I believe the car-free day is more of a symbol rather than an efficient way to
reduce air pollution. There will be air pollution all year long no matter how
much emissions of polluted air we reduce in that single day, but it’s still
helpful to draw some social attention to cleaner air circumstances.
Emissions are not truly countable. We would never say ‘1 emission’ so it’s better to use much instead of many. Also, ‘cleaner air circumstances’ doesn’t sound right. Maybe ‘the cause’ would be better, or ‘to how dirty our air is’...’cleaner air circumstances’ is a bit vague and I think that’s why it sounds odd.