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The subalpine and alpine areas in North-Western Italy and Southern Switzerland receive high deposition of atmospheric pollutants transported from emission sources in the Po Valley, one of the most urbanised and industrialised areas of Europe. Long-term studies, covering a 30-year period, on atmospheric deposition and its effects on surface water bodies have been performed in this area in the framework of the ICP WATERS (BOX 1) and of national projects. Studies on both deposition and surface water chemistry have been performed since the beginning (early 1980’s) through a cooperation between Swiss and Italian research institutions. Fig.1a: Annual sulphur dioxide, nitrogen oxides and ammonia emissions in Switzerland, 1900-2020 The study area is located in the southern part of the Alps, in Canton of Ticino, Switzerland, and in the Piedmont Region, Italy (Fig. 2). Precipitation in this region is mainly determined by warm, humid air masses originating from the Mediterranean Sea, passing over the Po Plain and colliding with the Alps In total, 15 atmospheric deposition sampling sites (wet-only) operate in this area 1. Sites span a wide latitudinal and altitudinal range, from the subalpine sites of Pallanza, in Italy, and Lugano, in Switzerland (about 200 m a.s.l.) to the high-altitude sites of Devero and Robiei (1640 and 1890 m a.s.l., respectively).

Methods

Major findings

Assessment of long-term trends in atmospheric deposition chemistry along the Italian-Swiss border: an example of transboundary cooperation

Luca Colombo1, Michela Rogora2, Sandra Steingruber3

1 luca.colombo@supsi.ch: University of applied sciences and arts of southern Switzerland 2 m.rogora@ise.cnr.it: CNR Institute of Ecosystem Study, I-28922 Verbania Pallanza, Italy 3 sandra.steingruber@ti.ch, Air, climate and renewable energies office of Canton Ticino, Switzerland

References EMEP status report 1_2015, Rogora et al. The role of nitrogen deposition in the recent nitrate decline in lakes and rivers in Northern Italy. Science of the Total Environment 417-418C: 219-228, 2012. Rogora et al. Thirty years of chemical changes in alpine acid-sensitive lakes in the Alps, Water, Air, Soil Pollution, 224:1746, 2013 Steingruber: Acidifying deposition in southern Switzerland, Assessment of the trend 1988-2012; Technical report. Dipartimento del territorio del Cantone Ticino, Bellinzona, 11.5.2015.

ü  The clearest decrease in deposition through time can be observed for potential acidity, mainly determined by the significant decrease in deposition of sulphate. A decrease in deposition of nitrate and ammonium is more difficult to observe.

ü  Deposition of base cations tend to be constant over time but can increase significantly in correspondence of alkaline rain events. These events significantly contribute to the total deposition of calcium, magnesium and alkalinity but also of other chemical compounds such as nitrogen and phosphorus.

ü  The pattern of deposition still show a north-south gradient in recent year, with higher values in the southern part, even if much less evident with respect to the previous period

ü  At present, nitrate and ammonium are the dominant acidifying agent at these sites. Further reductions of N emissions and deposition, especially reduced N, are needed.

Introduction

BOX 4 - Focus on base cations and alkalinity deposition

Long-term trends and spatial patterns ü  The analysis of the data at a spatial level (Fig. 3) highlighted a gradient in the deposition of acidity, sulphate and nitrogen

compounds, with decreasing values with altitude and in the northern part of the area. ü  Deposition data show a high interannual variability, due to the highly variable precipitation amount affecting this area (from

1200-1300 mm in dry years up to 3000 mm) (BOX2). ü  The analysis of long-term trends revealed a substantial reduction in sulfate deposition, which almost halved since the 80’s (BOX2). ü  Deposition of oxidised and reduced nitrogen has not changed to the same extent, showing a slight tendency to decrease only in

the most recent period. Nitrogen deposition is still high, also at the alpine sites (32 and 27 meq m-2 y-1 at Devero and 42 and 48 meq m-2 y-1 at Robiei for ammonium and nitrate, respectively) (BOX 3).

ü  The NH4:NO3 ratio in atmospheric deposition has increased in time at most of the sites, from about 1.2 in the 1984-2002 period up to 1.4-1.5 in recent years, indicating an increasing importance of NH4 deposition (BOX3).

BOX1 - International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Rivers and Lakes

BOX 3 - Focus on nitrogen deposition

ICP Waters was established in July 1985 under the United Nation Economic Comission for Europe (UNECE) Executive Body of the Convention on Long-Range Transboundary Air Pollution (LRTAP). ICP Waters is a programme for monitoring of the effects of acid rain and air pollution on water and water courses. Twenty countries (18 European countries, USA and Canada) participate and supply monitoring data to the programme's central database at NIVA.

Fig. 2a - Study area and typical meteorological situation of the southern Alps: wet currents from south to north

Fig. 3 Map of the deposition of acidity, sulphate, nitrate and ammonium over the study area in two periods: 1993-97 and 2008-12. Details on the method used can be found in Steingruber (2014).

Samples are analysed for pH, conductivity, alkalinity, major anions and cations. Details on sampling and analytical procedures can be found in Rogora et al. 2012 and Steingruber 2015. The quality of the data was assured by regular participation to national and international intercomparisons. Weekly sampling of rainwater with wet-only samplers started in 1988. Monthly and yearly mean concentrations in precipitation were calculated by weighting weekly concentrations with the sampled precipitation volume, while monthly and yearly wet deposition were calculated by multiplying monthly and yearly mean concentrations with the precipitation volume measured at the site itself or at a meteorological sampling station close to the sampling site.

Fig. 4: Present-day network of ICPW sites

BOX 2 - Long-term trends

0"

100"

200"

300"

400"

1982" 1986" 1990" 1994" 1998" 2002" 2006" 2010" 2014"

mm"

""Domodossola"""Lugano"""Locarno"""Pallanza"

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3"

6"

9"

12"

15"

1982" 1986" 1990" 1994" 1998" 2002" 2006" 2010" 2014"

H+#(meq#m(2)#

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6"

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1982" 1986" 1990" 1994" 1998" 2002" 2006" 2010" 2014"

N"NO3%(meq%m"2)%

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2"

4"

6"

8"

10"

12"

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16"

18"

20"

1982" 1986" 1990" 1994" 1998" 2002" 2006" 2010" 2014"

N"NH4%(meq%m"2)%

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5"

10"

15"

20"

25"

1982" 1986" 1990" 1994" 1998" 2002" 2006" 2010" 2014"

SO4$(meq$m)2)$

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5"

10"

15"

20"

25"

1982" 1986" 1990" 1994" 1998" 2002" 2006" 2010" 2014"

Base%ca'ons%(meq%m-2)%

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20%

40%

60%

80%

100%

1984

1986

1988

1990

1992

1994

1996

1998

2000

2002

2004

2006

2008

2010

2012

2014

POTENTIAL ACIDITY - Pallanza

NH4 NO3 SO4

0%

20%

40%

60%

80%

100%

1988

1990

1992

1994

1996

1998

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2002

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Locarno

NH4 NO3 SO4

Acknowledgments This study was supported and funded by the italian Ministry of the Environment and Protection of Land and Sea and the swiss federal Office for the Environment, FOEN, through the ICP Waters Programme. We would like to thank Beat Achermann and Gaston Theis, FOEN, for their support. We would also like to thank all the staff involved in sampling and chemical analysis.

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

1982 1986 1990 1994 1998 2002 2006 2010 2014

NH4:NO3ra*o

Fig. 1b Wet deposition of nitrogen in mg(N) m−2

in 2013 (EMEP 2015)

2008-2012

Potential acidity [meq m-2 yr-1]8 - 2728 - 5455 - 8182 - 108109 - 135136 - 162163 - 189190 - 216217 - 243244 - 270

1993-1997

Potential acidity [meq m-2 yr-1]8 - 2728 - 5455 - 8182 - 108109 - 135136 - 162163 - 189190 - 216217 - 243244 - 270

1993-1997

Sulphate [meq m-2 yr-1]0 - 1314 - 2627 - 3940 - 5253 - 6566 - 7879 - 9192 - 104105 - 117118 - 130

2008-2012

Sulphate [meq m-2 yr-1]0 - 1314 - 2627 - 3940 - 5253 - 6566 - 7879 - 9192 - 104105 - 117118 - 130

1993-1997

Nitrate [meq m-2 yr-1]0 - 1010 - 2020 - 3030 - 4040 - 5050 - 6060 - 7070 - 8080 - 9090 - 100

2008-2012

Nitrate [meq m-2 yr-1]0 - 1010 - 2020 - 3030 - 4040 - 5050 - 6060 - 7070 - 8080 - 9090 - 100

1993-1997

Ammonium [meq m-2 yr-1]0 - 1213 - 2425 - 3637 - 4849 - 6061 - 7273 - 8485 - 9697 - 108109 - 120

2008-2012

Ammonium [meq m-2 yr-1]0 - 1213 - 2425 - 3637 - 4849 - 6061 - 7273 - 8485 - 9697 - 108109 - 120

These events, often of Saharan origin, occurred at a frequency of 2/3 per year. They are usually characterised by pH above 7 and high concentrations of base cations and alkalinity (up to 8-10 times higher than average values). They can contribute up to 60% of the total annual deposition of alkalinity and calcium, respectively.

The relative contribution of acidifying compounds to potential acidity at the sites of Pallanza (Italy) and Locarno (Switzerland) showed a decreasing contribution of SO4, while NO3 and N H 4 b e c a m e m o r e a n d m o r e important. Furthermore, ammonium has become the dominant form of N in deposition at most of the sites.

Weekly data collected at the site of Domodossola showed the occurrence of alkaline events, with extremely high calcium and alkalinity concentrations.

Monthly deposition values (12-point running averages) at 4 sites in the study area

Station Country Altitude, m.a.s.l. Data sincePallanza I 208 1975Domodossola I 270 1986Devero I 1634 1996Lunecco I 415 1988Bellinzago I 190 1985S.M. Orta I 360 1984Lugano CH 350 1982Piotta CH 1007 1990Acquarossa CH 575 1990Stabio CH 353 1990Robiei CH 1890 1996Monte Bré CH 925 1995

Fig. 2b – Location and characteristics of the deposition sampling sites along the swiss-italian border