Seasonal variations and long-term trend of dust particle number concentration over the Northeastern US

The strength and frequency of extreme precipitations over the Northeastern US has been observed to increase significantly in the past decades. Previous studies indicated that mineral dust aerosol can impact cloud and precipitation by serving as ice nucleus. In this study, we use a global chemical transport model (GEOS-Chem/APM) to simulate seasonal variations and long-term trend of dust aerosol over Northeastern US in the past 18 years (2000 - 2017). By comparing with observations and previous case studies of dust events, we show that GEOS-Chem/APM appears to capture well the strong long-range transported dust events in the US. We find that the dominate source of dust in the middle and upper troposphere over the Northeastern US is the long-range transport. The dust seasonal development can be divided into two patterns: I) Strong dust events in spring and early summer months (AMJ) which are mainly transported to the Northeastern US at high altitudes (4 km or higher) from Asia. In this season, the number concentration of dust particles larger than 500 nm (n_D,d>500) varies from background level of 0.05 #/cm³ to 1 #/cm³ (during strong dust events). II) Relatively weak transported dust events happen in late summer, fall and winter months (ASON) which are mainly transported at about 2 km level. n_D,d>500 during this period varies from 0.002 #/cm³ (background) to 0.5 #/cm³ (in strong dust events) and Asia dust appears to have limited influence. Based on the dust seasonal properties, dust simulation in each year is divided into AMJ (90 days) and ASON (120 days) periods, and the dust days with top 20% strongest n_D,d>500 of each period are chosen to study long-term trend of days with strong dust over the Northeastern US. Number of days with strong dust during AMJ period (total 90 days) varies from 8 in 2016 to 32 in 2002, and the mean n_D,d>500 during these strong days varies from 0.18 #/cm³ in 2016 to 1.0 #/cm³ in 2001. From 2000 to 2017, both the number of strong dust days and mean n_D,d>500 during AMJ have a decrease trend over the Northeastern US. Top 20% strong dust days during ASON (total 120 days) in each year varies from 5 in 2012 to 25 in 2002, with averaged n_D,d>500 ranging from 0.15 #/cm³ in 2010 to 0.33 #/cm³ in 2004. ASON dust days and averaged n_D,d>500 do not have obvious trend in the past eighteen years.