Objectives and Overview

In preparation of Cal/Val activities for upcoming satellite missions a number of gaps in current and expected validation infrastructure have been identified, e.g. during the ACVE workshop held in March 2013. This is mainly due to the focus of these missions on air quality, aerosols, and clouds in the Troposphere (0-15 km height), which is different from previous ESA atmospheric chemistry missions (ENVISAT or ERS) that have a strong focus on ozone (O3) chemistry in the stratosphere (10-50 km height). Currently, there is no standardized measurement and processing system in Europe capable of providing a homogeneous validation dataset for tropospheric products retrieved from the S5p and EarthCARE missions.

The existing global Dobson/Brewer network is a reliable resource measuring total column O3 from direct sun observations. However, the costs per instrument (acquisition, operation, maintenance, and calibration) are large and the ability to retrieve tropospheric O3 or column amounts of other trace gases is very limited. Alternatively, there are some networks retrieving trace gas slant columns from sky radiance observations only. The sky radiance retrieval technique has the disadvantage that the air mass factor (AMF), used to obtain vertical column amounts, is rather uncertain, since one needs to make assumptions about the state of the atmosphere (e.g. aerosol loading) for its calculation. Similar assumptions are also needed for the AMF from satellite observations, meaning that the two data-sets from ground and space are not truly independent. The rather large uncertainty in the AMF is especially problematic for those trace gases, where high accuracy is needed (O3-accuracy should be <3%, nitrogen dioxide (NO2)-accuracy <10%). To obtain both accurate vertical column amounts and profile information, a system able to measure both direct sun and sky radiance is necessary.

Past studies have shown that due to the intrinsic difference in the field of view between satellite and ground data, large statistics (long time series in many locations) are needed in order to perform meaningful Cal/Val activities. This is especially true for highly variable (spatially and temporally) species such as NO2 and other tropospheric components. Therefore a global network with as many as possible (quasi-)autonomous stations is the preferred solution.

The objective of this project build a homogenous ground-based remote sensing network measuring trace gas amounts and aerosol properties. The network will be ready for ground-validation of Earth Observation (EO) missions, in particular for air-quality monitoring and aerosols. The main data products provided by the network will be total O3, total NO2, their separation in a stratospheric and tropospheric column, as well as spectral aerosol optical depth over the UV and visible range, overlapping significantly with the spectral ranges of the S5p instrument as well as the MSI and ATLID on-board EarthCARE. The feasibility for additional data products such as column amounts of water vapor, formaldehyde and sufure dioxide will be investigated.