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Fog occurrence and chemical composition in the Po valley over the last twenty years
Atmospheric Environment 98 (2014) 394e401
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Fog occurrence and chemical composition in the Po valley over the last twenty years L. Giulianelli*, S. Gilardoni, L. Tarozzi, M. Rinaldi, S. Decesari, C. Carbone 1, M.C. Facchini, S. Fuzzi Institute of Atmospheric Sciences and Climate e National Research Council (ISAC-CNR), Via P. Gobetti 101, 40129 Bologna, Italy
h i g h l i g h t s 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. SO2 4 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.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2014 Received in revised form 23 July 2014 Accepted 26 August 2014 Available online 6 September 2014
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 2 ionic load of the droplets. Specifically, the three major inorganic ions (NHþ 4 , SO4 , 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%e56% 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. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Po Valley Fog water composition Fog frequency
1. Introduction During the fallewinter 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 * Corresponding author. E-mail address: [email protected] (L. Giulianelli). 1 Present affiliation: Proambiente S.c.r.l., CNR Research Area, Bologna, Italy. http://dx.doi.org/10.1016/j.atmosenv.2014.08.080 1352-2310/© 2014 Elsevier Ltd. All rights reserved.
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 fallewinter months, high pressure and clear sky conditions,
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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 southeastern 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 fallewinter season (NovembereMarch), in conditions of dense fog occurrence (Fuzzi et al., 1996, 1992, 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. 2. Experimental methods 2.1. 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, 1988, 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.
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Fog water is sampled using an automated, computer driven active string collector extensively described in Fuzzi et al. (1997). The collector consists of a system in which a fan located at the rear part of a short wind tunnel creates an air stream containing fog droplets that impact on a series of strings. The collected droplets coalesce with each other and drain off the strings into a funnel and the sampling bottle. The air flow through the tunnel is of ca. 17 m3 min1 and the 50% of collection efficiency of individual string is ca. 3 mm radius. In 1997, the fog collector was modified to allow fog water analysis of organic compounds and all parts coming into contact with the fog droplets, including the sampling strings originally made of teflon, were made out of stainless steel to avoid problems of artefact formation and adsorption on the surfaces for these compounds. A Particulate Volume Monitor PVM-100, used to determine fog liquid water content (LWC) with a time resolution of 1 min, is used to activate the string collector on the basis of a protocol described in the above reference. A LWC threshold of 0.08 g m3 was chosen for the activation. Even if no direct physical relationship between LWC and visibility can be provided for our data, empirical analysis showed that this threshold usually corresponds to ca. 200 m visibility (Tomasi and Tampieri, 1976), that is our definition of dense fog (the meteorological definition of fog is visibility < 1 km). A total of 605 fog water samples were collected from 1989/90 to 2010/2011, with a seasonal median number of samples of 25. 2.2. Sample handling and chemical analysis Fog water samples are filtered through 47 mm quartz-fibre filters within a few hours after collection to remove the suspended particles. Conductivity and pH measurements are carried out with a Crison microCM 2201 conductimeter and Crison micropH 2002 pH meter, respectively. The samples are then stored frozen until the analysis. Gravimetrical analysis was performed on the quartz-fibre filters to determine the total insoluble mass in fog water. Filters are analysed by Sunset EC-OC to quantify the concentration of elemental carbon (EC) and particulate organic carbon (POC). þ þ Liquid samples are analysed for inorganic ions (NHþ 4 , Na , K , 2 Ca2þ, Mg2þ, Cl, NO 3 , SO4 ) and low molecular weight organic acids (acetate, formate, methanesulfonate and oxalate) by ion chromatography (Matta et al., 2003). Since 1997 also water-soluble organic carbon (WSOC) has been determined in fog water samples. A carbon analyser Shimadzu TOC-5000A was used until 2003 (Decesari et al., 2001). Thenceforth the instrument was replaced by a nitrogen and carbon analyser Analytik Jena Multi N/C 2100S (Rinaldi et al., 2007). Both instruments operate in TOC modality, and WSOC is calculated as the difference between total soluble carbon and inorganic carbonate concentration, directly measured by the analyser. 2.3. Time trends statistical analysis In this paper, long time series of data have been used to investigate the occurrence of temporal trends in the observed chemical and physical parameters. The statistical approach selected to detect and estimate the trends was the Ordinary Least Squares (OLS) regression method, which represents a suitable method when yearly averages are used as variables. The method is extensively described in Hess et al. (2001). 3. Results and discussion 3.1. Fog frequency
Fig. 1. Satellite view of a foggy day in the Po Valley with the location of the station of San Pietro Capofiume. Some main cities are indicated for reference (Credit: Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC).
A decrease of the frequency and persistence of fog throughout Europe over the last decades was described by Vautard et al. (2009).
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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 improvements, i.e. reduction in the aerosol sulphate loading which act as condensation nuclei for fog formation. Similar conclusions were reached by van Oldenborgh et al. (2010), who restricted the study of Vautard et al. (2009) to dense fog (visibility < 200 m). This study reported that the relative temporal trends of the number of days with dense fog are comparable to the trends of days with presence of haze and mist (2 km visibility and lower), although the scatter around the mean values is much larger in the former case. These studies address particularly the regions of eastern and central Europe, north of the Alps. For the Po Valley area, Mariani (2009) reported a decreasing frequency of foggy days (visibility < 1 km) in the Milan area. A reduction of 73% was observed within the city and of 52% at the Linate airport, 7 km from the city centre, over the decade 1991e2000 with respect to the decade 1960e1969. A reduction of fog frequency of about 50% over the period 1949e1990 was also documented by Sachweh and Koepke (1995) for the metropolitan area of Munich. These authors attribute this reduction to the urban heat island that increases the heating of the air and causes a moisture deficit. Visibility data collected by the Regional Environmental Agency of Emilia Romagna in the area of Bologna airport are available since 1984. They show a reduction of 47% of annual foggy hours in the period 2004/05e2012/13 with respect to the decade 1984/ 85e1993/94, with fog defined as visibility < 1 km. Visibility data at the field station of San Pietro Capofiume are available for a shorter period: fallewinter season 1986/87e1998/99. The trend is consistent with that of Bologna airport, even if the absolute values are higher, as expected for a rural site, not affected by the urban heat island effect and characterised by higher relative humidity. Both trends are reported in Fig. 2. A reduction in fog occurrence is evident until the first half of the 1990's. Thenceforth, fog occurrence during the fallewinter season levels around the mean value of 10% of the time. Fig. 2 reports the time trend of dense fog events (visibility < 200 m) in Bologna and San Pietro Capofiume. These trends are similar to the trends reported for visibility < 1 km.
Fig. 3. Conductivity and ionic strength time trends. Chromatographic data from 1989/ 90 to 1992/93 were rejected because of analytical problems occurred during the analysis. Therefore, related ionic strength values could not be calculated. The dotted line indicates the extrapolation of the regression line over this period. Shaded areas represent standard deviations of the volume-weighted means.
Unfortunately, visibility data in San Pietro Capofiume are not available after 1999. A proxy of dense fog occurrence can be based on LWC data using the empirical relationships between visibility and LWC investigated by previous studies (Tomasi and Tampieri, 1976). Specifically a LWC corresponding to the threshold used in our study for fog water sampling (0.08 g m3) was shown to correspond to visibility range between 100 and 400 m, i.e. dense fog. In Fig. 2 seasonally-averaged total dense fog duration from visibility measurements between 1984 and 1999 are overlapped with the proxies of dense fog duration obtained by the LWC measurements between 1991 and 2013. During the period 1991e1999,
Fig. 2. Time evolution of fog (visibility < 1 km) and dense fog (visibility < 200 m) occurrence in Bologna airport and San Pietro Capofiume, expressed as percentage of foggy hours during the fallewinter season (NovembereMarch) and time evolution of dense fog occurrence in San Pietro Capofiume expressed as percentage of hours of dense fog estimated by LWC values (dense fog_ISAC). In San Pietro Capofiume, visibility data recording was terminated in 1999. Data referring to seasons 1996/97 and 1999/00 in Bologna have been rejected because measurements cover less than 60% of the time.
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Fig. 4. Statistical summary of NH4þ, NO3 and SO42 molar concentration (mmol L1). Box plots indicate the 25th, median and 75th percentiles, whiskers the 10th and 90th percentiles.
when both visibility and LWC data are available, frequency from LWC data approximately follows the same trend as frequency from visibility data. After the late 1990's, LWC data confirm that in San Pietro Capofiume, as in Bologna, dense fog occurrence did not show a significant decrease. 3.2. Fog chemical composition and trends An important objective of the long-term project discussed in this paper is to evaluate the changes in fog water chemical composition, which are expected to be impacted by the concurrent changes in emission of atmospheric pollutants, which have adverse effects on the environment and human health. Fig. 3 shows the trends of both conductivity and ionic strength (IS), expressed as seasonal volume-weighted means. IS has been calculated as a function of the concentration of all inorganic ions detected by ion chromatography, while conductivity has been measured directly. The trends of these two parameters, both indicative of the total pollution loading of the atmosphere where fog droplets form, exhibit a significant decrease (82% over the period for IS and 75% for conductivity). This trend is in line with other results obtained in Europe (Lange et al., 2003), where reduction of ionic concentration in fog was attributed to an improvement of air quality achieved since the 1990's. 2 þ NO 3 , SO4 and NH4 are the three major inorganic constituents of fog water that alone account for an average 86% ± 12% of total IS. A statistical summary of their molar concentration (mmol L1) from 1993/94 to 2010/11 is presented in Fig. 4.
Seasonal median concentrations (mmol L1) of all the detected ionic species and of WSOC and pH seasonal median values are reported in Table 1. As expected from the decreasing trend of ionic strength and 2 conductivity values, NHþ 4 , NO3 and SO4 concentrations exhibit a reduction over the considered period. In Fig. 5a, coloured bars represent the volume-weighted mean concentrations of the three ionic species. Emission data about the gas-phase precursors of the fog inorganic components (SO2, NH3 and NOx) are also shown in the figure (dotted lines), referring to the total emissions in the region Emilia-Romagna (which stretches from the Po Valley to the Apennines to the Adriatic Sea) from 1990 to 2010. A 90% reduction of SO2 emission has been documented from 1990 to 2010. Over the same period, the concentration of SO2 4 in fog water decreased 76%. NO3 concentration underwent a smaller decrease (43%) against a reduction of 44% of NOx emission. Ammonium concentration decreased as well (55%), most likely due to reduction in sulphate and nitrate, rather than reduction in ammonia, whose emissions decreased by 31%. The highest relative decrease of sulphate is also 2 supported by the increase of NO 3 /SO4 ratio (Fig. 6), close to 1 until the end of the 1990's, then increasing to values around 3 in 2009/10 and 2010/11. In the last ten years, the nitrate ion has become two or three times more abundant than sulphate in fog water, in connection to the much higher decrease of SO2 emissions with respect to those of NOx. Fig. 5b reports the time trend of atmospheric loading of nitrate, sulphate and ammonium, calculated as concentration in fog water multiplied by LWC. The atmospheric loading shows a decreasing trend from 1993 to 2011 as ionic concentration, confirming that
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Table 1 Seasonal median concentrations of ionic species and WSOC and seasonal median values of pH. Median concentration (mmol L1)
1989/1990 1991/1992 1992/1993 1993/1994 1993/1994 1994/1995 1995/1996 1996/1997 1997/1998 1998/1999 1999/2000 2000/2001 2001/2002 2002/2003 2003/2004 2004/2005 2005/2006 2006/2007 2007/2008 2008/2009 2009/2010 2010/2011
mgC L1
CH3COOH
HCOO
CH3SO 3
C2HOO
Cl
NO 2
NO 3
SO2 4
Naþ
NH4þ
Kþ
Mg2þ
Ca2þ
WSOC
n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.10 0.10 0.10 0.04 0.03 n.a. n.a. 0.06 0.03 0.04 0.01 0.03 0.03 0.01
n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.12 0.09 0.01 0.02 0.01 n.a. n.a. 0.06 0.04 0.02 0.01 0.03 0.03 0.01
n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.01 n.a. 0.07 0.05 0.05 n.a. n.a. 0.10 n.a. 0.02 0.01 0.01 0.03 0.01
n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.02 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01
n.a. n.a. n.a. n.a. 0.18 0.12 0.18 0.12 0.50 0.11 0.13 0.12 0.12 0.09 0.01 0.12 0.17 0.09 0.12 0.07 0.11 0.07
n.a. n.a. n.a. n.a. 0.04 0.03 0.03 0.04 0.02 0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.02 0.03 0.02 0.03 0.02 0.02
n.a. n.a. n.a. n.a. 1.2 1.1 0.95 1.7 3.1 1.0 1.8 0.93 1.1 0.7 0.57 1.4 0.88 0.72 1.2 0.74 0.73 0.45
n.a. n.a. n.a. n.a. 0.41 0.37 0.44 0.45 0.93 0.25 0.53 0.27 0.32 0.23 0.18 0.27 0.22 0.17 0.22 0.12 0.16 0.10
n.a. n.a. n.a. n.a. 0.04 0.04 0.03 0.06 0.11 0.09 0.08 0.04 0.04 0.05 0.10 0.02 0.07 0.04 0.06 0.06 0.07 0.04
n.a. n.a. n.a. n.a. 2.5 2.1 1.8 2.9 3.1 1.5 3.0 1.5 1.8 1.2 1.2 1.9 1.5 1.3 2.0 1.1 1.3 0.73
n.a. n.a. n.a. n.a. 0.05 0.04 0.04 0.04 0.04 0.04 0.05 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.02 0.01 0.02 0.02
n.a. n.a. n.a. n.a. 0.01 0.01 0.01 0.01 0.03 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.02 0.01 0.02
n.a. n.a. n.a. n.a. 0.04 0.04 0.07 0.04 0.19 0.08 0.06 0.04 0.03 0.05 0.05 0.03 0.06 0.05 0.05 0.05 0.02 0.02
n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 42 27 48 n.a. 31 15 22 45 30 25 16 20 21 15
reduction of fog water ion concentration is due to decrease of precursors emissions and not to a dilution effect. 3.2.1. Relationship between fog water solute concentration and fog LWC The influence of fog liquid water content on the concentration of the soluble species in fog water has been widely studied in literature. An inverse relationship has been observed in previous studies between ion concentrations and LWC, although with a different trend in different locations (Aleksic and Dukett, 2010; Elbert et al., 2000, 2002; Kasper-Giebl, 2002; Straub et al., 2012). In this work, the sum of the molalities of the three major anions (chloride, nitrate and sulphate) is plotted as a function of LWC for the whole series of data from 1993/94 to 2010/11 (Fig. 7). This graphical representation is chosen to allow a direct comparison with the plot reported from previous experiments concerning cloud and fog
2 Fig. 5. Panel a: Time trend of NHþ and NO 4 , SO4 3 concentration (coloured bars). Dotted lines indicate the time trend of emissions of NH3, NOx and SO2 in the EmiliaRomagna region. Emission data have been provided by the Italian Institute for the Environmental Protection and Research e ISPRA (http://www.isprambiente.gov.it/it/ 2 banche-dati/aria-ed-emissioni-in-atmosfera). Panel b: time trend of NHþ 4 , SO4 and NO 3 atmospheric loading calculated as concentration in fog water multiplied by LWC. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
pH
5.3 6.0 4.8 5.5 6.0 5.4 5.7 4.9 3.9 6.5 5.8 6.1 5.9 6.6 6.9 7.1 7.4 6.7 6.4 6.1 5.9 6.9
water characterisation by Elbert et al. (2000). The thick solid line in the plot represents the best fit for the data, calculated by the equation: m ¼ bL1 (Elbert et al., 2000). The thin lines indicate the curves whose slopes were derived from the 5th and 95th percentile molality values. The coefficient b is equal to 460 nmol m3, which is one order of magnitude higher than the values reported by Elbert et al. (2000) for cloud and fog water in clean environments (10e31 nmol m3). Even if a decreasing anion molality with increasing LWC can be discerned, the power law best fit shows a very low correlation (R2 ¼ 0.03). Similarly, a lack of correlation is reported by Herckes et al. (2013) when analysing WSOC as a function of LWC across different locations. This analysis shows that the variation in fog LWC affects only minimally ions concentrations in fog water. Furthermore, no statistically significant trend is found for LWC for the investigated period. This supports the hypothesis that the observed decreasing concentrations of main ionic species are mainly due to a change in atmospheric emissions instead of a simple dilution effect.
3.2.2. Fog water acidity Until the 1980's, many studies were carried out on the acidity of cloud water, fog water, rain and snow, especially in the industrialized countries of the northern hemisphere, where increasing combustion processes caused the acidification of the atmospheric liquid water phase (Jacobson, 1984; Saxena and Lin, 1990; Wisniewski, 1982). Fuzzi et al. (1983) focused on the pH trend during the evolution of a fog event in the Po Valley, stressing the 2 crucial role of NO 3 and SO4 in determining fog acidity. From then on, fog water pH has regularly been measured at the field station of San Pietro Capofiume. Fig. 8 shows the statistical summary of pH values from season 1989/90 to 2010/11. Despite the variability of pH values within each season (in 2004/05 pH ranged from 3.4 to 7.4), pH shows an increasing trend, at a 99% confidence interval, according to the OLS regression method. The calculated volumeweighted means show that during the 1990's the mean pH values never exceeded 5, ranging from the lowest average pH in 1995/96 (3.6) to the highest in 1998/99 (4.7). Over the last decade, the lowest average pH value was 4.1 in 2001/02 and the highest reached 6.9 in 2005/06 with values mostly exceeding 5. This reduction of fog acidity is what we expected from the observed
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2 Fig. 6. Time trend of volume-weighted means of NO 3 /SO4 equivalent ratio.
2 decrease of NO 3 and SO4 concentrations in fog water. Conditions close to neutrality are almost achieved in the last years, in agreement with the lower reduction of alkaline species emissions (ammonia) compared to those of SO2 and NOx, precursors of the two major acidic species in solution.
3.2.3. Water-soluble organic carbon The three main ionic species represent about 90% of the total ionic strength, but in terms of total solute mass, it was interesting to evaluate the fraction accounted for by organic matter. For this purpose, WSOC content has been systematically measured since the fallewinter season 1997/98. Fig. 9 reports the mean chemical composition of fog water from 1997/98 to 2010/11. The three main ionic species account for almost 70% and the organic fraction represents on average 25% of the total solute mass. The amount of WSOC can be highly variable within the samples, and no significant time trend was observed over the considered period. Fig. 10 reports a statistical summary of WSOC concentrations, which range from 3 mgC L1 to 350 mgC L1 and seasonal median concentrations range from 15 mgC L1 to 49 mgC L1. Specific studies have been carried out to characterise the organic fraction of the Po Valley fog samples. Facchini et al. (1992) focused on the gaseliquid phase partitioning of carbonyl compounds: formaldehyde (HCHO), formic acid (HCOOH) and acetic acid (CH3COOH), which in the Po Valley mainly originate from anthropogenic sources. Furthermore, an analytical method was developed
Fig. 7. Sum of sulphate, nitrate and chloride molalities as a function of LWC. Thick line is the power law best fit, while thin lines were calculated using the 5th and the 95th percentile values of the sum of nitrate, chloride and sulphate molalities. No samples were collected at LWC <0.08 g m3 (see experimental methods section).
to characterise the unspeciated organic fraction at the molecular level, combining HPLC and H NMR techniques (Decesari et al., 2000). Results obtained from this method indicate a predominance of WSOC having an acidic character, including a class of polycarboxylic acids similar to fulvic acids (Fuzzi et al., 2002). However this method was not systematically adopted over the years, therefore no time trends of the organic fraction chemical composition can be provided. Only low molecular weight carboxylic acids easily detectable by ion chromatography, namely acetic acid (CH3COOH), formic acid (HCOOH), methanesulfonic acid (CH3SO3H) and oxalic acid (C2H2O4) were systematically measured over the study period. Concentrations are highly variable from sample to sample, ranging from a few to hundreds mmol L1. The best correlation between these low-molecular weight species and WSOC was found for oxalate (R2 ¼ 0.54). Oxalic acid is the endproduct of several aqueous-phase oxidation reactions involving the formation of “aqueous SOA” (Ervens et al., 2011; Lim et al., 2013; Tan et al., 2012). Our findings suggest that such reaction mechanisms can be a significant source of organic solutes in fog droplets. 3.2.4. Carbonaceous matter suspended in fog droplets The quantification of EC and POC concentrations on quartz-fibre filters was performed on samples collected in 1997/98, 1998/99, 1999/2000 and 2000/2001. Carbonaceous matter accounts for a significant fraction of the insoluble material suspended in fog water. The water insoluble organic mass was estimated from POC assuming that the lower and
Fig. 8. Statistical summary of pH values. Box plots indicate the 25th, median and 75th percentiles, whiskers the 10th and 90th percentiles. The dotted line indicates the regression line.
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The total OC concentration in fog, calculated as the sum of POC and WSOC, and the median of the POC to WSOC ratio are reported in Table 2. The POC fraction decreases slightly from 1997 to 2000, but the limited number of samples analysed during the first season (only 8 samples in 1997/1998) does not allow any speculation about the temporal trends. Further studies are needed to evaluate the change in the relative contribution of soluble and insoluble carbonaceous species in fog water over the years. The contribution of WSOC to the TOC (calculated as the sum of the soluble and insoluble organic fractions) ranges from 52 to 95% with an average of 86%. WSOC correlate with TOC with R2 ¼ 0.99. These findings are in agreement with previous studies carried out in different sites, where WSOC represent from 77% to 96% of TOC (Herckes et al., 2013). The concentrations of carbonaceous material in the Po Valley fog water are up to one order of magnitude higher than values reported in previous works for rural sites (Herckes et al., 2013), but comparable to urban fog as reported by Capel et al. (1990). 4. Conclusions
1
þ
þ
2þ
2þ
Fig. 9. Average chemical composition of fog water (mg L ). Na , K , Ca , Mg , Cl have been grouped under the category “other ions”. A factor of 1.8 was used to convert WSOC to water-soluble organic matter (WSOM) (Matta et al., 2003).
Fig. 10. Statistical summary of WSOC values. Box plots indicate the 25th, median and 75th percentiles, whiskers the 10th and 90th percentiles. Data for the season 2000/ 2001 are not available.
the upper bound of the OM to OC ratio were equal to 1.1 and 1.4, respectively (Aiken et al., 2008; Yttri et al., 2007). The sum of EC and water insoluble organic mass accounts on average for 46%e56% of the mass of total suspended material. The contribution of the carbonaceous species is comparable to the upper bound of the variability range reported in a similar study (Capel et al., 1990), i.e. 28e57%. We assume that the remaining fraction of insoluble mass is composed of mineral particles. Insoluble carbonaceous material is composed mainly by organic matter, in fact, EC accounts on average only 19% of the insoluble carbon. Absolute concentrations of POC and EC are highly variable. POC ranges between 1.6 and 206 mgC L1 with a median value of 5.8 mgC L1; EC ranges over two orders of magnitude (0.24 and 49 mgC L1), with 1.2 mgC L1 as median value. The ratio EC to POC, 0.24 on average, exhibits little variability among samples (standard deviation 0.10). The correlation of EC and POC observed through the different years (R2 ¼ 0.98) suggests that anthropogenic combustion processes, which represent the main source of EC, are also the most important source of POC in fog droplets.
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 observed reduction. Potential causes can be increasing temperatures or the decline of available condensation nuclei, but further studies are necessary to this end. A clear decreasing trend has been shown for ionic strength and conductivity, indicating a reduction of the ionic load of fog water. SO2 4 is the ionic species that exhibits the highest decrease (76% from 1990 to 2010). NO 3 also exhibits a decreasing trend although to lower extent (43%), as confirmed by the increasing values of 2 NO 3 /SO4 ratio over the study period. The low correlation between ion concentrations and LWC and the lack of a statistically significant trend for LWC, indicate that the observed decreasing concentrations of main ionic species is mainly due to a reduction in SO2 and NOx atmospheric emissions instead of a simple dilution effect. As a consequence of the lower content of the two main acidic 2 species (NO 3 and SO4 ) fog water pH exhibits an increase with volume-weighted means higher than 5 over the last decade. Carbonaceous matter accounts for about 50% of the insoluble material suspended in fog droplets and for almost 25% of the total solute mass of fog water. WSOC dominates over POC. No significant trend of WSOC and short chain organic acids concentrations was observed over the period for which WSOC measurements are available (1997/98e2010/11). To better understand the role of organic matter in the fog chemistry of the Po Valley, specific studies are currently ongoing in addition to the already set up measurement program detailed in this paper. To summarize, the results shown in this paper reflect the effectiveness of the implemented air quality policies and show that
Table 2 Seasonal median values of soluble and insoluble carbonaceous species. POC (mgC L1) EC (mgC L1) POC þ WSOC (mgC L1) POC/WSOC 1997/98 11.9 1998/99 4.23 1999/00 5.65 2000/01 6.00
2.24 0.95 1.25 1.16
66 30 55 n.a.
0.19 0.17 0.14 n.a.
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monitoring the chemical composition of fog water over extended periods can be a valuable tool to test the effectiveness of future policy actions to limit pollutants emission over the whole Po Valley basin. 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 Aiken, A.C., Decarlo, P.F., Kroll, J.H., Worsnop, D.R., Huffman, J.A., Docherty, K.S., Ulbrich, I.M., Mohr, C., Kimmel, J.R., Sueper, D., Sun, Y., Zhang, Q., Trimborn, A., Northway, M., Ziemann, P.J., Canagaratna, M.R., Onasch, T.B., Alfarra, M.R., Prevot, A.S.H., Dommen, J., Duplissy, J., Metzger, A., Baltensperger, U., Jimenez, J.L., 2008. 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. 42, 4478e4485. Aleksic, N., Dukett, J.E., 2010. Probabilistic relationship between liquid water content and ion concentrations in cloud water. Atmos. Res. 98, 400e405. Balmes, J.R., Fine, J.M., Gordon, T., Sheppard, D., 1989. Potential bronchoconstrictor stimuli in acid fog. Environ. Health Perspect. 79, 163e166. Butler, C.D., Trumble, J.T., 2008. Effects of pollutants on bottom-up and top-down processes in insect-plant interactions. Environ. Pollut. 156, 1e10. Capel, P.D., Gunde, R., Zurcher, F., Giger, W., 1990. Carbon speciation and surfacetension of fog. Environ. Sci. Technol. 24, 722e727. Collett Jr., J.L., Herckes, P., Youngster, S., Lee, T., 2008. Processing of atmospheric organic matter by California radiation fogs. Atmos. Res. 87, 232e241. Dall'Osto, M., Harrison, R.M., Coe, H., Williams, P., 2009. Real-time secondary aerosol formation during a fog event in London. Atmos. Chem. Phys. 9, 2459e2469. Decesari, S., Facchini, M.C., Fuzzi, S., Tagliavini, E., 2000. Characterization of watersoluble organic compounds in atmospheric aerosol: a new approach. J. Geophys. Res. Atmos. 105, 1481e1489. Decesari, S., Facchini, M.C., Matta, E., Lettini, F., Mircea, M., Fuzzi, S., Tagliavini, E., Putaud, J.P., 2001. Chemical features and seasonal variation of fine aerosol water-soluble organic compounds in the Po Valley, Italy. Atmos. Environ. 35, 3691e3699. Dollard, G.J., Unsworth, M.H., Harve, M.J., 1983. Pollutant transfer in upland regions by occult precipitation. Nature 302, 241e243. Elbert, W., Hoffmann, M.R., Kramer, M., Schmitt, G., Andreae, M.O., 2000. Control of solute concentrations in cloud and fog water by liquid water content. Atmos. Environ. 34, 1109e1122. Elbert, W., Kramer, M., Andreae, M.O., 2002. Reply to discussion on “control of solute concentrations in cloud and fog water by liquid water content”. Atmos. Environ. 36, 1909e1910. Ervens, B., Turpin, B.J., Weber, R.J., 2011. Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): a review of laboratory, field and model studies. Atmos. Chem. Phys. 11, 11069e11102. Facchini, M.C., Fuzzi, S., Lind, J.A., Fierlingeroberlinninger, H., Kalina, M., Puxbaum, H., Winiwarter, W., Arends, B.G., Wobrock, W., Jaeschke, W., Berner, A., Kruisz, C., 1992. Phase-partitioning and chemical-reactions of lowmolecular-weight organic-compounds in fog. Tellus Ser. B Chem. Phys. Meteorol. 44, 533e544. Fenn, M.E., Geiser, L., Bachman, R., Blubaugh, T.J., Bytnerowicz, A., 2007. Atmospheric deposition inputs and effects on lichen chemistry and indicator species in the Columbia River Gorge, USA. Environ. Pollut. 146, 77e91. Fuzzi, S., Facchini, M.C., Decesari, S., Matta, E., Mircea, M., 2002. Soluble organic compounds in fog and cloud droplets: what have we learned over the past few years? Atmos. Res. 64, 89e98. Fuzzi, S., Facchini, M.C., Orsi, G., Bonforte, G., Martinotti, W., Ziliani, G., Mazzali, P., Rossi, P., Natale, P., Grosa, M.M., Rampado, E., Vitali, P., Raffaelli, R., Azzini, G., Grotti, S., 1996. The NEVALPA project: a regional network for fog chemical climatology over the Po Valley basin. Atmos. Environ. 30, 201e213. Fuzzi, S., Facchini, M.C., Orsi, G., Lind, J.A., Wobrock, W., Kessel, M., Maser, R., Jaeschke, W., Enderle, K.H., Arends, B.G., Berner, A., Solly, I., Kruisz, C., Reischl, G., Pahl, S., Kaminski, U., Winkler, P., Ogren, J.A., Noone, K.J., Hallberg, A., Fierlingeroberlinninger, H., Puxbaum, H., Marzorati, A., Hansson, H.C., Wiedensohler, A., Svenningsson, I.B., Martinsson, B.G., Schell, D., Georgii, H.W., 1992. The Po Valley fog experiment 1989 e an overview. Tellus Ser. B Chem. Phys. Meteorol. 44, 448e468.
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Environ. 37, 3731e3739. Lim, Y.B., Tan, Y., Turpin, B.J., 2013. Chemical insights, explicit chemistry, and yields of secondary organic aerosol from OH radical oxidation of methylglyoxal and glyoxal in the aqueous phase. Atmos. Chem. Phys. 13, 8651e8667. Mariani, L., 2009. Fog in the Po Valley:some meteo-climatic aspects. Italian J. Agrometeorol. 3, 35e44. Matta, E., Facchini, M.C., Decesari, S., Mircea, M., Cavalli, F., Fuzzi, S., Putaud, J.P., Dell'Acqua, A., 2003. Mass closure on the chemical species in size-segregated atmospheric aerosol collected in an urban area of the Po Valley, Italy. Atmos. Chem. Phys. 3, 623e637. Rinaldi, M., Emblico, L., Decesari, S., Fuzzi, S., Facchini, M.C., Librando, V., 2007. Chemical characterization and source apportionment of size-segregated aerosol collected at an urban site in sicily. Water Air Soil Pollut. 185, 311e321. Sachweh, M., Koepke, P., 1995. Radiation fog and urban climate. Geophys. Res. Lett. 22, 1073e1076. Saxena, V.K., Lin, N.H., 1990. 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Fog occurrence and chemical composition in the Po valley over the last twenty years L. Giulianelli*, S. Gilardoni, L. Tarozzi, M. Rinaldi, S. Decesari, C. Carbone 1, M.C. Facchini, S. Fuzzi Institute of Atmospheric Sciences and Climate e National Research Council (ISAC-CNR), Via P. Gobetti 101, 40129 Bologna, Italy
h i g h l i g h t s 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. SO2 4 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.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2014 Received in revised form 23 July 2014 Accepted 26 August 2014 Available online 6 September 2014
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 2 ionic load of the droplets. Specifically, the three major inorganic ions (NHþ 4 , SO4 , 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%e56% 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. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Po Valley Fog water composition Fog frequency
1. Introduction During the fallewinter 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 * Corresponding author. E-mail address: [email protected] (L. Giulianelli). 1 Present affiliation: Proambiente S.c.r.l., CNR Research Area, Bologna, Italy. http://dx.doi.org/10.1016/j.atmosenv.2014.08.080 1352-2310/© 2014 Elsevier Ltd. All rights reserved.
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 fallewinter months, high pressure and clear sky conditions,
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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 southeastern 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 fallewinter season (NovembereMarch), in conditions of dense fog occurrence (Fuzzi et al., 1996, 1992, 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. 2. Experimental methods 2.1. 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, 1988, 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.
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Fog water is sampled using an automated, computer driven active string collector extensively described in Fuzzi et al. (1997). The collector consists of a system in which a fan located at the rear part of a short wind tunnel creates an air stream containing fog droplets that impact on a series of strings. The collected droplets coalesce with each other and drain off the strings into a funnel and the sampling bottle. The air flow through the tunnel is of ca. 17 m3 min1 and the 50% of collection efficiency of individual string is ca. 3 mm radius. In 1997, the fog collector was modified to allow fog water analysis of organic compounds and all parts coming into contact with the fog droplets, including the sampling strings originally made of teflon, were made out of stainless steel to avoid problems of artefact formation and adsorption on the surfaces for these compounds. A Particulate Volume Monitor PVM-100, used to determine fog liquid water content (LWC) with a time resolution of 1 min, is used to activate the string collector on the basis of a protocol described in the above reference. A LWC threshold of 0.08 g m3 was chosen for the activation. Even if no direct physical relationship between LWC and visibility can be provided for our data, empirical analysis showed that this threshold usually corresponds to ca. 200 m visibility (Tomasi and Tampieri, 1976), that is our definition of dense fog (the meteorological definition of fog is visibility < 1 km). A total of 605 fog water samples were collected from 1989/90 to 2010/2011, with a seasonal median number of samples of 25. 2.2. Sample handling and chemical analysis Fog water samples are filtered through 47 mm quartz-fibre filters within a few hours after collection to remove the suspended particles. Conductivity and pH measurements are carried out with a Crison microCM 2201 conductimeter and Crison micropH 2002 pH meter, respectively. The samples are then stored frozen until the analysis. Gravimetrical analysis was performed on the quartz-fibre filters to determine the total insoluble mass in fog water. Filters are analysed by Sunset EC-OC to quantify the concentration of elemental carbon (EC) and particulate organic carbon (POC). þ þ Liquid samples are analysed for inorganic ions (NHþ 4 , Na , K , 2 Ca2þ, Mg2þ, Cl, NO 3 , SO4 ) and low molecular weight organic acids (acetate, formate, methanesulfonate and oxalate) by ion chromatography (Matta et al., 2003). Since 1997 also water-soluble organic carbon (WSOC) has been determined in fog water samples. A carbon analyser Shimadzu TOC-5000A was used until 2003 (Decesari et al., 2001). Thenceforth the instrument was replaced by a nitrogen and carbon analyser Analytik Jena Multi N/C 2100S (Rinaldi et al., 2007). Both instruments operate in TOC modality, and WSOC is calculated as the difference between total soluble carbon and inorganic carbonate concentration, directly measured by the analyser. 2.3. Time trends statistical analysis In this paper, long time series of data have been used to investigate the occurrence of temporal trends in the observed chemical and physical parameters. The statistical approach selected to detect and estimate the trends was the Ordinary Least Squares (OLS) regression method, which represents a suitable method when yearly averages are used as variables. The method is extensively described in Hess et al. (2001). 3. Results and discussion 3.1. Fog frequency
Fig. 1. Satellite view of a foggy day in the Po Valley with the location of the station of San Pietro Capofiume. Some main cities are indicated for reference (Credit: Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC).
A decrease of the frequency and persistence of fog throughout Europe over the last decades was described by Vautard et al. (2009).
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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 improvements, i.e. reduction in the aerosol sulphate loading which act as condensation nuclei for fog formation. Similar conclusions were reached by van Oldenborgh et al. (2010), who restricted the study of Vautard et al. (2009) to dense fog (visibility < 200 m). This study reported that the relative temporal trends of the number of days with dense fog are comparable to the trends of days with presence of haze and mist (2 km visibility and lower), although the scatter around the mean values is much larger in the former case. These studies address particularly the regions of eastern and central Europe, north of the Alps. For the Po Valley area, Mariani (2009) reported a decreasing frequency of foggy days (visibility < 1 km) in the Milan area. A reduction of 73% was observed within the city and of 52% at the Linate airport, 7 km from the city centre, over the decade 1991e2000 with respect to the decade 1960e1969. A reduction of fog frequency of about 50% over the period 1949e1990 was also documented by Sachweh and Koepke (1995) for the metropolitan area of Munich. These authors attribute this reduction to the urban heat island that increases the heating of the air and causes a moisture deficit. Visibility data collected by the Regional Environmental Agency of Emilia Romagna in the area of Bologna airport are available since 1984. They show a reduction of 47% of annual foggy hours in the period 2004/05e2012/13 with respect to the decade 1984/ 85e1993/94, with fog defined as visibility < 1 km. Visibility data at the field station of San Pietro Capofiume are available for a shorter period: fallewinter season 1986/87e1998/99. The trend is consistent with that of Bologna airport, even if the absolute values are higher, as expected for a rural site, not affected by the urban heat island effect and characterised by higher relative humidity. Both trends are reported in Fig. 2. A reduction in fog occurrence is evident until the first half of the 1990's. Thenceforth, fog occurrence during the fallewinter season levels around the mean value of 10% of the time. Fig. 2 reports the time trend of dense fog events (visibility < 200 m) in Bologna and San Pietro Capofiume. These trends are similar to the trends reported for visibility < 1 km.
Fig. 3. Conductivity and ionic strength time trends. Chromatographic data from 1989/ 90 to 1992/93 were rejected because of analytical problems occurred during the analysis. Therefore, related ionic strength values could not be calculated. The dotted line indicates the extrapolation of the regression line over this period. Shaded areas represent standard deviations of the volume-weighted means.
Unfortunately, visibility data in San Pietro Capofiume are not available after 1999. A proxy of dense fog occurrence can be based on LWC data using the empirical relationships between visibility and LWC investigated by previous studies (Tomasi and Tampieri, 1976). Specifically a LWC corresponding to the threshold used in our study for fog water sampling (0.08 g m3) was shown to correspond to visibility range between 100 and 400 m, i.e. dense fog. In Fig. 2 seasonally-averaged total dense fog duration from visibility measurements between 1984 and 1999 are overlapped with the proxies of dense fog duration obtained by the LWC measurements between 1991 and 2013. During the period 1991e1999,
Fig. 2. Time evolution of fog (visibility < 1 km) and dense fog (visibility < 200 m) occurrence in Bologna airport and San Pietro Capofiume, expressed as percentage of foggy hours during the fallewinter season (NovembereMarch) and time evolution of dense fog occurrence in San Pietro Capofiume expressed as percentage of hours of dense fog estimated by LWC values (dense fog_ISAC). In San Pietro Capofiume, visibility data recording was terminated in 1999. Data referring to seasons 1996/97 and 1999/00 in Bologna have been rejected because measurements cover less than 60% of the time.
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Fig. 4. Statistical summary of NH4þ, NO3 and SO42 molar concentration (mmol L1). Box plots indicate the 25th, median and 75th percentiles, whiskers the 10th and 90th percentiles.
when both visibility and LWC data are available, frequency from LWC data approximately follows the same trend as frequency from visibility data. After the late 1990's, LWC data confirm that in San Pietro Capofiume, as in Bologna, dense fog occurrence did not show a significant decrease. 3.2. Fog chemical composition and trends An important objective of the long-term project discussed in this paper is to evaluate the changes in fog water chemical composition, which are expected to be impacted by the concurrent changes in emission of atmospheric pollutants, which have adverse effects on the environment and human health. Fig. 3 shows the trends of both conductivity and ionic strength (IS), expressed as seasonal volume-weighted means. IS has been calculated as a function of the concentration of all inorganic ions detected by ion chromatography, while conductivity has been measured directly. The trends of these two parameters, both indicative of the total pollution loading of the atmosphere where fog droplets form, exhibit a significant decrease (82% over the period for IS and 75% for conductivity). This trend is in line with other results obtained in Europe (Lange et al., 2003), where reduction of ionic concentration in fog was attributed to an improvement of air quality achieved since the 1990's. 2 þ NO 3 , SO4 and NH4 are the three major inorganic constituents of fog water that alone account for an average 86% ± 12% of total IS. A statistical summary of their molar concentration (mmol L1) from 1993/94 to 2010/11 is presented in Fig. 4.
Seasonal median concentrations (mmol L1) of all the detected ionic species and of WSOC and pH seasonal median values are reported in Table 1. As expected from the decreasing trend of ionic strength and 2 conductivity values, NHþ 4 , NO3 and SO4 concentrations exhibit a reduction over the considered period. In Fig. 5a, coloured bars represent the volume-weighted mean concentrations of the three ionic species. Emission data about the gas-phase precursors of the fog inorganic components (SO2, NH3 and NOx) are also shown in the figure (dotted lines), referring to the total emissions in the region Emilia-Romagna (which stretches from the Po Valley to the Apennines to the Adriatic Sea) from 1990 to 2010. A 90% reduction of SO2 emission has been documented from 1990 to 2010. Over the same period, the concentration of SO2 4 in fog water decreased 76%. NO3 concentration underwent a smaller decrease (43%) against a reduction of 44% of NOx emission. Ammonium concentration decreased as well (55%), most likely due to reduction in sulphate and nitrate, rather than reduction in ammonia, whose emissions decreased by 31%. The highest relative decrease of sulphate is also 2 supported by the increase of NO 3 /SO4 ratio (Fig. 6), close to 1 until the end of the 1990's, then increasing to values around 3 in 2009/10 and 2010/11. In the last ten years, the nitrate ion has become two or three times more abundant than sulphate in fog water, in connection to the much higher decrease of SO2 emissions with respect to those of NOx. Fig. 5b reports the time trend of atmospheric loading of nitrate, sulphate and ammonium, calculated as concentration in fog water multiplied by LWC. The atmospheric loading shows a decreasing trend from 1993 to 2011 as ionic concentration, confirming that
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Table 1 Seasonal median concentrations of ionic species and WSOC and seasonal median values of pH. Median concentration (mmol L1)
1989/1990 1991/1992 1992/1993 1993/1994 1993/1994 1994/1995 1995/1996 1996/1997 1997/1998 1998/1999 1999/2000 2000/2001 2001/2002 2002/2003 2003/2004 2004/2005 2005/2006 2006/2007 2007/2008 2008/2009 2009/2010 2010/2011
mgC L1
CH3COOH
HCOO
CH3SO 3
C2HOO
Cl
NO 2
NO 3
SO2 4
Naþ
NH4þ
Kþ
Mg2þ
Ca2þ
WSOC
n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.10 0.10 0.10 0.04 0.03 n.a. n.a. 0.06 0.03 0.04 0.01 0.03 0.03 0.01
n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.12 0.09 0.01 0.02 0.01 n.a. n.a. 0.06 0.04 0.02 0.01 0.03 0.03 0.01
n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.01 n.a. 0.07 0.05 0.05 n.a. n.a. 0.10 n.a. 0.02 0.01 0.01 0.03 0.01
n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.02 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01
n.a. n.a. n.a. n.a. 0.18 0.12 0.18 0.12 0.50 0.11 0.13 0.12 0.12 0.09 0.01 0.12 0.17 0.09 0.12 0.07 0.11 0.07
n.a. n.a. n.a. n.a. 0.04 0.03 0.03 0.04 0.02 0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.02 0.03 0.02 0.03 0.02 0.02
n.a. n.a. n.a. n.a. 1.2 1.1 0.95 1.7 3.1 1.0 1.8 0.93 1.1 0.7 0.57 1.4 0.88 0.72 1.2 0.74 0.73 0.45
n.a. n.a. n.a. n.a. 0.41 0.37 0.44 0.45 0.93 0.25 0.53 0.27 0.32 0.23 0.18 0.27 0.22 0.17 0.22 0.12 0.16 0.10
n.a. n.a. n.a. n.a. 0.04 0.04 0.03 0.06 0.11 0.09 0.08 0.04 0.04 0.05 0.10 0.02 0.07 0.04 0.06 0.06 0.07 0.04
n.a. n.a. n.a. n.a. 2.5 2.1 1.8 2.9 3.1 1.5 3.0 1.5 1.8 1.2 1.2 1.9 1.5 1.3 2.0 1.1 1.3 0.73
n.a. n.a. n.a. n.a. 0.05 0.04 0.04 0.04 0.04 0.04 0.05 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.02 0.01 0.02 0.02
n.a. n.a. n.a. n.a. 0.01 0.01 0.01 0.01 0.03 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.02 0.01 0.02
n.a. n.a. n.a. n.a. 0.04 0.04 0.07 0.04 0.19 0.08 0.06 0.04 0.03 0.05 0.05 0.03 0.06 0.05 0.05 0.05 0.02 0.02
n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 42 27 48 n.a. 31 15 22 45 30 25 16 20 21 15
reduction of fog water ion concentration is due to decrease of precursors emissions and not to a dilution effect. 3.2.1. Relationship between fog water solute concentration and fog LWC The influence of fog liquid water content on the concentration of the soluble species in fog water has been widely studied in literature. An inverse relationship has been observed in previous studies between ion concentrations and LWC, although with a different trend in different locations (Aleksic and Dukett, 2010; Elbert et al., 2000, 2002; Kasper-Giebl, 2002; Straub et al., 2012). In this work, the sum of the molalities of the three major anions (chloride, nitrate and sulphate) is plotted as a function of LWC for the whole series of data from 1993/94 to 2010/11 (Fig. 7). This graphical representation is chosen to allow a direct comparison with the plot reported from previous experiments concerning cloud and fog
2 Fig. 5. Panel a: Time trend of NHþ and NO 4 , SO4 3 concentration (coloured bars). Dotted lines indicate the time trend of emissions of NH3, NOx and SO2 in the EmiliaRomagna region. Emission data have been provided by the Italian Institute for the Environmental Protection and Research e ISPRA (http://www.isprambiente.gov.it/it/ 2 banche-dati/aria-ed-emissioni-in-atmosfera). Panel b: time trend of NHþ 4 , SO4 and NO 3 atmospheric loading calculated as concentration in fog water multiplied by LWC. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
pH
5.3 6.0 4.8 5.5 6.0 5.4 5.7 4.9 3.9 6.5 5.8 6.1 5.9 6.6 6.9 7.1 7.4 6.7 6.4 6.1 5.9 6.9
water characterisation by Elbert et al. (2000). The thick solid line in the plot represents the best fit for the data, calculated by the equation: m ¼ bL1 (Elbert et al., 2000). The thin lines indicate the curves whose slopes were derived from the 5th and 95th percentile molality values. The coefficient b is equal to 460 nmol m3, which is one order of magnitude higher than the values reported by Elbert et al. (2000) for cloud and fog water in clean environments (10e31 nmol m3). Even if a decreasing anion molality with increasing LWC can be discerned, the power law best fit shows a very low correlation (R2 ¼ 0.03). Similarly, a lack of correlation is reported by Herckes et al. (2013) when analysing WSOC as a function of LWC across different locations. This analysis shows that the variation in fog LWC affects only minimally ions concentrations in fog water. Furthermore, no statistically significant trend is found for LWC for the investigated period. This supports the hypothesis that the observed decreasing concentrations of main ionic species are mainly due to a change in atmospheric emissions instead of a simple dilution effect.
3.2.2. Fog water acidity Until the 1980's, many studies were carried out on the acidity of cloud water, fog water, rain and snow, especially in the industrialized countries of the northern hemisphere, where increasing combustion processes caused the acidification of the atmospheric liquid water phase (Jacobson, 1984; Saxena and Lin, 1990; Wisniewski, 1982). Fuzzi et al. (1983) focused on the pH trend during the evolution of a fog event in the Po Valley, stressing the 2 crucial role of NO 3 and SO4 in determining fog acidity. From then on, fog water pH has regularly been measured at the field station of San Pietro Capofiume. Fig. 8 shows the statistical summary of pH values from season 1989/90 to 2010/11. Despite the variability of pH values within each season (in 2004/05 pH ranged from 3.4 to 7.4), pH shows an increasing trend, at a 99% confidence interval, according to the OLS regression method. The calculated volumeweighted means show that during the 1990's the mean pH values never exceeded 5, ranging from the lowest average pH in 1995/96 (3.6) to the highest in 1998/99 (4.7). Over the last decade, the lowest average pH value was 4.1 in 2001/02 and the highest reached 6.9 in 2005/06 with values mostly exceeding 5. This reduction of fog acidity is what we expected from the observed
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399
2 Fig. 6. Time trend of volume-weighted means of NO 3 /SO4 equivalent ratio.
2 decrease of NO 3 and SO4 concentrations in fog water. Conditions close to neutrality are almost achieved in the last years, in agreement with the lower reduction of alkaline species emissions (ammonia) compared to those of SO2 and NOx, precursors of the two major acidic species in solution.
3.2.3. Water-soluble organic carbon The three main ionic species represent about 90% of the total ionic strength, but in terms of total solute mass, it was interesting to evaluate the fraction accounted for by organic matter. For this purpose, WSOC content has been systematically measured since the fallewinter season 1997/98. Fig. 9 reports the mean chemical composition of fog water from 1997/98 to 2010/11. The three main ionic species account for almost 70% and the organic fraction represents on average 25% of the total solute mass. The amount of WSOC can be highly variable within the samples, and no significant time trend was observed over the considered period. Fig. 10 reports a statistical summary of WSOC concentrations, which range from 3 mgC L1 to 350 mgC L1 and seasonal median concentrations range from 15 mgC L1 to 49 mgC L1. Specific studies have been carried out to characterise the organic fraction of the Po Valley fog samples. Facchini et al. (1992) focused on the gaseliquid phase partitioning of carbonyl compounds: formaldehyde (HCHO), formic acid (HCOOH) and acetic acid (CH3COOH), which in the Po Valley mainly originate from anthropogenic sources. Furthermore, an analytical method was developed
Fig. 7. Sum of sulphate, nitrate and chloride molalities as a function of LWC. Thick line is the power law best fit, while thin lines were calculated using the 5th and the 95th percentile values of the sum of nitrate, chloride and sulphate molalities. No samples were collected at LWC <0.08 g m3 (see experimental methods section).
to characterise the unspeciated organic fraction at the molecular level, combining HPLC and H NMR techniques (Decesari et al., 2000). Results obtained from this method indicate a predominance of WSOC having an acidic character, including a class of polycarboxylic acids similar to fulvic acids (Fuzzi et al., 2002). However this method was not systematically adopted over the years, therefore no time trends of the organic fraction chemical composition can be provided. Only low molecular weight carboxylic acids easily detectable by ion chromatography, namely acetic acid (CH3COOH), formic acid (HCOOH), methanesulfonic acid (CH3SO3H) and oxalic acid (C2H2O4) were systematically measured over the study period. Concentrations are highly variable from sample to sample, ranging from a few to hundreds mmol L1. The best correlation between these low-molecular weight species and WSOC was found for oxalate (R2 ¼ 0.54). Oxalic acid is the endproduct of several aqueous-phase oxidation reactions involving the formation of “aqueous SOA” (Ervens et al., 2011; Lim et al., 2013; Tan et al., 2012). Our findings suggest that such reaction mechanisms can be a significant source of organic solutes in fog droplets. 3.2.4. Carbonaceous matter suspended in fog droplets The quantification of EC and POC concentrations on quartz-fibre filters was performed on samples collected in 1997/98, 1998/99, 1999/2000 and 2000/2001. Carbonaceous matter accounts for a significant fraction of the insoluble material suspended in fog water. The water insoluble organic mass was estimated from POC assuming that the lower and
Fig. 8. Statistical summary of pH values. Box plots indicate the 25th, median and 75th percentiles, whiskers the 10th and 90th percentiles. The dotted line indicates the regression line.
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The total OC concentration in fog, calculated as the sum of POC and WSOC, and the median of the POC to WSOC ratio are reported in Table 2. The POC fraction decreases slightly from 1997 to 2000, but the limited number of samples analysed during the first season (only 8 samples in 1997/1998) does not allow any speculation about the temporal trends. Further studies are needed to evaluate the change in the relative contribution of soluble and insoluble carbonaceous species in fog water over the years. The contribution of WSOC to the TOC (calculated as the sum of the soluble and insoluble organic fractions) ranges from 52 to 95% with an average of 86%. WSOC correlate with TOC with R2 ¼ 0.99. These findings are in agreement with previous studies carried out in different sites, where WSOC represent from 77% to 96% of TOC (Herckes et al., 2013). The concentrations of carbonaceous material in the Po Valley fog water are up to one order of magnitude higher than values reported in previous works for rural sites (Herckes et al., 2013), but comparable to urban fog as reported by Capel et al. (1990). 4. Conclusions
1
þ
þ
2þ
2þ
Fig. 9. Average chemical composition of fog water (mg L ). Na , K , Ca , Mg , Cl have been grouped under the category “other ions”. A factor of 1.8 was used to convert WSOC to water-soluble organic matter (WSOM) (Matta et al., 2003).
Fig. 10. Statistical summary of WSOC values. Box plots indicate the 25th, median and 75th percentiles, whiskers the 10th and 90th percentiles. Data for the season 2000/ 2001 are not available.
the upper bound of the OM to OC ratio were equal to 1.1 and 1.4, respectively (Aiken et al., 2008; Yttri et al., 2007). The sum of EC and water insoluble organic mass accounts on average for 46%e56% of the mass of total suspended material. The contribution of the carbonaceous species is comparable to the upper bound of the variability range reported in a similar study (Capel et al., 1990), i.e. 28e57%. We assume that the remaining fraction of insoluble mass is composed of mineral particles. Insoluble carbonaceous material is composed mainly by organic matter, in fact, EC accounts on average only 19% of the insoluble carbon. Absolute concentrations of POC and EC are highly variable. POC ranges between 1.6 and 206 mgC L1 with a median value of 5.8 mgC L1; EC ranges over two orders of magnitude (0.24 and 49 mgC L1), with 1.2 mgC L1 as median value. The ratio EC to POC, 0.24 on average, exhibits little variability among samples (standard deviation 0.10). The correlation of EC and POC observed through the different years (R2 ¼ 0.98) suggests that anthropogenic combustion processes, which represent the main source of EC, are also the most important source of POC in fog droplets.
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 observed reduction. Potential causes can be increasing temperatures or the decline of available condensation nuclei, but further studies are necessary to this end. A clear decreasing trend has been shown for ionic strength and conductivity, indicating a reduction of the ionic load of fog water. SO2 4 is the ionic species that exhibits the highest decrease (76% from 1990 to 2010). NO 3 also exhibits a decreasing trend although to lower extent (43%), as confirmed by the increasing values of 2 NO 3 /SO4 ratio over the study period. The low correlation between ion concentrations and LWC and the lack of a statistically significant trend for LWC, indicate that the observed decreasing concentrations of main ionic species is mainly due to a reduction in SO2 and NOx atmospheric emissions instead of a simple dilution effect. As a consequence of the lower content of the two main acidic 2 species (NO 3 and SO4 ) fog water pH exhibits an increase with volume-weighted means higher than 5 over the last decade. Carbonaceous matter accounts for about 50% of the insoluble material suspended in fog droplets and for almost 25% of the total solute mass of fog water. WSOC dominates over POC. No significant trend of WSOC and short chain organic acids concentrations was observed over the period for which WSOC measurements are available (1997/98e2010/11). To better understand the role of organic matter in the fog chemistry of the Po Valley, specific studies are currently ongoing in addition to the already set up measurement program detailed in this paper. To summarize, the results shown in this paper reflect the effectiveness of the implemented air quality policies and show that
Table 2 Seasonal median values of soluble and insoluble carbonaceous species. POC (mgC L1) EC (mgC L1) POC þ WSOC (mgC L1) POC/WSOC 1997/98 11.9 1998/99 4.23 1999/00 5.65 2000/01 6.00
2.24 0.95 1.25 1.16
66 30 55 n.a.
0.19 0.17 0.14 n.a.
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monitoring the chemical composition of fog water over extended periods can be a valuable tool to test the effectiveness of future policy actions to limit pollutants emission over the whole Po Valley basin. 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 Aiken, A.C., Decarlo, P.F., Kroll, J.H., Worsnop, D.R., Huffman, J.A., Docherty, K.S., Ulbrich, I.M., Mohr, C., Kimmel, J.R., Sueper, D., Sun, Y., Zhang, Q., Trimborn, A., Northway, M., Ziemann, P.J., Canagaratna, M.R., Onasch, T.B., Alfarra, M.R., Prevot, A.S.H., Dommen, J., Duplissy, J., Metzger, A., Baltensperger, U., Jimenez, J.L., 2008. 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. 42, 4478e4485. Aleksic, N., Dukett, J.E., 2010. Probabilistic relationship between liquid water content and ion concentrations in cloud water. Atmos. Res. 98, 400e405. Balmes, J.R., Fine, J.M., Gordon, T., Sheppard, D., 1989. Potential bronchoconstrictor stimuli in acid fog. Environ. Health Perspect. 79, 163e166. Butler, C.D., Trumble, J.T., 2008. Effects of pollutants on bottom-up and top-down processes in insect-plant interactions. Environ. Pollut. 156, 1e10. Capel, P.D., Gunde, R., Zurcher, F., Giger, W., 1990. Carbon speciation and surfacetension of fog. Environ. Sci. Technol. 24, 722e727. Collett Jr., J.L., Herckes, P., Youngster, S., Lee, T., 2008. Processing of atmospheric organic matter by California radiation fogs. Atmos. Res. 87, 232e241. Dall'Osto, M., Harrison, R.M., Coe, H., Williams, P., 2009. Real-time secondary aerosol formation during a fog event in London. Atmos. Chem. Phys. 9, 2459e2469. Decesari, S., Facchini, M.C., Fuzzi, S., Tagliavini, E., 2000. Characterization of watersoluble organic compounds in atmospheric aerosol: a new approach. J. Geophys. Res. Atmos. 105, 1481e1489. Decesari, S., Facchini, M.C., Matta, E., Lettini, F., Mircea, M., Fuzzi, S., Tagliavini, E., Putaud, J.P., 2001. Chemical features and seasonal variation of fine aerosol water-soluble organic compounds in the Po Valley, Italy. Atmos. Environ. 35, 3691e3699. Dollard, G.J., Unsworth, M.H., Harve, M.J., 1983. Pollutant transfer in upland regions by occult precipitation. Nature 302, 241e243. Elbert, W., Hoffmann, M.R., Kramer, M., Schmitt, G., Andreae, M.O., 2000. Control of solute concentrations in cloud and fog water by liquid water content. Atmos. Environ. 34, 1109e1122. Elbert, W., Kramer, M., Andreae, M.O., 2002. Reply to discussion on “control of solute concentrations in cloud and fog water by liquid water content”. Atmos. Environ. 36, 1909e1910. Ervens, B., Turpin, B.J., Weber, R.J., 2011. Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): a review of laboratory, field and model studies. Atmos. Chem. Phys. 11, 11069e11102. Facchini, M.C., Fuzzi, S., Lind, J.A., Fierlingeroberlinninger, H., Kalina, M., Puxbaum, H., Winiwarter, W., Arends, B.G., Wobrock, W., Jaeschke, W., Berner, A., Kruisz, C., 1992. Phase-partitioning and chemical-reactions of lowmolecular-weight organic-compounds in fog. Tellus Ser. B Chem. Phys. Meteorol. 44, 533e544. Fenn, M.E., Geiser, L., Bachman, R., Blubaugh, T.J., Bytnerowicz, A., 2007. Atmospheric deposition inputs and effects on lichen chemistry and indicator species in the Columbia River Gorge, USA. Environ. Pollut. 146, 77e91. Fuzzi, S., Facchini, M.C., Decesari, S., Matta, E., Mircea, M., 2002. Soluble organic compounds in fog and cloud droplets: what have we learned over the past few years? Atmos. Res. 64, 89e98. Fuzzi, S., Facchini, M.C., Orsi, G., Bonforte, G., Martinotti, W., Ziliani, G., Mazzali, P., Rossi, P., Natale, P., Grosa, M.M., Rampado, E., Vitali, P., Raffaelli, R., Azzini, G., Grotti, S., 1996. The NEVALPA project: a regional network for fog chemical climatology over the Po Valley basin. Atmos. Environ. 30, 201e213. Fuzzi, S., Facchini, M.C., Orsi, G., Lind, J.A., Wobrock, W., Kessel, M., Maser, R., Jaeschke, W., Enderle, K.H., Arends, B.G., Berner, A., Solly, I., Kruisz, C., Reischl, G., Pahl, S., Kaminski, U., Winkler, P., Ogren, J.A., Noone, K.J., Hallberg, A., Fierlingeroberlinninger, H., Puxbaum, H., Marzorati, A., Hansson, H.C., Wiedensohler, A., Svenningsson, I.B., Martinsson, B.G., Schell, D., Georgii, H.W., 1992. The Po Valley fog experiment 1989 e an overview. Tellus Ser. B Chem. Phys. Meteorol. 44, 448e468.
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