I've tried searching on my own but I'm not finding anything concrete. It's a tad frustrating, as I'd like to include them in a major setting.
You could probably get away with describing it as smelling like lemons? I'm not much help, but personally I might pay your local garden centre a visit - there're bound to be people there who'd know. Have you tried looking up fruit farms and emailing the farmers directly for the information? A lot of people love to share their expertise and have much respect for writers who want to get this sort of details accurate, so you'd probably find help there. Beyond that, have you looked in gardening or botanical forums? There may be people there who can assist you. The only thing I know about lemons is that they're supposed to be sweet. My husband said he picked one fresh off the tree in Italy once and it tasted sweet. Sounds like heaven to me. Wish I could eat a sweet lemon!
My recollection is that lemon trees bear fruit over a very long period--maybe not year-round, but it's not a single fruiting period of a few weeks like most other fruit trees. I used to live in California and passed lemon trees fairly frequently. I don't remember them having a smell. Obviously the fruit had a smell, and the blossoms have a smell, but I don't remember there ever being enough of a smell to notice when passing close to the tree. Returning to add: And lemons don't grow where it's cold. (Something to keep in mind for the setting. If it gets snowy in the winter, probably no lemons.)
We had a huge lemon tree in our backyard when we first moved to California. It may depend on the variety of lemon, (ours was an older variety of the typical large lemon you'd juice for recipes), but the blossoms didn't have a fragrance at all and were so unnoticeable it usually seemed like the lemons suddenly just appeared. ETA: They weren't fragrant on the tree at all, and neither were the leaves. It wasn't until you grated the peel or cut into them that you could smell the lemons. ETA: We could pick the first lemons in November. I always made lemonade for Thanksgiving, because I could. The blossoms of the orange trees in our front yard were the fragrant ones. The fragrance of the blossoms was heady and sweet, and the wind carries a diluted version of it several blocks away. The oranges themselves didn't have a fragrance when the fruit was on the tree. Occasionally, one could smell the fruit of the Keifer lime tree, but you had to be really close to the tree. The blossoms had no fragrance. Same with our grapefruit tree, and to smell the fruit the tree had to be loaded with ripe fruit and the day very warm.
Seasoned Floridian here coming to agree with @Shenanigator: The tree itself smells like nothing, like a tree. The flowers have a smell that is reminiscent of the fruit itself, but don't think "lemon flowers", and you would literally have to be in an orchard (common where I lived) to even get the smell. It's not like "Oh, there must be a lemon tree somewhere nearby" as you're walking through a park. No. One tree would be undetectable by the human nose. As stated above, orange trees have a heady scent when in bloom.
Where I spent my childhood in Sicily, our landlord grew lemon trees, orange trees, and wine grapes on the property. My mom would send my brother and I out to the orchard to pick the lemons or the oranges for juice. I don't remember them having a smell. The only time the orchard smelled like lemon and oranges was when the fruit fell off the tree and started to rot or got stepped on. I used to hate that because it attracted bees. I don't really remember them growing, to be honest. It was always hot where we lived (in the "winter" is just rained), and suddenly it was time to pick the fruits again.
I've grown several variety of citrus tree including lemons. The trees don't smell and the flowers are barely noticeable. You'd have to stick one up your nose to smell anything and even then I'm pretty sure it was more very slightly astringent sap smell than flower smell. The fruit would only smell when it fell off the tree and either rotted or was stepped on. More often the latter. As for fruit we grew them in greenhouses and they would produce fruit year round as long as you watered and fertilized them consistently. They're tropical and the fruits take a long time to mature, especially for larger lemon varieties. If they grew in an environment with large seasonal shifts in temperature I doubt they'd survive long or produce any fruit at all.
Thank you, everyone, for the informed answers. I think I'll add a green house in the setting, without lemon trees (or maybe I'll still add them), but I still really want something fragrant to enhance scenes. I'm pretty sure evergreens have a strong enough smell for what I want. Whatever I end up using will be, at first, portrayed negatively before being shown as positive to show my heroine's development.
