Elsevier

Atmospheric Environment

Volume 98, December 2014, Pages 394-401
Atmospheric Environment

Fog occurrence and chemical composition in the Po valley over the last twenty years

https://doi.org/10.1016/j.atmosenv.2014.08.080Get rights and content

Highlights

  • A 20 year long database of fog water chemical composition has been assembled.

  • Fog frequency at Bologna airport has decreased over the last three decades.

  • Ionic strength and conductivity trends indicate a reduction of ionic load in fog.

  • SO42− exhibits the highest decrease because of the high decrease of SO2 emissions.

  • A reduction of fog water acidity has been observed over the last two decades.

Abstract

Frequency of fog events together with fog water chemical composition, pH, conductivity and liquid water content have systematically been measured from the end of the 1980's at the field station of San Pietro Capofiume, in the eastern Po Valley, Northern Italy. In agreement with what has been observed in other regions in Europe, fog frequency (visibility < 1 km) has decreased over the last three decades. Ionic strength and conductivity of fog samples also decreased over the period indicating a reduction of the ionic load of the droplets. Specifically, the three major inorganic ions (NH4+, SO42−, NO3), accounting for 86% of the total fog water ionic strength, show a decreasing trend in concentration over the period, which can be linked to the decreasing trend of NH3, SO2 and NOx emissions registered in northern Italy over the same period. Sulphate exhibits the highest relative decrease (76%). Seasonal volume-weighted means of pH show an increasing trend over the observed period. The available data of total water-soluble organic matter concentrations indicate that organic compounds represent a considerable fraction (25% on average) of the total solute mass of fog water. Fog water samples often contain suspended insoluble particles, which were collected by filtering fog water through quartz fibre filters. EC-OC analysis performed on the filters collected over a four-year period, show that the sum of elemental carbon (EC) and water-insoluble organic mass accounts on average for 46%–56% of the total suspended material mass. Insoluble carbonaceous material is composed mainly of organic matter, with EC accounting on average only for 19% of the insoluble carbon.

Introduction

During the fall–winter season, fog is a very frequent phenomenon in the Po Valley (northern Italy) (Fuzzi et al., 1996). Beside the well-known detrimental effects of the reduced visibility on the transportation system, the interaction between fog droplets and atmospheric pollutants may have also an important impact on the local environment, agriculture and human health (Balmes et al., 1989, Butler and Trumble, 2008, Dollard et al., 1983, Fenn et al., 2007, Shigihara et al., 2009, Waldman et al., 1985). The Po Valley area is characterised by the highest population density in Italy, approximately 30% of the Italian population lives there, and by intensive industrial, agricultural and trading activities. Consequently, a high load of pollutants is emitted into the atmosphere. Furthermore the orography of the area, which is surrounded by the Alpine chain to the north and west sides and by the Apennines to the south, frequently favours stagnation of the pollutants in the air. In the fall–winter months, high pressure and clear sky conditions, together with a sufficient amount of humidity in the air, provide the favourable conditions for the formation of radiation fogs. The interaction of aerosols and gases with fog droplets can affect the chemical composition as well as the lifetime of atmospheric pollutants (Collett et al., 2008, Dall'Osto et al., 2009, Gilardoni et al., 2014, Kaul et al., 2011, Wang et al., 2012, Yang et al., 2012).

At the beginning of the 1980's, the first fog samples were collected at the field station of San Pietro Capofiume (Fuzzi et al., 1983), which is located in an agricultural area in the south-eastern part of the Po Valley, 30 km north-east of Bologna (Fig. 1).

Initially fog droplets were sampled occasionally on a field campaign basis. Starting from November 1989, a regular program of fog sampling has been implemented every year in the fall–winter season (November–March), in conditions of dense fog occurrence (Fuzzi et al., 1996, Fuzzi et al., 1992, Fuzzi et al., 1997). This systematic activity, which is still on-going, enabled us to build an over twenty years long database of fog water chemical composition, pH, conductivity and liquid water content (LWC). This database represents a valuable tool to test the effectiveness of the implemented air quality policies, and can also be used to support future policy actions in the region. The spatial variability of fog chemistry over the whole Po Valley has also been investigated during shorter-term experiments (Fuzzi et al., 1996). These studies indicate a relatively homogeneous fog chemical composition across the valley. Based on such observations, the trends presented in this paper can reasonably be considered representative for the whole Po Valley basin.

This paper summarizes fog measurements results collected over twenty years in San Pietro Capofiume, discusses temporal trends in fog chemical composition, and reviews results of short term studies on specific fog chemical components.

Section snippets

Fog water collection

At the field station of San Pietro Capofiume, monitoring of fog occurrence and fog water collection have been performed from November to March systematically since 1989. At this station, several experiments had already been carried out earlier during the 1980's (Fuzzi et al., 1983, Fuzzi et al., 1988, Fuzzi et al., 1985, Winiwarter et al., 1988), but they were set up within intensive field campaigns and the available results cannot be considered representative for the whole season.

Fog water is

Fog frequency

A decrease of the frequency and persistence of fog throughout Europe over the last decades was described by Vautard et al. (2009). These authors report that the frequency of low-visibility conditions such as fog, mist and haze has declined in Europe over the past 30 years, for all seasons and all visibility ranges between distances of 0 and 8 km. This decline is spatially and temporally correlated with trends in sulphur dioxide emissions, suggesting a significant contribution of air-quality

Conclusions

A more than twenty year long database of fog chemical composition, pH, conductivity, LWC and frequency of dense fog events (visibility < 200 m) has been assembled, based on measurements carried out at the field station of San Pietro Capofiume in the eastern part of the Po Valley, Italy.

A reduction in fog occurrence over the last thirty years has been registered at the Bologna airport, in agreement with what has been observed in the rest of Europe. Anyway, we can not establish the reason of the

Acknowledgements

Giordano Orsi designed the entire fog water collection program and followed its functioning over the years. The Authors acknowledge the contribution of Valeriana Mancinelli in the elaboration of the database used for the preparation of this paper. The access to visibility data was kindly provided by Dr. Sandro Nanni (ARPA Emilia Romagna, Servizio Idro-Meteo-Clima of Bologna). Jean-Philippe Putaud is gratefully acknowledged for the carbon analysis performed on quartz filters.

References (47)

  • P. Herckes et al.

    A review of observations of organic matter in fogs and clouds: origin, processing and fate

    Atmos. Res.

    (2013)
  • A. Hess et al.

    Linear trend analysis: a comparison of methods

    Atmos. Environ.

    (2001)
  • A. Kasper-Giebl

    Control of solute concentrations in cloud and fog water by liquid water content

    Atmos. Environ.

    (2002)
  • C.A. Lange et al.

    Fog frequency and chemical composition of fog water – a relevant contribution to atmospheric deposition in the eastern Erzgebirge, Germany

    Atmos. Environ.

    (2003)
  • V.K. Saxena et al.

    Cloud chemistry measurements and estimates of acidic deposition on an above cloudbase coniferous forest

    Atmos. Environ. A General Top.

    (1990)
  • D.J. Straub et al.

    Measurements of fog composition at a rural site

    Atmos. Environ.

    (2012)
  • Z. Wang et al.

    Formation of secondary organic carbon and cloud impact on carbonaceous aerosols at Mount Tai, North China

    Atmos. Environ.

    (2012)
  • A.C. Aiken et al.

    O/C and OM/OC ratios of primary, secondary, and ambient organic aerosols with high-resolution time-of-flight aerosol mass spectrometry

    Environ. Sci. Technol.

    (2008)
  • J.R. Balmes et al.

    Potential bronchoconstrictor stimuli in acid fog

    Environ. Health Perspect.

    (1989)
  • P.D. Capel et al.

    Carbon speciation and surface-tension of fog

    Environ. Sci. Technol.

    (1990)
  • M. Dall'Osto et al.

    Real-time secondary aerosol formation during a fog event in London

    Atmos. Chem. Phys.

    (2009)
  • S. Decesari et al.

    Characterization of water-soluble organic compounds in atmospheric aerosol: a new approach

    J. Geophys. Res. Atmos.

    (2000)
  • G.J. Dollard et al.

    Pollutant transfer in upland regions by occult precipitation

    Nature

    (1983)
  • Cited by (66)

    View all citing articles on Scopus
    1

    Present affiliation: Proambiente S.c.r.l., CNR Research Area, Bologna, Italy.

    View full text