I am considering buying a house in a valley with pesticide-intensive agriculture on both sides. Schematically, it looks like this:
It won't win any design awards, but it should make things clear.
My question is how the pesticide vapor drift would behave? Wouldn't there be a high risk that the cold air with the vapor gets trapped there?
While @Gordon Stanger gave an overall perspective about pesticides in general in his answer you can deduce some key points about the micrometeorological conditions of the area.
Assuming pesticides as relatively large aerosol particles/droplets spayed quite close to ground they are mainly dry deposited to surfaces. If your house is as close to the pesticide spaying areas as your schematic implies, you might have a problem (assuming pesticides are dangerous to you, more in @Gordon Stangers answer) only if during the spraying air gets trapped into the valley.
Assuming the spraying is only done in daytime and summertime the chances of an inversion in the valley are smaller and they would only stabilize the air preventing mixing with your valley air and pesticide air.
Strong winds (if the location is near a coast (probable sea breeze) or it's otherwise known to be windy) might carry some pesticides to your direction, but overall adding vertical mixing dramatically lowering the average concentrations.
Your most worst case scenario is if calm (not windy) very sunny days are very often. Sun will in different times of days heat at least either of the slopes of the valley causing air to rise on that side. Replacing air is sucked from the other side creating a steady flow from pesticide rich agricultural areas to your valley. Pesticide being assumed a large aerosol will deposit most likely in the valley (if not in the bottom then in the drafts uphill).
Also rainy weather is good for you as the pesticides will most likely wet deposit near their source.
First, check out the dominant pesticides being used. There are three main groups: organochlorides, which are relatively safe for people but bad for the environment (very long lasting); organophosphates, which are seriously toxic for people, but short-lived, so safe for the environment; and pyrethroids, which are pretty much safe for both humans and environment. If it is dominantly organophosphates then maybe you have cause for concern.
Second, check out the proximity. Are they spraying right up to the walls of your house? If yes, then you may have a problem, but it is really only a potential problem within a few tens of metres of repeated exposure.
Third check the weather. Obviously if the winds are blowing straight off the fields and into your kitchen, that is not a good scenario.
Fourth, check out the disposal of pesticide containers. If there is a good waste disposal system for contaminated bottles, cartons, etc., and if farmers are reading the labels and sticking to the rules, then no problem.
Many people get very uptight about pesticides without understanding them. People do not fall ill from minute traces in groundwater and air. Farmers and their families get ill - generally from chemical violation of their central nervous system - when the farmer doesn't take appropriate precautions or follow the instructions. It is worrying when farmers in their fields walk into their own spray instead of spraying to the side, or when they mix up pesticides without gloves. No apron, no face-mask, careless discarding of pesticide containers into drainage ditches, etc. When the farmers wife washes the farmer's clothes - soaked in pesticide, then she's also at risk. Organophosphates in particular can be absorbed through the skin.
It is impossible to comment about wind drift without much more detailed local knowledge of conditions. My advice is don't get paranoid, but do use common sense about repeated exposure at high concentrations.
