PARENT SESSION
TA9 - Atmospheric Fate Processes
Chair: Simonich, Staci 1, 1 1141 ALS, Corvallis, OR, USA
Co-chair: Landers, Dixon 2, 2 200 SW 35th Street, Corvallis, OR, USA
8:00 AM to 12:00 PM - Tuesday, 19 November 2002
Room Ballroom E
(408) Vapor particle partitioning of metolachlor and atrazine: deviation from model predictions.
McConnell, Laura*,1, Kuang, Zhihua2, Torrents, Alba3, Harman-Fetcho, Jennifer1, 1 U. S. Department of Agriculture, Agricultural Research Service, Beltsville, MD, USA2 Gorge, Miles & Buhr, LLC, Salisbury, MD, USA3 University of Maryland College Park, College Park, MD
ABSTRACT- The environmental fate of airborne organic pollutants, i.e., atmospheric lifetime, is often governed by their distribution between the vapor and particle phases. An accurate understanding of the chemical-specific air-particle interactions is necessary to predict the distance a particular chemical may travel from its source and the loadings it may contribute to non-target regions. Air samples were collected in two different agricultural watersheds within the larger Chesapeake Bay system. Atrazine and metolachlor were examined with respect to trends in concentration, vapor-particle distribution, and a comparison of observed vs. predicted particle-phase fractions using the Junge-Pankow and Octanol-Air approaches. A comparison of 
(pred) and (obs) over time values were approximately 1.4-15 and 0.67-53 times as high as (pred) values for atrazine and metolachlor, respectively, indicating that the model underestimates the sorption of these polar chemicals to aerosols. Alternatively, this may indicate that atrazine and metolachlor are not at equilibrium in the atmosphere. A further examination of the data revealed that deviation from model predictions was largest during the spring and early summer when these chemicals were applied. For instance, in late spring and in the summer, the ratio (obs)/(pred) for metalochlor ranged from 3.1 to 53 compared with 0.9-1.5 in April and 0.67-2.0 in November and December. A similar trend was also found for atrazine, where the ratio (obs)/(pred) was 4.1-15 from May to July compared with 2.0-2.8 in April and 1.5-1.8 in winter. There are two main mechanisms by which pesticide residues are transported into the atmosphere from soil after application: one is volatilization, and the other is wind erosion. Results of this study suggest that particle-bound pesticide residues emitted via wind erosion may be an important source to the atmosphere in areas close to intense agricultural activity.
Key words: Pesticide, Atmospheric Transport, Vapor-Particle Partitioning, Herbicides