Seasonal Variations and Long-Term Trend of Dust Particle Number Concentration Over the Northeastern United States

摘要

Mineral dust is known to affect cloud and precipitation by serving as ice nuclei, and an increasing number of modeling studies have explicitly related ice nucleation with the dust number concentrations. We examine the seasonal variations and the long-term trend of dust number concentrations over the northeastern United States (NEUS) during 2000-2017, based on results from a global chemical transport model with size-resolved particle microphysics (GEOS-Chem/APM). Comparisons with observations show that GEOS-Chem/APM can capture most of the strong dust events in the NEUS. The model results indicate that mineral dusts over the NEUS are dominated by dust transported in the lower and middle troposphere (from 2 to 6 km). The number concentrations of dust larger than 500 nm (N-D,N-d > 500nm) vary by over 2 orders of magnitude from nondust-transport days to event days, and N-D,N-d > 500nm in different seasons can vary by 1 order of magnitude. The frequencies of dust events also show seasonal variation, with most annually top 50 dust events in spring and early summer (March-June) and in fall and early winter (September-December), and stronger dust events mainly in March-June. From 2000 to 2017, the springtime dust event days decrease by about 50 (above-average dust events), 76 (top 10 events), and 85 (top 5 events), and N-D,N-d > 500nm of strong events decreases by about 50. Our analysis indicates that these decrease trends are caused by the declining Asian dust loading. The frequencies of dust events in summer have a weaker decreasing trend, and there is no obvious tendency in other seasons.