I noticed that too. Some suggestions.
A similiarity with ponds: they too are much clearer following a spell of rain. Chemistry must be the reason for this.
My dad, an engineer, had an orchid greenhouse and collected only water form the roofs during easterly blowing winds (he considered the westerly winds in Holland too salty and knowing him, he must have tested that!)
Then there is the sensitivity of your own detection system: your nostrils. After rain in the area, the atmosphere in your garden will be cleared of particles, like traffic dust or pollen. Your nostrils will become more sensitive and pick up all sorts of smells, otherwise suppressed.
It's biochemistry and physics in the end. And a lovely experience.