Institute of Environmental Physics and Institute of Remote Sensing University of Bremen

1. Validation Strategy for Reactive Gases SCILOV final meeting Frascati, February 27, 2014 Andreas Richter, F. Wittrock, M. Weber, and J. P. Burrows Institute of Environmental Physics and Institute of Remote Sensing University of Bremen

2. Whathavebeen the activities in SCILOV? Background: • No actual validation can be performed for the minor trace gases for lack of appropriate independent measurements of the right quantities • The idea was to collect all available studies and proposals to come to a strategy on how to improve the situation in the future Actions • A first collection of ideas and challenges was put together early in the project • A more detailed investigation of the situation was presented at the mid-term review • The ideas were then iterated in the community and in particular in SCIAVALIG • The results were presented at the ACVE and published in the proceedings • The discussions are continued in the S5P MAG and elsewhere

3. What is the Task? Background: • Satellite data need to be validated with independent observations • This is a job to be done continuously on all sensors and products • For many products such as stratospheric absorbers this is standard procedure Task: • To ensure that in the future, similar rigorous validation is possible for tropospheric species as for stratospheric species • To define which data and methods are needed • To develop and support the reference measurements needed • To create the necessary data sets and validation tools

4. Whatisthe Challenge? Tropospheric species are characterised by • High variability in space and time • Large horizontal and vertical gradients • Large diurnal variation • Localised sources, often in inaccessible areas • Small signals (exception: volcanic SO2) • Strongly varying sensitivity of the retrievals • Large dependence of results on a priori used in retrieval • Large dependence of results on cloud fraction and altitude

5. Whatisthe Challenge? A typical tropospheric validation measurement is • Much more localised in space than the satellite measurement • Providing • either the concentration in one altitude • or the total column • or the vertical profile with a sensitivity and vertical resolution different from that of the satellite • Limited in temporal coverage • Limited in accuracy and precision • Located in an area characterised by background conditions => more often than not, we are measuring the wrong quantity in the wrong place not often enough and with inappropriate sampling and accuracy

6. Whatisneeded? An ideal tropospheric validation measurement would provide • Vertical profiles • Diurnal profiles • Seasonal profiles • Good spatial sampling • Good coverage of range of situations • Decent statistics • Sufficient accuracy and precision • Validation of other input data

7. What should be the Strategy for the Future? Identify and address the gaps: • Continue acquiring new data (DOAS, FTS, lidar, in-situ) • Go to the right places (polluted places, rain forests, oceans, deserts, polar regions) • Take the right measurements (profiles, columns, support data) • Accumulate enough data This can be achieved by • Continuing and extending the networks (NDACC, TCON, …) • Initiating dedicated campaigns • Developing new techniques and platforms (NO2sonde, UAV) • Piggy backing on existing platforms (trains, ships, cars, …) • Facilitating data access (NORS, GEOMS, …) • Integrating with assimilation models (MACC++)

8. For example NO2: A network of the future: • A small number of high-quality MAX-DOAS instruments in selected locations (not too affected by local pollution) • A larger number of good quality monitoring instruments (PANDORA?) at locations with variable pollution levels • A large number of simple instruments distributed over a large area and integrated in buildings, cars, trains, ships, feeding a centralised data base • Integration of in-situ NO2 measurements into commercial aircrafts • Regular sonde measurements in a small number of selected locations, if possible covering different pollution levels • Regular campaigns for instrument and algorithm comparisons • Irregular science campaigns with differing focus • International co-operation (Europe, US, Asia) to create global data sets

9. For example NO2: What could happen in the coming years? • S5P / TROPOMI and later S4 will become drivers of validation needs • High spatial resolution will make life easier for validation but create new problems for a priori data used by retrievals • The users of the data products will change from mainly “scientific” to • Data assimilation (COPERNICUS) • Air quality • This will change the requirements • Good error characterisation is more important then small errors => good statistics of validation data is needed • NO2 surface concentrations are needed, not tropospheric columns => strict validation is not as relevant as good link to surface concentrations

10. Summary and Conclusions • Validation is a continuous activity that needs continuous support for • Long-term measurements • Campaigns • Data analysis • Development of new capabilities • For tropospheric species in particular, it is crucial to have • Good and adapted spatial coverage • Coverage of the right quantities • A combination of long-term observations and dedicated campaigns • We need to change our mind-set for validation from the “stratospheric view” (few high precision profiling campaigns, O3sondes, Brewer / Dobson, background monitoring) to a “tropospheric view” (many continuous measurements, good spatial and temporal coverage, accuracy of individual measurement is not always the key)