Understanding the Tropospheric Transport and Fate of Semivolatile

Chapter 7

Understanding the Tropospheric Transport and Fate of Semivolatile Pest Management Chemicals Downloaded by NORTH CAROLINA STATE UNIV on September 7, 2012 | http://pubs.acs.org Publication Date: March 8, 2005 | doi: 10.1021/bk-2005-0899.ch007

Vincent R. Hebert Department of Entomology, Food and Environmental Quality Laboratory, Washington State University, Richland, WA 99352

Various stable tracer and elevated temperature laboratory approaches have been recently developed to assess the photochemical and oxidative fate of semi-to— low volatility agrochemicals. These specialized systems have been used to ascertain environmentally relevant reaction rates for various organochlorine, organophosphorus, and dinitroaniline agrochemicals. The use of tracer and elevated temperature approaches can provide the most useful information on tropospheric reaction rates for various semivolatile agrochemicals, subject to certain atmospheric dilution considerations and temperature constraints. When more detailed exposure information is desired for a specific agrochemical, especially in high-use agricultural areas, these specialized atmospheric fate evaluations together with structure activity relation modeling should be considered. The integration of laboratory estimates of reaction rates, atmospheric models, together with real world monitoring data will provide the best available scientific information for assessing exposure risks from airborne agrochemicals and their reaction products.

Introduction Pesticides are applied at rates that can exceed 2 billion kilograms each year in the United States (1). Based on their physicochemical properties, it should be expected that a considerable portion of this total amount can directly enter the atmosphere. Besides the direct entry into the air at application, post-application 70

© 2005 American Chemical Society In Environmental Fate and Safety Management of Agrochemicals; Clark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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71 volatilization and wind erosion also represents a second and significant source of sustained tropospheric loadingfromthe land surface. A large data set has been gathered which details tropospheric loading resulting from drift (2) and post-application volatilization (3, 4). Majewski and Capel (5) have also published a detailed review of regional and national atmospheric pesticide levels along with aerosol particulate and vapor distributions. Unfortunately, research studies that address environmentally relevant atmospheric fate processes of pesticides are relatively few in comparison to studies that measure transformations on land surfaces and in water. This scarcity of fate information is related to the difficulty in attaining relevant tropospheric photochemical and oxidative information under both environment and controlled laboratory conditions. Only a limited number of studies exist that have measured airborne pesticide reactivity under actual sunlight conditions (6,7,8). These studies employed photochemically stable tracer compounds of similar volatility and atmospheric mobility to compensate for physical dilution. The examined airborne sunlight-exposed pesticides in these limited studies had to react quickly to provide environmentally measurable reaction rate constants. The field examination of tropospheric reaction rates for the vast majority of agricultural pesticides is impractical since reaction rates for many of these compounds are probably too slow to yield reliable rate constant information. Problems encountered in acquiring stable gas-phase conditions in the laboratory also contribute to the relative lack of atmospheric pesticide reaction rate and product data. Semi-volatile organics, which comprise the majority of pesticides, can sorb onto the surfaces of the laboratory reaction vessel. The "wall interference" reaction rates and products may or may not be similar to those occurring under actual atmospheric vapor-phase conditions (9,10). Experimental designs that can provide environmentally relevant reaction rates, characterization of gas-phase and oxidative transformation products, and maintain material balance at environmental temperatures has yet to be established. Of the field and laboratory air studies that have been performed, sunlightinduced chemical oxidations and photochemical reaction pathways usually render pesticide residues less toxic, more polar, and more susceptible to being washed-out of the air mass (71,12,13,14,15). Field and laboratory atmospheric pesticide fate studies have also reported the formation of photooxidation products that can have equal or higher toxicity and/or greater environmental persistence than the parent pesticide (16,17,18,19). Because of the limited number of attempted atmospheric fate studies, there remains a substantial degree of uncertainty in regards to the mechanistic behavior and possible fate of many pesticide groups that can reside in the lower atmosphere. Due to the inherent difficulties in acquiring quantifiable pesticide reactivity data, structure-activity atmospheric oxidation models are often employed by regulatory agencies for estimating reaction rates of existing and newly registered pesticides (20,21,22). For volatile organic air pollutants and fumigants, structure-activity relationship models can reliably provide chemical oxidation rate constant data within an acceptable (i.e., factor of 2 or less) margin of uncertainty (22). These bimolecular reactivity models usually rely on global

In Environmental Fate and Safety Management of Agrochemicals; Clark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.


72 averaged oxidant concentrations that are invariant with changes in regional climate and solar intensity. When extended to estimate reaction rate constants for semivolatile multifunctional pesticides, model estimation power may diminish. Because many pesticides are structurally complex, relying on structure-activity model may possibly provide erroneous rate constant information on the tropospheric fate for many pesticide groups (23). Once in the air, pesticides can exist as vapor, liquid aerosols or be sorbed/partitioned on dust and particulates. Even though in-depth information exists for particle phase distributions of organics in the atmosphere (24, 25, 26, 27, 28), few studies have appeared in the literature that assesses the vapor/aerosol distribution of pesticides under actual tropospheric conditions (29). Further research to determine how pesticides are distributed among atmospheric phases will be needed to better gauge the overall significance of wet/dry deposition versus gas and particle-phase transformation processes occurring in air. The remainder of this proceeding summarizes and integrates pesticide transport and fate investigations, focusing on the more recent scientific advances in understanding the behavior of pesticides in air. Although pesticides in air present a major human and ecological community exposure route (30,31), our knowledge of distributional and fate processes in this environmental medium still remains poorly understood.

Laboratory Photochemical Oxidation Pathways Sunlight-induced transformations that lead to chemical oxidation or photolysis are likely to dominate the removal of pesticides, especially in their gaseous forms, from the lower troposphere (10,11,12,32,33). The residence time for an airborne pesticide will depend on the rate at which it will undergo direct photo-transformation and/or chemical oxidation by tropospheric reactants such as hydroxyl/peroxy radicals, N O radicals, or ozone. For pesticides that absorb sunlight, the lossfromdirect photolysis will be a function of both the extent of sunlight absorbance and the efficiency of transformation after absorbance. Figure 1 outlines various pathways can lead to the deactivation of the excited pesticide molecule (P*) through luminescence, physical quenching, or by collisionally transferring energy to other gaseous molecules (M). Thisfigurealso illustrates electron transfer, photoionization, or direct chemical reaction processes of the excited state that can lead to dissociation and subsequent product formation. x


73

Physical Quenching

Luminescence


hv

Ρ· ΑP- D Electron Transfer +

•>

p*

•> P + M*

Energy Transfer

Products

β +

Photoionization

Figure 1: Photochemical Pathways Atmospheric removal processes involving tropospheric oxidants will likely dominate for pesticides that are transparent to sunlight wavelengths. Of these oxidants, hydroxyl-radical (OH) reactions will be the primary mode for the chemical oxidation of the majority of pesticides in air (7/). The rate of reaction by hydroxyl radical mediated processes can be on the order of minutes to days or years and will depend upon the reactivity of the pesticide's functional groups with the oxidant and tropospheric concentrations of OH available for reaction. Pesticides with abstractable hydrogens, or can undergo OH addition will be most susceptible to transformation. Chemical oxidation rates will decrease rapidly for electron deficient compounds, such as chlorinated aromatics or nitroaromatics (20). In many cases, heteroatoms may provide for greater reactivity. For example, Atkinson et al. (34) have reported that phosphorothio systems (P=S) typical of many organophosphorus insecticides are highly reactive towards OH. Pesticides belonging to this group can usually be expected to have tropospheric lifetimes less than four hours (32).

Tropospheric Laboratory Reactivity Evaluations Photochemical Evaluations: Although fumigants and sterilants have vapor pressures (i.e., >1 Pa) that might allow relatively uncomplicated experimental



74 determination of rates and products, the majority of pesticides have lower vapor pressures ( then their relative rates of loss can be used to empirically determine the test substance's rate of reaction using the following expression:

This relative rate expression is very useful since the oxidation reaction rate of the reference compound (kj) is known and the natural log of the simultaneous loss of test and reference compound can be experimentally determined. The oxidative reaction rate for the test substance (k ) is the lone unknown variable and can be calculatedfromthe slope (k /k ) when In ([R]o/[R]t) is plotted against ln([?]J\?] )(42). The major advantage of the relative rate estimation method is that it is relatively easy to employ and can avoid the need for difficult experimental evaluations that must rely on estimating absolute OH concentrations over time. A disadvantage is that tropospheric OH radical concentrations can globally vary to a factor of five (77). The precision of calculated reaction rates may therefore be limited unless tropospheric OH concentrations can be more accurately estimated in the region of interest. Uncertainties also increase when extrapolating beyond the current database information for volatile compounds to {

x

2

t



77 more complex semivolatile organics. In studies performed on a series of OH reactions for dimethyl phosphoroamidates and dimethyl phosphorothioamidates, Goodman et al. (36) found that electronic effects between structural units may have been responsible for discrepancies up to a factor of three between measured and calculated rate constants. In their assessment, the loss in estimation power may well represent "the inherent uncertainties for more complex chemicals containing multiple substituent groups." Starting in the 1990's, elevated temperature gas flow-through and static systems have been used to assess chemical oxidation rates for low volatility organochlorine and organophosphorus pesticides. In these systems, multiple measurements at different temperatures were performed to also evaluate the temperature effects on observed reaction rates. Anderson and Hites (37) elevated the temperatures within a small volume gas flow-through system with in-line MS detection to assess the gas-please degradation of hexachlorobenzene relative to the rate of loss of cyclohexane, a reference compound with well-characterized OH reactivity. Plotting observed rate estimates for hexachlorobenzene at various elevated temperatures, these investigators observed excellent agreement to atmospheric oxidation structure-activity model predictions for this substance when extrapolated to 25 C. This system design also has the advantage of taking multiple readings over a very short time interval to increase reliability of measurement precision. Since 254 nm UV light is required for generating OH within the elevated temperature reaction flow cell, this system may be limited to evaluating thermally stable compounds that do not undergo rapid photodegradation under intense irradiation. The use of relative rate/elevated temperature experiments have also been employed to determine tropospheric OH radical reactivity rates for the atmospherically reactive organophosphorus (OP) insecticides chlorpyrifos and diazinon employing large-volume chamber conditions as illustrated in Figure 2 (14). In these evaluations, the gaseous OP pesticides, photo and oxidatively stable tracer compounds, and reference compounds with well-characterized OH reaction rates were simultaneously introduced into the chamber. Hydroxyl radicals were generated by the gas-phase photolysis of methyl nitrite at wavelengths greater than 290-nm using xenon arc irradiation after the methods of Atkinson and co-workers (75, 36, 40) C H O N O + hv -> C H 3 O 3

CH3O

+ NO

+ 0 -» HCHO + H 0 H 0 + NO ΟΗ·+ N 0 2

2

2

2

NO was added to the reaction mixture to avoid the formation of 0 and N 0 radicals. After dosing the vessel with the OH radical precursor, the xenon arc lamp was illuminated over the 2-hour experimental time frame. Under these conditions, sufficient methyl nitrite was present to generate OH for approximately 100 minutes. 3


3


78 Both of the above organophosphorus pesticides should have similar gasphase tropospheric lifetimes based on structural activity relationship model predictions (27). OH rate measurements for the two OPs when conducted at 5° C increments between 60 and 85° C, however, showed a significant difference in reactivity. The rate of OH oxidation for diazinon was found to be ca. three to four times more rapid than for chlorpyrifos with observed tropospheric lifetimes of ca. 1 and 4 hours, respectively. The difference in observed reactivity was not due to wall sorption since both compounds behaved similarly in the gas-phase. While OH radical reaction rate constants have been successfully generated to extend structure-activity predictive capability for thiocarbamates and chlorinated aromatics, further experimental work will be important to assess OH reaction rates for the more complex chemistries that comprise the majority of high-use urban and agricultural pesticides.

Conclusions Manyfield-monitoringstudies have indicated the substantial role of the troposphere as a transport medium and potential sink for semivolatile pesticides. But at the same time, it is the least studied and understood environmental compartment in regards to pesticide fate. While the fundamental principles behind tropospheric reactivity is well understood, it is becoming increasingly apparent that the ability to quantitatively measure reaction rates and product distributions from the air will continue to pose problems for researchers. To date most deterministic models try to simulate real-world conditions but may fall short in providing tropospherically relevant reaction rates since it is virtually impossible to scale down all possible interactions occurring in a near-infinite reservoir, especially for multifunctional semivolatile pesticides. More experimental data will be needed for verifying model rate predictions for the more complex chemistries that are representative of the vast majority of current and emerging pesticides. Basic laboratory and applied field research that can provide environmentally relevant reaction rate and fate information remains an urgent need in human and ecologicalriskassessment.

Acknowledgements I would like to express my sincere appreciation to both Glenn Miller at the University of Nevada and James Seiber from the USDA Western Regional Agricultural Research Station, Albany CA in providing support and mentoring in this area of research. I also express my thanks to Aldos Barefoot from Dupont Agrochemical Division and Cheryl Cleveland from Dow AgroSciences for their support of pesticide research in the troposphere.


79

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Understanding the Tropospheric Transport and Fate of Semivolatile

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