One of the most prevalent scents during the Regency would come from the horses. I like horse; they’re beautiful and back then were an essential part of transportation but let’s face it; horses smell. They build up sweat that froths wherever the reins rubs. It stinks. This is why people always changed clothes after riding and refused to enter the drawing room “in all their dirt.” And the stables smelled the same then as they do now—of hay, straw, horse, and manure, not to mention the smell of the stable boys who worked there all day.
The fashionable Mayfair area did not have the sewage smells of others areas. Houses had cesspits in the back where slops were emptied. These were cleaned out periodically by nightmen, but there would be some lingering smell. However, other areas had no drainage, with slops emptied into the street, so add human waste to the mix.
But not everything smelled bad, and after a couple of days in town, most people no longer noticed the manure smells or heavy perfumes. The streets abounded with other scents. Piemen offered hot pies, girls sold flowers, and bakeries always smelled of fresh bread. As our characters walk down the street, they might notice the smell of beer and food as they go by a pub. They might also pass a perfumery and notice the bouquet of perfumes.
As I set a scene in my Regency romance novels, I try to include the pleasant scents the character would notice, especially if it’s during a romantic or pleasant interlude between characters, and I downplay the other, less-pleasant odors. After all, writing historical romance involves a bit of fantasy.