Multiple new-particle growth pathways observed and modelled at the DOE Southern Great Plains field site

 

Authors

Jeffrey Robert Pierce — Colorado State University
Anna Lily Hodshire — Colorado State University
James Smith — University of California, Irvine
Peter H McMurry — University of Minnesota
Kelley C. Barsanti — University of California Riverside
David R. Hanson — Augsburg College
Michael Joseph Lawler — University of Colorado, Boulder
Jun Zhao — National Center for Atmospheric Research (NCAR)
Coty Nanchien Jen — University of California Berkeley

Category

General topics – Aerosols

Description

New-particle formation (NPF) is a significant source of aerosol particle number in the atmosphere. However, these particles are initially too small to have climatic importance and must grow, primarily through net uptake of low-volatility species, from diameters 1 nm to 30-100 nm in order to impact climate. There are currently uncertainties in the physical and chemical processes associated with the growth of these particles that lead to uncertainties in aerosol-climate modeling. It is currently theorized that new-particle growth can be modelled by the sum of the following: condensation of sulfuric acid vapor; condensation of organic vapors; uptake of organic acids by acid-base chemistry; and accretion of organic molecules in the particle-phase. The 2013 campaign at the DOE Southern Great Plains (SGP) field site provided measurements of gas-phase growth precursors, size distributions of new-particle growth events, and the composition of the growing aerosols. In this study, we analyse three new-particle formation events: (1) April 19 show growth by organics alone; (2) May 9 shows growth by sulfuric-acid/ammonia; and (3) May 11 shows growth by sulfuric-acid/amines/organics. To supplement the measurements, we use the detailed particle-growth model MABNAG to gain further insight into the growth processes on these three days at SGP. MABNAG can simulate growth from sulfuric-acid condensation and subsequent salt formation with ammonia or amines; near-irreversible condensation from neutral extremely-low-volatility organic compounds (EL-VOCs); and organic acid condensation and subsequent salt formation with ammonia or amines. We do not consider the contribution from accretion reactions in MABNAG. MABNAG is able to corroborate the observed differing growth pathways: for April 19, the model reproduces the dominance of organics in the growth, and predicts that EL-VOC condensation dominates this organic growth with organic salts as a minor contributor; for May 9, MABNAG predicts that sulfuric acid, with associated bases, explain nearly all of the growth on this day; and for May 11, the model shows that sulfuric acid with associated bases and organics are all important for growth, with EL-VOC condensation again dominating organic growth. However, most model simulations tend to underpredict the observed growth rates; this underprediction may come from not including the contributions to growth from semi-to-low-volatility species or accretion reactions.