Physicochemical Properties of Nitrate Aerosols:  Implications for the Atmosphere

Elizabeth R. Gibson received her undergraduate degree in Chemistry in 2002 from Coe College where she graduated Magna Cum Laude. She is currently a Ph.D. candidate with Professor Vicki Grassian at the University of Iowa. Her current research focus is on the heterogeneous chemistry of mineral dust aerosol and trace atmospheric gases and the climate implications of these reactions. In 2005, Ms. Gibson was selected as one of 50 graduate students from across the nation to attend the 55th annual meeting of Nobel Laureates held in Lindau, Germany. More recently, Ms. Gibson was awarded a 2006 GCEP Graduate Research Environmental Fellowship from the Department of Energy.

Paula K. Hudson received her B. A. in Environmental Science from the University of California, San Diego in 1995 and her doctorate from the University of Colorado in 2001 under the direction of Professor Maggie Tolbert. From 2001 to 2004, Dr. Hudson worked as a postdoctoral researcher at the National Oceanic and Atmospheric Administration in Boulder, CO, with Dr. Daniel Murphy where she participated in several field campaigns, including CRYSTAL-FACE and ICARTT, focusing on single-particle composition measurements. Currently, she is a postdoctoral research associate in the Center for Global and Regional Environmental Research at the University of Iowa with Professor Vicki Grassian. Her research interests include aerosol research and its applications to climate and atmospheric chemistry.

Vicki H. Grassian received her B. S. degree in Chemistry, 1981, from the State University of New York at Albany. From there, she did her graduate studies at Rensselaer Polytechnic Institute (M. S., 1982) and the University of California-Berkeley (Ph.D., 1987). Following postdoctoral positions, she began her independent academic career at the University of Iowa as an Assistant Professor. Professor Grassian is currently a full professor in the Department of Chemistry and holds appointments in the Departments of Chemical and Biochemical Engineering and Occupational and Environmental Health. At the University of Iowa, she has received a Faculty Scholar Award (1999−2001), a Distinguished Achievement Award (2002), a James Van Allen Natural Sciences Faculty Fellowship (2004), and the Regents Award for Faculty Excellence (2006). Her research interests are in the areas of heterogeneous atmospheric chemistry, climate impact of atmospheric aerosols, environmental molecular surface science, and environmental and health aspects of nanoscience and nanotechnology. She has over 100 peer-reviewed publications and is the editor of two books. In 2003 she received an NSF Creativity Award, and in 2005 she was elected as a Fellow of the American Association for the Advancement of Science.

As aerosols, such as sea salt and mineral dust, are transported through the atmosphere they undergo heterogeneous reactions with nitrogen oxides to form nitrate salts. The nitrate salt can have quite different physicochemical properties than the original aerosol, resulting in an aerosol that will markedly differ in its climate impact, heterogeneous chemistry, and photoactivity. In this Feature Article, we will review some aspects of the importance of aqueous nitrate aerosols as well as describe a new multi-analysis aerosol reactor system (MAARS) that is used to measure the physicochemical properties of these atmospherically relevant aerosols. Here we show measurements of the hygroscopic properties, cloud condensation nuclei activity, and FTIR extinction of nitrate salt aerosol. In particular, we have measured the hygroscopic growth of 100 nm size-selected nitrate particles including NaNO₃, Ca(NO₃)₂, Mg(NO₃)₂, and a 1:1 mixture of Ca(NO₃)₂ and Mg(NO₃)₂ as a function of relative humidity (RH) at 298 K. Using Köhler theory, we have quantified the water content of these particles with increasing RH. FTIR extinction measurements of the full size distribution of each of the nitrate aerosols are analyzed to yield information about the local solvation environment of the nitrate ions and the long-wavelength light scattering of the particles at different RH. Furthermore, we have measured and compared the cloud condensation nuclei (CCN) activity of CaCO₃, a large component of mineral dust aerosol, and Ca(NO₃)₂, a product of atmospherically aged CaCO₃ through reaction with nitrogen oxides, at supersaturations from 0.1% to 0.9%. These quantitative physicochemical data are needed if we are to better understand the chemistry as well as the climate effects of atmospheric aerosols as they are entrained, transported, reacted, and aged in the atmosphere. Our studies here focus on aqueous nitrate salts, the products of the reaction of nitrogen oxides with sea salt and mineral dust aerosol.