Airborne measurement of atmospheric aerosol particles in the lower troposphere over the central United States

Abstract

A series of airborne measurements of atmospheric aerosols were made in four seasonal field periods over the central United States as a part of Central U.S. RADM Test and Assessment Intensives (CURTAIN). The primary objective of the CURTAIN aerosol measurements was to provide a data base for evaluation of the Regional Acid Deposition Model (RADM). Aerosol size distributions were measured at two altitudes of about 1450 and 2450 m above sea level (asl) over the size range 0.1–16 μm, using two Particle Measuring Systems, Inc., probes, an active scattering aerosol spectrometer probe and a forward scattering spectrometer probe, mounted on the National Oceanic and Atmospheric Administration King Air research aircraft. Aerosol samples were also collected by a 2.5‐cm filter pack system for subsequent elemental analysis by X ray fluorescence (XRF). The average aerosol number concentration reached a maximum in summer at both low and high altitudes. The average aerosol number concentration was minimized during fall at low altitude and during winter at high altitude. The coarse particle mode of the aerosol volume distributions displayed a drastic seasonal change, with a winter volume geometric median diameter (VGMD) of 4.34 μm and a summer VGMD of 9.21 μm. The summer peak in the coarse‐mode VGMD and volume concentration is attributed to increased windblown dust particles and the presence of liquid water as part of the aerosol volume under high relative humidity conditions. No significant latitudinal dependence was found in the CURTAIN aerosol data at low and high‐altitude, during the day or night, or from season to season. The CURTAIN aerosol data are influenced differently by the mixed layer from season to season. In winter, high‐altitude aerosol data were collected above the top of the mixed layer. Therefore the vertical profile of aerosol concentration shows a general decrease above the base of the mixed layer temperature inversion. In summer, high‐altitude aerosol data were collected within the mixed layer. The vertical distribution of aerosol number concentration was rather constant during summer. XRF elemental analysis performed on the collected aerosol samples shows that fine‐fraction Fe and K are fairly strongly correlated with accumulation mode mass during summer. This indicates that soil and agricultural fertilizer aerosol sources are important contributors. Correlations for fine and coarse masses with various atmospheric parameters are calculated. Positive correlations with temperature and relative humidity are found. Low wind speed and east winds are correlated with higher fine mass.