Question

I know this is off subject. I just really need help.
Which of the following scenarios would be an ideal time to assess plant population decreases as part of an investigation into episodic acidification around a city at the base of the Rocky Mountains?

in the autumn, but only when fog is present
in the spring, along the streams formed from snowmelt
during the heaviest traffic periods, such as the summer
after factory production in nearby regions has peaked for the year
only when smog is visible, confirming the presence of particulate matter