At present, it is the responsibility of the IWG to establish and update the Protocols and of each PI to comply
with the Protocols. However tools and resources are lacking in the IWGs to verify the PIs' compliance.
Several research performing organisations (RPOs) contribute to the QA/QC processes but there is no
dedicated and sustainable QA/QC service provider.

Europe contributes very significantly to this Network: 45 out of 66 active NDACC Principal Investigators for O3 DIAL, UVVIS and FTIR instruments are affiliated to a European RPO. Also the governance structure of NDACC as a whole and of the IWG is such that all responsibilities are shared among a European and a non-European co-chair.

At many NDACC observatories, collocated in-situ measurements of trace gases and/or in-situ or remote sensing measurements of aerosol, or meteorological measurements are implemented. The synergy and complementarity between in-situ and remote sensing data of reactive trace gases has been the subject of collaborative investigation between both communities, e.g., in the ACTRIS-1 project by Dils et al, Atmos. Chem. Phys., 11, 6735-6748, https://doi.org/10.5194/acp-11-6735-2011, 2011, and ‘Derived tropospheric column data’ by Henne et al., http://nors.aeronomie.be/projectdir/PDF/D5.2_NORS_DTCD.pdf. Synergies and complementarities also exist between aerosol measurements from dedicated aerosol instruments and UVVIS MAXDOAS instruments, see e.g., Piters et al., 2012¹.

## Needs for a Centre for Reactive Trace Gases Remote Sensing (CREGARS) in ACTRIS

As said above, the reactive trace gases remote sensing 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.

Another important aspect to be characterised for making correct use of the target products is the representativeness of the data: the remote sensing technique probes a volume of air of which the exact location and extent depend on the technique and the observational conditions. The spatial (horizontal as well as vertical) and temporal resolution associated with the data are directly related to the representativeness.

Therefore, consistency across the RI regarding the reactive trace gases remote sensing data products and their characterisation, including the evaluation of the full uncertainty budget including the representativeness error, requires agreements about instruments and their set-up and operation, as well as standardized retrieval algorithms and agreed procedures for uncertainty and representativeness evaluation.

The Centre for Reactive Trace Gases Remote Sensing (CREGARS) shall serve the need to prescribe and support the instrumental set-up, operations, and retrieval and uncertainty evaluation procedures to achieve harmonised, high-quality, well-characterised remote sensing measurements of the targeted selection of reactive trace gases at the National Facilities (NFs), and to assure and control the quality of the reactive trace gases remote sensing data that will be submitted to the ACTRIS Data Centre.

<footnote n="1">¹ Piters, A.J.M., et al, Atmos. Meas. Tech., 5, 457–485, 2012, doi:10.5194/amt-5-457-2012</footnote>
At present, it is the responsibility of the IWG to establish and update the Protocols and of each PI to comply
with the Protocols. However tools and resources are lacking in the IWGs to verify the PIs' compliance.
Several research performing organisations (RPOs) contribute to the QA/QC processes but there is no
dedicated and sustainable QA/QC service provider.

Europe contributes very significantly to this Network: 45 out of 66 active NDACC Principal Investigators for O3 DIAL, UVVIS and FTIR instruments are affiliated to a European RPO. Also the governance structure of NDACC as a whole and of the IWG is such that all responsibilities are shared among a European and a non-European co-chair.

At many NDACC observatories, collocated in-situ measurements of trace gases and/or in-situ or remote sensing measurements of aerosol, or meteorological measurements are implemented. The synergy and complementarity between in-situ and remote sensing data of reactive trace gases has been the subject of collaborative investigation between both communities, e.g., in the ACTRIS-1 project by Dils et al, Atmos. Chem. Phys., 11, 6735-6748, https://doi.org/10.5194/acp-11-6735-2011, 2011, and ‘Derived tropospheric column data’ by Henne et al., http://nors.aeronomie.be/projectdir/PDF/D5.2_NORS_DTCD.pdf. Synergies and complementarities also exist between aerosol measurements from dedicated aerosol instruments and UVVIS MAXDOAS instruments, see e.g., Piters et al., 2012¹.

## Needs for a Centre for Reactive Trace Gases Remote Sensing (CREGARS) in ACTRIS

As said above, the reactive trace gases remote sensing 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.

Another important aspect to be characterised for making correct use of the target products is the representativeness of the data: the remote sensing technique probes a volume of air of which the exact location and extent depend on the technique and the observational conditions. The spatial (horizontal as well as vertical) and temporal resolution associated with the data are directly related to the representativeness.

Therefore, consistency across the RI regarding the reactive trace gases remote sensing data products and their characterisation, including the evaluation of the full uncertainty budget including the representativeness error, requires agreements about instruments and their set-up and operation, as well as standardized retrieval algorithms and agreed procedures for uncertainty and representativeness evaluation.

The Centre for Reactive Trace Gases Remote Sensing (CREGARS) shall serve the need to prescribe and support the instrumental set-up, operations, and retrieval and uncertainty evaluation procedures to achieve harmonised, high-quality, well-characterised remote sensing measurements of the targeted selection of reactive trace gases at the National Facilities (NFs), and to assure and control the quality of the reactive trace gases remote sensing data that will be submitted to the ACTRIS Data Centre.

<footnote n="1">¹ Piters, A.J.M., et al, Atmos. Meas. Tech., 5, 457–485, 2012, doi:10.5194/amt-5-457-2012</footnote>
As stated above, NDACC has already established various such protocols for measurements, instrument intercomparisons, data validation and data handling (see http://www.ndsc.ncep.noaa.gov/organize/protocols), however without any guarantee sustainability.

Secondly, additional target- and instrument-specific procedures are required to ensure the intra-network consistency and the long-term quality and homogeneity of the data in the database. E.g., in the Infrared WG (IRWG) and UVVIS WG, work has been done to harmonise the retrieval strategies for HCHO and NO₂ in the context of the S5P satellite validation project, but there is no sustainable follow-up of this effort. The establishment and maintenance of the procedures is another role for the Topical Centre.

Thirdly, the NDACC protocols are quite generic. The provision of tools that support the National Facilities and serve users to comply with the generic protocols and more specific guidelines established in ACTRIS is a third role for the Topical Centre. For example, the CREGARS must provide and maintain verified data retrieval software and associated templates.

Fourth, the TC must contribute to the education of future instrument operators and data providers by providing didactic material and dedicated support (including training) upon request and according to available resources.

For now, there is no official centre that is already operational in the ACTRIS sense. However, as described above, some services that the Centre for Reactive Trace Gases Remote Sensing must provide, are already provided on a voluntary or project-wise basis, and according to available resources, by some RPO involved in NDACC. This implies that currently the need for these services is well recognised, the procedures behind the services do exist in many cases and are known or are being established in the context of short-term projects<fnref n="2" />, but the provision of the services is done on an ad-hoc basis and not guaranteed on the long-term in a sustainable manner. This implies that the long-term quality of the remote sensing data of reactive trace gases is not guaranteed in a sustainable manner. The Centre for Reactive Trace Gases Remote Sensing will fill this gap for the ACTRIS target species.

## Mission

The mission of the Centre for Reactive Trace Gases Remote Sensing is to offer operational support to ACTRIS National Facilities operating FTIR, UVVIS spectrometers or LIDARS (O₃ DIAL).

Additionally, the Centre for Reactive Trace Gases Remote Sensing should offer specialized services for the above instruments and related ACTRIS variables, to ACTRIS users of various types: academia, research organisations, business, industry, and public services.

<footnote n="2">² On top of existing NDACC Protocols, recent and currently running EU projects like the FP7 projects NORS (nors.aeronomie.be) and QA4ECV (qa4ecv.eu) and the H2020 project GAIA-CLIM (gaia-clim.eu) include the establishment of quality assurance and harmonisation procedures, as well as procedures for establishing traceability to standards. Also the contract CAMS-27 will implement QA/QC procedures for the rapid delivery data stream from NDACC partners to CAMS.</footnote>
As stated above, NDACC has already established various such protocols for measurements, instrument intercomparisons, data validation and data handling (see http://www.ndsc.ncep.noaa.gov/organize/protocols), however without any guarantee sustainability.

Secondly, additional target- and instrument-specific procedures are required to ensure the intra-network consistency and the long-term quality and homogeneity of the data in the database. E.g., in the Infrared WG (IRWG) and UVVIS WG, work has been done to harmonise the retrieval strategies for HCHO and NO₂ in the context of the S5P satellite validation project, but there is no sustainable follow-up of this effort. The establishment and maintenance of the procedures is another role for the Topical Centre.

Thirdly, the NDACC protocols are quite generic. The provision of tools that support the National Facilities and serve users to comply with the generic protocols and more specific guidelines established in ACTRIS is a third role for the Topical Centre. For example, the CREGARS must provide and maintain verified data retrieval software and associated templates.

Fourth, the TC must contribute to the education of future instrument operators and data providers by providing didactic material and dedicated support (including training) upon request and according to available resources.

For now, there is no official centre that is already operational in the ACTRIS sense. However, as described above, some services that the Centre for Reactive Trace Gases Remote Sensing must provide, are already provided on a voluntary or project-wise basis, and according to available resources, by some RPO involved in NDACC. This implies that currently the need for these services is well recognised, the procedures behind the services do exist in many cases and are known or are being established in the context of short-term projects<fnref n="2" />, but the provision of the services is done on an ad-hoc basis and not guaranteed on the long-term in a sustainable manner. This implies that the long-term quality of the remote sensing data of reactive trace gases is not guaranteed in a sustainable manner. The Centre for Reactive Trace Gases Remote Sensing will fill this gap for the ACTRIS target species.

## Mission

The mission of the Centre for Reactive Trace Gases Remote Sensing is to offer operational support to ACTRIS National Facilities operating FTIR, UVVIS spectrometers or LIDARS (O₃ DIAL).

Additionally, the Centre for Reactive Trace Gases Remote Sensing should offer specialized services for the above instruments and related ACTRIS variables, to ACTRIS users of various types: academia, research organisations, business, industry, and public services.

<footnote n="2">² On top of existing NDACC Protocols, recent and currently running EU projects like the FP7 projects NORS (nors.aeronomie.be) and QA4ECV (qa4ecv.eu) and the H2020 project GAIA-CLIM (gaia-clim.eu) include the establishment of quality assurance and harmonisation procedures, as well as procedures for establishing traceability to standards. Also the contract CAMS-27 will implement QA/QC procedures for the rapid delivery data stream from NDACC partners to CAMS.</footnote>
3 Operation support provided to ACTRIS National Facilities

The operation support provided by the Centre for Reactive Trace Gases Remote Sensing are:

a) procedures and tools for quality assurance and quality control of ACTRIS measurements and data,

b) centralised processing from level-1 to 2 for UVVIS spectrometers,

c) transfer of knowledge and training to ACTRIS operators,

d) improvements of measurement methodologies for reactive trace gases (and aerosol if measured by the same technique).

The operation support is offered to the National Facilities for the measurement techniques described in
this concept document, also listed in the *Technical concepts and requirements for ACTRIS Observational
Platforms*.

The Centre for Reactive Trace Gases Remote Sensing 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 the Annex in Section
9. This annex will be updated each time it is necessary (e.g. development of new testing or calibration
methods and tools).

The Centre for Reactive Trace Gases Remote Sensing should operate at the state-of-the-art, fostering the
implementation of validated new techniques in ACTRIS. To sustain a high level of performance and to
stimulate the advancement of new techniques and methodologies, the Centre for Reactive Trace Gases
Remote Sensing contributes to expert collaboration networks like NDACC, as well as to dedicated research
projects.

Therefore, the ACTRIS Centre for Reactive Trace Gases Remote Sensing will establish procedures and
specifications for the ACTRIS target species that are more elaborated than they are in the more generic
NDACC protocols, but always compliant with the NDACC Protocols.
3 Operation support provided to ACTRIS National Facilities

The operation support provided by the Centre for Reactive Trace Gases Remote Sensing are:

a) procedures and tools for quality assurance and quality control of ACTRIS measurements and data,

b) centralised processing from level-1 to 2 for UVVIS spectrometers,

c) transfer of knowledge and training to ACTRIS operators,

d) improvements of measurement methodologies for reactive trace gases (and aerosol if measured by the same technique).

The operation support is offered to the National Facilities for the measurement techniques described in
this concept document, also listed in the *Technical concepts and requirements for ACTRIS Observational
Platforms*.

The Centre for Reactive Trace Gases Remote Sensing 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 the Annex in Section
9. This annex will be updated each time it is necessary (e.g. development of new testing or calibration
methods and tools).

The Centre for Reactive Trace Gases Remote Sensing should operate at the state-of-the-art, fostering the
implementation of validated new techniques in ACTRIS. To sustain a high level of performance and to
stimulate the advancement of new techniques and methodologies, the Centre for Reactive Trace Gases
Remote Sensing contributes to expert collaboration networks like NDACC, as well as to dedicated research
projects.

Therefore, the ACTRIS Centre for Reactive Trace Gases Remote Sensing will establish procedures and specifications for the ACTRIS target species that are more elaborated than they are in the more generic NDACC protocols, but always compliant with the NDACC Protocols.
**3.1 Measurement techniques covered by the Centre for Reactive Trace Gases Remote Sensing (CREGARS) and related ACTRIS variables**

Measurement techniques covered by the Centre for Aerosol Remote Sensing are described in detail in: *Technical concepts and requirements for ACTRIS Observational Platforms* and *Technical concepts and requirements for ACTRIS Exploratory Platforms*.

**3.1.1 FTIR solar absorption spectrometry**

**Typical set-up:**

A high-spectral-resolution FTIR spectrometer operating in the mid-infrared spectral region (1900-4100 cm⁻¹ for the minimum setup, 700-5000 cm⁻¹ for the optimum setup), and coupled to a sun tracker, preferably operated automatically or remotely controlled. The data are analysed with a standard line-by-line fitting algorithm. In order to perform high quality measurements liquid nitrogen-cooling of the detectors is necessary; preferably the liquid nitrogen filling of the detector is done automatically or remotely controlled.

**Data quality:**

The data quality is affected mainly by the spectral resolution and alignment of the spectrometer, the S/N ratio and possible artefacts of the recorded spectra (e.g., due to detector non-linearity), the pointing precision of the sun tracker, the temperature stability of the instrument/lab, the accuracy of the pressure-temperature profiles used as input as well as the retrieval algorithm itself and the selection of retrieval and spectroscopic parameters for each target species. FTIR solar absorption spectrometers require cloud-free conditions during the observation time, therefore the quality of the data can be affected by the sky-conditions (e.g., undetected presence of cirrus in the field-of-view).

**Retrieval products (FTIR L2 data) targeted in ACTRIS:**

<figure><img src="image_28.png" /></figure>

* $C_2H_6$ and HCHO total columns ("mandatory")

* NH₃ as well as NO₂ total columns when feasible ("specialising")

* O₃ partial and total columns when the instrument set-up is optimal ("optimum")

**3.1.2 UVVIS spectrometer**

**Typical set-up:**

A grating spectrometer detecting scattered sunlight, covering spectral bands in the range from 300 to 550 nm, with a spectral resolution better than 1.5 nm. Spectral data are analysed with a DOAS-type algorithm. UVVIS instruments can feature different levels of complexity depending on the target data products, ranging from simple zenith-sky systems (ZS-DOAS) for stratospheric total column measurements at twilight, to thermally controlled multi-axis systems (MAXDOAS) equipped with elevation scanners and/or sun-tracking devices for daytime high-frequency tropospheric column and profile measurements.

**Data quality:**

The data quality is affected by the accuracy of the calibration of the spectrometer (in particular the wavelength registration and the Instrumental Spectral Response Function (ISRF)), the S/N ratio and

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
**3.1 Measurement techniques covered by the Centre for Reactive Trace Gases Remote Sensing (CREGARS) and related ACTRIS variables**

Measurement techniques covered by the Centre for Aerosol Remote Sensing are described in detail in: *Technical concepts and requirements for ACTRIS Observational Platforms* and *Technical concepts and requirements for ACTRIS Exploratory Platforms*.

**3.1.1 FTIR solar absorption spectrometry**

**Typical set-up:**

A high-spectral-resolution FTIR spectrometer operating in the mid-infrared spectral region (1900-4100 cm⁻¹ for the minimum setup, 700-5000 cm⁻¹ for the optimum setup), and coupled to a sun tracker, preferably operated automatically or remotely controlled. The data are analysed with a standard line-by-line fitting algorithm. In order to perform high quality measurements liquid nitrogen-cooling of the detectors is necessary; preferably the liquid nitrogen filling of the detector is done automatically or remotely controlled.

**Data quality:**

The data quality is affected mainly by the spectral resolution and alignment of the spectrometer, the S/N ratio and possible artefacts of the recorded spectra (e.g., due to detector non-linearity), the pointing precision of the sun tracker, the temperature stability of the instrument/lab, the accuracy of the pressure-temperature profiles used as input as well as the retrieval algorithm itself and the selection of retrieval and spectroscopic parameters for each target species. FTIR solar absorption spectrometers require cloud-free conditions during the observation time, therefore the quality of the data can be affected by the sky-conditions (e.g., undetected presence of cirrus in the field-of-view).

**Retrieval products (FTIR L2 data) targeted in ACTRIS:**

<figure><img src="image_26.png" /></figure>

* $C_2H_6$ and HCHO total columns ("mandatory")

* NH₃ as well as NO₂ total columns when feasible ("specialising")

* O₃ partial and total columns when the instrument set-up is optimal ("optimum")

**3.1.2 UVVIS spectrometer**

**Typical set-up:**

A grating spectrometer detecting scattered sunlight, covering spectral bands in the range from 300 to 550 nm, with a spectral resolution better than 1.5 nm. Spectral data are analysed with a DOAS-type algorithm. UVVIS instruments can feature different levels of complexity depending on the target data products, ranging from simple zenith-sky systems (ZS-DOAS) for stratospheric total column measurements at twilight, to thermally controlled multi-axis systems (MAXDOAS) equipped with elevation scanners and/or sun-tracking devices for daytime high-frequency tropospheric column and profile measurements.

**Data quality:**

The data quality is affected by the accuracy of the calibration of the spectrometer (in particular the wavelength registration and the Instrumental Spectral Response Function (ISRF)), the S/N ratio and

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
possible artefacts of the recorded spectra (e.g., due to polarisation sensitivity, stray-light or detector non-linearity), the pointing precision of the elevation scanner for MAX-DOAS instruments and/or the sun-tracker for direct-sun systems, the temperature stability of the instrument, the accuracy of the a-priori profiles used as input for zenith-sky and MAX-DOAS instruments as well as the retrieval algorithms themselves and the selection of analysis settings and spectroscopic parameters for each target species.

### Retrieval products (UVVIS-ZS L2 data) targeted in ACTRIS:

* $O_3$ and $NO_2$ total columns at twilight

### Retrieval products (UVVIS-MAXDOAS L2 data) targeted in ACTRIS:

* $O_3$, $NO_2$ and HCHO total columns

* $NO_2$ and HCHO lower tropospheric profiles

* Total aerosol optical depth (AOD) at discrete UV and visible wavelengths

* Lower tropospheric profiles of aerosol extinction at discrete UV and visible wavelengths

### 3.1.3 O3 DIAL

#### Typical set-up:

Dual UV-wavelengths LIDAR, both wavelengths having different ozone absorption cross-sections. Lidar signals are collected by a telescope and spectrally separated by a spectrometer or Rmax filters.

#### Data quality:

The data quality is affected by the alignment of the laser beams in the telescope field of view and the S/N ratio. Lidar measurements need clear skies so the S/N ratio can be affected by the presence of cirrus clouds but the ozone profiles above or below the clouds are unaffected by the clouds.

For each measurement technique separately, it is the role of the CREGARS to prescribe the instrument requirements and operation procedures, and to provide data retrieval procedures, data characterisation procedures, and data submission protocols, as well as to provide tools to perform QA/QC of the data quality.

#### Retrieval products (O3 DIAL L2 data) targeted in ACTRIS:

* $O_3$ vertical profile

* Aerosol backscatter coefficient profile in upper troposphere and lower stratosphere

## 3.2 Estimation of the need

<table><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>FTIR</td><td>0</td><td>11</td><td>17</td></tr><tr><td>UVVIS</td><td>0</td><td>20</td><td>30</td></tr><tr><td>O3 DIAL</td><td>0</td><td>4</td><td>9</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
possible artefacts of the recorded spectra (e.g., due to polarisation sensitivity, stray-light or detector non-linearity), the pointing precision of the elevation scanner for MAX-DOAS instruments and/or the sun-tracker for direct-sun systems, the temperature stability of the instrument, the accuracy of the a-priori profiles used as input for zenith-sky and MAX-DOAS instruments as well as the retrieval algorithms themselves and the selection of analysis settings and spectroscopic parameters for each target species.

### Retrieval products (UVVIS-ZS L2 data) targeted in ACTRIS:

* $O_3$ and $NO_2$ total columns at twilight

### Retrieval products (UVVIS-MAXDOAS L2 data) targeted in ACTRIS:

* $O_3$, $NO_2$ and HCHO total columns

* $NO_2$ and HCHO lower tropospheric profiles

* Total aerosol optical depth (AOD) at discrete UV and visible wavelengths

* Lower tropospheric profiles of aerosol extinction at discrete UV and visible wavelengths

### 3.1.3 O3 DIAL

#### Typical set-up:

Dual UV-wavelengths LIDAR, both wavelengths having different ozone absorption cross-sections. Lidar signals are collected by a telescope and spectrally separated by a spectrometer or Rmax filters.

#### Data quality:

The data quality is affected by the alignment of the laser beams in the telescope field of view and the S/N ratio. Lidar measurements need clear skies so the S/N ratio can be affected by the presence of cirrus clouds but the ozone profiles above or below the clouds are unaffected by the clouds.

For each measurement technique separately, it is the role of the CREGARS to prescribe the instrument requirements and operation procedures, and to provide data retrieval procedures, data characterisation procedures, and data submission protocols, as well as to provide tools to perform QA/QC of the data quality.

#### Retrieval products (O3 DIAL L2 data) targeted in ACTRIS:

* $O_3$ vertical profile

* Aerosol backscatter coefficient profile in upper troposphere and lower stratosphere

## 3.2 Estimation of the need

<table><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>FTIR</td><td>0</td><td>11</td><td>17</td></tr><tr><td>UVVIS</td><td>0</td><td>20</td><td>30</td></tr><tr><td>O3 DIAL</td><td>0</td><td>4</td><td>9</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
For now, there are 11 FTIR that are operated by European RPO that comply with ACTRIS requirements and that are deployed at proposed ACTRIS National Facilities. When ACTRIS will become operational, CREGARS should also provide operational support to FTIR sites that are not yet supported as National Facilities but that comply with the ACTRIS requirements and that are operated by ACTRIS member countries. Therefore, the expected maximum number of users in 2025 is 17.

As regards UVVIS instruments, there are approximately 20 UVVIS stations (SAOZ, MAX-DOAS or Pandora) that currently operate at candidate national facilities in Europe. All of these systems must benefit from the operational support proposed within the TC. By 2025, it is anticipated that the number of instruments at NFs will grow by a few more units, to a maximum of 30.

Concerning LIDARS, there are currently very few operational systems for ozone in Europe. The potential for tropospheric ozone monitoring in Europe using LIDARS would amount to about 4 to 9 stations (, Payerne, Cabauw, Potenza, ...).

## 3.3 Timeline for implementation of the mandatory operation support

Considering the ACTRIS roadmap, CREGARS should consider the following implementation plan for the operation support.

After being selected as the Centre for Reactive Trace Gas Remote Sensing, CREGARS shall make sure that in the period 2020-2025, it

(1) Prepares and verifies all necessary software, tools and associated documentation for instrument QA/QC and site performance audits

(2) Prepares and verifies all necessary software, tools and associated tools and data for the L1 → L2 data retrieval

(3) Prepares and verifies all necessary software to perform the QA/QC of the L2 data submitted by the NFs

(4) Prepares the data flow to the Data Centre

(5) Prepares the facility(ies) for hands-on training of NF operators, for performing instrument tests and for performing tests of the S/W and the tools.

(6) Acquires an initial amount of spare parts, calibration cells, ...

(7) Hires and/or designates the required scientific, technical and management personnel of the TC

(8) Prepares the internal communication tools between the TC units and with the HO/SAMU

(9) Prepares the communication tools with the NFs and the users

(10)

In addition, the UVVIS units shall

(1) prepare an operational processing capacity

(2) establish the dataflow from the TC-unit to the Data Centre

(3) acquire and prepare a traveling standard instrument

(4) prepare the calibration and intercomparison facilities
For now, there are 11 FTIR that are operated by European RPO that comply with ACTRIS requirements and that are deployed at proposed ACTRIS National Facilities. When ACTRIS will become operational, CREGARS should also provide operational support to FTIR sites that are not yet supported as National Facilities but that comply with the ACTRIS requirements and that are operated by ACTRIS member countries. Therefore, the expected maximum number of users in 2025 is 17.

As regards UVVIS instruments, there are approximately 20 UVVIS stations (SAOZ, MAX-DOAS or Pandora) that currently operate at candidate national facilities in Europe. All of these systems must benefit from the operational support proposed within the TC. By 2025, it is anticipated that the number of instruments at NFs will grow by a few more units, to a maximum of 30.

Concerning LIDARS, there are currently very few operational systems for ozone in Europe. The potential for tropospheric ozone monitoring in Europe using LIDARS would amount to about 4 to 9 stations (, Payerne, Cabauw, Potenza, ...).

## 3.3 Timeline for implementation of the mandatory operation support

Considering the ACTRIS roadmap, CREGARS should consider the following implementation plan for the operation support.

After being selected as the Centre for Reactive Trace Gas Remote Sensing, CREGARS shall make sure that in the period 2020-2025, it

(1) Prepares and verifies all necessary software, tools and associated documentation for instrument QA/QC and site performance audits

(2) Prepares and verifies all necessary software, tools and associated tools and data for the L1 → L2 data retrieval

(3) Prepares and verifies all necessary software to perform the QA/QC of the L2 data submitted by the NFs

(4) Prepares the data flow to the Data Centre

(5) Prepares the facility(ies) for hands-on training of NF operators, for performing instrument tests and for performing tests of the S/W and the tools.

(6) Acquires an initial amount of spare parts, calibration cells, ...

(7) Hires and/or designates the required scientific, technical and management personnel of the TC

(8) Prepares the internal communication tools between the TC units and with the HO/SAMU

(9) Prepares the communication tools with the NFs and the users

(10)

In addition, the UVVIS units shall

(1) prepare an operational processing capacity

(2) establish the dataflow from the TC-unit to the Data Centre

(3) acquire and prepare a traveling standard instrument

(4) prepare the calibration and intercomparison facilities
## 3.4 Operation support for quality assurance and quality control

### 3.4.1 Definition and establishment of standard measurement guidelines

For all three measurement techniques, the CREGARS must formulate initial Standard Operating Procedures (SOP) and must provide tools for supporting their implementation at the NFs, by 2021. By 2025, CREGARS must be supporting all ACTRIS beneficiaries in an operational mode.

For all instruments, the SOP and tools will evolve in the course of ACTRIS, in line with technological and scientific developments, and the evolution of the research.

#### FTIR

Guidelines for the operation of the ACTRIS FTIR instruments must be established and updated as needs arise; at start NDACC IRWG Protocols will serve as a basis. These measurement guidelines or Standard Operating Procedures (SOP) must provide specific and detailed information for the operation of ACTRIS FTIR instruments for the observation of the ACTRIS target variables.

These SOP must include

* unified alignment procedures,

* procedures to perform calibration cell measurements and run the associated software (Linefit) to analyse the cell measurement for the retrieval of the Instrument Line Shape function (ILS); establish the criteria that the ILS must comply with;

* The Linefit software, that must be maintained and documented and must be available from the CF, including a user guide.

The CREGARS must also take care of providing the required sets of optical filters and instrument parameter settings (e.g., resolution, bandwidth...) for the measurement of each of the ACTRIS reactive trace gases

The CREGARS shall also have at its disposal an ACTRIS-compliant FTIR set-up as a test facility for the establishment of the procedures and as training facility for the ACTRIS beneficiaries and users.

#### UVVIS

Guidelines for the operation of UVVIS DOAS instruments have been established for zenith-sky systems as part of the NDACC UVVIS IWG Protocols. More recently, procedures covering the operation of MAXDOAS instruments have been developed in the context of EU and ESA projects dealing with the development of automated and quality-controlled UVVIS trace gas measurements in support of satellite validation. Those existing guidelines and protocols must be unified and further developed to SOP for all ACTRIS UVVIS DOAS instruments.

The following key elements will be addressed:

* Procedures for ISRF determination and stray-light level determination

* Procedures for polarization sensitivity characterization

* Procedures for detector performance assessment (noise, dark current, linearity)

* For the initial calibration of new instruments, the CREGARS must provide access to a calibration centre where all necessary reference light sources, filters, positioning elements must be available.
## 3.4 Operation support for quality assurance and quality control

### 3.4.1 Definition and establishment of standard measurement guidelines

For all three measurement techniques, the CREGARS must formulate initial Standard Operating Procedures (SOP) and must provide tools for supporting their implementation at the NFs, by 2021. By 2025, CREGARS must be supporting all ACTRIS beneficiaries in an operational mode.

For all instruments, the SOP and tools will evolve in the course of ACTRIS, in line with technological and scientific developments, and the evolution of the research.

#### FTIR

Guidelines for the operation of the ACTRIS FTIR instruments must be established and updated as needs arise; at start NDACC IRWG Protocols will serve as a basis. These measurement guidelines or Standard Operating Procedures (SOP) must provide specific and detailed information for the operation of ACTRIS FTIR instruments for the observation of the ACTRIS target variables.

These SOP must include

* unified alignment procedures,

* procedures to perform calibration cell measurements and run the associated software (Linefit) to analyse the cell measurement for the retrieval of the Instrument Line Shape function (ILS); establish the criteria that the ILS must comply with;

* The Linefit software, that must be maintained and documented and must be available from the CF, including a user guide.

The CREGARS must also take care of providing the required sets of optical filters and instrument parameter settings (e.g., resolution, bandwidth...) for the measurement of each of the ACTRIS reactive trace gases

The CREGARS shall also have at its disposal an ACTRIS-compliant FTIR set-up as a test facility for the establishment of the procedures and as training facility for the ACTRIS beneficiaries and users.

#### UVVIS

Guidelines for the operation of UVVIS DOAS instruments have been established for zenith-sky systems as part of the NDACC UVVIS IWG Protocols. More recently, procedures covering the operation of MAXDOAS instruments have been developed in the context of EU and ESA projects dealing with the development of automated and quality-controlled UVVIS trace gas measurements in support of satellite validation. Those existing guidelines and protocols must be unified and further developed to SOP for all ACTRIS UVVIS DOAS instruments.

The following key elements will be addressed:

* Procedures for ISRF determination and stray-light level determination

* Procedures for polarization sensitivity characterization

* Procedures for detector performance assessment (noise, dark current, linearity)

* For the initial calibration of new instruments, the CREGARS must provide access to a calibration centre where all necessary reference light sources, filters, positioning elements must be available.
* Specific operation parameters must also be formulated (e.g. frequency of MAXDOAS scans, which angles to use, typical accumulation time, data format, automatic QA, etc.)

## O3 DIAL

Guidelines for the operation of the ACTRIS O3 DIAL instruments must be established and updated as needed, in complement to the NDACC LIDAR protocols.

These guidelines must include:

* The comparison/harmonization of the retrieval of ozone vertical distribution from the LIDAR signals.

* The implementation of standardized definition of trace gas absorption cross-section, vertical resolution and measurement uncertainty in LIDAR retrieval algorithms

* A reference retrieval algorithm for ozone vertical distribution must be made available from the CF, including a user guide.

* Synthetic LIDAR signals must be made available from the CREGARS based on instrumental characteristics for the purpose of signal to noise ratio check.

### 3.4.2 Definition of measurement quality-assurance criteria and procedures

#### FTIR

Building upon the network-wide protocols dealing with the requirements for instrument quality and instrument intercomparisons, and the more specific quality criteria for the FTIR instruments that have been established by the NDCC IRWG, the CREGARS must establish specific criteria that the ACTRIS measurement data must comply with as, e.g., signal-to-noise ratio, ILS criteria, stability criteria, comparability with other instruments, etc.

The TC must also establish a procedure to control on a regular basis (e.g., yearly - see Sect. 9 Annex) whether the NFs comply with the quality criteria: well-defined sample spectra must be requested for verification at the CF.

Of course, the quality criteria and associated control procedures will evolve continuously, to keep up with the technological evolution of the instruments and the research requirements.

#### UVVIS

Specific quality criteria for the UVVIS zenith-sky instruments have long been established by the NDACC UVVIS WG and have, more recently, been extended to MAX-DOAS instruments. ACTRIS-specific quality criteria must be defined in terms of instrumental performance (e.g. polarisation response, signal-to-noise ratio, spectral resolution (ISRF), spectral stability generally related to thermal stability of instrument). Quality criteria must also be defined in terms of comparability level with other instruments. These must be established during intercomparison field campaigns where a large number of instruments are jointly operated. The CREGARS shall establish a reference site for UVVIS instrument intercalibrations. Such exercises should be conducted at the frequency of about one campaign every 5 to 10 years (see also Sect. 9 Annex).

Since large scale intercomparison campaigns cannot be organised very regularly (due to obvious reasons of cost and logistics), the TC must also establish a procedure to control on a regular basis (e.g., yearly)
* Specific operation parameters must also be formulated (e.g. frequency of MAXDOAS scans, which angles to use, typical accumulation time, data format, automatic QA, etc.)

## O3 DIAL

Guidelines for the operation of the ACTRIS O3 DIAL instruments must be established and updated as needed, in complement to the NDACC LIDAR protocols.

These guidelines must include:

* The comparison/harmonization of the retrieval of ozone vertical distribution from the LIDAR signals.

* The implementation of standardized definition of trace gas absorption cross-section, vertical resolution and measurement uncertainty in LIDAR retrieval algorithms

* A reference retrieval algorithm for ozone vertical distribution must be made available from the CF, including a user guide.

* Synthetic LIDAR signals must be made available from the CREGARS based on instrumental characteristics for the purpose of signal to noise ratio check.

## 3.4.2 Definition of measurement quality-assurance criteria and procedures

### FTIR

Building upon the network-wide protocols dealing with the requirements for instrument quality and instrument intercomparisons, and the more specific quality criteria for the FTIR instruments that have been established by the NDCC IRWG, the CREGARS must establish specific criteria that the ACTRIS measurement data must comply with as, e.g., signal-to-noise ratio, ILS criteria, stability criteria, comparability with other instruments, etc.

The TC must also establish a procedure to control on a regular basis (e.g., yearly - see Sect. 9 Annex) whether the NFs comply with the quality criteria: well-defined sample spectra must be requested for verification at the CF.

Of course, the quality criteria and associated control procedures will evolve continuously, to keep up with the technological evolution of the instruments and the research requirements.

### UVVIS

Specific quality criteria for the UVVIS zenith-sky instruments have long been established by the NDACC UVVIS WG and have, more recently, been extended to MAX-DOAS instruments. ACTRIS-specific quality criteria must be defined in terms of instrumental performance (e.g. polarisation response, signal-to-noise ratio, spectral resolution (ISRF), spectral stability generally related to thermal stability of instrument). Quality criteria must also be defined in terms of comparability level with other instruments. These must be established during intercomparison field campaigns where a large number of instruments are jointly operated. The CREGARS shall establish a reference site for UVVIS instrument intercalibrations. Such exercises should be conducted at the frequency of about one campaign every 5 to 10 years (see also Sect. 9 Annex).

Since large scale intercomparison campaigns cannot be organised very regularly (due to obvious reasons of cost and logistics), the TC must also establish a procedure to control on a regular basis (e.g., yearly)
whether the NFs comply with the quality criteria. This can be obtained through the exchange of sample
spectra to be verified at the CF. To quality assure new instruments and/or new NFs, the TC must also rely
on certified travelling standard instruments which must be deployed on-site for calibration transfer.

O3 DIAL

The CREGARS must establish guidelines to inform the ACTRIS NFs operating O3 DIAL instruments about the specific criteria that the measurement data must comply with as, e.g., signal-to-noise ratio or laser-telescope alignment procedures. The TC must also establish a procedure to control on a regular basis (e.g., yearly – see Sect. 9 Annex) whether the NFs comply with the quality criteria.

3.4.3 Development and provision of instrument-specific calibration

FTIR

The CREGARS must be responsible for the delivery of appropriate calibration cells for the FTIR instruments operated at the NFs, and for their regular calibration and maintenance. It must maintain and distribute the S/W (Linefit) including its documentation for the analysis of the cell spectra that are used for the ILS determination.

The gas cells that are distributed to the NFs must be calibrated regularly (annually) at the TC against a well-controlled N₂O cell. To that end, the NF operators must have to turn in their calibration cells on a regular basis to the CREGARS for re-calibrating it against the reference N₂O cell.

Also the ILS measurements taken at the NFs must be submitted to the CREGARS on a regular basis (see
Sect. 9) for verification.

UVVIS

Each instrument must receive initial calibration in the CREGARS. Such calibration must include:

* Wavelength calibration based on emission lines of spectral lamps (usually gas discharge lamps).

* In-band stray light calibration based on laser lines.

* For direct-sun systems, radiometric calibration based on calibrated high power tungsten halogen lamps.

* Determination of the field of view based on a moving small light source.

* For direct-sun systems, determination of initial reference spectra from solar observations on 2-3 clear sky days.

Once the instrument is at the NF, its calibration must be checked in the following way:

* A mobile Field Calibration Tool (mFCT) will be sent to the station at least once per year to check the radiometric stability of the system.

* A mobile reference instrument (CREGARS Certified Traveling Standard) will be sent to the station and measure side by side with the local instrument to compare the final data products and provide a certification transfer. This should happen every 2-4 years depending on the station and the status of participation to large-scale intercalibration campaigns at the reference site (see Annex).
whether the NFs comply with the quality criteria. This can be obtained through the exchange of sample
spectra to be verified at the CF. To quality assure new instruments and/or new NFs, the TC must also rely
on certified travelling standard instruments which must be deployed on-site for calibration transfer.

O3 DIAL

The CREGARS must establish guidelines to inform the ACTRIS NFs operating O3 DIAL instruments about the specific criteria that the measurement data must comply with as, e.g., signal-to-noise ratio or laser-telescope alignment procedures. The TC must also establish a procedure to control on a regular basis (e.g., yearly – see Sect. 9 Annex) whether the NFs comply with the quality criteria.

3.4.3 Development and provision of instrument-specific calibration

FTIR

The CREGARS must be responsible for the delivery of appropriate calibration cells for the FTIR instruments operated at the NFs, and for their regular calibration and maintenance. It must maintain and distribute the S/W (Linefit) including its documentation for the analysis of the cell spectra that are used for the ILS determination.

The gas cells that are distributed to the NFs must be calibrated regularly (annually) at the TC against a well-controlled N₂O cell. To that end, the NF operators must have to turn in their calibration cells on a regular basis to the CREGARS for re-calibrating it against the reference N₂O cell.

Also the ILS measurements taken at the NFs must be submitted to the CREGARS on a regular basis (see
Sect. 9) for verification.

UVVIS

Each instrument must receive initial calibration in the CREGARS. Such calibration must include:

* Wavelength calibration based on emission lines of spectral lamps (usually gas discharge lamps).

* In-band stray light calibration based on laser lines.

* For direct-sun systems, radiometric calibration based on calibrated high power tungsten halogen lamps.

* Determination of the field of view based on a moving small light source.

* For direct-sun systems, determination of initial reference spectra from solar observations on 2-3 clear sky days.

Once the instrument is at the NF, its calibration must be checked in the following way:

* A mobile Field Calibration Tool (mFCT) will be sent to the station at least once per year to check the radiometric stability of the system.

* A mobile reference instrument (CREGARS Certified Traveling Standard) will be sent to the station and measure side by side with the local instrument to compare the final data products and provide a certification transfer. This should happen every 2-4 years depending on the station and the status of participation to large-scale intercalibration campaigns at the reference site (see Annex).
## O3 DIAL

Since ozone DIAL vertical distribution measurements are self-calibrated, their quality is checked periodically with a traveling lidar instrument that must be provided by the CREGARS possibly through an agreement with an external partner. (An option to be investigated is to make an agreement with NASA that provided the international traveling standard instrument since the foundation of NDACC and has participated to more than 15 international intercomparison campaigns). In between intercomparison campaigns, the Centre for Reactive Trace Gases Remote Sensing must assist ACTRIS NFs in checking the quality of their measurements using comparisons with nearby independent ozone profiles, e.g. from ozone sondes or from satellite overpass measurements. These must be made available through the ACTRIS Data Centre.

### 3.4.4 Development and provision of in-house check-up tools

#### FTIR

The Centre for Reactive Trace Gases Remote Sensing must develop some tools that can be used at the NFs to assure the quality of their measurements, e.g., standardized visualization tools to check the zero baseline and out-of-band artefacts in the atmospheric spectra, tools for doing a preliminary QA/QC check of the data before submission, etc.

The CREGARS must also be a contact point for the instrument and peripheral equipment manufacturers to make the commercially provided equipment operation and handling software and tools evolve in line with the needs of the ACTRIS FTIR community.

And as mentioned above (Section 3.4.1.1) the CREGARS shall have an FTIR set-up at the disposal of the NF operators to experiment and become familiar with the measurement guidelines.

#### UVVIS

The Centre for Reactive Trace Gases Remote Sensing must develop tools that the NFs can use themselves to assure the quality of their measurements, e.g., standardized visualization and tools to check the instrumental spectral response function of the instrument, the wavelength stability, the stability of the signal to noise ratio, etc.

The CREGARS must also be a contact point for the instrument manufacturers to make the commercially provided instrument operation and spectra handling software and tools evolve corresponding to the needs of the ACTRIS UVVIS community.

## O3 DIAL

The Centre for Reactive Trace Gases Remote Sensing shall develop some tools that the PIs can use themselves to assure the quality of their measurements, e.g., visualization of raw lidar signals, provision of standardized modules to correct for saturation of the lidar photocounting signals, Rayleigh extinction correction, gluing of analogue and photocounting signals, etc...

The CREGARS must also be a contact point for some instrument manufacturer (e.g. laser or signal acquisition device providers) corresponding to the needs of the ACTRIS Lidar community.
## O3 DIAL

Since ozone DIAL vertical distribution measurements are self-calibrated, their quality is checked periodically with a traveling lidar instrument that must be provided by the CREGARS possibly through an agreement with an external partner. (An option to be investigated is to make an agreement with NASA that provided the international traveling standard instrument since the foundation of NDACC and has participated to more than 15 international intercomparison campaigns). In between intercomparison campaigns, the Centre for Reactive Trace Gases Remote Sensing must assist ACTRIS NFs in checking the quality of their measurements using comparisons with nearby independent ozone profiles, e.g. from ozone sondes or from satellite overpass measurements. These must be made available through the ACTRIS Data Centre.

### 3.4.4 Development and provision of in-house check-up tools

#### FTIR

The Centre for Reactive Trace Gases Remote Sensing must develop some tools that can be used at the NFs to assure the quality of their measurements, e.g., standardized visualization tools to check the zero baseline and out-of-band artefacts in the atmospheric spectra, tools for doing a preliminary QA/QC check of the data before submission, etc.

The CREGARS must also be a contact point for the instrument and peripheral equipment manufacturers to make the commercially provided equipment operation and handling software and tools evolve in line with the needs of the ACTRIS FTIR community.

And as mentioned above (Section 3.4.1.1) the CREGARS shall have an FTIR set-up at the disposal of the NF operators to experiment and become familiar with the measurement guidelines.

#### UVVIS

The Centre for Reactive Trace Gases Remote Sensing must develop tools that the NFs can use themselves to assure the quality of their measurements, e.g., standardized visualization and tools to check the instrumental spectral response function of the instrument, the wavelength stability, the stability of the signal to noise ratio, etc.

The CREGARS must also be a contact point for the instrument manufacturers to make the commercially provided instrument operation and spectra handling software and tools evolve corresponding to the needs of the ACTRIS UVVIS community.

## O3 DIAL

The Centre for Reactive Trace Gases Remote Sensing shall develop some tools that the PIs can use themselves to assure the quality of their measurements, e.g., visualization of raw lidar signals, provision of standardized modules to correct for saturation of the lidar photocounting signals, Rayleigh extinction correction, gluing of analogue and photocounting signals, etc...

The CREGARS must also be a contact point for some instrument manufacturer (e.g. laser or signal acquisition device providers) corresponding to the needs of the ACTRIS Lidar community.
3.4.5 Development of data evaluation procedures and plausibility test

FTIR

Common data retrieval and uncertainties evaluation software, and associated retrieval strategies are
inherited from NDACC and related projects and do exist. Their evolution, including best practices for the
evaluation of the uncertainties and including the selection of the appropriate reference spectroscopic
data, are discussed in the NDACC IRWG, at least on a yearly basis during the annual plenary meetings. It is
planned that the Centre for Reactive Trace Gases Remote Sensing will take the lead in this evolution and
the corresponding discussions regarding the ACTRIS target species.

The data retrieval strategies must be established such that high-quality data are delivered for all the ACTRIS FTIR sites – knowing that the sites are located at different altitudes and are confronted with different climatological conditions. The CREGARS must therefore verify that the proposed procedures provide good results, not only for all ACTRIS FTIR sites but also for all NDACC FTIR sites worldwide, in order that the procedures established by the CREGARS can be adopted by the whole NDACC FTIR community for the ACTRIS target species.

The Centre for Reactive Trace Gases Remote Sensing must be the housekeeper of the retrieval strategies, of the master copies of the official-version retrieval codes and of the spectroscopic linelists for the ACTRIS target species. It must also verify whether the NFs have applied the strategies as expected before data submission. The CREGARS must actively maintain and enhance the retrieval software codes and their documentation.

Although the NF's are responsible for the data retrieval and the QA/QC, the NF's may delegate the data
retrieval and QA/QC to the TC, if the TC has the capacity to perform it.

The CREGARS in collaboration with the ACTRIS Data Centre must also make sure that the NFs have easy access to the required auxiliary data (e.g., NCEP or ECMWF P/T profiles).

It must also provide support for automating the data analysis at the NFs in order to enable the required
rapid data delivery (within one month after data acquisition).

UVVIS

The evolution towards standardization of retrieval settings and ancillary data sets (e.g. spectral windows, absorption cross-sections, solar reference spectra, data base of reference atmospheric profiles, etc.) has started several years ago in the framework of several European projects such as GeoMon, NORS, QA4ECV and GAIA-CLIM. The most recent developments are taking place within the ESA Pandonia and FRM4DOAS projects, where capacities for centralised processing of multiple instruments using common data retrieval methods, uncertainty evaluation and automated quality control are being developed. It is planned that the ACTRIS Centre for Reactive Trace Gases Remote Sensing must integrate these systems to operationally deliver the ACTRIS target species in NRT mode with a latency of 1 day. The central processing environment must also offer facilities for coherent reprocessing of historical data sets according to retrieval algorithm evolution.

Considering the large variety of the existing UVVIS instruments, data retrieval strategies must be established with enough flexibility to accommodate differences in technical performances and also the large differences in the sites themselves (altitude, location with respect to sources, cloud and aerosol regimes, etc.). The CREGARS must therefore ensure that the procedures in place in the central processing
3.4.5 Development of data evaluation procedures and plausibility test

FTIR

Common data retrieval and uncertainties evaluation software, and associated retrieval strategies are
inherited from NDACC and related projects and do exist. Their evolution, including best practices for the
evaluation of the uncertainties and including the selection of the appropriate reference spectroscopic
data, are discussed in the NDACC IRWG, at least on a yearly basis during the annual plenary meetings. It is
planned that the Centre for Reactive Trace Gases Remote Sensing will take the lead in this evolution and
the corresponding discussions regarding the ACTRIS target species.

The data retrieval strategies must be established such that high-quality data are delivered for all the ACTRIS FTIR sites – knowing that the sites are located at different altitudes and are confronted with different climatological conditions. The CREGARS must therefore verify that the proposed procedures provide good results, not only for all ACTRIS FTIR sites but also for all NDACC FTIR sites worldwide, in order that the procedures established by the CREGARS can be adopted by the whole NDACC FTIR community for the ACTRIS target species.

The Centre for Reactive Trace Gases Remote Sensing must be the housekeeper of the retrieval strategies, of the master copies of the official-version retrieval codes and of the spectroscopic linelists for the ACTRIS target species. It must also verify whether the NFs have applied the strategies as expected before data submission. The CREGARS must actively maintain and enhance the retrieval software codes and their documentation.

Although the NF's are responsible for the data retrieval and the QA/QC, the NF's may delegate the data
retrieval and QA/QC to the TC, if the TC has the capacity to perform it.

The CREGARS in collaboration with the ACTRIS Data Centre must also make sure that the NFs have easy access to the required auxiliary data (e.g., NCEP or ECMWF P/T profiles).

It must also provide support for automating the data analysis at the NFs in order to enable the required
rapid data delivery (within one month after data acquisition).

UVVIS

The evolution towards standardization of retrieval settings and ancillary data sets (e.g. spectral windows, absorption cross-sections, solar reference spectra, data base of reference atmospheric profiles, etc.) has started several years ago in the framework of several European projects such as GeoMon, NORS, QA4ECV and GAIA-CLIM. The most recent developments are taking place within the ESA Pandonia and FRM4DOAS projects, where capacities for centralised processing of multiple instruments using common data retrieval methods, uncertainty evaluation and automated quality control are being developed. It is planned that the ACTRIS Centre for Reactive Trace Gases Remote Sensing must integrate these systems to operationally deliver the ACTRIS target species in NRT mode with a latency of 1 day. The central processing environment must also offer facilities for coherent reprocessing of historical data sets according to retrieval algorithm evolution.

Considering the large variety of the existing UVVIS instruments, data retrieval strategies must be established with enough flexibility to accommodate differences in technical performances and also the large differences in the sites themselves (altitude, location with respect to sources, cloud and aerosol regimes, etc.). The CREGARS must therefore ensure that the procedures in place in the central processing
systems provide optimal results for a large range of sites. It is the aim that the procedures established by
the CREGARS can be adopted by the whole UVVIS community worldwide (beyond the ACTRIS community).

O3 DIAL

The evolution of the verified data retrieval and uncertainties evaluation software and retrieval strategies,
including best practices for the evaluation of the uncertainties and including the selection of the
appropriate reference spectroscopic data, are discussed in the NDACC Lidar WG, during the bi-annual
plenary meetings. It is planned that the Centre for Reactive Trace Gases Remote Sensing will play a very
active role in these discussions regarding the ACTRIS target species.

The data retrieval strategies must be established such that high-quality data are delivered for all the NDACC Lidar WG sites – knowing that the sites are located at different altitudes and are confronted with different climatological conditions. The CREGARS must therefore verify that the proposed procedures provide good results, not only for all ACTRIS lidar sites but also for all NDACC lidar sites worldwide, in order that the procedures established by the CREGARS can be adopted by the whole NDACC lidar community for the ACTRIS target species.

The CREGARS must be the housekeeper of the retrieval strategies for the ACTRIS target species, and it will
verify whether the NFs have applied the strategies as expected before data submission.

3.4.6 Realization of observational site performance audits with reference samples or mobile systems

FTIR

The audits of an NF must be done on the basis of submitted sample data:

1. The NFs must be requested to submit their ILS measurements to the CREGARS on a regular basis for the verification of the instrument alignment.

2. The CREGARS must also verify the quality of the spectral data (e.g., Signal to Noise Ratio – time series) collected from the FTIRs at the NFs on a regular basis to assess the instruments performances.

3. The data products submitted by the NFs must be verified on a regular basis for assuring their long-term homogeneity and for verifying the network consistency, e.g., through comparisons of CO₂ and/or N₂ column measurements.

Any deviation from expectations must be discussed between the TC and the FTIR PIs at the NFs, and the
TC must provide guidance to improve where needed/possible.

Particular attention must be paid to:

* instrumental line shape parameters retrieved from gas cell measurements

- column measurements of known CO₂ column concentrations

* atmospheric spectra at each site for verification of zero baseline, spectral artefacts like channelling, and out-of-band artefacts
systems provide optimal results for a large range of sites. It is the aim that the procedures established by
the CREGARS can be adopted by the whole UVVIS community worldwide (beyond the ACTRIS community).

O3 DIAL

The evolution of the verified data retrieval and uncertainties evaluation software and retrieval strategies, including best practices for the evaluation of the uncertainties and including the selection of the appropriate reference spectroscopic data, are discussed in the NDACC Lidar WG, during the bi-annual plenary meetings. It is planned that the Centre for Reactive Trace Gases Remote Sensing will play a very active role in these discussions regarding the ACTRIS target species.

The data retrieval strategies must be established such that high-quality data are delivered for all the NDACC Lidar WG sites – knowing that the sites are located at different altitudes and are confronted with different climatological conditions. The CREGARS must therefore verify that the proposed procedures provide good results, not only for all ACTRIS lidar sites but also for all NDACC lidar sites worldwide, in order that the procedures established by the CREGARS can be adopted by the whole NDACC lidar community for the ACTRIS target species.

The CREGARS must be the housekeeper of the retrieval strategies for the ACTRIS target species, and it will verify whether the NFs have applied the strategies as expected before data submission.

3.4.6 Realization of observational site performance audits with reference samples or mobile systems

FTIR

The audits of an NF must be done on the basis of submitted sample data:

1. The NFs must be requested to submit their ILS measurements to the CREGARS on a regular basis for the verification of the instrument alignment.

2. The CREGARS must also verify the quality of the spectral data (e.g., Signal to Noise Ratio – time series) collected from the FTIRs at the NFs on a regular basis to assess the instruments performances.

3. The data products submitted by the NFs must be verified on a regular basis for assuring their long-term homogeneity and for verifying the network consistency, e.g., through comparisons of CO₂ and/or N₂ column measurements.

Any deviation from expectations must be discussed between the TC and the FTIR PIs at the NFs, and the
TC must provide guidance to improve where needed/possible.

Particular attention must be paid to:

* instrumental line shape parameters retrieved from gas cell measurements

- column measurements of known CO₂ column concentrations

* atmospheric spectra at each site for verification of zero baseline, spectral artefacts like channelling, and out-of-band artefacts
UVVIS

At present, UVVIS instruments are not deployed in a coordinated and systematic way, and therefore instrument performance is essentially verified during large scale intercalibration campaigns, such as the recent CINDI-2 campaign. As part of the ESA Pandonia project, mobile reference systems are being developed with the aim to use them to regularly verify instruments at the different sites of the network. Within ACTRIS, it is planned to extend this approach using certified mobile instruments designed for performance audits of both zenith-sky and MAX-DOAS systems.

Moreover the data products submitted by the NFs must be verified on a regular basis for assuring their long-term homogeneity and for verifying the network consistency, e.g., through comparisons with a global model or with climatological satellite data.

Any deviation from expectations must be discussed between the CREGARS and the NFs, and the CREGARS must provide guidance to improve where needed/possible.

Finally a more global discussion on NFs performances must continue to take place on a yearly basis during the international meetings of the UVVIS remote-sensing community.

O3 DIAL

As mentioned in section 3.4.3.3, the quality of NF O3 DIAL measurements must be checked periodically with a traveling lidar instrument. If possible at the lidar site, simultaneous measurements with nearby balloon borne ozonesonde measurements must be reported to identify ozone profile inconsistencies. The Centre for Reactive Trace Gases Remote Sensing must support the NFs to organize and assess these periodic intercomparison campaigns.

3.4.7 Organization of regular exercises to assess the performances of the NFs, including instrument performance workshops

FTIR

For each NF FTIR instrument, the CREGARS must perform a 'peer review' exercise on a regular basis. To this end, the FTIR NFs will be asked to send pictures of the setup and a sample of recorded spectra for each target species and the corresponding retrieved target gas concentrations and uncertainty budget, as well as a sample of cell spectra and the corresponding ILS parameters, to the CREGARS. The scientific expert at the CREGARS must verify the quality of the spectra and of the retrieved products and their characterisation, by making an independent retrieval at the CREGARS. The CREGARS must then provide feedback to the NF, including recommendations for improvements and possibly for re-processing of data as appropriate.

The CREGARS must have the capacity to do one complete peer review exercise every two years – this will determine the schedule per NF. Additional random checks can be done if a need appears.

UVVIS

The main formal assessment process for UVVIS instruments must be based on the participation to large scale intercalibration campaigns, to be organized at the approximate frequency of one every 5 years. The main requirements for a suitable infrastructure hosting such campaigns are (1) the capacity to accommodate a large number of instruments and teams in a single place, (2) to provide a widely open
UVVIS

At present, UVVIS instruments are not deployed in a coordinated and systematic way, and therefore instrument performance is essentially verified during large scale intercalibration campaigns, such as the recent CINDI-2 campaign. As part of the ESA Pandonia project, mobile reference systems are being developed with the aim to use them to regularly verify instruments at the different sites of the network. Within ACTRIS, it is planned to extend this approach using certified mobile instruments designed for performance audits of both zenith-sky and MAX-DOAS systems.

Moreover the data products submitted by the NFs must be verified on a regular basis for assuring their long-term homogeneity and for verifying the network consistency, e.g., through comparisons with a global model or with climatological satellite data.

Any deviation from expectations must be discussed between the CREGARS and the NFs, and the CREGARS must provide guidance to improve where needed/possible.

Finally a more global discussion on NFs performances must continue to take place on a yearly basis during the international meetings of the UVVIS remote-sensing community.

O3 DIAL

As mentioned in section 3.4.3.3, the quality of NF O3 DIAL measurements must be checked periodically with a traveling lidar instrument. If possible at the lidar site, simultaneous measurements with nearby balloon borne ozonesonde measurements must be reported to identify ozone profile inconsistencies. The Centre for Reactive Trace Gases Remote Sensing must support the NFs to organize and assess these periodic intercomparison campaigns.

3.4.7 Organization of regular exercises to assess the performances of the NFs, including instrument performance workshops

FTIR

For each NF FTIR instrument, the CREGARS must perform a 'peer review' exercise on a regular basis. To this end, the FTIR NFs will be asked to send pictures of the setup and a sample of recorded spectra for each target species and the corresponding retrieved target gas concentrations and uncertainty budget, as well as a sample of cell spectra and the corresponding ILS parameters, to the CREGARS. The scientific expert at the CREGARS must verify the quality of the spectra and of the retrieved products and their characterisation, by making an independent retrieval at the CREGARS. The CREGARS must then provide feedback to the NF, including recommendations for improvements and possibly for re-processing of data as appropriate.

The CREGARS must have the capacity to do one complete peer review exercise every two years – this will determine the schedule per NF. Additional random checks can be done if a need appears.

UVVIS

The main formal assessment process for UVVIS instruments must be based on the participation to large scale intercalibration campaigns, to be organized at the approximate frequency of one every 5 years. The main requirements for a suitable infrastructure hosting such campaigns are (1) the capacity to accommodate a large number of instruments and teams in a single place, (2) to provide a widely open
field of view at the horizon in many azimuthal directions and (3) to be equipped with a large set of relevant ancillary measurements.

For each NF UVVIS instrument participating to an intercalibration campaign, the CREGARS must perform a formal peer-reviewed assessment based on pre-established comparison procedure involving calibration steps and in-the-field measurements following a strict acquisition protocol. Typically, intercalibration campaigns must be conducted over a period of two weeks of measurements, weather-permitting; a semi-blind intercomparison protocol must be applied.

For logistical and cost reasons, intercalibration campaigns at the reference site can only be organized at the frequency of once every 5-6 years. As a consequence, an alternative certification approach must be offered for newly installed sites based on the use of certified travelling standard instruments. Additionally, procedures relying on the exchange of raw spectra to be evaluated by the CF will be used. The results of the performance analyses must be discussed at specific workshops on a yearly basis.

### O3 DIAL

Regular workshops must be organized in order to train the ozone lidar operators to the latest improvement in the hardware and software of ozone lidar systems. In addition, the technical expert at the TC must check each individual lidar profile submitted to the ACTRIS data centre and compare it to nearby ozone profile measurements with adequate methodology. The TC must provide feedback to the NF with recommendation for improvement if needed.

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

#### FTIR

The CREGARS must make use of traceability diagrams and associated documentation for the FTIR data products that have been developed in the EU H2020 GAIA-CLIM project (see http://www.gaia-clim.eu/page/ftir-traceability-diagram) to document the traceability of the L0 to L3 ACTRIS FTIR data products. The diagrams must be further completed and updated in agreement with the progress made in the context of ACTRIS. For now, the traceability diagrams cover from L0 to L2; If L3 products will be delivered to ACTRIS DC, then the traceability chains must be extended to the L3 products.

#### UVVIS

The TC must make use of traceability diagrams and associated documentation for the UVVIS observations according to standards developed in the EU H2020 GAIA-CLIM and QA4ECV projects (see http://www.gaia-clim.eu/page/uv-vis-traceability-model-diagram) to document the traceability of the L0 to L3 ACTRIS UVVIS data products. The diagrams must be further completed and updated in agreement with the progress made in the context of ACTRIS.

#### O3 DIAL

The TC must determine standardized procedures to document the level 0 to level 3 data products in cooperation with the ACTRIS data centre. These procedures must use standardized definitions of the vertical resolution and uncertainty of the ozone lidar profiles that have been determined by the NDACC lidar working group.
field of view at the horizon in many azimuthal directions and (3) to be equipped with a large set of relevant ancillary measurements.

For each NF UVVIS instrument participating to an intercalibration campaign, the CREGARS must perform a formal peer-reviewed assessment based on pre-established comparison procedure involving calibration steps and in-the-field measurements following a strict acquisition protocol. Typically, intercalibration campaigns must be conducted over a period of two weeks of measurements, weather-permitting; a semi-blind intercomparison protocol must be applied.

For logistical and cost reasons, intercalibration campaigns at the reference site can only be organized at the frequency of once every 5-6 years. As a consequence, an alternative certification approach must be offered for newly installed sites based on the use of certified travelling standard instruments. Additionally, procedures relying on the exchange of raw spectra to be evaluated by the CF will be used. The results of the performance analyses must be discussed at specific workshops on a yearly basis.

### O3 DIAL

Regular workshops must be organized in order to train the ozone lidar operators to the latest improvement in the hardware and software of ozone lidar systems. In addition, the technical expert at the TC must check each individual lidar profile submitted to the ACTRIS data centre and compare it to nearby ozone profile measurements with adequate methodology. The TC must provide feedback to the NF with recommendation for improvement if needed.

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

#### FTIR

The CREGARS must make use of traceability diagrams and associated documentation for the FTIR data products that have been developed in the EU H2020 GAIA-CLIM project (see http://www.gaia-clim.eu/page/ftir-traceability-diagram) to document the traceability of the L0 to L3 ACTRIS FTIR data products. The diagrams must be further completed and updated in agreement with the progress made in the context of ACTRIS. For now, the traceability diagrams cover from L0 to L2; If L3 products will be delivered to ACTRIS DC, then the traceability chains must be extended to the L3 products.

#### UVVIS

The TC must make use of traceability diagrams and associated documentation for the UVVIS observations according to standards developed in the EU H2020 GAIA-CLIM and QA4ECV projects (see http://www.gaia-clim.eu/page/uv-vis-traceability-model-diagram) to document the traceability of the L0 to L3 ACTRIS UVVIS data products. The diagrams must be further completed and updated in agreement with the progress made in the context of ACTRIS.

#### O3 DIAL

The TC must determine standardized procedures to document the level 0 to level 3 data products in cooperation with the ACTRIS data centre. These procedures must use standardized definitions of the vertical resolution and uncertainty of the ozone lidar profiles that have been determined by the NDACC lidar working group.
### 3.4.9 Contribution to CEN, ISO, or similar standardization activities

**FTIR**

The need for further standardization of the ACTRIS FTIR remote sensing products, e.g., traceability to WMO or other metrological standards, is recognized – especially for supporting research that relies on a combination of data from different techniques. There are ideas in the community to develop tools for calibration of the FTIR data products to WMO standards. By 2025, the Centre for Reactive Trace Gases Remote Sensing must develop procedures and tools to further contribute to such standardization efforts.

**UVVIS**

Likewise, standardization efforts regarding ACTRIS UVVIS remote sensing products must rely on the development of dedicated tools and procedures aiming to calibrate measurements against WMO standards. This might rely on the operation of in-situ profiling techniques (e.g. sondes) allowing to establish the link between remote-sensing data and established standards. The Centre for Reactive Trace Gases Remote Sensing must contribute to the development of adequate procedures and tools necessary to this aim.

**O3 DIAL**

Standardization efforts regarding ACTRIS O3 DIAL ozone profiles and other remote sensing products must rely on the development of dedicated tools and procedures, in close relationship with what is developed within the WMO GCOS Reference Upper Air Network.

## 3.5 Operation support for knowledge transfer and training

### 3.5.1 Training of operators and scientists

Training of operators and scientists who use the ACTRIS data will be provided in three ways:

* By providing didactic material, dedicated guidelines, procedures, user guides, templates, examples, etc. Much material is already available, e.g., for the FTIR and UVVIS instruments operations and data analysis at:

i. www2.acom.ucar.edu/irwg/links,

ii. nors.aeronomie.be/index.php/documents

iii. ndacc-uvvis-wg.aeronomie.be/tools.php

iv. http://pandonia.net/docs/ http://www.gaia-clim.eu/page/deliverables.

However, the existing material is not centrally available nor maintained/ updated, therefore the ACTRIS CREGARS must ensure the centralized availability and maintenance of the material and develop missing material as needs arise.

* By providing hands-on training upon request by an ACTRIS PI - if resources permit. Such hands-on training is already provided now by some RPO operating an FTIR, UVVIS or O3 DIAL on a voluntary basis.

* By having annual workshops during which new or updated procedures, tools, etc. will be presented.
### 3.4.9 Contribution to CEN, ISO, or similar standardization activities

**FTIR**

The need for further standardization of the ACTRIS FTIR remote sensing products, e.g., traceability to WMO or other metrological standards, is recognized – especially for supporting research that relies on a combination of data from different techniques. There are ideas in the community to develop tools for calibration of the FTIR data products to WMO standards. By 2025, the Centre for Reactive Trace Gases Remote Sensing must develop procedures and tools to further contribute to such standardization efforts.

**UVVIS**

Likewise, standardization efforts regarding ACTRIS UVVIS remote sensing products must rely on the development of dedicated tools and procedures aiming to calibrate measurements against WMO standards. This might rely on the operation of in-situ profiling techniques (e.g. sondes) allowing to establish the link between remote-sensing data and established standards. The Centre for Reactive Trace Gases Remote Sensing must contribute to the development of adequate procedures and tools necessary to this aim.

**O3 DIAL**

Standardization efforts regarding ACTRIS O3 DIAL ozone profiles and other remote sensing products must rely on the development of dedicated tools and procedures, in close relationship with what is developed within the WMO GCOS Reference Upper Air Network.

## 3.5 Operation support for knowledge transfer and training

### 3.5.1 Training of operators and scientists

Training of operators and scientists who use the ACTRIS data will be provided in three ways:

* By providing didactic material, dedicated guidelines, procedures, user guides, templates, examples, etc. Much material is already available, e.g., for the FTIR and UVVIS instruments operations and data analysis at:

i. www2.acom.ucar.edu/irwg/links,

ii. nors.aeronomie.be/index.php/documents

iii. ndacc-uvvis-wg.aeronomie.be/tools.php

iv. http://pandonia.net/docs/ http://www.gaia-clim.eu/page/deliverables.

However, the existing material is not centrally available nor maintained/ updated, therefore the ACTRIS CREGARS must ensure the centralized availability and maintenance of the material and develop missing material as needs arise.

* By providing hands-on training upon request by an ACTRIS PI - if resources permit. Such hands-on training is already provided now by some RPO operating an FTIR, UVVIS or O3 DIAL on a voluntary basis.

* By having annual workshops during which new or updated procedures, tools, etc. will be presented.
In case of significant updates to any of the measurement or data analysis procedures, the CREGARS should
judge the appropriateness of organizing a dedicated workshop for training the PIs and staff at the NFs.
Such workshops must be open to the concerned community, at the world-wide scale.

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

The needs to setup new measurements sites and observational capabilities are driven by the distribution
of the sources of the trace gases that can be monitored by reactive trace gas remote sensing instruments.
It is essential that monitoring sites sample areas close to emission sources, but also background/rural and
semi-rural/sub-urban conditions must be met. The characteristics of both natural and anthropogenic
sources must be considered in the process of defining new national facilities. In order to provide informed
consultancy on the setting up of new observational sites, the CREGARS will use model-based sources of
information (e.g. 3D chemical transport models, inverse modelling, statistical methods, evaluation of
representativeness areas) as well as satellite data to identify the most relevant observational gaps.
Information must be provided to the ACTRIS beneficiaries and users in the form of time-dependent maps
of the different trace gases, which will help to assess the relevance of proposed new observational or
exploratory platforms.

Examples of such evaluations can be found, e.g., in D1.10 and D1.11 of the GAIA-CLIM project
(http://www.gaia-clim.eu/page/deliverables).

The role of the CREGARS must be to maintain the contacts with the research communities and industrial partners to provide a forum for discussing opportunities for new instruments, new observation or exploratory platforms, and possibly to solicit research partners to perform some dedicated studies in support of the evolution of the reactive trace gases remote sensing component of the Research Infrastructure.

3.6 Operation support for improvement of measurement methodologies

3.6.1 Testing of new measurement instruments and procedures

If a new or upgraded instrument is proposed as alternative for certified ACTRIS instruments, the CREGARS must assess its performances by organizing an intercomparison campaign during which the new / upgraded instrument will be co-located with the reference ACTRIS instrument at the CREGARS and in which the TC will play the role of independent referee.

In particular for the UVVIS instruments:

First an initial calibration must be performed in the CREGARS Calibration Centre, which must cover
wavelength registration, stray-light characterization, ISRF and field of view determination and radiometric
calibration. Further evaluation of the instrument performance must be regularly obtained through
participation to intercalibration campaigns (once every 5 years) and/or through on-site comparison with
the CREGARS certified traveling standard instrument. This should happen every 2-4 years depending on
the station.
In case of significant updates to any of the measurement or data analysis procedures, the CREGARS should
judge the appropriateness of organizing a dedicated workshop for training the PIs and staff at the NFs.
Such workshops must be open to the concerned community, at the world-wide scale.

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

The needs to setup new measurements sites and observational capabilities are driven by the distribution
of the sources of the trace gases that can be monitored by reactive trace gas remote sensing instruments.
It is essential that monitoring sites sample areas close to emission sources, but also background/rural and
semi-rural/sub-urban conditions must be met. The characteristics of both natural and anthropogenic
sources must be considered in the process of defining new national facilities. In order to provide informed
consultancy on the setting up of new observational sites, the CREGARS will use model-based sources of
information (e.g. 3D chemical transport models, inverse modelling, statistical methods, evaluation of
representativeness areas) as well as satellite data to identify the most relevant observational gaps.
Information must be provided to the ACTRIS beneficiaries and users in the form of time-dependent maps
of the different trace gases, which will help to assess the relevance of proposed new observational or
exploratory platforms.

Examples of such evaluations can be found, e.g., in D1.10 and D1.11 of the GAIA-CLIM project
(http://www.gaia-clim.eu/page/deliverables).

The role of the CREGARS must be to maintain the contacts with the research communities and industrial partners to provide a forum for discussing opportunities for new instruments, new observation or exploratory platforms, and possibly to solicit research partners to perform some dedicated studies in support of the evolution of the reactive trace gases remote sensing component of the Research Infrastructure.

3.6 Operation support for improvement of measurement methodologies

3.6.1 Testing of new measurement instruments and procedures

If a new or upgraded instrument is proposed as alternative for certified ACTRIS instruments, the CREGARS must assess its performances by organizing an intercomparison campaign during which the new / upgraded instrument will be co-located with the reference ACTRIS instrument at the CREGARS and in which the TC will play the role of independent referee.

In particular for the UVVIS instruments:

First an initial calibration must be performed in the CREGARS Calibration Centre, which must cover
wavelength registration, stray-light characterization, ISRF and field of view determination and radiometric
calibration. Further evaluation of the instrument performance must be regularly obtained through
participation to intercalibration campaigns (once every 5 years) and/or through on-site comparison with
the CREGARS certified traveling standard instrument. This should happen every 2-4 years depending on
the station.
### 3.6.2 Development of strategies to increase the duty cycle of instruments operated at the NFs, to reduce breakdown and maintenance downtime

**FTIR**

For FTIR, it is not possible to have a backup instrument at the CREGARS, but some spare parts like a laser, optics components, and alignment tools... must be available for the ACTRIS beneficiaries. Especially the special optical filters that are required with each FTIR instrument must be available from the CREGARS that must care for grouped purchases. Similarly, the calibration cells must be provided by the CREGARS. Also spare mirrors for the sun tracker might be worthwhile to have at the CREGARS (although these are less standardised). The NFs should reimburse the CREGARS for parts that they obtain from the CREGARS, probably in-kind.

**UVVIS**

It is generally not possible to plan for backup instruments at all sites. However different strategies can be developed to reduce breakdown and maintenance downtime at remote or demanding sites. In particular, it is a common practice to equip remote Antarctic stations with backup instruments (or alternatively key spare parts). Another approach is to operate multi-channel instruments having overlaps in key spectral regions (e.g. the visible blue range where NO₂ is retrieved). This provides an inherent duplication of the measurements reducing the risks of interruption in case of instrumental breakdown affecting only one channel. For zenith-sky sites at Northern latitudes, repair is guaranteed within less than two months and full replacement possible within a six-month delay. The possibility to borrow a spare instrument during the repair period is also offered.

**O3 DIAL**

Considering the cost of O3 DIALs, it is not possible to have a backup instrument at the CREGARS. However optical and electronic parts that could serve as a back-up for NFs in case of breakdown must be available at the CREGARS.

### 3.6.3 Development of new technological products and methods

New measurement techniques, technological products and methods are developed by the Centre in collaboration with the NFs. Depending on the situation, the Centre 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 National Facilities.

**FTIR**

The TC must keep contact with the FTIR instruments manufacturers for a bi-directional exchange of information: new developments by the manufacturer will first be assessed at the TC, and vice-versa, the TC will pass on to the manufacturer demands from NF PIs for improvements that appear useful for the whole ACTRIS community. Instruments manufacturers will be invited by the TC to the annual workshops to discuss instrument evolution. Similarly technological developments proposed by individual NFs must be assessed by the CREGARS; if the development is validated, the CREGARS can suggest a path towards implementation at all NFs equipped with an FTIR instrument and/or towards commercialization.
### 3.6.2 Development of strategies to increase the duty cycle of instruments operated at the NFs, to reduce breakdown and maintenance downtime

**FTIR**

For FTIR, it is not possible to have a backup instrument at the CREGARS, but some spare parts like a laser, optics components, and alignment tools... must be available for the ACTRIS beneficiaries. Especially the special optical filters that are required with each FTIR instrument must be available from the CREGARS that must care for grouped purchases. Similarly, the calibration cells must be provided by the CREGARS. Also spare mirrors for the sun tracker might be worthwhile to have at the CREGARS (although these are less standardised). The NFs should reimburse the CREGARS for parts that they obtain from the CREGARS, probably in-kind.

**UVVIS**

It is generally not possible to plan for backup instruments at all sites. However different strategies can be developed to reduce breakdown and maintenance downtime at remote or demanding sites. In particular, it is a common practice to equip remote Antarctic stations with backup instruments (or alternatively key spare parts). Another approach is to operate multi-channel instruments having overlaps in key spectral regions (e.g. the visible blue range where NO₂ is retrieved). This provides an inherent duplication of the measurements reducing the risks of interruption in case of instrumental breakdown affecting only one channel. For zenith-sky sites at Northern latitudes, repair is guaranteed within less than two months and full replacement possible within a six-month delay. The possibility to borrow a spare instrument during the repair period is also offered.

**O3 DIAL**

Considering the cost of O3 DIALs, it is not possible to have a backup instrument at the CREGARS. However optical and electronic parts that could serve as a back-up for NFs in case of breakdown must be available at the CREGARS.

### 3.6.3 Development of new technological products and methods

New measurement techniques, technological products and methods are developed by the Centre in collaboration with the NFs. Depending on the situation, the Centre 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 National Facilities.

**FTIR**

The TC must keep contact with the FTIR instruments manufacturers for a bi-directional exchange of information: new developments by the manufacturer will first be assessed at the TC, and vice-versa, the TC will pass on to the manufacturer demands from NF PIs for improvements that appear useful for the whole ACTRIS community. Instruments manufacturers will be invited by the TC to the annual workshops to discuss instrument evolution. Similarly technological developments proposed by individual NFs must be assessed by the CREGARS; if the development is validated, the CREGARS can suggest a path towards implementation at all NFs equipped with an FTIR instrument and/or towards commercialization.
The evolution of measurement guidelines and data analysis procedures must follow a similar path: initial
assessment at the CREGARS and –when the findings are positive – adoption of a path towards acceptance
by the CREGARS and implementation at all NFs equipped with an FTIR instrument.

The CREGARS – in collaboration with the ACTRIS community and ACTRIS users – can investigate the option
to include additional target species in the list of mandatory ACTRIS trace gases. If such option is agreed by
the ACTRIS member countries and the associated funding is secured, it shall develop the required
additional procedures and guidelines, and design a roadmap towards implementation at all NFs equipped
with an FTIR instrument.

UVVIS

It is anticipated that UVVIS instruments will continue to be further developed with the aim to improve their performance and reliability as well as the range of the retrievable data products. E.g. polarization channels might be added to current systems in order to increase their information content for aerosol retrieval. Such developments will typically take place within research organizations, in contact with the TC. The relevance of newly developed techniques must first be assessed at the TC before being transferred to operational instruments and/or central processing systems. The progress with respect to new instrumental developments must be reviewed during annual workshops in order to plan for appropriate and timely evolution of the baselines in place at the TC.

O3 DIAL

The TC must keep contact with laser source and electronic cards manufacturers in order to follow all
technical developments linked to the lidar technology. New developments are presently made concerning
the retrieval of geophysical parameters from the lidar signals using the Optimal Estimation Method (OEM).
The TC must follow these developments and if they are found suitable for ozone profile retrieval, a
standardized algorithm based on this method must be proposed to the users.

In all cases, new developments that are ‘validated’ at the CREGARS and for which a roadmap towards
implementation has been drawn, must first be proposed by the CREGARS to the ACTRIS RI Committee and
Council. Upon approval, the CREGARS must take all the necessary measures for its implementation.

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

When new Radiative Transfer developments or improved databases (e.g., for spectroscopic reference
data) or new insights in the measurement and data analysis processes become available in the remote
sensing community, the TC must contribute to the verification of the impacts of the new/improved
knowledge on the ACTRIS data products and upgrade the retrieval algorithms as appropriate, after
discussion and consensus agreement in the wider remote sensing community.

If such upgrades happen, the TC must support the NFs to implement these changes and provide updates
to the procedures and documentation including the didactic material to maintain compliancy with the
new developments. Development of level 3 products and exploitation of instrument synergies require
research activities that will be carried out in the context of dedicated R&D projects. As soon as a project
comes to a conclusion, and the product becomes mature for operational delivery, the CREGARS must
study how this product can be implemented in ACTRIS and what the associated cost would be. The results
The evolution of measurement guidelines and data analysis procedures must follow a similar path: initial
assessment at the CREGARS and –when the findings are positive – adoption of a path towards acceptance
by the CREGARS and implementation at all NFs equipped with an FTIR instrument.

The CREGARS – in collaboration with the ACTRIS community and ACTRIS users – can investigate the option
to include additional target species in the list of mandatory ACTRIS trace gases. If such option is agreed by
the ACTRIS member countries and the associated funding is secured, it shall develop the required
additional procedures and guidelines, and design a roadmap towards implementation at all NFs equipped
with an FTIR instrument.

UVVIS

It is anticipated that UVVIS instruments will continue to be further developed with the aim to improve their performance and reliability as well as the range of the retrievable data products. E.g. polarization channels might be added to current systems in order to increase their information content for aerosol retrieval. Such developments will typically take place within research organizations, in contact with the TC. The relevance of newly developed techniques must first be assessed at the TC before being transferred to operational instruments and/or central processing systems. The progress with respect to new instrumental developments must be reviewed during annual workshops in order to plan for appropriate and timely evolution of the baselines in place at the TC.

O3 DIAL

The TC must keep contact with laser source and electronic cards manufacturers in order to follow all
technical developments linked to the lidar technology. New developments are presently made concerning
the retrieval of geophysical parameters from the lidar signals using the Optimal Estimation Method (OEM).
The TC must follow these developments and if they are found suitable for ozone profile retrieval, a
standardized algorithm based on this method must be proposed to the users.

In all cases, new developments that are ‘validated’ at the CREGARS and for which a roadmap towards
implementation has been drawn, must first be proposed by the CREGARS to the ACTRIS RI Committee and
Council. Upon approval, the CREGARS must take all the necessary measures for its implementation.

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

When new Radiative Transfer developments or improved databases (e.g., for spectroscopic reference
data) or new insights in the measurement and data analysis processes become available in the remote
sensing community, the TC must contribute to the verification of the impacts of the new/improved
knowledge on the ACTRIS data products and upgrade the retrieval algorithms as appropriate, after
discussion and consensus agreement in the wider remote sensing community.

If such upgrades happen, the TC must support the NFs to implement these changes and provide updates
to the procedures and documentation including the didactic material to maintain compliancy with the
new developments. Development of level 3 products and exploitation of instrument synergies require
research activities that will be carried out in the context of dedicated R&D projects. As soon as a project
comes to a conclusion, and the product becomes mature for operational delivery, the CREGARS must
study how this product can be implemented in ACTRIS and what the associated cost would be. The results
of this study must then be presented to the ACTRIS RI Committee and Council where the decision to implement it will be taken.

## 4 Services provided to ACTRIS users

The services that CREGARS should provide to ACTRIS users is of the same nature as the operational support provided to ACTRIS NFs. In particular, ACTRIS users will have free access to ACTRIS data, and to the same digital tools and software and associated documentation that CREGARS provides to the ACTRIS NFs. For other types of service that require dedicated manpower from the CREGARS like training, QA/QC, site performance audits...) or services that imply delivery of hardware (spare parts...) the user shall submit a request to HO/SAMU. The request shall be evaluated taking into account the scientific justification and the capacity of the CREGARS.

In the case of the CREGARS, the expected users are:

* The members of the NDACC FTIR, UVVIS and O3 DIAL communities that comply with the ACTRIS requirements

* The candidate NDACC FTIR, UVVIS and O3 DIAL PIs that agree to comply with the ACTRIS requirements

* Any FTIR, UVVIS or O3 DIAL operator who is willing to comply with ACTRIS requirements.

The above users are essentially academic or public service users.

Other users from industry or the business world may request access to CREGARS services; their requests shall be evaluated by HO/SAMU the same way as for any other user.

Access is regulated by both the ACTRIS access policy and the ACTRIS data policy, respectively.

### 4.1 Estimation of the need

<table><thead><tr><th rowspan="3">Type of ACTRIS user</th><th colspan="4">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 rowspan="3">Academia</td><td>FTIR</td><td>0</td><td>14</td><td>33</td></tr><tr><td>UVVIS</td><td>0</td><td>30</td><td>40</td></tr><tr><td>O3 DIAL</td><td>7</td><td>7</td><td>7</td></tr><tr><td rowspan="3">Business</td><td></td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>...</td><td>...</td><td>...</td></tr><tr><td rowspan="3">Industry</td><td>FTIR</td><td>0</td><td>0</td><td>1</td></tr><tr><td>UVVIS</td><td>...</td><td>...</td><td>...</td></tr><tr><td>O3 DIAL</td><td>...</td><td>...</td><td>...</td></tr><tr><td rowspan="3">Public services</td><td></td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>...</td><td>...</td><td>...</td></tr></tbody></table>
of this study must then be presented to the ACTRIS RI Committee and Council where the decision to implement it will be taken.

## 4 Services provided to ACTRIS users

The services that CREGARS should provide to ACTRIS users is of the same nature as the operational support provided to ACTRIS NFs. In particular, ACTRIS users will have free access to ACTRIS data, and to the same digital tools and software and associated documentation that CREGARS provides to the ACTRIS NFs. For other types of service that require dedicated manpower from the CREGARS like training, QA/QC, site performance audits...) or services that imply delivery of hardware (spare parts...) the user shall submit a request to HO/SAMU. The request shall be evaluated taking into account the scientific justification and the capacity of the CREGARS.

In the case of the CREGARS, the expected users are:

* The members of the NDACC FTIR, UVVIS and O3 DIAL communities that comply with the ACTRIS requirements

* The candidate NDACC FTIR, UVVIS and O3 DIAL PIs that agree to comply with the ACTRIS requirements

* Any FTIR, UVVIS or O3 DIAL operator who is willing to comply with ACTRIS requirements.

The above users are essentially academic or public service users.

Other users from industry or the business world may request access to CREGARS services; their requests shall be evaluated by HO/SAMU the same way as for any other user.

Access is regulated by both the ACTRIS access policy and the ACTRIS data policy, respectively.

### 4.1 Estimation of the need

<table><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 rowspan="3">Academia</td><td>FTIR</td><td>0</td><td>14</td><td>33</td></tr><tr><td>UVVIS</td><td>0</td><td>30</td><td>40</td></tr><tr><td>O3 DIAL</td><td>7</td><td>7</td><td>7</td></tr><tr><td rowspan="3">Business</td><td></td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>...</td><td>...</td><td>...</td></tr><tr><td rowspan="3">Industry</td><td>FTIR</td><td>0</td><td>0</td><td>1</td></tr><tr><td>UVVIS</td><td>...</td><td>...</td><td>...</td></tr><tr><td>O3 DIAL</td><td>...</td><td>...</td><td>...</td></tr><tr><td rowspan="3">Public services</td><td></td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>...</td><td>...</td><td>...</td></tr><tr><td></td><td>...</td><td>...</td><td>...</td></tr></tbody></table>
The estimated minimum number of users of the CREGARS FTIR units is based on the current number of NDACC FTIR operated by non-European countries and the number of planned NDACC FTIR that are compliant with ACTRIS requirements. The maximum number takes into account the additional number of TCCON FTIR instruments operated worldwide. The manufacturer of the standard NDACC and TCCON FTIR instruments may also become a user.

The number of users of the CREGARS UVVIS units might potentially reach 30 to 40 by 2025 (see section 4).

In the USA a network of five tropospheric ozone lidars has been established in the recent years: they may become users of the CREGARS O3 DIAL unit.

## 4.2 Provision of measurement quality assurance and quality control procedures and tools

Generally speaking, the CREGARS will offer the same services to the ACTRIS users as the operational support offered to ACTRIS beneficiaries as described in the previous chapter, - at least to the academic/research users affiliated or candidate members of NDACC -, but these services will be offered upon request and will depend on the available resources. Moreover, the services will not necessarily be free of charge.

## 4.3 Instrument-specific calibration

All calibration tools and procedures described in the previous chapter will be available to ACTRIS users, upon request and as a function of the available resources. The participation to intercomparison campaigns and a site visit with a mobile standard instrument will also be offered to ACTRIS users, upon request and resources permitting. Moreover, the services will not necessarily be free of charge.

## 4.4 Knowledge transfer and operator training

Training workshops are open to all ACTRIS beneficiaries and users. Dedicated operator training and knowledge transfer will be accessible to ACTRIS users upon request, resources permitting.

Costs for participation or dedicated service may be charged to ACTRIS users (to be decided).

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

ACTRIS users requesting more advanced measurements and retrieval methods will have to justify their requests (research arguments). The CREGARS will evaluate the feasibility of the request, in terms of resources (manpower effort, costs), of technical feasibility, and of future perspectives, in consultation with the NFs that are involved. If the request turns out to be acceptable, the CREGARS together with the involved NFs will provide the service, but possibly charge for it.
The estimated minimum number of users of the CREGARS FTIR units is based on the current number of NDACC FTIR operated by non-European countries and the number of planned NDACC FTIR that are compliant with ACTRIS requirements. The maximum number takes into account the additional number of TCCON FTIR instruments operated worldwide. The manufacturer of the standard NDACC and TCCON FTIR instruments may also become a user.

The number of users of the CREGARS UVVIS units might potentially reach 30 to 40 by 2025 (see section 4).

In the USA a network of five tropospheric ozone lidars has been established in the recent years: they may become users of the CREGARS O3 DIAL unit.

## 4.2 Provision of measurement quality assurance and quality control procedures and tools

Generally speaking, the CREGARS will offer the same services to the ACTRIS users as the operational support offered to ACTRIS beneficiaries as described in the previous chapter, - at least to the academic/research users affiliated or candidate members of NDACC -, but these services will be offered upon request and will depend on the available resources. Moreover, the services will not necessarily be free of charge.

## 4.3 Instrument-specific calibration

All calibration tools and procedures described in the previous chapter will be available to ACTRIS users, upon request and as a function of the available resources. The participation to intercomparison campaigns and a site visit with a mobile standard instrument will also be offered to ACTRIS users, upon request and resources permitting. Moreover, the services will not necessarily be free of charge.

## 4.4 Knowledge transfer and operator training

Training workshops are open to all ACTRIS beneficiaries and users. Dedicated operator training and knowledge transfer will be accessible to ACTRIS users upon request, resources permitting.

Costs for participation or dedicated service may be charged to ACTRIS users (to be decided).

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

ACTRIS users requesting more advanced measurements and retrieval methods will have to justify their requests (research arguments). The CREGARS will evaluate the feasibility of the request, in terms of resources (manpower effort, costs), of technical feasibility, and of future perspectives, in consultation with the NFs that are involved. If the request turns out to be acceptable, the CREGARS together with the involved NFs will provide the service, but possibly charge for it.
## 5 Governance and management structure of the Centre for Reactive Trace Gases Remote Sensing

The Units of the Centre for Reactive Trace Gases Remote Sensing shall be organized according to the specific role of the TC, assuring that the TC 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 Remote Sensing 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 TC Director and managed by a TC Management Board, which consists of the TC Director and the TC 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 Remote Sensing.

### 6.1 General requirements

In order to be labelled as the Centre for Reactive Trace Gases Remote Sensing, 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 Remote Sensing

The Units of the Centre for Reactive Trace Gases Remote Sensing shall be organized according to the specific role of the TC, assuring that the TC 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 Remote Sensing 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 TC Director and managed by a TC Management Board, which consists of the TC Director and the TC 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 Remote Sensing.

## 6.1 General requirements

In order to be labelled as the Centre for Reactive Trace Gases Remote Sensing, 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 CF should have available:

I. For the **FTIR** component of the CF:

I.1. a standard certified ACTRIS FTIR set-up (FTIR instrument coupled to a solar tracker and equipped with the required peripherals (Liq. N2 filling system, dry air flushing,) that complies with the ACTRIS requirements, that is operational and that can be used to perform some tests and hands-on training. This implies that some optical test equipment and expertise in FTIR spectrometry are also available, and that the instrument is not committed to continuous operations as part of a NF;

I.2. the equipment to align the FTIR instrument according to the standard alignment procedures that are mandatory in ACTRIS;

I.3. the equipment and resources to test and deliver calibrated cells to the NFs as well as the equipment and resources to re-calibrate these cells on a regular basis to a well-calibrated N₂O gas cell;

I.4. The required IT infrastructure to develop and verify upgrades of the standard ACTRIS FTIR retrieval software, retrieval strategies and associated tools, like Linefit, GEOMS HDF data formatting tools, data visualization tools, etc.;

II. For the **UVVIS** component of the CF:

II.1. Standard certified reference UVVIS instruments for MAX-DOAS and zenith-sky instrument types

II.2. A Calibration Centre equipped with all necessary light sources and optical elements necessary to characterize UVVIS instruments, for ISRF, wavelength registration, stray-light, polarization response, field-of-view, elevation scanner/sun-tracker accuracy

II.3. Delocalized central processing units dedicated to MAX-DOAS and zenith-sky data processing with Rapid-Delivery capabilities

II.4. The required IT infrastructure and IT and scientific expert skills to develop and verify upgrades of the standard ACTRIS UVVIS retrieval codes, retrieval strategies and associated tools, data formatting tools, data visualization tools, etc.;

II.5. The required expertise to develop, verify, upgrade and maintain retrieval strategies for the current and potential future ACTRIS target species

II.6. A facility allowing for the intercalibration of a large number of UVVIS instruments (>35). This facility should be equipped with all relevant ancillary measurements (sun photometer, in-situ monitors, ceilometer, aerosol extinction profile Raman lidar, and meteorological parameters). Moreover, a wide range of viewing angles needs to be accessible for multiple instruments at the same time. Ideally, full hemispherical view and a view of the horizon at low elevation angles need to be accessible.
## 6.2 Technical requirements

### 6.2.1 Facilities

The CF should have available:

I. For the **FTIR** component of the CF:

I.1. a standard certified ACTRIS FTIR set-up (FTIR instrument coupled to a solar tracker and equipped with the required peripherals (Liq. N2 filling system, dry air flushing,) that complies with the ACTRIS requirements, that is operational and that can be used to perform some tests and hands-on training. This implies that some optical test equipment and expertise in FTIR spectrometry are also available, and that the instrument is not committed to continuous operations as part of a NF;

I.2. the equipment to align the FTIR instrument according to the standard alignment procedures that are mandatory in ACTRIS;

I.3. the equipment and resources to test and deliver calibrated cells to the NFs as well as the equipment and resources to re-calibrate these cells on a regular basis to a well-calibrated N₂O gas cell;

I.4. The required IT infrastructure to develop and verify upgrades of the standard ACTRIS FTIR retrieval software, retrieval strategies and associated tools, like Linefit, GEOMS HDF data formatting tools, data visualization tools, etc.;

II. For the **UVVIS** component of the CF:

II.1. Standard certified reference UVVIS instruments for MAX-DOAS and zenith-sky instrument types

II.2. A Calibration Centre equipped with all necessary light sources and optical elements necessary to characterize UVVIS instruments, for ISRF, wavelength registration, stray-light, polarization response, field-of-view, elevation scanner/sun-tracker accuracy

II.3. Delocalized central processing units dedicated to MAX-DOAS and zenith-sky data processing with Rapid-Delivery capabilities

II.4. The required IT infrastructure and IT and scientific expert skills to develop and verify upgrades of the standard ACTRIS UVVIS retrieval codes, retrieval strategies and associated tools, data formatting tools, data visualization tools, etc.;

II.5. The required expertise to develop, verify, upgrade and maintain retrieval strategies for the current and potential future ACTRIS target species

II.6. A facility allowing for the intercalibration of a large number of UVVIS instruments (>35). This facility should be equipped with all relevant ancillary measurements (sun photometer, in-situ monitors, ceilometer, aerosol extinction profile Raman lidar, and meteorological parameters). Moreover, a wide range of viewing angles needs to be accessible for multiple instruments at the same time. Ideally, full hemispherical view and a view of the horizon at low elevation angles need to be accessible.
I. For the **O3 DIAL** component of the CF:

III.1. Technical skills to assist DIAL O3 users in the set-up and operation of the lidar (e.g. alignment procedures).

III.2. The equipment and resources to assist NFs in case of lidar breakdown.

III.3. The required IT infrastructure to develop and verify upgrades of the standard ACTRIS lidar retrieval software, retrieval strategies and associated tools, GEOMS HDF data formatting tools, data visualization tools, etc.

## 6.2.2 Human resources

### Skills, expertise

The CREGARS personnel should have the IT, scientific, technical and managerial expert skills required to

* Verify and maintain the instruments' performances

* perform the measurements, data analysis and data submission to the Data Centre

* develop, verify, upgrade and maintain retrieval strategies and spectroscopic reference data (linelists, cross-sections) for the current and potential future ACTRIS target species;

* assure QA/QC of the measurements, the data analysis and the data products delivered by the NFs;

* maintain documentation

* interface to the NFs, to the Data Centre and the Head Office, and maintain appropriate communication channels

* organize trainings, workshops, ...

* manage the financial aspects of the CREGARS

The director of the CREGARS shall coordinate the Units that constitute the TC.

### Manpower (in FTE):

In the operational phase:

<table><thead><tr><th></th><th></th><th>FTIR</th><th>UVVIS</th><th>LIDAR</th><th>TOTAL</th></tr></thead><tbody><tr><td rowspan="3">Management / Administration</td><td>L1 Expert Manager</td><td>FTE 0,3</td><td>0,55</td><td>0,05</td><td>0,9</td></tr><tr><td>L2 Qualified Officer</td><td>FTE 0,1</td><td>0,5</td><td>0</td><td>0,6</td></tr><tr><td>L3 Administrative assistant</td><td>FTE 0,3</td><td>0,2</td><td>0,05</td><td>0,55</td></tr><tr><td rowspan="3">Scientific / Technical</td><td>L1 Expert Scientist</td><td>FTE 1,8</td><td>1,4</td><td>0</td><td>3,2</td></tr><tr><td>L2 Qualified Operator</td><td>FTE 0,1</td><td>2,5</td><td>0,2</td><td>2,8</td></tr><tr><td>L3 Technician</td><td>FTE 0,75</td><td>1,3</td><td>0</td><td>2,05</td></tr></tbody></table>
I. For the **O3 DIAL** component of the CF:

III.1. Technical skills to assist DIAL O3 users in the set-up and operation of the lidar (e.g. alignment procedures).

III.2. The equipment and resources to assist NFs in case of lidar breakdown.

III.3. The required IT infrastructure to develop and verify upgrades of the standard ACTRIS lidar retrieval software, retrieval strategies and associated tools, GEOMS HDF data formatting tools, data visualization tools, etc.

## 6.2.2 Human resources

### Skills, expertise

The CREGARS personnel should have the IT, scientific, technical and managerial expert skills required to

* Verify and maintain the instruments' performances

* perform the measurements, data analysis and data submission to the Data Centre

* develop, verify, upgrade and maintain retrieval strategies and spectroscopic reference data (linelists, cross-sections) for the current and potential future ACTRIS target species;

* assure QA/QC of the measurements, the data analysis and the data products delivered by the NFs;

* maintain documentation

* interface to the NFs, to the Data Centre and the Head Office, and maintain appropriate communication channels

* organize trainings, workshops, ...

* manage the financial aspects of the CREGARS

The director of the CREGARS shall coordinate the Units that constitute the TC.

### Manpower (in FTE):

In the operational phase:

<table><thead><tr><th></th><th></th><th>FTIR</th><th>UVVIS</th><th>LIDAR</th><th>TOTAL</th></tr></thead><tbody><tr><td rowspan="3">Management / Administration</td><td>L1 Expert Manager</td><td>FTE 0,3</td><td>0,55</td><td>0,05</td><td>0,9</td></tr><tr><td>L2 Qualified Officer</td><td>FTE 0,1</td><td>0,5</td><td>0</td><td>0,6</td></tr><tr><td>L3 Administrative assistant</td><td>FTE 0,3</td><td>0,2</td><td>0,05</td><td>0,55</td></tr><tr><td rowspan="3">Scientific / Technical</td><td>L1 Expert Scientist</td><td>FTE 1,8</td><td>1,4</td><td>0</td><td>3,2</td></tr><tr><td>L2 Qualified Operator</td><td>FTE 0,1</td><td>2,5</td><td>0,2</td><td>2,8</td></tr><tr><td>L3 Technician</td><td>FTE 0,75</td><td>1,3</td><td>0</td><td>2,05</td></tr></tbody></table>
6.2.3 Other requirements

1. The CREGARS should have access to at least one conference facility for organizing the training workshops.

2. Upon request by an NF operating an FTIR instrument, the CREGARS may offer to do the FTIR data processing at the CREGARS; it should therefore have this capability for at least a few NFs that do not have the resources themselves.

3. The CREGARS should have good contact with experts in laboratory spectroscopy – in order to be informed of the latest improvements to the reference spectroscopic data and/or to be able to possibly request some dedicated laboratory spectroscopy experiments or evaluations.

7 Basic criteria for the selection of the Centre for Reactive Trace Gases Remote Sensing

The applications to host the Centre for Reactive Trace Gases Remote Sensing 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 must be detailed in the associated call documents.
6.2.3 Other requirements

1. The CREGARS should have access to at least one conference facility for organizing the training workshops.

2. Upon request by an NF operating an FTIR instrument, the CREGARS may offer to do the FTIR data processing at the CREGARS; it should therefore have this capability for at least a few NFs that do not have the resources themselves.

3. The CREGARS should have good contact with experts in laboratory spectroscopy – in order to be informed of the latest improvements to the reference spectroscopic data and/or to be able to possibly request some dedicated laboratory spectroscopy experiments or evaluations.

7 Basic criteria for the selection of the Centre for Reactive Trace Gases Remote Sensing

The applications to host the Centre for Reactive Trace Gases Remote Sensing 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 must be detailed in the associated call documents.
# 8 Obligations of the Centre for Reactive Trace Gases Remote Sensing

## 8.1 General obligations

The following general obligations shall apply to the Centre for Reactive Trace Gases Remote Sensing 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.

## 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 Remote Sensing 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 Remote Sensing 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.

**Guidelines, quality assurance criteria and procedures**

The Centre for Reactive Trace Gases Remote Sensing is responsible for the quality assurance procedures and guidelines of the ACTRIS measurements conducted at the NFs operating FTIR, UVVIS or O3 DIAL instruments. For this, the TC should implement the following operation support for each of the Measurement technique under its topics:

* Definition and establishment of standard operation procedures (as described in sections 3.4.1.1 for FTIR, 3.4.1.2 for UVVIS and 3.4.1.3 for O3 DIAL);

* Definition of measurement quality-assurance criteria and procedures (as described in sections 3.4.2.1 for FTIR, 3.4.2.2 for UVVIS and 3.4.2.3 for O3 DIAL)
# 8 Obligations of the Centre for Reactive Trace Gases Remote Sensing

## 8.1 General obligations

The following general obligations shall apply to the Centre for Reactive Trace Gases Remote Sensing 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.

## 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 Remote Sensing 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 Remote Sensing 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.

**Guidelines, quality assurance criteria and procedures**

The Centre for Reactive Trace Gases Remote Sensing is responsible for the quality assurance procedures and guidelines of the ACTRIS measurements conducted at the NFs operating FTIR, UVVIS or O3 DIAL instruments. For this, the TC should implement the following operation support for each of the Measurement technique under its topics:

* Definition and establishment of standard operation procedures (as described in sections 3.4.1.1 for FTIR, 3.4.1.2 for UVVIS and 3.4.1.3 for O3 DIAL);

* Definition of measurement quality-assurance criteria and procedures (as described in sections 3.4.2.1 for FTIR, 3.4.2.2 for UVVIS and 3.4.2.3 for O3 DIAL)
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.

## Tools, tests and consultancy for quality control of the measurements

It is the duty of the Centre for Reactive Trace Gases Remote Sensing to assist the related NFs in the quality control of their measurements by providing the following operation support for each of the measurement techniques under its topics:

* Development and provision of instrument-specific calibration (as described in sections 3.4.3.1 for FTIR, 3.4.3.2 for UVVIS and 3.4.3.3 for O3 DIAL);

* Development and provision of in-house check-up tools (as described in sections 3.4.4.1 for FTIR, 3.4.4.2 for UVVIS and 3.4.4.3 for O3 DIAL);

* Development of data evaluation procedures and plausibility test (as described in sections 3.4.5.1 for FTIR, 3.4.5.2 for UVVIS and 3.4.5.3 for O3 DIAL);

* Consultancy for setting-up new observational or exploratory platforms and new instruments at the NFs (as described in section 3.5.2 for FTIR, UVVIS and O3 DIAL);

* Testing of new measurement instruments and procedures (as described in section 3.6.1 for FTIR, UVVIS and O3 DIAL);

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.

## Assessment of performances, measurement flagging

The Centre for Reactive Trace Gases Remote Sensing 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 3.4.6.1 for FTIR, 3.4.6.2 for UVVIS and 3.4.6.3 for O3 DIAL);

* Organizing regular exercises to assess the performances of the NFs, including instrument performance workshops (as described in sections 3.4.7.1 for FTIR, 3.4.7.2 for UVVIS and 3.4.7.3 for O3 DIAL);

* Contributing to documentation and traceability of level 0 to level 3 data products (as described in section 3.4.8.1 for FTIR, 3.4.8.2 for UVVIS and 3.4.8.3 for O3 DIAL);

The activities involving directly the NFs operating FTIR, UVVIS or O3 DIAL instruments 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 Remote Sensing and agreed together with the ACTRIS Scientific Advisory Board (SAB).

## Transfer of knowledge and training

In addition to the training organized for the implementation of guidelines, procedures, and tools developed in support of the quality control of the measurements, the Centre for Reactive Trace Gases Remote Sensing should organize training sessions with the NFs operating FTIR, UVVIS and O3 DIAL instruments as needed (see sections 3.5.1).
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.

## Tools, tests and consultancy for quality control of the measurements

It is the duty of the Centre for Reactive Trace Gases Remote Sensing to assist the related NFs in the quality control of their measurements by providing the following operation support for each of the measurement techniques under its topics:

* Development and provision of instrument-specific calibration (as described in sections 3.4.3.1 for FTIR, 3.4.3.2 for UVVIS and 3.4.3.3 for O3 DIAL);

* Development and provision of in-house check-up tools (as described in sections 3.4.4.1 for FTIR, 3.4.4.2 for UVVIS and 3.4.4.3 for O3 DIAL);

* Development of data evaluation procedures and plausibility test (as described in sections 3.4.5.1 for FTIR, 3.4.5.2 for UVVIS and 3.4.5.3 for O3 DIAL);

* Consultancy for setting-up new observational or exploratory platforms and new instruments at the NFs (as described in section 3.5.2 for FTIR, UVVIS and O3 DIAL);

* Testing of new measurement instruments and procedures (as described in section 3.6.1 for FTIR, UVVIS and O3 DIAL);

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.

## Assessment of performances, measurement flagging

The Centre for Reactive Trace Gases Remote Sensing 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 3.4.6.1 for FTIR, 3.4.6.2 for UVVIS and 3.4.6.3 for O3 DIAL);

* Organizing regular exercises to assess the performances of the NFs, including instrument performance workshops (as described in sections 3.4.7.1 for FTIR, 3.4.7.2 for UVVIS and 3.4.7.3 for O3 DIAL);

* Contributing to documentation and traceability of level 0 to level 3 data products (as described in section 3.4.8.1 for FTIR, 3.4.8.2 for UVVIS and 3.4.8.3 for O3 DIAL);

The activities involving directly the NFs operating FTIR, UVVIS or O3 DIAL instruments 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 Remote Sensing and agreed together with the ACTRIS Scientific Advisory Board (SAB).

## Transfer of knowledge and training

In addition to the training organized for the implementation of guidelines, procedures, and tools developed in support of the quality control of the measurements, the Centre for Reactive Trace Gases Remote Sensing should organize training sessions with the NFs operating FTIR, UVVIS and O3 DIAL instruments as needed (see sections 3.5.1).
Improvement of measurement methodologies for Reactive Trace Gases Remote Sensing

Although it is not an obligation, the Centre for Reactive Trace Gases Remote Sensing should work to
sustain a high level of performance and to stimulate the advancement of new techniques and
methodologies in the field of remote sensing of reactive trace gases, 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 3.6.2.1 for FTIR, 3.6.2.2 for UVVIS and 3.6.2.3 for O3 DIAL)

* Development of new technological products and methods (as described in section 3.6.3.1 for FTIR, 3.6.3.2 for UVVIS and 3.6.3.3 for O3 DIAL);

* Development of retrieval algorithms or tools for producing level 1 to 3 data, including the exploitation of instrument synergies (as described in section 3.6.4 for FTIR, UVVIS and O3 DIAL)

* 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 3.4.9.1 for FTIR, 3.4.9.2 for UVVIS and 3.4.9.3 for O3 DIAL);

8.2.2 Technical obligations in relation with the ACTRIS Data Centre

I. For the FTIR component:

* The data analysis algorithms are maintained by the Centre for Reactive Trace Gases Remote Sensing where the expertise is located.

* The data processing is done at the NFs, because it are the NF operators who know best the housekeeping data, and who know best how to handle their measurement data (formats, ancillary information, etc.). Moreover, in this way, the NF keeps full responsibility for its retrieved L2 data.

However, NFs that prefer that the L1 to L2 data processing is done at the TC can request so, and the TC can offer this if it has the required capacity; there is no obligation for the TC to offer such processing capability.

* A site specific QA/QC of level 2 data is performed at every NF before they are transferred to the TC.

* In an additional QA/QC step the consistency of the measurement data and of the retrieved data is done at the Centre for Reactive Trace Gases Remote Sensing, to ensure compliance with the instrument SOP and harmonised retrieval strategies and to ensure intra-network consistency.

* The QA/QC of the data files before submission to the ACTRIS Data Centre is done at the Centre for Reactive Trace Gases Remote Sensing because it requires specific FTIR expertise. Therefore the L2 data will pass from the NFs to the TC for quality control, and then from the TC to the Data Centre. Once the QA/QC procedures of the data files to be submitted are (fully) automated, they could be transferred to the DC, as long as the required expertise is available at the DC.
Improvement of measurement methodologies for Reactive Trace Gases Remote Sensing

Although it is not an obligation, the Centre for Reactive Trace Gases Remote Sensing should work to
sustain a high level of performance and to stimulate the advancement of new techniques and
methodologies in the field of remote sensing of reactive trace gases, 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 3.6.2.1 for FTIR, 3.6.2.2 for UVVIS and 3.6.2.3 for O3 DIAL)

* Development of new technological products and methods (as described in section 3.6.3.1 for FTIR, 3.6.3.2 for UVVIS and 3.6.3.3 for O3 DIAL);

* Development of retrieval algorithms or tools for producing level 1 to 3 data, including the exploitation of instrument synergies (as described in section 3.6.4 for FTIR, UVVIS and O3 DIAL)

* 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 3.4.9.1 for FTIR, 3.4.9.2 for UVVIS and 3.4.9.3 for O3 DIAL);

8.2.2 Technical obligations in relation with the ACTRIS Data Centre

I. For the FTIR component:

* The data analysis algorithms are maintained by the Centre for Reactive Trace Gases Remote Sensing where the expertise is located.

* The data processing is done at the NFs, because it are the NF operators who know best the housekeeping data, and who know best how to handle their measurement data (formats, ancillary information, etc.). Moreover, in this way, the NF keeps full responsibility for its retrieved L2 data.

However, NFs that prefer that the L1 to L2 data processing is done at the TC can request so, and the TC can offer this if it has the required capacity; there is no obligation for the TC to offer such processing capability.

* A site specific QA/QC of level 2 data is performed at every NF before they are transferred to the TC.

* In an additional QA/QC step the consistency of the measurement data and of the retrieved data is done at the Centre for Reactive Trace Gases Remote Sensing, to ensure compliance with the instrument SOP and harmonised retrieval strategies and to ensure intra-network consistency.

* The QA/QC of the data files before submission to the ACTRIS Data Centre is done at the Centre for Reactive Trace Gases Remote Sensing because it requires specific FTIR expertise. Therefore the L2 data will pass from the NFs to the TC for quality control, and then from the TC to the Data Centre. Once the QA/QC procedures of the data files to be submitted are (fully) automated, they could be transferred to the DC, as long as the required expertise is available at the DC.
II. For the UVVIS component:

* The data analysis algorithms and the corresponding processing software codes are developed and maintained by the CREGARS in consultation with the UVVIS scientific community.

* The retrieval algorithms and processing tools selected for operational implementation are built as community tools using open-source software freely available to the scientific community.

* The data processing is performed in the UVVIS central processing units of the CF. PIs are responsible for the operation, calibration, maintenance and performance monitoring of their instrument(s) according to SOPs defined by the CF. They are also responsible for the timely delivery of Level-1 data sets (i.e. wavelength calibrated spectral radiance spectra) are to the central processing units, using a standardized file format including all necessary metadata and ancillary data. Although the level-1 to -2 processing is performed within the CF, the PI keeps full responsibility and ownership for the delivered data. Moreover, it has full authority to stop or delay the processing at any time upon appropriateness. The level-1 data sets remain internal to the central processing units and are not further redistributed unless explicit consent to do so is expressed by the PI.

* The basic QA/QC of the level-1 data set is under the responsibility of the PIs.

* Additional QA/QC is performed at the CREGARS, to ensure compliance with the SOPs, identify possible anomalies with respect to expected instrument performance and to ensure intra-network consistency.

* The final QA/QC of the data files before submission to the ACTRIS Data Centre is done as part of the central processing units. It is however envisaged that once the procedures will be fully validated and automated, they can be transferred to the DC.

III. For the LIDAR component:

* The data analysis algorithms are developed and maintained by the CREGARS where the expertise is located.

* The data processing is done at the NFs, by the NF operators, because it are the NF operators who know best the housekeeping data, and who know best how to handle their measurement data (formats, ancillary information, etc.). Moreover, in this way, the NF keeps full responsibility for his retrieved data.

* The QA/QC of the measurement data and of the retrieved data is done at the CREGARS, to ensure compliance with the instrument SOP and harmonised retrieval strategies and to ensure intra-network consistency.

* The QA/QC of the data files before submission to the ACTRIS Data Centre is done at the CREGARS because it requires specific Lidar expertise. Once these QA/QC procedures are (fully) automated, they could be transferred to the DC, as long as the required expertise is available at the DC.
II. For the UVVIS component:

* The data analysis algorithms and the corresponding processing software codes are developed and maintained by the CREGARS in consultation with the UVVIS scientific community.

* The retrieval algorithms and processing tools selected for operational implementation are built as community tools using open-source software freely available to the scientific community.

* The data processing is performed in the UVVIS central processing units of the CF. PIs are responsible for the operation, calibration, maintenance and performance monitoring of their instrument(s) according to SOPs defined by the CF. They are also responsible for the timely delivery of Level-1 data sets (i.e. wavelength calibrated spectral radiance spectra) are to the central processing units, using a standardized file format including all necessary metadata and ancillary data. Although the level-1 to -2 processing is performed within the CF, the PI keeps full responsibility and ownership for the delivered data. Moreover, it has full authority to stop or delay the processing at any time upon appropriateness. The level-1 data sets remain internal to the central processing units and are not further redistributed unless explicit consent to do so is expressed by the PI.

* The basic QA/QC of the level-1 data set is under the responsibility of the PIs.

* Additional QA/QC is performed at the CREGARS, to ensure compliance with the SOPs, identify possible anomalies with respect to expected instrument performance and to ensure intra-network consistency.

* The final QA/QC of the data files before submission to the ACTRIS Data Centre is done as part of the central processing units. It is however envisaged that once the procedures will be fully validated and automated, they can be transferred to the DC.

III. For the LIDAR component:

* The data analysis algorithms are developed and maintained by the CREGARS where the expertise is located.

* The data processing is done at the NFs, by the NF operators, because it are the NF operators who know best the housekeeping data, and who know best how to handle their measurement data (formats, ancillary information, etc.). Moreover, in this way, the NF keeps full responsibility for his retrieved data.

* The QA/QC of the measurement data and of the retrieved data is done at the CREGARS, to ensure compliance with the instrument SOP and harmonised retrieval strategies and to ensure intra-network consistency.

* The QA/QC of the data files before submission to the ACTRIS Data Centre is done at the CREGARS because it requires specific Lidar expertise. Once these QA/QC procedures are (fully) automated, they could be transferred to the DC, as long as the required expertise is available at the DC.
### 8.2.3 Technical obligations in relation with other ACTRIS Topical Centres

I. In its attempts to improve the traceability of the remotely-sensed reactive trace gas products to community standards (e.g., the WMO standards), the CREGARS shall collaborate with the Reactive Trace Gases in-situ Centre, in order to use the same standards.

II. Concerning the traceability and quality assessment of the aerosol products of the UVVIS and O3 DIAL components of the reactive trace gases remote sensing component of ACTRIS, the CREGARS shall collaborate with the aerosol in-situ and aerosol remote sensing topical centres.

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

The Centre for Reactive Trace Gases Remote Sensing should commit to provide a minimum amount of user services as described in section 4.

As stated in Section 4, the CREGARS will provide the same services to the ACTRIS users than to the ACTRIS beneficiaries, but the services must be provided upon request with a different time schedule, and the charged costs may be different. The CREGARS must keep the SAMU and the Head Office informed about the services that it has provided.

## 8.3 Evaluation of the activity of the Centre for Reactive Trace Gases Remote Sensing

Once established and operational, the Centre for Reactive Trace Gases Remote Sensing will be annually evaluated for its performances, against the following KPIs:

<table><thead><tr><th>No.</th><th>Criteria</th><th>Indicators</th><th>Score</th><th>Weight</th></tr></thead><tbody><tr><td rowspan="4">1</td><td rowspan="4">Usefulness for ACTRIS NFs</td><td>No. of operational support units provided to ACTRIS NFs</td><td></td><td rowspan="4">35%</td></tr><tr><td>Number of innovative updates to SOPs, documentation, QA/QC procedures, ...</td><td></td></tr><tr><td>Number of publications from the ACTRIS beneficiaries</td><td></td></tr><tr><td><strong>Average of Usefulness for ACTRIS</strong></td><td></td></tr><tr><td rowspan="4">2</td><td rowspan="4">Usefulness for ACTRIS users</td><td>No. of service units (calibration, training, data QA/QC, ...)</td><td></td><td rowspan="4">25%</td></tr><tr><td>Number of ACTRIS users involved in intercomparison campaigns</td><td></td></tr><tr><td>Number of users involved in workshops</td><td></td></tr><tr><td>Number of RTGRS data units accessed at the DC</td><td></td></tr><tr><td rowspan="5">3</td><td rowspan="5">Impact on science & technology</td><td><strong>Average of Usefulness for external users</strong></td><td></td><td rowspan="5">25%</td></tr><tr><td>No. of peer-review papers that make use of ACTRIS data</td><td></td></tr><tr><td>Technological progress</td><td></td></tr><tr><td>Scientific progress (e.g., new target species added, enhanced accuracy/precision of a target data product, ...)</td><td></td></tr><tr><td>Increase in size of the user community</td><td></td></tr><tr><td rowspan="4">4</td><td rowspan="4">Integration into ACTRI-RI</td><td><strong>Average of S&T Impact</strong></td><td></td><td rowspan="4">15%</td></tr><tr><td>Level of collaboration with other nodes and ACTRIS structures (NFs, TCs, DC, SAMU, ...)</td><td></td></tr><tr><td>Quality and readiness of the reports</td><td></td></tr><tr><td><strong>Average of Integration into ACTRI-RI</strong></td><td></td></tr><tr><td colspan="4"><strong>Total score</strong></td><td></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
### 8.2.3 Technical obligations in relation with other ACTRIS Topical Centres

I. In its attempts to improve the traceability of the remotely-sensed reactive trace gas products to community standards (e.g., the WMO standards), the CREGARS shall collaborate with the Reactive Trace Gases in-situ Centre, in order to use the same standards.

II. Concerning the traceability and quality assessment of the aerosol products of the UVVIS and O3 DIAL components of the reactive trace gases remote sensing component of ACTRIS, the CREGARS shall collaborate with the aerosol in-situ and aerosol remote sensing topical centres.

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

The Centre for Reactive Trace Gases Remote Sensing should commit to provide a minimum amount of user services as described in section 4.

As stated in Section 4, the CREGARS will provide the same services to the ACTRIS users than to the ACTRIS beneficiaries, but the services must be provided upon request with a different time schedule, and the charged costs may be different. The CREGARS must keep the SAMU and the Head Office informed about the services that it has provided.

## 8.3 Evaluation of the activity of the Centre for Reactive Trace Gases Remote Sensing

Once established and operational, the Centre for Reactive Trace Gases Remote Sensing will be annually evaluated for its performances, against the following KPIs:

<table><thead><tr><th>No.</th><th>Criteria</th><th>Indicators</th><th>Score</th><th>Weight</th></tr></thead><tbody><tr><td rowspan="4">1</td><td rowspan="4">Usefulness for ACTRIS NFs</td><td>No. of operational support units provided to ACTRIS NFs</td><td></td><td rowspan="4">35%</td></tr><tr><td>Number of innovative updates to SOPs, documentation, QA/QC procedures, ...</td><td></td></tr><tr><td>Number of publications from the ACTRIS beneficiaries</td><td></td></tr><tr><td><strong>Average of Usefulness for ACTRIS</strong></td><td></td></tr><tr><td rowspan="4">2</td><td rowspan="4">Usefulness for ACTRIS users</td><td>No. of service units (calibration, training, data QA/QC, ...)</td><td></td><td rowspan="4">25%</td></tr><tr><td>Number of ACTRIS users involved in intercomparison campaigns</td><td></td></tr><tr><td>Number of users involved in workshops</td><td></td></tr><tr><td>Number of RTGRS data units accessed at the DC</td><td></td></tr><tr><td rowspan="5">3</td><td rowspan="5">Impact on science & technology</td><td><strong>Average of Usefulness for external users</strong></td><td></td><td rowspan="5">25%</td></tr><tr><td>No. of peer-review papers that make use of ACTRIS data</td><td></td></tr><tr><td>Technological progress</td><td></td></tr><tr><td>Scientific progress (e.g., new target species added, enhanced accuracy/precision of a target data product, ...)</td><td></td></tr><tr><td>Increase in size of the user community</td><td></td></tr><tr><td rowspan="4">4</td><td rowspan="4">Integration into ACTRI-RI</td><td><strong>Average of S&T Impact</strong></td><td></td><td rowspan="4">15%</td></tr><tr><td>Level of collaboration with other nodes and ACTRIS structures (NFs, TCs, DC, SAMU, ...)</td><td></td></tr><tr><td>Quality and readiness of the reports</td><td></td></tr><tr><td><strong>Average of Integration into ACTRI-RI</strong></td><td></td></tr><tr><td colspan="4"><strong>Total score</strong></td><td></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
9 List of acronyms

AOD – Aerosol Optical Depth

CF – Central Facility

CREGARS – Centre for reactive Trace Gases Remote Sensing

DIAL – Differential Absorption LIDAR

DOAS – Differential Optical Absorption Spectroscopy

FTIR – Fourier Transform Infrared Spectrometer

ISRF - Instrumental Spectral Response Function

mFCT - mobile Field Calibration Tool

MAXDOAS – Multi-Axis Differential Optical Absorption Spectroscopy

NF – National Facility

NRT – Near Real Time

QA – Quality Assurance

QC – Quality Control

SAB – Scientific Advisory Board

SOP – Standard Operation Procedure

TC – Topical Centre

UVVIS – UV-visible

UV - Ultraviolet

ZS – Zenith-Sky
9 List of acronyms

AOD – Aerosol Optical Depth

CF – Central Facility

CREGARS – Centre for reactive Trace Gases Remote Sensing

DIAL – Differential Absorption LIDAR

DOAS – Differential Optical Absorption Spectroscopy

FTIR – Fourier Transform Infrared Spectrometer

ISRF - Instrumental Spectral Response Function

mFCT - mobile Field Calibration Tool

MAXDOAS – Multi-Axis Differential Optical Absorption Spectroscopy

NF – National Facility

NRT – Near Real Time

QA – Quality Assurance

QC – Quality Control

SAB – Scientific Advisory Board

SOP – Standard Operation Procedure

TC – Topical Centre

UVVIS – UV-visible

UV - Ultraviolet

ZS – Zenith-Sky
## 10 Glossary

**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.

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

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

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

* **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.

* **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.

**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)
## 10 Glossary

**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.

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

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

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

* **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.

* **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.

**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](#).

**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 (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.

**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:

* **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.),

* **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.

<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 variables** - the measured atmospheric variables as described in the [ACTRIS Data Management Plan](#).

**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 (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.

**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:

* **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.),

* **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.

<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>
# 11 Reference documents

Please include here the complete list of documents referenced in the text.

- ACTRIS-PPP proposal

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

- ACTRIS Concept Documents

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

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

- Baseline document for the Concepts of ACTRIS Central Facilities

- Technical concepts and requirements for ACTRIS Observational Platforms

- Technical concepts and requirements for ACTRIS Exploratory Platforms
# 11 Reference documents

Please include here the complete list of documents referenced in the text.

- ACTRIS-PPP proposal

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

- ACTRIS Concept Documents

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

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

- Baseline document for the Concepts of ACTRIS Central Facilities

- Technical concepts and requirements for ACTRIS Observational Platforms

- Technical concepts and requirements for ACTRIS Exploratory Platforms
# Annex: Provision of the operation support

## Scheduled support

The CREGARS must have a repository where guidelines, training support, example data, software, input files for running the software, spectroscopic reference data, etc. are available to all ACTRIS beneficiaries and users.

<table><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">Provision of measurement guidelines and software for ILS characterization</td><td rowspan="5">Provision of retrieval strategies and software</td><td>Provision of Linefit software</td><td>With any update</td><td>Available from the CREGARS repository</td></tr><tr><td>Provision of outline of the retrieval strategy per target species</td><td>Upon every update</td><td>Available from the CREGARS repository</td></tr><tr><td>Provision of the standard retrieval S/W</td><td>Upon every update</td><td>Available from the CREGARS repository</td></tr><tr><td>Provision of retrieval control files and reference spectroscopic data</td><td>Upon every update</td><td>Available from the CREGARS repository</td></tr><tr><td>QA/QC</td><td>QA/QC of instrument setup and measurements (operations)</td><td>Every 2 year and with any major upgrade of measurement guidelines or Linefit</td><td>Sample spectra and linefit results must be requested from each NF</td></tr><tr><td rowspan="2">Provision of retrieval strategies and software</td><td>QA/QC of data analysis</td><td>Every 2 year and with any major upgrade of retrieval strategies or S/W</td><td>Sample data (L2) must be requested from each NF</td></tr><tr><td>QA/QC of data products before submission to DC</td><td>Upon every delivery (at least monthly)</td><td>This process must be automated as much as possible and</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
# Annex: Provision of the operation support

## Scheduled support

The CREGARS must have a repository where guidelines, training support, example data, software, input files for running the software, spectroscopic reference data, etc. are available to all ACTRIS beneficiaries and users.

<table><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></td><td>Provision of measurement guidelines and software for ILS characterization</td><td>Provision of Linefit software</td><td>With any update</td><td>Available from the CREGARS repository</td></tr><tr><td rowspan="3">Provision of retrieval strategies and software</td><td></td><td>Provision of outline of the retrieval strategy per target species</td><td>Upon every update</td><td>Available from the CREGARS repository</td></tr><tr><td></td><td>Provision of the standard retrieval S/W</td><td>Upon every update</td><td>Available from the CREGARS repository</td></tr><tr><td></td><td>Provision of retrieval control files and reference spectroscopic data</td><td>Upon every update</td><td>Available from the CREGARS repository</td></tr><tr><td rowspan="3">QA/QC</td><td></td><td>QA/QC of instrument setup and measurements (operations)</td><td>Every 2 year and with any major upgrade of measurement guidelines or Linefit</td><td>Sample spectra and linefit results must be requested from each NF</td></tr><tr><td></td><td>QA/QC of data analysis</td><td>Every 2 year and with any major upgrade of retrieval strategies or S/W</td><td>Sample data (L2) must be requested from each NF</td></tr><tr><td></td><td>QA/QC of data products before submission to DC</td><td>Upon every delivery (at least monthly)</td><td>This process must be automated as much as possible and</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><tr><td></td><td></td><td></td><td>may run at the DC.</td></tr><tr><td></td><td>Verification of the calibration cells</td><td>Every 2 year</td><td>NFs must be requested to send their cell to the CREGARS for evaluation against a calibrated N2O cell.</td></tr><tr><td></td><td>Training of operators and scientists</td><td>Annual workshop</td><td>Co-located with NDACC annual working group meetings</td></tr></table>
<table><tr><td></td><td></td><td></td><td>may run at the DC.</td></tr><tr><td></td><td>Verification of the calibration cells</td><td>Every 2 year</td><td>NFs must be requested to send their cell to the CREGARS for evaluation against a calibrated N2O cell.</td></tr><tr><td></td><td>Training of operators and scientists</td><td>Annual workshop</td><td>Co-located with NDACC annual working group meetings</td></tr></table>
<table><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="12">UWIS</td><td rowspan="4">Laboratory calibration of instrument in CREGARS Calibration Center</td><td>Wavelength calibration and ISRF determination</td><td>Once initially</td><td rowspan="4">Before installation at NF</td></tr><tr><td>In-band stray light calibration based on Laser sources</td><td>Once initially</td></tr><tr><td>Radiometric calibration</td><td>Once initially</td></tr><tr><td>Field of view determination</td><td>Once initially</td></tr><tr><td rowspan="2">Provision of measurement guidelines and associated software tools</td><td>On-site calibration and monitoring procedures</td><td>At start</td><td rowspan="2"></td></tr><tr><td>Data acquisition protocol</td><td>At start</td></tr><tr><td rowspan="4">Central processing</td><td>Standardised slant column retrieval</td><td>Operational</td><td>Optimisation needed for individual (non-standard) instrument</td></tr><tr><td>Standardised trace gas column and profile retrieval</td><td>Operational</td><td></td></tr><tr><td>Automated QA/QC incl. feedback to data providers</td><td>Operational</td><td>Access to dashboard displaying processing status, e-mail alerts in case of anomalies</td></tr><tr><td>Generation of final products in standardised GEOMS format</td><td>Operational</td><td></td></tr></tbody></table>
<table><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="10">UWIS</td><td rowspan="4">Laboratory calibration of instrument in CREGARS Calibration Center</td><td>Wavelength calibration and ISRF determination</td><td>Once initially</td><td rowspan="4">Before installation at NF</td></tr><tr><td>In-band stray light calibration based on Laser sources</td><td>Once initially</td></tr><tr><td>Radiometric calibration</td><td>Once initially</td></tr><tr><td>Field of view determination</td><td>Once initially</td></tr><tr><td rowspan="2">Provision of measurement guidelines and associated software tools</td><td>On-site calibration and monitoring procedures</td><td>At start</td><td rowspan="2"></td></tr><tr><td>Data acquisition protocol</td><td>At start</td></tr><tr><td rowspan="4">Central processing</td><td>Standardised slant column retrieval</td><td>Operational</td><td>Optimisation needed for individual (non-standard) instrument</td></tr><tr><td>Standardised trace gas column and profile retrieval</td><td>Operational</td><td></td></tr><tr><td>Automated QA/QC incl. feedback to data providers</td><td>Operational</td><td>Access to dashboard displaying processing status, e-mail alerts in case of anomalies</td></tr><tr><td>Generation of final products in standardised GEOMS format</td><td>Operational</td><td></td></tr></tbody></table>
<table><tr><td></td><td>Rapid-delivery with latency &lt;= 1 day</td><td>Operational</td><td>Required for satellite validation</td></tr><tr><td rowspan="3">Intercalibration campaign</td><td>Provide assessment of performance by intercomparison</td><td>Every 5 years</td><td>Needs adequate facility to host large number of instruments</td></tr><tr><td>Provide certification label for NDACC/ACTRIS operation</td><td>Every 5 years</td><td></td></tr><tr><td>Provide forum for discussion involving the whole community</td><td>Every 5 years</td><td>Forster advances in measurement technique and data retrieval</td></tr></table>

<table><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="3">O3 DIAL</td><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><tr><td></td><td>Rapid-delivery with latency &lt;= 1 day</td><td>Operational</td><td>Required for satellite validation</td></tr><tr><td rowspan="3">Intercalibration campaign</td><td>Provide assessment of performance by intercomparison</td><td>Every 5 years</td><td>Needs adequate facility to host large number of instruments</td></tr><tr><td>Provide certification label for NDACC/ACTRIS operation</td><td>Every 5 years</td><td></td></tr><tr><td>Provide forum for discussion involving the whole community</td><td>Every 5 years</td><td>Forster advances in measurement technique and data retrieval</td></tr></table>

<table><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="3">O3 DIAL</td><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td></td></tr></tbody></table>
Operation support on request

<table><thead><tr><th>Technique</th><th>Type of support</th><th>Specific support</th><th>Comments</th></tr></thead><tbody><tr><td rowspan="4">FTIR, UVVIS and O3 DIAL</td><td>Site performance audits with reference samples or mobile systems*</td><td>Peer review verification of NF performance at the CREGARS</td><td>NFs must be requested to provide sample L1 and corresponding L2 data – to be assessed by the CREGARS</td></tr><tr><td>Training of operators and scientists*</td><td>Organization of dedicated training session at the NF or at the CREGARS</td><td>Training for operation of the instrument or training for the data analysis</td></tr><tr><td>Consultancy for setting-up new observational or exploratory platforms and new instruments at the NFs*</td><td></td><td>Based on existing satellite or model analyses or model analyses upon requests</td></tr><tr><td>Testing of new measurement instruments, operating procedures or data analysis procedures*</td><td>Direct comparison with the reference system or standard data analysis at the CREGARS</td><td></td></tr><tr><td rowspan="2">FTIR</td><td rowspan="2">Provision of measurement guidelines and software (+ H/W) for ILS characterization</td><td>Provision of calibration cells</td><td>Spare sets must always be available to satisfy requests</td></tr><tr><td>Provision of optical filters</td><td>Spare sets must always be available to satisfy requests</td></tr><tr><td rowspan="3">UVVIS</td><td rowspan="3">Provision of measurement guidelines and associated software tools</td><td>Provision of lines sources and optical filters</td><td>On-site ISRF, stray-light and polarization tests</td></tr><tr><td>Provision of community retrieval software codes</td><td>For local processing</td></tr><tr><td>Provision of level-1 data formatting tools</td><td>Support to inclusion in central processing</td></tr><tr><td>O3 DIAL</td><td></td><td></td><td></td></tr></tbody></table>
Operation support on request

<table><thead><tr><th>Technique</th><th>Type of support</th><th>Specific support</th><th>Comments</th></tr></thead><tbody><tr><td rowspan="4">FTIR, UVVIS and O3 DIAL</td><td>Site performance audits with reference samples or mobile systems*</td><td>Peer review verification of NF performance at the CREGARS</td><td>NFs must be requested to provide sample L1 and corresponding L2 data – to be assessed by the CREGARS</td></tr><tr><td>Training of operators and scientists*</td><td>Organization of dedicated training session at the NF or at the CREGARS</td><td>Training for operation of the instrument or training for the data analysis</td></tr><tr><td>Consultancy for setting-up new observational or exploratory platforms and new instruments at the NFs*</td><td></td><td>Based on existing satellite or model analyses or model analyses upon requests</td></tr><tr><td>Testing of new measurement instruments, operating procedures or data analysis procedures*</td><td>Direct comparison with the reference system or standard data analysis at the CREGARS</td><td></td></tr><tr><td rowspan="2">FTIR</td><td rowspan="2">Provision of measurement guidelines and software (+ H/W) for ILS characterization</td><td>Provision of calibration cells</td><td>Spare sets must always be available to satisfy requests</td></tr><tr><td>Provision of optical filters</td><td>Spare sets must always be available to satisfy requests</td></tr><tr><td rowspan="3">UVVIS</td><td rowspan="3">Provision of measurement guidelines and associated software tools</td><td>Provision of lines sources and optical filters</td><td>On-site ISRF, stray-light and polarization tests</td></tr><tr><td>Provision of community retrieval software codes</td><td>For local processing</td></tr><tr><td>Provision of level-1 data formatting tools</td><td>Support to inclusion in central processing</td></tr><tr><td>O3 DIAL</td><td></td><td></td><td></td></tr></tbody></table>