Impacts of current and projected oil palm plantation expansion on air
quality over Southeast Asia

Silva, Sam J.; Heald, Colette L.; Geddes, Jeffrey A.; Austin, Kemen G.; Kasibhatla, Prasad S.; Marlier, Miriam E.

doi:https://doi.org/10.5194/acp-16-10621-2016

Articles | Volume 16, issue 16

https://doi.org/10.5194/acp-16-10621-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-16-10621-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 16, issue 16

Research article

|

26 Aug 2016

Research article |

| 26 Aug 2016

Impacts of current and projected oil palm plantation expansion on air quality over Southeast Asia

Sam J. Silva, Colette L. Heald, Jeffrey A. Geddes, Kemen G. Austin, Prasad S. Kasibhatla, and Miriam E. Marlier

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Final revised paper (published on 26 Aug 2016)
Preprint (discussion started on 26 Feb 2016)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of “Impacts of current and projected oil palm plantation expansion on air quality over southeast Asia” by Silva et al.', Anonymous Referee #1, 25 Mar 2016
- AC1: 'Response to Anonymous Referee #1', Sam Silva, 10 Jun 2016
RC2: 'Reviewer's comment on "Impacts of current and projected oil palm plantation expansion on air quality over Southeast Asia', Anonymous Referee #2, 11 Apr 2016
- AC2: 'Response to Anonymous Referee #2', Sam Silva, 10 Jun 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Sam Silva on behalf of the Authors (10 Jun 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (19 Jun 2016) by Alex B. Guenther

RR by Anonymous Referee #2 (25 Jun 2016)

Suggestions for revision or reasons for rejection

I thank the authors for their detailed response to my concerns; they do not significantly alter my view of the manuscript as a whole. As the most novel aspect of the study is the analysis of the efficacy of current satellite observations in deducing air quality changes in the region I would suggest that this become the main focus of the report.

Replies to specific points made by the authors are shown below.

Response to authors’ comments:
To reviewer 1:
“We … completed simulations using 2007 and 2008 meteorology and emissions. These results indicate that the absolute magnitude of changes presented in this work are not highly sensitive to the choice of model year. The relative changes are more sensitive to the choice of model year ... We have added a sentence on P4 L8-9 in the manuscript: “Additional simulations using emissions and meteorology from 2007 and 2008 indicate that the choice of model year does not substantially influence the results of this work.” ”

Please quantify the statement added to the manuscript so that the reader can judge how substantial the interannual variability.

“The reduction in isoprene emission factors over broadleaf evergreen trees is confined to only over Southeast Asia. As such, it is likely not a major issue for global studies.”

Can the authors provide a reference for this assertion that it is not of global importance? The OP3 campaign found that the emission factors used for broadleaf evergreen trees in MEGANv2.0, which were based on measurements in the Amazon, were a factor of 4 too high for SE Asia, a previously under-studied region. However the tropics as a whole, outside of Amazonia, could be considered under-studied. How confident are we of emission factors for African ecosystems for example? In which case, while the over- / under-estimation of emissions for other regions may differ from SE Asia uncertainty in emission factors could be a global issue.

To reviewer 2:
“We have updated the text to further emphasize the novelty of the work”
The current work is now better contextualized in terms of previous studies.

“We use a more realistic land map for both modern and near-term future oil palm distributions, including the distribution of oil palm on Sumatra and the Malay Peninsula.”

As stated in my previous review, this is the main difference between this and previous work, other than the satellite observations (see below).

“Ashworth et al. (2012) scaled the isoprene emissions of only broadleaf evergreen trees within a given grid box based on the fraction of oil palm expected within that grid box. The scaling factor used was a ratio of measured emission factors for oil palm and broadleaf evergreen trees: 50/35. … We additionally directly calculate the isoprene emissions through the MEGANv2.1 algorithm using the measured basal isoprene emission factors from OP3. By using the MEGAN algorithm in conjunction with the measured emission factors, we are able to more robustly estimate oil palm emissions outside of the timeframe of the OP3 field campaign (accounting for seasonal temperature differences, PAR, LAI, etc.) This was not explicitly accounted for in Ashworth et al.
(2012) ....”

It is my understanding that emissions in Ashworth et al. (2012) were also calculated on-line although using scaled emission factors (which are in themselves merely a scaling factor of emissions at non-standard conditions). Temperature and PAR effects were therefore explicitly accounted for, although changes in LAI would not have been included. I would expect seasonal changes in LAI for broadleaf evergreen trees and oil palm to be small however although that may not be the case in global models such as GEOS-Chem and HadGEM.
Please alter the amended text in Section 2.2 to reflect that the Ashworth et al. (2012) emissions were also calculated on-line as the added sentences imply off-line emissions were used without changes in meteorology being considered.

“We further note that the reviewer did not discuss the novel satellite analysis presented in Section 4, wherein we describe how and why current observing systems are not capable of detecting the air quality impacts of the massive land use change signature of oil palm. Our conclusions are critically important for implementation of observing systems of the future. If we cannot observe the impacts of the massive change in biogenic emissions associated with oil palm plantations based on current satellite measurement capabilities, we are likely to struggle to detect the impacts of any forest to agriculture land use conversions in the future.”

If this is the novel aspect of the study presented in this manuscript it seems strange that it is given so little emphasis. Furthermore the “massive signature” of oil palm expansion in the region that the authors refer to has been deduced entirely through model simulations. The authors have not demonstrated that the model has been able to successfully simulate historical changes in air quality in the region and that model future projections are reliable. My understanding is that the OP3 campaign did not detect substantial differences in air quality above the oil palm plantations compared with native rainforest in spite of large differences in basal emission rates. This was ascribed to “NOx-limitation” and the authors deduced that in higher NOx regions air quality changes could be large. I agree that with our current understanding we would expect to see substantial changes in levels of secondary pollutants in the vicinity of urban areas downwind of the expansion but it has not been demonstrated. However, as suggested previously, I would urge the authors to re-frame the manuscript to focus on this aspect of their study as it is here that the novelty lies.

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RR by Anonymous Referee #3 (04 Jul 2016)

ED: Reconsider after minor revisions (Editor review) (08 Jul 2016) by Alex B. Guenther

AR by Sam Silva on behalf of the Authors (16 Jul 2016) Author's response Manuscript

ED: Publish as is (20 Jul 2016) by Alex B. Guenther

Short summary

We investigate the impacts of current (2010) and future (2020) oil palm plantations across Southeast Asia on surface–atmosphere exchange and air quality using satellite data, land maps, and a chemical transport model. These changes lead to increases in surface ozone and particulate matter. Oil palm plantations are likely to continue to degrade regional air quality in the coming decade and hinder efforts to achieve air quality regulations in major urban areas such as Kuala Lumpur and Singapore.