Measuring the city
The official Dutch institute that is measuring air quality, RIVM, has made a handy brochure for those interested in measuring air quality themselves, titled 'Meten voor een gezonde stad' (pdf). It explains the role of citizen science and clearly explains what substances have an influence on air quality. It does so in Dutch.
So, if you read Dutch, it is very useful to know more about standards like “parts per billion/million” (ppb/ppm) or “microgram per cubic meter air” (μg/m3). If you are involved in any measuring project, you better have this publication within reach!