In our example, osallistuja appears in its partitive form, osallistujaa, to mean ”participants.” (Note: Singular partitive is used with numbers greater than 1 and with 0 when it comes after the number.)
Osallistuja is a noun created from the verb osallistua, which means ”to participate in” or ”to take part in.” Here we have another example of how you can add the suffix –ja to a verb root to create a noun meaning a person who does such an action.
Our entire headline reads ”In the Helsinki City Run, 17,000 participants in sunny weather.” (There’s no verb in the phrase as is common in headlines.)
Aurinkoisessa säässä is in the inessive. It means ”in sunny weather.”
In Helsinki City Runilla, the word ”run” in English has been Fennicized (made into Finnish) by adding the adessive ending to indicate the entire English phrase should be read in the adessive. So here it is ”in the Helsinki City Run,” suggesting the run is a location. (Note: Finnish endings must be added in Finnish even when words are in other languages because there are no prepositions in Finnish.)
You can read the original news story and watch the video here.