In the field study monitoring PM from a farm field, which parameter would best explain the variation in PM concentrations across sites and over time?

• A. Wind speed
• B. Humidity levels
• C. Field distance from field edge
• D. Wind direction and speed

Answer: (D)

The key idea is how particles travel through the air. Particulate matter from a farm field is carried and dispersed by the wind. The combination of wind direction and wind speed determines not only which sites will receive the plume of particles but also how much the plume is diluted as it moves. When the wind is blowing from the field toward a monitoring site, PM concentrations at that site tend to be higher; if the wind shifts or slows, those concentrations drop. Humidity can affect particle size or hygroscopic growth, but it doesn’t explain why concentrations rise and fall across many sites and times as effectively as how the wind is moving. The distance from the field edge matters mostly for nearby, near-field measurements but not for broad spatial and temporal variation driven by transport. So, wind direction and speed best account for the patterns you’d see across sites and over time.