EMEP (European Monitoring and Evaluation Programme):
https://emep.int/

The EMEP is a cooperative program for monitoring and evaluating the long-range transmission of air pollutants in Europe.

Global Atmosperic Watch Program (GAW Station Information System):
community.wmo.int/activity-areas/gaw 

The Global Atmosphere Watch Station Information System (GAWSIS) is the directory of ground-based GAW stations worldwide and contains information about stations, measurements and contacts. 

German Environmental Specimen Bank (ESB): 
https://www.umweltprobenbank.de/en/documents

As an important component of environmental monitoring, the Environmental Specimen Bank serves as a long-term observation of the chemical, physical and biological development processes taking place in the environment in time and space by providing evidence for ecological and toxicological evidence through the archiving (chemical modification-free storage) of representative soil, plant, animal and human samples and their initial characterization. Both analytes and biometric data can be searched for in the ESB information system. 

Sensor Web:
https://fluggs.wupperverband.de/swc/

The fully functional prototype shows how various environmental data (including discharge, water level, precipitation, temperatures, chem. Parameters, wind) from different data sources (Wupperverband, PEGELONLINE) can be displayed together in one client. A notification service is currently being developed which will allow users to monitor the various environmental data according to their own rules.

Further examples of Internet presentations of air quality data (Humer & Schimack article):

https://www.umweltbundesamt.at/umweltthemen/luft/daten-luft 

www.wien.gv.at/ma22-lgb/luftgi.htm

www.stadtklima-stuttgart.de/index.php?luft_messdaten_station_smz

www.hlug.de/start/luft/luftmesswerte.html

www.lfu.bayern.de/luft/lueb/index.htm

www.lrapa.org/

www.airparif.asso.fr/

www.luft.rlp.de/de/zentrales-immissionsmessnetz-zimen/luftschadstoffe/