the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Feb 2025
Global Perspectives on Nitrate Aerosol Dynamics: A Comprehensive Sensitivity Analysis
Abstract. In recent years, nitrate aerosols have emerged as a dominant component of atmospheric composition, surpassing sulfate aerosols in both concentration and climatic impact. However, accurately simulating nitrate aerosols remains a significant challenge for global atmospheric models due to the complexity of their formation and regional variability. This study investigates key factors influencing nitrate aerosol formation to improve simulation accuracy in highly polluted regions. Using the advanced EMAC climate and chemistry model, we assess the effects of grid resolution, emission inventories, and thermodynamic, chemical, and aerosol scavenging processes. The ISORROPIA II thermodynamic model is employed to simulate the formation of inorganic aerosols. Model predictions are compared with surface observations of particulate nitrate in PM1 and PM2.5 size fractions, including PM2.5 data from filter-based observational networks and PM1 data from aerosol mass spectrometer field campaigns across Europe, North America, East Asia, and India. Results show that the model overestimates PM2.5 nitrate concentrations, especially in East Asia, with biases up to a factor of three. Increasing grid resolution, adjusting N2O5 hydrolysis uptake coefficient, and utilizing an appropriate emission database (e.g., CMIP6) improve performance. However, these adjustments do not necessarily enhance PM1 predictions, which remain underestimated, especially in urban downwind sites. Seasonal variations and diurnal trends reveal discrepancies in model performance, especially in Europe and urban downwind locations. In Europe, model bias is driven by an unrealistically sharp decrease in nitrate aerosol levels from morning maxima to evening minima. Sensitivity tests show relatively small impact on total tropospheric nitrate burden, with variations within 25 %.
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RC1: 'Comment on egusphere-2025-313', Anonymous Referee #1, 24 Apr 2025
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Review of “Global perspectives on nitrate aerosol dynamics: a comprehensive sensitivity analysis” by Milousis et al.
Nitrate aerosols have recently emerged as a more dominant component of atmospheric composition than sulfate aerosols, yet accurately simulating them remains a significant challenge. This study employs the EMAC climate-chemistry model in combination with the ISORROPIA II thermodynamic module to investigate key factors influencing nitrate aerosol formation, particularly in highly polluted regions. The manuscript presents a comprehensive analysis of sensitivity simulations, which will be of strong interest to the atmospheric chemistry modeling community. However, before I can recommend this manuscript for publication, several issues need to be addressed to improve clarity and readability for the broader audience. These are outlined below:
L35 – “Sensitivity tests show ...”, Please clarify this sentence. What sensitivity tests do you mean? Please be more specific!
L230 – “even at low relative humidities”, Is it true for lower humidities than CRH?
L230 – “The first cast is used …”, I wonder why the authors chose this option. As far as I understand, many climate and air quality models employ a “metastable assumption” for the phase of aerosols in thermodynamic calculations.
Section 2.2 – A significant part is focused on comparing simulated PM1 and PM2.5 nitrate with the observations. Therefore, please elaborate on how the simulated PM1 and PM2.5 nitrate are calculated in the model for broader readers.
L304 – “PM2.5 accounts for more than …”, Please revise this sentence for clarity, such that “PM2.5 nitrate accounts for … 80% of the total nitrate concentrations”
L332 – A scatter plot is of particular interest for readers to understand the model performance, so it is worthwhile to be included in the main text.
L353-354 – “While the model …., the mean bias and .. be relatively unaffected”, What do you mean by this?
L431 – Section title is misleading. Please revise it to embrace the contents appropriately.
L465 – In summary, the lower resolution model simulates higher PM2.5 and PM1 nitrate than the base model. A brief summary would be highly appreciated, with a likely cause for this change, here and elsewhere in this section.
L495 – Using CMIP6 vs. HTAP in the model shows differences in simulated nitrate concentrations. I wonder what drives this change. It may be a bit difficult to examine year-by-year emissions, but authors can provide an insight into the apparent differences in simulated nitrate by looking at NOx and NH3 emissions from the two inventories.
Figures 5 and 6 – The model appears to have a smaller bias for PM1 nitrate than PM2.5 nitrate compared to the observations. Could you provide a determining factor for this?
L575 – Well, the difference could be minor in nitrate mass concentrations, but it could be very large and important in terms of aerosol optical depths because of hygroscopic growth. Could you comment on this?
L637 – Can authors make a statement about a recommended uptake coefficient of N2O5 in the model from the analysis using HYDRO results? Or at least possible causes for the different performance of the model with different values depending on regions.
L653 – I think that “SCAV” results in the underprediction of winter observation by the model.
L668 – Even in East Asia, the “SCAV” model appears to be better than the base for spring and autumn, as shown in Figure 5.
L762 – Capturing diurnal variation of nitrate aerosol can be associated with hourly emission of NOx and PBL variation, which is much more complicated issue. I would recommend that the authors omit this from the text. If they want to keep this, please include discussion of those two factors in the text.
L901 – I do not think that Table 5 is necessary because the text already discusses nitrate budgets in detail, along with Figure 9.
Citation: https://doi.org/10.5194/egusphere-2025-313-RC1
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