From the journal Environmental Science: Atmospheres Peer review history

16-Aug-2022

Dear Dr Bi:

Manuscript ID: EA-ART-07-2022-000091
TITLE: Estimated Timescales for Wet Deposition of Organic Compounds as a Function of Henry’s Law Constants

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

After careful evaluation of your manuscript and the reviewers’ reports, I will be pleased to accept your manuscript for publication after revisions. However, as you will see below, one reviewer thought that the paper was lacking evaluation against observations, such as concentration in rainwater samples. I understand that the manuscript is a theoretical calculation, but if you are able to add such an evaluation please do.

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Dr Tzung-May Fu
Associate Editor
Environmental Science: Atmospheres
Royal Society of Chemistry

************

Reviewer 1

Authors could consider verifying the results with some real-world measurement data (e.g. the concentrations of certain organic species in both air samples and precipitation samples). Authors could also consider adding more discussions about any significant potential for practical application of the conclusions of this work or/and the proposed approach for estimating wet deposition timescales.

Minor comments:
Introduction, para 2: “…a relative dearth of studies on the timescales for deposition. 10 found that deposition…” The sentence is not complete.

“Estimation of wet deposition timescales”, para 1, last sentence, the second “similarly” shall be deleted.

“Wet deposition timescales during rain events”, para 2, last few sentences about the comparison between MAO and the four sites, the meaning and purpose of the comparison are not clear.

Reviewer 2

Bi and Isaacman-VanWertz estimated wet deposition timescales of atmospheric organic gases as a function of Henry’s law constants in five sites. They found that wet deposition timescales are independent of H for highly soluble compounds with H > 10^5 M/atm, and are mainly driven by the frequency and duration of precipitation, rather than precipitation intensity or droplet size distribution. Based on these findings, the authors proposed a rapid estimate of wet scavenging timescales from basic precipitation information. The manuscript is clearly structured with solid quantitative analysis. I would recommend publication and only have a few clarifying questions.

What’s the spread of timescale estimates? Could Figure 1 include a 25/75 percentile of estimated timescale values?

Page 6 last sentence – timescales of less soluble gases should be days (rather than weeks) to months based on Figure 1 (at H<10^4 M/atm)?

Page 9 last paragraph –It’s said that the simple approach (dash line in Figure 5) applies 5 hours for compounds with H>10^5 M/atm. For the less soluble compounds, does the estimate use the wet deposition timescales averaged over the 5 sites? For less soluble compounds, their wet deposition timescales vary a lot, and the difference between the simple and the more accurate estimates are apparently larger than 20% (mentioned at the beginning of Page 10). How reliable is the simple approach for less soluble compounds with H<10^5 M/atm?

It seems to me that the proposed simple estimate applies mainly to compounds with H>10^5 M/atm at longer time scales. If the authors would agree, this should be emphasized in several places throughout the discussion and conclusion. (Please note that the current layout of the manuscript without line numbers is not friendly to reviewers.)

It would be better to include some discussion on the timescales of chemical loss versus wet/dry deposition for typical organic gases (e.g. from oxidation of aromatics/isoprene/monoterpenes).

Reviewer 3

This paper estimates the wet deposition timescale for atmospheric organic gas compounds as a function of Henry’s law constants (H) using precipitation data at five sites over the globe. The wet deposition timescale is found to decrease with increasing H with a stable minimum for H>10^5 M/atm. The median wet deposition timescale for very soluble gases is estimated to be about 5hr for a continuous rain event at all sites. The authors conclude that the timescales depend mainly on the precipitation frequency and duration rather than intensity or droplet size. They thus propose that wet deposition timescale for gases can be estimated at any location using only basic precipitation information without detailed precipitation measurements. This paper deals with an important topic, wet deposition of organic gases, for which current understanding is still limited. The results are useful in assessing the relative roles of wet deposition, dry deposition, and oxidation in the budgets of organic gases. The paper is well written. I have only a few minor comments for the authors to consider during revision.

Page 5, left: “10 found that….” -- Do you mean “Hodzic et al. found that…”?

Page 7, Fig.2: It’s not clear, among these five sites, which ones are of tropical and are mainly subject to convective (versus stratiform) precipitation, and which ones are of mid-/high- latitudes. Do the data set used allow the authors to categorize the events based on precipitation types? Are snow events excluded?

Fig. S2: caption should say “…for varying precipitation height”.

The authors would like to thank the reviewers for the feedback on the manuscript. We have made revisions to the manuscript according to the reviewers’ comments.

Editors’ comments: After careful evaluation of your manuscript and the reviewers’ reports, I will be pleased to accept your manuscript for publication after revisions. However, as you will see below, one reviewer thought that the paper was lacking evaluation against observations, such as concentration in rainwater samples. I understand that the manuscript is a theoretical calculation, but if you are able to add such an evaluation please do.

Response: We thank the editor for the comment. We agree that it is important to compare modeling results with experimental measurements. Unfortunately, we are not aware of experimental measurements of wet deposition timescales for semivolatile organic species across this range of Henry’s constants against which to compare, or wet deposition measurements at the precipitation measurement sites examined in this work. Our group is currently working on some experimental measurements to address this lack, but such measurements are out of the scope of this work. These issues are mentioned in our response to Reviewer 1.

Referee: 1

Comments to the Author
Comment 1: Authors could consider verifying the results with some real-world measurement data (e.g. the concentrations of certain organic species in both air samples and precipitation samples). Authors could also consider adding more discussions about any significant potential for practical application of the conclusions of this work or/and the proposed approach for estimating wet deposition timescales.

Response: We would like to thank the reviewer for the suggestions. We agree with the reviewer that the results would be improved by comparing the modeled results with experimental measurements. However, to the best of our knowledge, there are few if any experimental measurements of wet deposition timescales for semivolatile organic species across the modeled range of Henry’s constants to compare. There are also unfortunately no available measurements of gas-phase wet deposition at the measurement sites examined in this work. Our group is currently working on some experimental measurements but such measurements are not yet available.
The reviewer also suggests adding more discussions about the potential significant practical application of the conclusions. To make the estimations more practical, we have added a fitted equation for extrapolating timescales for less soluble chemical species from the estimation of more soluble species on Page 7:

“Since the shapes of the predictions (dashed lines in Figure 5) are similar across sites, we further simplify the approach and provide a fitted equation of the curves as Equation S1. While the details of the fit are described in the supporting information, in Figure S4, we demonstrate that the estimation of wet deposition timescales as a function of Henry’s law constants can be achieved by calculating only the timescales for compounds with H > 105 M atm-1 and fitting the timescales for less soluble species as an exponential curve .”

We have also added additional descriptions in the supporting information:

“Estimation of timescales for less soluble species
Because the relationship between wet deposition timescales and Henry’s law constants are generally similar between sites (Figure 5), timescales for compounds with H < 105 M atm-1 can be estimated as a function of wet deposition timescales for more soluble species. It is consequently only necessary to calculate the time taken to reach cumulative 5 hours of rain at a given location as an estimate for compounds with H > 105 M atm-1, reducing the computational load. The equation describing the average wet deposition timescale during a precipitation event is approximated as an exponential decay. Because timescales below H = 103 M atm-1 are poorly constrained due to long computational times, the equation is presented here referenced to this lower bound for solubility.

log⁡(t_H )=log⁡(t_min )+log⁡〖(t_(H=10^3 )/t_min ) e^(-τ log⁡(H/10^3 ) ) 〗 (S1)

Where tH is the wet deposition timescale (hr) for a given Henry’s law constants, H (M atm-1), and tmin is the estimated wet deposition timescale for compounds with H>10^5 M atm-1 (hr), beyond which timescales plateau at their minimum. The decay constant, τ, is estimated as 1.0, and the ratio of average timescales at H = 10^3 M atm-1 and H > 10^5 M atm-1, t_(H=10^3 )/t_min , is equal to 8.7. With these two estimated constants, which are constrained by the averaged data (dashed lines shown in Figure 5), timescales can be calculated for any compound using only an observationally constrained tmin, as shown in Figure S4. Uncertainty increases substantially for solubilities of H < 10^3 M atm-1, but wet deposition timescales for such compounds are likely significantly longer than timescales for other removal processes.”

Minor comments:
Comment 2: Introduction, para 2: “…a relative dearth of studies on the timescales for deposition. 10 found that deposition…” The sentence is not complete.

Response: Thanks for pointing out the mistake. We have revised the sentence as follows.
“…in part due to a relative dearth of studies on the timescales for deposition. Hodzic et. al.10 found that deposition is insensitive to…”

Comment 3: “Estimation of wet deposition timescales”, para 1, last sentence, the second “similarly” shall be deleted.

Response: We have deleted the word “similarly” on Page 3:

“…which are often treated as similarly first-order though they are not always continuous processes.”

Comment 4: “Wet deposition timescales during rain events”, para 2, last few sentences about the comparison between MAO and the four sites, the meaning and purpose of the comparison are not clear.

Response: We have improved the clarity of the comparison on Page 4:

“However, MAO, a site located near the Amazon Forest, has more frequent heavy rainfalls (i.e., high frequency of rain from 10 to 100 mm/hr in Figure 2), which contributes to slightly higher scavenging rates thus resulting in lower wet deposition timescales than other sites in Figure 1. The small differences in timescales between MAO and the other four sites in Figure 1 suggest that higher precipitation intensities may have some effects on wet deposition timescales, but the differences in precipitation intensity between sites are not big enough to have substantial influences on the wet deposition timescales.”

Referee: 2

Comments to the Author
Bi and Isaacman-VanWertz estimated wet deposition timescales of atmospheric organic gases as a function of Henry’s law constants in five sites. They found that wet deposition timescales are independent of H for highly soluble compounds with H > 10^5 M/atm, and are mainly driven by the frequency and duration of precipitation, rather than precipitation intensity or droplet size distribution. Based on these findings, the authors proposed a rapid estimate of wet scavenging timescales from basic precipitation information. The manuscript is clearly structured with solid quantitative analysis. I would recommend publication and only have a few clarifying questions.

Comment 1: What’s the spread of timescale estimates? Could Figure 1 include a 25/75 percentile of estimated timescale values?

Response: We have added values between 25th to 75th percentiles as shaded areas in Figure 1 to improve the clarity of the spread of estimated timescales.

Comment 2: Page 6 last sentence – timescales of less soluble gases should be days (rather than weeks) to months based on Figure 1 (at H<10^4 M/atm)?

Response: We would like to thank the reviewer for the correction. However, our previous sentence might create some misunderstandings and we would like to clarify it. We actually want to discuss the uncertainty of timescales between different sites here. while uncertainty for low solubility gases is an order of magnitude, the timescales are weeks to months. For H=10^4 M/atm, timescales are indeed days as suggested by the reviewer, but uncertainty is relatively small. We would like to conclude that uncertainty is scaling with timescale and Henry’s constant to some extent. We have revised the sentence as follows.
“Differences between deposition timescales for less soluble gases are somewhat larger, up to nearly an order of magnitude for the least soluble gases, but timescales are so long for the least soluble compounds studied (weeks to months) that wet deposition is unlikely to be the dominant loss process”

Comment 3: Page 9 last paragraph –It’s said that the simple approach (dash line in Figure 5) applies 5 hours for compounds with H>10^5 M/atm. For the less soluble compounds, does the estimate use the wet deposition timescales averaged over the 5 sites? For less soluble compounds, their wet deposition timescales vary a lot, and the difference between the simple and the more accurate estimates are apparently larger than 20% (mentioned at the beginning of Page 10). How reliable is the simple approach for less soluble compounds with H<10^5 M/atm?

Response: As described by the reviewer, we indeed used the wet deposition timescales averaged over the 5 sites for predictions in Figure 5 for less soluble compounds . Upon consideration, we agreed with the reviewer that the simple approach will create errors for less soluble compounds that may be larger than 20%, particularly for compounds with H<10^3 M/atm. Predictions using over the global average timescales generate larger than 20% errors for compounds with H<10^5 M/atm (Figure 5) at some sites, but the majority of sites still exhibit low uncertainty with this approach, and predictions using site-specific, instead of averaged, timescales reduces the errors to within 20% for compounds with H from 10^3 to 10^5 M/atm. It is true that for compounds with H< 10^3 M/atm, both approaches lose skill in controlling the errors. However, we note that the timescales for compounds with H<10^3 M/atm are larger than 1 month, which for many compounds is expected to be orders of magnitude slower than other atmospheric removal processes such as dry deposition and oxidation. We therefore expect this approach to at least have reasonable uncertainty for compounds showing competitive timescales between dry and wet depositions.

We have revised the manuscript on Page 7:

“For compounds with H > 10^5 M atm-1 , timescales predicted using this size- and intensity-blind approach are within 20% of the more accurate approach using the complete precipitation information (Figure 5). For compounds with H between 10^3 and 10^5 M atm-1, the simplified approach still yields less than 20% errors at sites that tend more toward the central tendency (SGP, COR, and HOU), but can lead to somewhat higher uncertainties at other sites (at the most extreme end., a factor of 3 at ENA for H = 10^3 M atm- 1). For this simplified estimation approach, uncertainty gets significantly higher for compounds with H < 10^3 M atm-1. However, for most of these less soluble compounds, wet deposition timescales on the order of months to years suggest that other processes such as dry deposition and oxidation will dominate atmospheric removal, making accuracy less important. This novel approach allows rapid estimations of wet deposition timescales for more soluble compounds at any site for which the presence or absence of precipitation is known with reasonable time resolution. We note that using a site-specific value for timescales during precipitation instead of global values can improve accuracy to be within 5% for compounds with H > 10^5 M atm-1 and within 20% for compounds with H between 10^3 and 10^5 M atm-1 (Figure S3), but this approach negates much of the advantage of the approach as it would still require a detailed analysis of precipitation size distribution and intensity at each site.”

We have also revised the sentence on Page 8:

“We demonstrate that this approach can yield accuracy within 20% of error for most sites and most compounds with H > 10^3 M atm-1, though uncertainty in the most extreme case for compounds with H between 10^3 and 10^5 M atm-1 can be a factor of 3, which could be applied to rapidly estimate the global wet deposition timescale in the future.”

Comment 4: It seems to me that the proposed simple estimate applies mainly to compounds with H>10^5 M/atm at longer time scales. If the authors would agree, this should be emphasized in several places throughout the discussion and conclusion. (Please note that the current layout of the manuscript without line numbers is not friendly to reviewers.)

Response: We originally used the template provided by the journal but have now added the line number in the revised manuscript. We agree with the reviewer that the accuracy of estimated timescales for less soluble compounds needs to be further discussed. We have emphasized them in multiple places in the discussions and conclusion and shown our revisions in the responses to Comment 3. The simplified approach is expanded beyond H>10^5 M atm-1 by using equation S1 to estimate timescales for lower solubility compounds from those of highly soluble compounds. This detail has been added in this revision, with a demonstration of the approach in the newly added Figure S4.

Comment 5: It would be better to include some discussion on the timescales of chemical loss versus wet/dry deposition for typical organic gases (e.g. from oxidation of aromatics/isoprene/monoterpenes).

Response: We agree with the reviewer that adding the comparisons of timescales between different atmospheric removal pathways would be very interesting thus increasing the practical implication of this work. However, the competition between those pathways is quite complex and governed by many factors such as the physical-chemical properties of the chemical species, oxidant concentrations, precipitation frequency and duration, leaf area index, friction velocity, and surface types. We hope this work provides more detailed insight into the wet deposition aspect in order to facilitate future more detailed examinations of these competitions, which require significant additional discussion and examination beyond the scope of this manuscript.

Referee: 3

Comments to the Author
This paper estimates the wet deposition timescale for atmospheric organic gas compounds as a function of Henry’s law constants (H) using precipitation data at five sites over the globe. The wet deposition timescale is found to decrease with increasing H with a stable minimum for H>10^5 M/atm. The median wet deposition timescale for very soluble gases is estimated to be about 5hr for a continuous rain event at all sites. The authors conclude that the timescales depend mainly on the precipitation frequency and duration rather than intensity or droplet size. They thus propose that wet deposition timescale for gases can be estimated at any location using only basic precipitation information without detailed precipitation measurements. This paper deals with an important topic, wet deposition of organic gases, for which current understanding is still limited. The results are useful in assessing the relative roles of wet deposition, dry deposition, and oxidation in the budgets of organic gases. The paper is well written. I have only a few minor comments for the authors to consider during revision.

Comment 1: Page 5, left: “10 found that….” -- Do you mean “Hodzic et al. found that…”?

Response: Thanks for pointing out the mistake. We have revised the sentence as follows.
“…in part due to a relative dearth of studies on the timescales for deposition. Hodzic et. al.10 found that deposition is insensitive to…”

Comment 2: Page 7, Fig.2: It’s not clear, among these five sites, which ones are of tropical and are mainly subject to convective (versus stratiform) precipitation, and which ones are of mid-/high- latitudes. Do the data set used allow the authors to categorize the events based on precipitation types? Are snow events excluded?

Response: We agree that it is worth adding precipitation type as a factor for estimating wet deposition timescale. However, the current data set do not differentiate convective, stratiform, and orographic precipitation. Snow events are excluded from the analysis. We have revised the sentence to emphasize the exclusion of snow on Page 2:

“Because determination of droplet size by disdrometer may have large inaccuracies when observing of snowflakes and scavenging by snow involves separate processes than that by rain, data with snow and hail are excluded from the analysis.”

Comment 3: Fig. S2: caption should say “…for varying precipitation height”.

Response: We thank the reviewer for correcting the typo. We have revised the caption of Figure S2
“Figure S2. The uncertainty of wet deposition timescale in rain estimated for varying precipitation height using data collected from Lamont, Oklahoma, US (SGP).”

11-Oct-2022

Dear Dr Bi:

Manuscript ID: EA-ART-07-2022-000091.R1
TITLE: Estimated Timescales for Wet Deposition of Organic Compounds as a Function of Henry’s Law Constants

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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Environmental Science: Atmospheres
Royal Society of Chemistry

Reviewer 2

The comments have been clearly addressed by the authors. I would recommend publication of this work.

Reviewer 3

The authors addressed reviewers' comments very well. Acceptance for publication is recommended.