7.3 Subprogramme PC: Precipitation chemistry

• 7.3.1 Introduction
• 7.3.2 Sampling methods
• 7.3.2.1 Siting and number of collectors
• 7.3.2.2 Type of collector
• 7.3.2.3 Sampling frequency
• 7.3.2.4 Collection and handling of precipitation samples
• 7.3.3 Chemical analyses
• 7.3.4 Quality assurance/Quality control
• 7.3.5 Data reporting
• 7.3.6 References

7.3.1 Introduction

The purpose of the precipitation chemistry (PC) subprogramme is to quantify the input of precipitation and ions in precipitation (wet deposition) to the integrated monitoring area. The PC subprogramme shall also give information compatible with the throughfall (TF) subprogramme and further be comparable with other UNECE activities under the Convention on Long Range Transboundary Air Pollution (LRTAP). The deposition of pollutants to the ecosystems by precipitation is assumed to be a major factor affecting the natural processes in the environment. The main focus of the PC subprogramme is the sampling and chemical analysis of precipitation with particular emphasis on the acidifying compounds and on nutrients. By the simultaneous use of information from the subprogrammes on meteorology, air chemistry, throughfall, and stemflow, even total deposition to the site or parts of the site may be inferred for some compound.

At sites frequently influenced by fog and clouds, a significant fraction of the deposition input may deposit by fog (occult deposition). Since the usual precipitation sampler usually do not collect fog, other and more sophisticated methods are needed. At a workshop (Lövblad et al. 1993) current knowledge of cloud and fog deposition was evaluated. It should be noted, however, that the throughfall sampling will be influenced by fog deposition and may serve as an indicator of the amount of fog deposition.

It is strongly recommended that the participants of the ICP IM programme follow the recommended methods as described below. If methods other than those recommended in this manual are used, their comparability to the recommended method should be demonstrated by the data originators.

7.3.2 Sampling methods

7.3.2.1 Siting and number of collectors

Besides the general siting criteria given in Chapter 5.2, the specific siting criteria as described by EMEP, Chapter 2.2 should be followed.

The site should represent the deposition for the whole IM site. There should be no obstacles, such as trees, above 30° from the rim of the precipitation collector, and buildings, hedges, or topographical features which may give rise to updraughts or downdraughts should be avoided.

A detailed description of site requirements are given in the EMEP manual EMEP, Chapter 3.1.3. Data from neighbouring monitoring stations meeting the above mentioned criteria (e.g. EMEP sites) may be used for ICP IM purposes, provided that they can be shown to be representative for the IM site.

It is recommended to control the measurements of precipitation with a standard rain gauge. It is also recommended to have at least two parallel collectors if weekly samples are taken.

7.3.2.2 Type of collector

Wet deposition may be determined using a bulk precipitation sampler (sampler open also in dry periods) or a wet only sampler. The advantage of the wet only sampler is the reduced deposition of gases and particulates compared to the bulk, which may be a problem at sites with local emission sources and particularly for the coarse particulates. On the other hand the bulk sampler has several advantages; e.g. comparability to the throughfall measurements, no need for power supply, and the aerodynamical design may be better. In addition the bulk sampler has several advantages over the wet only sampler during the winter period. If the bulk sampler is used, it is important to verify to which extent gases and particles are deposited to the bulk sampler, and to take measures to avoid/ reduce dry deposition to the sampler.

Due to the easy maintenance of the bulk precipitation sampler it is recommended as the minimum mandatory equipment to be used at the IM sites.

The wet only sampling equipment is described in detail by EMEP, Chapter 3.1.4. including some general information about other types of collectors.

The construction principles for bulk deposition gauges are relatively simple. The sampler should not be too large or bulky, because this will obstruct the air flow around the sampler. On the other hand, the diameter of the collector must be large enough to provide samples large enough for chemical analysis. In practice, a diameter of 20 cm is sufficient for weekly sampling.

The sampling equipment normally consists of a funnel and a receiving vessel. If a funnel is used, there should be a vertical section of at least 5 cm height. An example of a bulk precipitation sampler is shown in figure 7.3.1. When precipitation is in the form of snow, it is advisable to use a special snow collector, an open cylinder of diameter 20 cm. The height of the cylinder should be at least twice the diameter to prevent "blow-out". The snow collector should be equipped with a tight-fitting lid, which is put on when the collector and sample is brought indoors for the sample to melt.

In order for the sample not to be contaminated from the ground during heavy rain, the rim of the funnel should be positioned 1.5-2 m above the ground level. Collectors should be designed to prevent contamination by bird droppings by e.g. a bird ring.

The material of the funnel and collection bottle should not in any way alter the chemical composition of the sample, and give a reliable measure of the amount of precipitation. Both funnel and sampling bottle should be cleaned with deionized water when samples are taken (and at least weekly to avoid contribution from dry deposition) even if no precipitation events have occurred.

Evaporation changes can be particularly serious, and may result in a significant enhancement of the concentration of the sample. Electrical heating of the precipitation collector in order to melt snow is therefore not recommended. Precipitation amount in the collector may by influenced by both low catch efficiency or blow off (particularly of snow) as well as evaporation. It is important the data originator verifies which of these processes have caused the deviation, and further reports the relevant amount for the determination of wet deposition (in case of evaporation, the observed amount should be reported whereas in case of lost sample, corrections should be made based on other available information). The simultaneous use of an automatic rain gauge and bulk deposition collectors is strongly recommended, as this will provide an indication of evaporative losses.

7.3.2.3 Sampling frequency

It is recommended that precipitation samples are taken so that correct monthly values can be derived. Due to quality assurance/quality control reasons individual sampling period should be as short as possible , weekly is recommended, daily if possible. If samples in addition are collected on the first of the month, monthly values can be determined. The sampling integration time should be harmonized with throughfall and stemflow and if practical with other relevant subprogrammes e.g. stream water.

Weekly sampling may to some extent cause biodegradation of the samples. By shielding the samplers using aluminium foil, this degradation will be strongly reduced. It is, however, not recommended to add preservatives. If samples are taken daily and kept cool, the decomposition, particularly of ammonium and nitrate, is strongly reduced.

Weekly samples can be analysed as they are or, in order to save expenses, mixed to monthly samples before analysis. If samples are mixed they must be mixed in proportion to the total sample volume. Special care must be taken in the mixing procedure in order to avoid contamination and errors.

7.3.2.4 Collection and handling of precipitation samples

The sampling procedure is described in EMEP, Chapter 3.1.5. There are, however, special demands for the trace metal sampling. Recommendations on sampling of heavy metals in precipitaion are included in the EMEP manual (see Chapter 3.10 in EMEP web-manual).

The general procedures for collection and handling of all water samples are described in Chapter 8.2.

7.3.3 Chemical analyses

The set of mandatory parameters for the PC subprogramme are: sulphate, nitrate, ammonium, chloride, sodium, potassium, calcium, magnesium and alkalinity (depending on pH). However, it is also recommended for quality assurance reasons to determine the electrical conductivity.

The use of adequate methods is the responsibility of the national institutes. A list of available standards is given in section 8.5.

The recommended method for determination of the major ions is ion chromatography. Suitable alternative methods are, for example, atomic absorption spectrometry for Na, K, Ca, Mg and spectrometric methods for ammonium. The recommended method is described in the EMEP manual, Chapter 4.1, alternative methods are described in EMEP, Chapters 4.2 - 4.6.

The recommended method for determination of pH, strong and weak acids is potentiometry, as described in the EMEP manual, Chapter 4.7. An alternative method for the determination of strong and weak acids is the coulometric titration method (modified Gran''s titration). This method is described in the EMEP manual, Chapter 4.8.

The recommended method for determination of conductivity is conductometry. The method is described in detail in the EMEP manual, Chapter 4.9.

The EMEP has implemented heavy metals in its monitoring programme in 1999. Recommendations on sampling and chemical analysis of heavy metals are now included in the EMEP manual (see EMEP web-manual).

7.3.4 Quality assurance/Quality control

It is very important to have a good quality of data, both being consistent in time (in order to assess trends) and space (for the comparisons between different sites and countries). The general procedures for quality assurance given by EMEP, Chapter 3.1.8 as well as procedures in Chapter 8 of this manual should be followed. The QA/QC procedures should include all parts of the activities performed at the site, and in the laboratory.

Standard operation procedures should be followed for all activities. Necessary equipment, cleaning materials, sufficient supply of spare parts etc. must be available. All operators should be well trained and sites and equipment must be inspected/controlled at least once a year by the quality assurance manager/data originator. The QA/QC routines in the field include addition of field blanks and control samples, and also requirements for sample transportation.

It is expected that the chemical laboratory is accredited under one of the laboratory accreditation systems, or is performing close to these standards, e.g. EN 45001 and ISO/IEC guide 25. The laboratory must check on its performance, with respect to detection limits, precision and repeatability, by repeated analyses of control solutions etc.

It is strongly recommended to participate annually in international intercomparisons for all analysed compounds. It is also recommended to participate in field intercomparisons. The ICP IM Programme Centre will be able to give information about relevant intercalibrations. All data should be verified and validated following the instructions given by EMEP, Chapter 5 and 6.

7.3.5 Data reporting

Example files

PC example Excel file
PC example ASCII file

• File identifier SUBPROG states the subprogramme.
• MEDIUM refers to either bulk deposition (BULK) or wet deposition (WET) sampler.
• LEVEL is given as the distance of collectors from the ground (cm).
• Spatial pool SPOOL refers to the number of individual samplers used for each parameter.
• Deposition values, except precipitation amount, are reported as monthly volume weighted means, status flag is W, monthly samples without status. Precipitation amount is reported as monthly sum, status flag is S. General information on flags is given in Chapter 4.
• Sampling year and month are given as YYYYMM, day field is left blank.
• No corrections for sea-salt derived sulphate should be made, rather it is emphasised that both sodium, magnesium and chloride should be reported. For calculation of volume weighted means, please see Annex 7. 

7.3.6 References

EMEP web-manual: EMEP manual for sampling and analysis
http://www.nilu.no/projects/ccc/manual/

EMEP manual for sampling and chemical analysis, EMEP/CCC-Report 1/95, NILU, Kjeller, Norway, March 1996.

ICP Forests Manual, 2016
http://icp-forests.net/page/icp-forests-manual

ICP Forests manual, 1997. Manual on methods and criteria for harmonized sampling, assessment, monitoring and analysis of the effects of air pollution on forests, 4th edition. Edited in 1997 by the Programme Coordination Centre Federal Research Centre for Forestry and Forest Products (BFH), Hamburg, Germany.

ICP Forests manual, 1994. Manual on methods and criteria for harmonized sampling, assessment, monitoring and analysis of the effects of air pollution on forests, 1994 edition. Edited by the Programme Coordination Centres Hamburg and Prague.

Lövblad, G., Erisman, J.W. and Fowler, D. (Eds), 1993. Models and methods for the quantification of atmospheric input to ecosystems. Report from a workshop held in Gothenburg, Sweden, 3-6. November 1992. Copenhagen, Nordic Council of Ministers (Nordiske Seminar- og Arbeidsrapporter 1993:573).