This may be something you've investigated already but in case not: have any potential sources of standing water been checked, aside from what you noted about not being near natural water sources? For example, roof gutters that may be clogged or partially clogged and slow to drain, or child/pet equipment that may be holding rainwater left out in the yard? (Mosquitoes need very small amounts of water in which to breed...as little as a teaspoon.) If not a problem in your yard, perhaps a neighbor has one of those issues and is the source of the mosquito population. (Does someone have a bird bath that isn't regularly emptied and refilled? A corrugated pipe carrying downspout water out into the yard, whose pipe end is uncovered and accessible to mosquitoes? Does anyone have an empty pool or pool cover that is catching rainwater?)

Spraying would be a last-resort option, but even then, it's not a very long-lasting solution. Aerial spraying targets adult mosquitoes, and doesn't address the problem closer to its source, which is the water breeding site. Plus, the chemicals (even organic ingredients) that are used for spraying for adult mosquitoes are not specific to only mosquitoes; they can harm other insects as well, including pollinators and fireflies. They might also harm other animals sensitive to pesticides, like amphibians. You can learn more on the page linked above. Wearing personal repellent and/or long-sleeved clothing is one of the most reliable approaches in lieu of spraying when habitat modification isn't working well enough.

Miri