Contrast and correlations between coarse and fine particulate matter in the United States

Abstract

The characteristics of concentrations of PM₁₀₋₂.₅, PM₂.₅, and PM₁₀ at 77 sites in the United States are evaluated. PM₁₀ concentrations show strong spatial variability, with highest levels occurring in the southwestern United States, driven primarily by PM₁₀₋₂.₅. PM₁₀₋₂.₅ and PM₂.₅ concentrations show different spatial patterns. The highest concentrations of PM₁₀₋₂.₅ were observed at sites in the southwestern US, leading to the highest PM₁₀ concentrations there. The PM₂.₅ concentrations are the major contributors to the average PM₁₀ concentrations at many sites in the eastern United States. Poor correlations were generally found between PM₁₀₋₂.₅ and PM₂.₅, suggesting that PM₁₀₋₂.₅ and PM₂.₅ are generally influenced by different sources. PM₁₀₋₂.₅ is generally more variable than PM₂.₅ because PM₁₀₋₂.₅ has a higher deposition velocity and is primarily emitted from mechanical processes (e.g. agricultural harvest and construction) that are more influenced by factors including human operation and wind speed leading to a strong episodic nature. As a result of its high variability, PM₁₀₋₂.₅ acts as the major driver for PM₁₀ extremes. PM₁₀₋₂.₅ is significantly correlated with PM₁₀ at all investigated sites, with the average correlation value R(2)=0.79. Correlations of PM₂.₅ with PM₁₀ (average of 0.37) are overall considerably lower than those between PM₁₀₋₂.₅ and PM₁₀. Different seasonal, weekly, and diurnal patterns were observed between PM₁₀₋₂.₅ and PM₂.₅ at agricultural, on-road traffic, quarrying, airport, and marine sites. At investigated agricultural sites, while the concentrations of PM₂.₅ are higher in winter when there are few agricultural activities, PM₁₀₋₂.₅ concentrations are lower in winter months than in summer and autumn months, with highest levels corresponding to harvest and planting. The harvest and planting signatures were not observed in PM₂.₅ concentrations at any of these sites, suggesting that agricultural activities do not have a strong influence on PM₂.₅ concentrations.