the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Aug 2024
The lifetimes and potential change in planetary albedo owing to the oxidation of organic films extracted from atmospheric aerosol by hydroyxl (OH) radical oxidation at the air-water interface of aerosol particles
Abstract. Water insoluble organic material extracted from atmospheric aerosol samples collected in urban (Royal Holloway, University of London, UK) and remote (Halley, Antarctica) locations were shown to form stable thin films at an air-water interface, these organic films reacted quickly with gas-phase OH radicals which may impact planetary albedo. The x-ray reflectivity measurements additionally indicate that the film may be consistent with having a structure with increased electron density of film molecules towards the water, suggesting amphiphilic behaviour. Bimolecular rate constants for gas-phase OH radical oxidation of urban or remote aerosol extracts were typically of the order ∼1010 cm3 molecule−1 s−1, giving atmospheric lifetimes of the film with respect to gas-phase OH radical oxidation of minutes at typical atmospheric OH radical concentrations. Kinetic modelling of core-shell droplet dynamics suggests film lifetime of a few minutes, depending on ambient OH radical mixing ratio. Modelling the oxidation kinetics with KM SUB suggests half-lives of minutes to an hour and values of ksurf of ∼ 2 × 10−7 and ∼ 5 × 10−5 cm2 s−1 for urban and remote aerosol film extracts respectively. The lifetimes and half-lives calculated at typical OH atmospheric ambient mixing ratios are smaller than the typical residence time of atmospheric aerosols and thus oxidation of organic material should be considered in atmospheric modelling. Thin organic films at the air-water interface of atmospheric aerosol or cloud droplets may alter the light scattering properties of the aerosol. X-ray reflectivity measurements of atmospheric aerosol film material at the air-water interface resulted in calculated film thickness values to be either ∼10 Å or ∼17 Å for remote or urban aerosol extracts respectively and oxidation did not remove the films completely. One dimensional radiative transfer-modelling suggest the oxidation of thin organic films on atmospheric particles by OH radicals may reduce the planetary albedo by a small, but potentially significant amount.
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RC1: 'Comment on egusphere-2024-2367', Anonymous Referee #1, 15 Sep 2024
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This manuscript reports X-ray reflectivity (XR) measurements of organic material extracted from authentic aerosol samples and applied to a liquid water surface. XR yields information on the thickness, the electron density and the vertical structure of the film. The aerosol samples were collected in an urban environment to represent polluted air and in a remote location in Antarctica to represent clean air. The XR measurements indicated the resulting films on water to be less than 2 nm thick and application of fits to simulate the XR profiles suggest a 2-layer structure, the one on the water-side of the film exhibiting higher electron density than the one on the air-side, indicating hygrophilic functionalities towards the water surface. The latter was more pronounced for the sample from the remote location. Exposure of the films to OH lead to degradation of the film at a rate close to the OH collision limit and a change in the structure. This was supported by multilayer modelling to obtain an apparent surface reaction rate coefficient for OH with the film. In addition, model calculations are presented to demonstrate that expected changes to film thickness and optical properties due to aging (thus degradation of organic films) will affect aerosol scattering albedo globally.
The work is clearly interesting and provides insight into the surface properties of organic material likely to assemble on atmospheric aerosols. The methods used are unique and the direct conclusions from the XR measurements appear sound. It is clear that, given limited access to synchrotron X-ray facilities, not a larger amount of samples could be investigated to expand the scope of the work. More specific questions and concerns related to these are given below. On a more general level, the calculations with respect to the optical properties appear rather disconnected from the other parts of the work. Film thicknesses measured on water in this work cannot be used to infer film thicknesses in atmospheric particles (see details below), nor can their life time from the measurement on the water surface be extrapolated directly. Therefore, the claim of the impact of aging of surface layers on Earth's albedo are rather speculative, and this section of the manuscript is not really useful for assessing the implications of the present work. Those calculations would likely be better presented in a separate manuscript, where changes to optical properties of particles due to aging are reviewed based on a broad literature overview, then used to assess potential impacts on aerosol optical properties.
Specific comments
Somewhere in the experimental information sections, the authors should give more quantitative information on how much organic was extracted and deposited onto the water in the trough. Over what area was the material spreading? Do the authors assume that it fills the entire water surface area, or is it forming a floating island? Was the mass deposited similar for the two samples? or what is the basis for the comparison of thickness derived later from XR if it cannot be normalized to mass? Have the authors done some calibration experiments with well defined surfactants with different properties, such as fatty acids or similar? Specifically, on line 134, what was the x-ray beam footprint?
line 205, equation (5): this rate law is a bit strange, as [OH] is provided as gas phase concentration. What is the meaning of the value for k4? OH and the molecules (presented as surface coverage) are not homogeneously mixed. This equation should either contain the collision rate, or the OH concentration treated as a surface concentration, as the authors do when they use the mulilayer model.
line 243: here comes the point of transferring the thickness retrieved from the water surface by XR to that of atmospheric aerosols. First, the extraction only takes a fraction of surface active material from the sample. 2nd, there is no calibration of mass of organic vs thickness in the trough: on the water surface, the extract spreads until it obtains an equilibrium configuration on an extended water surface. In the atmosphere, the same material may by forced into a separate phase forming a much thicker layer.
line 368, back to kinetics: as mentioned above, the meaning of k4 is not clear. This is not a homogeneous reaction. What is the diffusion limit? for gas phase? do you mean the collision limit?
line 389: How it is possible to have an apparent loss rate that then translates to an uptake coefficient so close to one? Given the geometry of the setup, this should be gas phase diffusion limited and thus considerably lower.
line 414: applying OH uptake observed to degradation of organics in the real atmosphere: in the atmosphere, oxidation targets are not at the surface only, while the reaction with gas phase OH is limited to surface and by the supply of OH from the gas phase, which is in the end determining the lifetime. This comes back to the point made above: if in the real aerosol, the organics are forming a thicker phase, their degradation (on average) is much slower.
in the last lines of the manuscript, line 468 - 470, the authors reflect on these concerns, but still, the last sentence reiterates the impact on albedo.
Technical comments
line 46: something is wrong with this sentence: 'oxidation between sub-micron aerosols'?
lines 74-79: consider splitting this overly long sentence into 2 or 3. From 'to study oxidation...' to 'by aqueous-phase OH' takes too long, and the reader looses orientation, at least this reviewer.
line 101: the filters were not collected, the samples on the filters were collected.
line 124: ... in the dark until use for the X-ray reflectivity measurements. Not clear at this point what a beam line is.
line 141: already mention here 'Generation of gas-phase OH radical by O3 photolysis...'
line 224: a small side remark: if the model indicates photolysis of molecular O2 contributing to OH production, can that be from 254 nm radiation or is there some 185 nm light from the Hg discharge leaking through the lamp tube?
line 232: why are the authors running a global optimisation algorithm if only one parameter was varied?
line 290: state more explicitly which type of functional groups exhibit higher electron density and which ones lower.
line 470: to microscopic areas, not macroscopic, isn't it.
Citation: https://doi.org/10.5194/egusphere-2024-2367-RC1
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