Potential particulate impacts at the Grand Canyon from northwestern Mexico

Project MOHAVE was a major air quality and visibility research program cond ucted from 1990 to 1999 to investigate the causes of visibility impairment in the Grand Canyon National Park region. At Meadview, a remote monitoring site just west of the Grand Canyon National Park, on September 1 and 2, 199 2, the concentrations of sulfate (3.1 and 4.3 mug sulfate/m(3)) were the hi ghest seen in 6 years of monitoring at this site. During this period, the c oncentrations of SO, at Meadview were also abnormally high and approximatel y three times the sulfate concentrations, on a nmol/m(3) basis. High concen trations of sulfate and SO2 extended south into southern Arizona and northw estern Mexico. Based on ambient atmospheric conditions, emissions from the Mohave Power Project (MPP) 110 km upwind of Meadview could not have been re sponsible for the majority of the regionally observed sulfur oxides. The ge ographical distribution of SO2 and sulfate, and available source informatio n suggest that northwestern Mexico was a significant source of the unusuall y high observed sulfur oxides. A CMB model developed during Project MOHAVE was used to apportion sulfur oxides at Meadview and other sampling sites th roughout the study region for August 31-September 2, 1992. The results indi cate that the contribution of MPP to sulfate at Meadview was typical. Howev er, the transport of SOx from northwestern Mexico was elevated throughout m uch of the region during this time period. This led to the large increase i n sulfate concentrations at Meadview on September 1 and 2. These results in dicate that emissions from Mexico can be a significant source of particulat e material in the Grand Canyon. (C) 2001 Elsevier Science B.V. All rights r eserved.