Does sea salt influence air chemistry in the central U.S.?
Bulk and size-segregated aerosols were sampled 22 m AGL at the Boulder Atmospheric Observatory (40°N, 105°W, 1563 m ASL) from 18 February to 13 March 2011 during the Nitrogen, Aerosol Composition, and Halogens on a Tall Tower (NACHTT) Campaign. Concentrations of total Na, Mg, Al, Cl, V, Mn, Br and I in bulk aerosol were determined by neutron activation analysis. Ionic compositions of all size-segregated and a subset of bulk samples were determined by ion chromatography of aqueous extracts. Mg, Al, V and Mn concentrations were highly correlated and present in ratios similar to those in surface soils. Total Na and Cl were less well correlated with these soil elements but, after correction for soil contributions, were highly correlated with each other, suggesting the presence of a soluble NaCl "salt" component. Br and I were somewhat correlated and appeared to represent a third aerosol component. The average bulk Cl–:total Cl ratio was 0.99 ± 0.03 (n = 44) indicating that essentially all aerosol chlorine was water-soluble. Na+ and Cl– mass distributions were bimodal with most of the masses (medians 75% and 78%, respectively, n = 45) associated with supermicrometer diameter particles. Possible oceanic origin of the "salt" component will be evaluated based on consideration of forward FLEXPART model runs with explicit wind speed-dependent sea salt emissions along with other information on characteristics of sampled air masses. Implications of "salt" aerosols at this mid-continent site will be discussed.