The UVVIS instruments satisfying these optimum requirements are referred to as **UVVIS-MAXDOAS** instruments.

Moreover, the ACTRIS UVVIS instruments have to comply with specific quality criteria defined by the Centre for Reactive Trace Gases Remote Sensing as well as with certification criteria established as part of regular intercalibration campaigns, organised by the Centre for Reactive Trace Gases Remote Sensing. ACTRIS quality and certification criteria will be compliant with NDACC criteria, but may be more strict. The operations also have to comply with the Standard Operating Procedures to be developed and maintained by the Topical Centre.

Instrument intercalibration campaigns are considered necessary every 5 to 10 years. Such exercises need to be organised in a location that allows optimal observing conditions with respect to the required viewing geometry. Auxiliary observations of the atmospheric column, aerosol profile and cloud properties, as well as *in situ* trace gas observations should be available during such campaigns. The Centre for Reactive Trace Gases Remote Sensing should identify an ACTRIS site for this purpose. More technical details are provided in the concept document of the Centre for Reactive Trace Gases Remote Sensing (see D4.1).

#### 4.3.3.3 $O_3$ DIAL

The **DIAL technique** is used for the measurement of the **ozone vertical distribution**. This technique requires the simultaneous emission of two laser beams with different wavelengths characterised by different ozone absorption cross-sections. The laser wavelengths are chosen in the ultraviolet where ozone absorption is most efficient. Tropospheric and stratospheric measurements require different wavelength pairs and laser sources. Since the ozone profile is derived from the derivatives of the lidar signals, the technique does not require any calibration, but correction terms in the DIAL equation require the use of auxiliary total number density and temperature profile data.

ACTRIS $O_3$ DIAL instruments that are included in the optimum setup of an ACTRIS Observational Platform for remote sensing of reactive trace gases have to comply with the specific procedures defined by the Centre for Reactive Trace Gases Remote Sensing. Such procedures include periodic intercomparison campaigns with a reference mobile lidar instrument, regular comparisons with nearby satellite measurements or ozonesonde profiles and evaluation of the retrieval algorithm using synthetic data.
### 4.3.4 Operation

#### 4.3.4.1 FTIR

The measurement mode is the solar absorption mode. The Centre for Reactive Trace Gases Remote Sensing will prescribe the specific operation procedures for the ACTRIS target gases, i.e., spectral resolution and optical band of the solar spectrum to be recorded for each target species.

For the regular monitoring, measurements are recorded during daytime under clear-sky conditions – preferably from sunrise to sunset, and alternating between the different target gases. For the ACTRIS targeted short-lived climate pollutants, it is important to have several measurements per day, in order to cover the diurnal cycle. The automation of the observations, with remote-control access, is highly recommended, in order to have a good temporal coverage (diurnal, seasonal, interannual) of the observations at an affordable cost.

#### 4.3.4.2 UVVIS

The standard measurement mode of UVVIS instruments consists in scanning the sky from the zenith down to the horizon. The simplest instruments (zenith-sky systems) measure the sky light scattered at the zenith only and are only sensitive to total integrated columns. Best sensitivity to the near-surface trace-gas concentrations is obtained when observing at multiple elevation angles close to the horizon. This geometry is known as the MAXDOAS geometry. In addition, some systems also include a solar absorption mode (or direct-sun mode).

For the regular monitoring at ACTRIS Observational Platforms, measurements are recorded during daytime under all weather conditions from sunrise to sunset, and alternating between sky-light scanning and direct-sun modes, if the latter one is available. For the MAXDOAS type instruments, the standard operation mode typically consists of 3 scans hourly at solar zenith angles smaller than 85°.

All MAXDOAS systems must be fully automated, preferable with remote-control access, and setup to deliver spectral data to the centralised data base at least once in a day. The Centre for Reactive Trace Gases Remote Sensing will prescribe the specific operation procedures for the ACTRIS target gases, i.e., spectral intervals to be recorded for each target species, elevation angles to be scanned, repetition frequency of the elevation scans, reference spectra acquisition, etc.

For the instruments of the MAXDOAS type for which profile inversion is feasible, the selection of data suitable for profile inversion is performed as part of the retrieval process using cloud detection methods making use of intensity ratios (colour index) and O₄ measurements. In case of direct-sun observations, the presence of the sun is determined using measured intensities.

#### 4.3.4.3 O₃ DIAL

Stratospheric ozone lidar measurements are performed during night time and require clear-sky meteorological conditions – laser radiation is rapidly attenuated by clouds and only cirrus can be tolerated for accurate lidar measurements. Tropospheric lidar measurements are also performed at night to be used with the stratospheric data. Requirements for clear sky are less stringent for tropospheric profiling which can be less than 30 min and be obtained below the strongly attenuating cloud layers. Both stratospheric
### 4.3.4 Operation

#### 4.3.4.1 FTIR

The measurement mode is the solar absorption mode. The Centre for Reactive Trace Gases Remote Sensing will prescribe the specific operation procedures for the ACTRIS target gases, i.e., spectral resolution and optical band of the solar spectrum to be recorded for each target species.

For the regular monitoring, measurements are recorded during daytime under clear-sky conditions – preferably from sunrise to sunset, and alternating between the different target gases. For the ACTRIS targeted short-lived climate pollutants, it is important to have several measurements per day, in order to cover the diurnal cycle. The automation of the observations, with remote-control access, is highly recommended, in order to have a good temporal coverage (diurnal, seasonal, interannual) of the observations at an affordable cost.

#### 4.3.4.2 UVVIS

The standard measurement mode of UVVIS instruments consists in scanning the sky from the zenith down to the horizon. The simplest instruments (zenith-sky systems) measure the sky light scattered at the zenith only and are only sensitive to total integrated columns. Best sensitivity to the near-surface trace-gas concentrations is obtained when observing at multiple elevation angles close to the horizon. This geometry is known as the MAXDOAS geometry. In addition, some systems also include a solar absorption mode (or direct-sun mode).

For the regular monitoring at ACTRIS Observational Platforms, measurements are recorded during daytime under all weather conditions from sunrise to sunset, and alternating between sky-light scanning and direct-sun modes, if the latter one is available. For the MAXDOAS type instruments, the standard operation mode typically consists of 3 scans hourly at solar zenith angles smaller than 85°.

All MAXDOAS systems must be fully automated, preferable with remote-control access, and setup to deliver spectral data to the centralised data base at least once in a day. The Centre for Reactive Trace Gases Remote Sensing will prescribe the specific operation procedures for the ACTRIS target gases, i.e., spectral intervals to be recorded for each target species, elevation angles to be scanned, repetition frequency of the elevation scans, reference spectra acquisition, etc.

For the instruments of the MAXDOAS type for which profile inversion is feasible, the selection of data suitable for profile inversion is performed as part of the retrieval process using cloud detection methods making use of intensity ratios (colour index) and O₄ measurements. In case of direct-sun observations, the presence of the sun is determined using measured intensities.

#### 4.3.4.3 O₃ DIAL

Stratospheric ozone lidar measurements are performed during night time and require clear-sky meteorological conditions – laser radiation is rapidly attenuated by clouds and only cirrus can be tolerated for accurate lidar measurements. Tropospheric lidar measurements are also performed at night to be used with the stratospheric data. Requirements for clear sky are less stringent for tropospheric profiling which can be less than 30 min and be obtained below the strongly attenuating cloud layers. Both stratospheric
and tropospheric lidar profiles are necessary for the measurement of ozone in the upper troposphere/lower stratosphere.

Lidar sites measuring composition are chosen for their quality for atmospheric transparency and allow quasi-daily operation capabilities. Generally, one measurement per day is provided with a duration ranging from one to several hours depending on the altitude range of the target profile.

### 4.3.5 Data production and data products

#### 4.3.5.1 FTIR

The Level 0 data acquired at the NF that operate FTIR instruments are interferogrammes and corresponding housekeeping data. The conversion of Level 0 to Level 1 data is done at the NFs and consists of (1) the rejection of bad interferogrammes, taking into account the housekeeping data and the meteorological conditions (for example, interferogrammes contaminated by clouds are rejected) and (2) conversion of the interferogrammes to solar absorption spectra accompanied by relevant metadata. The quality of the Level 1 data will be verified regularly by the Centre for Reactive Trace Gases Remote Sensing; if issues with the Level 1 data are detected, the Topical Centre may request access to the Level 0 and/or Level 1 data for verifying the quality. It is the responsibility of the NF to care for the long-term archiving of the Level 1 data, including the housekeeping data and metadata. Long-term archiving of the Level 0 data at the NFs is also recommended.

Data processing from Level 1 to the Level 2 data, i.e., the retrieval of the target gases' concentrations, is done generally at the NF, but the data processing software is standardised. The comparison between the two existing standard retrieval software packages (SFIT4 and PROFFIT) will be done in the Centre for Reactive Trace Gases Remote Sensing to ensure that both software codes deliver consistent retrieval products and associated uncertainties (Level 2 data). In recent years, a lot of effort has been spent on improving and harmonising the procedures for the evaluation of the uncertainties (see, e.g., the documents 'NORS uncertainty budgets' and 'Spatial representativeness of NORS observations' on http://nors.aeronomie.be/index.php/documents). More recent additional efforts have been made in the EU H2020 projects QA4ECV and GAIA-CLIM and a publication is in preparation (Langerock et al., in preparation). The progress made in these recent projects will be implemented at the Centre for Reactive Trace Gases Remote Sensing in the retrieval procedures and software that will be distributed to the ACTRIS NFs.

The required auxiliary data are the vertical profiles of pressure and temperature that can be taken from NCEP or ECMWF, if local observations are not available. Also some *a priori* climatological information is required in the algorithms using the Optimal Estimation Method, which is taken from global models or historical data.

In ACTRIS, the NFs will have to comply with strict harmonised processing requirements including retrieval strategies, *a priori* information, processing parameters and auxiliary data, established by the Centre for Reactive Trace Gases Remote Sensing, and the Topical Centre will quality-control the retrieval products. The NFs will also have to respond to requests for re-processing of the time series from the Centre for
and tropospheric lidar profiles are necessary for the measurement of ozone in the upper troposphere/lower stratosphere.

Lidar sites measuring composition are chosen for their quality for atmospheric transparency and allow quasi-daily operation capabilities. Generally, one measurement per day is provided with a duration ranging from one to several hours depending on the altitude range of the target profile.

### 4.3.5 Data production and data products

#### 4.3.5.1 FTIR

The Level 0 data acquired at the NF that operate FTIR instruments are interferogrammes and corresponding housekeeping data. The conversion of Level 0 to Level 1 data is done at the NFs and consists of (1) the rejection of bad interferogrammes, taking into account the housekeeping data and the meteorological conditions (for example, interferogrammes contaminated by clouds are rejected) and (2) conversion of the interferogrammes to solar absorption spectra accompanied by relevant metadata. The quality of the Level 1 data will be verified regularly by the Centre for Reactive Trace Gases Remote Sensing; if issues with the Level 1 data are detected, the Topical Centre may request access to the Level 0 and/or Level 1 data for verifying the quality. It is the responsibility of the NF to care for the long-term archiving of the Level 1 data, including the housekeeping data and metadata. Long-term archiving of the Level 0 data at the NFs is also recommended.

Data processing from Level 1 to the Level 2 data, i.e., the retrieval of the target gases' concentrations, is done generally at the NF, but the data processing software is standardised. The comparison between the two existing standard retrieval software packages (SFIT4 and PROFFIT) will be done in the Centre for Reactive Trace Gases Remote Sensing to ensure that both software codes deliver consistent retrieval products and associated uncertainties (Level 2 data). In recent years, a lot of effort has been spent on improving and harmonising the procedures for the evaluation of the uncertainties (see, e.g., the documents 'NORS uncertainty budgets' and 'Spatial representativeness of NORS observations' on http://nors.aeronomie.be/index.php/documents). More recent additional efforts have been made in the EU H2020 projects QA4ECV and GAIA-CLIM and a publication is in preparation (Langerock et al., in preparation). The progress made in these recent projects will be implemented at the Centre for Reactive Trace Gases Remote Sensing in the retrieval procedures and software that will be distributed to the ACTRIS NFs.

The required auxiliary data are the vertical profiles of pressure and temperature that can be taken from NCEP or ECMWF, if local observations are not available. Also some *a priori* climatological information is required in the algorithms using the Optimal Estimation Method, which is taken from global models or historical data.

In ACTRIS, the NFs will have to comply with strict harmonised processing requirements including retrieval strategies, *a priori* information, processing parameters and auxiliary data, established by the Centre for Reactive Trace Gases Remote Sensing, and the Topical Centre will quality-control the retrieval products. The NFs will also have to respond to requests for re-processing of the time series from the Centre for
Reactive Trace Gases Remote Sensing, whenever the latter will distribute a significant update of the processing software, spectroscopic linelist and/or retrieval strategies.

The Level 2 data will be submitted in the GEOMS HDF format (https://avdc.gsfc.nasa.gov/index.php?site=1178067684) to the TC, where the QA/QC is done, and the TC will then submit them to the ACTRIS Data Centre.

NFs can request that their data are processed at the TC, however, this option is no general obligation of the TC. It is highly recommended to automate to a large extent the data processing procedures, to enable the timely delivery of the data.

No standardized L3 data are foreseen.

#### 4.3.5.2 UVVIS

The Level 0 data acquired at NFs operating UVVIS instruments are uncalibrated radiance spectra and corresponding housekeeping data. Like for FTIR instruments, the conversion of Level 0 to Level 1 data is done at the NF and consists of various calibration steps (wavelength registration, dark-current, non-linearity and stray-light correction) as well as filtering for possible anomalies during the spectral acquisition process (e.g., saturation). The quality of the Level 1 data submitted to the Centre for Reactive Trace Gases Remote Sensing will be further checked as part of the QA/QC procedure of the central processing unit, leading to possible additional screening of the Level 1 data.

Data processing from Level 1 to the Level 2 data, i.e., the retrieval of the target atmospheric species concentrations, is performed using standardised algorithms implemented in the central processing units operated by the Centre for Reactive Trace Gases Remote Sensing. Centralised processing units are based on systems developed for the different UVVIS instrument types as part of NDACC and within the ESA-funded Pandonia and FRM₄DOAS projects. Common retrieval methods selected after round-robin intercomparison and peer-reviewed publication are being implemented allowing for a high level of traceability and standardisation in the data processing and delivery. The central processing units also execute systematic QA/QC procedures at each step of the data processing, resulting in the generation of various quality flags attached to the data products.

In ACTRIS, the NFs will have to comply with the requirements defined to join in the centralised processing units. Such requirements concern the operation, calibration and quality control of the instruments providing spectral data to central processors. In turn, the central processing units will provide feedback to the data providers including alerts in case of instrument failure identified as part of the QA/QC procedures. All UVVIS Level 2 data products will be generated using a common data format following the internationally recognised GEOMS metadata standard. They will be made available via the ACTRIS Data Centre.

#### 4.3.5.3 O₃ DIAL

The Level 0 data acquired at the NFs that operate lidar instruments are individual raw signal data files corresponding to signal averages of a few thousands lidar pulses. The conversion of Level 0 to Level 1 data is done at the NFs and consists of (1) the rejection of spikes in lidar signals and rejection of files with
Reactive Trace Gases Remote Sensing, whenever the latter will distribute a significant update of the processing software, spectroscopic linelist and/or retrieval strategies.

The Level 2 data will be submitted in the GEOMS HDF format (https://avdc.gsfc.nasa.gov/index.php?site=1178067684) to the TC, where the QA/QC is done, and the TC will then submit them to the ACTRIS Data Centre.

NFs can request that their data are processed at the TC, however, this option is no general obligation of the TC. It is highly recommended to automate to a large extent the data processing procedures, to enable the timely delivery of the data.

No standardized L3 data are foreseen.

#### 4.3.5.2 UVVIS

The Level 0 data acquired at NFs operating UVVIS instruments are uncalibrated radiance spectra and corresponding housekeeping data. Like for FTIR instruments, the conversion of Level 0 to Level 1 data is done at the NF and consists of various calibration steps (wavelength registration, dark-current, non-linearity and stray-light correction) as well as filtering for possible anomalies during the spectral acquisition process (e.g., saturation). The quality of the Level 1 data submitted to the Centre for Reactive Trace Gases Remote Sensing will be further checked as part of the QA/QC procedure of the central processing unit, leading to possible additional screening of the Level 1 data.

Data processing from Level 1 to the Level 2 data, i.e., the retrieval of the target atmospheric species concentrations, is performed using standardised algorithms implemented in the central processing units operated by the Centre for Reactive Trace Gases Remote Sensing. Centralised processing units are based on systems developed for the different UVVIS instrument types as part of NDACC and within the ESA-funded Pandonia and FRM₄DOAS projects. Common retrieval methods selected after round-robin intercomparison and peer-reviewed publication are being implemented allowing for a high level of traceability and standardisation in the data processing and delivery. The central processing units also execute systematic QA/QC procedures at each step of the data processing, resulting in the generation of various quality flags attached to the data products.

In ACTRIS, the NFs will have to comply with the requirements defined to join in the centralised processing units. Such requirements concern the operation, calibration and quality control of the instruments providing spectral data to central processors. In turn, the central processing units will provide feedback to the data providers including alerts in case of instrument failure identified as part of the QA/QC procedures. All UVVIS Level 2 data products will be generated using a common data format following the internationally recognised GEOMS metadata standard. They will be made available via the ACTRIS Data Centre.

#### 4.3.5.3 O₃ DIAL

The Level 0 data acquired at the NFs that operate lidar instruments are individual raw signal data files corresponding to signal averages of a few thousands lidar pulses. The conversion of Level 0 to Level 1 data is done at the NFs and consists of (1) the rejection of spikes in lidar signals and rejection of files with
insufficient signal to noise ratio due to meteorological conditions (arrival of clouds during the measurement period) and (2) summation of these individual files. The quality of the Level 1 data will be verified regularly by the Centre for Reactive Trace Gases Remote Sensing; if issues with the Level 1 data are detected, the Topical Centre may request NFs for verifying the quality of the Level 0 data. It is the responsibility of the NF to care for the long-term archiving of the Level 0 and Level 1 data, including the housekeeping data and metadata.

The Level 2 data processing, e.g., retrieval of the ozone number density profile from the average lidar signal data (Level 1 data) is done at the NF with proven and standardised processing software tested at the Centre for Reactive Trace Gases Remote Sensing. Efforts have been made for improving and harmonising the procedures for the evaluation of the uncertainties and vertical resolution, as discussed in Leblanc et al. (2016a, b, and c).

### 4.3.6 Data delivery and quality control

ACTRIS FTIR, UVVIS and O₃ DIAL data handling must deliver the data products to the associated unit of the ACTRIS Data Centre within three months after data acquisition. More rapid data delivery is recommended; in particular NRT delivery is recommended whenever feasible (e.g., the UVVIS data will probably be delivered by the Centre for Reactive Trace Gases Remote Sensing in NRT).

The data format will be the NDACC GEOMS HDF format (https://avdc.gsfc.nasa.gov/index.php?site=1178067684). For each instrument type, specific GEOMS templates have been defined and the submitted data files must be formatted according to these templates (https://avdc.gsfc.nasa.gov/index.php?site=1989220925 for FTIR; https://avdc.gsfc.nasa.gov/index.php?site=1876901039 for UVVIS and https://avdc.gsfc.nasa.gov/index.php?site=455555165 for O₃ DIAL). For data collected at ACTRIS NFs, the entry 'ACTRIS' will be added in the global file attribute 'FILE_ASSOCIATION' in the file's metadata.

The quality of the submitted data files will be controlled by the Centre for Reactive Trace Gases Remote Sensing, not only as to format but also as to file content – in order to avoid that files in the ACTRIS Data Centre provide erroneous information to the data users. Experience from the past has demonstrated that this quality control is a very useful step in the data archiving process. This means that data will transit through the Centre for Reactive Trace Gases Remote Sensing on their way from the data provider to the ACTRIS Data Centre, while it guarantees that all the data in the Data Centre satisfy the quality requirements.
insufficient signal to noise ratio due to meteorological conditions (arrival of clouds during the measurement period) and (2) summation of these individual files. The quality of the Level 1 data will be verified regularly by the Centre for Reactive Trace Gases Remote Sensing; if issues with the Level 1 data are detected, the Topical Centre may request NFs for verifying the quality of the Level 0 data. It is the responsibility of the NF to care for the long-term archiving of the Level 0 and Level 1 data, including the housekeeping data and metadata.

The Level 2 data processing, e.g., retrieval of the ozone number density profile from the average lidar signal data (Level 1 data) is done at the NF with proven and standardised processing software tested at the Centre for Reactive Trace Gases Remote Sensing. Efforts have been made for improving and harmonising the procedures for the evaluation of the uncertainties and vertical resolution, as discussed in Leblanc et al. (2016a, b, and c).

### 4.3.6 Data delivery and quality control

ACTRIS FTIR, UVVIS and O₃ DIAL data handling must deliver the data products to the associated unit of the ACTRIS Data Centre within three months after data acquisition. More rapid data delivery is recommended; in particular NRT delivery is recommended whenever feasible (e.g., the UVVIS data will probably be delivered by the Centre for Reactive Trace Gases Remote Sensing in NRT).

The data format will be the NDACC GEOMS HDF format (https://avdc.gsfc.nasa.gov/index.php?site=1178067684). For each instrument type, specific GEOMS templates have been defined and the submitted data files must be formatted according to these templates (https://avdc.gsfc.nasa.gov/index.php?site=1989220925 for FTIR; https://avdc.gsfc.nasa.gov/index.php?site=1876901039 for UVVIS and https://avdc.gsfc.nasa.gov/index.php?site=455555165 for O₃ DIAL). For data collected at ACTRIS NFs, the entry 'ACTRIS' will be added in the global file attribute 'FILE_ASSOCIATION' in the file's metadata.

The quality of the submitted data files will be controlled by the Centre for Reactive Trace Gases Remote Sensing, not only as to format but also as to file content – in order to avoid that files in the ACTRIS Data Centre provide erroneous information to the data users. Experience from the past has demonstrated that this quality control is a very useful step in the data archiving process. This means that data will transit through the Centre for Reactive Trace Gases Remote Sensing on their way from the data provider to the ACTRIS Data Centre, while it guarantees that all the data in the Data Centre satisfy the quality requirements.
## 4.4 Aerosol in situ measurements

### 4.4.1 Introduction

Aerosol particles are highly important actors in air pollution and climate radiative forcing. Their size and/or shape and/or chemical composition determine their deposition in the respiratory tract, the diseases they trigger, and their impacts on ecosystems (acidification, eutrophication). Moreover, these particle properties determine the ice or snow covered area reflectance, the particle interaction with radiation (direct radiative forcing, visibility) and the effect of particles on the formation and microphysical properties of clouds (indirect radiative forcing, precipitation probability). Impacts such as particle light scattering and absorption, cloud formation potential, etc. can be inferred from the particle number size distribution and other independently measured physical variables. Conversely, a detailed knowledge of the chemical composition is essential to determine the sources of aerosol particles, and therefore develop efficient emission abatement measures. All the most relevant variables related to aerosol climate forcing and air pollution shall be measured at ACTRIS Observational Platforms for aerosol in situ observations. This section describes the respective technical concepts and requirements.

### 4.4.2 Observational capabilities

#### 4.4.2.1 Optimum setup

For the Observational Platforms connected to the Centre for Aerosol In Situ Measurements there is no optimum setup, as each Observational Platform has a specific science case. Instead, minimum requirements are defined below, which account for this broad range of science cases. Therefore, it is distinguished here between mandatory, specializing, added-value and auxiliary variables, listed in Tab. 1.

#### 4.4.2.2 Mandatory and specializing variables

The minimum requirement for ACTRIS aerosol in situ measurement stations includes measurements of three mandatory variables (**particle number size distribution, particle light scattering & backscattering coefficients** and **particle light absorption coefficient & equivalent black carbon concentration**) and at least two free-choice specializing variables (see Tab. 1 for the two complete list). The reasons that these three variables are mandatory are i) their importance in climate and health issues, ii) the possibility to account for the specific science case of each Observational Platform and iii) the long-term records for these variables at several sites around Europe using validated SOPs developed by the ACTRIS community and endorsed by the Centre for Aerosol In Situ Measurements. Among the two free-choice variables, at least one must be a chemical variable. Chemical composition is not as well characterized as physical parameters of aerosol particles, yet it is as important.

#### 4.4.2.3 Added-value variables

Complementary added-value variables permit the full characterization of aerosol particle properties and providing insight to many processes linked to emission, transformation or ageing of particles. Measurements of **trace gases** – O₃, CO, NOₓ (NO+NO₂) and SO₂ – should be performed which are useful
## 4.4 Aerosol in situ measurements

### 4.4.1 Introduction

Aerosol particles are highly important actors in air pollution and climate radiative forcing. Their size and/or shape and/or chemical composition determine their deposition in the respiratory tract, the diseases they trigger, and their impacts on ecosystems (acidification, eutrophication). Moreover, these particle properties determine the ice or snow covered area reflectance, the particle interaction with radiation (direct radiative forcing, visibility) and the effect of particles on the formation and microphysical properties of clouds (indirect radiative forcing, precipitation probability). Impacts such as particle light scattering and absorption, cloud formation potential, etc. can be inferred from the particle number size distribution and other independently measured physical variables. Conversely, a detailed knowledge of the chemical composition is essential to determine the sources of aerosol particles, and therefore develop efficient emission abatement measures. All the most relevant variables related to aerosol climate forcing and air pollution shall be measured at ACTRIS Observational Platforms for aerosol in situ observations. This section describes the respective technical concepts and requirements.

### 4.4.2 Observational capabilities

#### 4.4.2.1 Optimum setup

For the Observational Platforms connected to the Centre for Aerosol In Situ Measurements there is no optimum setup, as each Observational Platform has a specific science case. Instead, minimum requirements are defined below, which account for this broad range of science cases. Therefore, it is distinguished here between mandatory, specializing, added-value and auxiliary variables, listed in Tab. 1.

#### 4.4.2.2 Mandatory and specializing variables

The minimum requirement for ACTRIS aerosol in situ measurement stations includes measurements of three mandatory variables (**particle number size distribution, particle light scattering & backscattering coefficients** and **particle light absorption coefficient & equivalent black carbon concentration**) and at least two free-choice specializing variables (see Tab. 1 for the two complete list). The reasons that these three variables are mandatory are i) their importance in climate and health issues, ii) the possibility to account for the specific science case of each Observational Platform and iii) the long-term records for these variables at several sites around Europe using validated SOPs developed by the ACTRIS community and endorsed by the Centre for Aerosol In Situ Measurements. Among the two free-choice variables, at least one must be a chemical variable. Chemical composition is not as well characterized as physical parameters of aerosol particles, yet it is as important.

#### 4.4.2.3 Added-value variables

Complementary added-value variables permit the full characterization of aerosol particle properties and providing insight to many processes linked to emission, transformation or ageing of particles. Measurements of **trace gases** – O₃, CO, NOₓ (NO+NO₂) and SO₂ – should be performed which are useful
tracers for anthropogenic pollution events and therefore parallel measurements of these gases are a clear
asset.

4.4.2.4 Auxiliary variables

Aerosol in situ observations should be accompanied by measurements of **meteorological parameters** (wind speed, wind direction, temperature, and relative humidity) and **radiation** (UV, IR, total radiation) in order to interpret the measured data in an appropriate manner. At least the meteorological auxiliary variables must be measured at an Observational Platform connected to the Centre for Aerosol In Situ Measurements.

4.4.3 Instrumentation and calibration

Before describing the requirements in more detail, it is noted that in general the Centre for Aerosol In Situ
Measurement sets the quality objectives for the instrument QA, and that the NF is responsible for having
instruments quality-assured at the TC at given interval. In addition, the NF is responsible for conducting
and documenting QC of both instrument and data, where instrument QC is defined by TC and data QC
defined jointly by the TC and DC. As in most cases there is a need to move the instruments during the
calibration against the reference at the TC, an extra one to cover up the data while calibrating or in cases
of failure would be useful.

4.4.3.1 Particle number size distribution – mobility diameter (0.01 – 0.8 µm)

A mobility particle size spectrometer (MPSS) is used to measure the **particle number size distribution** of the submicrometer size range from approximately 0.010 to 0.8 µm. The technology is well established and is commercially available, but there are also custom-designed models. The MPSS is robust and designed for long-term operations. It needs regular checks on-site to quality-assure the measurements. Into the MPSS, a **Condensation Particle Counter** (CPC) is included, which also has to undergo frequent on-site checks. Additionally, MPSS systems must be calibrated every one to two years against a reference instruments at the TC.

4.4.3.2 Particle light scattering & backscattering coefficients (multi-wavelength)

A multi-wavelength integrating nephelometer (InNe) is used to measure the multi-wavelength particle
light scattering & backscattering coefficients. The technology is well established and a few instruments
are commercially available. The InNe is robust and designed for long-term operations. It needs regular
checks on-site to quality-assure the measurement. Additionally, InNe instruments must be calibrated
every one to two years against a reference instruments at the TC.

4.4.3.3 Particle light absorption coefficient & equivalent black carbon concentration

An absorption photometer (AP) is used to measure the **particle light absorption coefficient & equivalent black carbon concentration**. The technology is well established and instruments are commercially available. The existing APs are robust and designed for long-term operations and yearly or every two years calibrations against a reference instruments at the TC are required.
tracers for anthropogenic pollution events and therefore parallel measurements of these gases are a clear
asset.

4.4.2.4 Auxiliary variables

Aerosol in situ observations should be accompanied by measurements of **meteorological parameters** (wind speed, wind direction, temperature, and relative humidity) and **radiation** (UV, IR, total radiation) in order to interpret the measured data in an appropriate manner. At least the meteorological auxiliary variables must be measured at an Observational Platform connected to the Centre for Aerosol In Situ Measurements.

4.4.3 Instrumentation and calibration

Before describing the requirements in more detail, it is noted that in general the Centre for Aerosol In Situ
Measurement sets the quality objectives for the instrument QA, and that the NF is responsible for having
instruments quality-assured at the TC at given interval. In addition, the NF is responsible for conducting
and documenting QC of both instrument and data, where instrument QC is defined by TC and data QC
defined jointly by the TC and DC. As in most cases there is a need to move the instruments during the
calibration against the reference at the TC, an extra one to cover up the data while calibrating or in cases
of failure would be useful.

4.4.3.1 Particle number size distribution – mobility diameter (0.01 – 0.8 µm)

A mobility particle size spectrometer (MPSS) is used to measure the **particle number size distribution** of the submicrometer size range from approximately 0.010 to 0.8 µm. The technology is well established and is commercially available, but there are also custom-designed models. The MPSS is robust and designed for long-term operations. It needs regular checks on-site to quality-assure the measurements. Into the MPSS, a **Condensation Particle Counter** (CPC) is included, which also has to undergo frequent on-site checks. Additionally, MPSS systems must be calibrated every one to two years against a reference instruments at the TC.

4.4.3.2 Particle light scattering & backscattering coefficients (multi-wavelength)

A multi-wavelength integrating nephelometer (InNe) is used to measure the multi-wavelength particle
light scattering & backscattering coefficients. The technology is well established and a few instruments
are commercially available. The InNe is robust and designed for long-term operations. It needs regular
checks on-site to quality-assure the measurement. Additionally, InNe instruments must be calibrated
every one to two years against a reference instruments at the TC.

4.4.3.3 Particle light absorption coefficient & equivalent black carbon concentration

An absorption photometer (AP) is used to measure the **particle light absorption coefficient & equivalent black carbon concentration**. The technology is well established and instruments are commercially available. The existing APs are robust and designed for long-term operations and yearly or every two years calibrations against a reference instruments at the TC are required.
<table><caption>Tab. 1: List of variables for ACTRIS aerosol in situ Observational Platforms</caption><thead><tr><th>Variable status</th><th>Variable Name</th><th>Instrument type</th><th>Recommended methodology</th></tr></thead><tbody><tr><td rowspan="3">Mandatory aerosol variables - Required for an ACTRIS Observational Platform connected to the Centre For Aerosol In Situ Measurements</td><td>Particle number size distribution - mobility diameter</td><td>Mobility Particle Size Spectrometer</td><td>Wiedensohler et al. (2017)</td></tr><tr><td>Particle light scattering & backscattering coefficient</td><td>Integrating Nephelometer</td><td>GAW report 227</td></tr><tr><td>Particle light absorption coefficient & equivalent black carbon concentration</td><td>Absorption Photometer</td><td>GAW report 227</td></tr><tr><td rowspan="6">Specializing aerosol variables - Provision of at least two specializing variable is mandatory for an ACTRIS Observational Platform connected to the Centre For Aerosol In Situ Measurements</td><td>Mass concentration of particulate organic & elemental carbon</td><td>Thermo-optical method on quartz filters</td><td>EUSAAR-2 / Can be replaced by on-line OC/EC for specific conditions</td></tr><tr><td>Particle number size distribution - optical and aerodynamic diameter</td><td>Aerodynamic & Optical Particle Size Spectrometer</td><td>in preparation</td></tr><tr><td>Particle number concentration</td><td>Condensation Particle Counter</td><td>CEN</td></tr><tr><td>Mass concentration of particulate elements</td><td>Filter-based XRF/PIXE/ICP_OES/ICP_MS</td><td>in preparation</td></tr><tr><td>Mass concentration of particulate organic tracers</td><td>Filter-based IC, GC-MS HPLC-MS, LC/MS</td><td>in preparation</td></tr><tr><td>Number concentration of cloud condensation nuclei</td><td>Cloud Condensation Nuclei Counter</td><td>Rose et al. (2008)</td></tr></tbody></table>
<table><caption>Tab. 1: List of variables for ACTRIS aerosol in situ Observational Platforms</caption><thead><tr><th>Variable status</th><th>Variable Name</th><th>Instrument type</th><th>Recommended methodology</th></tr></thead><tbody><tr><td rowspan="3">Mandatory aerosol variables - Required for an ACTRIS Observational Platform connected to the Centre For Aerosol In Situ Measurements</td><td>Particle number size distribution - mobility diameter</td><td>Mobility Particle Size Spectrometer</td><td>Wiedensohler et al. (2017)</td></tr><tr><td>Particle light scattering & backscattering coefficient</td><td>Integrating Nephelometer</td><td>GAW report 227</td></tr><tr><td>Particle light absorption coefficient & equivalent black carbon concentration</td><td>Absorption Photometer</td><td>GAW report 227</td></tr><tr><td rowspan="6">Specializing aerosol variables - Provision of at least two specializing variable is mandatory for an ACTRIS Observational Platform connected to the Centre For Aerosol In Situ Measurements</td><td>Mass concentration of particulate organic & elemental carbon</td><td>Thermo-optical method on quartz filters</td><td>EUSAAR-2 / Can be replaced by on-line OC/EC for specific conditions</td></tr><tr><td>Particle number size distribution - optical and aerodynamic diameter</td><td>Aerodynamic & Optical Particle Size Spectrometer</td><td>in preparation</td></tr><tr><td>Particle number concentration</td><td>Condensation Particle Counter</td><td>CEN</td></tr><tr><td>Mass concentration of particulate elements</td><td>Filter-based XRF/PIXE/ICP_OES/ICP_MS</td><td>in preparation</td></tr><tr><td>Mass concentration of particulate organic tracers</td><td>Filter-based IC, GC-MS HPLC-MS, LC/MS</td><td>in preparation</td></tr><tr><td>Number concentration of cloud condensation nuclei</td><td>Cloud Condensation Nuclei Counter</td><td>Rose et al. (2008)</td></tr></tbody></table>
<table><tr><td></td><td>Mass concentration of non-refractory particulate organics and inorganics</td><td>Aerosol Mass Spectrometer</td><td>in preparation</td></tr><tr><td></td><td>Nanoparticle number concentration</td><td>nCNC</td><td>in preparation</td></tr><tr><td></td><td>Nanoparticle number size distribution</td><td>Scanning PSM, (N)AIS, N-MPSS</td><td>in preparation</td></tr><tr><td rowspan="7">Added-value variables - Not required at ACTRIS Observational Platforms connected to the Centre For Aerosol In Situ Measurements, but recommended for comprehensive studies of aerosol processes</td><td>Refractory black carbon</td><td>SP2</td><td>to be prepared</td></tr><tr><td>Atmospheric concentration of SO<sub>2</sub></td><td>UV-Absorption</td><td>CEN</td></tr><tr><td>Atmospheric concentration of CO</td><td>Multiple techniques</td><td>CEN/ICOS</td></tr><tr><td>Particle mass concentration PM<sub>10</sub> and/or PM<sub>2.5</sub></td><td>Filter-based gravimetric / On-line equivalent technique</td><td>CEN</td></tr><tr><td>Mass of major ions in PM<sub>10</sub> or PM<sub>2.5</sub></td><td>Filter-based Ion chromatography</td><td>WMO/EMEP</td></tr><tr><td>Atmospheric concentrations of O<sub>3</sub>, NO</td><td>Chemiluminescence</td><td rowspan="2">CEN</td></tr><tr><td>Atmospheric concentrations of NO<sub>2</sub></td><td>Multiple techniques</td></tr></table>

<table><tr><td rowspan="2">Auxiliary variables</td><td>Meteorological Measurements (RH, T, Wind)</td><td>Anemometer, P, T, RH probes</td><td>WMO CIMO standards</td></tr><tr><td>UV, IR, Total radiation</td><td>Pyranometers</td><td>possibly WMO CIMO standards</td></tr></table>
<table><tr><td></td><td>Mass concentration of non-refractory particulate organics and inorganics</td><td>Aerosol Mass Spectrometer</td><td>in preparation</td></tr><tr><td></td><td>Nanoparticle number concentration</td><td>nCNC</td><td>in preparation</td></tr><tr><td></td><td>Nanoparticle number size distribution</td><td>Scanning PSM, (N)AIS, N-MPSS</td><td>in preparation</td></tr><tr><td rowspan="7">Added-value variables - Not required at ACTRIS Observational Platforms connected to the Centre For Aerosol In Situ Measurements, but recommended for comprehensive studies of aerosol processes</td><td>Refractory black carbon</td><td>SP2</td><td>to be prepared</td></tr><tr><td>Atmospheric concentration of SO<sub>2</sub></td><td>UV-Absorption</td><td>CEN</td></tr><tr><td>Atmospheric concentration of CO</td><td>Multiple techniques</td><td>CEN/ICOS</td></tr><tr><td>Particle mass concentration PM<sub>10</sub> and/or PM<sub>2.5</sub></td><td>Filter-based gravimetric / On-line equivalent technique</td><td>CEN</td></tr><tr><td>Mass of major ions in PM<sub>10</sub> or PM<sub>2.5</sub></td><td>Filter-based Ion chromatography</td><td>WMO/EMEP</td></tr><tr><td>Atmospheric concentrations of O<sub>3</sub>, NO</td><td>Chemiluminescence</td><td rowspan="2">CEN</td></tr><tr><td>Atmospheric concentrations of NO<sub>2</sub></td><td>Multiple techniques</td></tr></table>

<table><tr><td rowspan="2">Auxiliary variables</td><td>Meteorological Measurements (RH, T, Wind)</td><td>Anemometer, P, T, RH probes</td><td>WMO CIMO standards</td></tr><tr><td>UV, IR, Total radiation</td><td>Pyranometers</td><td>possibly WMO CIMO standards</td></tr></table>
#### 4.4.3.4 Mass concentration of particulate organic & elemental carbon

Thermal-optical analysers are used to determine the amount of organic carbon (OC) and elemental carbon (EC) deposited on quartz fibre filters after active sampling. Thermal-optical analysers can be calibrated for total carbon measurements by the user, but not for OC and EC due to a lack of suitable commercial standard materials. ACTRIS stations delivering OC and EC concentrations shall participate in round-robin inter-laboratory exercises at least once a year.

#### 4.4.3.5 Particle number size distribution – optical and aerodynamic diameter (0.7 – 10 µm)

Particle number size distribution of particles in the upper accumulation and in the coarse mode range can be measured either by an **optical particle size spectrometer** (OPSS) based on the intensity of particle light scattering (optical diameter) or by an **aerodynamic particle size spectrometer** (APSS) based on time-of-flight in an accelerated flow (aerodynamic diameter). Yearly or every two years calibrations of OPSS and APSS against a reference instruments at the TC are required.

#### 4.4.3.6 Particle number concentration (>$0.010$ µm)

The **particle number concentration** can be determined by CPCs, which are commercially available. Modern CPCs are based on a continuous aerosol flow, work in a single counting mode, and use most often butanol as working fluid. They need regular checks on-site to quality-assure the measurement. Additionally, yearly or every two years calibrations against a reference instruments at the TC are required.

#### 4.4.3.7 Mass concentration of particulate elements

Mineral dust particles consist of crustal material originating mostly from suspension of exposed soil by wind in arid and semi-arid areas such as desert and agricultural regions, but also from re-suspended road dust. Mineral dust particles are primarily in the coarse mode. It is recommended that a **multi-elemental analysis** approach should be used to determine the **mineral dust and associated heavy metal components**, using filter or impactor samples and techniques such as PIXE, INAA, XRF, AAS, ICP-MS and ICP-OES. It is also recommended that ISO standard procedures are followed, whenever available when using these techniques.

#### 4.4.3.8 Mass concentration of particulate organic tracers

Knowledge of specific tracers in aerosol particles provides information on their sources. The ACTRIS measurement program includes the determination of the **mass concentration of specific organic tracers** from **filter samples**. Organic tracers include different type of compounds that can trace biomass-burning activities (levoglucosan), industrial processes or road traffic.

#### 4.4.3.9 Cloud condensation nuclei number concentration (at various supersaturations)

Cloud Condensation Nuclei (CCN) are particles, which are capable to form cloud droplets at a given supersaturation. The **number concentration of CCN** can be determined by a **Cloud Condensation Nuclei Counter** (CCNC). Commercial CCNC measure the CCN number concentration over a limited range of supersaturations. CCNC need regular calibrations and checks on-site to quality-assure the measurement. Additionally, yearly or every two years calibrations against a reference instruments at the TC are required.
#### 4.4.3.4 Mass concentration of particulate organic & elemental carbon

Thermal-optical analysers are used to determine the amount of organic carbon (OC) and elemental carbon (EC) deposited on quartz fibre filters after active sampling. Thermal-optical analysers can be calibrated for total carbon measurements by the user, but not for OC and EC due to a lack of suitable commercial standard materials. ACTRIS stations delivering OC and EC concentrations shall participate in round-robin inter-laboratory exercises at least once a year.

#### 4.4.3.5 Particle number size distribution – optical and aerodynamic diameter (0.7 – 10 µm)

Particle number size distribution of particles in the upper accumulation and in the coarse mode range can be measured either by an **optical particle size spectrometer** (OPSS) based on the intensity of particle light scattering (optical diameter) or by an **aerodynamic particle size spectrometer** (APSS) based on time-of-flight in an accelerated flow (aerodynamic diameter). Yearly or every two years calibrations of OPSS and APSS against a reference instruments at the TC are required.

#### 4.4.3.6 Particle number concentration (≥ 0.010 µm)

The **particle number concentration** can be determined by CPCs, which are commercially available. Modern CPCs are based on a continuous aerosol flow, work in a single counting mode, and use most often butanol as working fluid. They need regular checks on-site to quality-assure the measurement. Additionally, yearly or every two years calibrations against a reference instruments at the TC are required.

#### 4.4.3.7 Mass concentration of particulate elements

Mineral dust particles consist of crustal material originating mostly from suspension of exposed soil by wind in arid and semi-arid areas such as desert and agricultural regions, but also from re-suspended road dust. Mineral dust particles are primarily in the coarse mode. It is recommended that a **multi-elemental analysis** approach should be used to determine the **mineral dust and associated heavy metal components**, using filter or impactor samples and techniques such as PIXE, INAA, XRF, AAS, ICP-MS and ICP-OES. It is also recommended that ISO standard procedures are followed, whenever available when using these techniques.

#### 4.4.3.8 Mass concentration of particulate organic tracers

Knowledge of specific tracers in aerosol particles provides information on their sources. The ACTRIS measurement program includes the determination of the **mass concentration of specific organic tracers** from **filter samples**. Organic tracers include different type of compounds that can trace biomass-burning activities (levoglucosan), industrial processes or road traffic.

#### 4.4.3.9 Cloud condensation nuclei number concentration (at various supersaturations)

Cloud Condensation Nuclei (CCN) are particles, which are capable to form cloud droplets at a given supersaturation. The **number concentration of CCN** can be determined by a **Cloud Condensation Nuclei Counter** (CCNC). Commercial CCNC measure the CCN number concentration over a limited range of supersaturations. CCNC need regular calibrations and checks on-site to quality-assure the measurement. Additionally, yearly or every two years calibrations against a reference instruments at the TC are required.
#### 4.4.3.10 Mass concentration of non-refractory organic and inorganic aerosols

Aerosol Mass Spectrometers (AMS) measure concentrations of particulate sulphate, nitrate, ammonium, chloride, and organic mass at the same time. Upon contact with the heater, the non-refractory components of the particles are vaporized and ionized by electron impact, resulting in charged mass fragments that are detected either by a quadrupole or a time-of-flight mass spectrometer. Some of the current commercially available AMS are robust and designed for long-term operations. They need regular calibrations and checks on-site to quality-assure the measurement. Additionally, yearly or every two years calibrations against a reference instruments at the TC are required.

#### 4.4.3.11 Nanoparticle number concentration ($< 0.01 \ \mu m$)

Nano-CNCs (nCNC) are used for counting particles in the range of 1–10 nm. The working principle of a Nano-CNC is to mix turbulently cooled sample flow with heated clean airflow saturated by the working fluid. The resulting supersaturations grow the particles to sizes which can be measured by a standard CPC. Annual or every two years calibrations against a reference instruments at the TC are required.

#### 4.4.3.12 Nanoparticle number size distribution (0.001 – 0.02 µm)

The size distribution below 20 nm can be measured with N-MPSS systems, where the sampling and instrumental losses are minimized and a suitable CPC is used for a detector. Furthermore, ion spectrometers (Balanced Scanning Mobility Analyser, BSMA, and Neutral Cluster & Air Ion Spectrometer, NAIS, N-MPSS) can be used to measure ions down to 0.8 nm in diameter. NAIS can additionally detect neutral particles down to ~2 nm. Annual or every two years calibrations against a reference instruments at the TC are required.

### 4.4.4 Operation

Operations at ACTRIS Observational Platforms are expected to follow guidelines recommended in the reports and papers listed in Tab. 1 and any additional material provided by the Centre for Aerosol In Situ Measurements.

It is expected that data coverage should allow robust documenting of variability at different scales: diurnal, monthly, seasonal and annual. This implies that instruments should be operated at least 70% of the time under normal operating conditions. Participation to the calibration exercise organized by the TC is mandatory.

Within ACTRIS aerosol in situ data production for on-line data, by default and whenever possible a RRT schedule submission should be set up, according to the agreed principles.
#### 4.4.3.10 Mass concentration of non-refractory organic and inorganic aerosols

Aerosol Mass Spectrometers (AMS) measure concentrations of particulate sulphate, nitrate, ammonium, chloride, and organic mass at the same time. Upon contact with the heater, the non-refractory components of the particles are vaporized and ionized by electron impact, resulting in charged mass fragments that are detected either by a quadrupole or a time-of-flight mass spectrometer. Some of the current commercially available AMS are robust and designed for long-term operations. They need regular calibrations and checks on-site to quality-assure the measurement. Additionally, yearly or every two years calibrations against a reference instruments at the TC are required.

#### 4.4.3.11 Nanoparticle number concentration ($< 0.01 \ \mu m$)

Nano-CNCs (nCNC) are used for counting particles in the range of 1–10 nm. The working principle of a Nano-CNC is to mix turbulently cooled sample flow with heated clean airflow saturated by the working fluid. The resulting supersaturations grow the particles to sizes which can be measured by a standard CPC. Annual or every two years calibrations against a reference instruments at the TC are required.

#### 4.4.3.12 Nanoparticle number size distribution ($0.001 – 0.02 \ \mu m$)

The size distribution below 20 nm can be measured with N-MPSS systems, where the sampling and instrumental losses are minimized and a suitable CPC is used for a detector. Furthermore, ion spectrometers (Balanced Scanning Mobility Analyser, BSMA, and Neutral Cluster & Air Ion Spectrometer, NAIS, N-MPSS) can be used to measure ions down to 0.8 nm in diameter. NAIS can additionally detect neutral particles down to ~2 nm. Annual or every two years calibrations against a reference instruments at the TC are required.

### 4.4.4 Operation

Operations at ACTRIS Observational Platforms are expected to follow guidelines recommended in the reports and papers listed in Tab. 1 and any additional material provided by the Centre for Aerosol In Situ Measurements.

It is expected that data coverage should allow robust documenting of variability at different scales: diurnal, monthly, seasonal and annual. This implies that instruments should be operated at least 70% of the time under normal operating conditions. Participation to the calibration exercise organized by the TC is mandatory.

Within ACTRIS aerosol in situ data production for on-line data, by default and whenever possible a RRT schedule submission should be set up, according to the agreed principles.
### 4.4.5 Data production and data products

The final data product (ACTRIS data Level 1 and Level 2) consists of various ACTRIS aerosol particle properties with the respective precision and accuracy, metadata and time dependent flags as specified by the Data Centre and in the standard operating procedures. All operational and planned aerosol in situ variables are listed in Annex A.4.

Raw measurement data (Level 0) at temperature and pressure conditions as provided by the on-line instruments (in situ measurement) has to be stored together with the relevant standard and calibration measurements and all information and metadata, which is needed to calculate the higher-level data products. Off-line measurements (in situ sampling) do not deliver Level 0 data. Details for the data production are given in the ACTRIS Data Management Plan (D4.2) and the associated data production descriptions and documents provided by the Data Centre and the Centre for Aerosol In Situ Measurements. All data products, pre-products and software tools are version controlled and identified by the associated DC unit.

ACTRIS aerosol in situ data production follows one of two main workflows, on-line or off-line. For the on-line workflow, data production is based on Level 0 data, i.e., all raw data and signals provided by an instrument in its native time resolution and native conditions of temperature and pressure, brought to a harmonized format, and annotated with all discovery and metadata needed in the further data production process. Level 0 data are to be submitted and archived at the Data Centre unless the ACTRIS Data Management Plan (D4.2), with the associated data production descriptions, defines an exception caused by, e.g., large raw data volumes. In this case, an alternative storage procedure has to be agreed between the Centre for Aerosol In Situ Measurements and the Data Centre, which meets long-term archive criteria. The Level 0 data submission includes relevant calibration measurements and measurements of traceable standards needed for data quality control, which are flagged as such. From Level 0 data, Level 1 and Level 2 data are produced. Level 1 data are quality-controlled, calibrations are applied, final variable calculated, native time resolution, invalid and quality control measurements removed, and transferred to standard conditions of temperature and pressure where applicable. Level 2 data is averaged Level 1 data to typically hourly averages, with measure of atmospheric variability included. The Level 1 and Level 2 data can be produced either by the NF or the associated Data Centre unit, depending on variable. The distribution of work is included in the associated data production descriptions available from the Data Centre, and indicated in Annex A.4. The quality control step between Levels 0 and 1 can be automatic (RRT data production) or manual, performed by the NF. Data level identifiers distinguish between data having received automatic or manual QC. The details of the steps producing Level 1 from Level 0, and Level 2 from Level 1, are specified in the associated data production descriptions and the standard operating procedures.

For the off-line workflow, the steps of sample medium pre-exposure treatment, exposure, preparation, sampling, and analysis are documented with standard operating procedures and protocols including discovery and use metadata in machine-readable form. Items to be included are specified in the associated data production descriptions and the standard operating procedures. The archive for the protocols is located at the NF, its operating protocols coordinated with and approved by the DC and the TC. From these protocols, the Level 2 data products are produced in the temporal resolution determined by the
### 4.4.5 Data production and data products

The final data product (ACTRIS data Level 1 and Level 2) consists of various ACTRIS aerosol particle properties with the respective precision and accuracy, metadata and time dependent flags as specified by the Data Centre and in the standard operating procedures. All operational and planned aerosol in situ variables are listed in Annex A.4.

Raw measurement data (Level 0) at temperature and pressure conditions as provided by the on-line instruments (in situ measurement) has to be stored together with the relevant standard and calibration measurements and all information and metadata, which is needed to calculate the higher-level data products. Off-line measurements (in situ sampling) do not deliver Level 0 data. Details for the data production are given in the ACTRIS Data Management Plan (D4.2) and the associated data production descriptions and documents provided by the Data Centre and the Centre for Aerosol In Situ Measurements. All data products, pre-products and software tools are version controlled and identified by the associated DC unit.

ACTRIS aerosol in situ data production follows one of two main workflows, on-line or off-line. For the on-line workflow, data production is based on Level 0 data, i.e., all raw data and signals provided by an instrument in its native time resolution and native conditions of temperature and pressure, brought to a harmonized format, and annotated with all discovery and metadata needed in the further data production process. Level 0 data are to be submitted and archived at the Data Centre unless the ACTRIS Data Management Plan (D4.2), with the associated data production descriptions, defines an exception caused by, e.g., large raw data volumes. In this case, an alternative storage procedure has to be agreed between the Centre for Aerosol In Situ Measurements and the Data Centre, which meets long-term archive criteria. The Level 0 data submission includes relevant calibration measurements and measurements of traceable standards needed for data quality control, which are flagged as such. From Level 0 data, Level 1 and Level 2 data are produced. Level 1 data are quality-controlled, calibrations are applied, final variable calculated, native time resolution, invalid and quality control measurements removed, and transferred to standard conditions of temperature and pressure where applicable. Level 2 data is averaged Level 1 data to typically hourly averages, with measure of atmospheric variability included. The Level 1 and Level 2 data can be produced either by the NF or the associated Data Centre unit, depending on variable. The distribution of work is included in the associated data production descriptions available from the Data Centre, and indicated in Annex A.4. The quality control step between Levels 0 and 1 can be automatic (RRT data production) or manual, performed by the NF. Data level identifiers distinguish between data having received automatic or manual QC. The details of the steps producing Level 1 from Level 0, and Level 2 from Level 1, are specified in the associated data production descriptions and the standard operating procedures.

For the off-line workflow, the steps of sample medium pre-exposure treatment, exposure, preparation, sampling, and analysis are documented with standard operating procedures and protocols including discovery and use metadata in machine-readable form. Items to be included are specified in the associated data production descriptions and the standard operating procedures. The archive for the protocols is located at the NF, its operating protocols coordinated with and approved by the DC and the TC. From these protocols, the Level 2 data products are produced in the temporal resolution determined by the
sampling schedule, and transferred to the DC using the tools provided for data submission to ensure
required documentation, flagging and metadata. The details of the procedure are specified in the
associated data production descriptions.

4.4.6 Data delivery and quality control

It is compulsory for the NF to produce at least the Level 1 and Level 2 data for archiving at the associated Data Centre unit. Archiving of Level 0 data will depend on the specific variable. For the submission of aerosol *in situ* data, required procedures are described in the ACTRIS Data Management Plan (D4.2) and the associated data production descriptions. Manually quality controlled data has to be submitted on a regular, scheduled frequency, at least yearly to the ACTRIS DC following the scheme in the ACTRIS Data Management Plan (D4.2), and associated documents. Submission to the DC has to be done before 31 May, using the submission tools and automatic quality control software available within the DC. Data originators at the NF are responsible for checking the data using the procedures in the associated data production descriptions. Data is then reviewed by the Centre for Aerosol In Situ Measurements and will be discussed at the annual data quality meeting or teleconference, in collaboration with the DC. An issue tracker is operated in the process of data evaluation to ensure full documentation and traceability of the data production and quality control process. This comes into place yearly following the initial data submission by stations to the ACTRIS DC. Upon errors, the data has to be re-submitted and will be reviewed again for compliance with suggested changes.

Whenever possible by measurement principle and connectivity to the station, Level 0 data must be
transmitted to the Data Centre in RRT, i.e., latest within 3 hours of measurement, for delivery of an RRT
data product for operational services.
sampling schedule, and transferred to the DC using the tools provided for data submission to ensure
required documentation, flagging and metadata. The details of the procedure are specified in the
associated data production descriptions.

4.4.6 Data delivery and quality control

It is compulsory for the NF to produce at least the Level 1 and Level 2 data for archiving at the associated Data Centre unit. Archiving of Level 0 data will depend on the specific variable. For the submission of aerosol *in situ* data, required procedures are described in the ACTRIS Data Management Plan (D4.2) and the associated data production descriptions. Manually quality controlled data has to be submitted on a regular, scheduled frequency, at least yearly to the ACTRIS DC following the scheme in the ACTRIS Data Management Plan (D4.2), and associated documents. Submission to the DC has to be done before 31 May, using the submission tools and automatic quality control software available within the DC. Data originators at the NF are responsible for checking the data using the procedures in the associated data production descriptions. Data is then reviewed by the Centre for Aerosol In Situ Measurements and will be discussed at the annual data quality meeting or teleconference, in collaboration with the DC. An issue tracker is operated in the process of data evaluation to ensure full documentation and traceability of the data production and quality control process. This comes into place yearly following the initial data submission by stations to the ACTRIS DC. Upon errors, the data has to be re-submitted and will be reviewed again for compliance with suggested changes.

Whenever possible by measurement principle and connectivity to the station, Level 0 data must be
transmitted to the Data Centre in RRT, i.e., latest within 3 hours of measurement, for delivery of an RRT
data product for operational services.
## 4.5 Cloud in situ measurements

### 4.5.1 Introduction

Clouds are an important component of the atmosphere, influencing a large number of physical and chemical properties of the Earth's atmosphere and thereby strongly impacting climate, air quality, ecosystems and associated interactions. Cloud observations in ACTRIS and preceding networks have so far focussed on remote-sensing techniques, delivering information on temporal and vertical variation of cloud properties and structures. *In situ* observations of clouds and cloud-related parameters provide additional, very detailed insights into important processes related to the formation, the structure, and the chemical composition of clouds, which feedback to associated aerosol properties and thus impact the atmosphere as a whole.

Cloud *in situ* measurements will therefore be an important new observational component within ACTRIS. They will be performed at ground-based field stations, typically on mountains, which are frequently covered in clouds. Being a multiphase system consisting of gas, particle, liquid-water and ice phases with complex interactions, clouds represent a challenging system to study. ACTRIS cloud *in situ* observations will focus on a set of easily accessible variables and extend to technically more demanding parameters in the future. In contrast to other observational components, ground-based cloud *in situ* measurements still lack a long record of standardisation efforts. These will, however, be enforced within the ACTRIS implementation phase, such that this gap will be closed and high-quality data will be ensured until the start of the operational phase of the RI in 2025.

### 4.5.2 Observational capabilities

#### 4.5.2.1 Optimum setup

The optimum setup of cloud *in situ* observations consists of a number of **microphysical and chemical measurements**, which provide crucial parameters to study cloud processes, see also Annex A.5: *Liquid water content (LWC), effective droplet diameter (D<sub>eff</sub>), cloud droplet number concentration and size distribution, ice particle number concentration and size distribution* (in mixed-phase clouds), *ice nucleating particles (INP) number concentration and temperature spectra* (outside cloud periods), *cloud condensation nuclei (CCN) number concentration, interstitial particle number concentration and size distribution, cloud residuals particle number concentration and composition, total particle number concentration and size distribution, bulk cloud water chemical composition*.

As the microphysical parameters can be measured in an automated fashion by the instrumentation given below, they should be provided for all clouds occurring at the site. In contrast, the chemical measurements need manual interaction and are more labour intensive, which is why they should be provided at least for a 6-weeks campaign each year with sampling of all occurring clouds and a maximum collection time per sample of 24 hours or – alternatively – over a period of 4 months per year with more infrequent sampling of occurring clouds, but at least one cloud water sample per cloud event.
## 4.5 Cloud in situ measurements

### 4.5.1 Introduction

Clouds are an important component of the atmosphere, influencing a large number of physical and chemical properties of the Earth's atmosphere and thereby strongly impacting climate, air quality, ecosystems and associated interactions. Cloud observations in ACTRIS and preceding networks have so far focussed on remote-sensing techniques, delivering information on temporal and vertical variation of cloud properties and structures. *In situ* observations of clouds and cloud-related parameters provide additional, very detailed insights into important processes related to the formation, the structure, and the chemical composition of clouds, which feedback to associated aerosol properties and thus impact the atmosphere as a whole.

Cloud *in situ* measurements will therefore be an important new observational component within ACTRIS. They will be performed at ground-based field stations, typically on mountains, which are frequently covered in clouds. Being a multiphase system consisting of gas, particle, liquid-water and ice phases with complex interactions, clouds represent a challenging system to study. ACTRIS cloud *in situ* observations will focus on a set of easily accessible variables and extend to technically more demanding parameters in the future. In contrast to other observational components, ground-based cloud *in situ* measurements still lack a long record of standardisation efforts. These will, however, be enforced within the ACTRIS implementation phase, such that this gap will be closed and high-quality data will be ensured until the start of the operational phase of the RI in 2025.

### 4.5.2 Observational capabilities

#### 4.5.2.1 Optimum setup

The optimum setup of cloud *in situ* observations consists of a number of **microphysical and chemical measurements**, which provide crucial parameters to study cloud processes, see also Annex A.5: *Liquid water content (LWC), effective droplet diameter (D<sub>eff</sub>), cloud droplet number concentration and size distribution, ice particle number concentration and size distribution* (in mixed-phase clouds), *ice nucleating particles (INP) number concentration and temperature spectra* (outside cloud periods), *cloud condensation nuclei (CCN) number concentration, interstitial particle number concentration and size distribution, cloud residuals particle number concentration and composition, total particle number concentration and size distribution, bulk cloud water chemical composition*.

As the microphysical parameters can be measured in an automated fashion by the instrumentation given below, they should be provided for all clouds occurring at the site. In contrast, the chemical measurements need manual interaction and are more labour intensive, which is why they should be provided at least for a 6-weeks campaign each year with sampling of all occurring clouds and a maximum collection time per sample of 24 hours or – alternatively – over a period of 4 months per year with more infrequent sampling of occurring clouds, but at least one cloud water sample per cloud event.
### 4.5.2.2 Minimum requirements

The minimum requirements to qualify as an ACTRIS cloud *in situ* observational site are (i) the **occurrence of clouds** at the site, defined by periodical LWC > 0.05 g m⁻³ at least during two seasons of a year, (ii) the measurements of LWC and D-eff, and (iii) the measurements of at least two other specializing cloud variables, which are listed in Annex A.5 and for which the Centre for Cloud In Situ Measurements is providing operation support.

### 4.5.2.3 Complementary added-value observations

Measurements of **visibility, precipitation intensity, diffuse radiation** and **vertical wind speeds** can provide additional value to the observation of cloud *in situ* parameters and are therefore recommended, as well as the measurement of **total condensed water content** in mixed-phase clouds.

### 4.5.2.4 Auxiliary measurements

Measurements of **meteorological parameters** (temperature, pressure, relative humidity, wind speed, wind direction, global radiation) can supplement cloud *in situ* data and support dedicated process studies. Stations should therefore be equipped with automatic weather stations, ideally including sensors for precipitation.

## 4.5.3 Instrumentation and calibration

### 4.5.3.1 In situ observations of cloud liquid water content

A number of instruments are available to measure the LWC, commonly expressed in gram of liquid water per cubic meter of air. Instruments can be divided into two types: (i) **integrating probes** like the Particle Volume Monitor (PVM) and the Present Weather Detector (PWD) that integrate the scattering of cloud droplets over a given open path, and (ii) **size distribution probes** like the Forward Scattering Spectrometer Probe (FSSP), Fog Monitors (FM) and Cloud Droplet Probes (CDP) that actively operate with a controlled flow entering the probe. The most commonly used of these instruments at ground-based stations is the PVM probe, which was shown to give a stable and reliable measurement most adapted for monitoring purposes and does not suffer from bias due to the orientation of the probe into the wind direction (Guyot et al. 2015). There are approximately 5 of these instruments operating on a regular basis at ACTRIS facilities throughout Europe. These probes have been the subject of intercomparison campaigns during ACTRIS-2 (deliverable D3.12).

The ground-based PVM probes are usually installed on a fixed platform on the roof of the sampling site. The PVM measures the laser light (at $\lambda = 0.780 \ \mu m$) scattered in the forward direction by an ensemble of cloud droplets which crosses the probe's sampling volume of 3 cm³. The light scattered is collected by a system of lenses and directed through two spatial filters. The first filter converts scattered light to a signal proportional to the particle volume density (or LWC).

Recommended SOPs for the liquid water content measurements have been supplied in ACTRIS-2 (deliverable D3.12).
### 4.5.2.2 Minimum requirements

The minimum requirements to qualify as an ACTRIS cloud *in situ* observational site are (i) the **occurrence of clouds** at the site, defined by periodical LWC > 0.05 g m⁻³ at least during two seasons of a year, (ii) the measurements of LWC and D-eff, and (iii) the measurements of at least two other specializing cloud variables, which are listed in Annex A.5 and for which the Centre for Cloud In Situ Measurements is providing operation support.

### 4.5.2.3 Complementary added-value observations

Measurements of **visibility, precipitation intensity, diffuse radiation** and **vertical wind speeds** can provide additional value to the observation of cloud *in situ* parameters and are therefore recommended, as well as the measurement of **total condensed water content** in mixed-phase clouds.

### 4.5.2.4 Auxiliary measurements

Measurements of **meteorological parameters** (temperature, pressure, relative humidity, wind speed, wind direction, global radiation) can supplement cloud *in situ* data and support dedicated process studies. Stations should therefore be equipped with automatic weather stations, ideally including sensors for precipitation.

## 4.5.3 Instrumentation and calibration

### 4.5.3.1 In situ observations of cloud liquid water content

A number of instruments are available to measure the LWC, commonly expressed in gram of liquid water per cubic meter of air. Instruments can be divided into two types: (i) **integrating probes** like the Particle Volume Monitor (PVM) and the Present Weather Detector (PWD) that integrate the scattering of cloud droplets over a given open path, and (ii) **size distribution probes** like the Forward Scattering Spectrometer Probe (FSSP), Fog Monitors (FM) and Cloud Droplet Probes (CDP) that actively operate with a controlled flow entering the probe. The most commonly used of these instruments at ground-based stations is the PVM probe, which was shown to give a stable and reliable measurement most adapted for monitoring purposes and does not suffer from bias due to the orientation of the probe into the wind direction (Guyot et al. 2015). There are approximately 5 of these instruments operating on a regular basis at ACTRIS facilities throughout Europe. These probes have been the subject of intercomparison campaigns during ACTRIS-2 (deliverable D3.12).

The ground-based PVM probes are usually installed on a fixed platform on the roof of the sampling site. The PVM measures the laser light (at $\lambda = 0.780 \ \mu m$) scattered in the forward direction by an ensemble of cloud droplets which crosses the probe's sampling volume of 3 cm³. The light scattered is collected by a system of lenses and directed through two spatial filters. The first filter converts scattered light to a signal proportional to the particle volume density (or LWC).

Recommended SOPs for the liquid water content measurements have been supplied in ACTRIS-2 (deliverable D3.12).
#### 4.5.3.2 In situ observation of cloud droplet effective diameter

$D_{\text{eff}}$ is a single variable that gives an indication on the average size of a given cloud droplet population. It is commonly expressed in µm, and it is derived from **integrated measurements** performed by PVM-type instrumentation. In the PVM, the first filter converts scattered light into LWC, while the second filter converts scattered light to a signal proportional to the particle surface area density (PSA, Gerber et al. 1994). From the ratio of these two quantities, $D_{\text{eff}}$ is derived. These two filters guarantee a linear relationship between scattering intensity and LWC or PSA for droplet diameters from 3 to 50 µm (Gerber et al. 1994). Calibration for instrument droplet diameter is the same as that above for LWC.

Recommended SOPs for cloud droplet distributions measurements ($D_{\text{eff}}$) have been supplied in ACTRIS-2 (deliverable D3.12).

#### 4.5.3.3 In situ observation of cloud droplet number concentration and size distribution

The **number size distribution of cloud droplets** varies and depends on many things like CCN concentration, CCN chemical composition and water supersaturation. The mean cloud droplet diameters are typically 5–15 µm, the size distribution varies between 1 and 100 µm. Clouds typically contain few hundred droplets in one cubic centimetre, varying from few tens to few thousands in a cubic centimetre (e.g., Henning et al. 2002, Miles et al. 2000). Number size distributions of cloud droplets are measured usually optically taking advance of forward scattering of light from liquid droplets (e.g., Spiegel et al. 2012). There are several **cloud droplet probes** available, most of them are designed to be installed into aircraft (e.g., Lance et al. 2010). Such cloud probes need special inlet arrangements to get droplets through the measurement area when applied in *in situ* cloud measurements. The alignment of the inlet of the probes should also be facing directly towards the wind. The probes typically measure the number size distribution between about 3 and 50 µm. Probes are usually calibrated with known sized glass beads and spinning pinholes.

Recommended SOPs will be developed within the implementation and pre-operational phases by the Centre for Cloud In Situ Measurements.

#### 4.5.3.4 In situ observation of ice particle number concentration and size distribution

The **ice particle number concentration** is important for those platforms where mixed-phase or ice clouds occur naturally. It is usually measured with the following cloud-ice probes:

* **Photodiode imaging spectrometers**, e.g., 2D-Stereoscopic spectrometers (2DS, 2DS-GS) and Cloud/Precipitation Imaging Probes (CIP-15, CIP-15, PIP), Cloud Aerosol Precipitation Spectrometers (CAPS), High Volume Precipitation Spectrometer (HVPS);

* **CCD Imaging Probes**, e.g., Cloud Particle Imager (µCPI, µCOPP);

* **2D spatial scattering imaging probes**, e.g., Small Ice Detectors and Phase Particle Discriminators (SID, PPD-2);

* **Holographic Spectrometers**, e.g., HaloHolo, Holimo;
#### 4.5.3.2 In situ observation of cloud droplet effective diameter

$D_{\text{eff}}$ is a single variable that gives an indication on the average size of a given cloud droplet population. It is commonly expressed in µm, and it is derived from **integrated measurements** performed by PVM-type instrumentation. In the PVM, the first filter converts scattered light into LWC, while the second filter converts scattered light to a signal proportional to the particle surface area density (PSA, Gerber et al. 1994). From the ratio of these two quantities, $D_{\text{eff}}$ is derived. These two filters guarantee a linear relationship between scattering intensity and LWC or PSA for droplet diameters from 3 to 50 µm (Gerber et al. 1994). Calibration for instrument droplet diameter is the same as that above for LWC.

Recommended SOPs for cloud droplet distributions measurements ($D_{\text{eff}}$) have been supplied in ACTRIS-2 (deliverable D3.12).

#### 4.5.3.3 In situ observation of cloud droplet number concentration and size distribution

The **number size distribution of cloud droplets** varies and depends on many things like CCN concentration, CCN chemical composition and water supersaturation. The mean cloud droplet diameters are typically 5–15 µm, the size distribution varies between 1 and 100 µm. Clouds typically contain few hundred droplets in one cubic centimetre, varying from few tens to few thousands in a cubic centimetre (e.g., Henning et al. 2002, Miles et al. 2000). Number size distributions of cloud droplets are measured usually optically taking advance of forward scattering of light from liquid droplets (e.g., Spiegel et al. 2012). There are several **cloud droplet probes** available, most of them are designed to be installed into aircraft (e.g., Lance et al. 2010). Such cloud probes need special inlet arrangements to get droplets through the measurement area when applied in *in situ* cloud measurements. The alignment of the inlet of the probes should also be facing directly towards the wind. The probes typically measure the number size distribution between about 3 and 50 µm. Probes are usually calibrated with known sized glass beads and spinning pinholes.

Recommended SOPs will be developed within the implementation and pre-operational phases by the Centre for Cloud In Situ Measurements.

#### 4.5.3.4 In situ observation of ice particle number concentration and size distribution

The **ice particle number concentration** is important for those platforms where mixed-phase or ice clouds occur naturally. It is usually measured with the following cloud-ice probes:

* **Photodiode imaging spectrometers**, e.g., 2D-Stereoscopic spectrometers (2DS, 2DS-GS) and Cloud/Precipitation Imaging Probes (CIP-15, CIP-15, PIP), Cloud Aerosol Precipitation Spectrometers (CAPS), High Volume Precipitation Spectrometer (HVPS);

* **CCD Imaging Probes**, e.g., Cloud Particle Imager (µCPI, µCOPP);

* **2D spatial scattering imaging probes**, e.g., Small Ice Detectors and Phase Particle Discriminators (SID, PPD-2);

* **Holographic Spectrometers**, e.g., HaloHolo, Holimo;
* **Depolarisation Particle Spectrometers**, e.g., Cloud Aerosol Spectrometer with Polarisation (CAS-POL), backscatter Cloud Probe with Depolarisation (BCP-D).

Cloud-ice particle phase monitoring is problematic due to the large range of particle sizes and shapes, from 1 µm to 2 cm or more, and the definition of ice particle size. No one instrument can fully define the necessary ice cloud parameters, and field experiments use a combination of the above instruments. Ice cloud parameters include ice crystal number size distribution, Nice(D), and ice crystal shape/habit. At present, although some information on the response of the bulk cloud instrument, PVM, to ice particles is available, more work is required to deliver quantitative information, e.g., total ice particle concentration, Nice, Ice Water Content (IWC) and ice particle size distribution, Nice(D). Most of the above instruments have been developed for airborne applications, but have been used successfully in laboratory, chamber and ground-based field applications using appropriate aspirated inlets. These have been well characterised, documented and intercompared at ground-based field sites (Lloyd et al. 2015) and recommendations for specific instrument operation is available based on feedback from extensive cloud instrument workshops.

Field operation and protocols follow those for the cloud liquid water spectrometers for calibration particles and particle generator gun, with the addition of spinning disk rigs for simulated large ice particle size, absolute ice number concentrations and ice particle shape calibration. Annual to biennial calibration and maintenance with operator training at a chamber facility is recommended using repeatable, simulated ice cloud conditions.

#### 4.5.3.5 In situ observation of ice nucleating particle number concentration and temperature spectra

INP can currently only episodically be measured with **aerosol sampling** based methods and **off-line analysis** of the temperature spectra of ambient concentrations. These methods have a poor time resolution of only a few hours to a few days, but can deliver accurate INP concentrations in the temperature range from about -5°C to -25°C. These techniques, though relatively labour intensive, should be used for both exploratory and observational ACTRIS activities, but require a thorough assessment and calibration of the aerosol sampling methods (e.g., filters or impingers) and the INP analysis systems. Furthermore, the storage of the samples as well as their shipment to the laboratory should be standardized to assure comparability between different sampling locations and systems. It is anticipated that in the near future new INP instruments with higher time resolution and more automated operation may be available. The Centre for Cloud In Situ Measurements has to provide guidance for such new developments and develop calibration protocols and procedures.

#### 4.5.3.6 In situ observation of total and interstitial particle number concentration and size distribution

The simultaneous measurement of the **total and interstitial aerosol particle size distribution** provides the opportunity to derive the activation diameter ($D_{50}$) and (with assumption/knowledge of the particle hygroscopicity) the maximum supersaturation ($SS_{max}$) of the cloud (e.g., Krüger et al. 2014). Both parameters are important for cloud microphysical modelling purposes (Reutter et al. 2009). The needed size distribution of the activated particles (CCN) is then inferred by the difference of the total and
* **Depolarisation Particle Spectrometers**, e.g., Cloud Aerosol Spectrometer with Polarisation (CAS-POL), backscatter Cloud Probe with Depolarisation (BCP-D).

Cloud-ice particle phase monitoring is problematic due to the large range of particle sizes and shapes, from 1 µm to 2 cm or more, and the definition of ice particle size. No one instrument can fully define the necessary ice cloud parameters, and field experiments use a combination of the above instruments. Ice cloud parameters include ice crystal number size distribution, Nice(D), and ice crystal shape/habit. At present, although some information on the response of the bulk cloud instrument, PVM, to ice particles is available, more work is required to deliver quantitative information, e.g., total ice particle concentration, Nice, Ice Water Content (IWC) and ice particle size distribution, Nice(D). Most of the above instruments have been developed for airborne applications, but have been used successfully in laboratory, chamber and ground-based field applications using appropriate aspirated inlets. These have been well characterised, documented and intercompared at ground-based field sites (Lloyd et al. 2015) and recommendations for specific instrument operation is available based on feedback from extensive cloud instrument workshops.

Field operation and protocols follow those for the cloud liquid water spectrometers for calibration particles and particle generator gun, with the addition of spinning disk rigs for simulated large ice particle size, absolute ice number concentrations and ice particle shape calibration. Annual to biennial calibration and maintenance with operator training at a chamber facility is recommended using repeatable, simulated ice cloud conditions.

#### **4.5.3.5 In situ observation of ice nucleating particle number concentration and temperature spectra**

INP can currently only episodically be measured with **aerosol sampling** based methods and **off-line analysis** of the temperature spectra of ambient concentrations. These methods have a poor time resolution of only a few hours to a few days, but can deliver accurate INP concentrations in the temperature range from about -5°C to -25°C. These techniques, though relatively labour intensive, should be used for both exploratory and observational ACTRIS activities, but require a thorough assessment and calibration of the aerosol sampling methods (e.g., filters or impingers) and the INP analysis systems. Furthermore, the storage of the samples as well as their shipment to the laboratory should be standardized to assure comparability between different sampling locations and systems. It is anticipated that in the near future new INP instruments with higher time resolution and more automated operation may be available. The Centre for Cloud In Situ Measurements has to provide guidance for such new developments and develop calibration protocols and procedures.

#### **4.5.3.6 In situ observation of total and interstitial particle number concentration and size distribution**

The simultaneous measurement of the **total and interstitial aerosol particle size distribution** provides the opportunity to derive the activation diameter (D₅₀) and (with assumption/knowledge of the particle hygroscopicity) the maximum supersaturation (SSmax) of the cloud (e.g., Krüger et al. 2014). Both parameters are important for cloud microphysical modelling purposes (Reutter et al. 2009). The needed size distribution of the activated particles (CCN) is then inferred by the difference of the total and
interstitial particle size distribution. Both direct *in situ* observations require the combination of a dedicated inlet and an aerosol sizing instrument, which is able to measure down to at least 20 nm.

This approach was already successfully applied in numerous *in situ* cloud studies (Anttila et al. 2009, Anttila et al. 2012, Asmi et al. 2012, Ditas et al. 2012, Hammer et al. 2014, Henning et al. 2002) at different research field stations (Jungfraujoch, Pallas, Puy de Dôme). The **dedicated total and interstitial inlets** used at these field sites were individual versions and thus their sampling specifications might be different. For future cloud studies the total inlet should follow the design of Weingartner et al. (1999) to sample particles up to a size of 40 µm for wind speeds up to 20 m s⁻¹. The interstitial inlet should have a sharp cut-off at 2.5 µm either as a cyclone (Verheggen et al. 2007) or an impactor (Schwarzenböck et al. 2000). Further standards and SOPs regarding the inlets will be developed within the implementation and pre-operational phases by the Centre for Cloud In Situ Measurements.

The second component is the aerosol particle size distribution measurement device. In order to derive the D₅₀ at high supersaturation and the presence of highly soluble aerosol particles, the **scanning mobility particle sizer** (SMPS) would be the best choice for this purpose. This kind of size distribution instrument has been frequently intercompared, calibrated and technically standardized (Wiedensohler et al. 2012) and the respective standards and SOPs are defined by Centre for Aerosol In Situ Measurements and will need to be followed. Both inlets and SMPS systems can be operated in an autonomous way, so that their deployment is proposed as a minimum requirement at each future cloud station.

#### 4.5.3.7 In situ observation of cloud residuals number concentration and composition

Special inlets with inertial separation of large hydrometeors form smaller, interstitial aerosol particles, so-called **counterflow virtual impactors** (CVI), have been developed and designed in order to selectively sample droplets and ice particles into dry, particle-free air, evaporate the condensed water and analyse the **residual particles** with various techniques (Ogren et al. 1985, Noone et al. 1988, Twohy et al. 1997, Mertes et al. 2001). Based on the CVI principle, Boulter et al. (2006) have developed a pumped CVI (PCVI) with for stationary operation at both laboratory and observational platforms. This device has cut-off diameters of only a few µm for separating small cloud droplets and ice crystals from interstitial aerosols. It was further improved and modified towards the larger ice-selective PCVI (IS-PCVI) with cut-off diameters larger than 10 µm for separating ice crystals from droplets in mixed-phase clouds (Hiranuma et al. 2016). The residuals can either be sampled for off-line physical and chemical analysis, e.g., with electron microscopy (Worringen et al. 2015), or directly be analysed with proper instruments like CPC, SMPS or single particle mass spectrometers continuously sampling from the PCVI (Cziczo et al. 2009). PCVI residual sampling and analysis provides valuable insight into the formation mechanisms of droplets and ice crystals, but require proper control and maintenance for setup, sampling connection and operation procedures in order to achieve high quality data.

#### 4.5.3.8 In situ observation of cloud water chemical composition

The **chemical composition of cloud water** is most often determined from **bulk cloud water collectors** that rely on the principle of droplet inertial impaction and the collection of impacted droplets of all sizes into one collection vessel. For passive samplers (e.g., Reynolds et al. 1996), droplet impaction typically takes
interstitial particle size distribution. Both direct *in situ* observations require the combination of a dedicated inlet and an aerosol sizing instrument, which is able to measure down to at least 20 nm.

This approach was already successfully applied in numerous *in situ* cloud studies (Anttila et al. 2009, Anttila et al. 2012, Asmi et al. 2012, Ditas et al. 2012, Hammer et al. 2014, Henning et al. 2002) at different research field stations (Jungfraujoch, Pallas, Puy de Dôme). The **dedicated total and interstitial inlets** used at these field sites were individual versions and thus their sampling specifications might be different. For future cloud studies the total inlet should follow the design of Weingartner et al. (1999) to sample particles up to a size of 40 µm for wind speeds up to 20 m s⁻¹. The interstitial inlet should have a sharp cut-off at 2.5 µm either as a cyclone (Verheggen et al. 2007) or an impactor (Schwarzenböck et al. 2000). Further standards and SOPs regarding the inlets will be developed within the implementation and pre-operational phases by the Centre for Cloud In Situ Measurements.

The second component is the aerosol particle size distribution measurement device. In order to derive the D₅₀ at high supersaturation and the presence of highly soluble aerosol particles, the **scanning mobility particle sizer** (SMPS) would be the best choice for this purpose. This kind of size distribution instrument has been frequently intercompared, calibrated and technically standardized (Wiedensohler et al. 2012) and the respective standards and SOPs are defined by Centre for Aerosol In Situ Measurements and will need to be followed. Both inlets and SMPS systems can be operated in an autonomous way, so that their deployment is proposed as a minimum requirement at each future cloud station.

#### 4.5.3.7 In situ observation of cloud residuals number concentration and composition

Special inlets with inertial separation of large hydrometeors form smaller, interstitial aerosol particles, so-called **counterflow virtual impactors** (CVI), have been developed and designed in order to selectively sample droplets and ice particles into dry, particle-free air, evaporate the condensed water and analyse the **residual particles** with various techniques (Ogren et al. 1985, Noone et al. 1988, Twohy et al. 1997, Mertes et al. 2001). Based on the CVI principle, Boulter et al. (2006) have developed a pumped CVI (PCVI) with for stationary operation at both laboratory and observational platforms. This device has cut-off diameters of only a few µm for separating small cloud droplets and ice crystals from interstitial aerosols. It was further improved and modified towards the larger ice-selective PCVI (IS-PCVI) with cut-off diameters larger than 10 µm for separating ice crystals from droplets in mixed-phase clouds (Hiranuma et al. 2016). The residuals can either be sampled for off-line physical and chemical analysis, e.g., with electron microscopy (Worringen et al. 2015), or directly be analysed with proper instruments like CPC, SMPS or single particle mass spectrometers continuously sampling from the PCVI (Cziczo et al. 2009). PCVI residual sampling and analysis provides valuable insight into the formation mechanisms of droplets and ice crystals, but require proper control and maintenance for setup, sampling connection and operation procedures in order to achieve high quality data.

#### 4.5.3.8 In situ observation of cloud water chemical composition

The **chemical composition of cloud water** is most often determined from **bulk cloud water collectors** that rely on the principle of droplet inertial impaction and the collection of impacted droplets of all sizes into one collection vessel. For passive samplers (e.g., Reynolds et al. 1996), droplet impaction typically takes
place on strings or meshes and is driven by the ambient wind only. So-called rotating arm collectors force droplet impaction by rotating a rod around its centre point at high speed (e.g., Krämer und Schütz 1994). In other active samplers, droplet-laden air is drawn through the device to ensure sufficient droplet velocities for impaction even under calm conditions. Impaction then takes place on a set of vertically clamped strings (e.g., Demoz et al. 1996) or on flat surfaces (e.g., Berner 1988). As active samplers show less dependency of collection efficiencies on wind speeds, they are the preferred instruments for ground-based cloud water sampling. After sampling, the integral cloud water sample is typically analysed off-line, i.e., in a laboratory away from the sampling device, for its chemical composition. For inorganic ions, ion chromatography is most often applied, while for the organic composition a range of analytical separation techniques are available, including gas and liquid chromatography and capillary electrophoresis, coupled either to optical or mass spectrometry detectors. Such combinations have been frequently applied in cloud chemistry campaigns (e.g., Collett et al. 1990, Lihavainen et al. 2008, Marinoni et al. 2004, van Pinxteren et al. 2005) and efforts to harmonize and standardize both the collection and the chemical analysis will be reinforced within the ACTRIS implementation and pre-operational phases to define standards and SOPs for the determination of cloud water chemical composition.

#### 4.5.4 Operation

Measurement strategies given in the following will serve as a starting point to further refine standard operation procedures within the implementation and pre-operational phases of the RI (until 2024).

The LWC as well as $D_{eff}$ should be measured continuously for all clouds. A PVM is capable of generating data at a 1-second time resolution. For ground-based observations, data averaging over 1 minute provides sufficient temporal resolution, however. The droplet size distribution should also be measured for all clouds with a time resolution of 1 min, covering a size range of at least 5–50 µm diameter. Interstitial, residual and total particle size distributions need to be obtained during all clouds occurring at a site as well, with a cycle time of 20 min. It is suggested to run the particle spectrometers continuously, i.e., also outside cloud periods, to support data interpretation of INP and CCN measurements, taking place outside clouds as well. The chemical composition of cloud water is more difficult to obtain in an automated fashion. It should be provided at least for a 6-week campaign each year with sampling of all occurring clouds and a maximum collection time per sample of 24 hours, but preferably much shorter (1 h). Alternatively, it can be determined over a period of 4 months per year with more infrequent sampling of occurring clouds, but at least one cloud water sample per cloud event. Regarding the inorganic composition, the minimum set of constituents to be analysed comprises the ions sulphate, nitrate, chloride, ammonium, potassium, calcium and sodium. Regarding the organic cloud water composition, marker compounds that will be defined by the Centre for Cloud In Situ Measurements should be provided as a minimum set of constituents.

#### 4.5.5 Data production and data products

All data processing from raw data to the different data levels will be done at the NFs according to the standards developed and defined by the Centre for Cloud In Situ Measurements.

#### 4.5.6 Data delivery and quality control
place on strings or meshes and is driven by the ambient wind only. So-called rotating arm collectors force droplet impaction by rotating a rod around its centre point at high speed (e.g., Krämer und Schütz 1994). In other active samplers, droplet-laden air is drawn through the device to ensure sufficient droplet velocities for impaction even under calm conditions. Impaction then takes place on a set of vertically clamped strings (e.g., Demoz et al. 1996) or on flat surfaces (e.g., Berner 1988). As active samplers show less dependency of collection efficiencies on wind speeds, they are the preferred instruments for ground-based cloud water sampling. After sampling, the integral cloud water sample is typically analysed off-line, i.e., in a laboratory away from the sampling device, for its chemical composition. For inorganic ions, ion chromatography is most often applied, while for the organic composition a range of analytical separation techniques are available, including gas and liquid chromatography and capillary electrophoresis, coupled either to optical or mass spectrometry detectors. Such combinations have been frequently applied in cloud chemistry campaigns (e.g., Collett et al. 1990, Lihavainen et al. 2008, Marinoni et al. 2004, van Pinxteren et al. 2005) and efforts to harmonize and standardize both the collection and the chemical analysis will be reinforced within the ACTRIS implementation and pre-operational phases to define standards and SOPs for the determination of cloud water chemical composition.

#### 4.5.4 Operation

Measurement strategies given in the following will serve as a starting point to further refine standard operation procedures within the implementation and pre-operational phases of the RI (until 2024).

The LWC as well as $D_{eff}$ should be measured continuously for all clouds. A PVM is capable of generating data at a 1-second time resolution. For ground-based observations, data averaging over 1 minute provides sufficient temporal resolution, however. The droplet size distribution should also be measured for all clouds with a time resolution of 1 min, covering a size range of at least 5–50 µm diameter. Interstitial, residual and total particle size distributions need to be obtained during all clouds occurring at a site as well, with a cycle time of 20 min. It is suggested to run the particle spectrometers continuously, i.e., also outside cloud periods, to support data interpretation of INP and CCN measurements, taking place outside clouds as well. The chemical composition of cloud water is more difficult to obtain in an automated fashion. It should be provided at least for a 6-week campaign each year with sampling of all occurring clouds and a maximum collection time per sample of 24 hours, but preferably much shorter (1 h). Alternatively, it can be determined over a period of 4 months per year with more infrequent sampling of occurring clouds, but at least one cloud water sample per cloud event. Regarding the inorganic composition, the minimum set of constituents to be analysed comprises the ions sulphate, nitrate, chloride, ammonium, potassium, calcium and sodium. Regarding the organic cloud water composition, marker compounds that will be defined by the Centre for Cloud In Situ Measurements should be provided as a minimum set of constituents.

#### 4.5.5 Data production and data products

All data processing from raw data to the different data levels will be done at the NFs according to the standards developed and defined by the Centre for Cloud In Situ Measurements.

#### 4.5.6 Data delivery and quality control
Data delivery and quality control given in the following will serve as a starting point to further refine standard operation procedures within the implementation and pre-operational phases of the RI (until 2024).

Quality controlled data has to be submitted on a regular, scheduled frequency, at least yearly to the ACTRIS *in situ* data repository following the scheme in the ACTRIS Data Management Plan (D4.2) and associated documents. Data originators at the NF are responsible for checking the data using the procedures in the associated data production descriptions and the QA/QC tools provided by the Centre for Cloud In Situ Measurements.
Data delivery and quality control given in the following will serve as a starting point to further refine standard operation procedures within the implementation and pre-operational phases of the RI (until 2024).

Quality controlled data has to be submitted on a regular, scheduled frequency, at least yearly to the ACTRIS *in situ* data repository following the scheme in the ACTRIS Data Management Plan (D4.2) and associated documents. Data originators at the NF are responsible for checking the data using the procedures in the associated data production descriptions and the QA/QC tools provided by the Centre for Cloud In Situ Measurements.
## 4.6 Reactive trace gases in situ measurements

### 4.6.1 Introduction

Reactive gas species in the Earth's lower atmosphere are a large group of compounds with lifetimes between minutes and a few months. These gases are the primary source of the highly reactive free radical species that drive the gas-phase chemistry of the atmosphere. This radical chemistry is essential for cleansing the atmosphere of a range of pollutants and greenhouse gases by initiating their oxidation. Key reactive gases, which are also readily observable, are tropospheric ozone, carbon monoxide, volatile organic compounds (VOCs), nitrogen oxides ($NO_x$), and reactive sulfur species. Assessing the role and impacts of reactive gases in the atmosphere requires knowledge of the global distribution and long-term changes in their abundance.

This section describes the technical concepts and requirements for the ACTRIS Observational Platforms that perform in situ $NO_x$ and VOC measurements. $NO_x$ and VOCs are key components in the formation of tropospheric ozone and secondary aerosols, which affect human health, food security and climate change. Accurate and precise measurements of $NO_x$ and VOCs are crucial for improving air-quality models and emission inventories and to monitor long-term trends as an independent measure for air quality and anthropogenic emission mitigation strategies. Additionally, climate change alters the temperature driven biospheric emission patterns (especially of VOCs) and its impact is still difficult to predict due to missing long-term observations.

**Nitrogen oxides, $NO_x$,** are the combination of NO and $NO_2$. NO is mainly emitted from anthropogenic sources such as combustion processes. It is rapidly oxidized in the atmosphere to $NO_2$ by ozone and peroxyradicals, originating from the degradation of VOCs. NO and $NO_2$ are the key substances for ozone formation in the presence of sunlight leading to photochemical smog formation. $NO_x$ is removed from the atmosphere by dry and wet deposition or by formation of organic nitrates. Having a longer lifetime than $NO_x$ itself, these organic nitrates can be subject to long-range transport in the atmosphere and can act as $NO_x$ sources in remote regions when ultimately decomposed. The atmospheric lifetime of $NO_x$ ranges from a few hours to one or two days. Since both NO and $NO_2$ are involved in essential reactions in the troposphere they have to be measured individually.

**VOCs** summarizes species of short-chain, non-methane hydrocarbons (NMHCs; alkanes, alkenes, alkynes and aromatics, terpenes) and oxygenated hydrocarbons (OVOCs; e.g., alcohols, ketones, aldehydes) with a high enough vapour pressure to be gaseous under tropospheric conditions. They are emitted from anthropogenic and biogenic sources and are subsequently oxidised mostly by OH radicals, resulting in formation of ozone and secondary organic aerosols. VOC oxidation products include also highly oxygenated molecules (HOM) that partition to the aerosol phase and contribute to gas-to-particle conversion in concert with e.g., sulfuric acid.

The atmospheric lifetimes of VOCs are linked to their reactivity and range from minutes (alkenes) to a few months (light alkanes). Together with long-term measurement of anthropogenic and biogenic VOCs as well as their degradation products, this information can be used to assess the reactivity of the atmosphere and to study atmospheric processes.
## 4.6 Reactive trace gases in situ measurements

### 4.6.1 Introduction

Reactive gas species in the Earth's lower atmosphere are a large group of compounds with lifetimes between minutes and a few months. These gases are the primary source of the highly reactive free radical species that drive the gas-phase chemistry of the atmosphere. This radical chemistry is essential for cleansing the atmosphere of a range of pollutants and greenhouse gases by initiating their oxidation. Key reactive gases, which are also readily observable, are tropospheric ozone, carbon monoxide, volatile organic compounds (VOCs), nitrogen oxides ($NO_x$), and reactive sulfur species. Assessing the role and impacts of reactive gases in the atmosphere requires knowledge of the global distribution and long-term changes in their abundance.

This section describes the technical concepts and requirements for the ACTRIS Observational Platforms that perform in situ $NO_x$ and VOC measurements. $NO_x$ and VOCs are key components in the formation of tropospheric ozone and secondary aerosols, which affect human health, food security and climate change. Accurate and precise measurements of $NO_x$ and VOCs are crucial for improving air-quality models and emission inventories and to monitor long-term trends as an independent measure for air quality and anthropogenic emission mitigation strategies. Additionally, climate change alters the temperature driven biospheric emission patterns (especially of VOCs) and its impact is still difficult to predict due to missing long-term observations.

**Nitrogen oxides, $NO_x$,** are the combination of $NO$ and $NO_2$. $NO$ is mainly emitted from anthropogenic sources such as combustion processes. It is rapidly oxidized in the atmosphere to $NO_2$ by ozone and peroxyradicals, originating from the degradation of VOCs. $NO$ and $NO_2$ are the key substances for ozone formation in the presence of sunlight leading to photochemical smog formation. $NO_x$ is removed from the atmosphere by dry and wet deposition or by formation of organic nitrates. Having a longer lifetime than $NO_x$ itself, these organic nitrates can be subject to long-range transport in the atmosphere and can act as $NO_x$ sources in remote regions when ultimately decomposed. The atmospheric lifetime of $NO_x$ ranges from a few hours to one or two days. Since both $NO$ and $NO_2$ are involved in essential reactions in the troposphere they have to be measured individually.

**VOCs** summarizes species of short-chain, non-methane hydrocarbons (NMHCs; alkanes, alkenes, alkynes and aromatics, terpenes) and oxygenated hydrocarbons (OVOCs; e.g., alcohols, ketones, aldehydes) with a high enough vapour pressure to be gaseous under tropospheric conditions. They are emitted from anthropogenic and biogenic sources and are subsequently oxidised mostly by OH radicals, resulting in formation of ozone and secondary organic aerosols. VOC oxidation products include also highly oxygenated molecules (HOM) that partition to the aerosol phase and contribute to gas-to-particle conversion in concert with e.g., sulfuric acid.

The atmospheric lifetimes of VOCs are linked to their reactivity and range from minutes (alkenes) to a few months (light alkanes). Together with long-term measurement of anthropogenic and biogenic VOCs as well as their degradation products, this information can be used to assess the reactivity of the atmosphere and to study atmospheric processes.
## 4.6.2 Observational capabilities

### 4.6.2.1 Optimum setup

**NOₓ specific setup:**

In addition to NO and NO₂, ozone and relative humidity have to be measured to enable the correction of interferences. Additional J(NO₂) data are needed for further data analysis. Ideally and in addition to final data (Level 2), RRT or at least NRT data should be submitted, i.e., within 3 hours or 3 days of measurement, respectively. Automated quality control has to be used, based on tools specified in SOP-NOx (SOP-NOx, 2014; Deliverable D4.10 of ACTRIS-I3 project: Standardized Operating Procedures (SOPs) for NOₓ Measurements).

**VOC specific setup:**

Measurement of all relevant non-methane hydrocarbons (NMHCs; alkanes, alkenes, alkynes, aromatic compounds) in the C₂–C₉ range (around 30 compounds) should be performed.

Measurement of all relevant oxygenated VOCs (OVOCs; alcohols, ketones, aldehydes, ethers, esters) in the C₁–C₅ range (around 10 compounds) should be performed.

Measurements of biogenic VOCs (BVOCs; monoterpenes) should be performed.

Measurements of direct aerosol precursors like HOM and sulfuric acid should be performed (e.g., by online techniques, such as CI-APi-TOF) when the station also performs aerosol and gas-to-particle conversion studies.

Ideally and in addition to final data (Level 2), RRT or NRT data should be submitted whenever possible at reasonable quality, i.e., within 3 hours or 3 days of measurement, respectively. Automated quality control has to be used, based on tools specified in SOP-VOC (SOP-VOC, 2014; Deliverable D4.9 of ACTRIS-I3 project: Standardized Operating Procedures (SOPs) for VOC Measurements).

### 4.6.2.2 Minimum requirements

Observational Platforms for in situ measurements of reactive trace gases must have collocated measurements of VOCs and NOₓ as specified below and in Annex A.6.

For calibration, a standard has to be used which is traceable to the Centre for Reactive Trace Gases In Situ Measurements, which provides the link to the Central Calibration Laboratories (CCLs) of the World Meteorological Organisations' Global Atmosphere Watch program (WMO-GAW) holding the SI traceable scales for VOCs and NOₓ.

The performance of the measurement instrument has to be checked against a target gas in regular intervals (e.g., monthly). The target gas consists either of an ambient air standard with known compound concentrations or of an artificial standard within the ambient concentration range of the target compounds.

Reported measurement data have to include the measured mixing ratios, their associated precisions and uncertainties as specified in SOP-VOC (2014) and SOP-NOₓ (2014), and flags as commonly accepted by the
## 4.6.2 Observational capabilities

### 4.6.2.1 Optimum setup

**NOₓ specific setup:**

In addition to NO and NO₂, ozone and relative humidity have to be measured to enable the correction of interferences. Additional J(NO₂) data are needed for further data analysis. Ideally and in addition to final data (Level 2), RRT or at least NRT data should be submitted, i.e., within 3 hours or 3 days of measurement, respectively. Automated quality control has to be used, based on tools specified in SOP-NOx (SOP-NOx, 2014; Deliverable D4.10 of ACTRIS-I3 project: Standardized Operating Procedures (SOPs) for NOₓ Measurements).

**VOC specific setup:**

Measurement of all relevant non-methane hydrocarbons (NMHCs; alkanes, alkenes, alkynes, aromatic compounds) in the C₂–C₉ range (around 30 compounds) should be performed.

Measurement of all relevant oxygenated VOCs (OVOCs; alcohols, ketones, aldehydes, ethers, esters) in the C₁–C₅ range (around 10 compounds) should be performed.

Measurements of biogenic VOCs (BVOCs; monoterpenes) should be performed.

Measurements of direct aerosol precursors like HOM and sulfuric acid should be performed (e.g., by online techniques, such as CI-APi-TOF) when the station also performs aerosol and gas-to-particle conversion studies.

Ideally and in addition to final data (Level 2), RRT or NRT data should be submitted whenever possible at reasonable quality, i.e., within 3 hours or 3 days of measurement, respectively. Automated quality control has to be used, based on tools specified in SOP-VOC (SOP-VOC, 2014; Deliverable D4.9 of ACTRIS-I3 project: Standardized Operating Procedures (SOPs) for VOC Measurements).

### 4.6.2.2 Minimum requirements

Observational Platforms for in situ measurements of reactive trace gases must have collocated measurements of VOCs and NOₓ as specified below and in Annex A.6.

For calibration, a standard has to be used which is traceable to the Centre for Reactive Trace Gases In Situ Measurements, which provides the link to the Central Calibration Laboratories (CCLs) of the World Meteorological Organisations' Global Atmosphere Watch program (WMO-GAW) holding the SI traceable scales for VOCs and NOₓ.

The performance of the measurement instrument has to be checked against a target gas in regular intervals (e.g., monthly). The target gas consists either of an ambient air standard with known compound concentrations or of an artificial standard within the ambient concentration range of the target compounds.

Reported measurement data have to include the measured mixing ratios, their associated precisions and uncertainties as specified in SOP-VOC (2014) and SOP-NOₓ (2014), and flags as commonly accepted by the
ACTRIS reactive trace gases measurement community and specified by the Centre for Reactive Trace Gases In Situ Measurements.

During the course of the years, data have to be regularly (e.g., monthly) reviewed for consistency with existing data from the same measurement site and against similar European measurement sites. Tools for these checks are provided by the ACTRIS Data Centre and the Centre for Reactive Trace Gases In Situ Measurements. Data are submitted following strictly the data submission procedures as outlined in Section 4.6.5 together with metadata and data flagging.

The data providers of the Observational Platforms have to participate in round-robin exercises when organized by the Centre for Reactive Trace Gases In Situ Measurements. In case of on-going non-compliance with the Key Performance Indicators (KPI) set by Centre for Reactive Trace Gases In Situ Measurements they have to accept and organize performance audits by the Centre for Reactive Trace Gases In Situ Measurements.

**NOₓ specific requirements:**

For NOₓ both NO and NO₂ measurements have to be reported separately. NO₂ has to be measured mandatorily, either with chemiluminescence detection (CLD) and photolytic converter (PLC), Blue Light converter (BLC), with Cavity Attenuated Phase Shift instrumentation (CAPS) or with any other NO₂-specific measurement technique approved by the Topical Centre. Explicitly not considered as ACTRIS compatible are NO₂ measurements using a molybdenum-based converter. NO and NO₂ values have to be corrected for conversion caused by the reaction with ozone inside of the inlet line. If this is not done by the instrument directly, also O₃ and relative humidity have to be measured and submitted to allow for data correction, as specified by SOP-NOx (2014).

**VOC specific requirements:**

At least 6 different VOCs have to be measured at an Observational Platform for in situ measurements of reactive trace gases.

Atmospheric VOCs have to be preconcentrated, subsequently separated by gas chromatography and finally analysed by mass spectrometry (GC-MS) or flame ionization detection (GC-FID). Alternatively, VOCs can also be measured without preconcentration, using specific detection such as proton transfer reaction–mass spectrometry (e.g., PTR-MS). Analysis can be performed on-line (at the site) or off-line (i.e., sampling at the site and analysis remotely in the lab).

Off-line sampling intervals have to be at least twice a week and sampling has to follow a station-specific protocol, where station-characteristic air masses are sampled with little contamination due to local sources. This will allow obtaining a consistent climatology of VOCs and assessing the influence of emission mitigation measures or changes of climate and transport regimes (e.g., El Niño-Southern Oscillation – ENSO, North Atlantic Oscillation – NAO) in the atmosphere.

On-line sampling has to be performed continuously, with an interval of at least two times daily but preferably in hourly intervals. With respect to off-line sampling, this setup allows obtaining high-resolution
ACTRIS reactive trace gases measurement community and specified by the Centre for Reactive Trace Gases In Situ Measurements.

During the course of the years, data have to be regularly (e.g., monthly) reviewed for consistency with existing data from the same measurement site and against similar European measurement sites. Tools for these checks are provided by the ACTRIS Data Centre and the Centre for Reactive Trace Gases In Situ Measurements. Data are submitted following strictly the data submission procedures as outlined in Section 4.6.5 together with metadata and data flagging.

The data providers of the Observational Platforms have to participate in round-robin exercises when organized by the Centre for Reactive Trace Gases In Situ Measurements. In case of on-going non-compliance with the Key Performance Indicators (KPI) set by Centre for Reactive Trace Gases In Situ Measurements they have to accept and organize performance audits by the Centre for Reactive Trace Gases In Situ Measurements.

**NOₓ specific requirements:**

For NOₓ both NO and NO₂ measurements have to be reported separately. NO₂ has to be measured mandatorily, either with chemiluminescence detection (CLD) and photolytic converter (PLC), Blue Light converter (BLC), with Cavity Attenuated Phase Shift instrumentation (CAPS) or with any other NO₂-specific measurement technique approved by the Topical Centre. Explicitly not considered as ACTRIS compatible are NO₂ measurements using a molybdenum-based converter. NO and NO₂ values have to be corrected for conversion caused by the reaction with ozone inside of the inlet line. If this is not done by the instrument directly, also O₃ and relative humidity have to be measured and submitted to allow for data correction, as specified by SOP-NOx (2014).

**VOC specific requirements:**

At least 6 different VOCs have to be measured at an Observational Platform for in situ measurements of reactive trace gases.

Atmospheric VOCs have to be preconcentrated, subsequently separated by gas chromatography and finally analysed by mass spectrometry (GC-MS) or flame ionization detection (GC-FID). Alternatively, VOCs can also be measured without preconcentration, using specific detection such as proton transfer reaction–mass spectrometry (e.g., PTR-MS). Analysis can be performed on-line (at the site) or off-line (i.e., sampling at the site and analysis remotely in the lab).

Off-line sampling intervals have to be at least twice a week and sampling has to follow a station-specific protocol, where station-characteristic air masses are sampled with little contamination due to local sources. This will allow obtaining a consistent climatology of VOCs and assessing the influence of emission mitigation measures or changes of climate and transport regimes (e.g., El Niño-Southern Oscillation – ENSO, North Atlantic Oscillation – NAO) in the atmosphere.

On-line sampling has to be performed continuously, with an interval of at least two times daily but preferably in hourly intervals. With respect to off-line sampling, this setup allows obtaining high-resolution
information on the impact of specific processes/events and provides time series useful for source quantification by inverse modelling.

#### 4.6.2.3 Complementary added-value observations

CO, CO₂, CH₄, SO₂ and NOₓ are useful tracers for anthropogenic pollution events and therefore parallel measurements of these gases are highly recommended. The analysis of the NOₓ and VOCs also benefits from co-located measurements of **ozone** for photochemical studies and of **aerosol components** for studies on formation of secondary aerosols.

In addition to the *in situ* data, **remote-sensing measurements of trace gases and aerosol parameters** can be used to further enhance the potential of the observational platform.

#### 4.6.2.4 Auxiliary measurements

**Ozone** and **water vapour** must be measured according to the rules of WMO-GAW at Observational Platforms for reactive trace gases.

Furthermore, measurements of reactive trace gases have to be accompanied by measurements of **meteorological parameters** in high time resolution near the place of the air intake port for the analysers.
The minimum set of parameters includes wind speed, wind direction, air temperature, and air pressure.
These auxiliary data are essential for checking the quality of the data and for advanced data products (e.g., emission estimation, long-term trend analysis). In addition, so-called footprints from meteorological transport models should be available in order to connect measured data with potential source regions.

### 4.6.3 Instrumentation and calibration

#### 4.6.3.1 *In situ* observations of NOₓ

The **chemiluminescence detection** (CLD) is the state-of-the-art technique for **NO** observations. No other technique for direct detection of NO is recommended so far (SOP-NOx, 2014). **NO₂** can either be detected indirectly by converting NO₂ to NO or by newly evolving measurement techniques, e.g., **Cavity Enhanced Phase Shift (CAPS)** or **Quantum Cascade Laser systems (QCLAS)**.

The instrument performance should target to fulfil the ACTRIS Data Quality Objectives (SOP-NOx, 2014) for NO and NO₂ dependent on the atmospheric regimes (e.g., urban, rural, remote). Sensitivity drifts of the instrument response have to be corrected by regular calibrations, which need to be adapted to the specific instrument. The detection limit has to be at least <50 ppt and <100 ppt for NO and NO₂, respectively.

For the NO observations, low-maintenance CLDs instruments are commercially available from different manufacturers. In principle, no supply gases are needed during operation. However, the precision is greatly enhanced by supplying pure oxygen for the in-built ozone generator. Though reaching the ACTRIS quality goals, the ACTRIS quality assurance and quality control procedures have to be strictly followed (SOP-NOx, 2014).
information on the impact of specific processes/events and provides time series useful for source quantification by inverse modelling.

#### 4.6.2.3 Complementary added-value observations

CO, CO₂, CH₄, SO₂ and NOₓ are useful tracers for anthropogenic pollution events and therefore parallel measurements of these gases are highly recommended. The analysis of the NOₓ and VOCs also benefits from co-located measurements of **ozone** for photochemical studies and of **aerosol components** for studies on formation of secondary aerosols.

In addition to the *in situ* data, **remote-sensing measurements of trace gases and aerosol parameters** can be used to further enhance the potential of the observational platform.

#### 4.6.2.4 Auxiliary measurements

**Ozone** and **water vapour** must be measured according to the rules of WMO-GAW at Observational Platforms for reactive trace gases.

Furthermore, measurements of reactive trace gases have to be accompanied by measurements of **meteorological parameters** in high time resolution near the place of the air intake port for the analysers.
The minimum set of parameters includes wind speed, wind direction, air temperature, and air pressure.
These auxiliary data are essential for checking the quality of the data and for advanced data products (e.g., emission estimation, long-term trend analysis). In addition, so-called footprints from meteorological transport models should be available in order to connect measured data with potential source regions.

### 4.6.3 Instrumentation and calibration

#### 4.6.3.1 *In situ* observations of NOₓ

The **chemiluminescence detection** (CLD) is the state-of-the-art technique for **NO** observations. No other technique for direct detection of NO is recommended so far (SOP-NOx, 2014). **NO₂** can either be detected indirectly by converting NO₂ to NO or by newly evolving measurement techniques, e.g., **Cavity Enhanced Phase Shift (CAPS)** or **Quantum Cascade Laser systems (QCLAS)**.

The instrument performance should target to fulfil the ACTRIS Data Quality Objectives (SOP-NOx, 2014) for NO and NO₂ dependent on the atmospheric regimes (e.g., urban, rural, remote). Sensitivity drifts of the instrument response have to be corrected by regular calibrations, which need to be adapted to the specific instrument. The detection limit has to be at least <50 ppt and <100 ppt for NO and NO₂, respectively.

For the NO observations, low-maintenance CLDs instruments are commercially available from different manufacturers. In principle, no supply gases are needed during operation. However, the precision is greatly enhanced by supplying pure oxygen for the in-built ozone generator. Though reaching the ACTRIS quality goals, the ACTRIS quality assurance and quality control procedures have to be strictly followed (SOP-NOx, 2014).
For NO₂ observations the photolytic conversion of NO₂ to NO using **Blue Light** (BLC) or **photolytic converters** (PLC) and subsequent detection by CLD is the recommended measurement technique.
Molybdenum converters are not recommended due to their insufficient selectiveness to NO₂. Direct NO₂ detection methods (CAPS, QCLAS) are able to reach the same performance as BLC/PLC-CLD instruments and are also recommended.

Calibration for NO has to be performed by dynamic dilution of a reference standard (low mixture of NO in N₂, in the order of 5 ppm). The reference standard has to be traceable to standards provided by the Centre for Reactive Trace Gases In Situ Measurements. Regular calibrations are necessary to correct for drifting instruments. Two calibration points (zero and one concentration) are recommended to be performed on a weekly basis at least.

Calibration for NO₂ has to be performed by using a gas phase titration system. As for the NO calibration, a dynamic dilution of an NO in N₂ reference standard is produced. Ozone, produced by UV-lamps is added to the mixture, quantitatively converting NO in NO₂. Depending on the concentrations, different NO₂ concentrations are achieved. The converter efficiency of the BLC/PLC is checked with one NO₂ concentration after the regular NO-calibration on a weekly basis. Since dilution is a key component of the calibration process, a specific device for zero air generation and a systematic check of the mass flow controller calibration must be conducted in agreement with SOP-NOx (2014).

#### 4.6.3.2 In situ observations of VOC species

The equipment for analysing VOCs in ambient air varies in specification and performance depending on the measured species. A sample of atmospheric VOCs can be introduced to the analytical system directly from ambient air (on-line), or via a canister or an adsorptive sampling tube (off-line).

**On-line sampling** avoids storage issues and minimizes leak issues, however, requires an analytical system at the sampling site. The air sample is directly transferred via a sampling line into the instrument. Use of on-line systems is encouraged for ACTRIS stations if the required well-trained personal, the appropriate equipment and the resources necessary for QA/QC including regular zero, calibration and target gas measurements are available. Otherwise, it is recommended to use **off-line sampling** and conduct the sample analyses in an experienced laboratory. For precursor gases measured with the CI-APi-TOF or similar instrumentation, such as HOM and sulfuric acid, on-line sampling is mandatory in order avoid condensation to the walls and thus loss of analysed compounds. Filter inlets such as FIGAERO may be used for simultaneous measurements of gas and condense phase HOM.

For on-line and off-line analyses of VOCs from ambient air gas chromatography (GC) systems equipped with flame ionization detection (GC-FID) and/or mass spectrometry (GC-MS) are key instruments. For light OVOCs (aldehydes and ketones) a possible off-line method combines sampling through a silica gel adsorbent coated with 2,4-Dinitrophenylhydrazine (DNPH) and analysis with the High Performance Liquid Chromatography and UV detection (HPLC-UV). For measuring formaldehyde in air an on-line analyser with fluorimetric detection of the product formed by the Hantzsch-Reaction may be used.

An alternative are the PTR-MS systems with high-time-resolution measurements. PTR-MS systems are capable of measuring only compounds with higher proton affinity than water molecules, thus selected
For NO₂ observations the photolytic conversion of NO₂ to NO using **Blue Light** (BLC) or **photolytic converters** (PLC) and subsequent detection by CLD is the recommended measurement technique. Molybdenum converters are not recommended due to their insufficient selectiveness to NO₂. Direct NO₂ detection methods (CAPS, QCLAS) are able to reach the same performance as BLC/PLC-CLD instruments and are also recommended.

Calibration for NO has to be performed by dynamic dilution of a reference standard (low mixture of NO in N₂, in the order of 5 ppm). The reference standard has to be traceable to standards provided by the Centre for Reactive Trace Gases In Situ Measurements. Regular calibrations are necessary to correct for drifting instruments. Two calibration points (zero and one concentration) are recommended to be performed on a weekly basis at least.

Calibration for NO₂ has to be performed by using a gas phase titration system. As for the NO calibration, a dynamic dilution of an NO in N₂ reference standard is produced. Ozone, produced by UV-lamps is added to the mixture, quantitatively converting NO in NO₂. Depending on the concentrations, different NO₂ concentrations are achieved. The converter efficiency of the BLC/PLC is checked with one NO₂ concentration after the regular NO-calibration on a weekly basis. Since dilution is a key component of the calibration process, a specific device for zero air generation and a systematic check of the mass flow controller calibration must be conducted in agreement with SOP-NOx (2014).

#### 4.6.3.2 In situ observations of VOC species

The equipment for analysing VOCs in ambient air varies in specification and performance depending on the measured species. A sample of atmospheric VOCs can be introduced to the analytical system directly from ambient air (on-line), or via a canister or an adsorptive sampling tube (off-line).

**On-line sampling** avoids storage issues and minimizes leak issues, however, requires an analytical system at the sampling site. The air sample is directly transferred via a sampling line into the instrument. Use of on-line systems is encouraged for ACTRIS stations if the required well-trained personal, the appropriate equipment and the resources necessary for QA/QC including regular zero, calibration and target gas measurements are available. Otherwise, it is recommended to use **off-line sampling** and conduct the sample analyses in an experienced laboratory. For precursor gases measured with the CI-APi-TOF or similar instrumentation, such as HOM and sulfuric acid, on-line sampling is mandatory in order avoid condensation to the walls and thus loss of analysed compounds. Filter inlets such as FIGAERO may be used for simultaneous measurements of gas and condense phase HOM.

For on-line and off-line analyses of VOCs from ambient air gas chromatography (GC) systems equipped with flame ionization detection (GC-FID) and/or mass spectrometry (GC-MS) are key instruments. For light OVOCs (aldehydes and ketones) a possible off-line method combines sampling through a silica gel adsorbent coated with 2,4-Dinitrophenylhydrazine (DNPH) and analysis with the High Performance Liquid Chromatography and UV detection (HPLC-UV). For measuring formaldehyde in air an on-line analyser with fluorimetric detection of the product formed by the Hantzsch-Reaction may be used.

An alternative are the PTR-MS systems with high-time-resolution measurements. PTR-MS systems are capable of measuring only compounds with higher proton affinity than water molecules, thus selected
OVOC, unsaturated NMHCs (aromatics and few alkenes) can be measured. Furthermore, they cannot separate compounds with identical molecular masses, though recent developments in high resolving time-of-flight (TOF) MS have overcome this problem for isobaric but not for isomeric compounds. The main purpose of PTR-MS is the measurement of OVOCs and BVOCs.

A list of prior species has been defined regarding their interest in atmospheric science (WMO, 2017). The Observational Platform has to define its measurement strategy considering this list. C₂–C₉ VOC species have to be measured with a detection limit of at least 15 ppt for individual VOC species.

For details of GC-FID and GC-MS systems consult SOP-VOC (2014), the new measurement guidelines for VOCs of WMO (to be published in 2018) and the Section 3 of the concept document for the Centre for Reactive Trace Gases In Situ Measurements. Furthermore, an overview of available systems for the analysis of VOCs from ambient air and their performance is available from Hoerger et al. (2015). For the PTR-MS system a guideline is under progress within ACTRIS-2. Precursor gases system (CI-API-TOF) and calibration procedures and schedules and measurement guidelines are under progress.

For VOCs, calibration has to be performed using a multicomponent mixture, containing all analysed species in the lower ppb-range. The target concentrations must be traceable to the WMO-GAW scale for VOC, with the Centre for Reactive Trace Gases In Situ Measurements as anchor point. For NMHC and BVOCs, scales are available by the Central Calibration Laboratories (CCLs) for NMHC (NPL, UK) and Monoterpenes (NIST, USA), whereas for OVOC and ELVOCs no CCL has been approved yet. Thus, in case of OVOCs, the Centre for Reactive Trace Gases In Situ Measurements will host a preliminary scale for these compounds in close cooperation with the CCQM (Consultative Committee for Amount of Substance: Metrology in Chemistry and Biology) Working Group on Gas Analysis (GAWG) of the International Office for Weight and Measures (BIPM). Calibration has to be performed in regular periods. For the observations with a GC-FID a monthly calibration cycle is recommended, if no manual interference or system break occurs. For the GC-MS systems a daily calibration is a minimum requirement. In addition, a target sample has to be measured at least monthly. This standard sample consists of a pressurized air cylinder which is again traceable to the WMO-GAW scale. The Centre for Reactive Trace Gases In Situ Measurements provides both calibration gas and target gas samples for regular checks. Recalibration is performed after the approved lifetime of the gas cylinder or when the pressure of the gas/air in the cylinder becomes less than 5 bar.

#### 4.6.4 Operation

For NOₓ measurements are performed continuously (i.e., native raw data aggregated on 1-minute time resolution). For VOCs, a minimum of two measurements per week is required. Hourly measurements are however preferred. Standard operation procedures are outlined in the SOP-NOx (2014) and the SOP-VOC (2014), as well as their updates in ACTRIS-2 deliverable D3.17. Updated Measurement Guidelines for NOₓ and VOCs are in progress and will be available at least by end of 2018. Measurement Guidelines for aerosol precursor gases are being prepared and will be available by the end of 2020.
OVOC, unsaturated NMHCs (aromatics and few alkenes) can be measured. Furthermore, they cannot separate compounds with identical molecular masses, though recent developments in high resolving time-of-flight (TOF) MS have overcome this problem for isobaric but not for isomeric compounds. The main purpose of PTR-MS is the measurement of OVOCs and BVOCs.

A list of prior species has been defined regarding their interest in atmospheric science (WMO, 2017). The Observational Platform has to define its measurement strategy considering this list. C₂–C₉ VOC species have to be measured with a detection limit of at least 15 ppt for individual VOC species.

For details of GC-FID and GC-MS systems consult SOP-VOC (2014), the new measurement guidelines for VOCs of WMO (to be published in 2018) and the Section 3 of the concept document for the Centre for Reactive Trace Gases In Situ Measurements. Furthermore, an overview of available systems for the analysis of VOCs from ambient air and their performance is available from Hoerger et al. (2015). For the PTR-MS system a guideline is under progress within ACTRIS-2. Precursor gases system (CI-API-TOF) and calibration procedures and schedules and measurement guidelines are under progress.

For VOCs, calibration has to be performed using a multicomponent mixture, containing all analysed species in the lower ppb-range. The target concentrations must be traceable to the WMO-GAW scale for VOC, with the Centre for Reactive Trace Gases In Situ Measurements as anchor point. For NMHC and BVOCs, scales are available by the Central Calibration Laboratories (CCLs) for NMHC (NPL, UK) and Monoterpenes (NIST, USA), whereas for OVOC and ELVOCs no CCL has been approved yet. Thus, in case of OVOCs, the Centre for Reactive Trace Gases In Situ Measurements will host a preliminary scale for these compounds in close cooperation with the CCQM (Consultative Committee for Amount of Substance: Metrology in Chemistry and Biology) Working Group on Gas Analysis (GAWG) of the International Office for Weight and Measures (BIPM). Calibration has to be performed in regular periods. For the observations with a GC-FID a monthly calibration cycle is recommended, if no manual interference or system break occurs. For the GC-MS systems a daily calibration is a minimum requirement. In addition, a target sample has to be measured at least monthly. This standard sample consists of a pressurized air cylinder which is again traceable to the WMO-GAW scale. The Centre for Reactive Trace Gases In Situ Measurements provides both calibration gas and target gas samples for regular checks. Recalibration is performed after the approved lifetime of the gas cylinder or when the pressure of the gas/air in the cylinder becomes less than 5 bar.

#### 4.6.4 Operation

For NOₓ measurements are performed continuously (i.e., native raw data aggregated on 1-minute time resolution). For VOCs, a minimum of two measurements per week is required. Hourly measurements are however preferred. Standard operation procedures are outlined in the SOP-NOx (2014) and the SOP-VOC (2014), as well as their updates in ACTRIS-2 deliverable D3.17. Updated Measurement Guidelines for NOₓ and VOCs are in progress and will be available at least by end of 2018. Measurement Guidelines for aerosol precursor gases are being prepared and will be available by the end of 2020.
4.6.5 Data production and data products

The final data product (ACTRIS Level 1 and Level 2 data) consists of measured concentrations of the reactive trace gases listed in Annex A.6 connected with the respective measurement uncertainty and precision, metadata and time dependent flags as specified by the DC, and in the standard operating procedures. Raw ambient measurement data has to be stored together with the relevant standard measurements and all information and metadata, which is needed to recalculate the higher-level data products. Details for the data production will be given in the ACTRIS Data Management Plan (D4.2) and the associated data production descriptions and documents provided by ACTRIS Data Centre and the Centre for Reactive Trace Gases In Situ Measurements. All data products, per-products and software tools are version controlled and identified by the DC.

ACTRIS *in situ* data production follows one of two main workflows, on-line or off-line. For the **on-line workflow**, data production is based on Level 0 data, i.e., all raw data and signals provided by an instrument in its native time resolution, brought to a harmonised format, and annotated with all discovery and metadata needed in the further data production process. Level 0 data are to be submitted and archived at the Data Centre, unless the ACTRIS Data Management Plan and the associated data production descriptions define an exception caused by large raw data volumes. In this case, an archive procedure has to be agreed between the Centre for Reactive Trace Gases In Situ Measurements and the Data Centre. The Level 0 data submission includes relevant calibration measurements and measurements of traceable standards needed for data quality control which are flagged as such. From Level 0 data, data Level 1 and Level 2 are produced. Level 1 data are quality controlled and calibrations are applied, final concentrations calculated, native time resolution, invalid and quality control measurements removed, and transferred to standard conditions of temperature and pressure where applicable. Level 2 data is averaged Level 1 data to typically hourly averages, measure of atmospheric variability included. The Level 1 and Level 2 data can be produced either by NF or the respective unit of the Data Centre, depending on variable, the distribution of work is included in the associated data production descriptions available from the Data Centre, and indicated in Annex A.6. The quality control step between Levels 0 and 1 can be automatic (RRT data production) or manual, performed by the Observational Platform. Data level identifiers distinguish between data with automatic or manual QC. The details of the steps producing Level 1 data from Level 0, and Level 2 from Level 1, are specified in the associated data production descriptions and the standard operating procedures.

For the **off-line workflow**, the steps of sample medium pre-exposure treatment, exposure, sampling time, preparation, and analysis are documented following standard operating procedures and protocols including metadata in machine-readable form. Items to be included are specified in the associated data production descriptions and the standard operating procedures. The archive for these protocols is located at the Observational Platform, its operating protocols coordinated with and approved by the Data Centre and the Centre for Reactive Trace Gases In Situ Measurements. From these protocols, the NF produces the Level 2 data product in the temporal resolution determined by the sampling schedule, and submits it to the Data Centre using the tools provided for data submission to ensure required documentation, flagging and metadata. The details of the procedure are specified in the associated data production descriptions.
### 4.6.5 Data production and data products

The final data product (ACTRIS Level 1 and Level 2 data) consists of measured concentrations of the reactive trace gases listed in Annex A.6 connected with the respective measurement uncertainty and precision, metadata and time dependent flags as specified by the DC, and in the standard operating procedures. Raw ambient measurement data has to be stored together with the relevant standard measurements and all information and metadata, which is needed to recalculate the higher-level data products. Details for the data production will be given in the ACTRIS Data Management Plan (D4.2) and the associated data production descriptions and documents provided by ACTRIS Data Centre and the Centre for Reactive Trace Gases In Situ Measurements. All data products, per-products and software tools are version controlled and identified by the DC.

ACTRIS in situ data production follows one of two main workflows, on-line or off-line. For the **on-line workflow**, data production is based on Level 0 data, i.e., all raw data and signals provided by an instrument in its native time resolution, brought to a harmonised format, and annotated with all discovery and metadata needed in the further data production process. Level 0 data are to be submitted and archived at the Data Centre, unless the ACTRIS Data Management Plan and the associated data production descriptions define an exception caused by large raw data volumes. In this case, an archive procedure has to be agreed between the Centre for Reactive Trace Gases In Situ Measurements and the Data Centre. The Level 0 data submission includes relevant calibration measurements and measurements of traceable standards needed for data quality control which are flagged as such. From Level 0 data, data Level 1 and Level 2 are produced. Level 1 data are quality controlled and calibrations are applied, final concentrations calculated, native time resolution, invalid and quality control measurements removed, and transferred to standard conditions of temperature and pressure where applicable. Level 2 data is averaged Level 1 data to typically hourly averages, measure of atmospheric variability included. The Level 1 and Level 2 data can be produced either by NF or the respective unit of the Data Centre, depending on variable, the distribution of work is included in the associated data production descriptions available from the Data Centre, and indicated in Annex A.6. The quality control step between Levels 0 and 1 can be automatic (RRT data production) or manual, performed by the Observational Platform. Data level identifiers distinguish between data with automatic or manual QC. The details of the steps producing Level 1 data from Level 0, and Level 2 from Level 1, are specified in the associated data production descriptions and the standard operating procedures.

For the **off-line workflow**, the steps of sample medium pre-exposure treatment, exposure, sampling time, preparation, and analysis are documented following standard operating procedures and protocols including metadata in machine-readable form. Items to be included are specified in the associated data production descriptions and the standard operating procedures. The archive for these protocols is located at the Observational Platform, its operating protocols coordinated with and approved by the Data Centre and the Centre for Reactive Trace Gases In Situ Measurements. From these protocols, the NF produces the Level 2 data product in the temporal resolution determined by the sampling schedule, and submits it to the Data Centre using the tools provided for data submission to ensure required documentation, flagging and metadata. The details of the procedure are specified in the associated data production descriptions.
4.6.6 Data delivery and quality control

It is compulsory for the Observational Platform to produce the Level 1 and Level 2 data for archiving at the Data Centre. Archiving of Level 0 data will depend on specific variables. For the submission of trace-gas in *situ* data required procedures are described in ACTRIS Data Management Plan (D4.2) and the associated data production descriptions. Manually quality controlled data has to be submitted on a regular, scheduled frequency, at least yearly to the ACTRIS *in situ* data repository following the scheme in the ACTRIS Data Management Plan, and associated documents. Preliminary, manually quality controlled data has to be submitted by the end of March in the following year, the latest. Data originators at the NF are responsible for checking the data using the procedures in the associated data production descriptions.

Data is then reviewed by the Centre for Reactive Trace Gases In Situ Measurements and will be discussed at the annual data quality meeting in May, in collaboration with the respective unit of the Data Centre. An issue tracker is operated in the process of data evaluation to ensure full documentation and traceability of the data production and quality control process. This comes into place yearly following the initial data submission by stations to the DC. After review of the data according to the suggestions of the Centre for Reactive Trace Gases In Situ Measurements and the measurement community attending the meeting, data has to be resubmitted and will be reviewed again for compliance with suggested changes. Final submission to the DC has to be done before 31 May, using the submission tools and automatic quality control software available within the DC. Figure 1 shows the cooperation between the Observational Platforms, the Topical Centre and the Data Centre.

<figure><img src="image_3.png" /><figcaption>Figure 1: Scheme of the QA-system envisioned for the Operational Platforms together with the Centre for Reactive Trace Gases In Situ Measurements and the Data Centre.</figcaption></figure>
4.6.6 Data delivery and quality control

It is compulsory for the Observational Platform to produce the Level 1 and Level 2 data for archiving at the Data Centre. Archiving of Level 0 data will depend on specific variables. For the submission of trace-gas in *situ* data required procedures are described in ACTRIS Data Management Plan (D4.2) and the associated data production descriptions. Manually quality controlled data has to be submitted on a regular, scheduled frequency, at least yearly to the ACTRIS *in situ* data repository following the scheme in the ACTRIS Data Management Plan, and associated documents. Preliminary, manually quality controlled data has to be submitted by the end of March in the following year, the latest. Data originators at the NF are responsible for checking the data using the procedures in the associated data production descriptions.

Data is then reviewed by the Centre for Reactive Trace Gases In Situ Measurements and will be discussed at the annual data quality meeting in May, in collaboration with the respective unit of the Data Centre. An issue tracker is operated in the process of data evaluation to ensure full documentation and traceability of the data production and quality control process. This comes into place yearly following the initial data submission by stations to the DC. After review of the data according to the suggestions of the Centre for Reactive Trace Gases In Situ Measurements and the measurement community attending the meeting, data has to be resubmitted and will be reviewed again for compliance with suggested changes. Final submission to the DC has to be done before 31 May, using the submission tools and automatic quality control software available within the DC. Figure 1 shows the cooperation between the Observational Platforms, the Topical Centre and the Data Centre.

<figure><img src="image_4.png" /><figcaption>Figure 1: Scheme of the QA-system envisioned for the Operational Platforms together with the Centre for Reactive Trace Gases In Situ Measurements and the Data Centre.</figcaption></figure>
# 5 Organizational and strategic concepts for ACTRIS Observational Platforms

Experience from ACTRIS heritage shows that typically two kinds of Observational Platforms evolve in the research infrastructure: a) advanced, well-equipped Observational Platforms, which contribute to the observational strategy with optimized and combined observations leading to substantial synergistic effects (described in Sec. 5.1), and b) specialized Observational Platforms, which deliver a limited number of variables with focus on regional coverage and specific topics (described in Sec. 5.2). In the following, a brief description of respective requirements for these two types of Observational Platforms is given. Details of requirements and procedures that will be used in the respective labelling of ACTRIS Observational Platforms are discussed in D5.3 (Documentation on ACTRIS National Facility labelling principles).

## 5.1 ACTRIS Observational Platforms focussing on synergy

### 5.1.1 Observational requirements

Advanced ACTRIS Observational Platforms apply sophisticated and optimized instrumentation in a synergistic manner in order to deliver a comprehensive set of aerosol, cloud and/or reactive-trace-gas data, together with auxiliary parameters. It is expected that several of the observational components described in Sec. 4.1–4.6 are joined at such stations or that ACTRIS observational components are deployed together with an extensive set of non-ACTRIS instrumentation, e.g., at existing national observatories or at sites shared with another RI. Instruments at these advanced ACTRIS Observational Platforms should run continuously (24 hours/7 days a week if applicable; otherwise whenever measurement conditions are suitable) and data should be delivered in RRT or NRT in order to allow for data assimilation, on-line validation of model or satellite products and immediate information of the public.

### 5.1.2 Geographical requirements

Advanced ACTRIS Observational Platforms require substantial implementation and operation efforts, i.e., it is expected that only a limited number of such stations can be realized. Most probably, they will be set up at already existing national observatories, often also called “supersites” (a term that is however not adopted by ACTRIS). Thus, the geographical location of these sites, in many cases, has been already chosen in the past. It may be supposed that major general requirements have been considered in the selection, e.g., representativeness for a larger area or homogeneity of the surrounding in terms of orography, vegetation, cultivation, population density, etc. The sites should usually not be located in an isolated micro-environment with very specific conditions, except if the performed research is specifically dedicated to such an environment, which could be the case, e.g., for high-mountain sites. When new synergistic Observational Platforms are planned the locations should be chosen accordingly.

Moreover, synergistic Observational Platforms should be well distributed across Europe, covering the major climatic conditions and all typical air masses present over the continent. Due to the limited number of the sites, typical distances between them will be of the order of several hundred to thousand
# 5 Organizational and strategic concepts for ACTRIS Observational Platforms

Experience from ACTRIS heritage shows that typically two kinds of Observational Platforms evolve in the research infrastructure: a) advanced, well-equipped Observational Platforms, which contribute to the observational strategy with optimized and combined observations leading to substantial synergistic effects (described in Sec. 5.1), and b) specialized Observational Platforms, which deliver a limited number of variables with focus on regional coverage and specific topics (described in Sec. 5.2). In the following, a brief description of respective requirements for these two types of Observational Platforms is given. Details of requirements and procedures that will be used in the respective labelling of ACTRIS Observational Platforms are discussed in D5.3 (Documentation on ACTRIS National Facility labelling principles).

## 5.1 ACTRIS Observational Platforms focussing on synergy

### 5.1.1 Observational requirements

Advanced ACTRIS Observational Platforms apply sophisticated and optimized instrumentation in a synergistic manner in order to deliver a comprehensive set of aerosol, cloud and/or reactive-trace-gas data, together with auxiliary parameters. It is expected that several of the observational components described in Sec. 4.1–4.6 are joined at such stations or that ACTRIS observational components are deployed together with an extensive set of non-ACTRIS instrumentation, e.g., at existing national observatories or at sites shared with another RI. Instruments at these advanced ACTRIS Observational Platforms should run continuously (24 hours/7 days a week if applicable; otherwise whenever measurement conditions are suitable) and data should be delivered in RRT or NRT in order to allow for data assimilation, on-line validation of model or satellite products and immediate information of the public.

### 5.1.2 Geographical requirements

Advanced ACTRIS Observational Platforms require substantial implementation and operation efforts, i.e., it is expected that only a limited number of such stations can be realized. Most probably, they will be set up at already existing national observatories, often also called “supersites” (a term that is however not adopted by ACTRIS). Thus, the geographical location of these sites, in many cases, has been already chosen in the past. It may be supposed that major general requirements have been considered in the selection, e.g., representativeness for a larger area or homogeneity of the surrounding in terms of orography, vegetation, cultivation, population density, etc. The sites should usually not be located in an isolated micro-environment with very specific conditions, except if the performed research is specifically dedicated to such an environment, which could be the case, e.g., for high-mountain sites. When new synergistic Observational Platforms are planned the locations should be chosen accordingly.

Moreover, synergistic Observational Platforms should be well distributed across Europe, covering the major climatic conditions and all typical air masses present over the continent. Due to the limited number of the sites, typical distances between them will be of the order of several hundred to thousand
kilometres. Consequently, each advanced Observation Platform will be of major importance as key
location and anchor point for observations in a larger region.

## 5.2 ACTRIS Observational Platforms focussing on regional coverage

### 5.2.1 Observational requirements

Specialized ACTRIS Observational Platforms contribute to at least one observational component described in Sec. 4.1–4.6 following at least the minimum ACTRIS requirements discussed above. Specific operation procedures may be applied at such stations, if they are in line with the ACTRIS standards.

### 5.2.2 Geographical requirements

Specialized ACTRIS Observational Platforms require less implementation and operation efforts. Therefore, a larger number of sites, involving also a larger number of national institutions, are expected. These Observational Platforms may be dedicated to the observation of distinct short-lived species under certain conditions. For instance, aerosols and short-lived trace gases relevant for human health need to be observed in and around industrialized urban regions, whereas for cloud observations rural mountainous sites are of much higher interest. Specialized ACTRIS Observational Platforms are important for geographical coverage and the investigation of the variability of different ACTRIS species in between the bigger observatories.

## 5.3 Access requirements

The major task of Observational Platforms is the collection and delivery of quality-controlled data for provision to users via the ACTRIS Data Centre. In addition, selected ACTRIS Observational Platforms, in particular large observatories, may offer physical access to users. Adequate personnel, equipment, logistics and resources at the respective sites are required, i.e.:

* Capabilities to run large scientific experiments;

* Possibilities to run instruments on user request for specific investigations, in specific modes, etc.;

* Possibilities to set up and run instruments of the user, including supply of power, water, technical gases, internet connection, etc.;

* Logistics to perform specific campaigns, e.g., with mobile ground-based or airborne platforms;

* Resources to host users, including working (office) and storage space and all necessary auxiliary facilities and supplies;

* Capabilities to train and support users by experienced staff;

* Resources to support users in administrative and logistic issues, e.g., customs, shipping, travel, transport, accommodation, etc.;

* Capabilities to ensure the ACTRIS observational requirements during physical access provision.

Physical access to Observational Platforms will be managed by the SAMU. The selection of Observational Platforms that will provide physical access will be part of the labelling process. Details are specified in D5.3, D2.6 and D6.3.
kilometres. Consequently, each advanced Observation Platform will be of major importance as key
location and anchor point for observations in a larger region.

## 5.2 ACTRIS Observational Platforms focussing on regional coverage

### 5.2.1 Observational requirements

Specialized ACTRIS Observational Platforms contribute to at least one observational component described in Sec. 4.1–4.6 following at least the minimum ACTRIS requirements discussed above. Specific operation procedures may be applied at such stations, if they are in line with the ACTRIS standards.

### 5.2.2 Geographical requirements

Specialized ACTRIS Observational Platforms require less implementation and operation efforts. Therefore, a larger number of sites, involving also a larger number of national institutions, are expected. These Observational Platforms may be dedicated to the observation of distinct short-lived species under certain conditions. For instance, aerosols and short-lived trace gases relevant for human health need to be observed in and around industrialized urban regions, whereas for cloud observations rural mountainous sites are of much higher interest. Specialized ACTRIS Observational Platforms are important for geographical coverage and the investigation of the variability of different ACTRIS species in between the bigger observatories.

## 5.3 Access requirements

The major task of Observational Platforms is the collection and delivery of quality-controlled data for provision to users via the ACTRIS Data Centre. In addition, selected ACTRIS Observational Platforms, in particular large observatories, may offer physical access to users. Adequate personnel, equipment, logistics and resources at the respective sites are required, i.e.:

* Capabilities to run large scientific experiments;

* Possibilities to run instruments on user request for specific investigations, in specific modes, etc.;

* Possibilities to set up and run instruments of the user, including supply of power, water, technical gases, internet connection, etc.;

* Logistics to perform specific campaigns, e.g., with mobile ground-based or airborne platforms;

* Resources to host users, including working (office) and storage space and all necessary auxiliary facilities and supplies;

* Capabilities to train and support users by experienced staff;

* Resources to support users in administrative and logistic issues, e.g., customs, shipping, travel, transport, accommodation, etc.;

* Capabilities to ensure the ACTRIS observational requirements during physical access provision.

Physical access to Observational Platforms will be managed by the SAMU. The selection of Observational Platforms that will provide physical access will be part of the labelling process. Details are specified in D5.3, D2.6 and D6.3.
## 5.4 Recommendations for national strategies

Setting up ACTRIS Observational Platforms following the concepts described above requires coordination and funding efforts at the national level. Thus, cost-effective solutions for maximum scientific value are demanded. The more instrumental and data synergy is achieved the more valuable information on atmospheric processes can be extracted from the observations. Thus, bundling efforts at national level or even collaboration of member states in order to set up high-sophisticated, synergistic Observational Platforms is strongly recommended. Such strategies may also consider co-location of sites with other environmental RIs, if added value can be achieved in this way.

ACTRIS focusses on highly variable short-lived species originating from various sources and undergoing vast atmospheric processes. Therefore, as discussed above, observational sites should be located such that, on the one hand, data representative for a larger area can be collected, and, on the other hand, sufficient information on the variability of atmospheric parameters in different environments is gathered. Therefore, the coverage of scientifically relevant geographical areas and ecosystems should be considered in national strategies and Observational Platforms distributed accordingly. Decisions on locations of observational sites, in particular when new sites are planned, should be reconciled within ACTRIS to allow for a coordination of efforts to enclose the scientific areas of interest. Furthermore, national and common initiatives for setting up observational sites at global hot spots are highly appreciated. Collaboration with local partners at such locations outside of Europe and respective outreach strategies will be strongly supported by ACTRIS.
## 5.4 Recommendations for national strategies

Setting up ACTRIS Observational Platforms following the concepts described above requires coordination and funding efforts at the national level. Thus, cost-effective solutions for maximum scientific value are demanded. The more instrumental and data synergy is achieved the more valuable information on atmospheric processes can be extracted from the observations. Thus, bundling efforts at national level or even collaboration of member states in order to set up high-sophisticated, synergistic Observational Platforms is strongly recommended. Such strategies may also consider co-location of sites with other environmental RIs, if added value can be achieved in this way.

ACTRIS focusses on highly variable short-lived species originating from various sources and undergoing vast atmospheric processes. Therefore, as discussed above, observational sites should be located such that, on the one hand, data representative for a larger area can be collected, and, on the other hand, sufficient information on the variability of atmospheric parameters in different environments is gathered. Therefore, the coverage of scientifically relevant geographical areas and ecosystems should be considered in national strategies and Observational Platforms distributed accordingly. Decisions on locations of observational sites, in particular when new sites are planned, should be reconciled within ACTRIS to allow for a coordination of efforts to enclose the scientific areas of interest. Furthermore, national and common initiatives for setting up observational sites at global hot spots are highly appreciated. Collaboration with local partners at such locations outside of Europe and respective outreach strategies will be strongly supported by ACTRIS.
# Annex A: ACTRIS Variables and Related Instruments

Version 23 April 2018

This section comprises a catalogue with ACTRIS Level 1 (L1) and Level 2 (L2) data from Observational Platforms associated to recommended instrument techniques.

## Abbreviations and symbols

<table><thead><tr><th colspan="2">Colour codes</th></tr></thead><tbody><tr><td>Variable produced from a single measurement technique</td><td></td></tr><tr><td>Variable produced from synergy of measurement techniques</td><td></td></tr><tr><td>Variable produced from synergy of measurement techniques where combinations are possible</td><td></td></tr></tbody></table>

<table><thead><tr><th colspan="2">Abbreviations used in the tables</th></tr></thead><tbody><tr><td>NF</td><td>National Facility</td></tr><tr><td>DC</td><td>Data Centre</td></tr><tr><td>TC</td><td>Topical Centre</td></tr><tr><td>CARS</td><td>Centre for Aerosol Remote Sensing</td></tr><tr><td>CAIS</td><td>Centre for Aerosol In Situ Measurements</td></tr><tr><td>CGas-SiM</td><td>Centre for Reactive Trace Gases In Situ Measurements</td></tr><tr><td>CIS</td><td>Centre for Cloud In Situ Measurements</td></tr><tr><td>CCRES</td><td>Centre for Cloud Remote Sensing</td></tr><tr><td>CREGARS</td><td>Centre for Reactive Trace Gases Remote Sensing</td></tr><tr><td>RRT-O</td><td>Real real time (RRT) data stream implemented and operational. Most cases less than 3 h (real real time) but can be near real time (less than 3 days)</td></tr><tr><td>RRT-S</td><td>Real real time (RRT) possible and scheduled</td></tr><tr><td>M</td><td>Mandatory variables required for ACTRIS site</td></tr><tr><td>S</td><td>Specializing variable provision of at least 1 specializing variable are mandatory for ACTRIS sites connected to Aerosol in-situ TC - Optimum - recommended</td></tr><tr><td>O</td><td></td></tr></tbody></table>
# Annex A: ACTRIS Variables and Related Instruments

Version 23 April 2018

This section comprises a catalogue with ACTRIS Level 1 (L1) and Level 2 (L2) data from Observational Platforms associated to recommended instrument techniques.

## Abbreviations and symbols

<table><thead><tr><th colspan="2">Colour codes</th></tr></thead><tbody><tr><td>Variable produced from a single measurement technique</td><td></td></tr><tr><td>Variable produced from synergy of measurement techniques</td><td></td></tr><tr><td>Variable produced from synergy of measurement techniques where combinations are possible</td><td></td></tr></tbody></table>

<table><thead><tr><th colspan="2">Abbreviations used in the tables</th></tr></thead><tbody><tr><td>NF</td><td>National Facility</td></tr><tr><td>DC</td><td>Data Centre</td></tr><tr><td>TC</td><td>Topical Centre</td></tr><tr><td>CARS</td><td>Centre for Aerosol Remote Sensing</td></tr><tr><td>CAIS</td><td>Centre for Aerosol In Situ Measurements</td></tr><tr><td>CGas-SiM</td><td>Centre for Reactive Trace Gases In Situ Measurements</td></tr><tr><td>CIS</td><td>Centre for Cloud In Situ Measurements</td></tr><tr><td>CCRES</td><td>Centre for Cloud Remote Sensing</td></tr><tr><td>CREGARS</td><td>Centre for Reactive Trace Gases Remote Sensing</td></tr><tr><td>RRT-O</td><td>Real real time (RRT) data stream implemented and operational. Most cases less than 3 h (real real time) but can be near real time (less than 3 days)</td></tr><tr><td>RRT-S</td><td>Real real time (RRT) possible and scheduled</td></tr><tr><td>M</td><td>Mandatory variables required for ACTRIS site</td></tr><tr><td>S</td><td>Specializing variable provision of at least 1 specializing variable are mandatory for ACTRIS sites connected to Aerosol in-situ TC -</td></tr><tr><td>O</td><td>Optimum - recommended</td></tr></tbody></table>
## A.1 Aerosol remote sensing

<table><thead><tr><th>ACTRIS variable</th><th>NF requirements<br>Mandatory - M<br>Specializing - S<br>Optimum - O</th><th>Associated<br>Topical Centre</th><th>Data<br>producer<br>L1, L2</th><th>RRT-O=<br>operational in<br>RRT now<br>RRT-S =<br>scheduled<br>from 2020</th><th>Approx.<br>height<br>resolution</th><th>High-power aerosol lidar</th><th>Automatic sun/sky/lunar photometer</th></tr></thead><tbody><tr><td>Attenuated backscatter profile</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Volume depolarization profile</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Particle backscatter coefficient profile</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Particle extinction coefficient profile</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Lidar ratio profile</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Ångström exponent profile</td><td>O</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Backscatter-related Ångström exponent profile</td><td>O</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Particle depolarization ratio profile</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Particle layer geometrical properties (height and thickness)</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr></tbody></table>
## A.1 Aerosol remote sensing

<table><thead><tr><th>ACTRIS variable</th><th>NF requirements<br>Mandatory - M<br>Specializing - S<br>Optimum - O</th><th>Associated<br>Topical Centre</th><th>Data<br>producer<br>L1, L2</th><th>RRT-O=<br>operational in<br>RRT now<br>RRT-S =<br>scheduled<br>from 2020</th><th>Approx.<br>height<br>resolution</th><th>High-power aerosol lidar</th><th>Automatic sun/sky/lunar photometer</th></tr></thead><tbody><tr><td>Attenuated backscatter profile</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Volume depolarization profile</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Particle backscatter coefficient profile</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Particle extinction coefficient profile</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Lidar ratio profile</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Ångström exponent profile</td><td>O</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Backscatter-related Ångström exponent profile</td><td>O</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Particle depolarization ratio profile</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Particle layer geometrical properties (height and thickness)</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr></tbody></table>
<table><thead><tr><th>ACTRIS variable</th><th>NF requirements<br>Mandatory - M<br>Specializing - S<br>Optimum - O</th><th>Associated<br>Topical Centre</th><th>Data<br>producer<br>L1, L2</th><th>RRT-O=<br>operational in<br>RRT now<br>RRT-S =<br>scheduled<br>from 2020</th><th>Approx.<br>height<br>resolution</th><th>High-power aerosol lidar</th><th>Automatic sun/sky/lunar photometer</th></tr></thead><tbody><tr><td>Particle layer optical properties (extinction, backscatter, lidar ratio, Ångström exponent, depolarization ratio, optical depth)</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Column integrated extinction</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Planetary boundary layer height</td><td>O</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Spectral Downward Sky Radiances</td><td>M</td><td>CARS</td><td>TC</td><td>NRT-O</td><td>NA</td><td></td><td></td></tr><tr><td>Direct Sun/Moon Extinction Aerosol Optical Depth (column)</td><td>M</td><td>CARS</td><td>TC</td><td>NRT-O</td><td>NA</td><td></td><td></td></tr></tbody></table>
<table><thead><tr><th>ACTRIS variable</th><th>NF requirements<br>Mandatory - M<br>Specializing - S<br>Optimum - O</th><th>Associated<br>Topical Centre</th><th>Data<br>producer<br>L1, L2</th><th>RRT-O=<br>operational in<br>RRT now<br>RRT-S =<br>scheduled<br>from 2020</th><th>Approx.<br>height<br>resolution</th><th>High-power aerosol lidar</th><th>Automatic sun/sky/lunar photometer</th></tr></thead><tbody><tr><td>Particle layer optical properties (extinction, backscatter, lidar ratio, Ångström exponent, depolarization ratio, optical depth)</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Column integrated extinction</td><td>M</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Planetary boundary layer height</td><td>O</td><td>CARS</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td></tr><tr><td>Spectral Downward Sky Radiances</td><td>M</td><td>CARS</td><td>TC</td><td>NRT-O</td><td>NA</td><td></td><td></td></tr><tr><td>Direct Sun/Moon Extinction Aerosol Optical Depth (column)</td><td>M</td><td>CARS</td><td>TC</td><td>NRT-O</td><td>NA</td><td></td><td></td></tr></tbody></table>
## A.2 Cloud remote sensing

<table><thead><tr><th>ACTRIS variable</th><th>NF requirements Mandatory - M Specializing - S Optimum - O</th><th>Associated Topical Centre</th><th>Data producer L1, L2</th><th>RRT-O= operational in RRT now RRT-S = scheduled from 2020</th><th>Approx. height resolution</th><th>High-power aerosol lidar</th><th>Automatic low-power lidar and ceilometer</th><th>Doppler cloud radar</th><th>Radiosonde</th><th>Microwave radiometer</th><th>Doppler lidar</th><th>Drop-counting raingauge</th><th>Disdrometer</th><th>NWP model input required</th></tr></thead><tbody><tr><td>Radar reflectivity factor</td><td>M</td><td>CCRES</td><td>DC</td><td>RRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Radar Doppler velocity</td><td>M</td><td>CCRES</td><td>DC</td><td>RRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Radar Doppler spectral width</td><td>M</td><td>CCRES</td><td>DC</td><td>RRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Radar linear depolarisation ratio</td><td>O</td><td>CCRES</td><td>DC</td><td>RRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Attenuated backscatter profile</td><td>M</td><td>CARS</td><td>DC</td><td>RRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Cloud/aerosol target classification</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Drizzle drop size distribution</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Drizzle water content</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
## A.2 Cloud remote sensing

<table><thead><tr><th>ACTRIS variable</th><th>NF requirements Mandatory - M Specializing - S Optimum - O</th><th>Associated Topical Centre</th><th>Data producer L1, L2</th><th>RRT-O= operational in RRT now RRT-S = scheduled from 2020</th><th>Approx. height resolution</th><th>High-power aerosol lidar</th><th>Automatic low-power lidar and ceilometer</th><th>Doppler cloud radar</th><th>Radiosonde</th><th>Microwave radiometer</th><th>Doppler lidar</th><th>Drop-counting raingauge</th><th>Disdrometer</th><th>NWP model input required</th></tr></thead><tbody><tr><td>Radar reflectivity factor</td><td>M</td><td>CCRES</td><td>DC</td><td>RRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Radar Doppler velocity</td><td>M</td><td>CCRES</td><td>DC</td><td>RRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Radar Doppler spectral width</td><td>M</td><td>CCRES</td><td>DC</td><td>RRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Radar linear depolarisation ratio</td><td>O</td><td>CCRES</td><td>DC</td><td>RRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Attenuated backscatter profile</td><td>M</td><td>CARS</td><td>DC</td><td>RRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Cloud/aerosol target classification</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Drizzle drop size distribution</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Drizzle water content</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><thead><tr><th>ACTRIS variable</th><th>NF requirements Mandatory - M Specializing - S Optimum - O</th><th>Associated Topical Centre</th><th>Data producer L1, L2</th><th>RRT-O= operational in RRT now RRT-S = scheduled from 2020</th><th>Approx. height resolution</th><th>High-power aerosol lidar</th><th>Automatic low-power lidar and ceilometer</th><th>Doppler cloud radar</th><th>Radiosonde</th><th>Microwave radiometer</th><th>Doppler lidar</th><th>Drop-counting raingauge</th><th>Disdrometer</th><th>NWP model input required</th></tr></thead><tbody><tr><td>Drizzle water flux</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ice water content</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Liquid water content</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Dissipation rate of TKE (turbulent kinetic energy)</td><td>O</td><td>CCRES</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Atmospheric boundary layer classification</td><td>O</td><td>CCRES</td><td>DC</td><td>RRT-S</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Liquid water path</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>-</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Temperature profile</td><td>O</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>variable</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Relative humidity profile</td><td>O</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>variable</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Integrated water vapor path</td><td>O</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>-</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><thead><tr><th>ACTRIS variable</th><th>NF requirements Mandatory - M Specializing - S Optimum - O</th><th>Associated Topical Centre</th><th>Data producer L1, L2</th><th>RRT-O= operational in RRT now RRT-S = scheduled from 2020</th><th>Approx. height resolution</th><th>High-power aerosol lidar</th><th>Automatic low-power lidar and ceilometer</th><th>Doppler cloud radar</th><th>Radiosonde</th><th>Microwave radiometer</th><th>Doppler lidar</th><th>Drop-counting raingauge</th><th>Disdrometer</th><th>NWP model input required</th></tr></thead><tbody><tr><td>Drizzle water flux</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ice water content</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Liquid water content</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Dissipation rate of TKE (turbulent kinetic energy)</td><td>O</td><td>CCRES</td><td>DC</td><td>NRT-S</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Atmospheric boundary layer classification</td><td>O</td><td>CCRES</td><td>DC</td><td>RRT-S</td><td>60 m</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Liquid water path</td><td>M</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>-</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Temperature profile</td><td>O</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>variable</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Relative humidity profile</td><td>O</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>variable</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Integrated water vapor path</td><td>O</td><td>CCRES</td><td>DC</td><td>NRT-O</td><td>-</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
A.3 Reactive trace gases remote sensing

<table><thead><tr><th>ACTRIS variable</th><th>NF requirements Mandatory - M Specializing - S Optimum - O</th><th>Associated Topical Centre</th><th>Data producer L1, L2</th><th>RRT-O= operational in RRT now RRT-S = scheduled from 2020</th><th>Approx. height resolution</th><th>Single FTIR</th><th>Double FTIR</th><th>UVVIS-Zenith Sky</th><th>UVVIS MAXDOAS</th><th>O3 DIAL</th></tr></thead><tbody><tr><td>Ozone profile</td><td>O</td><td>CREGARS</td><td>NF</td><td>-</td><td>few hundred meters to few km</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ozone partial columns</td><td>O</td><td>CREGARS</td><td>NF</td><td>-</td><td>5 to 8 km</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ozone column</td><td>M</td><td>CREGARS</td><td>FTIR: NF; UVVIS: TC</td><td>UVVIS: NRT-S</td><td>NA</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Formaldehyde column</td><td>M</td><td>CREGARS</td><td>FTIR: NF; UVVIS-MAXDOAS: TC</td><td>UVVIS MAXDOAS: NRT-S</td><td>NA</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Formaldehyde lower tropospheric profile</td><td>O</td><td>CREGARS</td><td>TC</td><td>NRT-S</td><td>few hundred meters to few km</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>NO2 column</td><td>M</td><td>CREGARS</td><td>FTIR: NF; UVVIS: TC</td><td>FTIR: after 3 months; UVVIS: NRT-S</td><td>NA</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>NO2 partial columns</td><td>S</td><td>CREGARS</td><td>FTIR: NF; UVVIS: TC</td><td>FTIR: after 3 months; UVVIS: NRT-S</td><td>stratospheric column</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>NO2 lower tropospheric profile</td><td>O</td><td>CREGARS</td><td>TC</td><td>NRT-O</td><td>-</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>NH3 column</td><td>S</td><td>CREGARS</td><td>NF</td><td>NRT-O</td><td>NA</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>C2H6 column</td><td>M</td><td>CREGARS</td><td>NF</td><td>NRT-O</td><td>-</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
A.3 Reactive trace gases remote sensing

<table><thead><tr><th>ACTRIS variable</th><th>NF requirements Mandatory - M Specializing - S Optimum - O</th><th>Associated Topical Centre</th><th>Data producer L1, L2</th><th>RRT-O= operational in RRT now RRT-S = scheduled from 2020</th><th>Approx. height resolution</th><th>Single FTIR</th><th>Double FTIR</th><th>UVVIS-Zenith Sky</th><th>UVVIS MAXDOAS</th><th>O3 DIAL</th></tr></thead><tbody><tr><td>Ozone profile</td><td>O</td><td>CREGARS</td><td>NF</td><td>-</td><td>few hundred meters to few km</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ozone partial columns</td><td>O</td><td>CREGARS</td><td>NF</td><td>-</td><td>5 to 8 km</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ozone column</td><td>M</td><td>CREGARS</td><td>FTIR: NF; UVVIS: TC</td><td>UVVIS: NRT-S</td><td>NA</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Formaldehyde column</td><td>M</td><td>CREGARS</td><td>FTIR: NF; UVVIS-MAXDOAS: TC</td><td>UVVIS MAXDOAS: NRT-S</td><td>NA</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Formaldehyde lower tropospheric profile</td><td>O</td><td>CREGARS</td><td>TC</td><td>NRT-S</td><td>few hundred meters to few km</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>NO2 column</td><td>M</td><td>CREGARS</td><td>FTIR: NF; UVVIS: TC</td><td>FTIR: after 3 months; UVVIS: NRT-S</td><td>NA</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>NO2 partial columns</td><td>S</td><td>CREGARS</td><td>FTIR: NF; UVVIS: TC</td><td>FTIR: after 3 months; UVVIS: NRT-S</td><td>stratospheric column</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>NO2 lower tropospheric profile</td><td>O</td><td>CREGARS</td><td>TC</td><td>NRT-O</td><td>-</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>NH3 column</td><td>S</td><td>CREGARS</td><td>NF</td><td>NRT-O</td><td>NA</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>C2H6 column</td><td>M</td><td>CREGARS</td><td>NF</td><td>NRT-O</td><td>-</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>