Trends in deposition of acid particles and gases

The Clean Air Status and Trends Network (CASTNet) operates 79 primarily rural aerometric monitoring sites across the U.S. The network measures SO2, SO4, HNO3, NH4, NO3, O3, and several meteorological parameters in order to estimate dry deposition fluxes. The dry deposition fluxes are simulated using a micrometeorological model called the Multi-Layer Model (MLM). The MLM was evaluated using an extensive database of direct flux measurements that were collected at six sites during CASTNet. This paper summarizes trends in CASTNet concentration and atmospheric deposition data from 1987 through 1998.

Trends analyses indicate significant reductions in concentrations and depositions of SO2 and sulfate particles measured at eastern sites over the period. Concentrations and depositions follow generally trends in nationwide SO2 emissions. However, over the last couple years there has been an increase in concentrations as SO2 emissions have increased somewhat. Nitrate particles and HNO3 measured at the eastern sites show no trend. Similarly, data collected at the western sites show no trend.

Wet deposition measurements show significant reductions of SO4 in precipitation. Total deposition (wet plus dry) of sulfur has also declined. Dry deposition accounts for about 15 to 45 percent of total sulfur deposition, and 20 to 60 percent of total nitrogen deposition. These data also show that dry deposition is a more significant contributor in and near major source regions, and wet deposition is more significant in areas with heavy precipitation, such as the deep South and mountainous regions.

Analysis of O3 collected throughout the network indicate considerable geographic variability, but little year-to-year variability at individual stations. Deposition of ozone has decreased slightly over the last ten years.

PM2.5 data from the eight-station (eastern) visibility network show sulfate is the major contributor to fine mass. Organic carbon and nitrate are also important contributors. PM2.5 concentrations peak in summer and are highly correlated with fine sulfate.