I’m a huge believer in babies spending time outside in the middle of the day (properly shaded from sunburn if applicable) for the reason given in the Telegraph article above. The more daylight you can expose them to, the quicker their bodies will know the difference between day and night and you can all start getting more sleep.
Anecdotally, I went on holiday with both DC when they were very small indeed and spent all day outside sightseeing from 10am - 4pm which transformed their nighttime sleep, and, incidentally, my mild PND. Yes I know that’s of no evidential value without a control group but I found it persuasive. If a baby spends all their time in a twilight world with occasional 10 minute trips in the car to a supermarket/baby centre/friend’s house then it’s hardly surprising if they wake at 2 hour intervals all day and night.
If you’re out and about walking during the daytime then babies will get sunlight exposure naturally but if you’re confined to the house for whatever reason then a nap in the garden might be a good substitute.
On the subject of pollution, unless you live right by a main road, have a good filtration system, or are rigorous about excluding indoor pollution due to asthma then even city air in your garden is probably less polluted than the stuff in your house, because it will have all the external pollutants plus the internally generated stuff. www.epa.gov/indoor-air-quality-iaq/inside-story-guide-indoor-air-quality