the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Mar 2024
Observed and CMIP6 model simulated organic aerosol response to drought in the contiguous United States during summertime
Abstract. Drought events have been linked with the enhancements of organic aerosols (OA), but the mechanisms have not been comprehensively understood. This study investigates the relationships between the monthly standardized precipitation–evapotranspiration index (SPEI) and surface OA in the contiguous United States (CONUS) during the summertime from 1998 to 2019. OA under severe drought conditions shows a significant increase in mass concentrations across most of the CONUS relative to non-drought periods with the Pacific Northwest (PNW) and Southeastern United States (SEUS) experiencing the highest average enhancement of 1.79 µg m−3 (112 %) and 0.92 µg m−3 (33 %), respectively. In the SEUS, a linear regression approach between OA and sulfate was used to estimate the isoprene epoxydiols derived secondary organic aerosol (IEPOX SOA), which is the primary driver of the OA enhancements under droughts due to the simultaneous increase of isoprene and sulfate. The rise of sulfate is mainly caused by the reduced wet deposition because of the up to 62 % lower precipitation amount. In the PNW, OA enhancements are closely linked to intensified wildfire emissions, which raise OA mass concentrations to be four to eight times higher relative to non-fire conditions. All ten Earth system models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6) can capture the negative slopes between SPEI and OA in the PNW with CESM2-WACCM and GFDL-ESM4 performing the best and worst in predicting the OA enhancement under severe droughts. However, all models significantly underestimate the OA increase in the SEUS with Nor-ESM2-LM and MIRCO6 showing relatively better performance. This study reveals the key drivers of the elevated OA levels under droughts in the CONUS and underscores the deficiencies of current climate models in their predictive capacity for assessing the impact of future droughts on air quality.
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RC1: 'Comment on egusphere-2024-430', Anonymous Referee #1, 17 Apr 2024
This study focused on the organic aerosol response to drought in the contiguous United States during the summertime. The authors have carried out a long-term analysis, and recognized the different responding mechanisms in SEUS and PNW. The study also proposed a direction of model improvement through a comprehensive analysis of the ability of CMIP6 models to capture the correlation between OA and drought. The article is well-organized, but some problems still need to be addressed before consideration of publication.
- Line 51-52: The statement is confusing. SOA is a component of PM2.5, so why could it lead to a higher mortality rate than overall PM2.5?
- The abstract and introduction stated that the study period was 1998 to 2019, but in 2.1 and 2.2, the datasets only extend to 2018. In 3.1, line 124 and line 137 also pointed to different time periods. Please clarify the exact time period.
- Line 99-101: Sites with data records longer than 5 years were selected for data analysis in this study. However, sites with data records of shorter time range need interpolation of longer time range, which may cause larger uncertainty. Could you provide the distribution of site data records according to their duration? For example, what percentage of sites have data records spanning 6 years, 7 years, etc.?
- Line 124: What’s the frequencies of non-drought and drought summer during the study period? Is there large interannual variation of OA concentration in either of the cases? What’s the uncertainty of the mean OA concentration in these two cases?
- Section 3.2: Could you please provide a map showing which grids in the SEUS and PNW regions were included in the data analysis?
- Figure 5: Wildfire OA transports over long distances. What is the necessity to distinguish between with and without local fire within a single grid? If the same grid is classified into different categories in different months and years, will there be inconsistency in calculating the average OA with/without local fire?
- This study covers a long time scale, during which anthropogenic emissions of OC, SO2 and VOCs in the United States may all have a long-term trend of change, which may affect OA concentration. When classifying the time periods based on drought or non-drought, did you consider that these human factors might differ significantly between the two periods, which might interfere with the analysis of this study?
Citation: https://doi.org/10.5194/egusphere-2024-430-RC1 -
RC2: 'Comment on egusphere-2024-430', Anonymous Referee #2, 06 May 2024
Li and Wang present a modeling study of the response of CMIP6 modeled organic aerosol to drought in USA. A drought response to OA is a worthwhile study since it affects our understanding of how aerosols respond to droughts and interact with climate change. However, several points need clarifications (related to deriving IEPOX-SOA based on slopes of total OA versus sulfate in measurements) and the caveats related to mechanistic representations of IEPOX-SOA in CMIP6 models need to be described so that a reader can better understand why the models could not represent how IEPOX-SOA might increase in SE USA as drought intensifies. A clearer terminology needs to be used instead of “negative slopes versus SPEI” in terms of what this means with respect to response of OA and sulfate to intensifying drought.
- Line 42: “The abnormally high temperature and low humidity under droughts can enhance the volatility and oxidation of OA”
Increasing temperature will increase the volatility of OA and cause OA evaporation (reducing OA) but increased photochemistry will cause oxidation and reduce volatility (increasing SOA). Are the authors referring to OA evaporation at high temperature, or do they mean enhanced photochemical aging at high temperature and low humidity that can reduce volatility and increase SOA formation? Please clarify. Also how does drought affect hygroscopicity of OA?
2. Line 48: OA from wildfires is just POA or do the authors consider SOA formed due to oxidation of VOCs emitted by wildfires?
3. Line 149: Do the authors consider prescribed burning in SE USA? What is its change with drought compared to non-drought conditions?
4. Line 188: Increased OA per unit increase in sulfate as drought “deteriorates”? Should it be drought “intensifies”?
5. Line 212-213: In addition to IEPOX-SOA other biogenic SOA types (like monoterpene SOA), and anthropogenic SOA can also change with sulfate due to the formation of organosulfates. See following references:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JD032253
https://pubs.acs.org/doi/abs/10.1021/acs.est.9b06751
6. In addition, correlation between OA and sulfate does not necessarily imply causal mechanistic relations between OA and sulfate. These caveats need to be acknowledged.
7. Line 236-238: How does Change in pH from 4.98 to 4.87 affect IEPOX SOA formation? Can the authors quantify this with box model calculations representative of cloud chemistry? Also does pH in wet aerosols affect IEPOX-SOA in their simulations? What is the model predicted pH trend in wet aerosols in SE USA?
8. Line 270 and section 3.3: The authors should present brief discussions of how IEPOX-SOA is simulated within each of the 10 CMIP6 models. For example, do these models consider limitations of mass transfer to IEPOX gas on aqueous sulfate aerosols due to gas-phase diffusion, interfacial accommodation at gas-particle interface, diffusion limitations within SOA coatings and aqueous phase reaction rates of IEPOX forming 2-methyltetrols and organosulfates (key components of IEPOX-SOA)? Which of these models consider the effects of particle-phase (solid, liquid) as a function of relative humidity on IEPOX-SOA formation?
For example please see:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016RG000540
https://pubs.acs.org/doi/abs/10.1021/acsearthspacechem.1c00356
9. Line 275: When referring to strong negative slopes of OA per unit decrease of SPEI, the authors should clarify that this represents increase of OA as droughts intensify.
10. Line 318: Define OA-sulfate slopes to SPEI. How is this calculated from the models? Is it slope of OA versus sulfate (let’s say its X) and then the slope of X versus SPEI?
11. Can the authors evaluate sensitivity of IEPOX SOA in models to sulfate with long term observations for example the IMPROVE network and SEARCH sites ? For example Zheng et al. 2020: https://acp.copernicus.org/articles/20/13091/2020/acp-20-13091-2020.html
12. It was shown Zheng et al. 2020 (listed above) that a global model overestimates OA sensitivity to sulfate by atleast a factor of 2. But seems the CMIP6 models are underestimating the sensitivity of OA to sulfate? How does this sensitivity change with drought? The authors should consider plotting the long-term multi-year model predicted concentrations of IEPOX-SOA versus sulfate at least in the models that explicitly model IEPOX-SOA formation as function of RH, particle acidity, sulfate, organic coatings etc. Then the drought years can be marked on this plot. The same can be done for measurements.
Citation: https://doi.org/10.5194/egusphere-2024-430-RC2 -
AC1: 'Comment on egusphere-2024-430', Wei Li, 14 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-430/egusphere-2024-430-AC1-supplement.pdf
Status: closed
-
RC1: 'Comment on egusphere-2024-430', Anonymous Referee #1, 17 Apr 2024
This study focused on the organic aerosol response to drought in the contiguous United States during the summertime. The authors have carried out a long-term analysis, and recognized the different responding mechanisms in SEUS and PNW. The study also proposed a direction of model improvement through a comprehensive analysis of the ability of CMIP6 models to capture the correlation between OA and drought. The article is well-organized, but some problems still need to be addressed before consideration of publication.
- Line 51-52: The statement is confusing. SOA is a component of PM2.5, so why could it lead to a higher mortality rate than overall PM2.5?
- The abstract and introduction stated that the study period was 1998 to 2019, but in 2.1 and 2.2, the datasets only extend to 2018. In 3.1, line 124 and line 137 also pointed to different time periods. Please clarify the exact time period.
- Line 99-101: Sites with data records longer than 5 years were selected for data analysis in this study. However, sites with data records of shorter time range need interpolation of longer time range, which may cause larger uncertainty. Could you provide the distribution of site data records according to their duration? For example, what percentage of sites have data records spanning 6 years, 7 years, etc.?
- Line 124: What’s the frequencies of non-drought and drought summer during the study period? Is there large interannual variation of OA concentration in either of the cases? What’s the uncertainty of the mean OA concentration in these two cases?
- Section 3.2: Could you please provide a map showing which grids in the SEUS and PNW regions were included in the data analysis?
- Figure 5: Wildfire OA transports over long distances. What is the necessity to distinguish between with and without local fire within a single grid? If the same grid is classified into different categories in different months and years, will there be inconsistency in calculating the average OA with/without local fire?
- This study covers a long time scale, during which anthropogenic emissions of OC, SO2 and VOCs in the United States may all have a long-term trend of change, which may affect OA concentration. When classifying the time periods based on drought or non-drought, did you consider that these human factors might differ significantly between the two periods, which might interfere with the analysis of this study?
Citation: https://doi.org/10.5194/egusphere-2024-430-RC1 -
RC2: 'Comment on egusphere-2024-430', Anonymous Referee #2, 06 May 2024
Li and Wang present a modeling study of the response of CMIP6 modeled organic aerosol to drought in USA. A drought response to OA is a worthwhile study since it affects our understanding of how aerosols respond to droughts and interact with climate change. However, several points need clarifications (related to deriving IEPOX-SOA based on slopes of total OA versus sulfate in measurements) and the caveats related to mechanistic representations of IEPOX-SOA in CMIP6 models need to be described so that a reader can better understand why the models could not represent how IEPOX-SOA might increase in SE USA as drought intensifies. A clearer terminology needs to be used instead of “negative slopes versus SPEI” in terms of what this means with respect to response of OA and sulfate to intensifying drought.
- Line 42: “The abnormally high temperature and low humidity under droughts can enhance the volatility and oxidation of OA”
Increasing temperature will increase the volatility of OA and cause OA evaporation (reducing OA) but increased photochemistry will cause oxidation and reduce volatility (increasing SOA). Are the authors referring to OA evaporation at high temperature, or do they mean enhanced photochemical aging at high temperature and low humidity that can reduce volatility and increase SOA formation? Please clarify. Also how does drought affect hygroscopicity of OA?
2. Line 48: OA from wildfires is just POA or do the authors consider SOA formed due to oxidation of VOCs emitted by wildfires?
3. Line 149: Do the authors consider prescribed burning in SE USA? What is its change with drought compared to non-drought conditions?
4. Line 188: Increased OA per unit increase in sulfate as drought “deteriorates”? Should it be drought “intensifies”?
5. Line 212-213: In addition to IEPOX-SOA other biogenic SOA types (like monoterpene SOA), and anthropogenic SOA can also change with sulfate due to the formation of organosulfates. See following references:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JD032253
https://pubs.acs.org/doi/abs/10.1021/acs.est.9b06751
6. In addition, correlation between OA and sulfate does not necessarily imply causal mechanistic relations between OA and sulfate. These caveats need to be acknowledged.
7. Line 236-238: How does Change in pH from 4.98 to 4.87 affect IEPOX SOA formation? Can the authors quantify this with box model calculations representative of cloud chemistry? Also does pH in wet aerosols affect IEPOX-SOA in their simulations? What is the model predicted pH trend in wet aerosols in SE USA?
8. Line 270 and section 3.3: The authors should present brief discussions of how IEPOX-SOA is simulated within each of the 10 CMIP6 models. For example, do these models consider limitations of mass transfer to IEPOX gas on aqueous sulfate aerosols due to gas-phase diffusion, interfacial accommodation at gas-particle interface, diffusion limitations within SOA coatings and aqueous phase reaction rates of IEPOX forming 2-methyltetrols and organosulfates (key components of IEPOX-SOA)? Which of these models consider the effects of particle-phase (solid, liquid) as a function of relative humidity on IEPOX-SOA formation?
For example please see:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016RG000540
https://pubs.acs.org/doi/abs/10.1021/acsearthspacechem.1c00356
9. Line 275: When referring to strong negative slopes of OA per unit decrease of SPEI, the authors should clarify that this represents increase of OA as droughts intensify.
10. Line 318: Define OA-sulfate slopes to SPEI. How is this calculated from the models? Is it slope of OA versus sulfate (let’s say its X) and then the slope of X versus SPEI?
11. Can the authors evaluate sensitivity of IEPOX SOA in models to sulfate with long term observations for example the IMPROVE network and SEARCH sites ? For example Zheng et al. 2020: https://acp.copernicus.org/articles/20/13091/2020/acp-20-13091-2020.html
12. It was shown Zheng et al. 2020 (listed above) that a global model overestimates OA sensitivity to sulfate by atleast a factor of 2. But seems the CMIP6 models are underestimating the sensitivity of OA to sulfate? How does this sensitivity change with drought? The authors should consider plotting the long-term multi-year model predicted concentrations of IEPOX-SOA versus sulfate at least in the models that explicitly model IEPOX-SOA formation as function of RH, particle acidity, sulfate, organic coatings etc. Then the drought years can be marked on this plot. The same can be done for measurements.
Citation: https://doi.org/10.5194/egusphere-2024-430-RC2 -
AC1: 'Comment on egusphere-2024-430', Wei Li, 14 Jun 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-430/egusphere-2024-430-AC1-supplement.pdf
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