Response of organochlorine pesticides over the Great Lakes to climatic fluctuations

The relationships between dominant processes of the Northern Hemisphere climate variability, the El Niņo-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO), and atmospheric loadings of hexachlorobenzene (HCB) and α - and γ hexachlorocyclohexanes (HCHs) to the Great Lakes are investigated during spring and winter seasons of 1990s (from 1992 through 1998).

Climate fluctuations observed as changes in surface air temperature, air circulation pattern and precipitation affect the pollutant emission strength at source regions, transportation of pollutants and deposition to the Great lakes. Dry deposition, wet deposition and net gas exchange fluxes of HCB in winter seasons in the 1990s are calculated using measurement data around the Great Lakes ecosystem and the results show that dry deposition and gas exchange fluxes of HCB over the Great Lakes region associate fairly well with El Niņo events. Dry depositions of HCB increase during El Niņo events and decrease during La Niņa events. Spatial averaged seasonal net gas exchange over the lake surfaces (except for Lake Ontario) become negative during El Niņo, indicating air-to-water transfer, and tend to be positive under La Niņa conditions, indicating volatilization. Lowest wet deposition flux during 1992-1999 was found in winter 1998, corresponding to the lowest precipitation over the Great Lakes region observed in that winter during the 1990s associated with a strong El Niņo occurring in 1997/1998. Similar results were obtained for HCB loadings to the Great Lakes during spring season. Our analysis reveals that HCB loadings in the western portion of the Great Lakes have better response to ENSO events than the eastern portion. From climate related studies, it was found that changes in temperature and precipitation associated with ENSO are more significant in the regions west of the Great Lakes than the east of the lakes. Our loading results agree with the distributions of temperature and precipitation anomalies during El Niņo and La Niņa.

HCHs are being less volatile than HCBs and with well designated source regions in North America the response of HCH loadings to the Great Lakes with spring season NAO events exhibit strong association in our analysis. Both dry and wet deposition fluxes correlate positively with the standardized NAO index in springs from 1992 through 1998. They tend to increase during the positive phase of the NAO and decrease during the negative phase of the NAO in springs. Statistically significant correlations between the HCHs wet deposition and the NAO is attributed largely to response of seasonal precipitation around the Great Lakes to the NAO. The inter-annual variations in the HCHs depositional fluxes to the Great Lakes correspond to the increase in HCHs concentrations in the atmosphere associated with the increase in surface air temperature in the west of the Great Lakes and subsequent strong volatilization and transport associated with the positive NAO phase, and to the decrease in HCHs in colder than normal springs associated with the NAO negative phases. We show that high deposition (absorption in water / loss in the atmosphere) of HCHs occurs in the positive phase of the NAO and low deposition occurs in the negative phase of the NAO.