# 1 Purpose of the document

This document describes the relevance, mission, requirements and obligations of the ACTRIS Centre for
Reactive Trace Gases in Situ Measurements.

# 2 Description and role of the Centre for Reactive Trace Gases in Situ Measurements

## 2.1 Framework

ACTRIS is the European platform for fostering the use of research data and data analysis tools in the field of atmospheric aerosols, clouds, and reactive trace gases. ACTRIS Central Facilities (CFs) represent the key operative entities of this Research Infrastructure (RI) and have a fundamental role as they provide operation support to the National Facilities (NFs) i.e. the observational and exploratory platforms as well as services to the users according to the ACTRIS access policy. CFs are operated at the European level and are part of the governance and decision-making structure of the RI. Each CF may have several operational Units that can be located in the same or different locations, and are operated by research performing organizations (RPOs) or by ACTRIS ERIC.

The Centre for Reactive Trace Gases in Situ Measurements (CGas-SiM) is one of the six ACTRIS Topical Centres (TCs) organized around the main scientific themes of ACTRIS: aerosols, clouds, and reactive trace gases, each with a particular focus on either remote sensing or in situ measurement techniques.

The CGasSiM is related to the ACTRIS tasks for the Reactive Trace Gases Calibration Centre specified in the ACTRIS ESFRI proposal, i.e. "Science Case" and the "Terms of reference" by Global Atmosphere Watch (GAW) of the World Meteorological Organisation (WMO) for the World Calibration Centres (WCC). These needs have passed the ESFRI evaluation process and the selection and evaluation of expert teams, the GAW Scientific Advisory Group for Reactive Trace Gases (SAG-RG) and the Commission of Atmospheric Science (CAS) by WMO. Thus, these needs are the outcome of well justified processes and are high level approved.

Within ACTRIS and ACTRIS-2 projects, the services have been further tested and implemented. Currently, the WCC for VOC and NOx are able to provide limited services to users owing to the very limited resources available. It is foreseen that more resources are made available within the ACTRIS RI to achieve a higher coverage and higher level of the services in cooperation with the NFs to provide the highest quality level of the data. In addition, with the introduction of several units, the load of the services will be shared by several institutions in Europe which are well recognized for their high expertise in reactive trace gas measurements and quality management.
# 1 Purpose of the document

This document describes the relevance, mission, requirements and obligations of the ACTRIS Centre for
Reactive Trace Gases in Situ Measurements.

# 2 Description and role of the Centre for Reactive Trace Gases in Situ Measurements

## 2.1 Framework

ACTRIS is the European platform for fostering the use of research data and data analysis tools in the field of atmospheric aerosols, clouds, and reactive trace gases. ACTRIS Central Facilities (CFs) represent the key operative entities of this Research Infrastructure (RI) and have a fundamental role as they provide operation support to the National Facilities (NFs) i.e. the observational and exploratory platforms as well as services to the users according to the ACTRIS access policy. CFs are operated at the European level and are part of the governance and decision-making structure of the RI. Each CF may have several operational Units that can be located in the same or different locations, and are operated by research performing organizations (RPOs) or by ACTRIS ERIC.

The Centre for Reactive Trace Gases in Situ Measurements (CGas-SiM) is one of the six ACTRIS Topical Centres (TCs) organized around the main scientific themes of ACTRIS: aerosols, clouds, and reactive trace gases, each with a particular focus on either remote sensing or in situ measurement techniques.

The CGasSiM is related to the ACTRIS tasks for the Reactive Trace Gases Calibration Centre specified in the ACTRIS ESFRI proposal, i.e. "Science Case" and the "Terms of reference" by Global Atmosphere Watch (GAW) of the World Meteorological Organisation (WMO) for the World Calibration Centres (WCC). These needs have passed the ESFRI evaluation process and the selection and evaluation of expert teams, the GAW Scientific Advisory Group for Reactive Trace Gases (SAG-RG) and the Commission of Atmospheric Science (CAS) by WMO. Thus, these needs are the outcome of well justified processes and are high level approved.

Within ACTRIS and ACTRIS-2 projects, the services have been further tested and implemented. Currently, the WCC for VOC and NOx are able to provide limited services to users owing to the very limited resources available. It is foreseen that more resources are made available within the ACTRIS RI to achieve a higher coverage and higher level of the services in cooperation with the NFs to provide the highest quality level of the data. In addition, with the introduction of several units, the load of the services will be shared by several institutions in Europe which are well recognized for their high expertise in reactive trace gas measurements and quality management.
## 2.2 Scientific relevance

Reactive trace gas species in the Earth's lower atmosphere are a large group of compounds with lifetimes between minutes to a few months. These gases are the players of atmospheric chemistry and determine among others the abundance of free radicals, reactive climate gases, toxic pollutants, aerosol precursors and compounds relevant for atmospheric deposition. Assessing the role and impacts of reactive trace gases in the atmosphere requires knowledge of the global distribution and long-term changes in their abundance especially under a climate change context. Observations of their background levels are also essential for understanding long-range pollution transport, which in turn is crucial for assessing the success of regional to local emission reduction plans (see Schultz et al., 2015).

Key Reactive Trace Gases, which are readily observable, are tropospheric ozone, carbon monoxide, volatile organic compounds (VOCs), reactive nitrogen gases (e.g. NO, $NO_2$), and reactive sulphur gases. The CGas-SiM in particular should consider the ACTRIS variables volatile organic compounds (VOC), nitrogen oxides (NO, $NO_2$) and specific inorganic and organic precursors for particles with measurement techniques described in Chapter 3.1 of this document.

ACTRIS network wide in situ observations of key organic and inorganic compounds in the atmosphere are essential for providing reliable scientific data of the chemical composition of the atmosphere, its natural and anthropogenic change, and improving the understanding of interactions within the atmosphere. This requires high data compatibility within the ACTRIS observation network and thus a well-developed quality assurance and quality control (QA/QC) program. For key-VOC and NOx, this is further elaborated in the WMO-GAW program (WMO-GAW, 2017). In ACTRIS-1 and ACTRIS-2, specific VOC and NOx data have successfully been improved and better inter-linked by a comprehensive QA/QC program than before (see Hörger et al., 2013). The ACTRIS RI will provide such high quality data more and more requested by data users e.g. for validating “chemical weather” forecast models, Near Real Time (NRT) assimilation into Global-Chemistry-Climate Models, validating satellite retrievals for e.g. formaldehyde, validating ground remote sensing data products on e.g. ethane. In addition, process studies of VOC oxidation in simulation chambers with near real ambient conditions may benefit from common QA/QC procedures.

For ACTRIS the CGas-SiM should ensure traceability and compatibility of network data and should comprise the WMO-GAW World Calibration Centres for Volatile Organic Compounds (WCC-VOC) and for NOx (WCC-NOx). Both centres have been approved by the international science community and form one corner stone of the QA/QC framework defined within WMO-GAW and thus already include substantial capacities and know-how.

ACTRIS-RI should make use of that knowledge as a basis to implement the CGas-SiM as pan-European effort with units in Germany, Finland, Switzerland, and France at the frontier of science and technical innovations.
## 2.2 Scientific relevance

Reactive trace gas species in the Earth's lower atmosphere are a large group of compounds with lifetimes between minutes to a few months. These gases are the players of atmospheric chemistry and determine among others the abundance of free radicals, reactive climate gases, toxic pollutants, aerosol precursors and compounds relevant for atmospheric deposition. Assessing the role and impacts of reactive trace gases in the atmosphere requires knowledge of the global distribution and long-term changes in their abundance especially under a climate change context. Observations of their background levels are also essential for understanding long-range pollution transport, which in turn is crucial for assessing the success of regional to local emission reduction plans (see Schultz et al., 2015).

Key Reactive Trace Gases, which are readily observable, are tropospheric ozone, carbon monoxide, volatile organic compounds (VOCs), reactive nitrogen gases (e.g. NO, $NO_2$), and reactive sulphur gases. The CGas-SiM in particular should consider the ACTRIS variables volatile organic compounds (VOC), nitrogen oxides (NO, $NO_2$) and specific inorganic and organic precursors for particles with measurement techniques described in Chapter 3.1 of this document.

ACTRIS network wide in situ observations of key organic and inorganic compounds in the atmosphere are essential for providing reliable scientific data of the chemical composition of the atmosphere, its natural and anthropogenic change, and improving the understanding of interactions within the atmosphere. This requires high data compatibility within the ACTRIS observation network and thus a well-developed quality assurance and quality control (QA/QC) program. For key-VOC and NOx, this is further elaborated in the WMO-GAW program (WMO-GAW, 2017). In ACTRIS-1 and ACTRIS-2, specific VOC and NOx data have successfully been improved and better inter-linked by a comprehensive QA/QC program than before (see Hörger et al., 2013). The ACTRIS RI will provide such high quality data more and more requested by data users e.g. for validating “chemical weather” forecast models, Near Real Time (NRT) assimilation into Global-Chemistry-Climate Models, validating satellite retrievals for e.g. formaldehyde, validating ground remote sensing data products on e.g. ethane. In addition, process studies of VOC oxidation in simulation chambers with near real ambient conditions may benefit from common QA/QC procedures.

For ACTRIS the CGas-SiM should ensure traceability and compatibility of network data and should comprise the WMO-GAW World Calibration Centres for Volatile Organic Compounds (WCC-VOC) and for NOx (WCC-NOx). Both centres have been approved by the international science community and form one corner stone of the QA/QC framework defined within WMO-GAW and thus already include substantial capacities and know-how.

ACTRIS-RI should make use of that knowledge as a basis to implement the CGas-SiM as pan-European effort with units in Germany, Finland, Switzerland, and France at the frontier of science and technical innovations.
## 2.3 Mission

The key-mission of the CGas-SiM should be to offer operational support to ACTRIS NFs operating instrumentation for continuous long-term measurements of VOC, volatile and semi-volatile aerosol precursors, guide research and service development in the reactive gases field, and develop towards future user needs utilizing newly emerging methodologies. While the main activities should focus on the ACTRIS and GAW communities, specialized services should be offered to users from academia, business, industry, and public services depending on the respective resources.

The core services of CGas-SiM should be to ensure (1) sustainable and traceable high quality data and data products with known accuracy, (2) to implement advanced technologies and data evaluation algorithms, and (3) to assist in sustaining the high competence of the operative staff by implementing and training in advanced measurement technologies and data evaluation algorithms. Additionally, CGas-SiM should provide customized services to NFs and users including testing of prototypes. The key activities of the CGas-SiM for observational platforms are highlighted in Figure 1.

<figure><img src="image_24.png" /></figure>

Figure 1: Scheme of the QA/QC system for observational platforms envisioned for the CGas-SiM. The CGas-SiM together with the station assembly should develop the QA Tools (white: developed in cooperation with GAW), should provide plausibility checks performed by stations and CGas-SiM, should control the issue tracker by setting up issues, should discuss and control actions by stations, and should close the issues upon successful finalisation by stations.
## 2.3 Mission

The key-mission of the CGas-SiM should be to offer operational support to ACTRIS NFs operating instrumentation for continuous long-term measurements of VOC, volatile and semi-volatile aerosol precursors, guide research and service development in the reactive gases field, and develop towards future user needs utilizing newly emerging methodologies. While the main activities should focus on the ACTRIS and GAW communities, specialized services should be offered to users from academia, business, industry, and public services depending on the respective resources.

The core services of CGas-SiM should be to ensure (1) sustainable and traceable high quality data and data products with known accuracy, (2) to implement advanced technologies and data evaluation algorithms, and (3) to assist in sustaining the high competence of the operative staff by implementing and training in advanced measurement technologies and data evaluation algorithms. Additionally, CGas-SiM should provide customized services to NFs and users including testing of prototypes. The key activities of the CGas-SiM for observational platforms are highlighted in Figure 1.

<figure><img src="image_22.png" /></figure>

Figure 1: Scheme of the QA/QC system for observational platforms envisioned for the CGas-SiM. The CGas-SiM together with the station assembly should develop the QA Tools (white: developed in cooperation with GAW), should provide plausibility checks performed by stations and CGas-SiM, should control the issue tracker by setting up issues, should discuss and control actions by stations, and should close the issues upon successful finalisation by stations.
The CGas-SiM activities should

1. establish adequate procedures and methodologies for measurement of VOCs, volatile and semi-volatile aerosol precursors in close cooperation with international bodies (e.g. GAW) and the NFs;

2. perform quality assurance and quality control of the measurements and data in the RI in close cooperation with NFs and the DC and provide tools for the data providers for checking the data before submission;

3. review yearly submitted data after first submission and provide guidance for improvement;

4. organise yearly a data quality workshop, where data is finally quality checked before submission to the data base;

5. ensure the traceability of data from the NFs to the primary standard (see Figure 2);

6. check the consistency of data and measurement methods by organising intercomparison exercises;

7. ensure a high performance of NFs by conducting station audits and training sessions;

8. promote technological development and knowledge transfer to NFs and users;

9. foster up-to-date capacity-building in the RI by conducting own high quality research with latest technologies contributing to research and development projects.

<figure><img src="image_25.png" /><figcaption>Figure 2: Scheme demonstrating how the SI traceable scale of reactive trace gases should be maintained and transferred to the National Facilities.</figcaption></figure>

The CGas-SiM then should act as a living organisation with adapting its procedures and performance to future needs resulting from research and development projects with focus on analytical methods and traceability of new components in cooperation with NFs and other TCs within their capacity and depending on their available resources
The CGas-SiM activities should

1. establish adequate procedures and methodologies for measurement of VOCs, volatile and semi-volatile aerosol precursors in close cooperation with international bodies (e.g. GAW) and the NFs;

2. perform quality assurance and quality control of the measurements and data in the RI in close cooperation with NFs and the DC and provide tools for the data providers for checking the data before submission;

3. review yearly submitted data after first submission and provide guidance for improvement;

4. organise yearly a data quality workshop, where data is finally quality checked before submission to the data base;

5. ensure the traceability of data from the NFs to the primary standard (see Figure 2);

6. check the consistency of data and measurement methods by organising intercomparison exercises;

7. ensure a high performance of NFs by conducting station audits and training sessions;

8. promote technological development and knowledge transfer to NFs and users;

9. foster up-to-date capacity-building in the RI by conducting own high quality research with latest technologies contributing to research and development projects.

<figure><img src="image_23.png" /><figcaption>Figure 2: Scheme demonstrating how the SI traceable scale of reactive trace gases should be maintained and transferred to the National Facilities.</figcaption></figure>

The CGas-SiM then should act as a living organisation with adapting its procedures and performance to future needs resulting from research and development projects with focus on analytical methods and traceability of new components in cooperation with NFs and other TCs within their capacity and depending on their available resources
## 3 Operation support provided to ACTRIS National Facilities

The operation support to NFs which should be provided by the CGas-SiM include:
a) procedures and tools for quality assurance and quality control of measurements and data,
b) transfer of knowledge and training to operators, and
c) improvements of measurement methodologies. The operation support should focus on ACTRIS variables and measurement techniques described in this concept document, also listed in the *Technical Concepts and Requirements for ACTRIS Observational and Exploratory Platforms*.

The CGas-SiM is obliged to put in place and offer to NFs the specific operation support marked with * in this document. Other operation support described in this document is not mandatory for the TC, but recommended.

The operation support can be scheduled or on request. Participation at the scheduled activities is mandatory for the NFs. Measurements not following the mandatory actions are not considered as ACTRIS-conform and do not yield ACTRIS data.

In addition, NFs may request operation support which is not scheduled, depending on the identified need. Specific operation support offered as scheduled or offered on request is detailed in *Annex: Provision of the operation support*. This annex will be updated each time as it is necessary (e.g. development of new testing or calibration methods and tools).

The CGas-SiM should operate at the state-of-the-art, fostering the implementation of validated new techniques in the RI. To sustain a high level of performance and to stimulate the advancement of new techniques and methodologies, the CGas-SiM should contribute to research and expert collaboration networks.

### 3.1 Measurement techniques covered by the Centre for Reactive Trace Gases in Situ Measurements, and related ACTRIS variables

The ACTRIS variables covered by reactive trace gases in situ measurements should include tracers for anthropogenic/biogenic activities as well as chemical process indicators in the atmosphere. The compounds considered here focus on: (1) non-methane hydrocarbons (up to 40 NMHCs; e.g. ethane, ethyne, propane, i-/n-butane, i-/n-pentane, isoprene, benzene, toluene), (2) the biogenic VOCs (BVOC) with the monoterpenes (e.g. α-/β-pinene, 3-carene, 1,8-cineol, limonene), (3) the oxyVOC (e.g. formaldehyde, methanol, ethanol, acetone), (4) the direct aerosol precursors sulfuric acid and highly oxygenated molecules (HOM), and (5) NO and $NO_2$. In order to detect and quantify the target compounds in ambient air, specific sampling and analytical techniques should be implemented.
## 3 Operation support provided to ACTRIS National Facilities

The operation support to NFs which should be provided by the CGas-SiM include:
a) procedures and tools for quality assurance and quality control of measurements and data, b) transfer of knowledge and training to operators, and c) improvements of measurement methodologies. The operation support should focus on ACTRIS variables and measurement techniques described in this concept document, also listed in the Technical Concepts and Requirements for ACTRIS Observational and Exploratory Platforms.

The CGas-SiM is obliged to put in place and offer to NFs the specific operation support marked with * in this document. Other operation support described in this document is not mandatory for the TC, but recommended.

The operation support can be scheduled or on request. Participation at the scheduled activities is mandatory for the NFs. Measurements not following the mandatory actions are not considered as ACTRIS-conform and do not yield ACTRIS data.

In addition, NFs may request operation support which is not scheduled, depending on the identified need. Specific operation support offered as scheduled or offered on request is detailed in Annex: Provision of the operation support. This annex will be updated each time as it is necessary (e.g. development of new testing or calibration methods and tools).

The CGas-SiM should operate at the state-of-the-art, fostering the implementation of validated new techniques in the RI. To sustain a high level of performance and to stimulate the advancement of new techniques and methodologies, the CGas-SiM should contribute to research and expert collaboration networks.

### 3.1 Measurement techniques covered by the Centre for Reactive Trace Gases in Situ Measurements, and related ACTRIS variables

The ACTRIS variables covered by reactive trace gases in situ measurements should include tracers for anthropogenic/biogenic activities as well as chemical process indicators in the atmosphere. The compounds considered here focus on: (1) non-methane hydrocarbons (up to 40 NMHCs; e.g. ethane, ethyne, propane, i-/n-butane, i-/n-pentane, isoprene, benzene, toluene), (2) the biogenic VOCs (BVOC) with the monoterpenes (e.g. α-/β-pinene, 3-carene, 1,8-cineol, limonene), (3) the oxyVOC (e.g. formaldehyde, methanol, ethanol, acetone), (4) the direct aerosol precursors sulfuric acid and highly oxygenated molecules (HOM), and (5) NO and $NO_2$. In order to detect and quantify the target compounds in ambient air, specific sampling and analytical techniques should be implemented.
### 3.1.1 NMHC Gas chromatographic System

The measurement of NMHCs in air by gas chromatography (GC) is described in more detail in the corresponding Measurement Guideline (MG) document (SOP-VOC, 2014). Briefly, measurements can generally be performed in a series of steps with (1) taking a sample, (2) pre-concentration (PC), (3) thermo-desorption (TD), (4) gas chromatographic separation, (5) detection, and (6) data processing and data delivery. For taking an air sample, two options should be implemented: (a) taking the sample air directly to the pre-concentrating system with subsequent analysis (on-line) or (b) via a canister or adsorption trap and later analysis (off-line). The air sample treatment before analysis is prone to loss of target substances and artefact formation and needs careful assessment. For off-line sampling with stainless steel canisters the standard operation procedure of GAW (GAW-Report 204, WMO 2012) should be followed. The detection of NMHC by GC can be by flame ionization detection (FID) or mass spectrometry (MS).

The preferable technique should be on-line sampling for lower risk of artefacts. The QA/QC for NMHC-gas chromatographic system is complex and listed in the MG, it includes a.o. regular zero, calibration and target gas measurements with traceable standards. This measurement guideline should be updated according new developments.

### 3.1.2 BVOC Gas chromatographic System

The measurements of BVOC namely isoprene and monoterpenes (isoprenoids) in air samples can follow similar procedures as described for NMHC (see Chapter 3.1.1) by using a PC-TD-GC preferably with MS detection. FID is also possible with some peculiarities to consider.

The BVOC system should be tuned with respect to (a) pre-concentrating, (b) thermo-desorbing, (c) detecting, and (d) quantifying the target compounds. For pre-concentrating isoprenoids on adsorbents, special care is required to achieve total scrubbing of the target compounds and to avoid oxidation of the already adsorbed compounds during the sampling process. Further, during the thermo-desorption process isoprenoids may degrade when in contact with hot active adsorbents or metals surfaces which impact complete recovery. The complex compound matrix in ambient air may affect a sound identification of the isoprenoids. Thus a mass selective detection can be recommended, but in highly resolving GC systems also detection by FID can be acceptable. The sensitivity changes and drift of a MS detector can be accounted for by more frequent calibration. For this technique well advanced QA/QC procedures including calibration, artefacts, performance assessments and specific data validation processes should be developed and provided in dedicated measurement guidelines.

### 3.1.3 Oxy-VOC Measurement Systems

Oxygen containing volatile compounds in air can be analysed using GC-FID, GC-MS, or GC-FID/MS as described above. But, great care should be taken to minimize surface adsorptive losses and releases and to suppress oxidation reactions in the analytical system of VOC that can react with ozone or oxygen to form additional OVOCs as artefacts.
### 3.1.1 NMHC Gas chromatographic System

The measurement of NMHCs in air by gas chromatography (GC) is described in more detail in the corresponding Measurement Guideline (MG) document (SOP-VOC, 2014). Briefly, measurements can generally be performed in a series of steps with (1) taking a sample, (2) pre-concentration (PC), (3) thermo-desorption (TD), (4) gas chromatographic separation, (5) detection, and (6) data processing and data delivery. For taking an air sample, two options should be implemented: (a) taking the sample air directly to the pre-concentrating system with subsequent analysis (on-line) or (b) via a canister or adsorption trap and later analysis (off-line). The air sample treatment before analysis is prone to loss of target substances and artefact formation and needs careful assessment. For off-line sampling with stainless steel canisters the standard operation procedure of GAW (GAW-Report 204, WMO 2012) should be followed. The detection of NMHC by GC can be by flame ionization detection (FID) or mass spectrometry (MS).

The preferable technique should be on-line sampling for lower risk of artefacts. The QA/QC for NMHC-gas chromatographic system is complex and listed in the MG, it includes a.o. regular zero, calibration and target gas measurements with traceable standards. This measurement guideline should be updated according new developments.

### 3.1.2 BVOC Gas chromatographic System

The measurements of BVOC namely isoprene and monoterpenes (isoprenoids) in air samples can follow similar procedures as described for NMHC (see Chapter 3.1.1) by using a PC-TD-GC preferably with MS detection. FID is also possible with some peculiarities to consider.

The BVOC system should be tuned with respect to (a) pre-concentrating, (b) thermo-desorbing, (c) detecting, and (d) quantifying the target compounds. For pre-concentrating isoprenoids on adsorbents, special care is required to achieve total scrubbing of the target compounds and to avoid oxidation of the already adsorbed compounds during the sampling process. Further, during the thermo-desorption process isoprenoids may degrade when in contact with hot active adsorbents or metals surfaces which impact complete recovery. The complex compound matrix in ambient air may affect a sound identification of the isoprenoids. Thus a mass selective detection can be recommended, but in highly resolving GC systems also detection by FID can be acceptable. The sensitivity changes and drift of a MS detector can be accounted for by more frequent calibration. For this technique well advanced QA/QC procedures including calibration, artefacts, performance assessments and specific data validation processes should be developed and provided in dedicated measurement guidelines.

### 3.1.3 Oxy-VOC Measurement Systems

Oxygen containing volatile compounds in air can be analysed using GC-FID, GC-MS, or GC-FID/MS as described above. But, great care should be taken to minimize surface adsorptive losses and releases and to suppress oxidation reactions in the analytical system of VOC that can react with ozone or oxygen to form additional OVOCs as artefacts.
Some light oxy-VOC including formaldehyde, other aldehydes and ketones can be measured using an off-line method. Ambient air is sampled through a silica gel adsorbent coated with 2,4-Dinitrophenylhydrazine (DNPH). The corresponding derivates can then be analysed after liquid extraction with high performance liquid chromatography and UV detection (HPLC-UV). While this method is sensitive, it is subject to interferences and artefacts which should be carefully evaluated. Formaldehyde specifically can be measured on-line using the Hantzsch reaction. The detection of formaldehyde is based on the liquid phase reaction of formaldehyde with acetyl acetone (2,4-pentadione) and ammonia. This reaction produces 3,5-diacetly-1,4-dihydrolutidine (DDL), which absorbs light at 410 nm and shows a strong fluorescence around 510 nm, measured by a photomultiplier. This analytical method can achieve a high selectivity, avoiding mostly interferences of other chemical substances in the sample gas. The Hantzsch reaction is conducted in aqueous solution thus gaseous formaldehyde is transferred into aqueous solution first using a stripping coil. Size, temperature and flow rate of the stripping coil should be optimised for a quantitative reaction of HCHO. Details on all processes employed should be provided in a corresponding measurement guideline to be established.

### 3.1.4 Proton-Transfer-Mass-Spectroscopy

Proton Transfer Reaction-Mass Spectrometer (PTR-MS) is a fast on-line analytical technique for VOCs capable of measuring compounds with higher proton affinity than water molecules, e.g. selected OVOC, unsaturated NMHCs (aromatics, few alkenes, the sum of monoterpenes). In a drift tube VOCs are protonised and subsequently detected by a quadrupole or a time-of-flight (ToF) MS. No specific sample preparation is necessary but isobaric compounds cannot be separated and compounds with a lower proton-affinity than water cannot be detected. Thus, it should be carefully assured that all compounds of interest are detected and no other compound is co-detected together with the target compounds. Details should be provided in a corresponding measurement guideline to be established.

### 3.1.5 Direct Aerosol Precursor Measurement Systems

The chemical ionisation (CI) atmospheric pressure interface (API) time of flight (ToF) mass spectrometer can be used to characterise condensing aerosol precursor molecules and clusters comprehensively. This instrument is capable of resolving e.g. the concentrations of atmospheric sulfuric acid, iodic acid and oxidized organic compounds in a quantified manner. It can also resolve the chemical composition of neutral clusters. Various reagent ion chemistries can be applied for charging of the sample molecules and clusters (e.g. $NO_3^-$, $I^-$, $CH_3COO^-$). With the nitrate ion chemistry at ambient pressure, it is possible to ionize and quantify concentrations of e.g. sulfuric acid or iodine oxide containing clusters and detect Highly Oxygenated Molecules (HOM, e.g. $C_{10}H_{14}O_9$) in air samples, online and with fast time resolution. Details should be provided in corresponding measurement guidelines to be established.
Some light oxy-VOC including formaldehyde, other aldehydes and ketones can be measured using an off-line method. Ambient air is sampled through a silica gel adsorbent coated with 2,4-Dinitrophenylhydrazine (DNPH). The corresponding derivates can then be analysed after liquid extraction with high performance liquid chromatography and UV detection (HPLC-UV). While this method is sensitive, it is subject to interferences and artefacts which should be carefully evaluated. Formaldehyde specifically can be measured on-line using the Hantzsch reaction. The detection of formaldehyde is based on the liquid phase reaction of formaldehyde with acetyl acetone (2,4-pentadione) and ammonia. This reaction produces 3,5-diacetly-1,4-dihydrolutidine (DDL), which absorbs light at 410 nm and shows a strong fluorescence around 510 nm, measured by a photomultiplier. This analytical method can achieve a high selectivity, avoiding mostly interferences of other chemical substances in the sample gas. The Hantzsch reaction is conducted in aqueous solution thus gaseous formaldehyde is transferred into aqueous solution first using a stripping coil. Size, temperature and flow rate of the stripping coil should be optimised for a quantitative reaction of HCHO. Details on all processes employed should be provided in a corresponding measurement guideline to be established.

### 3.1.4 Proton-Transfer-Mass-Spectroscopy

Proton Transfer Reaction-Mass Spectrometer (PTR-MS) is a fast on-line analytical technique for VOCs capable of measuring compounds with higher proton affinity than water molecules, e.g. selected OVOC, unsaturated NMHCs (aromatics, few alkenes, the sum of monoterpenes). In a drift tube VOCs are protonised and subsequently detected by a quadrupole or a time-of-flight (ToF) MS. No specific sample preparation is necessary but isobaric compounds cannot be separated and compounds with a lower proton-affinity than water cannot be detected. Thus, it should be carefully assured that all compounds of interest are detected and no other compound is co-detected together with the target compounds. Details should be provided in a corresponding measurement guideline to be established.

### 3.1.5 Direct Aerosol Precursor Measurement Systems

The chemical ionisation (CI) atmospheric pressure interface (API) time of flight (ToF) mass spectrometer can be used to characterise condensing aerosol precursor molecules and clusters comprehensively. This instrument is capable of resolving e.g. the concentrations of atmospheric sulfuric acid, iodic acid and oxidized organic compounds in a quantified manner. It can also resolve the chemical composition of neutral clusters. Various reagent ion chemistries can be applied for charging of the sample molecules and clusters (e.g. $NO_3^-$, $I^-$, $CH_3COO^-$). With the nitrate ion chemistry at ambient pressure, it is possible to ionize and quantify concentrations of e.g. sulfuric acid or iodine oxide containing clusters and detect Highly Oxygenated Molecules (HOM, e.g. $C_{10}H_{14}O_9$) in air samples, online and with fast time resolution. Details should be provided in corresponding measurement guidelines to be established.
### 3.1.6 Nitrogen Oxides Measurement Systems

NO in air can be measured using the chemiluminescence detection (CLD) method. Ambient NO reacts with added ozone to yield excited $NO_2$, and emitted luminescence photons are detected by a photomultiplier.

$NO_2$ can be measured using either photolytical conversion (PLC) to NO subsequently measured as described above or employing the cavity attenuated phase shift (CAPS) technique.

The photolysis of $NO_2$ is conducted in a conversion cell. $NO_2$ is photolytically converted into NO and O. The NO is then detected by a chemiluminescence instrument while the O reacts with $O_2$ to yield $O_3$. For this photolytic conversion, either UV emitting lamps, e.g. Xe-lamps, or high-power-light emitting diodes (blue light converter; BLC) can be used.

The CAPS technique relies on producing very long optical paths (up to 2 km) using very high reflectivity mirrors in a sampling cell that is less than 30 cm in length. These monitors utilize a light emitting diode (LED) as an optical source and measure the resultant signal in the frequency domain using a phase shift technique. The CAPS technique is in theory an absolute method, since it provides a direct absorption measurement of nitrogen dioxide at 450 nm, but needs in reality calibration and offset correction. Details of the methods described are in part already reported in the $NO_x$ measurement guidelines (SOP-NOx, 2014) and should be updated according to new developments.

## 3.2 Estimation of the need

In Europe, the ACTRIS observational sites are part of the Emission Monitoring and Evaluation of the Long Range Transmission of Air (EMEP) network of the Convention on Long-Range Transboundary Air Pollution (CLRTAP). Currently, 26 Station are operating VOC and/or NOx measurement systems which require extended QA/QC measures for compatible data (Figure 3). Also the ACTRIS exploratory platform EUROCHAMP-2020 with its 18 atmospheric simulation chambers should benefit from the QA/QC activities of CGas-SiM. Already some ACTRIS and EMEP stations contribute QA/QC data to the WMO-GAW program, currently hosting 37 approved global stations and more than 600 regional and contributing stations with 250 in Europe (Figure 4). WMO-GAW with its contributing networks forms the largest global, atmospheric observation network for research and outreach products are used for science, policy and journalism advice.
### 3.1.6 Nitrogen Oxides Measurement Systems

NO in air can be measured using the chemiluminescence detection (CLD) method. Ambient NO reacts with added ozone to yield excited $NO_2$, and emitted luminescence photons are detected by a photomultiplier.

$NO_2$ can be measured using either photolytical conversion (PLC) to NO subsequently measured as described above or employing the cavity attenuated phase shift (CAPS) technique.

The photolysis of $NO_2$ is conducted in a conversion cell. $NO_2$ is photolytically converted into NO and O. The NO is then detected by a chemiluminescence instrument while the O reacts with $O_2$ to yield $O_3$. For this photolytic conversion, either UV emitting lamps, e.g. Xe-lamps, or high-power-light emitting diodes (blue light converter; BLC) can be used.

The CAPS technique relies on producing very long optical paths (up to 2 km) using very high reflectivity mirrors in a sampling cell that is less than 30 cm in length. These monitors utilize a light emitting diode (LED) as an optical source and measure the resultant signal in the frequency domain using a phase shift technique. The CAPS technique is in theory an absolute method, since it provides a direct absorption measurement of nitrogen dioxide at 450 nm, but needs in reality calibration and offset correction. Details of the methods described are in part already reported in the $NO_x$ measurement guidelines (SOP-NOx, 2014) and should be updated according to new developments.

## 3.2 Estimation of the need

In Europe, the ACTRIS observational sites are part of the Emission Monitoring and Evaluation of the Long Range Transmission of Air (EMEP) network of the Convention on Long-Range Transboundary Air Pollution (CLRTAP). Currently, 26 Station are operating VOC and/or NOx measurement systems which require extended QA/QC measures for compatible data (Figure 3). Also the ACTRIS exploratory platform EUROCHAMP-2020 with its 18 atmospheric simulation chambers should benefit from the QA/QC activities of CGas-SiM. Already some ACTRIS and EMEP stations contribute QA/QC data to the WMO-GAW program, currently hosting 37 approved global stations and more than 600 regional and contributing stations with 250 in Europe (Figure 4). WMO-GAW with its contributing networks forms the largest global, atmospheric observation network for research and outreach products are used for science, policy and journalism advice.
<figure><img src="image_26.png" /></figure>

Figure 3: ACTRIS observational sites providing quality assured reactive trace gas in situ data and complying with ACTRIS recommendations in 2016.

<figure><img src="image_27.png" /></figure>

Figure 4. GAW Global stations (left) and the European GAW Global- and Regional stations for VOC and/or (right; https://gawsis.meteoswiss.ch/GAWSIS/index.html).

Details on WMO-GAW are available in the recently published implementation plan for 2016 to 2023 (WMO-GAW 2017).
<figure><img src="image_24.png" /><figcaption>Figure 3: ACTRIS observational sites providing quality assured reactive trace gas in situ data and complying with ACTRIS recommendations in 2016.</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>Figure 4. GAW Global stations (left) and the European GAW Global- and Regional stations for VOC and/or (right; https://gawsis.meteoswiss.ch/GAWSIS/index.html).</figcaption></figure>

Details on WMO-GAW are available in the recently published implementation plan for 2016 to 2023 (WMO-GAW 2017).
Within these networks, currently 92 additional users of the CGas-SiM were identified.

The corner stone for good quality data is a well elaborated QA/QC framework as shown by. WMO-GAW with its central facilities (1) Central Calibration laboratories (CCL), (2) the Quality Assurance – Science Activity Centres (QA-SAC), (3) the World/Regional Calibration Centres (WCCs/RCCs) and (4) the World data Centre (WDCs). In ACTRIS, this quality management framework should form the basis for CGas-SiM. Up to now, for key reactive trace gases only the data quality of non-methane hydrocarbons targets (NMHCs; ethane, ethyne, propane, i-/n-butane, i-/n-pentane, isoprene, benzene, toluene) has been fully assed. The QA/QC for other VOC with biogenic VOCs (BVOC) like monoterpenes (α-/β-pinene, 3-carene, 1,8-cineol, limonene), oxyVOC (formaldehyde, methanol, ethanol, acetone), sulfurVOC (dimethylsuflide DMS), and nitroVOC (acetonitril) as well as NO and NO₂ still is on the agenda. Within the ACTRIS-RI the CGas-SiM should extend the QA/QC checks to the ACTRIS variables measured at ACTRIS observational and exploratory platforms. Table 3-1 provides a summary of the required measurement techniques and a number estimate of groups where potentially operational support is provided.

<table><caption>Table 3-1: Summary of the required potential of the CGAS-SiM</caption><thead><tr><th rowspan="3">Measurement technique</th><th colspan="3">Number of instruments/groups to which the TC should provide operational support</th></tr><tr><th rowspan="2">Now</th><th colspan="2">by 2025</th></tr><tr><th>Min.</th><th>Max.</th></tr></thead><tbody><tr><td>NMHC-GC</td><td>15</td><td>20</td><td>40</td></tr><tr><td>BVOC-GC</td><td>2</td><td>4</td><td>8</td></tr><tr><td>OVOC-GC</td><td>4</td><td>6</td><td>10</td></tr><tr><td>OVOC-DNPH-HPLC</td><td>4</td><td>6</td><td>10</td></tr><tr><td>HCHO-HANTZSCH</td><td>3</td><td>4</td><td>10</td></tr><tr><td>PTR-MS</td><td>4</td><td>6</td><td>10</td></tr><tr><td>CI-API-ToF</td><td>5</td><td>10</td><td>20</td></tr><tr><td>NO-CLD</td><td>20</td><td>25</td><td>40</td></tr><tr><td>NO₂-PLC/BLC-CLD</td><td>20</td><td>25</td><td>40</td></tr><tr><td>NO₂-CAPS</td><td>5</td><td>10</td><td>20</td></tr></tbody></table>
Within these networks, currently 92 additional users of the CGas-SiM were identified.

The corner stone for good quality data is a well elaborated QA/QC framework as shown by. WMO-GAW with its central facilities (1) Central Calibration laboratories (CCL), (2) the Quality Assurance – Science Activity Centres (QA-SAC), (3) the World/Regional Calibration Centres (WCCs/RCCs) and (4) the World data Centre (WDCs). In ACTRIS, this quality management framework should form the basis for CGas-SiM. Up to now, for key reactive trace gases only the data quality of non-methane hydrocarbons targets (NMHCs; ethane, ethyne, propane, i-/n-butane, i-/n-pentane, isoprene, benzene, toluene) has been fully assed. The QA/QC for other VOC with biogenic VOCs (BVOC) like monoterpenes (α-/β-pinene, 3-carene, 1,8-cineol, limonene), oxyVOC (formaldehyde, methanol, ethanol, acetone), sulfurVOC (dimethylsuflide DMS), and nitroVOC (acetonitril) as well as NO and NO₂ still is on the agenda. Within the ACTRIS-RI the CGas-SiM should extend the QA/QC checks to the ACTRIS variables measured at ACTRIS observational and exploratory platforms. Table 3-1 provides a summary of the required measurement techniques and a number estimate of groups where potentially operational support is provided.

<table><caption>Table 3-1: Summary of the required potential of the CGAS-SiM</caption><thead><tr><th rowspan="3">Measurement technique</th><th colspan="3">Number of instruments/groups to which the TC should provide operational support</th></tr><tr><th rowspan="2">Now</th><th colspan="2">by 2025</th></tr><tr><th>Min.</th><th>Max.</th></tr></thead><tbody><tr><td>NMHC-GC</td><td>15</td><td>20</td><td>40</td></tr><tr><td>BVOC-GC</td><td>2</td><td>4</td><td>8</td></tr><tr><td>OVOC-GC</td><td>4</td><td>6</td><td>10</td></tr><tr><td>OVOC-DNPH-HPLC</td><td>4</td><td>6</td><td>10</td></tr><tr><td>HCHO-HANTZSCH</td><td>3</td><td>4</td><td>10</td></tr><tr><td>PTR-MS</td><td>4</td><td>6</td><td>10</td></tr><tr><td>CI-API-ToF</td><td>5</td><td>10</td><td>20</td></tr><tr><td>NO-CLD</td><td>20</td><td>25</td><td>40</td></tr><tr><td>NO₂-PLC/BLC-CLD</td><td>20</td><td>25</td><td>40</td></tr><tr><td>NO₂-CAPS</td><td>5</td><td>10</td><td>20</td></tr></tbody></table>
### 3.3 Timeline for implementation of the mandatory operation support

Considering the ACTRIS roadmap, CGas-SiM should consider the following implementation plan for the mandatory operation support (including general tasks, scheduled and on request operation support, see Annex: Provision of the operation support):

<table><thead><tr><th rowspan="2">Operation support</th><th rowspan="2">Type</th><th>Preparation phase</th><th>Implementation phase</th><th>Operation phase</th></tr><tr><th>&lt; 2020</th><th>2020 - 2025</th><th>&gt; 2025</th></tr></thead><tbody><tr><td>Measurement Guidelines (MGs) and Standard Operation Procedures (SOPs)</td><td>General task</td><td>Basic MGs/SOPs including QA/QC procedures for operation of the instruments and data analysis (minimum requirements)</td><td>Advanced/New MGs/SOPs including QA/QC procedures for operation of the instruments and data analysis (minimum requirements) (optimum setup)</td><td>Updates and new MGs/SOPs</td></tr><tr><td>Scale transfer</td><td>Scheduled/ On request</td><td>Provision of scale to working standards for selected trace gases, on request only</td><td>Continue provision of scale to working standards of selected trace gases, on request only and update working standards with new scales</td><td>Continue as scheduled support every four years and on request including updates</td></tr><tr><td>New Instrument techniques</td><td>On request</td><td>Direct comparison of selected trace gases with fixed, state-of-the-art, reference instruments</td><td>Direct comparison of selected trace gases with fixed, state-of-the-art reference instruments</td><td>continue as support on request and consulting for implementing new techniques</td></tr></tbody></table>

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
### 3.3 Timeline for implementation of the mandatory operation support

Considering the ACTRIS roadmap, CGas-SiM should consider the following implementation plan for the mandatory operation support (including general tasks, scheduled and on request operation support, see Annex: Provision of the operation support):

<table><thead><tr><th rowspan="2">Operation support</th><th rowspan="2">Type</th><th>Preparation phase</th><th>Implementation phase</th><th>Operation phase</th></tr><tr><th>&lt; 2020</th><th>2020 - 2025</th><th>&gt; 2025</th></tr></thead><tbody><tr><td>Measurement Guidelines (MGs) and Standard Operation Procedures (SOPs)</td><td>General task</td><td>Basic MGs/SOPs including QA/QC procedures for operation of the instruments and data analysis (minimum requirements)</td><td>Advanced/New MGs/SOPs including QA/QC procedures for operation of the instruments and data analysis (minimum requirements) (optimum setup)</td><td>Updates and new MGs/SOPs</td></tr><tr><td>Scale transfer</td><td>Scheduled/ On request</td><td>Provision of scale to working standards for selected trace gases, on request only</td><td>Continue provision of scale to working standards of selected trace gases, on request only and update working standards with new scales</td><td>Continue as scheduled support every four years and on request including updates</td></tr><tr><td>New Instrument techniques</td><td>On request</td><td>Direct comparison of selected trace gases with fixed, state-of-the-art, reference instruments</td><td>Direct comparison of selected trace gases with fixed, state-of-the-art reference instruments</td><td>continue as support on request and consulting for implementing new techniques</td></tr></tbody></table>

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
<table><thead><tr><th rowspan="2">Operation support</th><th rowspan="2">Type</th><th>Preparation phase</th><th>Implementation phase</th><th>Operation phase</th></tr><tr><th>&lt; 2020</th><th>2020 - 2025</th><th>&gt; 2025</th></tr></thead><tbody><tr><td>Training of operators and scientists</td><td>Scheduled/ On request</td><td>Training sessions for a specific measurement technique and on pre-defined subjects on request only</td><td>Training sessions for a specific measurement technique and on pre-defined subjects on request only</td><td>Regular training sessions every two years for a specific measurement technique and on pre-defined subjects as scheduled/on request activity</td></tr><tr><td>Provision of target gases</td><td>Scheduled</td><td>Tests at CGas-SiM laboratories (minimum requirements)</td><td>Provide first target gases to stations (minimum requirements), tests at CGas-SiM laboratories (optimum requirements)</td><td>Provision of target gases on annual basis</td></tr><tr><td>Station comparison (round robin and parallel off-line sampling)</td><td>Scheduled/ On request</td><td>Selected round robin exercises</td><td>At least one regular round robin exercise and one parallel off-line sampling (VOC) per target compound in 5 years (minimum requirements)</td><td>Regular round robins and parallel off-line sampling (VOC) at least every two years</td></tr></tbody></table>

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
<table><thead><tr><th rowspan="2">Operation support</th><th rowspan="2">Type</th><th>Preparation phase</th><th>Implementation phase</th><th>Operation phase</th></tr><tr><th>&lt; 2020</th><th>2020 - 2025</th><th>&gt; 2025</th></tr></thead><tbody><tr><td>Training of operators and scientists</td><td>Scheduled/ On request</td><td>Training sessions for a specific measurement technique and on pre-defined subjects on request only</td><td>Training sessions for a specific measurement technique and on pre-defined subjects on request only</td><td>Regular training sessions every two years for a specific measurement technique and on pre-defined subjects as scheduled/on request activity</td></tr><tr><td>Provision of target gases</td><td>Scheduled</td><td>Tests at CGas-SiM laboratories (minimum requirements)</td><td>Provide first target gases to stations (minimum requirements), tests at CGas-SiM laboratories (optimum requirements)</td><td>Provision of target gases on annual basis</td></tr><tr><td>Station comparison (round robin and parallel off-line sampling)</td><td>Scheduled/ On request</td><td>Selected round robin exercises</td><td>At least one regular round robin exercise and one parallel off-line sampling (VOC) per target compound in 5 years (minimum requirements)</td><td>Regular round robins and parallel off-line sampling (VOC) at least every two years</td></tr></tbody></table>

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
<table><thead><tr><th rowspan="2">Operation support</th><th rowspan="2">Type</th><th>Preparation phase</th><th>Implementation phase</th><th>Operation phase</th></tr><tr><th>&lt; 2020</th><th>2020 - 2025</th><th>&gt; 2025</th></tr></thead><tbody><tr><td>Comparison using mobile reference instruments</td><td>Scheduled/ On request</td><td>Direct comparison with mobile reference system for selected trace gases, in few cases only</td><td>Direct comparison with mobile reference system for selected trace gases, in few cases only</td><td>Continue at least every four years as scheduled activity and on request</td></tr><tr><td>Central side-by-side comparisons</td><td>Scheduled/ On request</td><td>Selected comparisons for OVOCs, selected BVOC and NOx</td><td>Use of existing and establishment of a new platforms (EUROCHAMP) for conducting the comparisons every four years for selected participants</td><td>Continue as scheduled activity at least every four years</td></tr><tr><td>Station audits</td><td>Scheduled</td><td></td><td>Auditing the performance of the measurement systems at a site (minimum requirements) for selected sites</td><td>Continue and extend to optimum requirements as scheduled activity every four years</td></tr><tr><td>Survey of target and comparison results; checking compatibility</td><td>Scheduled</td><td>Assessment of reported target concentrations with QA/QC tools (minimum requirements)</td><td>Further developing QA/QC tools and assessment of reported target concentrations (optimum requirements)</td><td>Continue annually as scheduled activity</td></tr><tr><td>QC data-procedure and review (issue tracker)</td><td>Scheduled</td><td>Assessment of ambient air data with QA/QC tools (minimum requirements)</td><td>Further developing QA/QC tools and assessment of ambient data, operate issue tracker (optimum requirements)</td><td>Continue annually as scheduled activity</td></tr><tr><td>Consulting in procedures when</td><td>General task</td><td></td><td>Setting up procedures for ACTRIS label award</td><td>Consulting how to become an ACTRIS NF and to apply for the</td></tr></tbody></table>

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
<table><thead><tr><th rowspan="2">Operation support</th><th rowspan="2">Type</th><th>Preparation phase</th><th>Implementation phase</th><th>Operation phase</th></tr><tr><th>&lt; 2020</th><th>2020 - 2025</th><th>&gt; 2025</th></tr></thead><tbody><tr><td>Comparison using mobile reference instruments</td><td>Scheduled/ On request</td><td>Direct comparison with mobile reference system for selected trace gases, in few cases only</td><td>Direct comparison with mobile reference system for selected trace gases, in few cases only</td><td>Continue at least every four years as scheduled activity and on request</td></tr><tr><td>Central side-by-side comparisons</td><td>Scheduled/ On request</td><td>Selected comparisons for OVOCs, selected BVOC and NOx</td><td>Use of existing and establishment of a new platforms (EUROCHAMP) for conducting the comparisons every four years for selected participants</td><td>Continue as scheduled activity at least every four years</td></tr><tr><td>Station audits</td><td>Scheduled</td><td></td><td>Auditing the performance of the measurement systems at a site (minimum requirements) for selected sites</td><td>Continue and extend to optimum requirements as scheduled activity every four years</td></tr><tr><td>Survey of target and comparison results; checking compatibility</td><td>Scheduled</td><td>Assessment of reported target concentrations with QA/QC tools (minimum requirements)</td><td>Further developing QA/QC tools and assessment of reported target concentrations (optimum requirements)</td><td>Continue annually as scheduled activity</td></tr><tr><td>QC data-procedure and review (issue tracker)</td><td>Scheduled</td><td>Assessment of ambient air data with QA/QC tools (minimum requirements)</td><td>Further developing QA/QC tools and assessment of ambient data, operate issue tracker (optimum requirements)</td><td>Continue annually as scheduled activity</td></tr><tr><td>Consulting in procedures when</td><td>General task</td><td></td><td>Setting up procedures for ACTRIS label award</td><td>Consulting how to become an ACTRIS NF and to apply for the</td></tr></tbody></table>

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
<table><thead><tr><th rowspan="2">Operation support</th><th rowspan="2">Type</th><th>Preparation phase</th><th>Implementation phase</th><th>Operation phase</th></tr><tr><th>&lt; 2020</th><th>2020 - 2025</th><th>&gt; 2025</th></tr></thead><tbody><tr><td>applying for the ACTRIS label</td><td></td><td style="background-color: #808080;"></td><td style="background-color: #F5F5F5;"></td><td style="background-color: #FFFFE0;">ACTRIS label, control the conformity of existing ACTRIS NF to ACTRIS label</td></tr></tbody></table>

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
<table><thead><tr><th rowspan="2">Operation support</th><th rowspan="2">Type</th><th>Preparation phase</th><th>Implementation phase</th><th>Operation phase</th></tr><tr><th>&lt; 2020</th><th>2020 - 2025</th><th>&gt; 2025</th></tr></thead><tbody><tr><td>applying for the ACTRIS label</td><td></td><td style="background-color: #808080;"></td><td style="background-color: #F5F5DC;"></td><td style="background-color: #FFFFE0;">ACTRIS label, control the conformity of existing ACTRIS NF to ACTRIS label</td></tr></tbody></table>

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
## 3.4 Operation support for quality assurance and quality control

### 3.4.1 Definition and establishment of Measurement Guidelines (MG)

During ACTRIS-2, the first MGs for VOC by gas chromatography and for NOx (NO and NO₂) have been developed (ACTRIS 2011-2017). Since different instrument set ups are deployed at the stations which are generally considered to be appropriate to achieve the DQOs defined in ACTRIS and GAW, the MGs should be generic and not specific to just one instrumental set-up. The MGs for NOx and NMHC Gas chromatography are currently in review by the SAG-RG of WMO GAW and the wider scientific community to achieve high level guidelines commonly agreed upon. During the implementation of ACTRIS-RI, these shall be expanded to cover also the other variables and techniques (BVOC and OVOC by GC, DNPH-HPLC, HCHO-Hantzsch, PTR-MS, NO₂ by CAPS, and CI-API-ToF), and during future developments of ACTRIS-RI towards further, new analytical technologies.

The objective should be to compile the analytical state-of-the-art into measurement guidelines with the CGas-SiM as competence centre coordinating the expert knowledge transfer in that fields and having a leading role in writing and reviewing state-of-the-art MGs. The MGs should include a description of the measurement techniques and procedures as well as all fields of QA and QC of measurements, calibrations, data evaluation and data delivery. New and updated guidelines should go through a review process and should be approved by the WMO-GAW SAG-RG, the WMO-CAS, and then published. These MGs define how ACTRIS measurements of the corresponding variables should be performed in order to achieve the ACTIRS label. The CGas-SiM will decide in consultancy with the ACTRIS assemblies of NFs and the Research Infrastructure Committee about revisions and the development of new MGs.

#### 3.4.1.1 NMHC by GC

A corresponding MG is in place and currently in the review process (ACTRIS 2011-2017)

#### 3.4.1.2 BVOC by GC

This is partly included in the MG for NMHC monitoring (ACTRIS 2011-2017) and should be revised in a specific BVOC MG during the ACTRIS implementation phase.

#### 3.4.1.3 OVOC by GC, DNPH-HPLC, and HANTZSCH

This is partly included in MG for NMHC monitoring (ACTRIS 2011-2017) and should be revised in a specific OVOC MG during the ACTRIS implementation phase.

#### 3.4.1.4 VOC by PTR-MS

This is currently under development and should be available in the ACTRIS implementation phase.

#### 3.4.1.5 Direct Aerosol Precursors by CI-API-ToF

This is currently under development and will be available in the ACTRIS implementation phase.

#### 3.4.1.6 NOx by CLD and CAPS

Corresponding MGs exist for NO and NO₂ monitoring with CLD (ACTRIS 2011-2017) and are currently in the review process. A specific MG should be developed for NO₂ monitored by CAPS during the ACTRIS implementation phase.

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
## 3.4 Operation support for quality assurance and quality control

### 3.4.1 Definition and establishment of Measurement Guidelines (MG)

During ACTRIS-2, the first MGs for VOC by gas chromatography and for NOx (NO and NO₂) have been developed (ACTRIS 2011-2017). Since different instrument set ups are deployed at the stations which are generally considered to be appropriate to achieve the DQOs defined in ACTRIS and GAW, the MGs should be generic and not specific to just one instrumental set-up. The MGs for NOx and NMHC Gas chromatography are currently in review by the SAG-RG of WMO GAW and the wider scientific community to achieve high level guidelines commonly agreed upon. During the implementation of ACTRIS-RI, these shall be expanded to cover also the other variables and techniques (BVOC and OVOC by GC, DNPH-HPLC, HCHO-Hantzsch, PTR-MS, NO₂ by CAPS, and CI-APi-ToF), and during future developments of ACTRIS-RI towards further, new analytical technologies.

The objective should be to compile the analytical state-of-the-art into measurement guidelines with the CGas-SiM as competence centre coordinating the expert knowledge transfer in that fields and having a leading role in writing and reviewing state-of-the-art MGs. The MGs should include a description of the measurement techniques and procedures as well as all fields of QA and QC of measurements, calibrations, data evaluation and data delivery. New and updated guidelines should go through a review process and should be approved by the WMO-GAW SAG-RG, the WMO-CAS, and then published. These MGs define how ACTRIS measurements of the corresponding variables should be performed in order to achieve the ACTIRS label. The CGas-SiM will decide in consultancy with the ACTRIS assemblies of NFs and the Research Infrastructure Committee about revisions and the development of new MGs.

#### 3.4.1.1 NMHC by GC

A corresponding MG is in place and currently in the review process (ACTRIS 2011-2017)

#### 3.4.1.2 BVOC by GC

This is partly included in the MG for NMHC monitoring (ACTRIS 2011-2017) and should be revised in a specific BVOC MG during the ACTRIS implementation phase.

#### 3.4.1.3 OVOC by GC, DNPH-HPLC, and HANTZSCH

This is partly included in MG for NMHC monitoring (ACTRIS 2011-2017) and should be revised in a specific OVOC MG during the ACTRIS implementation phase.

#### 3.4.1.4 VOC by PTR-MS

This is currently under development and should be available in the ACTRIS implementation phase.

#### 3.4.1.5 Direct Aerosol Precursors by CI-APi-ToF

This is currently under development and will be available in the ACTRIS implementation phase.

#### 3.4.1.6 NOx by CLD and CAPS

Corresponding MGs exist for NO and NO₂ monitoring with CLD (ACTRIS 2011-2017) and are currently in the review process. A specific MG should be developed for NO₂ monitored by CAPS during the ACTRIS implementation phase.

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
3.4.2 Definition of measurement quality-assurance criteria and procedures

Measurement quality-assurance criteria and procedures should be included in the corresponding MGs and Standard Operation Procedures (SOPs). The QA processes are assumed to be similar for the different variables and techniques.

Briefly, QA processes should combine:

* Implementing Measurement Guidelines.

* Following MGs for sampling, calibration, data processing, data corrections and data delivery.

* Assuring documentation and data handling according to the MGs.

* Using target gas and standard gas addition measurements.

* Participating in round robin and side by side inter-comparisons proposed by CGas-SiM.

* Conducting parallel sampling with or without employing travelling reference instruments coordinated by CGas-SiM.

* Establishing plausibility plots and evaluations.

* Contributing to data quality workshops and to station performance audits.

* Participating in training activities.

In part, QA criteria and procedures for specific ACTRIS variables are already in place as described in the corresponding MGs (ACTRIS 2011-2017). Updated and new QA criteria and procedures should be developed and included in the corresponding MGs during the ACTRIS implementation phase according to needs identified.

3.4.3 Development and provision of instrument-specific calibration

Reactive Trace gases can be calibrated by a combination of high pressure reference gases with corresponding dilution systems, and appropriate zero-gas generation systems. Recommendations and mandatory components as well as procedures should be provided in the corresponding MGs. Furthermore, the CGas-SiM should provide services including (1) the compatibility of laboratory standards with the WMO-GAW CCL scale, (2) offer station audits with test gases to check the calibration, (3) offer round robin test gases and participation in inter-comparison exercises to check calibration and artefact free operation of the respective measurement systems. Depending on needs and availability of suitable instruments, mobile reference instruments should be used by the CGas-SiM in station audits and run in parallel to routine measurements (side-by-side intercomparisons, e.g. using EUROCHAMP capabilities) for a continuous check of calibrations and data quality on-site. Corresponding compliance reports should be provided. Calibration procedures are similar for all ACTRIS variables covered here.

Specifically, the nitrate Cl-APi-ToF calibration procedures should include: 1) transmission calibration, 2) controlled fragmentation experiments, 3) concentration calibration with a standardized flow tube experiment or suitable calibration gas, and 4) side-by-side ambient sampling.
3.4.2 Definition of measurement quality-assurance criteria and procedures

Measurement quality-assurance criteria and procedures should be included in the corresponding MGs and Standard Operation Procedures (SOPs). The QA processes are assumed to be similar for the different variables and techniques.

Briefly, QA processes should combine:

* Implementing Measurement Guidelines.

* Following MGs for sampling, calibration, data processing, data corrections and data delivery.

* Assuring documentation and data handling according to the MGs.

* Using target gas and standard gas addition measurements.

* Participating in round robin and side by side inter-comparisons proposed by CGas-SiM.

* Conducting parallel sampling with or without employing travelling reference instruments coordinated by CGas-SiM.

* Establishing plausibility plots and evaluations.

* Contributing to data quality workshops and to station performance audits.

* Participating in training activities.

In part, QA criteria and procedures for specific ACTRIS variables are already in place as described in the corresponding MGs (ACTRIS 2011-2017). Updated and new QA criteria and procedures should be developed and included in the corresponding MGs during the ACTRIS implementation phase according to needs identified.

3.4.3 Development and provision of instrument-specific calibration

Reactive Trace gases can be calibrated by a combination of high pressure reference gases with corresponding dilution systems, and appropriate zero-gas generation systems. Recommendations and mandatory components as well as procedures should be provided in the corresponding MGs. Furthermore, the CGas-SiM should provide services including (1) the compatibility of laboratory standards with the WMO-GAW CCL scale, (2) offer station audits with test gases to check the calibration, (3) offer round robin test gases and participation in inter-comparison exercises to check calibration and artefact free operation of the respective measurement systems. Depending on needs and availability of suitable instruments, mobile reference instruments should be used by the CGas-SiM in station audits and run in parallel to routine measurements (side-by-side intercomparisons, e.g. using EUROCHAMP capabilities) for a continuous check of calibrations and data quality on-site. Corresponding compliance reports should be provided. Calibration procedures are similar for all ACTRIS variables covered here.

Specifically, the nitrate Cl-APi-ToF calibration procedures should include: 1) transmission calibration, 2) controlled fragmentation experiments, 3) concentration calibration with a standardized flow tube experiment or suitable calibration gas, and 4) side-by-side ambient sampling.
3.4.4 Development and provision of in-house check-up tools

CGas-SiM should provide the MGs, the check of calibrations as listed above, and target gases to investigate the performance of their measurement systems. A set of plausibility check-tools should be made available to the NFs and should be used at the sites. Furthermore, advice should be provided upon request and at the yearly data control workshop. The implementation should proceed in a stepwise manner during the implementation phase after the corresponding MGs become available.

3.4.5 Development of data evaluation procedures and plausibility test

A number of evaluations procedures and plausibility tests have already been developed, tested and partly
implemented in ACTRIS and ACTRIS-2 projects. Details are available in the corresponding measurement
guidelines (ACTRIS 2011-2017). But data evaluation procedures and plausibility tests should be further
developed for all of the mentioned measurement techniques hand-in-hand with the measurement quality
assurance criteria in order to enable stations to perform mature plausibility checks and develop a good
understanding of the quality of the data. This should be an ongoing activity of the CGas-SiM.

The tests should be designed so that data backup procedure problems and instrumental malfunctioning
is eliminated. That includes also situations where everything technically is ok, but instruments are no
longer in the range of the calibration conditions. First level of plausibility tests should filter out the most
obvious and physically impossible conditions, like negative concentration values, or total ion count is lower
than any individual ion count. The plausibility tests should be performed also on operational parameters
like inlet voltages, inlet flows, detector pressures and detector operational voltages. Each operational
parameter should have its own acceptance window. Instrument specific information is required to
perform these tests. Operational parameters should be logged along the data and then processed without
specialised software.

The next level of data evaluation procedures should include tests checking the chemical conditions (e.g. recovery rate of target compounds) of the instrument. That requires instrument specific information to be logged. The magnitude and the ratio (e.g. for CI or PTR instruments) of target compounds should be investigated. Significantly altered ratios e.g. in calibration time series, indicate considerable change in the instrument-specific chemistry and thus data are not achieving the required quality level. Performing the tests for instrument-specific chemistry requires data pre-processing and cannot be performed on raw data. That means: The data evaluation procedure should include an automatic data processing procedure. This procedure should be backwards traceable and all parameters used for processing and evaluating should be recorded and stored.

Even though the parameters used for data pre-processing are very much instrument specific, the general
evaluation procedures are not. The software for data pre-processing should be able to perform the same
tasks with different input parameters in order to incorporate a large number of NFs.
3.4.4 Development and provision of in-house check-up tools

CGas-SiM should provide the MGs, the check of calibrations as listed above, and target gases to investigate the performance of their measurement systems. A set of plausibility check-tools should be made available to the NFs and should be used at the sites. Furthermore, advice should be provided upon request and at the yearly data control workshop. The implementation should proceed in a stepwise manner during the implementation phase after the corresponding MGs become available.

3.4.5 Development of data evaluation procedures and plausibility test

A number of evaluations procedures and plausibility tests have already been developed, tested and partly
implemented in ACTRIS and ACTRIS-2 projects. Details are available in the corresponding measurement
guidelines (ACTRIS 2011-2017). But data evaluation procedures and plausibility tests should be further
developed for all of the mentioned measurement techniques hand-in-hand with the measurement quality
assurance criteria in order to enable stations to perform mature plausibility checks and develop a good
understanding of the quality of the data. This should be an ongoing activity of the CGas-SiM.

The tests should be designed so that data backup procedure problems and instrumental malfunctioning
is eliminated. That includes also situations where everything technically is ok, but instruments are no
longer in the range of the calibration conditions. First level of plausibility tests should filter out the most
obvious and physically impossible conditions, like negative concentration values, or total ion count is lower
than any individual ion count. The plausibility tests should be performed also on operational parameters
like inlet voltages, inlet flows, detector pressures and detector operational voltages. Each operational
parameter should have its own acceptance window. Instrument specific information is required to
perform these tests. Operational parameters should be logged along the data and then processed without
specialised software.

The next level of data evaluation procedures should include tests checking the chemical conditions (e.g. recovery rate of target compounds) of the instrument. That requires instrument specific information to be logged. The magnitude and the ratio (e.g. for CI or PTR instruments) of target compounds should be investigated. Significantly altered ratios e.g. in calibration time series, indicate considerable change in the instrument-specific chemistry and thus data are not achieving the required quality level. Performing the tests for instrument-specific chemistry requires data pre-processing and cannot be performed on raw data. That means: The data evaluation procedure should include an automatic data processing procedure. This procedure should be backwards traceable and all parameters used for processing and evaluating should be recorded and stored.

Even though the parameters used for data pre-processing are very much instrument specific, the general
evaluation procedures are not. The software for data pre-processing should be able to perform the same
tasks with different input parameters in order to incorporate a large number of NFs.
3.4.6 Realization of observational site performance audits with reference samples or mobile systems

Site performance audits with reference samples or mobile systems should be provided by the CGas-SiM. A period between station visits of 4 years should be envisioned and should be supplemented by calibration checks, round robin exercises, target-gas checks and intercomparisons experiments. New stations and stations encountering problems with data quality and data flow should get more frequent audits upon necessity.

The performance audits check the conformity of a station to the QA requirements as specified in this document and the corresponding MGs. This should include the conformity of the measurements of CGAs-SiM test gases with targeted values within the DQOs (see definitions and procedures of performance and system audits in GAW Report No. 142 (WMO, 2004). The reference for conformity of a station should evolve as the QA system evolves. During the audit at a station the following parts should be evaluated:

* The sampling and instrument set-up.

* The calibration and zero gas systems.

* The QA/QC procedures implemented.

* The training and instructions sessions.

* The calibration, zero gas, target gas, and standard addition data.

* The data delivery.

* The results from intercomparison exercises.

* The uncertainty evaluation.

* The logbooks (instrument, measurements, station).

* The scientific use of the data.

* The overall equipment of the station.

Reference gas samples are already available for currently performed audits within WMO-GAW. Reference instruments are currently available only for a sub-set of reactive trace gases, e.g. formaldehyde and NOx. These instruments should be run in parallel together with the NF instruments during the audit. Results of the audits should be written in a standardized way and provide as comprehensive audit reports.

Specifically, the CGas-SiM should operate a reference CI-API-ToF in the laboratory, which can be used in side-by-side intercomparison experiments. A standard flow tube experiment should be developed, which should be used to check the performance of instruments participating in intercomparison workshops. For the CI-API-ToF performance audits, a mobile reference system for generating sulphuric acid vapour can be utilized. That system should be intercompared against the reference system located permanently at the CGas-SiM before and after each station audit. In addition, field deployable permeation systems for generating e.g. fluorinated acids for calibration should be manufactured and used together with the sulfuric acid generator in audits after the implementation phase.
### 3.4.6 Realization of observational site performance audits with reference samples or mobile systems

Site performance audits with reference samples or mobile systems should be provided by the CGas-SiM. A period between station visits of 4 years should be envisioned and should be supplemented by calibration checks, round robin exercises, target-gas checks and intercomparisons experiments. New stations and stations encountering problems with data quality and data flow should get more frequent audits upon necessity.

The performance audits check the conformity of a station to the QA requirements as specified in this document and the corresponding MGs. This should include the conformity of the measurements of CGAs-SiM test gases with targeted values within the DQOs (see definitions and procedures of performance and system audits in GAW Report No. 142 (WMO, 2004). The reference for conformity of a station should evolve as the QA system evolves. During the audit at a station the following parts should be evaluated:

* The sampling and instrument set-up.

* The calibration and zero gas systems.

* The QA/QC procedures implemented.

* The training and instructions sessions.

* The calibration, zero gas, target gas, and standard addition data.

* The data delivery.

* The results from intercomparison exercises.

* The uncertainty evaluation.

* The logbooks (instrument, measurements, station).

* The scientific use of the data.

* The overall equipment of the station.

Reference gas samples are already available for currently performed audits within WMO-GAW. Reference instruments are currently available only for a sub-set of reactive trace gases, e.g. formaldehyde and NOx. These instruments should be run in parallel together with the NF instruments during the audit. Results of the audits should be written in a standardized way and provide as comprehensive audit reports.

Specifically, the CGas-SiM should operate a reference CI-API-ToF in the laboratory, which can be used in side-by-side intercomparison experiments. A standard flow tube experiment should be developed, which should be used to check the performance of instruments participating in intercomparison workshops. For the CI-API-ToF performance audits, a mobile reference system for generating sulphuric acid vapour can be utilized. That system should be intercompared against the reference system located permanently at the CGas-SiM before and after each station audit. In addition, field deployable permeation systems for generating e.g. fluorinated acids for calibration should be manufactured and used together with the sulfuric acid generator in audits after the implementation phase.
### 3.4.7 Organization of regular exercises to assess the performances of the NFs, including instrument performance workshops

Yearly QA/QC workshops checking the plausibility and quality of the NF data should be performed. The objective is the conformity with the DQOs as defined in the MGs. That should include assessments of the year-to-year consistency of the reported data, within network station-to-station consistency, and should consider the impact of emission ratios, chemical processes, lifetime-variability and remoteness of the stations on reported data.

Instrument performance workshops or intercomparison experiments are envisioned on a regular bases aiming at a 4 years period.

Target gases are provided to the stations by CGas-SiM. Building on the experience in ICOS where target gases are successfully used as a major quality control tool, these techniques should be introduced to ACTRIS reactive trace gas measurement stations. CGas-SiM should further optimize the target gas measurement procedures with respect to frequency of measurements and analyses of deviations from reference values as well as in capturing the statistics of the measurements, e.g. repeatability and reproducibility.

In addition, the CGas-SiM should operate a data quality issue tracker system for NFs. That system should be activated on a yearly basis following the initial data submission by a station to the DC. The DC should compile and transfer the submitted data for quality and plausibility checks to CGas-SiM. Issues should then be defined wherever data quality is in question with respect to DQOs and achievable measurement uncertainty by a station. Defined issues should be iteratively solved by the station PI and experts of the CGas-SiM. When CGas-SiM considers conducted actions as sufficient, the issue is closed and revised data can be resubmitted by the station in final form to the DC for publication.

### 3.4.8 Contribution to documentation and traceability of level 0 to level 3 data products

The CGas-SiM with its different units should be linked to the corresponding international and approved scales maintained at the WMO-CCLs, which are related to SI units. The CGas-SiM should assure the compliance of the stations' laboratory standards with the scale. The CGas-SiM should contribute in key comparisons organized by the community of National Metrology Institutes (NMIs) assembled in the Gas Analysis Working Group (GAWG) of the Consulting Committee of Quantity and Matter (CCQM) of the International Office for Weight and Measures (BIPM).

The CGas-SiM should define together with the DC the data submission templates for the NFs according to the data flow documents developed between the DC and the CGas-SiM. This should include reporting the measurement uncertainty evaluated according the MGs in order assess the quality level of the data and to track any change in data quality. These documents, the MGs, and the data submission procedures regulate the appropriate documentation of the data. All the references on the used calibration gases should inter alia be reported as meta-data together with each data submission. In addition, bias and repeatability/reproducibility of target gas measurements and in round robin measurements, as well as results of intercomparisons should be made available in the data base.
### 3.4.7 Organization of regular exercises to assess the performances of the NFs, including instrument performance workshops

Yearly QA/QC workshops checking the plausibility and quality of the NF data should be performed. The objective is the conformity with the DQOs as defined in the MGs. That should include assessments of the year-to-year consistency of the reported data, within network station-to-station consistency, and should consider the impact of emission ratios, chemical processes, lifetime-variability and remoteness of the stations on reported data.

Instrument performance workshops or intercomparison experiments are envisioned on a regular bases aiming at a 4 years period.

Target gases are provided to the stations by CGas-SiM. Building on the experience in ICOS where target gases are successfully used as a major quality control tool, these techniques should be introduced to ACTRIS reactive trace gas measurement stations. CGas-SiM should further optimize the target gas measurement procedures with respect to frequency of measurements and analyses of deviations from reference values as well as in capturing the statistics of the measurements, e.g. repeatability and reproducibility.

In addition, the CGas-SiM should operate a data quality issue tracker system for NFs. That system should be activated on a yearly basis following the initial data submission by a station to the DC. The DC should compile and transfer the submitted data for quality and plausibility checks to CGas-SiM. Issues should then be defined wherever data quality is in question with respect to DQOs and achievable measurement uncertainty by a station. Defined issues should be iteratively solved by the station PI and experts of the CGas-SiM. When CGas-SiM considers conducted actions as sufficient, the issue is closed and revised data can be resubmitted by the station in final form to the DC for publication.

### 3.4.8 Contribution to documentation and traceability of level 0 to level 3 data products

The CGas-SiM with its different units should be linked to the corresponding international and approved scales maintained at the WMO-CCLs, which are related to SI units. The CGas-SiM should assure the compliance of the stations' laboratory standards with the scale. The CGas-SiM should contribute in key comparisons organized by the community of National Metrology Institutes (NMIs) assembled in the Gas Analysis Working Group (GAWG) of the Consulting Committee of Quantity and Matter (CCQM) of the International Office for Weight and Measures (BIPM).

The CGas-SiM should define together with the DC the data submission templates for the NFs according to the data flow documents developed between the DC and the CGas-SiM. This should include reporting the measurement uncertainty evaluated according the MGs in order assess the quality level of the data and to track any change in data quality. These documents, the MGs, and the data submission procedures regulate the appropriate documentation of the data. All the references on the used calibration gases should inter alia be reported as meta-data together with each data submission. In addition, bias and repeatability/reproducibility of target gas measurements and in round robin measurements, as well as results of intercomparisons should be made available in the data base.
3.4.9 Contribution to CEN, ISO, or similar standardization activities

The CGas-SiM should contribute to the corresponding standardizations within CEN and ISO and bring in
the needs of monitoring reactive trace gases at low air concentrations. By now, the standard operating
procedures and measurement guidelines in place focus on urban air quality networks with high
concentrations of reactive trace gases not always applicable in remote areas. These documents should
further be developed for monitoring background air as in the ACTRIS-RI.

**3.5 Operation support for knowledge transfer and training**

3.5.1 Training of operators and scientists

For improving and maintaining high data quality in the ACTRIS reactive trace gas network it is essential to train on a regular basis operators and scientists of the NFs. In training centres with thematic training courses, the necessary expertise should be shared with the corresponding station staff. The annual concept of the training schedule should be developed together with the NFs. Objectives of the training should cover all aspects of the MGs, QA/QC procedures and data management. Beside the specific training centres, the yearly data QC workshops, the audits, the side-by-side intercomparisons, and the topical NF assemblies should be used for training. Training is similar for all variables and techniques covered by the CGas-SiM. Besides schooling in technical aspects of the instruments, the training should also provide knowledge about diurnal and seasonal relationships between trace gas species so that the trainees will be able to discover conspicuous features in data series at an early stage either to solve technical problems of an instrument or to be on watch for possibly hazardous events in the environment.

3.5.2 Consultancy for setting-up new observational or exploratory platforms and new instruments at the NFs

The CGas-SiM together with the WMO SAG-RG and the UNECE CLRTAP Task Force on Measurement and Modelling (TFMM) should identify gaps in the measurement network in Europe. Thus, CGas-SiM should provide the respective consultancy to the corresponding decision bodies in ACTRIS and GAW. Furthermore, the CGas-SiM should consult new stations in setting up their monitoring infrastructure and quality management system. Consultancy is provided via the MGs, direct communication, reference station visits, and visits of the new to build stations. Consultancy will be available from the beginning of the implementation phase.

**3.6 Operation support for improvement of measurement methodologies**

3.6.1 Testing of new measurement instruments and procedures

The CGas-SiM should track new developments in measuring and analysing reactive trace gases in air and
should be involved in testing and developing new instrumentation for assessing the ACTRIS chemical
target variables. Specifically, the suitability of new instruments for long-term monitoring within ACTRIS
and the conformity with DQOs should be tested and characterized. Recommendations of the suitability of
new measurement technologies for achieving high data quality in the ACTRIS-RI should be provided. For

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
3.4.9 Contribution to CEN, ISO, or similar standardization activities

The CGas-SiM should contribute to the corresponding standardizations within CEN and ISO and bring in
the needs of monitoring reactive trace gases at low air concentrations. By now, the standard operating
procedures and measurement guidelines in place focus on urban air quality networks with high
concentrations of reactive trace gases not always applicable in remote areas. These documents should
further be developed for monitoring background air as in the ACTRIS-RI.

3.5 Operation support for knowledge transfer and training

3.5.1 Training of operators and scientists

For improving and maintaining high data quality in the ACTRIS reactive trace gas network it is essential to train on a regular basis operators and scientists of the NFs. In training centres with thematic training courses, the necessary expertise should be shared with the corresponding station staff. The annual concept of the training schedule should be developed together with the NFs. Objectives of the training should cover all aspects of the MGs, QA/QC procedures and data management. Beside the specific training centres, the yearly data QC workshops, the audits, the side-by-side intercomparisons, and the topical NF assemblies should be used for training. Training is similar for all variables and techniques covered by the CGas-SiM. Besides schooling in technical aspects of the instruments, the training should also provide knowledge about diurnal and seasonal relationships between trace gas species so that the trainees will be able to discover conspicuous features in data series at an early stage either to solve technical problems of an instrument or to be on watch for possibly hazardous events in the environment.

3.5.2 Consultancy for setting-up new observational or exploratory platforms and new instruments at the NFs

The CGas-SiM together with the WMO SAG-RG and the UNECE CLRTAP Task Force on Measurement and Modelling (TFMM) should identify gaps in the measurement network in Europe. Thus, CGas-SiM should provide the respective consultancy to the corresponding decision bodies in ACTRIS and GAW. Furthermore, the CGas-SiM should consult new stations in setting up their monitoring infrastructure and quality management system. Consultancy is provided via the MGs, direct communication, reference station visits, and visits of the new to build stations. Consultancy will be available from the beginning of the implementation phase.

3.6 Operation support for improvement of measurement methodologies

3.6.1 Testing of new measurement instruments and procedures

The CGas-SiM should track new developments in measuring and analysing reactive trace gases in air and
should be involved in testing and developing new instrumentation for assessing the ACTRIS chemical
target variables. Specifically, the suitability of new instruments for long-term monitoring within ACTRIS
and the conformity with DQOs should be tested and characterized. Recommendations of the suitability of
new measurement technologies for achieving high data quality in the ACTRIS-RI should be provided. For

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
this activity specific joint-projects should be envisaged considering further the maturity of the new
technologies. During the topical NFs assemblies, initiatives or requirements towards new instrumentation
can be discussed and decided.

The presently intended techniques include different ion chemistries for sample ionization in CI-APi-ToF instruments as well as the optical measurement of specific VOC using quantum cascade lasers.

3.6.2 Development of strategies to increase the duty cycle of instruments operated at the NFs, to reduce breakdown and maintenance downtime

The CGas-SiM should contribute to development of optimised methods and procedures in order to reduce breakdown and maintenance times to ensure maximum possible operation time of the instruments.

3.6.3 Development of new technological products and methods

New measurement techniques, technological products and methods should be developed by CGas-SiM in collaboration with the NFs. Depending on the situation, the CGas-SiM may recommend to the ACTRIS RI Committee to decide on the implementation of such new techniques at the RI level. The Centre has no obligation to develop the full set of operation support for such new techniques, unless they are embraced by ACTRIS and implemented as part of the NFs.

The CGas-SiM should work on improving data quality verification processes for example based on the uncertainty analysis. Considering the large datasets existing for VOCs measurements, multi-dimensional approaches could be developed to check the consistency of the measurements between sites. For example, factorial analysis (ex. PMF) utilising the co-variability of species commonly observed between sites.

Specifically for CI-APi-ToF instruments, halogen (chloride bromide, iodide) based CI-ion chemistries for charging should be developed and tested. Further, new methods for producing calibration vapours for the CI-APi-ToF should be developed, tested and validated and then distributed to NFs.

3.6.4 Development of retrieval algorithms or tools for producing level 1 to 3 data, including
the exploitation of instrument synergies

The CGas-SiM should contribute to developments of optimised procedures for level 1 and 2 products, as well for NRT data delivery with automated quality test tools.
this activity specific joint-projects should be envisaged considering further the maturity of the new
technologies. During the topical NFs assemblies, initiatives or requirements towards new instrumentation
can be discussed and decided.

The presently intended techniques include different ion chemistries for sample ionization in CI-APi-ToF instruments as well as the optical measurement of specific VOC using quantum cascade lasers.

3.6.2 Development of strategies to increase the duty cycle of instruments operated at the NFs, to reduce breakdown and maintenance downtime

The CGas-SiM should contribute to development of optimised methods and procedures in order to reduce breakdown and maintenance times to ensure maximum possible operation time of the instruments.

3.6.3 Development of new technological products and methods

New measurement techniques, technological products and methods should be developed by CGas-SiM in collaboration with the NFs. Depending on the situation, the CGas-SiM may recommend to the ACTRIS RI Committee to decide on the implementation of such new techniques at the RI level. The Centre has no obligation to develop the full set of operation support for such new techniques, unless they are embraced by ACTRIS and implemented as part of the NFs.

The CGas-SiM should work on improving data quality verification processes for example based on the uncertainty analysis. Considering the large datasets existing for VOCs measurements, multi-dimensional approaches could be developed to check the consistency of the measurements between sites. For example, factorial analysis (ex. PMF) utilising the co-variability of species commonly observed between sites.

Specifically for CI-APi-ToF instruments, halogen (chloride bromide, iodide) based CI-ion chemistries for charging should be developed and tested. Further, new methods for producing calibration vapours for the CI-APi-ToF should be developed, tested and validated and then distributed to NFs.

3.6.4 Development of retrieval algorithms or tools for producing level 1 to 3 data, including
the exploitation of instrument synergies

The CGas-SiM should contribute to developments of optimised procedures for level 1 and 2 products, as well for NRT data delivery with automated quality test tools.
# 4 Services provided to ACTRIS users

An ACTRIS user is a person, a team, or an institution using either ACTRIS data or any other ACTRIS service. ACTRIS users originate from academia, business, industry and public services, from ACTRIS member countries as well as from countries, which are not ACTRIS members, inside and outside Europe. All user employing or developing instruments for measuring the ACTRIS variables in reactive trace gases may benefit from the QA/QC capabilities developed and available for service by the CGas-SiM.

Access to some ACTRIS services is open and free, most importantly the access to ACTRIS data, data products, and digital tools provided by the DC, which is the point of entry for free ACTRIS data services. Other access to ACTRIS services is considered competitive access based on capacity or excellence and will require a review process that is centrally managed by the HO/SAMU. SAMU is the single point of entry for user access to all TC services and to some specific services and data products provided by the DC. Access is regulated by both the ACTRIS access policy and the ACTRIS data policy, respectively.

Within the CGas-SiM, service provided to ACTRIS users is only provided in a restricted manner considering the limited resources of the TC, the service fee covered by the user, and the priority to ACTRIS NFs support. The units of CGas-SiM will decide about provision of services to GAW, EMEP stations, and other specific users. CGas-SiM should agree on these issues with SAMU. This might include, for example, a service for helping municipalities to set up clean air plans to comply with European air quality regulations for NO₂.

## 4.1 Estimation of the need

The CGas-SiM is able to schedule some of its potential capabilities for service to ACTRIS users. It is envisaged that up to a maximum of 25% of the availably capacity is made available for use by user. Table 4-1 summarises the current available estimates.

<table><caption>Table 4-1: The CGas-SiM service estimation for potential ACTRIS users</caption><thead><tr><th rowspan="3">Type of ACTRIS user</th><th colspan="3">Number of users to which ACTRIS is providing services</th></tr><tr><th rowspan="2">Now</th><th colspan="2">by 2025</th></tr><tr><th>Min.</th><th>Max.</th></tr></thead><tbody><tr><td>Academia</td><td>0</td><td>1</td><td>4</td></tr><tr><td>Business</td><td>0</td><td>1</td><td>4</td></tr><tr><td>Industry</td><td>0</td><td>1</td><td>4</td></tr><tr><td>Public Services</td><td>0</td><td>1</td><td>4</td></tr></tbody></table>
## 4 Services provided to ACTRIS users

An ACTRIS user is a person, a team, or an institution using either ACTRIS data or any other ACTRIS service. ACTRIS users originate from academia, business, industry and public services, from ACTRIS member countries as well as from countries, which are not ACTRIS members, inside and outside Europe. All user employing or developing instruments for measuring the ACTRIS variables in reactive trace gases may benefit from the QA/QC capabilities developed and available for service by the CGas-SiM.

Access to some ACTRIS services is open and free, most importantly the access to ACTRIS data, data products, and digital tools provided by the DC, which is the point of entry for free ACTRIS data services. Other access to ACTRIS services is considered competitive access based on capacity or excellence and will require a review process that is centrally managed by the HO/SAMU. SAMU is the single point of entry for user access to all TC services and to some specific services and data products provided by the DC. Access is regulated by both the ACTRIS access policy and the ACTRIS data policy, respectively.

Within the CGas-SiM, service provided to ACTRIS users is only provided in a restricted manner considering the limited resources of the TC, the service fee covered by the user, and the priority to ACTRIS NFs support. The units of CGas-SiM will decide about provision of services to GAW, EMEP stations, and other specific users. CGas-SiM should agree on these issues with SAMU. This might include, for example, a service for helping municipalities to set up clean air plans to comply with European air quality regulations for NO₂.

### 4.1 Estimation of the need

The CGas-SiM is able to schedule some of its potential capabilities for service to ACTRIS users. It is envisaged that up to a maximum of 25% of the availably capacity is made available for use by user. Table 4-1 summarises the current available estimates.

<table><caption>Table 4-1: The CGas-SiM service estimation for potential ACTRIS users</caption><thead><tr><th rowspan="3">Type of ACTRIS user</th><th colspan="3">Number of users to which ACTRIS is providing services</th></tr><tr><th rowspan="2">Now</th><th colspan="2">by 2025</th></tr><tr><th>Min.</th><th>Max.</th></tr></thead><tbody><tr><td>Academia</td><td>0</td><td>1</td><td>4</td></tr><tr><td>Business</td><td>0</td><td>1</td><td>4</td></tr><tr><td>Industry</td><td>0</td><td>1</td><td>4</td></tr><tr><td>Public Services</td><td>0</td><td>1</td><td>4</td></tr></tbody></table>
## 4.2 Provision of measurement quality assurance and quality control procedures and tools

The CGas-SiM can offer all documents and data-plausibility control procedures to users. Furthermore, depending on resources, CGas-SiM can offer participation in round-robin, side-by-side intercomparison and yearly data quality control workshops. In individual cases and on short-term, also calibration of laboratory standards and the provision of target gases may be offered. However, on long-term, such stations should become part of the ACTRIS RI.

## 4.3 Instrument-specific calibration

The CGas-SiM can offer, depending on resources, calibration of laboratory standards, comparison to travelling reference instruments, and the participation in round-robin and side-by-side intercomparisons which should be used to check the calibration and performance of the instruments including ambient samples under realistic measurement conditions. Besides this, instruments for reactive trace gas monitoring can be calibrated by on-site reference gases, i.e. certified calibration standards, provided by NMIs or, more specifically, by the Central Calibration Laboratories (CCL) of WMO-GAW. Respective standards or gas mixtures traceable to these standards, can be made available on short-term by CGas-SiM.

## 4.4 Knowledge transfer and operator training

As outlined in 3.4.1, also knowledge transfer and training for users can be offered by CGas-SiM in restricted extent, mainly depending on resources, with priority for ACTRIS NFs support.

## 4.5 Improvement of measurement and retrieval methodologies for aerosols, clouds, and reactive trace gases

The CGas-SiM can contribute to developments relevant for specific requirements arising from the aerosol-cloud-reactive trace gases interaction, i.e. when processes at the boundaries of respective atmospheric states require the observation of new variables. Between gas- and aerosol-phase, this can already be addressed by the unit on aerosol precursor measurements for organic compounds with very low volatility. This illustrates gap-filling activities provided by the CGas-SiM. The magnitude of the different activities depends on the available resources, the overall importance for ACTRIS, and the network-wide use of such procedures.
## 4.2 Provision of measurement quality assurance and quality control procedures and tools

The CGas-SiM can offer all documents and data-plausibility control procedures to users. Furthermore, depending on resources, CGas-SiM can offer participation in round-robin, side-by-side intercomparison and yearly data quality control workshops. In individual cases and on short-term, also calibration of laboratory standards and the provision of target gases may be offered. However, on long-term, such stations should become part of the ACTRIS RI.

## 4.3 Instrument-specific calibration

The CGas-SiM can offer, depending on resources, calibration of laboratory standards, comparison to travelling reference instruments, and the participation in round-robin and side-by-side intercomparisons which should be used to check the calibration and performance of the instruments including ambient samples under realistic measurement conditions. Besides this, instruments for reactive trace gas monitoring can be calibrated by on-site reference gases, i.e. certified calibration standards, provided by NMIs or, more specifically, by the Central Calibration Laboratories (CCL) of WMO-GAW. Respective standards or gas mixtures traceable to these standards, can be made available on short-term by CGas-SiM.

## 4.4 Knowledge transfer and operator training

As outlined in 3.4.1, also knowledge transfer and training for users can be offered by CGas-SiM in restricted extent, mainly depending on resources, with priority for ACTRIS NFs support.

## 4.5 Improvement of measurement and retrieval methodologies for aerosols, clouds, and reactive trace gases

The CGas-SiM can contribute to developments relevant for specific requirements arising from the aerosol-cloud-reactive trace gases interaction, i.e. when processes at the boundaries of respective atmospheric states require the observation of new variables. Between gas- and aerosol-phase, this can already be addressed by the unit on aerosol precursor measurements for organic compounds with very low volatility. This illustrates gap-filling activities provided by the CGas-SiM. The magnitude of the different activities depends on the available resources, the overall importance for ACTRIS, and the network-wide use of such procedures.
## 5 Governance and management structure of the Centre for Reactive Trace Gases in Situ Measurements

The CGas-SiM shall be organized in different units according to the specific role of each Unit in addressing the ACTRIS variables, assuring that CGas-SiM complies with the requirements and obligations described in sections 6 and 8 of this document, and considering the general principles described in the *Baseline document for the concept of the Central Facilities*. The Centre for Reactive Trace Gases in Situ Measurements shall be organized such that:

* It has a well-defined structure of Units,

* It has a well-defined decision-making process,

* It is coordinated by a CF Director and managed by a CF Management Board, which consists of the CF Director and the CF Unit Heads,

* It has clear and cost-efficient task sharing between the Units,

* It has a risk management strategy,

* It participates in the ACTRIS decision making by sending representative(s) to the ACTRIS legal entity bodies, e.g. to the RI committee.

## 6 Requirements for the Centre for Reactive Trace gases in Situ Measurements

### 6.1 General requirements

In order to be labelled as the Centre for Reactive Trace gases in Situ Measurements, the candidate shall:

* Commit for long-term operation, at least 10 years starting with the next year after the implementation as a CF,

* Set-up an appropriate structure of Units, to ensure an adequate functionality of the CF, cost-effective operation, and scalability with respect to the user demands and advancement of the state-of-the-art technology,

* Demonstrate the capacity (in terms of material and personnel resources), to ensure the provision of the needed services and operation support, as described in section 6.2

* Provide a feasible implementation and operation plan for the first five years, starting with the next year after the selection as CF,

* Commit to provide a minimum amount of user services, as described in section 4

* Commit to provide the mandatory operation support to the ACTRIS NFs, as described in section 3.
## 5 Governance and management structure of the Centre for Reactive Trace Gases in Situ Measurements

The CGas-SiM shall be organized in different units according to the specific role of each Unit in addressing the ACTRIS variables, assuring that CGas-SiM complies with the requirements and obligations described in sections 6 and 8 of this document, and considering the general principles described in the *Baseline document for the concept of the Central Facilities*. The Centre for Reactive Trace Gases in Situ Measurements shall be organized such that:

* It has a well-defined structure of Units,

* It has a well-defined decision-making process,

* It is coordinated by a CF Director and managed by a CF Management Board, which consists of the CF Director and the CF Unit Heads,

* It has clear and cost-efficient task sharing between the Units,

* It has a risk management strategy,

* It participates in the ACTRIS decision making by sending representative(s) to the ACTRIS legal entity bodies, e.g. to the RI committee.

## 6 Requirements for the Centre for Reactive Trace gases in Situ Measurements

### 6.1 General requirements

In order to be labelled as the Centre for Reactive Trace gases in Situ Measurements, the candidate shall:

* Commit for long-term operation, at least 10 years starting with the next year after the implementation as a CF,

* Set-up an appropriate structure of Units, to ensure an adequate functionality of the CF, cost-effective operation, and scalability with respect to the user demands and advancement of the state-of-the-art technology,

* Demonstrate the capacity (in terms of material and personnel resources), to ensure the provision of the needed services and operation support, as described in section 6.2

* Provide a feasible implementation and operation plan for the first five years, starting with the next year after the selection as CF,

* Commit to provide a minimum amount of user services, as described in section 4

* Commit to provide the mandatory operation support to the ACTRIS NFs, as described in section 3.
## 6.2 Technical requirements

### 6.2.1 Facilities

The CGas-SiM should share the tasks between different units. All units should be established in well recognized institutes with long-term experience in the field of continuous long-term monitoring of reactive trace gases in situ in the atmosphere and superior experience in QA/QC. The various units should contribute with their specific and complementary competences to achieve a share of the multiple tasks. Sharing the tasks is necessary because no unit itself has the needed resources. The number of units should be a reasonable balance between available resources comprising personal, competence and capacity, and enhanced coordinative and administrative work. Furthermore, many of the calibration services require independent checks by various laboratories, i.e. the units of the CGas-SiM.

Since high pressure cylinders or air sample canisters should be exchanged between CGas-SiM and stations, the location of the CGas-SiM and its proximity to stations is generally not an issue. With respect to mobile reference instruments and intercomparisons, transport of instruments should be easily feasible in the EU. VOC instruments are more bulky and complex, thus, transport and intercomparison activities are more difficult to achieve. An allocation of units in different European countries would shorten transport time and distance to the NFs. But in general road transportation within Europe is well developed and the location of the units is not a big issue.

All units shall be able to provide calibration, QA/QC, and training services as outlined above. Otherwise, the intended services shall be complimentary, with the coordinating unit having capacity and know-how in the majority of services proposed.

Overall, in each unit laboratory space is needed with parts of the instrumentation covered under Section 3, i.e. at minimum one instrument needs to be present in the laboratory and available for ACTRIS support and users. The CGas-SiM should have training facilities with reference measurement systems (all systems listed in Section 3), side-by-side intercomparison facilities for a reasonable subsets of instruments, i.e. space and logistics for more than 5 VOC systems and more than 10 NOx systems, target gas storage and measurement facilities for compressed gas cylinders for NO and VOC, set-ups for travelling reference instruments or samplers for ambient air measurements, and computing facilities to analyse the corresponding data sets from NFs.

### 6.2.2 Human resources

The required human resources of CGas-SiM to cover the envisioned services and NFs are estimated to be:

- 5 full positions expert scientists (>5 years of experience in related field),

- 5 full positions engineer/qualified operator (>3 years of experience),

- 3 full positions of technician (> 1 year of experience),

and in the administrative area 0.5, 0.5 and 0.75 full positions of expert manager, qualified officer, and administrative assistance, respectively.
## 6.2 Technical requirements

### 6.2.1 Facilities

The CGas-SiM should share the tasks between different units. All units should be established in well recognized institutes with long-term experience in the field of continuous long-term monitoring of reactive trace gases in situ in the atmosphere and superior experience in QA/QC. The various units should contribute with their specific and complementary competences to achieve a share of the multiple tasks. Sharing the tasks is necessary because no unit itself has the needed resources. The number of units should be a reasonable balance between available resources comprising personal, competence and capacity, and enhanced coordinative and administrative work. Furthermore, many of the calibration services require independent checks by various laboratories, i.e. the units of the CGas-SiM.

Since high pressure cylinders or air sample canisters should be exchanged between CGas-SiM and stations, the location of the CGas-SiM and its proximity to stations is generally not an issue. With respect to mobile reference instruments and intercomparisons, transport of instruments should be easily feasible in the EU. VOC instruments are more bulky and complex, thus, transport and intercomparison activities are more difficult to achieve. An allocation of units in different European countries would shorten transport time and distance to the NFs. But in general road transportation within Europe is well developed and the location of the units is not a big issue.

All units shall be able to provide calibration, QA/QC, and training services as outlined above. Otherwise, the intended services shall be complimentary, with the coordinating unit having capacity and know-how in the majority of services proposed.

Overall, in each unit laboratory space is needed with parts of the instrumentation covered under Section 3, i.e. at minimum one instrument needs to be present in the laboratory and available for ACTRIS support and users. The CGas-SiM should have training facilities with reference measurement systems (all systems listed in Section 3), side-by-side intercomparison facilities for a reasonable subsets of instruments, i.e. space and logistics for more than 5 VOC systems and more than 10 NOx systems, target gas storage and measurement facilities for compressed gas cylinders for NO and VOC, set-ups for travelling reference instruments or samplers for ambient air measurements, and computing facilities to analyse the corresponding data sets from NFs.

### 6.2.2 Human resources

The required human resources of CGas-SiM to cover the envisioned services and NFs are estimated to be:

- 5 full positions expert scientists (>5 years of experience in related field),

- 5 full positions engineer/qualified operator (>3 years of experience),

- 3 full positions of technician (> 1 year of experience),

and in the administrative area 0.5, 0.5 and 0.75 full positions of expert manager, qualified officer, and administrative assistance, respectively.
6.2.3 Other requirements

As the CGas-SiM strongly builds on the experience and resources of the WMO-GAW WCCs for VOC and NOx it is strongly recommended that the WCCs become part of the CGas-SiM. Further, the WCCs are encouraged to head units and coordinate the activities for the respective compound groups. In section 2 and 3, the synergies between GAW and ACTRIS in the reactive trace gases area were identified. ACTRIS procedures and know-how will be used further to strengthen the GAW-WCC activities. It is extremely important to achieve compatible data of reactive trace gases for the ACTRIS-RI and for the GAW-network. By combining QA/QC activities of both networks exceptional synergies are formed thus leading to a world– class centre for QA/QC in the area of atmospheric reactive trace gases, procedure development and training.

7 Basic criteria for the selection of the Centre for Reactive Trace gases
in Situ Measurements

The applications to host the Centre for Reactive Trace gases in Situ Measurements are evaluated against
a set of criteria covering:

* The level of commitment for long-term operation

* The **capacity** to provide the operational support and the services described in sections 3 and 4

○ The availability of the necessary laboratories, instruments, equipment

o The availability of human resources (no., expertise)

* The efficiency in providing the operational support and the services

o The feasibility of the implementation plan (costs, resources, timeline)

o The feasibility of the operation plan (methodology, costs per service)

* The level of maturity

o Status of the development of the Units

o No. of existing users

o Adequacy of the decision-making

o Risk management strategy

These criteria will be detailed in the associated call documents.
### 6.2.3 Other requirements

As the CGas-SiM strongly builds on the experience and resources of the WMO-GAW WCCs for VOC and NOx it is strongly recommended that the WCCs become part of the CGas-SiM. Further, the WCCs are encouraged to head units and coordinate the activities for the respective compound groups. In section 2 and 3, the synergies between GAW and ACTRIS in the reactive trace gases area were identified. ACTRIS procedures and know-how will be used further to strengthen the GAW-WCC activities. It is extremely important to achieve compatible data of reactive trace gases for the ACTRIS-RI and for the GAW-network. By combining QA/QC activities of both networks exceptional synergies are formed thus leading to a world-class centre for QA/QC in the area of atmospheric reactive trace gases, procedure development and training.

## 7 Basic criteria for the selection of the Centre for Reactive Trace gases in Situ Measurements

The applications to host the Centre for Reactive Trace gases in Situ Measurements are evaluated against a set of criteria covering:

* The level of commitment for long-term operation

* The capacity to provide the operational support and the services described in sections 3 and 4

* The availability of the necessary laboratories, instruments, equipment

* The availability of human resources (no., expertise)

* The efficiency in providing the operational support and the services

* The feasibility of the implementation plan (costs, resources, timeline)

* The feasibility of the operation plan (methodology, costs per service)

* The level of maturity

* Status of the development of the Units

* No. of existing users

* Adequacy of the decision-making

* Risk management strategy

These criteria will be detailed in the associated call documents.
# 8 Obligations of the Centre for Reactive Trace gases in Situ Measurements

## 8.1 General obligations

The following general obligations shall apply to the Centre for Reactive Trace gases in Situ Measurements while operational:

* To organize its activities in a cost-efficient way, timely, and with high quality,

* To actively participate in the governance and sustainable development of ACTRIS,

* To provide services to users, according to the access procedures, availability of time, and material resources,

* To provide operation support to the NFs, according to the identified needs, agreed schedules, and procedures,

* To implement efficient user interaction activities by:

* Organizing workshops, to interact with users,

* Collecting user feed-backs,

* Maintaining the CF and ACTRIS websites,

* To document its activities, accesses provided, key performance indicators, and finances, and provide this information to the HO,

* To contribute to minimize and assess the uncertainties related to data and data products, according to their role and in collaboration with each other,

* To remain at the forefront of the technology for reactive trace gases in situ measurements.

## 8.2 Technical obligations

### 8.2.1 Technical obligations in relation with the ACTRIS National Facilities

Technical obligations of the Centre for Reactive Trace gases in Situ Measurements in relation with the ACTRIS National Facilities refer strictly to the measurement techniques described in this concept document and in *Technical concepts and requirements for ACTRIS Observational Platforms*. The operation of other techniques at the NFs does not imply a technical obligation for the Centre for Reactive Trace gases in Situ Measurements to provide operation support. New techniques / instrument types are only accepted as ACTRIS instruments after the approval of the RI Committee and after the development of the associated QA procedures by the CF.

#### 8.2.1.1 Guidelines, quality assurance criteria and procedures

The Centre for Reactive Trace gases in Situ Measurements is responsible for the quality assurance procedures and guidelines of the ACTRIS measurements conducted at the NFs operating as well as the procedure development as outlines in the previous sections of this concept paper. All the procedures, guidelines, quality assurance criteria, plausibility tests, etc. should be documented and accessible to all related NFs. Specific training sessions with the related NF operators should be organized to ensure a smooth and correct implementation of the above.
# 8 Obligations of the Centre for Reactive Trace gases in Situ Measurements

## 8.1 General obligations

The following general obligations shall apply to the Centre for Reactive Trace gases in Situ Measurements while operational:

* To organize its activities in a cost-efficient way, timely, and with high quality,

* To actively participate in the governance and sustainable development of ACTRIS,

* To provide services to users, according to the access procedures, availability of time, and material resources,

* To provide operation support to the NFs, according to the identified needs, agreed schedules, and procedures,

* To implement efficient user interaction activities by:

* Organizing workshops, to interact with users,

* Collecting user feed-backs,

* Maintaining the CF and ACTRIS websites,

* To document its activities, accesses provided, key performance indicators, and finances, and provide this information to the HO,

* To contribute to minimize and assess the uncertainties related to data and data products, according to their role and in collaboration with each other,

* To remain at the forefront of the technology for reactive trace gases in situ measurements.

## 8.2 Technical obligations

### 8.2.1 Technical obligations in relation with the ACTRIS National Facilities

Technical obligations of the Centre for Reactive Trace gases in Situ Measurements in relation with the ACTRIS National Facilities refer strictly to the measurement techniques described in this concept document and in *Technical concepts and requirements for ACTRIS Observational Platforms*. The operation of other techniques at the NFs does not imply a technical obligation for the Centre for Reactive Trace gases in Situ Measurements to provide operation support. New techniques / instrument types are only accepted as ACTRIS instruments after the approval of the RI Committee and after the development of the associated QA procedures by the CF.

#### 8.2.1.1 Guidelines, quality assurance criteria and procedures

The Centre for Reactive Trace gases in Situ Measurements is responsible for the quality assurance procedures and guidelines of the ACTRIS measurements conducted at the NFs operating as well as the procedure development as outlines in the previous sections of this concept paper. All the procedures, guidelines, quality assurance criteria, plausibility tests, etc. should be documented and accessible to all related NFs. Specific training sessions with the related NF operators should be organized to ensure a smooth and correct implementation of the above.
### 8.2.1.2 Tools, tests and consultancy for quality control of the measurements

It is duty of the Centre for Reactive Trace gases in Situ Measurements to assist the related NFs in the quality control of their measurements by providing the operation support listed in the previous sections, specifically:

* Development and provision of instrument-specific calibration (as described in section 3.3.3).

* Development and provision of in-house check-up tools (as described in section 3.3.4)

* Development of data evaluation procedures and plausibility test (as described in sections 3.3.5).

* Consultancy for setting-up new observational or exploratory platforms and new instruments at the NFs (as described in sections 3.4.2

* Testing of new measurement instruments and procedures (as described in sections 3.5.3).

This operation support should be offered to the related NFs whenever there is a need, or a significant progress in the development of these tools, and accompanied by the appropriate training given that sufficient resources are available at the CGas-SiM.

The CGas-SiM shall appoint yearly station workshops for quality control of data. At specified dates prior to the workshop, station PIs have to deliver the data including meta-data according to ACTRIS rules and ACTRIS templates. CGas-SiM will evaluate the data, discuss the evaluation results with station PIs at the workshop, and initiate an issue tracker in case of questionable data. The issues are solved by station PIs and either closed by CGas-SiM or, in case of still open issues, handed back to the station PIs for another review. This dialogue should be openly available at the ACTRIS data centre.

### 8.2.1.3 Assessment of performances, measurement flagging

The Centre for Reactive Trace gases in Situ Measurements is mandated to assist the Data Centre and the related NFs in the quality control of the measurements and data, by:

* Realization of observational site performance audits with reference samples or mobile systems (as described in sections 3.3.6).

* Organizing regular exercises to assess the performances of the NFs, including instrument performance workshops (as described in sections 3.3.7).

* Contributing to documentation and traceability of level 0 to level 3 data products (as described in sections 3.3.8).

The activities involving directly the NFs operating instruments for measuring reactive trace gases covered by the TC should be scheduled by the TC and provided regularly. The corresponding NFs should participate in these activities according to the plan proposed by the Centre for Reactive Trace gases in Situ Measurements and agreed on with the ACTRIS Scientific Advisory Board (SAB).

### 8.2.1.4 Transfer of knowledge and training

In addition to the training organized for the implementation of guidelines, procedures, and tools developed in support of QA/QC of the measurements and data, the Centre for Reactive Trace gases in Situ Measurements should organize training sessions with the NFs as needed (see sections 3.4.1).
### 8.2.1.2 Tools, tests and consultancy for quality control of the measurements

It is duty of the Centre for Reactive Trace gases in Situ Measurements to assist the related NFs in the quality control of their measurements by providing the operation support listed in the previous sections, specifically:

* Development and provision of instrument-specific calibration (as described in section 3.3.3).

* Development and provision of in-house check-up tools (as described in section 3.3.4).

* Development of data evaluation procedures and plausibility test (as described in sections 3.3.5).

* Consultancy for setting-up new observational or exploratory platforms and new instruments at the NFs (as described in sections 3.4.2).

* Testing of new measurement instruments and procedures (as described in sections 3.5.3).

This operation support should be offered to the related NFs whenever there is a need, or a significant progress in the development of these tools, and accompanied by the appropriate training given that sufficient resources are available at the CGas-SiM.

The CGas-SiM shall appoint yearly station workshops for quality control of data. At specified dates prior to the workshop, station PIs have to deliver the data including meta-data according to ACTRIS rules and ACTRIS templates. CGas-SiM will evaluate the data, discuss the evaluation results with station PIs at the workshop, and initiate an issue tracker in case of questionable data. The issues are solved by station PIs and either closed by CGas-SiM or, in case of still open issues, handed back to the station PIs for another review. This dialogue should be openly available at the ACTRIS data centre.

### 8.2.1.3 Assessment of performances, measurement flagging

The Centre for Reactive Trace gases in Situ Measurements is mandated to assist the Data Centre and the related NFs in the quality control of the measurements and data, by:

* Realization of observational site performance audits with reference samples or mobile systems (as described in sections 3.3.6).

* Organizing regular exercises to assess the performances of the NFs, including instrument performance workshops (as described in sections 3.3.7).

* Contributing to documentation and traceability of level 0 to level 3 data products (as described in sections 3.3.8).

The activities involving directly the NFs operating instruments for measuring reactive trace gases covered by the TC should be scheduled by the TC and provided regularly. The corresponding NFs should participate in these activities according to the plan proposed by the Centre for Reactive Trace gases in Situ Measurements and agreed on with the ACTRIS Scientific Advisory Board (SAB).

### 8.2.1.4 Transfer of knowledge and training

In addition to the training organized for the implementation of guidelines, procedures, and tools developed in support of QA/QC of the measurements and data, the Centre for Reactive Trace gases in Situ Measurements should organize training sessions with the NFs as needed (see sections 3.4.1).
**8.2.1.5 Improvement of measurement methodologies for Centre for Reactive Trace gases in Situ Measurements**

Although it is not an obligation, the Centre for Reactive Trace gases in Situ Measurements should work to sustain a high level of performance and to stimulate the advancement of new techniques and methodologies in the analysis of reactive trace gases in the atmosphere by:

* Development of strategies to increase the duty cycle of instruments operated at the NFs, to reduce breakdown and maintenance downtime (as described in section 3.5.2).

* Development of new technological products and methods (as described in section 3.5.3).

* Development of retrieval algorithms or tools for producing level 1 to 3 data, including the exploitation of instrument synergies (as described in section 3.5.4)

* Organizing regular events (at least once at 2 years) and other forums, in order to exchange knowledge with the NFs and other scientists.

* Contributing to CEN, ISO, or similar standardization activities (as described in sections 3.3.9).

**8.2.2 Technical obligations in relation with the ACTRIS Data Centre**

The workflow between CGAs-SiM is graphically shown in Figure 1. Further information is provided in sections 3.3.7 and 3.3.8. Data submitted by the NFs are reviewed by CGAs-SiM on a yearly basis in collaboration with the unit data centre for trace gas in situ data. Jointly with the unit data centre for trace gas in situ data the CGAs-SiM is developing automated QA/QC tools in particular for NRT data submission.

**8.2.3 Technical obligations in relation with other TC**

The CGas-SiM should establish links to other topical centres where appropriate and ACTRIS variables of reactive trace gases are assessed e.g. Centre for Reactive Trace Gases Remote Sensing.

**8.2.4 Technical obligations in relation with the ACTRIS users**

The Centre for Reactive Trace gases in Situ Measurements should commit to provide a minimum amount of user services as described in section 4.

**8.3 Evaluation of the activity of Centre for Reactive Trace gases in Situ Measurements**

Once established and operational, the Centre for Reactive Trace gases in Situ Measurements will be annually evaluated for its performance, against the following Key Performance indicators (KPIs). The KPI and the corresponding scores are listed in Table 8-1.
**8.2.1.5 Improvement of measurement methodologies for Centre for Reactive Trace gases in Situ Measurements**

Although it is not an obligation, the Centre for Reactive Trace gases in Situ Measurements should work to sustain a high level of performance and to stimulate the advancement of new techniques and methodologies in the analysis of reactive trace gases in the atmosphere by:

* Development of strategies to increase the duty cycle of instruments operated at the NFs, to reduce breakdown and maintenance downtime (as described in section 3.5.2).

* Development of new technological products and methods (as described in section 3.5.3).

* Development of retrieval algorithms or tools for producing level 1 to 3 data, including the exploitation of instrument synergies (as described in section 3.5.4)

* Organizing regular events (at least once at 2 years) and other forums, in order to exchange knowledge with the NFs and other scientists.

* Contributing to CEN, ISO, or similar standardization activities (as described in sections 3.3.9).

**8.2.2 Technical obligations in relation with the ACTRIS Data Centre**

The workflow between CGAs-SiM is graphically shown in Figure 1. Further information is provided in sections 3.3.7 and 3.3.8. Data submitted by the NFs are reviewed by CGAs-SiM on a yearly basis in collaboration with the unit data centre for trace gas in situ data. Jointly with the unit data centre for trace gas in situ data the CGAs-SiM is developing automated QA/QC tools in particular for NRT data submission.

**8.2.3 Technical obligations in relation with other TC**

The CGas-SiM should establish links to other topical centres where appropriate and ACTRIS variables of reactive trace gases are assessed e.g. Centre for Reactive Trace Gases Remote Sensing.

**8.2.4 Technical obligations in relation with the ACTRIS users**

The Centre for Reactive Trace gases in Situ Measurements should commit to provide a minimum amount of user services as described in section 4.

**8.3 Evaluation of the activity of Centre for Reactive Trace gases in Situ Measurements**

Once established and operational, the Centre for Reactive Trace gases in Situ Measurements will be annually evaluated for its performance, against the following Key Performance indicators (KPIs). The KPI and the corresponding scores are listed in Table 8-1.
<table><caption>Table 8-1: Key Performance indicators for the yearly evaluation of the CGas-SiM activities</caption><thead><tr><th>Criteria</th><th>Indicator</th><th>Planned value</th><th>Achieved value</th><th>Percentage of achievement<sup>1</sup></th><th>Weight</th></tr></thead><tbody><tr><td colspan="6" style="background-color: #d2b48c; font-weight: bold;">GENERAL SCORE</td></tr><tr><td rowspan="5" style="vertical-align: top;">Usefulness for ACTRIS NFs</td><td>CRITERIA TOTAL SCORE</td><td></td><td></td><td></td><td rowspan="5" style="text-align: center; vertical-align: middle;">50 %</td></tr><tr><td>No. of operation support units provided to ACTRIS NFs for quality assurance and quality control</td><td></td><td></td><td></td></tr><tr><td>No. of operation support units provided to ACTRIS NFs for knowledge transfer and training</td><td></td><td></td><td></td></tr><tr><td>No. of operation support units provided to ACTRIS NFs for Improvement of measurement and data processing methodologies</td><td></td><td></td><td></td></tr><tr><td>Average score of satisfaction received from NFs Including utility, fairness and timeliness of the activities for operation support</td><td></td><td></td><td></td></tr><tr><td rowspan="5" style="vertical-align: top;">Usefulness for ACTRIS users</td><td>CRITERIA TOTAL SCORE</td><td></td><td></td><td></td><td rowspan="5" style="text-align: center; vertical-align: middle;">20 %</td></tr><tr><td>No. of service units provided to ACTRIS users for quality assurance and quality control</td><td></td><td></td><td></td></tr><tr><td>No. of service units provided to ACTRIS users for knowledge transfer and training</td><td></td><td></td><td></td></tr><tr><td>No. of service units provided to ACTRIS users for improvement of measurement and data processing methodologies</td><td></td><td></td><td></td></tr><tr><td>Average score of satisfaction received from ACTRIS users Including utility, fairness and timeliness of the services performed</td><td></td><td></td><td></td></tr><tr><td colspan="6" style="background-color: #d2b48c; font-weight: bold;">Impac</td></tr><tr><td></td><td>CRITERIA TOTAL SCORE</td><td></td><td></td><td></td><td></td></tr></tbody></table>

<footnote n="1">¹ With regard to the planned and approved values</footnote>
<table><caption>Table 8-1: Key Performance indicators for the yearly evaluation of the CGas-SiM activities</caption><thead><tr><th>Criteria</th><th>Indicator</th><th>Planned value</th><th>Achieved value</th><th>Percentage of achievement<sup>1</sup></th><th>Weight</th></tr></thead><tbody><tr><td colspan="6" style="background-color: #D2B48C; font-weight: bold;">GENERAL SCORE</td></tr><tr><td rowspan="5" style="vertical-align: top;">Usefulness for ACTRIS NFs</td><td>CRITERIA TOTAL SCORE</td><td></td><td></td><td></td><td rowspan="5" style="text-align: center; vertical-align: middle;">50<br>%</td></tr><tr><td>No. of operation support units provided to ACTRIS NFs for quality assurance and quality control</td><td></td><td></td><td></td></tr><tr><td>No. of operation support units provided to ACTRIS NFs for knowledge transfer and training</td><td></td><td></td><td></td></tr><tr><td>No. of operation support units provided to ACTRIS NFs for Improvement of measurement and data processing methodologies</td><td></td><td></td><td></td></tr><tr><td>Average score of satisfaction received from NFs Including utility, fairness and timeliness of the activities for operation support</td><td></td><td></td><td></td></tr><tr><td rowspan="5" style="vertical-align: top;">Usefulness for ACTRIS users</td><td>CRITERIA TOTAL SCORE</td><td></td><td></td><td></td><td rowspan="5" style="text-align: center; vertical-align: middle;">20<br>%</td></tr><tr><td>No. of service units provided to ACTRIS users for quality assurance and quality control</td><td></td><td></td><td></td></tr><tr><td>No. of service units provided to ACTRIS users for knowledge transfer and training</td><td></td><td></td><td></td></tr><tr><td>No. of service units provided to ACTRIS users for improvement of measurement and data processing methodologies</td><td></td><td></td><td></td></tr><tr><td>Average score of satisfaction received from ACTRIS users Including utility, fairness and timeliness of the services performed</td><td></td><td></td><td></td></tr><tr><td>Impac</td><td>CRITERIA TOTAL SCORE</td><td></td><td></td><td></td><td></td></tr></tbody></table>

<footnote n="1">¹ With regard to the planned and approved values</footnote>
<table><tbody><tr><td></td><td>No. of new technological products, methods and algorithms developed / improved</td><td></td><td></td><td></td><td rowspan="4">20 %</td></tr><tr><td></td><td>No. of peer-review CF-related papers published</td><td></td><td></td><td></td></tr><tr><td></td><td>No. of CF-related communications at scientific conferences/workshops</td><td></td><td></td><td></td></tr><tr><td></td><td>No. of CF-related patents promoted</td><td></td><td></td><td></td></tr><tr><td rowspan="5">Integration into ACTRIS</td><td>CRITERIA TOTAL SCORE</td><td></td><td></td><td></td><td rowspan="5">10 %</td></tr><tr><td>No. of participations to ACTRIS committees and boards</td><td></td><td></td><td></td></tr><tr><td>No. of activities performed in collaboration with other TCs (joint SOPs, joint workshops, etc.)</td><td></td><td></td><td></td></tr><tr><td>Average score of satisfaction received from ACTRIS DC for the contribution to documentation and traceability of data products</td><td></td><td></td><td></td></tr><tr><td>Average score of satisfaction received from ACTRIS HO for the quality and readiness of the reports</td><td></td><td></td><td></td></tr></tbody></table>
<table><tbody><tr><td></td><td>No. of new technological products, methods and algorithms developed / improved</td><td></td><td></td><td></td></tr><tr><td></td><td>No. of peer-review CF-related papers published</td><td></td><td></td><td rowspan="2">20 %</td></tr><tr><td></td><td>No. of CF-related communications at scientific conferences/workshops</td><td></td><td></td></tr><tr><td></td><td>No. of CF-related patents promoted</td><td></td><td></td><td></td></tr><tr><td rowspan="5">Integration into ACTRIS</td><td>CRITERIA TOTAL SCORE</td><td></td><td></td><td rowspan="5">10 %</td></tr><tr><td>No. of participations to ACTRIS committees and boards</td><td></td><td></td></tr><tr><td>No. of activities performed in collaboration with other TCs (joint SOPs, joint workshops, etc.)</td><td></td><td></td></tr><tr><td>Average score of satisfaction received from ACTRIS DC for the contribution to documentation and traceability of data products</td><td></td><td></td></tr><tr><td>Average score of satisfaction received from ACTRIS HO for the quality and readiness of the reports</td><td></td><td></td></tr></tbody></table>
9 Glossary

**In situ measurements** - measured or sampled air and instrument are at the same location and in physical contact. In the context of ACTRIS, *in situ* measurements of aerosol, cloud, and reactive-trace-gas properties are performed at observational sites near the Earth surface, on mobile surface-based or airborne platforms, and in simulation chambers and laboratories.

**Remote sensing** - measured air and instrument are not at the same location and not in physical contact. In the context of ACTRIS, passive (radiometer) and active atmospheric remote-sensing techniques<fnref n="2" /> (lidar, radar) for the observation of aerosols, clouds, and trace gases are applied at observational sites or on mobile surface-based and airborne platforms.

**ACTRIS data** - are the ACTRIS variables resulting from measurements that fully comply with the standard operating procedures (SOP), measurement recommendations, and quality guidelines established within ACTRIS.

o **ACTRIS level 0 data:** Raw sensor output, either mV or physical units. Native resolution, metadata necessary for next level.

o **ACTRIS level 1 data:** Calibrated and quality assured data with minimum level of quality control.

o **ACTRIS level 2 data:** Approved and fully quality controlled ACTRIS data product or geophysical variable.

o **ACTRIS level 3 data:** Elaborated ACTRIS data products derived by post-processing of ACTRIS Level 0 -1 -2 data, and data from other sources. The data can be gridded or not.

o **ACTRIS syntheses product (Proper name to be defined later):** Data product from e.g. research activities, not under direct ACTRIS responsibility, but ACTRIS offer repository and access.

**ACTRIS Data Centre (DC)** - the Central Facility responsible for ACTRIS data curation, preservation, and distribution of data, value-added products and tools, and hosting the ACTRIS data portal.

**ACTRIS data originator** - entity operating instruments at a National Facility or Topical Centre, resulting in ACTRIS data and delivering ACTRIS data to the Data Centre.

**ACTRIS data provider** - the Data Centre offering the ACTRIS data and value-added data products and tools to users.

**ACTRIS digital tools and services** - tailored codes and software for processing and visualization of ACTRIS data, production of ACTRIS data products, and for data analysis and research.

**ACTRIS exploratory platform** - National Facility (simulation chambers, laboratories, or mobile facilities) operating on campaign basis and delivering dedicated data to the Data Centre.

<footnote n="2">² In contrast to remote sensing of the Earth's surface (terrestrial and oceanic remote sensing), which is typically done from satellite or aircraft, atmospheric remote sensing is performed from satellite-borne and airborne as well as from surface-based platforms. In ACTRIS, mainly surface-based remote observations are carried out.</footnote>
9 Glossary

**In situ measurements** - measured or sampled air and instrument are at the same location and in physical contact. In the context of ACTRIS, *in situ* measurements of aerosol, cloud, and reactive-trace-gas properties are performed at observational sites near the Earth surface, on mobile surface-based or airborne platforms, and in simulation chambers and laboratories.

**Remote sensing** - measured air and instrument are not at the same location and not in physical contact. In the context of ACTRIS, passive (radiometer) and active atmospheric remote-sensing techniques<fnref n="2" /> (lidar, radar) for the observation of aerosols, clouds, and trace gases are applied at observational sites or on mobile surface-based and airborne platforms.

**ACTRIS data** - are the ACTRIS variables resulting from measurements that fully comply with the standard operating procedures (SOP), measurement recommendations, and quality guidelines established within ACTRIS.

o **ACTRIS level 0 data:** Raw sensor output, either mV or physical units. Native resolution, metadata necessary for next level.

o **ACTRIS level 1 data:** Calibrated and quality assured data with minimum level of quality control.

o **ACTRIS level 2 data:** Approved and fully quality controlled ACTRIS data product or geophysical variable.

o **ACTRIS level 3 data:** Elaborated ACTRIS data products derived by post-processing of ACTRIS Level 0 -1 -2 data, and data from other sources. The data can be gridded or not.

o **ACTRIS syntheses product (Proper name to be defined later):** Data product from e.g. research activities, not under direct ACTRIS responsibility, but ACTRIS offer repository and access.

**ACTRIS Data Centre (DC)** - the Central Facility responsible for ACTRIS data curation, preservation, and distribution of data, value-added products and tools, and hosting the ACTRIS data portal.

**ACTRIS data originator** - entity operating instruments at a National Facility or Topical Centre, resulting in ACTRIS data and delivering ACTRIS data to the Data Centre.

**ACTRIS data provider** - the Data Centre offering the ACTRIS data and value-added data products and tools to users.

**ACTRIS digital tools and services** - tailored codes and software for processing and visualization of ACTRIS data, production of ACTRIS data products, and for data analysis and research.

**ACTRIS exploratory platform** - National Facility (simulation chambers, laboratories, or mobile facilities) operating on campaign basis and delivering dedicated data to the Data Centre.

<footnote n="2">² In contrast to remote sensing of the Earth's surface (terrestrial and oceanic remote sensing), which is typically done from satellite or aircraft, atmospheric remote sensing is performed from satellite-borne and airborne as well as from surface-based platforms. In ACTRIS, mainly surface-based remote observations are carried out.</footnote>
**ACTRIS General Assembly (GA)** - a council of ministry- and funding organization representatives of ACTRIS members after ACTRIS legal entity has been established, superior decision-making body of ACTRIS.

**ACTRIS Head Office (HO)** - a Central Facility coordinating and representing ACTRIS, and holding the statutory seat.

**ACTRIS label** - earmarks a data set or a measurement site as ACTRIS data or ACTRIS National Facility.

**ACTRIS observational platform** – ACTRIS National Facility performing long-term, regular observations and delivering standardized data to the Data Centre.

**ACTRIS Topical Centres (TCs)** – a Central Facility offering services and operation support for QA/QC of measurements and data (including training, calibration, QA/QC tools, and development of standard operation and evaluation procedures)

**ACTRIS variables** - the measured atmospheric variables as described in the ACTRIS Data Management Plan<fnref n="3" />.

**Central Facility (CF)** - a European level ACTRIS component that offers ACTRIS data or other ACTRIS services to users as well as operation support to ACTRIS National Facilities.

**Central Facility Unit** – part of a Central Facility located at, and operated by a research performing organization (RPO) or by ACTRIS ERIC.

**CF Director** – the person responsible for the coordination and representation of a Central Facility.

**CF Unit Head** – the person responsible for the coordination and representation of a Central Facility unit.

**CF Management Board** – consists of the CF Unit Heads and the CF Director; this board manages the Central Facility.

**Data curation** - the activity that stores, manages and ensures access to all persistent data sets produced within the infrastructure.

**Data traceability** - an unbroken chain of uniquely identified process steps leading from raw data to any kind of processed data, where identification of process steps follows the data.

**Interim ACTRIS Council** - a council of ministry- and funding organization representatives of ACTRIS members before ACTRIS legal entity has been established (during the ACTRIS Preparation and Transition Phase), superior decision-making body of ACTRIS.

**Measurement traceability** - an unbroken chain of comparisons relating an instrument's measurements to a known standard, in the ideal case SI units.

<footnote n="3">³The ACTRIS data management plan and list of variables were approved by ACTRIS-2 scientific steering committee, October 2015. These documents are expected to be refined during ACTRIS-PPP (WP5).</footnote>
**ACTRIS General Assembly (GA)** - a council of ministry- and funding organization representatives of ACTRIS members after ACTRIS legal entity has been established, superior decision-making body of ACTRIS.

**ACTRIS Head Office (HO)** - a Central Facility coordinating and representing ACTRIS, and holding the statutory seat.

**ACTRIS label** - earmarks a data set or a measurement site as ACTRIS data or ACTRIS National Facility.

**ACTRIS observational platform** – ACTRIS National Facility performing long-term, regular observations and delivering standardized data to the Data Centre.

**ACTRIS Topical Centres (TCs)** – a Central Facility offering services and operation support for QA/QC of measurements and data (including training, calibration, QA/QC tools, and development of standard operation and evaluation procedures)

**ACTRIS variables** - the measured atmospheric variables as described in the ACTRIS Data Management Plan<fnref n="3" />.

**Central Facility (CF)** - a European level ACTRIS component that offers ACTRIS data or other ACTRIS services to users as well as operation support to ACTRIS National Facilities.

**Central Facility Unit** – part of a Central Facility located at, and operated by a research performing organization (RPO) or by ACTRIS ERIC.

**CF Director** – the person responsible for the coordination and representation of a Central Facility.

**CF Unit Head** – the person responsible for the coordination and representation of a Central Facility unit.

**CF Management Board** – consists of the CF Unit Heads and the CF Director; this board manages the Central Facility.

**Data curation** - the activity that stores, manages and ensures access to all persistent data sets produced within the infrastructure.

**Data traceability** - an unbroken chain of uniquely identified process steps leading from raw data to any kind of processed data, where identification of process steps follows the data.

**Interim ACTRIS Council** - a council of ministry- and funding organization representatives of ACTRIS members before ACTRIS legal entity has been established (during the ACTRIS Preparation and Transition Phase), superior decision-making body of ACTRIS.

**Measurement traceability** - an unbroken chain of comparisons relating an instrument's measurements to a known standard, in the ideal case SI units.

<footnote n="3">³The ACTRIS data management plan and list of variables were approved by ACTRIS-2 scientific steering committee, October 2015. These documents are expected to be refined during ACTRIS-PPP (WP5).</footnote>
**National Facility (NF)** - an observational or exploratory platform providing data and/or physical access to the platform within ACTRIS.

**Quality assurance and control:** Quality assurance is process oriented and focuses on defect prevention; quality control is product oriented and focuses on defect identification:

o **Quality Assurance (QA):** The process or set of processes used to ensure the quality of a product (e.g. data series, instrument, sample, measured value of a variable, etc.),

o **Quality Control (QC):** The process and activities of ensuring products and services meet the expectations.

**RI committee** - consists of up to two representatives from each Central Facility and [three] representatives from the National Facilities Assembly; the RI committee supports the (Interim) Director or Board of Directors in operating the RI.

**Service and Access Management Unit (SAMU)** - a part of ACTRIS Head Office facilitating the access to ACTRIS services.

**User** - a person, a team, or an institution making use of ACTRIS data or other ACTRIS services, including access to ACTRIS facilities.
**National Facility (NF)** - an observational or exploratory platform providing data and/or physical access to the platform within ACTRIS.

**Quality assurance and control:** Quality assurance is process oriented and focuses on defect prevention; quality control is product oriented and focuses on defect identification:

o **Quality Assurance (QA):** The process or set of processes used to ensure the quality of a product (e.g. data series, instrument, sample, measured value of a variable, etc.),

o **Quality Control (QC):** The process and activities of ensuring products and services meet the expectations.

**RI committee** - consists of up to two representatives from each Central Facility and [three] representatives from the National Facilities Assembly; the RI committee supports the (Interim) Director or Board of Directors in operating the RI.

**Service and Access Management Unit (SAMU)** - a part of ACTRIS Head Office facilitating the access to ACTRIS services.

**User** - a person, a team, or an institution making use of ACTRIS data or other ACTRIS services, including access to ACTRIS facilities.
# 10 Reference documents

ACTRIS (2011-2017): Measurement Guidelines and Standard Operation Procedures available at: http://actris-nilu.no/Content/SOP).

ACTRIS Concept Documents

ACTRIS Data Management Plan, approved by ACTRIS-2 SSC 23 October 2015

ACTRIS-PPP proposal

Baseline document for the Concepts of ACTRIS Central Facilities

Hoerger C.C., Claude A., Plass-Duelmer C., Reimann S., Eckart E., Steinbrecher R., Aalto J., Arduini J., Bonnaire N., Cape J.N., Colomb A., Connolly R., Diskova J., Dumitrean P., Ehlers C., Gros V., Hakola H., Hill M., Hopkins J.R., Jäger J., Junek R., Kajos M.K., Klemp D., Leuchner M., Lewis A.C., Locoge N., Maione M., Martin D., Michl K., Nemitz E., O'Doherty S., PÉrez Ballesta P., Ruuskanen T.M., Sauvage S., Schmidbauer N., Spain T.G., Straube E., Vana, M., Vollmer M.K., Wegener R., Wenger, A. (2015): ACTRIS non-methane hydrocarbon intercomparison experiment in Europe to support WMO GAW and EMEP observation networks. Atmospheric Measurement Techniques, 8(7), 2715-2736.

ISO 10012:2003: Measurement management systems — Requirements for measurement processes and measuring equipment

ISO 9000:2015: Quality management systems—Fundamentals and vocabulary

Schultz M.G., Akimoto H., Bottenheim J., Buchmann B., Galbally I.E., Gilge S., Helmig D., Koide H., Lewis A.C., Novelli P.C., Plass-Dülmer C., Ryerson T.B., Steinbacher M., Steinbrecher R., Tarasova O., Tørseth K., Thouret V., Zellweger C. (2015): The Global Atmosphere Watch reactive gases measurement network. Elementa. Science of the Anthropocene, 3: 000067, doi: 10.12952/journal.elementa.000067.

SOP-NOx, 2014: Deliverable D4.10 of ACTRIS I3 in FP7 (2011-2015): Standardized operating procedures (SOPs) for NOxy Measurements, report available at: http://fp7.actris.eu/Portals/97/deliverables/PU/WP4_D4.10_M42.pdf

SOP-VOC, 2014: Deliverable D4.9 of ACTRIS I3 in FP7 (2011-2015): Final SOPs for VOCs measurements, report available at: http://fp7.actris.eu/Portals/97/deliverables/PU/WP4_D4.9_M42_v2.pdf. Technical concepts and requirements for ACTRIS Exploratory Platforms.

WMO 2004: Strategy for the implementation of the Global Atmosphere Watch Programme (2001-2007): a contribution to the implementation of the WMO Long-Term Plan, GAW Report No 142, available at: http://library.wmo.int/pmb_ged/wmo-td_1077.pdf.

WMO-GAW, 2017: GAW Implementation Plan available at: http://www.wmo.int/pages/prog/arep/gaw/documents/GAWIP2016-2023.pdf.
10 Reference documents

ACTRIS (2011-2017): Measurement Guidelines and Standard Operation Procedures available at: http://actris-nilu.no/Content/SOP).

ACTRIS Concept Documents

ACTRIS Data Management Plan, approved by ACTRIS-2 SSC 23 October 2015

ACTRIS-PPP proposal

Baseline document for the Concepts of ACTRIS Central Facilities

Hoerger C.C., Claude A., Plass-Duelmer C., Reimann S., Eckart E., Steinbrecher R., Aalto J., Arduini J., Bonnaire N., Cape J.N., Colomb A., Connolly R., Diskova J., Dumitrean P., Ehlers C., Gros V., Hakola H., Hill M., Hopkins J.R., Jäger J., Junek R., Kajos M.K., Klemp D., Leuchner M., Lewis A.C., Locoge N., Maione M., Martin D., Michl K., Nemitz E., O'Doherty S., PÉrez Ballesta P., Ruuskanen T.M., Sauvage S., Schmidbauer N., Spain T.G., Straube E., Vana, M., Vollmer M.K., Wegener R., Wenger, A. (2015): ACTRIS non-methane hydrocarbon intercomparison experiment in Europe to support WMO GAW and EMEP observation networks. Atmospheric Measurement Techniques, 8(7), 2715-2736.

ISO 10012:2003: Measurement management systems — Requirements for measurement processes and measuring equipment

ISO 9000:2015: Quality management systems—Fundamentals and vocabulary

Schultz M.G., Akimoto H., Bottenheim J., Buchmann B., Galbally I.E., Gilge S.,, Helmig D., Koide H., Lewis A.C., Novelli P.C., Plass-Dülmer C., Ryerson T.B., Steinbacher M., Steinbrecher R., Tarasova O., Tørseth K., Thouret V., Zellweger C. (2015): The Global Atmosphere Watch reactive gases measurement network. Elementa. Science of the Anthropocene, 3: 000067, doi: 10.12952/journal.elementa.000067.

SOP-NOx, 2014: Deliverable D4.10 of ACTRIS I3 in FP7 (2011-2015): Standardized operating procedures (SOPs) for NOxy Measurements, report available at: http://fp7.actris.eu/Portals/97/deliverables/PU/WP4_D4.10_M42.pdf

SOP-VOC, 2014: Deliverable D4.9 of ACTRIS I3 in FP7 (2011-2015): Final SOPs for VOCs measurements, report available at: http://fp7.actris.eu/Portals/97/deliverables/PU/WP4_D4.9_M42_v2.pdf.
Technical concepts and requirements for ACTRIS Exploratory Platforms.

WMO 2004: Strategy for the implementation of the Global Atmosphere Watch Programme (2001-2007): a contribution to the implementation of the WMO Long-Term Plan, GAW Report No 142, available at: http://library.wmo.int/pmb_ged/wmo-td_1077.pdf.

WMO-GAW, 2017: GAW Implementation Plan available at: http://www.wmo.int/pages/prog/arep/gaw/documents/GAWIP2016-2023.pdf.
# Annex: Provision of the operation support

## Scheduled support

The CGas-SiM should develop and discuss the scheduled support together with the NFs during the yearly assemblies. Fine tuning should be achieved by teleconferences on request. This support should be managed by the lead unit of the CGas-SiM.

QA/QC activities like round robin or intercomparison experiments which are open to all users, should be announced on the CGas-SiM web-page at least 2 months before the event, with number of potential participants and conditions for contribution. Applications should be selected by the CGas-SiM units in one of the bimonthly teleconferences of the CGas-SiM. Results of these activities should be made openly available on the CGas-SiM webpage. The scheduled support is listed in Table:

<table><caption>Table 9-1: Summary of the scheduled support to NFs by the CGas-SiM</caption><thead><tr><th>Technique</th><th>Type of support</th><th>Specific support</th><th>Frequency</th><th>Comments</th></tr></thead><tbody><tr><td rowspan="5">Reactive Trace Gas Measurement Techniques</td><td>Measurement Guidelines (MGs) and Standard Operation Procedures (SOPs)</td><td>Provision of MGs and SOPs</td><td>Once</td><td>and after each major upgrade</td></tr><tr><td>Scale transfer; Transfer standards</td><td>Provision of scale to working standards used at NFs</td><td>Once and then every four years</td><td>and after each major upgrade</td></tr><tr><td>New Instrument techniques</td><td>Consulting for implementing new techniques</td><td>Once</td><td>and after each major upgrade</td></tr><tr><td>Training of operators and scientists</td><td>Not specific at this stage</td><td>Every two years for a specific measurement technique used</td><td>At training centres or co-located with instrument performance workshops</td></tr><tr><td>Target-gases</td><td>Provision of target gases</td><td>Every year</td><td>Target gases have to be analysed by NFs</td></tr></tbody></table>
# Annex: Provision of the operation support

## Scheduled support

The CGas-SiM should develop and discuss the scheduled support together with the NFs during the yearly assemblies. Fine tuning should be achieved by teleconferences on request. This support should be managed by the lead unit of the CGas-SiM.

QA/QC activities like round robin or intercomparison experiments which are open to all users, should be announced on the CGas-SiM web-page at least 2 months before the event, with number of potential participants and conditions for contribution. Applications should be selected by the CGas-SiM units in one of the bimonthly teleconferences of the CGas-SiM. Results of these activities should be made openly available on the CGas-SiM webpage. The scheduled support is listed in Table:

<table><caption>Table 9-1: Summary of the scheduled support to NFs by the CGas-SiM</caption><thead><tr><th>Technique</th><th>Type of support</th><th>Specific support</th><th>Frequency</th><th>Comments</th></tr></thead><tbody><tr><td rowspan="5">Reactive Trace Gas Measurement Techniques</td><td>Measurement Guidelines (MGs) and Standard Operation Procedures (SOPs)</td><td>Provision of MGs and SOPs</td><td>Once</td><td>and after each major upgrade</td></tr><tr><td>Scale transfer; Transfer standards</td><td>Provision of scale to working standards used at NFs</td><td>Once and then every four years</td><td>and after each major upgrade</td></tr><tr><td>New Instrument techniques</td><td>Consulting for implementing new techniques</td><td>Once</td><td>and after each major upgrade</td></tr><tr><td>Training of operators and scientists</td><td>Not specific at this stage</td><td>Every two years for a specific measurement technique used</td><td>At training centres or co-located with instrument performance workshops</td></tr><tr><td>Target-gases</td><td>Provision of target gases</td><td>Every year</td><td>Target gases have to be analysed by NFs</td></tr></tbody></table>
<table><tr><td rowspan="7">Reactive Trace Gas Measurement Techniques</td><td>Station comparison RR/flasks/ads. tubes</td><td>Provision of target concentrations</td><td>Every four years</td><td>Station have to report target gas concentrations</td></tr><tr><td>Comparison using mobile reference instruments</td><td>Provision of reference instrument</td><td>Every four years</td><td>The instrument will be run in parallel with on-going measurements at the NF</td></tr><tr><td>Central side-by-side comparisons.</td><td>Provision of a platform for conducting the comparisons</td><td>Every four years</td><td>At one NF or CGas-SiM unit this kind of intercomparison will be conducted</td></tr><tr><td>Station audits</td><td>Auditing the performance of the measurement systems at the NFs</td><td>Every four years</td><td>Audits will be conducted using traceable standards and/or reference instruments</td></tr><tr><td>Survey of target and comparison results; checking compatibility</td><td>Provision of QA/QC tools to NFs</td><td>Every year</td><td>Level 0 data will be reviewed</td></tr><tr><td>QC data-procedure and review</td><td>Provision of QA/QC tools to NFs</td><td>Every year</td><td>Level 1 data will be reviewed</td></tr><tr><td>ACTRIS Label</td><td>Consulting NFs in procedures when applying for the ACTRIS label</td><td>once</td><td>Approval is based on achieving the DQOs for the ACTRIS variables applied for.</td></tr></table>
<table><tr><td rowspan="7">Reactive Trace Gas Measurement Techniques</td><td>Station comparison RR/flasks/ads. tubes</td><td>Provision of target concentrations</td><td>Every four years</td><td>Station have to report target gas concentrations</td></tr><tr><td>Comparison using mobile reference instruments</td><td>Provision of reference instrument</td><td>Every four years</td><td>The instrument will be run in parallel with on-going measurements at the NF</td></tr><tr><td>Central side-by-side comparisons.</td><td>Provision of a platform for conducting the comparisons</td><td>Every four years</td><td>At one NF or CGas-SiM unit this kind of intercomparison will be conducted</td></tr><tr><td>Station audits</td><td>Auditing the performance of the measurement systems at the NFs</td><td>Every four years</td><td>Audits will be conducted using traceable standards and/or reference instruments</td></tr><tr><td>Survey of target and comparison results; checking compatibility</td><td>Provision of QA/QC tools to NFs</td><td>Every year</td><td>Level 0 data will be reviewed</td></tr><tr><td>QC data-procedure and review</td><td>Provision of QA/QC tools to NFs</td><td>Every year</td><td>Level 1 data will be reviewed</td></tr><tr><td>ACTRIS Label</td><td>Consulting NFs in procedures when applying for the ACTRIS label</td><td>once</td><td>Approval is based on achieving the DQOs for the ACTRIS variables applied for.</td></tr></table>
Operation support on request

The operation support on request should managed by SAMU. SAMU should inform the lead unit of CGas-SiM about requests. The requests are discussed in bimonthly teleconferences of the unit heads. SAMU will then be informed about the decisions. The type of support available in CGas-SiM for support on request is shown in Table 9-2.

<table><caption>Table 9-2: Summary of support on request available in the CGas-SiM</caption><thead><tr><th>Technique</th><th>Type of support</th><th>Specific support</th><th>Comments</th></tr></thead><tbody><tr><td rowspan="6">Reactive Trace Gas Measurement Techniques</td><td>Measurement Guidelines (MGs) and Standard Operation Procedures (SOPs)</td><td>Provision of MGs and SOPs</td><td>and after each major upgrade</td></tr><tr><td>Scale transfer; Transfer standards</td><td>Provision of scale to working standards used at users</td><td>and after each major upgrade</td></tr><tr><td>New Instrument techniques</td><td>Testing to reference instruments</td><td>conducted at CGas-SiM</td></tr><tr><td>Training of operators and scientists</td><td>Not specific at this stage</td><td>At training centres or co-located with instrument performance workshops</td></tr><tr><td>Comparison with user instruments RR/flasks/ads. tubes</td><td>Provision of target concentrations</td><td>Conducted at the user</td></tr><tr><td>Central side-by-side comparisons.</td><td>Provision of a platform for conducting the comparisons</td><td>At a CGas-SiM unit this kind of intercomparison will be conducted and/or in cooperation with EUROCHAMP</td></tr><tr><td></td><td>ACTRIS Label</td><td>Consulting user how to become a ACTRIS member and to apply for the ACTRIS Label</td><td>Approval is based on achieving the DQOs for the ACTRIS variables applied for.</td></tr></tbody></table>
Operation support on request

The operation support on request should managed by SAMU. SAMU should inform the lead unit of CGas-SiM about requests. The requests are discussed in bimonthly teleconferences of the unit heads. SAMU will then be informed about the decisions. The type of support available in CGas-SiM for support on request is shown in Table 9-2.

<table><caption>Table 9-2: Summary of support on request available in the CGas-SiM</caption><thead><tr><th>Technique</th><th>Type of support</th><th>Specific support</th><th>Comments</th></tr></thead><tbody><tr><td rowspan="6">Reactive Trace Gas Measurement Techniques</td><td>Measurement Guidelines (MGs) and Standard Operation Procedures (SOPs)</td><td>Provision of MGs and SOPs</td><td>and after each major upgrade</td></tr><tr><td>Scale transfer; Transfer standards</td><td>Provision of scale to working standards used at users</td><td>and after each major upgrade</td></tr><tr><td>New Instrument techniques</td><td>Testing to reference instruments</td><td>conducted at CGas-SiM</td></tr><tr><td>Training of operators and scientists</td><td>Not specific at this stage</td><td>At training centres or co-located with instrument performance workshops</td></tr><tr><td>Comparison with user instruments RR/flasks/ads. tubes</td><td>Provision of target concentrations</td><td>Conducted at the user</td></tr><tr><td>Central side-by-side comparisons.</td><td>Provision of a platform for conducting the comparisons</td><td>At a CGas-SiM unit this kind of intercomparison will be conducted and/or in cooperation with EUROCHAMP</td></tr><tr><td></td><td>ACTRIS Label</td><td>Consulting user how to become a ACTRIS member and to apply for the ACTRIS Label</td><td>Approval is based on achieving the DQOs for the ACTRIS variables applied for.</td></tr></tbody></table>
Section 8

Concept of the Centre for Reactive Trace Gases
Remote Sensing (CREGARS)

ACTRIS PPP WP 4 task 4.1

21.2.2018

Public
Section 8

Concept of the Centre for Reactive Trace Gases
Remote Sensing (CREGARS)

ACTRIS PPP WP 4 task 4.1

21.2.2018

Public
Contents

<table><tr><td>1</td><td>Purpose of the document</td><td>4</td></tr><tr><td>2</td><td>Description and Role of the Centre for Reactive Trace Gases Remote Sensing</td><td>4</td></tr><tr><td>2.1</td><td>Framework</td><td>4</td></tr><tr><td>2.2</td><td>Scientific relevance</td><td>4</td></tr><tr><td>3</td><td>Operation support provided to ACTRIS National Facilities</td><td>8</td></tr><tr><td>3.1</td><td>Measurement techniques covered by the Centre for Reactive Trace Gases Remote Sensing (CREGARS) and related ACTRIS variables</td><td>9</td></tr><tr><td>3.1.1</td><td>FTIR solar absorption spectrometry</td><td>9</td></tr><tr><td>3.1.2</td><td>UVVIS spectrometer</td><td>9</td></tr><tr><td>3.1.3</td><td>O3 DIAL</td><td>10</td></tr><tr><td>3.2</td><td>Estimation of the need</td><td>10</td></tr><tr><td>3.3</td><td>Timeline for implementation of the mandatory operation support</td><td>11</td></tr><tr><td>3.4</td><td>Operation support for quality assurance and quality control</td><td>12</td></tr><tr><td>3.4.1</td><td>Definition and establishment of standard measurement guidelines</td><td>12</td></tr><tr><td>3.4.2</td><td>Definition of measurement quality-assurance criteria and procedures</td><td>13</td></tr><tr><td>3.4.3</td><td>Development and provision of instrument-specific calibration</td><td>14</td></tr><tr><td>3.4.4</td><td>Development and provision of in-house check-up tools</td><td>15</td></tr><tr><td>3.4.5</td><td>Development of data evaluation procedures and plausibility test</td><td>16</td></tr><tr><td>3.4.6</td><td>Realization of observational site performance audits with reference samples or mobile systems</td><td>17</td></tr><tr><td>3.4.7</td><td>Organization of regular exercises to assess the performances of the NFs, including instrument performance workshops</td><td>18</td></tr><tr><td>3.4.8</td><td>Contribution to documentation and traceability of level 0 to level 3 data products</td><td>19</td></tr><tr><td>3.4.9</td><td>Contribution to CEN, ISO, or similar standardization activities</td><td>20</td></tr><tr><td>3.5</td><td>Operation support for knowledge transfer and training</td><td>20</td></tr><tr><td>3.5.1</td><td>Training of operators and scientists</td><td>20</td></tr><tr><td>3.5.2</td><td>Consultancy for setting-up new observational or exploratory platforms and new instruments at the NFs</td><td>21</td></tr><tr><td>3.6</td><td>Operation support for improvement of measurement methodologies</td><td>21</td></tr><tr><td>3.6.1</td><td>Testing of new measurement instruments and procedures</td><td>21</td></tr><tr><td>3.6.2</td><td>Development of strategies to increase the duty cycle of instruments operated at the NFs, to reduce breakdown and maintenance downtime</td><td>22</td></tr><tr><td>3.6.3</td><td>Development of new technological products and methods</td><td>22</td></tr><tr><td>3.6.4</td><td>Development of retrieval algorithms or tools for producing level 1 to 3 data, including the exploitation of instrument synergies</td><td>23</td></tr><tr><td>4</td><td>Services provided to ACTRIS users</td><td>24</td></tr><tr><td>4.1</td><td>Estimation of the need</td><td>24</td></tr></table>

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
Contents

<table><tr><td>1</td><td>Purpose of the document</td><td>4</td></tr><tr><td>2</td><td>Description and Role of the Centre for Reactive Trace Gases Remote Sensing</td><td>4</td></tr><tr><td>2.1</td><td>Framework</td><td>4</td></tr><tr><td>2.2</td><td>Scientific relevance</td><td>4</td></tr><tr><td>3</td><td>Operation support provided to ACTRIS National Facilities</td><td>8</td></tr><tr><td>3.1</td><td>Measurement techniques covered by the Centre for Reactive Trace Gases Remote Sensing (CREGARS) and related ACTRIS variables</td><td>9</td></tr><tr><td>3.1.1</td><td>FTIR solar absorption spectrometry</td><td>9</td></tr><tr><td>3.1.2</td><td>UVVIS spectrometer</td><td>9</td></tr><tr><td>3.1.3</td><td>O3 DIAL</td><td>10</td></tr><tr><td>3.2</td><td>Estimation of the need</td><td>10</td></tr><tr><td>3.3</td><td>Timeline for implementation of the mandatory operation support</td><td>11</td></tr><tr><td>3.4</td><td>Operation support for quality assurance and quality control</td><td>12</td></tr><tr><td>3.4.1</td><td>Definition and establishment of standard measurement guidelines</td><td>12</td></tr><tr><td>3.4.2</td><td>Definition of measurement quality-assurance criteria and procedures</td><td>13</td></tr><tr><td>3.4.3</td><td>Development and provision of instrument-specific calibration</td><td>14</td></tr><tr><td>3.4.4</td><td>Development and provision of in-house check-up tools</td><td>15</td></tr><tr><td>3.4.5</td><td>Development of data evaluation procedures and plausibility test</td><td>16</td></tr><tr><td>3.4.6</td><td>Realization of observational site performance audits with reference samples or mobile systems</td><td>17</td></tr><tr><td>3.4.7</td><td>Organization of regular exercises to assess the performances of the NFs, including instrument performance workshops</td><td>18</td></tr><tr><td>3.4.8</td><td>Contribution to documentation and traceability of level 0 to level 3 data products</td><td>19</td></tr><tr><td>3.4.9</td><td>Contribution to CEN, ISO, or similar standardization activities</td><td>20</td></tr><tr><td>3.5</td><td>Operation support for knowledge transfer and training</td><td>20</td></tr><tr><td>3.5.1</td><td>Training of operators and scientists</td><td>20</td></tr><tr><td>3.5.2</td><td>Consultancy for setting-up new observational or exploratory platforms and new instruments at the NFs</td><td>21</td></tr><tr><td>3.6</td><td>Operation support for improvement of measurement methodologies</td><td>21</td></tr><tr><td>3.6.1</td><td>Testing of new measurement instruments and procedures</td><td>21</td></tr><tr><td>3.6.2</td><td>Development of strategies to increase the duty cycle of instruments operated at the NFs, to reduce breakdown and maintenance downtime</td><td>22</td></tr><tr><td>3.6.3</td><td>Development of new technological products and methods</td><td>22</td></tr><tr><td>3.6.4</td><td>Development of retrieval algorithms or tools for producing level 1 to 3 data, including the exploitation of instrument synergies</td><td>23</td></tr><tr><td>4</td><td>Services provided to ACTRIS users</td><td>24</td></tr><tr><td>4.1</td><td>Estimation of the need</td><td>24</td></tr></table>

ACTRIS PPP (www.actris.eu) is supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme, H2020-INFRADEV-2016-2017, Grant Agreement number: 739530
<table><tr><td>4.2</td><td>Provision of measurement quality assurance and quality control procedures and tools</td><td>25</td></tr><tr><td>4.3</td><td>Instrument-specific calibration</td><td>25</td></tr><tr><td>4.4</td><td>Knowledge transfer and operator training</td><td>25</td></tr><tr><td>4.5</td><td>Improvement of measurement and retrieval methodologies for aerosols, clouds, and reactive trace gases</td><td>25</td></tr><tr><td>5</td><td>Governance and management structure of the Centre for Reactive Trace Gases Remote Sensing</td><td>26</td></tr><tr><td>6</td><td>Requirements for the Centre for Reactive Trace Gases Remote Sensing</td><td>26</td></tr><tr><td>6.1</td><td>General requirements</td><td>26</td></tr><tr><td>6.2</td><td>Technical requirements</td><td>27</td></tr><tr><td>6.2.1</td><td>Facilities</td><td>27</td></tr><tr><td>6.2.2</td><td>Human resources</td><td>28</td></tr><tr><td>6.2.3</td><td>Other requirements</td><td>29</td></tr><tr><td>7</td><td>Basic criteria for the selection of the Centre for Reactive Trace Gases Remote Sensing</td><td>29</td></tr><tr><td>8</td><td>Obligations of the Centre for Reactive Trace Gases Remote Sensing</td><td>30</td></tr><tr><td>8.1</td><td>General obligations</td><td>30</td></tr><tr><td>8.2</td><td>Technical obligations</td><td>30</td></tr><tr><td>8.2.1</td><td>Technical obligations in relation with the ACTRIS National Facilities</td><td>30</td></tr><tr><td>8.2.2</td><td>Technical obligations in relation with the ACTRIS Data Centre</td><td>32</td></tr><tr><td>8.2.3</td><td>Technical obligations in relation with other ACTRIS Topical Centres</td><td>34</td></tr><tr><td>8.2.4</td><td>Technical obligations in relation with the ACTRIS users</td><td>34</td></tr><tr><td>8.3</td><td>Evaluation of the activity of the Centre for Reactive Trace Gases Remote Sensing</td><td>34</td></tr><tr><td>9</td><td>List of acronyms</td><td>35</td></tr><tr><td>10</td><td>Glossary</td><td>36</td></tr><tr><td>11</td><td>Reference documents</td><td>38</td></tr><tr><td colspan="2">Annex: Provision of the operation support</td><td>39</td></tr><tr><td colspan="2">Scheduled support</td><td>39</td></tr><tr><td colspan="2">Operation support on request</td><td>43</td></tr></table>
<table><tr><td>4.2</td><td>Provision of measurement quality assurance and quality control procedures and tools</td><td>25</td></tr><tr><td>4.3</td><td>Instrument-specific calibration</td><td>25</td></tr><tr><td>4.4</td><td>Knowledge transfer and operator training</td><td>25</td></tr><tr><td>4.5</td><td>Improvement of measurement and retrieval methodologies for aerosols, clouds, and reactive trace gases</td><td>25</td></tr><tr><td>5</td><td>Governance and management structure of the Centre for Reactive Trace Gases Remote Sensing</td><td>26</td></tr><tr><td>6</td><td>Requirements for the Centre for Reactive Trace Gases Remote Sensing</td><td>26</td></tr><tr><td>6.1</td><td>General requirements</td><td>26</td></tr><tr><td>6.2</td><td>Technical requirements</td><td>27</td></tr><tr><td>6.2.1</td><td>Facilities</td><td>27</td></tr><tr><td>6.2.2</td><td>Human resources</td><td>28</td></tr><tr><td>6.2.3</td><td>Other requirements</td><td>29</td></tr><tr><td>7</td><td>Basic criteria for the selection of the Centre for Reactive Trace Gases Remote Sensing</td><td>29</td></tr><tr><td>8</td><td>Obligations of the Centre for Reactive Trace Gases Remote Sensing</td><td>30</td></tr><tr><td>8.1</td><td>General obligations</td><td>30</td></tr><tr><td>8.2</td><td>Technical obligations</td><td>30</td></tr><tr><td>8.2.1</td><td>Technical obligations in relation with the ACTRIS National Facilities</td><td>30</td></tr><tr><td>8.2.2</td><td>Technical obligations in relation with the ACTRIS Data Centre</td><td>32</td></tr><tr><td>8.2.3</td><td>Technical obligations in relation with other ACTRIS Topical Centres</td><td>34</td></tr><tr><td>8.2.4</td><td>Technical obligations in relation with the ACTRIS users</td><td>34</td></tr><tr><td>8.3</td><td>Evaluation of the activity of the Centre for Reactive Trace Gases Remote Sensing</td><td>34</td></tr><tr><td>9</td><td>List of acronyms</td><td>35</td></tr><tr><td>10</td><td>Glossary</td><td>36</td></tr><tr><td>11</td><td>Reference documents</td><td>38</td></tr><tr><td colspan="2">Annex: Provision of the operation support</td><td>39</td></tr><tr><td colspan="2">Scheduled support</td><td>39</td></tr><tr><td colspan="2">Operation support on request</td><td>43</td></tr></table>
# 1 Purpose of the document

This document describes the relevance, mission, requirements and obligations of the ACTRIS Centre for
Reactive Trace Gases Remote Sensing.

# 2 Description and Role of the Centre for Reactive Trace Gases Remote Sensing

## 2.1 Framework

The Centre for Reactive Trace Gases Remote Sensing is one of the six ACTRIS Topical Centres (TC) organized around the main scientific themes of ACTRIS: aerosols, clouds, and reactive trace gases, each with a particular focus on either remote sensing or in situ measurement techniques.

The following documents are completing the current concept of the Centre for Reactive Trace Gases
Remote Sensing (CREGARS):

* The ACTRIS framework is described in the following documents:

* ACTRIS ESFRI proposal

* ACTRIS-PPP project

* ACTRIS Stakeholder Handbook

* ACTRIS Science Case document

* The general rules and principles for the ACTRIS Central Facilities are described in the Baseline document for the Concepts of ACTRIS Central Facilities.

* The measurements techniques operated by the ACTRIS National Facilities for which the operation support should be offered, along with the specific requirements, are described in:

* Technical concepts and requirements for ACTRIS Observational Platforms

* Technical concepts and requirements for ACTRIS Exploratory Platforms

## 2.2 Scientific relevance

### Targeted ACTRIS Variables

The Centre for Reactive Trace Gases Remote Sensing (CREGARS) must deal with total columns and/or
vertical profiles of a number of atmospheric trace gases that are within the scope of ACTRIS, meaning (1)
that they are short-lived climate pollutants, for which ACTRIS will provide the whole value-chain, from the
raw measurement data to quality-controlled and traceable geophysical data (L2) and possibly derived
higher-level (L3) data, (2) that they are solicited by a large user community, and (3) that they are not yet
provided on a sustainable basis by another European Research Infrastructure (RI) or organisation. The
variables that are targeted at start are total columns and/or vertical profiles of NO2, HCHO, C2H6, O3 and
NH3. Aerosol extinction profiles in the lowermost troposphere, as well as aerosol backscatter coefficient
profiles and total aerosol optical depth will also be provided. Which products/species are to be provided
by the 3 different techniques (FTIR, UVVIS, and O3 DIAL) as "mandatory", "optimum", or "specialising" will
be specified in Section 3.1 below. The list of targeted variables will expand when the associated
# 1 Purpose of the document

This document describes the relevance, mission, requirements and obligations of the ACTRIS Centre for
Reactive Trace Gases Remote Sensing.

# 2 Description and Role of the Centre for Reactive Trace Gases Remote Sensing

## 2.1 Framework

The Centre for Reactive Trace Gases Remote Sensing is one of the six ACTRIS Topical Centres (TC) organized around the main scientific themes of ACTRIS: aerosols, clouds, and reactive trace gases, each with a particular focus on either remote sensing or in situ measurement techniques.

The following documents are completing the current concept of the Centre for Reactive Trace Gases Remote Sensing (REGARS):

* The ACTRIS framework is described in the following documents:

* ACTRIS ESFRI proposal

* ACTRIS-PPP project

* ACTRIS Stakeholder Handbook

* ACTRIS Science Case document

* The general rules and principles for the ACTRIS Central Facilities are described in the Baseline document for the Concepts of ACTRIS Central Facilities.

* The measurements techniques operated by the ACTRIS National Facilities for which the operation support should be offered, along with the specific requirements, are described in:

* Technical concepts and requirements for ACTRIS Observational Platforms

* Technical concepts and requirements for ACTRIS Exploratory Platforms

## 2.2 Scientific relevance

### Targeted ACTRIS Variables

The Centre for Reactive Trace Gases Remote Sensing (REGARS) must deal with total columns and/or vertical profiles of a number of atmospheric trace gases that are within the scope of ACTRIS, meaning (1) that they are short-lived climate pollutants, for which ACTRIS will provide the whole value-chain, from the raw measurement data to quality-controlled and traceable geophysical data (L2) and possibly derived higher-level (L3) data, (2) that they are solicited by a large user community, and (3) that they are not yet provided on a sustainable basis by another European Research Infrastructure (RI) or organisation. The variables that are targeted at start are total columns and/or vertical profiles of NO₂, HCHO, C₂H₆, O₃ and NH₃. Aerosol extinction profiles in the lowermost troposphere, as well as aerosol backscatter coefficient profiles and total aerosol optical depth will also be provided. Which products/species are to be provided by the 3 different techniques (FTIR, UVVIS, and O3 DIAL) as "mandatory", "optimum", or "specialising" will be specified in Section 3.1 below. The list of targeted variables will expand when the associated
measurement and data analysis techniques and the user demands evolve; e.g., additional Volatile Organic Compounds like CHOCHO, C₂H₂, ... may be included in the future, if funding allows.

## Rationale

Observations related to reactive trace gases concentrations can be done either in-situ, at the surface or on a tower or on a moving platform like an aircraft, or by remote sensing techniques, again from the same platforms as well as from space-borne platforms. The in-situ observations provide local concentration values of the target reactive trace gases. The remote sensing techniques do not measure the concentrations directly, but they typically measure an electromagnetic spectrum from which the concentrations must be retrieved. The retrieval results generally are vertical profiles (with a variable vertical resolution) or total or partial columns of target trace gases. They are therefore better comparable to the corresponding remotely sensed satellite data than the corresponding local in-situ data, and are used extensively by the satellite community for the validation of their products. Also the atmospheric modelling communities, e.g., the ones involved in the Copernicus Atmospheric Monitoring Service (CAMS) or the ones involved in CMIP (Coupled Model Intercomparison Project) are keen on using observational data of columns and/or vertical profiles of reactive trace gases to validate the models and (re-)analyses of the atmospheric composition. Another important use of the remotely sensed reactive trace gases data is the detection and evaluation of long-term trends, which will be an important application in the Copernicus Climate Change Service (C3S).

## Relevant measurement techniques

The remote sensing measurement techniques that will be considered in ACTRIS for the provision of the target variables are:

* the Fourier-transform Infrared (FTIR) solar absorption technique,

* the UV-visible Differential Optical Absorption Spectroscopy (DOAS) technique, called UVVIS hereafter, including the zenith-sky (ZS-) and the Multi-Axis (MAX-)DOAS systems,

* the LIDAR technique for trace gas vertical profiling, in particular for Ozone in the troposphere up to the upper stratosphere, also called O₃ DIAL (Differential Absorption LIDAR)

Because the techniques do not measure the variables directly, but derive (retrieve) them from spectral measurements using dedicated retrieval algorithms, both the observing systems as well as the retrieval algorithms must satisfy stringent specifications for ensuring that the retrieved products are accurate and precise, high-quality profiles or columns, and for ensuring consistency among the different contributing observation facilities.

## State-of-the-art

At the international level, the reference network for high-quality remote sensing of reactive trace gases is the Network for the Detection of Atmospheric Composition Change (NDACC). This network started as a research network and evolved bottom-up. For each of the instrumental techniques involved in NDACC, among which FTIR, UVVIS and LIDAR techniques, Instrument Working Groups (IWG) guide the members operating these instruments through various Protocols (http://www.ndsc.ncep.noaa.gov/organize/protocols/) with which the members must comply. The purpose of the Protocols is to ensure the high-quality and intra-network consistency of the NDACC data.
measurement and data analysis techniques and the user demands evolve; e.g., additional Volatile Organic Compounds like CHOCHO, C₂H₂, ... may be included in the future, if funding allows.

## Rationale

Observations related to reactive trace gases concentrations can be done either in-situ, at the surface or on a tower or on a moving platform like an aircraft, or by remote sensing techniques, again from the same platforms as well as from space-borne platforms. The in-situ observations provide local concentration values of the target reactive trace gases. The remote sensing techniques do not measure the concentrations directly, but they typically measure an electromagnetic spectrum from which the concentrations must be retrieved. The retrieval results generally are vertical profiles (with a variable vertical resolution) or total or partial columns of target trace gases. They are therefore better comparable to the corresponding remotely sensed satellite data than the corresponding local in-situ data, and are used extensively by the satellite community for the validation of their products. Also the atmospheric modelling communities, e.g., the ones involved in the Copernicus Atmospheric Monitoring Service (CAMS) or the ones involved in CMIP (Coupled Model Intercomparison Project) are keen on using observational data of columns and/or vertical profiles of reactive trace gases to validate the models and (re-)analyses of the atmospheric composition. Another important use of the remotely sensed reactive trace gases data is the detection and evaluation of long-term trends, which will be an important application in the Copernicus Climate Change Service (C3S).

## Relevant measurement techniques

The remote sensing measurement techniques that will be considered in ACTRIS for the provision of the target variables are:

* the Fourier-transform Infrared (FTIR) solar absorption technique,

* the UV-visible Differential Optical Absorption Spectroscopy (DOAS) technique, called UVVIS hereafter, including the zenith-sky (ZS-) and the Multi-Axis (MAX-)DOAS systems,

* the LIDAR technique for trace gas vertical profiling, in particular for Ozone in the troposphere up to the upper stratosphere, also called O₃ DIAL (Differential Absorption LIDAR)

Because the techniques do not measure the variables directly, but derive (retrieve) them from spectral measurements using dedicated retrieval algorithms, both the observing systems as well as the retrieval algorithms must satisfy stringent specifications for ensuring that the retrieved products are accurate and precise, high-quality profiles or columns, and for ensuring consistency among the different contributing observation facilities.

## State-of-the-art

At the international level, the reference network for high-quality remote sensing of reactive trace gases is the Network for the Detection of Atmospheric Composition Change (NDACC). This network started as a research network and evolved bottom-up. For each of the instrumental techniques involved in NDACC, among which FTIR, UVVIS and LIDAR techniques, Instrument Working Groups (IWG) guide the members operating these instruments through various Protocols (http://www.ndsc.ncep.noaa.gov/organize/protocols/) with which the members must comply. The purpose of the Protocols is to ensure the high-quality and intra-network consistency of the NDACC data.