Effects of Relative Humidity on Chloroacetanilide and Dinitroaniline Herbicide Desorption from Agricultural PM_2.5 on Quartz Fiber Filters

This study quantified the release of seven relatively polar preemergence herbicides to the gas phase from soil-generated PM_2.5-loaded quartz fiber filters (QFFs) and bare QFF as a function of relative humidity (RH). A 48-hour desorption fraction, F₄₈, was defined to evaluate the relative desorption behavior of herbicides from two families, chloroacetanilide (alachlor, butachlor, metolachlor, and propachlor) and dinitroaniline (pendimethalin, prodiamine, and trifluralin) using temperature- (8 °C) and humidity- (10−64% RH) controlled air at a flow rate of 4 L/min. With increasing RH, an increase in F₄₈ by a factor of 2−8 was observed for all herbicides, except metolachlor and butachlor, which showed significantly strong sorption to both sorbents. The conjugate carbonyl oxygen and amide nitrogen in the chloroacetanilide structure enables stronger specific interactions with the sorbents, leading to lower desorption compared to the dinitroaniline herbicides. Desorption of chloroacetanilides decreased in the order propachlor > alachlor > metolachlor ∼ butachlor, and desorption of dinitroanilines decreased in the order trifluralin > pendimethalin > prodiamine. These orders are consistent with the different substituents in the herbicide molecules for each family and their relative tendencies to coordinate with surface moieties as indicated by electron-donating capacity. Henry's law constant and Abraham's H-acceptor parameter were found to be useful empirical parameters for describing the F₄₈ desorption behavior for all seven herbicides.