Article pubs.acs.org/JPCA
Depositional Ice Nucleation on Monocarboxylic Acids: Effect of the O:C Ratio Gregory P. Schill and Margaret A. Tolbert* Cooperative Institute for Research in Environmental Sciences and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, United States S Supporting Information *
ABSTRACT: The heterogeneous ice nucleation efficiency of a series of thin C3−C6 monocarboxylic acid films between 180 and 200 K has been investigated using a Knudsen cell flow reactor. At each temperature, the critical ice saturation ratio for depositional nucleation as well as the effective contact angle was found to be strongly dependent on the chemical nature of the film. For the organic acids used in this study, increasing the O:C ratio lowered the supersaturation required for the onset of heterogeneous ice nucleation and decreased the effective angle of contact. This could be the result of an increase in surface hydrophilicity, which allows the film to better adsorb a metastable, icelike layer of water that serves as a template for the new phase of ice. These ice nucleation results are in excellent agreement with ice nucleation on laboratory generated α-pinene secondary organic aerosol as well as on predominantly organic particles collected in Mexico City.
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INTRODUCTION Cirrus clouds, composed of water ice, cover approximately 30% of the Earth’s surface and have a predominantly warming effect on climate.1 Despite their ubiquity in the atmosphere, ice clouds are not accurately represented in current global climate models and provide one of the largest uncertainties in predictions of climate change.2 This uncertainty is due, in part, to an incomplete understanding of ice formation mechanisms. Ice nucleation in the atmosphere has been shown to take place via homogeneous or heterogeneous nucleation as reviewed by Cantrell and Heymsfield. 3 Homogeneous nucleation occurs when icelike clusters, or nucleation embryos, within an aqueous solution droplet reach a critical size and stimulate ice formation. In contrast, heterogeneous ice nucleation occurs at the surface of solid ice-nuclei (IN), which act as a catalytic surface for the formation of ice embryos. Traditionally, it has been thought that homogeneous ice nucleation is the dominant mechanism governing cirrus cloud formation.4,5 However, recent field6−8 and modeling9 studies have underlined the importance of heterogeneous nucleation. In the past, extensive work has been done to study the efficacy of various heterogeneous IN. It is now generally accepted that mineral dust10,11 as well as bacteria and pollen12,13 are all exceptionally good IN. However, recent field studies have shown that aerosols at cirrus altitudes are dominated by sulfates and organics.8,14 In addition, subvisible cirrus residues are enhanced in sulfates and organics and depleted in mineral dust and bacteria. While ammonium sulfate has been shown to be a sufficient IN,15 single particle measurements have determined that up to 80% of the dry aerosol mass can be attributed to organic material.16 Thus, it is © 2012 American Chemical Society
clear that heterogeneous ice nucleation on organic species cannot be ignored. Previously, it has been shown that organics suppress ice nucleation;17−19 some laboratory and modeling studies, however, indicate that organic species can also be efficient IN.20−24 Furthermore, Knopf et al.25 have shown that anthropogenic particles collected in Mexico City are predominantly organic in nature and can nucleate ice as efficiently as ammonium sulfate at temperatures relevant to cirrus formation. Given the diversity, complexity, and mutability of organics in the atmosphere, it is no longer sufficient to bin organics as either generally “good” or “poor” IN, nor can studies of organic IN focus solely on likely nucleation candidates. Systematic experiments to unveil general trends in organic IN efficacy may provide useful data and offer predictive ability. Here, we have studied the depositional nucleation of ice on a series of thin films of monocarboxylic acids between 180 and 200 K as a first attempt to parametrize organic IN. Carboxylic acids were chosen as they are expected to be an important part of aged aerosol.26
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EXPERIMENTAL SECTION Knudsen Cell. A schematic of the high vacuum Knudsen cell used in these studies is shown in Figure 1. This setup has been described in detail previously.27 Briefly, the apparatus consists of a single stainless steel chamber that houses a vertically mounted gold substrate (d = 2.34 cm) upon which thin organic films are deposited. The temperature of the substrate is controlled by resistively heating against a Received: February 22, 2012 Revised: May 17, 2012 Published: May 30, 2012 6817
dx.doi.org/10.1021/jp301772q | J. Phys. Chem. A 2012, 116, 6817−6822
The Journal of Physical Chemistry A
Article
gold surface is covered and does not take part in the nucleation process. The growth and stability of the films are monitored by FTIR-RAS. In separate experiments, FTIR-RAS was also used to determine that these organic films are crystalline rather than amorphous. A sample spectrum of butyric acid is shown in Figure 2a. The lack of scattering in the high energy end of the
Figure 1. Schematic of the Knudsen cell flow reactor setup.
differentially pumped liquid nitrogen cryostat. Temperature is monitored by three T-type thermocouples, which are calibrated by referencing the measured ice vapor pressure at the frost point to literature data;28 this results in a temperature error
