Transport of Po Valley aerosol pollution to the northwestern Alps – Part 2: Long-term impact on air quality

Abstract

Abstract. This work evaluates the impact of trans-regional aerosol transport from the Po basin on particulate matter levels (PM10) and physico-chemical characteristics in the northwestern Alps. To this purpose, we exploited a multi-sensor, multi-platform database over a 3-year period (2015–2017) accompanied by a series of numerical simulations. The experimental setup included operational (24/7) vertically resolved aerosol profiles by an automated lidar ceilometer (ALC), vertically integrated aerosol properties by a Sun/sky photometer, and surface measurements of aerosol mass concentration, size distribution and chemical composition. This experimental set of observations was then complemented by modelling tools, including numerical weather prediction (NWP), trajectory statistical (TSM) and chemical transport (CTM) models, plus positive matrix factorisation (PMF) on both the PM10 chemical speciation analyses and particle size distributions. In a first companion study, we showed and discussed through detailed case studies the 4-D phenomenology of recurrent episodes of aerosol transport from the polluted Po basin to the northwestern Italian Alps. Here we draw more general and statistically significant conclusions on the frequency of occurrence of this phenomenon, and on the quantitative impact of this regular, wind-driven, aerosol-rich “atmospheric tide” on PM10 air-quality levels in this alpine environment. Based on an original ALC-derived classification, we found that an advected aerosol layer is observed at the receptor site (Aosta) in 93 % of days characterized by easterly winds (i.e. from the Po basin) and that the longer the time spent by air masses over the Po plain the higher this probability. Frequency of these advected aerosol layers was found to be rather stable over the seasons with about 50 % of the days affected. Duration of these advection events ranges from few hours up to several days, while aerosol layer thickness ranges from 500 up to 4000 m. Our results confirm this phenomenon to be related to non-local emissions, to act at the regional scale and to largely impact both surface levels and column-integrated aerosol properties. In Aosta, PM10 and aerosol optical depth (AOD) values increase respectively up to factors of 3.5 and 4 in dates under the Po Valley influence. Pollution transport events were also shown to modify the mean chemical composition and typical size of particles in the target region. In fact, increase in secondary species, and mainly nitrate- and sulfate-rich components, were found to be effective proxies of the advections, with the transported aerosol responsible for at least 25 % of the PM10 measured in the urban site of Aosta, and adding up to over 50 µg m−3 during specific episodes, thus exceeding alone the EU established daily limit. From a modelling point of view, our CTM simulations performed over a full year showed that the model is able to reproduce the phenomenon, but markedly underestimates its impact on PM10 levels. As a sensitivity test, we employed the ALC-derived identification of aerosol advections to re-weight the emissions from outside the boundaries of the regional domain in order to match the observed PM10 field. This simplified exercise indicated that an increase in such “external” emissions by a factor of 4 in the model is needed to halve the model PM10 maximum deviations and to significantly reduce the PM10 normalised mean bias forecasts error (from −35 % to 5 %).

Transport of Po Valley aerosol pollution to the northwestern Alps. Part 1: phenomenology

“…Although the available dataset is rather short, the OC/EC ratio during the event almost doubles (values of 6.1-7.2) compared to the value before (3.7) and after (3.3) the event. This increase of the OC fraction during transport episodes is confirmed by the long-term analysis (Diémoz et al, 2018).…”

“…Apart from one spike (80 µg m −3 ) on 26 August of local origin, the concentrations measured in Aosta-Downtown are generally higher during daytime compared to the night and do not show any noticeable increase corresponding to the arrival of the layer. This feature, however, can be connected to the fact that mass loss occurs in TEOM due to secondary aerosol volatility, as also found by Diémoz et al (2018) by comparing the daily PM 10 cycle from this instrument and the Fidas OPC in Aosta-Saint Christophe. Besides, FARM estimates at the surface are again lower than measurements (-60%, on average).…”

“…The case studies were also selected among those showing long sequences of days characterised by the recurrent appearance of a thick aerosol layer from the ALC, to emphasise the periodicity of the phenomenon. Indeed, as explained in more detail by Diémoz et al (2018), the elevated aerosol layer can be observed very frequently, i.e. at least 40-50% of the days, depending on the season.…”

“…. Samples collected on quartz fibre filters by a co-located Micro-PNS automatic lowvolume sampling system (10 µm cutoff diameter, 2.3 m 3 h −1 ) are analysed alternatively for elemental/organic carbon (EC/OC, 4/10 days) and for metals (6/10 days, not used in the present study, but discussed by Diémoz et al (2018)). The carbonaceous aerosol mass is determined with a Sunset Laboratory Inc. instrument (Birch and Cary, 1996) on portions of 1 cm 2 punches using a thermal-optical transmission (TOT) method with transmission correction for the split point and following the EUSAAR-2 protocol (Cavalli et al, 2010), according to the EN 16909:2017.…”

Solar UV Irradiance in a Changing Climate: Trends in Europe and the Significance of Spectral Monitoring in Italy

“…OMI underestimated the measured UV index during the first half of the year, with a peak underestimation in April and May, and overestimated during the second half of the year. The role of aerosol in Aosta was not as significant as in Rome, although under particular conditions highly absorbing aerosols may either be transported to the Aosta Valley or produced locally [185,186]. The absorbing effect of these aerosols cannot be easily described by the model or the satellite algorithm, and possibly is responsible for, at least part of, the overestimation of the UV index in summer, autumn, and winter.…”