How often does a google car go past a location? I always assumed it was less than once a year based on how often they update street view where I live.

Plus, you have no idea what time of day the car will be passing through a location.

So you can't compare the data points over time - they're not taken the same time of day. And I'm not sure if you can even compare different regions - your data may tell you that one part of the city has cleaner air than another, but it's only because one part of the city was measured in the morning and another part of the city in the afternoon.