the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Jun 2023
pH-Dependence of Brown Carbon Optical Properties in Cloud Water
Christopher J. Hennigan
Michael McKee
Vikram Pratap
Bryanna Boegner
Jasper Reno
Lucia Garcia
Madison McLaren
Sara M. Lance
Abstract. Light-absorbing organic species present in aerosols, collectively called brown carbon (BrC), contribute important but highly uncertain effects on climate. Clouds likely represent a significant medium for secondary BrC production and for bleaching reactions, though the relative importance of formation and loss processes in clouds is unknown at present. The acidity (or pH) of atmospheric particles and clouds affects the optical properties of BrC and bleaching rates. Given the wide variability of pH in the atmosphere (pH in particles and clouds ranges from -1 to 8), the optical properties of BrC and its bleaching behavior are expected to vary significantly, and the link between pH and BrC is yet another uncertainty in attempts to constrain its climate forcing effects. In this work, we characterize the pH-dependence of BrC optical properties – including light absorption at 365 nm (Abs365), mass absorption coefficient (MAC365), and the absorption Ångström exponent (AAE) – in bulk cloud water sampled from the summit of Whiteface Mountain, NY. In all samples (n = 17), Abs365 and MAC365 increased linearly with increasing pH, highlighting the importance of reporting pH in studies of BrC in aqueous media. There was strong variability in the sensitivity of Abs365 to pH, with normalized slopes that ranged from 5.1 % to 17.2 % per pH unit. The normalized slope decreased strongly with increasing cloud water [K+], suggesting that the non-biomass burning BrC has optical properties that are more sensitive to pH than BrC associated with biomass burning. AAE also showed a distinct pH-dependence, as it was relatively flat between pH 1.5 – 5, then decreased significantly above pH 5. The cloud water composition was used to inform thermodynamic predictions of aerosol pH upwind/downwind of Whiteface Mountain and the subsequent changes in BrC optical properties. Overall, these results show that, in addition to secondary BrC production, photobleaching, and the altitudinal distribution, the climate forcing of BrC is quite strongly affected by its pH-dependent absorption.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
Christopher J. Hennigan et al.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-854', Anonymous Referee #3, 22 Jun 2023
This paper describes observations of the absorption characteristics of brown carbon in cloud water samples obtained from orographic clouds at Whiteface Mountain. Recent work has demonstrated that brown carbon absorption is highly pH dependent, however, the number of studies that report optical properties in aqueous samples as a function of pH is limited. Furthermore, the majority of these studies have analysed aerosol samples in aqueous solution but cloudwater is more dilute, typically has a higher pH and therefore demonstrates stronger absorption. However, to date few studies exist. This work presents such a set of observations. The work is well described and the methods carefully detailed in the paper. The results are carefully described and logically presented and the authors have provided a well considered and detailed discussion, comparing their results with those of other studies and discussing the ramifications of their work. Overall, this is a well presented paper that is of merit scientifically and offers some new results and important insight. I have some small suggestions that the authors should address, but these are minor.
Line 46-49: “Unlike BC and dust, which are removed from the atmosphere only through wet and dry deposition, it also undergoes chemical losses initiated by oxidants and direct photolysis (collectively termed bleaching), that can rapidly diminish its light absorbing properties (Hems and Abbatt, 2018)”. This sentence needs a re-word. The processes discussed for BrC are in addition to the physical processes controlling BC and dust.
It is worth emphasising in the introduction as well as in the conclusions that many studies only focus on the optical properties of brown carbon under dry conditions and also that multiple studies considered absorption at ambient humidity have not reported the aerosol pH at which the determinations have been made.
Line 95-97: How were non-precipitating clouds selected?Line 147-149 and table 1: “with the exception that Ca2+ and Mg2+ concentrations were excluded because a decadal analysis of WFM cloud composition revealed that these species likely derive predominantly from coarse particles” If these cations are predominately in the coarse mode, which anions correspond to the coarse mode? How can it be discounted that the Na+, Cl- and NO3- do not have a significant coarse mode contribution? What role would this play on the aerosol pH calculation?
Line 155-159: To what extent does the Mountain affect airflow and therefore the ability of HYSPLIT capture the airmass history accurately?
Lines 195-196: The authors comment on the greater variability in their results compared to the aerosol measurements of Phillips et al and suggest this may be due to ageing of air masses at the sample site. Another plausible explanation is that the activation characteristics of the aerosol are a source of variability. Unlike Phillips et al, who studied aerosol, the cloud water samples only observe activated aerosols. Since this is a strong function of both the aerosol size distribution and the updraft velocity and BrC is likely to be prevalent in the unactivated aerosol, greater variability may be induced in the observations. This is worth commentary.
Lines 280-284: This discussion also implies that it is also important that it is important to quantify the available activated fraction of BrC from a range of important sources as a function of age.
Citation: https://doi.org/10.5194/egusphere-2023-854-RC1 -
RC2: 'Comment on egusphere-2023-854', Anonymous Referee #1, 06 Jul 2023
This manuscript prepared by Hennigan et al. represents a novel study to provide insights into the impact of pH on the light absorptivity of atmospheric Brown Carbon (BrC). Through measurement of actual cloudwater samples, they have demonstrated that the light absorptivity of water-soluble BrC in cloudwater samples is highly pH dependent. While pH dependence of BrC absorptivity has been shown to some degree by previous studies, this work is the first to provide systematic insights into this phenomenon. BrC belongs to a class of short-lived climate forcers that has uncertain radiative effects and atmospheric lifetime. Meanwhile, a changing pH in cloud and fog water in North America and Europe has been reported. These facts make the current work highly relevant and important. The manuscript is very well written and should be considered for publication in ACP. I have the following comments and questions for the authors. I only have one major comment, with the rest considered minor or technical.
Major comment
- Generally, I think the manuscript can benefit from a little more discussion regarding potential mechanisms via which the observed pH dependence is attained. In Line 335, the authors mention hydrophobic organic acid. Do we expect this magnitude of changes in absorbance when the pH swing across their pKa value(s)? Have any previous studies on Suwanee River samples discussed how DOM exhibits pH dependence? I think such a mechanism is important in connecting a few key observations/conclusions in the manuscript: Ageing of BB aerosol, relative changes in absorption spectra, etc.
Minor comments
- As the authors pointed out themselves, aerosol liquid water represents a highly concentrated medium that is not considered ideal aqueous solution. Are acid-base equilibria, hence BrC absorption, differ in non-ideal solutions compared to ideal aqueous solutions? I do not know the answer. I am just asking.
- I wonder if the pH dependence observed in the current work is repeatable. In other words, if the authors would acidify the solution but then basify it again (or vice versa), do you expect the absorptivity to follow the same pH-dependence?
- Related to my previous comment, certain aqueous-phase reactions (e.g., hydrolysis) and equilibria (imine formation) are acid and/or base-catalyzed. Is there any chance that acidifying or basifying the sample induces any irreversible artifact to the composition?
- The authors mentioned that this study ignores water-insoluble BrC chromophores. Do the authors think water-insoluble chromophores also exhibit pH dependence? They do not dissolve or interact with water very much. I do not know the answer. I am just asking.
- Page 5. It seems that the LWCC measurement was intentionally done using two channels, one to record absorbance and the other to track light source stability. How is this approach more advantageous compared to a single-channel measurement? I have seen previous studies using only one channel.
- Line 215. “The present results suggest that one such change not previously reported is that atmospheric ageing reduces the sensitivity of biomass burning BrC optical properties to pH ". I feel like this sentence is an overstatement and would ask the authors to consider relaxing the statement. I don't think the results really suggested it. It is the authors' speculation.
- Line 254. “Therefore, measurements of BrC in aqueous environments need to include and report pH in order to facilitate interstudy comparisons and to assess the climate forcing effects of BrC.” I agree with the authors and also believe that this is one of the most important implications they are making from this manuscript. Given that the authors demonstrated that the pH dependence also varies from sample to sample. Shouldn't we report absorbance at least two pH to constrain the slope? This may or may not always be feasible, but I wanted to hear the reviewer's ideas.
- Discussion related to Figure 7. I think it would be beneficial if the authors could include a little discussion on what functional groups are likely (or unlikely) contributing to the observed relative spectral change. E.g., carbonyl is not likely contributing due to minimal changes at around 300 nm.
Technical Comment
- I feel like the use of Figure X and Fig. X is inconsistent throughout the manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-854-RC2 - AC1: 'Author Response to Reviewer Comments: egusphere-2023-854', Christopher Hennigan, 25 Aug 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-854', Anonymous Referee #3, 22 Jun 2023
This paper describes observations of the absorption characteristics of brown carbon in cloud water samples obtained from orographic clouds at Whiteface Mountain. Recent work has demonstrated that brown carbon absorption is highly pH dependent, however, the number of studies that report optical properties in aqueous samples as a function of pH is limited. Furthermore, the majority of these studies have analysed aerosol samples in aqueous solution but cloudwater is more dilute, typically has a higher pH and therefore demonstrates stronger absorption. However, to date few studies exist. This work presents such a set of observations. The work is well described and the methods carefully detailed in the paper. The results are carefully described and logically presented and the authors have provided a well considered and detailed discussion, comparing their results with those of other studies and discussing the ramifications of their work. Overall, this is a well presented paper that is of merit scientifically and offers some new results and important insight. I have some small suggestions that the authors should address, but these are minor.
Line 46-49: “Unlike BC and dust, which are removed from the atmosphere only through wet and dry deposition, it also undergoes chemical losses initiated by oxidants and direct photolysis (collectively termed bleaching), that can rapidly diminish its light absorbing properties (Hems and Abbatt, 2018)”. This sentence needs a re-word. The processes discussed for BrC are in addition to the physical processes controlling BC and dust.
It is worth emphasising in the introduction as well as in the conclusions that many studies only focus on the optical properties of brown carbon under dry conditions and also that multiple studies considered absorption at ambient humidity have not reported the aerosol pH at which the determinations have been made.
Line 95-97: How were non-precipitating clouds selected?Line 147-149 and table 1: “with the exception that Ca2+ and Mg2+ concentrations were excluded because a decadal analysis of WFM cloud composition revealed that these species likely derive predominantly from coarse particles” If these cations are predominately in the coarse mode, which anions correspond to the coarse mode? How can it be discounted that the Na+, Cl- and NO3- do not have a significant coarse mode contribution? What role would this play on the aerosol pH calculation?
Line 155-159: To what extent does the Mountain affect airflow and therefore the ability of HYSPLIT capture the airmass history accurately?
Lines 195-196: The authors comment on the greater variability in their results compared to the aerosol measurements of Phillips et al and suggest this may be due to ageing of air masses at the sample site. Another plausible explanation is that the activation characteristics of the aerosol are a source of variability. Unlike Phillips et al, who studied aerosol, the cloud water samples only observe activated aerosols. Since this is a strong function of both the aerosol size distribution and the updraft velocity and BrC is likely to be prevalent in the unactivated aerosol, greater variability may be induced in the observations. This is worth commentary.
Lines 280-284: This discussion also implies that it is also important that it is important to quantify the available activated fraction of BrC from a range of important sources as a function of age.
Citation: https://doi.org/10.5194/egusphere-2023-854-RC1 -
RC2: 'Comment on egusphere-2023-854', Anonymous Referee #1, 06 Jul 2023
This manuscript prepared by Hennigan et al. represents a novel study to provide insights into the impact of pH on the light absorptivity of atmospheric Brown Carbon (BrC). Through measurement of actual cloudwater samples, they have demonstrated that the light absorptivity of water-soluble BrC in cloudwater samples is highly pH dependent. While pH dependence of BrC absorptivity has been shown to some degree by previous studies, this work is the first to provide systematic insights into this phenomenon. BrC belongs to a class of short-lived climate forcers that has uncertain radiative effects and atmospheric lifetime. Meanwhile, a changing pH in cloud and fog water in North America and Europe has been reported. These facts make the current work highly relevant and important. The manuscript is very well written and should be considered for publication in ACP. I have the following comments and questions for the authors. I only have one major comment, with the rest considered minor or technical.
Major comment
- Generally, I think the manuscript can benefit from a little more discussion regarding potential mechanisms via which the observed pH dependence is attained. In Line 335, the authors mention hydrophobic organic acid. Do we expect this magnitude of changes in absorbance when the pH swing across their pKa value(s)? Have any previous studies on Suwanee River samples discussed how DOM exhibits pH dependence? I think such a mechanism is important in connecting a few key observations/conclusions in the manuscript: Ageing of BB aerosol, relative changes in absorption spectra, etc.
Minor comments
- As the authors pointed out themselves, aerosol liquid water represents a highly concentrated medium that is not considered ideal aqueous solution. Are acid-base equilibria, hence BrC absorption, differ in non-ideal solutions compared to ideal aqueous solutions? I do not know the answer. I am just asking.
- I wonder if the pH dependence observed in the current work is repeatable. In other words, if the authors would acidify the solution but then basify it again (or vice versa), do you expect the absorptivity to follow the same pH-dependence?
- Related to my previous comment, certain aqueous-phase reactions (e.g., hydrolysis) and equilibria (imine formation) are acid and/or base-catalyzed. Is there any chance that acidifying or basifying the sample induces any irreversible artifact to the composition?
- The authors mentioned that this study ignores water-insoluble BrC chromophores. Do the authors think water-insoluble chromophores also exhibit pH dependence? They do not dissolve or interact with water very much. I do not know the answer. I am just asking.
- Page 5. It seems that the LWCC measurement was intentionally done using two channels, one to record absorbance and the other to track light source stability. How is this approach more advantageous compared to a single-channel measurement? I have seen previous studies using only one channel.
- Line 215. “The present results suggest that one such change not previously reported is that atmospheric ageing reduces the sensitivity of biomass burning BrC optical properties to pH ". I feel like this sentence is an overstatement and would ask the authors to consider relaxing the statement. I don't think the results really suggested it. It is the authors' speculation.
- Line 254. “Therefore, measurements of BrC in aqueous environments need to include and report pH in order to facilitate interstudy comparisons and to assess the climate forcing effects of BrC.” I agree with the authors and also believe that this is one of the most important implications they are making from this manuscript. Given that the authors demonstrated that the pH dependence also varies from sample to sample. Shouldn't we report absorbance at least two pH to constrain the slope? This may or may not always be feasible, but I wanted to hear the reviewer's ideas.
- Discussion related to Figure 7. I think it would be beneficial if the authors could include a little discussion on what functional groups are likely (or unlikely) contributing to the observed relative spectral change. E.g., carbonyl is not likely contributing due to minimal changes at around 300 nm.
Technical Comment
- I feel like the use of Figure X and Fig. X is inconsistent throughout the manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-854-RC2 - AC1: 'Author Response to Reviewer Comments: egusphere-2023-854', Christopher Hennigan, 25 Aug 2023
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Christopher J. Hennigan et al.
Christopher J. Hennigan et al.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2640 KB) - Metadata XML