A challenge champion or ambassador is someone from your community that also believe in your mission to clean up your area. They're someone who tend to be a hand raiser who will take on your cause. Here is one way of identifying them.
Start a broader local challenge and communicate it out to your community. After your challenge is complete, look at your leaderboard to see if there are any standouts. If so, you might try to reach out to them while planning another challenge and do the following:
Provide them with a litter grabber and encourage app download/testing long before event.
Solicit feedback from them about the litter picking process, which can be used to better inform participant recruitment strategy.