The smaller the cloud you wish to detect the higher resolution You need. Like on the camera picture with better resolution You can see smaller details. If the detail you are looking at is smaller than your pixel it will be invisible for you. We are not talking literally about the visible graphic on the display but rather about Field of View (FoV) and the smallest cloud (vapor) we are able to identify.
For high resolution you need small pixels. Field of View is our smallest pixel. The smallest pixel within which we can still see and detect chemical vapor in the air. And because we are doing stand-off detection the distance comes into play - the further the cloud, the smaller it is on the picture. Thus relative size of our smallest pixel (FoV) depends on the distance as well.
I will stop at the Field of View for now. What is the ideal size of the “circle” for long range (stand-off) detection? There is a consensus on the market that 5km is what should be achieved as minimum. That is our first limit: see minimum 5000 meters. Second limit is set by natural behavior of the clouds themselves. Chemical warfare agents (CWA) are heavy gasses. Same is true for many Toxic Industrial Chemicals (TIC). We know that CWA will reach maximum height of 10m. In fact, some barely 5 meters. But let’s stick to 10m for the simplicity. That is our second limit. Taking these 2 limits (5km distance, 10m height of the cloud) it can be easily calculated that our smallest pixel/FoV should be 2 mRad (0,11°).
If your imaginary detection pixel is bigger than 2 mRad, you will not be able to see any cloud formed by heavy gasses in the distance of 5km, no matter how wide or dense the cloud is. Scanning the circle of 500m in diameter requires minimum size Field of View to be 20 mRad (1,14°) to see same detail in 5km you need 2 mRad.
360 is not always 360
If we apply this knowledge on the scanning rate we have to take one more aspect into account: The length of the circle. Bigger circle has longer circuit, obviously. You have 10 times more to scan at 5km compared to 500m. Your speed will be 10 times slower.
There are stand-off detectors that scan very fast, but only because they see at a short distance only, or very large clouds far away (large in width and height). At a greater distance a large FoV detector is unable to detect even clouds with significant size that are low (btw majority of the clouds are low, as mentioned previously). So yes, the rotation with bigger Field of View is fast but only because you are blind to details in the distance.
It is not the same to turn around when we look at 500 meters or when we scan circle with 5000 meters diameter. Another words, to turn around 360 is not the same as turning around 360. It depends on what we do, what we scan, what is our reach and what sensitivity is crucial for us.
Combat use of CWA
In case of an attack in the standard conflict we can assume it will be massive one. N x 100 kg of CWA substance used. Hot zone or contaminated area will be in square km. Detecting a contaminated area is therefore an easy task. About 30 measurement lines must be sufficient to detect within 5km radius. If the stand-off detector operator does apply a reason and logic, weather and terrain information, it will simplify the situation and speed up scanning rate tremendously.
The difficult task here is seeing the edge of the cloud, where the concentration of chemical substance is smaller but can still be lethal. High sensitivity is needed.
Toxic gas in hands of a terrorist
Terrorist attack by chemical gasses is (probably) more complex scenario. We can expect small amounts of toxic substance being used. The beginning of an attack is very likely to be marked by a triggering event such as an explosion or spray from the drone. The location of the contamination can be identified from the triggering event or from the first effect gasses have on people. Long range recoinassance or stand-off detector can aim at such location. Single or very few measurements will be enough. In scenario where small group of adversaries applies small amount of chemical gas You need, again, high sensitivity and ability to scan tiny areas. More so, if the contaminated area is inaccessible. Long range reconnaissance. Scanning will be made within a very limited zone. Speed will be much faster than standard scanning/