Assessment of isoprene and near surface ozone sensitivities to water stress over the Euro-Mediterranean region

Strada, Susanna; Pozzer, Andrea; Giorgi, Filippo; Giuliani, Graziano; Coppola, Erika; Solmon, Fabien; Jiang, Xiaoyan; Guenther, Alex

doi:https://doi.org/10.5194/egusphere-2022-1522

Preprints

https://doi.org/10.5194/egusphere-2022-1522

Preprints

20 Feb 2023

| 20 Feb 2023

Assessment of isoprene and near surface ozone sensitivities to water stress over the Euro-Mediterranean region

Susanna Strada, Andrea Pozzer, Filippo Giorgi, Graziano Giuliani, Erika Coppola, Fabien Solmon, Xiaoyan Jiang, and Alex Guenther

Abstract. Plants emit biogenic volatile organic compounds (BVOCs) in response to changes in environmental conditions (e.g., temperature, radiation, soil moisture). In the large family of BVOCs, isoprene is by far the largest emitted compounds and plays an important role in ozone chemistry, thus affecting both climate and air quality. In turn, climate change may alter isoprene emissions by increasing the occurrence and intensity of severe water stresses that alter plant functioning.

The Model of Emissions of Gases and Aerosols from Nature (MEGAN) provides different parameterizations to account for the impact of water stress on isoprene emissions, which essentially reduces emissions in response to the effect of soil moisture deficit on plant productivity.

By applying the regional climate-chemistry model RegCM4chem coupled to the Community Land Model CLM4.5 and MEGAN2.1, we thus performed sensitivity simulations to assess the effects of water stress on isoprene emissions and near-surface ozone levels over the Euro-Mediterranean region and across the drier/wetter summers over the period 1992–2016 using two different parametrizations of the impact of water stress implemented in the MEGAN model.

Over the Euro-Mediterranean region and across the simulated summers, water stress reduces isoprene emissions on average by nearly 6 %. However, during the warmest and driest selected summers (e.g., 2003, 2010, 2015) and over large isoprene-source area (e.g., the Balkans), decreases in isoprene emissions range from −20 to −60 % and co-occur with negative anomalies in precipitation, soil moisture and plant productivity. Sustained decreases in isoprene emissions also occur after prolonged or repeated dry anomalies, as observed for the summers of 2010 and 2012. Although the decrease in isoprene emissions due to water stress may be important, it only reduce near-surface ozone levels by few percents due to a dominant NOx-limited regime over southern Europe and the Mediterranean Basin. Overall, over the selected analysis region, compared to the old MEGAN parameterization, the new one leads to localized and 25–50 % smaller decreases in isoprene emissions, and 3–8 % smaller reduction in near-surface ozone levels.

Received: 30 Dec 2022 – Discussion started: 20 Feb 2023

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23 Oct 2023

Assessment of isoprene and near-surface ozone sensitivities to water stress over the Euro-Mediterranean region

Susanna Strada, Andrea Pozzer, Graziano Giuliani, Erika Coppola, Fabien Solmon, Xiaoyan Jiang, Alex Guenther, Efstratios Bourtsoukidis, Dominique Serça, Jonathan Williams, and Filippo Giorgi

Atmos. Chem. Phys., 23, 13301–13327, https://doi.org/10.5194/acp-23-13301-2023,https://doi.org/10.5194/acp-23-13301-2023, 2023

Short summary

Susanna Strada et al.

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-1522', Anonymous Referee #3, 14 Mar 2023

Assessment of isoprene and near surface ozone sensitivities to water stress over the Euro-Mediterranean region
This is a detailed study of how emission of isoprene are impacted by the newer MEGAN v3 soil moisture activity parameterisation compared to the previous v2.1 parameterisation. Comparisons are also made using satellite formaldehyde columns and surface ozone measurements. Results were compared over a number of summer seasons from 1992 to 2015, when isoprene is expected to be at peak concentrations in Europe.
One of the interesting parts of this paper was the demonstration of how ‘smooth’ the soil moisture activity function is using the MEGANv2.1 parameterisation compared to MEGANv3. The latter parameterisation allowed for more spatially varying reductions in isoprene, which were often less than those calculated using MEGANv2.1 and more localised. By contrast, a soil moisture activity function of ~0.4-0.5 covers most of Europe in summer using MEGANv2.1 which causes a very even, but perhaps too high a reduction in isoprene emissions.
I thought the methods, model and observations section was very well detailed, with all datasets well documented and described.
I only have a few comments before publication is recommended. They mainly relate to difficulties reading the figures.
Line 139. Would be good to have these values tabulated somewhere, or even refer to Oleson et al (2013) table 8.1 which is where I finally found them. Would be useful if others wanted to implement the new scheme.
Line 343. 76 mg/m2/day is a huge reduction. I wondered where abouts this was located, and what was the underlying vegetation type?
Line 481 there looks to be a co-author comment (?) still in the text.
Conclusions section. There are a lot of new references introduced here which isn’t usual – they’re more suited to the introduction where previous literature is more commonly reviewed.
Figures: Most were too small to see properly.
In figure 1 the axis text is too feint to read.
Figure 4 needs the units putting on the y-axis. The orange line is also too feint to see.
Figure 5: I was confused by the legend which has pointers indicating the scale goes below 0 and above 1. There is a lot of white areas in the 12 maps which look to be above 1 and suggests that gamma2018 is higher than the default (which it can’t be)?
Figure 9a: numbers on y-axis are bunched together and overlap

Oleson et al (2013). Technical Description of version 4.5 of the Community Land Model (CLM). https://opensky.ucar.edu/islandora/object/technotes:515

Citation: https://doi.org/10.5194/egusphere-2022-1522-RC1
- AC1: 'Reply on RC1', Susanna Strada, 30 Jul 2023
  
  Publisher’s note: this comment is a copy of AC3 and its content was therefore removed.
  
  Citation: https://doi.org/10.5194/egusphere-2022-1522-AC1
- AC3:
  'Reply on RC1', Susanna Strada, 30 Jul 2023
  GENERAL COMMENT:
  This study applies a regional vegetation-climate-chemistry model to investigate the influence of water stress on isoprene emissions and surface ozone over Europe in 1992-2016. This is done by coupling the land module and biogenic emission module to derive the soil water stress function, which is then used to determine soil moisture activity factor in the parameterization of isoprene emission. Simulation results show that water stress reduces summertime isoprene emissions on average by nearly 6%, and by -20 to -60% in extreme dry summers, but influence on ozone is relatively small. This study is well-designed, easy to follow, and the results are useful for the community. It has room to be improved by addressing the following comments.
  Authors’ response: We thank Reviewer #3 for this positive evaluation of the study and for the insightful comments. Below, we separately reply to each comment and, when necessary, we precise where and how the manuscript has been modified. Supplementary figures produced to answer to reviewers’ comments have been gathered in a separated document uploaded as a supplement (“Responses_to_Reviewers_fig.pdf”). To avoid confusion with figures in the manuscript and in the Supplementary Material, we customized the counter as Figure R.*. While reviewing the manuscript, we found an error in the conversion of ozone and formaldehyde mixing ratios that has been corrected in the revised manuscript.
  
  SPECIFIC COMMENTS
  1. My major concern is that this study lacks direct evaluation with observed isoprene emissions. Does the inclusion of water stress effect improve the simulation of isoprene emissions? Does the new scheme outperform the old scheme?
  Authors’ response: We agree with the Reviewer that direct evaluation with observed isoprene emissions would be the best choice to assess the model performance and compare the old and new schemes that link the effect of soil moisture on isoprene emissions. However, since there is no network over Europe routinely measuring isoprene emissions, or isoprene concentrations, in vegetated areas, we focused the model evaluation on a proxy of isoprene emissions such as formaldehyde (HCHO). Nevertheless, we have now also a limited evaluation of isoprene concentrations as simulated by the RegCM4chem-CLM4.5-MEGAN2.1 model in the GAMMA-SMoff simulation. The model output have been compared against observations collected during two field campaigns. Figure R.1 shows the comparison against isoprene concentrations measured in south-eastern France (site: La Verdière; Latitude: 43.63° N, Longitude: 5.93° E) during the summer 2000 (from June 21 to July 6) in the framework of the ESCOMPTE field campaign (Cros et al., 2004) when isoprene concentrations had been measured every 30 minutes using a Fast Isoprene Sensor. In Figure R.2, model output have been compared against data collected in Cyprus (site: Ineia; Latitude: 34.96° N, Longitude: 32.39° E) during the summer 2014 (from July 7 to August 3; data collected nearly every 30 minutes) using techniques of gas chromatography - mass spectrometry.
  At both sites, RegCM underestimates isoprene concentrations, which is consistent with RegCM underestimating concentrations of a proxy of isoprene such as formaldehyde, as shown in Section 3.1.3 in the manuscript. Sometimes, the model reproduces a delayed peak in isoprene concentrations compared to observations. Differences between observations and model output could result from multiple factors, for example:
  
  The cold and wet model bias (see Fig. 1 in the manuscript) that limits isoprene emissions;
  
  Differences between the dominant vegetation types on the field and in the model grid-cell. For example, in La Verdière, vegetation is mainly characterized by Mediterranean oak forest (more than 80% of Quercus Pubescens, which is a deciduous tree), while the RegCM grid-cell is mainly covered with needle-leaf evergreen temperate trees (36%), and C3 grass (37%), which are both low isoprene emitters, and has only a small amount of broadleaf deciduous trees (6%).
  
  Different scales: a model grid-cell spans over a surface of around 25x25 km², while station measurements have a footprint of a few hundreds of meters, depending on the terrain where the observations have been collected. For example, Ineia is located close to the seacoast leading to use a model grid-cell that is located over the sea.
  
  Based on these results, we have added the comparison between model output and in-situ measurements of isoprene concentrations in Section 3.1.3 in the revised manuscript.
  Cros, B., et al.: The ESCOMPTE Program: an overview. Atmospheric Research, 2004, 69 (3-4), pp.241-279, DOI: 10.1016/j.atmosres.2003.05.001_xffff_.
  
  2. I also wonder how the water stress effect changes the relationship between isoprene emissions and temperature. Previous studies have revealed the decrease in isoprene emission in extreme high temperature, but would the water stress effect further change the turning point of the T-emission curve? Some discussions would be useful.
  Authors’ response: This is an interesting question.
  The version of the MEGAN model (2.1) implemented in the RegCM4.7 model accounts for the effect of past temperatures on isoprene emissions (over the past ten days): the warmer the temperatures, the higher the emissions (see Fig. 4 in Guenther et al., 2006). However, this version does not account for the decrease in isoprene emissions due to extreme high temperatures.
  Extreme high temperatures often co-occur with droughts. It is, then, not trivial to separate the effect of these climate extremes on isoprene emissions. In our simulations, we observed the largest decreases in isoprene emissions in the summers 2003 and 2012 (see Table 4) when the observation-based summer temperatures are nearly 4–5 standard deviations above (warmer than) the 1970–1990 climatology, as shown by Figure S.6 in the Supplementary Material. These results and those mentioned by the Reviewer, which reveal the decrease in isoprene emissions under extreme high temperatures, suggest that isoprene emissions would be strongly reduced when heat wave and drought co-occur. In the revised manuscript, we added the following comment in Sect. 3.2:
  “The largest decreases in isoprene emissions occur in the summers 2003 and 2012 (Table 4), when the observation-based summer temperatures are nearly 4–5 standard deviations above (warmer than) the 1970–1990 climatology (Fig. S.6). These results suggest that isoprene emissions would be strongly reduced when heat wave and drought co-occur.”
  
  3. Related to point 2, I think some discussions on the extreme ozone episodes would be helpful to evaluate the effect of water stress on ozone concentration.
  Authors’ response: Extreme high temperatures often exacerbate ozone pollution and lead to extreme ozone episodes. In a second paper (in preparation), we investigated the ozone climate penalty by performing simulations under both present-day (1990–2004) and future climates (2035–2049). To assess the impact on ozone concentration of both the direct effect of high temperatures and the indirect effect of water stress on isoprene emissions, we designed two sensitivity simulations over the summer 2003 with and without the soil moisture activity factor activated. In these simulations, we artificially increased air temperature across the whole atmospheric column by a fixed amount varying with atmospheric levels. Results show that an average increase of 2.5°C in air temperature leads to an increase in surface-ozone level smaller than 2 ppbv (1%), regardless the soil moisture activity factor is activated or not. Although the paper will be submitted soon, we added the following comment in Sect. 4:
  “In a future study, we aim to explore the ozone climate penalty over the Euro-Mediterranean region under both present-day and future climates and to assess the impact on ozone concentration of both the direct effect of high temperatures and the indirect effect of water stress on isoprene emissions.”
  
  4. I feel that the model evaluation of chemical fields is rather insufficient. It only shows the mean magnitude of observed and simulated HCHO column and ozone concentrations. How well does the model capture the spatial and temporal pattern of HCHO and ozone? It might be also important to evaluate the ozone chemical regime (NOx-limited or VOCs-limited) somewhere.
  Authors’ response: We agree with the Reviewer that the model evaluation for chemical species could be extended. In our study, we focused on the analysis of differences between sensitivity simulations, therefore possible systematic biases in the model would cancel out, thus giving robust results. However, we performed some additional evaluation for both ozone and formaldehyde by comparing model output against re-analyses from the Copernicus Atmosphere Monitoring Service (CAMS) for the period 2003–2007. In addition, surface ozone concentrations have been evaluated against observations collected in La Verdière (France) in the summer 2000 (from June 21 to July 6) using an conventional UV absorption ozone analyzer (Environement S.A., Poissy, France, model O3 41M). Ozone concentrations have been measured during the ESCOMPTE field campaign, together with isoprene measurements shown in Figure R.1.
  For near surface ozone, model output are lower than CAMS re-analyses with differences between 10 and 20 ppbv over the Mediterranean Basin, with some summers and few grid-cells showing differences between 20 and 30 ppbv (Fig. R.3). The model underestimates near-surface ozone as well when compared to in-situ measurements (Fig. R.4); in particular, model output shows a smaller variability than in-situ measurements. For formaldehyde, model outputs near the surface are also lower than CAMS reanalyses with differences between -1 and -4 ppbv (Fig. R.5).
  We also assessed the ozone chemical regime using the ratio between formaldehyde (HCHO) and nitrogen dioxide (NO₂) as presented in Duncan et al. (2010). Results show that the model reproduces a VOC-limited regime over the whole domain, with a HCHO/NO₂ ratio lower than 1 (Fig. R.6). Based on this analysis, in the revised manuscript we updated all discussions about the ozone regime over the model domain.
  Duncan, B. N. et al.: Application of OMI observations to a space-based indicator of NOx and VOC controls on surface ozone formation, Atmospheric Environment, Volume 44, Issue 18, 2010, Pages 2213-2223, DOI: https://doi.org/10.1016/j.atmosenv.2010.03.010.
  
  5. The figure quality can be improved. For example, the size of the figure is often too small compared to the colorbar (Figs 11 and 12), and in some cases the label is missing (Fig.5).
  Authors’ response: We improved all figures. We added axis labels to Figure 5 (now Fig. 6), we increased the size of figures compared to the colorbars, and we implemented colorblind-friendly colormaps in Figures 6 (now Fig. 7) and 11 (now Fig. 12), as requested by the EGU journals.
  
  Citation: https://doi.org/10.5194/egusphere-2022-1522-AC3
RC2:
'Comment on egusphere-2022-1522', Anonymous Referee #1, 03 Apr 2023

This study applies a regional vegetation-climate-chemistry model to investigate the influence of water stress on isoprene emissions and surface ozone over Europe in 1992-2016. This is done by coupling the land module and biogenic emission module to derive the soil water stress function, which is then used to determine soil moisture activity factor in the parameterization of isoprene emission. Simulation results show that water stress reduces summertime isoprene emissions on average by nearly 6%, and by -20 to -60% in extreme dry summers, but influence on ozone is relatively small. This study is well-designed, easy to follow, and the results are useful for the community. It has room to be improved by addressing the following comments.
1. My major concern is that this study lacks direct evaluation with observed isoprene emissions. Does the inclusion of water stress effect improve the simulation of isoprene emissions? Does the new scheme outperform the old scheme?
2. I also wonder how the water stress effect changes the relationship between isoprene emissions and temperature. Previous studies have revealed the decrease in isoprene emission in extreme high temperature, but would the water stress effect further change the turning point of the T-emission curve? Some discussions would be useful.
3. Related to point 2, I think some discussions on the extreme ozone episodes would be helpful to evaluate the effect of water stress on ozone concentration.
4. I feel that the model evaluation of chemical fields is rather insufficient. It only shows the mean magnitude of observed and simulated HCHO column and ozone concentrations. How well does the model capture the spatial and temporal pattern of HCHO and ozone? It might be also important to evaluate the ozone chemical regime (NOx-limited or VOCs-limited) somewhere.
5. The figure quality can be improved. For example, the size of the figure is often too small compared to the colorbar (Figs 11 and 12), and in some cases the label is missing (Fig.5).

Citation: https://doi.org/10.5194/egusphere-2022-1522-RC2
- AC2: 'Reply on RC2', Susanna Strada, 30 Jul 2023
  
  Please see the attached file for the 1-on-1 response.
  
  Citation: https://doi.org/10.5194/egusphere-2022-1522-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-1522', Anonymous Referee #3, 14 Mar 2023

Assessment of isoprene and near surface ozone sensitivities to water stress over the Euro-Mediterranean region
This is a detailed study of how emission of isoprene are impacted by the newer MEGAN v3 soil moisture activity parameterisation compared to the previous v2.1 parameterisation. Comparisons are also made using satellite formaldehyde columns and surface ozone measurements. Results were compared over a number of summer seasons from 1992 to 2015, when isoprene is expected to be at peak concentrations in Europe.
One of the interesting parts of this paper was the demonstration of how ‘smooth’ the soil moisture activity function is using the MEGANv2.1 parameterisation compared to MEGANv3. The latter parameterisation allowed for more spatially varying reductions in isoprene, which were often less than those calculated using MEGANv2.1 and more localised. By contrast, a soil moisture activity function of ~0.4-0.5 covers most of Europe in summer using MEGANv2.1 which causes a very even, but perhaps too high a reduction in isoprene emissions.
I thought the methods, model and observations section was very well detailed, with all datasets well documented and described.
I only have a few comments before publication is recommended. They mainly relate to difficulties reading the figures.
Line 139. Would be good to have these values tabulated somewhere, or even refer to Oleson et al (2013) table 8.1 which is where I finally found them. Would be useful if others wanted to implement the new scheme.
Line 343. 76 mg/m2/day is a huge reduction. I wondered where abouts this was located, and what was the underlying vegetation type?
Line 481 there looks to be a co-author comment (?) still in the text.
Conclusions section. There are a lot of new references introduced here which isn’t usual – they’re more suited to the introduction where previous literature is more commonly reviewed.
Figures: Most were too small to see properly.
In figure 1 the axis text is too feint to read.
Figure 4 needs the units putting on the y-axis. The orange line is also too feint to see.
Figure 5: I was confused by the legend which has pointers indicating the scale goes below 0 and above 1. There is a lot of white areas in the 12 maps which look to be above 1 and suggests that gamma2018 is higher than the default (which it can’t be)?
Figure 9a: numbers on y-axis are bunched together and overlap

Oleson et al (2013). Technical Description of version 4.5 of the Community Land Model (CLM). https://opensky.ucar.edu/islandora/object/technotes:515

Citation: https://doi.org/10.5194/egusphere-2022-1522-RC1
- AC1: 'Reply on RC1', Susanna Strada, 30 Jul 2023
  
  Publisher’s note: this comment is a copy of AC3 and its content was therefore removed.
  
  Citation: https://doi.org/10.5194/egusphere-2022-1522-AC1
- AC3:
  'Reply on RC1', Susanna Strada, 30 Jul 2023
  GENERAL COMMENT:
  This study applies a regional vegetation-climate-chemistry model to investigate the influence of water stress on isoprene emissions and surface ozone over Europe in 1992-2016. This is done by coupling the land module and biogenic emission module to derive the soil water stress function, which is then used to determine soil moisture activity factor in the parameterization of isoprene emission. Simulation results show that water stress reduces summertime isoprene emissions on average by nearly 6%, and by -20 to -60% in extreme dry summers, but influence on ozone is relatively small. This study is well-designed, easy to follow, and the results are useful for the community. It has room to be improved by addressing the following comments.
  Authors’ response: We thank Reviewer #3 for this positive evaluation of the study and for the insightful comments. Below, we separately reply to each comment and, when necessary, we precise where and how the manuscript has been modified. Supplementary figures produced to answer to reviewers’ comments have been gathered in a separated document uploaded as a supplement (“Responses_to_Reviewers_fig.pdf”). To avoid confusion with figures in the manuscript and in the Supplementary Material, we customized the counter as Figure R.*. While reviewing the manuscript, we found an error in the conversion of ozone and formaldehyde mixing ratios that has been corrected in the revised manuscript.
  
  SPECIFIC COMMENTS
  1. My major concern is that this study lacks direct evaluation with observed isoprene emissions. Does the inclusion of water stress effect improve the simulation of isoprene emissions? Does the new scheme outperform the old scheme?
  Authors’ response: We agree with the Reviewer that direct evaluation with observed isoprene emissions would be the best choice to assess the model performance and compare the old and new schemes that link the effect of soil moisture on isoprene emissions. However, since there is no network over Europe routinely measuring isoprene emissions, or isoprene concentrations, in vegetated areas, we focused the model evaluation on a proxy of isoprene emissions such as formaldehyde (HCHO). Nevertheless, we have now also a limited evaluation of isoprene concentrations as simulated by the RegCM4chem-CLM4.5-MEGAN2.1 model in the GAMMA-SMoff simulation. The model output have been compared against observations collected during two field campaigns. Figure R.1 shows the comparison against isoprene concentrations measured in south-eastern France (site: La Verdière; Latitude: 43.63° N, Longitude: 5.93° E) during the summer 2000 (from June 21 to July 6) in the framework of the ESCOMPTE field campaign (Cros et al., 2004) when isoprene concentrations had been measured every 30 minutes using a Fast Isoprene Sensor. In Figure R.2, model output have been compared against data collected in Cyprus (site: Ineia; Latitude: 34.96° N, Longitude: 32.39° E) during the summer 2014 (from July 7 to August 3; data collected nearly every 30 minutes) using techniques of gas chromatography - mass spectrometry.
  At both sites, RegCM underestimates isoprene concentrations, which is consistent with RegCM underestimating concentrations of a proxy of isoprene such as formaldehyde, as shown in Section 3.1.3 in the manuscript. Sometimes, the model reproduces a delayed peak in isoprene concentrations compared to observations. Differences between observations and model output could result from multiple factors, for example:
  
  The cold and wet model bias (see Fig. 1 in the manuscript) that limits isoprene emissions;
  
  Differences between the dominant vegetation types on the field and in the model grid-cell. For example, in La Verdière, vegetation is mainly characterized by Mediterranean oak forest (more than 80% of Quercus Pubescens, which is a deciduous tree), while the RegCM grid-cell is mainly covered with needle-leaf evergreen temperate trees (36%), and C3 grass (37%), which are both low isoprene emitters, and has only a small amount of broadleaf deciduous trees (6%).
  
  Different scales: a model grid-cell spans over a surface of around 25x25 km², while station measurements have a footprint of a few hundreds of meters, depending on the terrain where the observations have been collected. For example, Ineia is located close to the seacoast leading to use a model grid-cell that is located over the sea.
  
  Based on these results, we have added the comparison between model output and in-situ measurements of isoprene concentrations in Section 3.1.3 in the revised manuscript.
  Cros, B., et al.: The ESCOMPTE Program: an overview. Atmospheric Research, 2004, 69 (3-4), pp.241-279, DOI: 10.1016/j.atmosres.2003.05.001_xffff_.
  
  2. I also wonder how the water stress effect changes the relationship between isoprene emissions and temperature. Previous studies have revealed the decrease in isoprene emission in extreme high temperature, but would the water stress effect further change the turning point of the T-emission curve? Some discussions would be useful.
  Authors’ response: This is an interesting question.
  The version of the MEGAN model (2.1) implemented in the RegCM4.7 model accounts for the effect of past temperatures on isoprene emissions (over the past ten days): the warmer the temperatures, the higher the emissions (see Fig. 4 in Guenther et al., 2006). However, this version does not account for the decrease in isoprene emissions due to extreme high temperatures.
  Extreme high temperatures often co-occur with droughts. It is, then, not trivial to separate the effect of these climate extremes on isoprene emissions. In our simulations, we observed the largest decreases in isoprene emissions in the summers 2003 and 2012 (see Table 4) when the observation-based summer temperatures are nearly 4–5 standard deviations above (warmer than) the 1970–1990 climatology, as shown by Figure S.6 in the Supplementary Material. These results and those mentioned by the Reviewer, which reveal the decrease in isoprene emissions under extreme high temperatures, suggest that isoprene emissions would be strongly reduced when heat wave and drought co-occur. In the revised manuscript, we added the following comment in Sect. 3.2:
  “The largest decreases in isoprene emissions occur in the summers 2003 and 2012 (Table 4), when the observation-based summer temperatures are nearly 4–5 standard deviations above (warmer than) the 1970–1990 climatology (Fig. S.6). These results suggest that isoprene emissions would be strongly reduced when heat wave and drought co-occur.”
  
  3. Related to point 2, I think some discussions on the extreme ozone episodes would be helpful to evaluate the effect of water stress on ozone concentration.
  Authors’ response: Extreme high temperatures often exacerbate ozone pollution and lead to extreme ozone episodes. In a second paper (in preparation), we investigated the ozone climate penalty by performing simulations under both present-day (1990–2004) and future climates (2035–2049). To assess the impact on ozone concentration of both the direct effect of high temperatures and the indirect effect of water stress on isoprene emissions, we designed two sensitivity simulations over the summer 2003 with and without the soil moisture activity factor activated. In these simulations, we artificially increased air temperature across the whole atmospheric column by a fixed amount varying with atmospheric levels. Results show that an average increase of 2.5°C in air temperature leads to an increase in surface-ozone level smaller than 2 ppbv (1%), regardless the soil moisture activity factor is activated or not. Although the paper will be submitted soon, we added the following comment in Sect. 4:
  “In a future study, we aim to explore the ozone climate penalty over the Euro-Mediterranean region under both present-day and future climates and to assess the impact on ozone concentration of both the direct effect of high temperatures and the indirect effect of water stress on isoprene emissions.”
  
  4. I feel that the model evaluation of chemical fields is rather insufficient. It only shows the mean magnitude of observed and simulated HCHO column and ozone concentrations. How well does the model capture the spatial and temporal pattern of HCHO and ozone? It might be also important to evaluate the ozone chemical regime (NOx-limited or VOCs-limited) somewhere.
  Authors’ response: We agree with the Reviewer that the model evaluation for chemical species could be extended. In our study, we focused on the analysis of differences between sensitivity simulations, therefore possible systematic biases in the model would cancel out, thus giving robust results. However, we performed some additional evaluation for both ozone and formaldehyde by comparing model output against re-analyses from the Copernicus Atmosphere Monitoring Service (CAMS) for the period 2003–2007. In addition, surface ozone concentrations have been evaluated against observations collected in La Verdière (France) in the summer 2000 (from June 21 to July 6) using an conventional UV absorption ozone analyzer (Environement S.A., Poissy, France, model O3 41M). Ozone concentrations have been measured during the ESCOMPTE field campaign, together with isoprene measurements shown in Figure R.1.
  For near surface ozone, model output are lower than CAMS re-analyses with differences between 10 and 20 ppbv over the Mediterranean Basin, with some summers and few grid-cells showing differences between 20 and 30 ppbv (Fig. R.3). The model underestimates near-surface ozone as well when compared to in-situ measurements (Fig. R.4); in particular, model output shows a smaller variability than in-situ measurements. For formaldehyde, model outputs near the surface are also lower than CAMS reanalyses with differences between -1 and -4 ppbv (Fig. R.5).
  We also assessed the ozone chemical regime using the ratio between formaldehyde (HCHO) and nitrogen dioxide (NO₂) as presented in Duncan et al. (2010). Results show that the model reproduces a VOC-limited regime over the whole domain, with a HCHO/NO₂ ratio lower than 1 (Fig. R.6). Based on this analysis, in the revised manuscript we updated all discussions about the ozone regime over the model domain.
  Duncan, B. N. et al.: Application of OMI observations to a space-based indicator of NOx and VOC controls on surface ozone formation, Atmospheric Environment, Volume 44, Issue 18, 2010, Pages 2213-2223, DOI: https://doi.org/10.1016/j.atmosenv.2010.03.010.
  
  5. The figure quality can be improved. For example, the size of the figure is often too small compared to the colorbar (Figs 11 and 12), and in some cases the label is missing (Fig.5).
  Authors’ response: We improved all figures. We added axis labels to Figure 5 (now Fig. 6), we increased the size of figures compared to the colorbars, and we implemented colorblind-friendly colormaps in Figures 6 (now Fig. 7) and 11 (now Fig. 12), as requested by the EGU journals.
  
  Citation: https://doi.org/10.5194/egusphere-2022-1522-AC3
RC2:
'Comment on egusphere-2022-1522', Anonymous Referee #1, 03 Apr 2023

This study applies a regional vegetation-climate-chemistry model to investigate the influence of water stress on isoprene emissions and surface ozone over Europe in 1992-2016. This is done by coupling the land module and biogenic emission module to derive the soil water stress function, which is then used to determine soil moisture activity factor in the parameterization of isoprene emission. Simulation results show that water stress reduces summertime isoprene emissions on average by nearly 6%, and by -20 to -60% in extreme dry summers, but influence on ozone is relatively small. This study is well-designed, easy to follow, and the results are useful for the community. It has room to be improved by addressing the following comments.
1. My major concern is that this study lacks direct evaluation with observed isoprene emissions. Does the inclusion of water stress effect improve the simulation of isoprene emissions? Does the new scheme outperform the old scheme?
2. I also wonder how the water stress effect changes the relationship between isoprene emissions and temperature. Previous studies have revealed the decrease in isoprene emission in extreme high temperature, but would the water stress effect further change the turning point of the T-emission curve? Some discussions would be useful.
3. Related to point 2, I think some discussions on the extreme ozone episodes would be helpful to evaluate the effect of water stress on ozone concentration.
4. I feel that the model evaluation of chemical fields is rather insufficient. It only shows the mean magnitude of observed and simulated HCHO column and ozone concentrations. How well does the model capture the spatial and temporal pattern of HCHO and ozone? It might be also important to evaluate the ozone chemical regime (NOx-limited or VOCs-limited) somewhere.
5. The figure quality can be improved. For example, the size of the figure is often too small compared to the colorbar (Figs 11 and 12), and in some cases the label is missing (Fig.5).

Citation: https://doi.org/10.5194/egusphere-2022-1522-RC2
- AC2: 'Reply on RC2', Susanna Strada, 30 Jul 2023
  
  Please see the attached file for the 1-on-1 response.
  
  Citation: https://doi.org/10.5194/egusphere-2022-1522-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Susanna Strada on behalf of the Authors (31 Jul 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (14 Aug 2023) by Qi Chen

AR by Susanna Strada on behalf of the Authors (24 Aug 2023) Author's response Manuscript

Post-review adjustments

AA: Author's adjustment | EA: Editor approval

AA by Susanna Strada on behalf of the Authors (13 Oct 2023) Author's adjustment Manuscript

EA: Adjustments approved (17 Oct 2023) by Qi Chen

Journal article(s) based on this preprint

23 Oct 2023

Assessment of isoprene and near-surface ozone sensitivities to water stress over the Euro-Mediterranean region

Susanna Strada, Andrea Pozzer, Graziano Giuliani, Erika Coppola, Fabien Solmon, Xiaoyan Jiang, Alex Guenther, Efstratios Bourtsoukidis, Dominique Serça, Jonathan Williams, and Filippo Giorgi

Atmos. Chem. Phys., 23, 13301–13327, https://doi.org/10.5194/acp-23-13301-2023,https://doi.org/10.5194/acp-23-13301-2023, 2023

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Susanna Strada et al.

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https://doi.org/10.5194/egusphere-2022-1522-supplement

Susanna Strada et al.

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Short summary

Water deficit modifies emissions of isoprene, an aromatic compound released by plants that influence the production of a pollutant such as surface ozone. Numerical modeling shows that, during the warmest and driest summers, isoprene decreases between −20 to −60 % over the Euro-Mediterranean region, while surface ozone only diminishes by few percents. Decreases in isoprene emissions not only happen simultaneously of dry conditions, but could also occur after prolonged or repeated water deficit.


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