the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Mar 2024
Ozone deposition measurements over wheat fields in the North China Plain: variability and related factors of deposition flux and velocity
Abstract. Ozone (O3) deposition is closely related to air quality, ecosystem and climate changes. Due to the instrument and method shortage, O3 deposition was less observed and investigated in China, experiencing significantly increasing O3 exposure. Here, we conducted a comprehensive measurement of O3 deposition over the wheat canopy at a typical polluted agricultural site in the North China Plain using a newly developed relaxed eddy accumulation system. For the main wheat growing season in 2023, O3 deposition flux and velocity (Vd) averaged -0.25 ± 0.39 μg m-2 s-1 and 0.29 ± 0.33 cm s-1, respectively. Daytime Vd (0.40 ± 0.38 cm s-1) was obviously higher than in the nighttime (0.17 ± 0.26 cm s-1). The temporal changes of Vd were mainly determined by crop growth, with predominant contribution of stomatal uptake. Both daytime and nighttime Vd exhibited significant increases with decreasing relative humidity, and increasing friction velocity and soil water content, enhanced by higher leaf area index. With rapid increases of soil moisture, simultaneous and following overall increments in Vd were detected, attributed to remarkably strengthening O3 stomatal uptake under increased stomatal conductance and extended opening to the night, and more non-stomatal O3 removal at night resulted from strengthened soil NO emission at moist conditions. This study confirms the leading effects of crop growth on O3 deposition modulated by environmental conditions and the non-negligible influences of nocturnal plant activities, and emphasizes the needs for O3 deposition observation over different surfaces and accurate evaluation of O3 agricultural impacts based on deposition fluxes.
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RC1: 'Comment on egusphere-2024-643', Anonymous Referee #1, 19 Apr 2024
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General comment
This paper presents ozone flux measurements over one growing season in 2023 of winter wheat crop in North China. The flux measurement technique used here has not been applied to ozone before and the paper includes a description of the experimental set up. Additional flux measurements of momentum, CO2, H2O and sensible heat are used to interpret the ozone flux measurements and their quality. Other chemically relevant species such as NOx (NO and NO2) are also measured. The ozone flux measurements are investigated in relation to the growth stage of the wheat crop, meteorological and soil conditions or events such as precipitation and irrigation events. Different deposition pathways (stomatal and non-stomatal) are considered, but the partitioning is not estimated. Since ozone flux measurements are not as ubiquitous and standardised as e.g. CO2 flux measurements this paper makes a useful contribution and provides sufficient ancillary measurements for detailed investigation. Using a novel measurement technique that does not require fast response ozone analysers extends the variety of techniques that are available for ozone deposition studies.
The following changes should be made to improve the manuscript with respect to clarity, information content and thus value to the reader.
Specific questions/issues
1 Introduction
The introduction could benefit from revision. Content in lines 61 to 67 could be shortened as a method is described that is not used in this study. Instead extend the introduction (including benefits and disadvantages) of the relaxed REA method. Include references to recent studies, e.g. REA over crop surfaces and review of the REA method.
2 Observation and method
2.1 Site description
Add information on the characteristics and thus suitability (e.g. fetch and footprint, topography, closest sites of e.g. anthropogenic NOx emissions) of the site for ecosystem flux measurements, adding references if available. The referenced figure 1a in Zhang et al, 2022b is not sufficient for this as a scale is missing.
2.2. Relaxed eddy accumulation (REA) technique
2.2.1. Add general reference on eddy covariance technique and its variant, relaxed eddy accumulation.
2.2.2 Line 112, height of mounting (4.5 m) is provided, also add information somewhere in the text on the maximum height of the crop.
Figure 1 and section 2.2.2. More technical detail will be useful to include in Figure 1 and/or the text. Materials used should be fully described: type/model, manufacturer of e.g. particle filters, valves, pumps, air compressor. Also, length of tubing should be provided for the inlet section (entry point up to ozone analyser). The sample flows of the ozone analysers will provide salient information. Residence time from system inlet to 3-way valves are given as 18 ms, however the residence time within the updraft/downdraft tube sections is also relevant to specify. Is this residence time constant or depending on the up/downdraft switch frequency? Total residence time from tip of the inlet to point of ozone detection is important as ozone is a reactive species and in a polluted environment such as this study, the sampled air will also include ozone reactants (e.g. up to 4 ppb NO were measured, see Fig S5). This potential effect should be considered and quantified as a measurement uncertainty. Please add information on the zero air, is it only ozone free air, or is it also scrubbed for other trace species (e.g. NOx), as well as possible drying. If the air is dry, it would be useful to include observations on whether the switching between ambient humid air and zero dry air causes effects on the ozone concentration measurements.
On the comparison of the channels by simultaneous measurements and the calibrations: Since two separate analysers are used and concentration measurements are subtracted from each other, measurement uncertainties from each analyser have to be considered and estimated. The Thermo Fisher 49i analysers are expected to be very stable in their sensitivity, so the monthly multi-point calibration is commendable. Nevertheless, the stability of the two instruments against each other is of critical importance for the quality of the calculated fluxes, therefore I would suggest adding this information on sensitivities and offsets from the multipoint calibrations in the supplementary material. Information on uncertainties in sensitivity should be added so that Figure S2 (and the slope of 1.02 and offset of 1.05) can be evaluated as to whether they are within the measurement uncertainty (2 sigma) or not. Does Figure S2 show 1-minute or 30-minute averaged data? Over what time frame and at what frequency where these comparison measurements made?
2.3. Field measurements and ancillary data
NOx measurements: add more information on the location/distance/height of the NOx inlet with respect to the ozone measurements. No plot in the manuscript shows a direct comparison (timeseries) of both ozone and NOx (NO and NO2 speciated) for the campaign time period, however this could orient the reader as to the nature of pollution regime at the site. Consider adding such information in the supplementary material.
3 Results and discussion
3.1 Meteorological conditions
Is information available from the farmers when irrigation events occurred and also how they compare to the recorded precipitation events (volume water applied)? Figure 2 is incomplete in that respect and the reader can only speculate as to how much irrigation water was applied to achieve the steep increases in soil VWC.
3.2 O3 flux, deposition and concentration
Figure 3. see comment above on the additional information of NOx. Relevance here, ozone concentrations show a considerable range from 0 to 280 ug/m3.
Please provide an interpretation of the observed ozone emission flux on 15 March 2023.
Line 220. Consider the word “uncertainties” of the reported mean values. Are uncertainties or observed variability around the mean reported in the studies mentioned?
Figure 4. Considering that the time period February to June 2023 shows changes in time in ozone concentration and also meteorological and soil variables (Fig. 2), is the inference robust, i.e. do the median values show the same pattern? Since H2O and CO2 flux measurements were also made, the statement that stomatal opening is driving the ozone deposition could be supported if the CO2 and H2O flux variation is consistent with Figure 4. The partitioning into stomatal and non-stomatal ozone deposition could be attempted (see e.g. Fares, S., Matteucci, G., Scarascia Mugnozza, G., Morani, A., Calfapietra, C., Salvatori, E., Fusaro, L., Manes, F., Loreto, F., 2013b. Testing of models of stomatal ozone fluxes with field measurements in a mixed Mediterranean forest. Atmos. Environ. 67, 242–251.)
3.4 O3 deposition relation to environmental factors
Results from the stepwise MLR model are described for each growth stage (page 12), but elaboration on possible reasons (physical, physiological, chemical) for the factors and differences between the stages could be helpful to understand processes driving such relationships.
Line 290-295. Can any further information from the literature be provided on stomatal response in wheat and effects related to humidity (not the already quoted Zhu et al., 2014; Zhu et al., 2015 in line 288)?
Line 355-370. As suggested above, it would enhance the paper if stomatal conductance (stomatal ozone flux) is estimated, which would strengthen the evidence for statements made on relationships and dependencies. Partitioning/estimating also non-stomatal ozone deposition would allow to address and provide possible support on NO soil emission, as conjectured here.
4 Conclusions and implications
Make changes to overall conclusions in accordance with revisions in the main text.
Line 414. Assessment of deposition parameterisation schemes have so far not been content of this manuscript, therefore the statement on possible large errors in simulation results does not follow from the analysis presented in this study. Some of the statements in this and final paragraph are rather vague or expressed in too general a way. Consider revising this and the final paragraph to more directly follow from the results shown here.
Data availability. Please refer to the ACP data policy to make data available through a publicly available depository or else state reason (to journal editor) for not being able to make data publicly available.
Supplemental material
Table S1. This table is by no means comprehensive, a good review of different measurements can be found in Clifton et al. (2020), considering referring to this review here, too. This table S1 however is useful as values of vd are listed for direct comparison, consider changing title to “Summary of selected O3 deposition velocity (…)”
Figure S4. As night time ozone can be seen to reach zero, the importance of chemical reactions needs to be considered. Adding NOx (NO and NO2) data here could provide additional information.
Technical corrections
Abstract. Revise the first two sentences, they are vague, making not clear how ozone is related to air quality, ecosystems and climate change, and who/what is experiencing increased ozone exposure.
1 Introduction.
Line 30, delete “etc.” as it is too unspecific.
Line 34, consider rephrasing second part of the sentence “and in the budget of tropospheric O3” including a verb.
Line 36, Insert “plant” before “uptake” and substitute “higher” with “large”
Line 37, Substitute “hazardous” with “deleterious”
Line 40/41, sentence on ozone deposition and its contribution to the tropospheric ozone budget should be merged with similar sentence in line 34/35
Line 44, consider rewording, unclear what is exactly meant by “ecological environment”
Line 49, delete “the” before “environmental factors”
Line 53, providing a decimal place “31.2%” seems over-precise in this context
Line 55, consider adding a reference or two, e.g. Altimir et al., 2006; Clifton et al., 2020
Line 72, change “wildly” to “widely”
2.2. Relaxed eddy accumulation (REA) technique.
Line 106, add year of measurements, i.e. “February to June 2023”
3.1 Meteorological conditions
Figure 2. the label “u*” on the second panel seems cut off at the bottom
Supplemental material.
Table S1. Delete “recent years” as measurements/references are from 90s or early 2000s
Citation: https://doi.org/10.5194/egusphere-2024-643-RC1
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