10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Errors in Nanoparticle Growth Rates Inferred from Measurements in Chemically Reacting Aerosol Systems
CHENXI LI, Peter H. McMurry, University of Minnesota
Abstract Number: 811 Working Group: Aerosol Physics
Abstract In systems where aerosols are being formed by chemical transformations, individual particles grow due to the addition of molecular species. Other processes, such as coagulation, may also lead to particle growth, but coagulation is a well understood process that is described by the coagulation integrals in the aerosol general dynamics equation. Growth, in contrast, is not always sufficiently well understood to be accurately described in models, especially for chemically complex systems such as the atmosphere. Processes including condensation & evaporation, acid-base reactions, organic salt formation, liquid phase reactions, and the accretion of two or more organic molecules to form a larger compound having lower volatility may contribute to growth. Efforts to improve our understanding of growth due to chemical transformations often focus on attempts to reconcile observed growth rates with values calculated from models.
However, because it is typically not possible to measure the growth rates of individual particles in chemically reacting systems, they must be inferred from measurements of aerosol properties such as size distributions, particle number concentrations, etc. For example, atmospheric nucleation leads to the formation and growth of a nucleation mode. Growth rates are sometimes assumed equal to the growth rate of the geometric mean size of this mode. However, this modal growth rate is also affected by self-coagulation of particles within the mode as well as condensation of smaller molecular clusters that are often not measured. To determine growth rates from such observations, it would be necessary to correct for the effects of coagulation on modal size. This complicates data analysis procedures and may require information that was not measured or is otherwise unknown for the system under study.
This presentation quantifies errors in growth rates obtained using methods that are commonly employed for analyzing atmospheric data. We analyze "data" obtained by simulating the formation of aerosols in a system where a single chemical species is formed at a constant rate, R. The advantage of this approach is that we understand the system perfectly, so can unambiguously determine true particle growth rates, which are due solely to condensation and evaporation. We examine the effects of a pre-existing aerosol and of evaporation. We show that the maximum possible error in measured growth rates occurs for collision-controlled nucleation in a single-component system in the absence of a pre-existing aerosol, wall losses, evaporation or dilution, as this leads to the highest concentrations of nucleated particles. Those high concentrations lead to high coagulation rates that cause the nucleation mode to grow faster than would be caused by vapor condensation alone. Scavenging by preexisting aerosol, wall losses, evaporation and dilution decrease concentrations of nucleated particles, thereby decreasing errors associated with coagulation. Although we do not explicitly study the effects of growth pathways other than condensation (i.e., contributions to growth by other chemical species or processes), they would lead to higher growth rates and therefore smaller relative errors in growth rates due to coagulation.
Our analysis is based on the non-dimensional formulation described by McMurry and Li (2017). This allows us to draw general conclusions that are independent of the chemical species and of the rate of aerosol formation. We show that for collision-controlled nucleation, the modal growth rate can be up to a factor of five higher than the true growth rate.
McMurry, P. H. and C. Li (2017). "The Dynamic Behavior of Nucleating Aerosols In Constant Reaction Rate Systems: Dimensional Analysis and Generic Numerical Solutions." Aerosol Sci. Technol. 51(9): 1057-1070. doi: 10.1080/02786826.2017.1331292.