the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Nov 2024
Representing improved tropospheric ozone distribution by including lightning NOx emissions in CHIMERE
Abstract. Estimating nitrogen oxide emissions from lightning (LNOx) in models is highly uncertain, affecting the accuracy of atmospheric composition and air quality assessments. Still, it is essential to include the emissions in model to increase the realism in representing the model outcomes. LNOx emissions have recently been incorporated into the updated version of the CHIMERE model (v2023r2). In the present study, we evaluate the present state of modelling the lightning flashes and the LNOx emissions, using a classical scheme based on cloud top height (CTH) and the model CHIMERE. We asses the impact of LNOx on tropospheric ozone (O3) concentration over the northern hemisphere (NH) through a detailed evaluation of simulated tropospheric O3. The total NO emission from lightning is estimated as 8.82 Tg N yr−1 over the NH. There is an overall increase in O3 concentration due to inclusion of LNOx. The increase is highest in the mid to upper troposphere, specifically over the tropics. The comparison of the simulated O3 to measurements shows that the inclusion of LNOx emissions substantially improves the tropospheric O3 distribution, reducing bias significantly. This is particularly true for the free troposphere over the tropical region. The LNOx emissions hence critically influence the O3 concentration as well as the concentration of hydroxyl radicals (OH). There are 15 % and 40 % increases, respectively, in O3 and OH burden as observed due to the inclusion of LNOx in model, which further impact the atmospheric lifetime of trace gas methane (CH4) by reducing it by 24 %.
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Status: open (until 17 Dec 2024)
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RC1: 'Comment on egusphere-2024-3087', Anonymous Referee #1, 19 Nov 2024
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Summary:
The authors implement a cloud-height based flash rate scheme into the regional model CHIMERE and then evaluate the impact of lightning-NOx production on ambient ozone and NO2 concentrations, ozone profiles, the tropospheric OH burden, and the methane lifetime.
The magnitude of the lightning-NOx source is biased high, which has implications for the LNOx increment, OH burden, and CH4 lifetime. Thus, I think the authors need to re-run the simulation with a lower LNOx source, and then spend more time evaluating the lightning parameterization. As the LNOx scheme is not new, it would help if they modify it to better simulate the seasonal cycle. Thus, I would suggest rejecting the manuscript – at least for now.
Main Comments:
The evaluation of the flash rate scheme needs to be enhanced.
L8 & L202: 8.82 Tg N yr-1 is too high of a production rate for just the Northern Hemisphere. It is consistent with ~15 Tg N yr-1 for the globe, a value that is well outside of the typically cited 2-8 Tg N yr-1 range. You need to justify why you chose that value or choose a lower value.
L119: I don’t understand why the flash rate you obtain is so high given that you’ve scaled it to match a satellite product. What satellite data are you comparing to? What is the estimated flash rate of the satellite-based observation? What is the Northern Hemisphere flash rate after applying the scaling factor?
L210: You note that the model does not capture the seasonality of flash rates. Could you compare the monthly model flashes for 0 to 35 N with OTD/LIS climatology. It would also be interesting to compare the flash rates with the diel OTD/LIS climatology. Is this a problem with the model’s cloud top heights or with the scheme itself?
L125: Typically, the “land-based” parameterization is used for grid boxes near land as well, i.e. continental convection extends a few grid boxes downwind of land.
L132: Could you give values of Hf for 0, 30, and 60 N.
L135: What is the ratio of CG to IC flashes that you obtain over the NH with this parameterization?
L140: Include a plot showing the percent of CG and IC LNOx emissions input into each 1-km layer for land and water over the tropics and midlatitudes.
L187: By what metric is the flash rate in agreement with the ISS-LIS data set? How was the ISS-LIS May-August “climatology” obtained? What years were used? Was there a correction for viewing time? Was there a correction for detection efficiency? How many total flashes are there during May-August (and the year) for ISS-LIS and the model. How do the flash totals compare to the OTD/LIS climatology for this period.
L189: Yes, putting too many flashes in the tropics is a fairly well known bias of cloud-height based schemes. Do you have any suggestions as to how this bias could be reduced while still retaining the cloud-height scheme?
You need to add comparisons with OMI/TROPOMI and SHADOZ.
L214: The impact of lightning-NOx on atmospheric composition is larger in the mid- and upper-troposphere than it is at the surface, yet you begin with a surface comparison. I would suggest saving that comparison for last.
You compare with surface concentrations and profiles. You should also compare model NO2 with satellite-retrieved columns from OMI (January-December 2018) or TROPOMI (April – December 2018).
The SHADOZ ozonesondes are a wonderful source of profile information and should be compared to. Despite the acronym, there are a number of Northern Hemisphere sites.
It should be more revealing to calculate biases as a function of season as opposed to annually.
You need to improve the model simulation. This will add value to the comparison with observations.
L315-320: CH4 lifetime of 4.89 years is considerably less than range of 7-14. Obviously, your large LNOx source is contributing to this low bias in lifetime.
You need to be careful with generalized statements.
There are numerous reasons why a model may have a high-or-low bias in ozone at the surface. The magnitude of the LNOx source plays only a small role here.
Stratosphere-troposphere exchange is poorly resolved in this model due to its low model top. This may make comparison with mid- and high-latitude ozone profiles problematic.
Minor Comments:
L24: The range you cite here (8 to 4000 moles NO / flash) is for individual flashes. Estimates of the mean production per flash are usually between 125 and 500 moles per flash.
L73: Is 998 hPa the mean surface pressure?
L74: A 200 hPa top layer is when evaluating O3 profiles. What is the pressure of the model top? Clearly, this model was designed for tropospheric chemistry.
L160 and L201: Is this a climatological flash data set from ISS-LIS? If so, what years does it cover?
L192: Are you saying that the modeled tropopause height is too high in the tropics? If yes, please show.
L200: Why are you comparing May-August flash rates with the annual mean?
L207: You need to be clear here. 8.8 Tg N yr-1 for the NH is consistent with ~15 Tg N yr-1 for the globe, a value that is well outside of the 2-8 range but below 25.
L214: Why do you think emissions are underestimated?
L265: Do any of these studies give the NH burden? Or do they just list the total burden?
L269: Are you comparing to climatological ozone sondes or ozone sondes from 2018? You should consider comparing to individual SHADOZ sondes as several of the sites are located in NH.
L279: Stratosphere-troposphere exchange is one cause for the very low biases you are seeing in the extratropics and at high latitudes.
Grammatical Comments:
L19: such as lightning --- such as lightning and soil-NOx emissions
L24: 4000 mol NO per flash --- 4000 mol NO per individual flash
L25: suggest a value of 250 --- suggest a mean value of 250
L42: most frequently used CTH scheme --- the frequently used CTH scheme
L75: for chemical mechanisms --- for the chemical mechanism
L91: deep convection fluxes --- deep convective fluxes
L160: … but ISS-LIS only sees a small subset of total flashes …
L197: inconsistancies --- inconsistencies.
L223: represents a spatial variation --- varies spatially
L225: rest part of NH --- rest of the NH
L233: 10-25 ppbv enhancement of annual mean surface ozone due to lightning –
Figure 3d: 500-200 hPa or 500-300 hPa. Caption is not consistent with figure.
Citation: https://doi.org/10.5194/egusphere-2024-3087-RC1
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