) volatile organic compounds (VOCs) were 1.6-1.8 ppbv and NO[sub x] was 0.7-0.8 ppbv. Downwind of London, we encountered elevations in all species with CO>100 ppbv, VOCs 2.8-3.8 ppbv, CH[sub 4]>2080 ppbv and NO[sub x]>4 ppbv, and peak concentrations in individual pollution events were higher still. Levels of O[sub 3] were inversely correlated with NO[sub x] during the first flight, with O[sub 3] concentrations of 37 ppbv upwind falling to ~ 26 ppbv in the well-defined London plume. Total pollutant fluxes from London were estimated through a vertical plane downwind of the city. Our calculated CO[sub 2] fluxes are within the combined uncertainty of those estimated previously, but there was a greater disparity in our estimates of CH[sub 4] and CO. On the second day, winds were lighter and downwind O[sub 3] concentrations were elevated to ~ 39-43 ppbv (from ~ 32 to 35 ppbv upwind), reflecting the contribution of more aged pollution to the regional background. Elevations in pollutant concentrations were dispersed over a wider area than the first day, although we also encountered a number of clear transient enhancements from local sources. This series of flights demonstrated that even in a region of megacity outflow, such as the south-east of the UK, local fresh emissions and more distant UK sources of pollution can all contribute substantially to pollution events. In the highly complex atmosphere around a megacity where a high background level of pollution mixes with a variety of local sources at a range of spatial and temporal scales and atmospheric dynamics are further complicated by the urban heat island, the use of pollutant ratios to track and determine the ageing of air masses may not be valid. The individual sources must therefore all be well-characterised and constrained to understand air quality around megacities such as London. Research aircraft offer that capability through targeted sampling of specific sources and longitudinal studies monitoring trends in emission strength and profiles over time. [ABSTRACT FROM AUTHOR]

The StratoClim airborne campaign took place in Nepal from 27 July to 10 August 2017 to document the physical and chemical properties of the South Asian Upper Troposphere Lower Stratosphere (UTLS) during the Asian Summer Monsoon (ASM). In the present paper, simulations with the Meso-NH cloud-chemistry model at a horizontal resolution of 15 km are performed over the Asian region to characterize the impact of monsoon deep convection on the composition of Asian Monsoon Anticyclone (AMA) and on the formation of the Asian Tropopause Aerosol Layer (ATAL) during the StratoClim campaign. StratoClim took place during a break phase of the monsoon with an intense convective activity over south China and Sichuan. Comparisons between Brightness Temperature (BT) at 10.8 microns observed by satellite sensors and simulated by Meso-NH highlight the ability of the model to correctly reproduce the life cycle of deep convective clouds. Comparison between CO and O3 concentrations from Meso-NH and airborne observations (StratoClim and IAGOS) demonstrates that the model captures most of the observed variabilities. Nevertheless, for both gases, the model tends to overestimate the concentrations and misses some thin CO plumes related to local convective events probably because of a too coarse resolution, but the convective uplift of pollution is very well captured by the model. We have therefore focused on the impact of Sichuan convection on the AMA composition. A dedicated sensitivity simulation showed that the 7 August convective event brought large amounts of CO deep into the AMA and even across the 380 K isentropic level located at 17.8 km. This Sichuan contribution enhanced the CO concentration by ~15 % to reach more than 180 ppbv over a large area around 15 km height. Noteworthy, Meso-NH captures the impact of the diluted Sichuan plume on the CO concentration during a StratoClim flight south of Kathmandu highlighting its ability to reproduce the transport pathway of Sichuan pollution. According to the model, primary organic aerosol and black carbon particles originating from Sichuan are transported following the same pathway as CO. The large particles are heavily scavenged within the precipitating part of the convective clouds but remain the most important contributor to the particle mass in the AMA. Over the whole AMA region, the 7 August convective event resulted in a 0.5% increase of CO over the 10-20 km range that lasted about 2 days. The impact of pollution uplift from three regions (India, China and Sichuan) averaged over the first 10 days of August has also been evaluated with sensitivity simulations. Even during this monsoon break phase, the results confirm the predominant role of India relative to China with respective contributions of 11 and 7 % to CO in the 10[sup -1]5 km layer. Moreover, during this period a large part (35 %) of the Chinese contribution comes from the Sichuan basin alone. [ABSTRACT FROM AUTHOR]

We present the first high resolution measurements of pollutant trace gases in the Asian Summer Monsoon Upper Troposphere and Lowermost Stratosphere (UTLS) from the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) during the StratoClim (Stratospheric and upper tropospheric processes for better climate predictions) campaign with base in Kathamandu, Nepal, 2017. Measurements of peroxyacetyl nitrate (PAN), acetylene (C2H2), and formic acid (HCOOH) show strong local enhancements up to altitudes of 16 km. More than 500 pptv of PAN, more than 200 pptv of C2H2, and more than 200 pptv of HCOOH are observed. An observed local maximum of PAN and C2H2 at altitudes up to 18 km, reaching to the lowermost stratosphere, instead has been transported for a longer time. A local minimum of HCOOH is correlated with a maximum of ammonia (NH3), which suggests different wash out efficiencies of these species in the same air masses. To study the influence of convective transport to the measured pollution trace gas occurrences in detail, a trajectory analysis of the models ATLAS and TRACZILLA examined backward trajectories, starting at geolocations of GLORIA measurements with enhanced pollution trace gases. Both trajectory schemes implemented advanced techniques for detection of convective events. These convective events along trajectories leading to GLORIA measurements with enhanced pollutants are located close to regions, where satellite measurements by OMI show enhanced tropospheric columns of nitrogen dioxide (NO2) in the days prior to the observation. As an application of these highly resolved measurements, a comparison to the atmospheric models CAMS and EMAC is performed. It is demonstrated that these simulation results are able to reproduce large scale structures of the pollution trace gas distributions if the convective influence on the measured air masses is captured by the meteorological fields used by these simulations. Both models do not have sufficient horizontal resolution to capture all the convective events that are necessary to reproduce the fine structures measured by GLORIA. To investigate the influence of the strength of non-methane volatile organic compounds (NMVOCs) emissions in the EMAC model, sensitivity studies with artificially enhanced NMVOC emissions are performed. With these enhanced emissions, the simulation results succeed to reproduce the measured peak values of the pollutants, but do not improve the comparison of spatial distributions. [ABSTRACT FROM AUTHOR]

The StratoClim stratospheric aircraft campaign took place in summer 2017 in Nepal (the 27th of July-10th of August) and provided a wide dataset of observations of air composition inside the Asian Monsoon Anticyclone. In the framework of this project, with the purpose of modelling the injection of pollutants and natural compounds into the stratosphere, we performed a series of diffusive back-trajectories runs along the flights' tracks. The availability of in-situ measurements of trace gases has been exploited to evaluate the capability of the trajectory system to reproduce the transport in the Upper Troposphere-Lower Stratosphere (UTLS) region. The diagnostics of the convective sources and mixing in the air parcel samples have been derived by integrating the trajectories output with high-resolution observations of cloud tops from the Meteosat Second Generation (MSG1) and Himawari geostationary satellites. Back-trajectories have been calculated using meteorological fields from European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA-Interim and ERA-5) at 3 h and 1 h resolution, using both kinematic and diabatic vertical motion. The comparison among the different trajectory runs shows, in general, a higher consistency with observed data, as well as a better agreement between the diabatic and kinematic version, when using ERA-5 based runs with respect to ERA-Interim. Overall, a better capacity in reproducing the pollution features is finally found in the diabatic version of the ERA-5 runs. Adopting this setting for the analysis, a large variety of transport conditions have been individuated during the 8 flights of the campaign. The larger influence by convective injections is found from the continental sources of China and India. Only a small contribution appears to be originated from maritime regions, in particular the South Pacific and the Bay of Bengal that, unexpectedly, was not particularly active during the period of the campaign. Thin filamentary structures of polluted air, characterized by peaks in CO, are observed, mostly associated with young convective air (age less than a few days) and a predominant South-China origin. Observed air from continental India, on the contrary, is often linked to a lower concentration of the trace gas and to air masses that recirculated within the anticyclone for 10 to 20 days. Vertical stratification in the age of air is observed: up to 15 km, the age is less than 3 days and these fresh air masses constitute almost the totality of the air composition. A transition layer is then individuated between 15 km and 17 km, where the convective contribution is still dominant and the ages vary between one and two weeks. Above this level, the mean age of the air sampled by the aircraft is estimated to be 20 days. There, the convective contribution rapidly decreases with height, and finally became negligible around 20 km. [ABSTRACT FROM AUTHOR]

We study the transport pathways from the top of convective clouds to the lower tropical stratosphere during the Asian monsoon, using massive Lagrangian trajectories driven by observed clouds and the two reanalysis ERA-Interim and ERA5 with diabatic and kinematic vertical motions. We find that the upward propagation of convective impact is very similar for the kinematic and diabatic calculations using ERA5 while the two cases strongly differ for ERA-Interim. The separation of descending and ascending motion occurs at a crossover level which is slightly above the all sky zero level of radiative heating rate, except over the Tibetan plateau. The parcels that stay confined within the Asian monsoon anticyclone and reach 380 K are mostly of continental origin while maritime sources are dominating when the whole global 380 K surface is considered. We find that the strong impact of the Tibetan plateau with respect to its share of high clouds is entirely due to its elevated proportion of high clouds above the crossover. We find no trace of a vertical conduit above convection over the Tibetan plateau; parcels are rather entrained into an ascending spiral motion that spans the whole anticyclone. The mean age of parcels with respect to convection exhibits a minimum at the centre of the Asian monsoon anticyclone due to the permanent renewal by fresh convective air and largest values on the periphery as air spirals out. The contrast is reduced by dilution for increasing potential temperature. We find that the confinement above 360 K can be represented, on the average, by a simple 1D process of diabatic advection with loss. The mean loss time is about 13 days and uniform over the range 360 K to 420 K which is to be compared with a total circulation time of two to three weeks around the anticyclone. The vertical dilution is consequently exponential with an e-folding potential temperature scale of 15 K (about 3 km). [ABSTRACT FROM AUTHOR]

In July 2017 three research flights circumnavigating the megacity of London were conducted as a part of the STANCO training school for students and early career researchers organised by EUFAR (European Facility for Aircraft Research). Measurements were made from the UK's Facility for Airborne Atmospheric Measurements (FAAM) BAe-146-301 Atmospheric Research Aircraft with the aim to sample, characterise and quantify the impact of megacity outflow pollution on air quality in the surrounding region. Conditions were extremely favourable for airborne measurements and all three flights were able to observe clear pollution events along the flight path. A small change in wind direction provided sufficiently different airmass origins over the two days such that a distinct pollution plume from London, attributable marine emissions and a double-peaked dispersed area of pollution resulting from a combination of local and transported emissions were measured. We were able to analyse the effect of London emissions on air quality in the wider region and the extent to which local sources contribute to pollution events. The background air upwind of London was relatively clean during both days; concentrations of CO were 88-95 ppbv, total (measured) volatile organic compounds (VOCs) were 1.6-1.8 ppbv, and NOx were 0.7-0.8 ppbv. Downwind of London, we encountered elevations in all species with CO > 100 ppbv, VOCs 2.8-3.8 ppbv, CH4 > 2080 ppbv and NOx > 4 ppbv, and peak concentrations in individual pollution events higher still. Levels of O3 were inversely correlated with NOx during the first flight, with O3 concentrations of 37 ppbv upwind falling to ~ 26 ppbv in the well-defined London plume. Mass balance techniques were applied to estimate pollutant fluxes from London. Our calculated CO2 fluxes are within 10 % of those estimated previously, but there was a greater disparity in our estimates of CH4 and CO. On the second day, winds were lighter and downwind O3 concentrations were elevated to ~ 39-43 ppbv (from ~ 32-35 ppbv upwind), reflecting the contribution of more aged pollution to the regional background. Elevations in pollutant concentrations were dispersed over a wider area than the first day, although we also encountered a number of clear spikes from local sources. This series of flights demonstrated that megacity outflow, local fresh emissions and more distant UK sources of pollution all contribute to pollution events in the southeast of the UK. These sources must therefore all be well-characterised and constrained to understand air quality around London. [ABSTRACT FROM AUTHOR]

We show that a fire plume originating at high northern latitudes during the Canadian wildfire event in July/August 2017 reached the tropics, and subsequently the tropical stratosphere via the ascending branch of the Brewer-Dobson Circulation (BDC). The transport from high to low latitudes in the upper troposphere and lowermost stratosphere was mediated by the anticyclonic flow of the Asian monsoon circulation. The fire plume reached the Asian monsoon area in late August/early September, when the Asian Monsoon Anticyclone (AMA) was still in place. While there is no evidence of mixing into the center of the AMA, we show that a substantial part of the fire plume is entrained into the anticyclonic flow at the AMA edge, and is transported into the tropical Upper-Troposphere–Lower-Stratosphere (UTLS), and possibly the Southern Hemisphere particularly following the north-south flow on the eastern side. In the tropics the fire plume is lifted by ~1.5 km per month. Inside the AMA we find evidence of the Asian Tropopause Aerosol Layer (ATAL) in August, doubling background aerosol conditions with a calculated top of the atmosphere shortwave radiative forcing (RF) of −0.05 W/m2. The regional climate impact of the fire signal in the wider Asian monsoon area in September exceeds the impact of the ATAL by a factor of 2–4 and compares to that of a plume coming from an advected moderate volcanic eruption. The stratospheric, trans-continental transport of this plume to the tropics and the related regional climate impact point at the importance of long-range dynamical interconnections of pollution sources. [ABSTRACT FROM AUTHOR]

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]