the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Trends and seasonal variability of ammonia across major biomes inferred from long-term series of ground-based and satellite measurements
Jonathan E. Hickman
Lieven Clarisse
Pierre-François Coheur
Martin Damme
Marcellin Adon
Véronique Yoboué
Eric Gardrat
Maria Dias Alvès
Corinne Galy-Lacaux
Abstract. Ammonia (NH3) is the most abundant alkaline component in the atmosphere. Changes in NH3 concentrations have important implications for atmospheric chemistry, air quality, and ecosystem integrity. We present a long-term ammonia (NH3) assessment in the Western and Central Africa region within the framework of the International Network to study Deposition and Atmospheric chemistry in Africa (INDAAF) program. We analyze seasonal variations and trends of NH3 concentrations and total columns densities along an African ecosystem transect spanning dry savannas in Banizoumbou, Niger and Katibougou, Mali, wet savannas in Djougou, Benin and Lamto, Côte d’Ivoire, and forests in Bomassa, Republic of Congo and Zoétélé, Cameroon. We use a 21-year record of observations (1998–2018) from INDAAF passive samplers and 11-year record of observations (2008–2018) of atmospheric vertical column densities from the Infrared Atmospheric Sounding Interferometer (IASI) to evaluate NH3 ground-based concentrations and total column densities, respectively. Annual mean ground-based NH3 concentrations are around 5.7–5.8 ppb in dry savannas, 3.5–4.7 ppb in wet savannas and 3.4–5.6 ppb in forests. These results suggest that NH3 emissions from precipitation-induced pulses and volatilization from animal excreta are important emission sources in dry savannas, and biomass burning and agricultural sources are important sources in wet savanna and forest ecosystems. NH3 total column densities clearly show that the biomass burning source is the most important source in the Lamto wet savanna ecosystem. Annual IASI NH3 total column densities are 10.1–11.0x1015 molec cm-2 in dry savanna, 16.5–21.4x1015 molec cm-2 in wet savanna and 14.3–15.1x1015 molec cm-2 in forest stations. Non-parametric statistical Mann-Kendall trend tests applied to annual data show that ground-based NH3 concentrations increase at Bomassa (+2.56 % yr-1), but decrease at Zoétélé (-2.95 % yr-1) over the 21-year period. The 11-year period of IASI NH3 total column density measurements show yearly increasing trends at Katibougou (+3.98 % yr-1) and Djougou (+2.24 % yr-1). At Zoétélé, we calculated an increasing trend of leaf area index associated to a significant anticorrelation with ground-based NH3 concentrations. Leaf area index increase could enhance deposition processes and could contribute to the decrease of ground-based NH3 concentrations.
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Status: final response (author comments only)
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RC1: 'Comment on acp-2022-793', Anonymous Referee #1, 03 Mar 2023
Summary
The authors present an assessment of satellite-based (IASI) and ground-based (INDAAF) monitoring records of NH3 across three ecosystems in Western and Central Africa. Africa has been an understudied region for NH3 emissions due to the lack of surface monitoring infrastructure. This study attempts to provide insights on NH3 emissions and sources in this region using two independent measurements. However, there are major issues that need to be addressed. The key question this study is trying to answer does not appear clear to me. For example, it is mentioned that a prior study already found biomass burning emissions drive seasonal variation in NH3 total column densities in West Africa (Line 68-70). The authors also mention that IASI and INDAAF have been compared to each other and used to study seasonal cycles and trends of NH3 in this region (Line 75-82; Line 167-169). What new information do we gain from this study? I am also concerned about the disagreement between IASI and ground-based observations in many aspects. The scientific implications of these disagreements are never discussed. How do we reconcile contradicting measurements and use them to improve long-term NH3 assessments? These are the fundamental questions the authors need to think more carefully about.
Specific comments
Title: I suggest modifying to “across major biomes in Western and Central Africa” or something similar to clarify that you are only looking at certain parts of Africa, and in fact, only six locations rather than the whole region.
Line 53: Define “soil emissions” more precisely. Soil emissions of NH3, reactive nitrogen or total nitrogen?
Line 100: Among all the INDAAF stations, is NH3 only measured at the six stations shown in Figure 1?
Line 136: The 14.3% reproducibility between duplicates is technically the precision of the measurements, not accuracy.
Line 140: Was there a reason for the higher data loss at Bomassa?
Line 155: MetOp-A retired in late 2021. Did you see any worsening in instrument performance over time? If instrument performance was not an issue, why did this study only include observations up to 2018?
Line 162: You mentioned that satellite observations are for 1°x1° around each site, then why regrid to 0.25° first? Also, how many IASI observations do you usually find near the ground stations on a monthly basis? Do you see seasonal differences in the availability of satellite observations?
Table 2:
- This is not really a fair comparison. 21 years of ground data could be very different from 11 years of satellite data. Your low correlation between the Katibougou station and IASI could be a result of this. The time series plots are fine, but if you want to compare the statistics, I suggest only looking at periods when IASI and ground stations overlapped (i.e., 2008-2018). You can add another table comparing ground data before 2008 and ground data after 2008.
- It would be more meaningful if you could label the corresponding month and year where you saw the minima/maxima.
- I noticed the minimum ground NH3 concentrations were same across all the sites (0.7 ppb), which happened to be equal to the samplers’ detection limit. Do you think these just reflect the minimum detectable level, while the real minimum concentrations of NH3 in the air may be lower?
- The same comments apply to Tables 3 and 4.
Line 217: Aren’t the dashed lines for the GFED4 emissions?
Figure 3: The three NH3 datasets (IASI, INDAAF and GFED4) have very different units, making it somewhat difficult to associate them together. I know they represent different quantities, but is there a way you can translate the units into more meaningful contexts?
Line 238: You only showed rainfall data among all meteorological parameters. I would like to see at least how temperature changes throughout the year in each of the climate zone. Can you show the monthly temperature since it is closely related to NH3 volatilization potential?
Line 255: I see the amount of rainfall is drastically higher later in the wet season, but I don’t see any evidence telling whether it is erratic or evenly distributed within each month. Do you have data to support this?
Line 270: Can you provide a more quantitative overview of biomass burning events in this region, such as the average number of fires in each month in dry savannas, wet savannas and forests?
Line 284: Again, this looks like a red flag to me if IASI didn’t correlate with ground-based concentrations at all, and no theory was proposed to explain why. The anticorrelation during the wet season (Figure 5a) was not explained either. There could be a highly localized source near the site biasing the ground measurements, or something could be off in your comparison methodology, which goes back to my earlier comment that you need to provide more information on how you mapped the IASI data to the ground sites.
Line 303: Do you mean the seasonal fluctuations are less remarkable at Djougou than Lamto?
Line 337: I found the first part of this sentence confusing: “Ground-based NH3 concentrations are high or low in both the dry and wet seasons, with no clear seasonality”. You can simply say that ground-based concentrations show no clear seasonality.
Line 357: If biomass burning is the largest contributor to NH3 in wet savannas and forests, why did IASI and INDAAF not peak in the month where you see highest GFED4 emissions at Djougou and Lamto?
Line 359: This statement seems exactly opposite of what I see on Figure 3, where IASI and ground concentrations both reach their maxima in the early wet season (May-June) and minima in the dry season (December-January).
Line 370: What are the correlations between the two NH3 datasets and GFED4 at other sites?
Line 380: I suggest saying “which is not the case at Djougou” than “which is unusual at Djougou”. One other thing to note is that canopy heights at Djougou and Lamto look very different on Figure 1. Dry deposition can be affected by both LAI and the vertical distance between canopy and instrument. The canopy looks much shorter at Djougou, with lots of vegetation being lower than the instrument. This could be another reason why NH3 emissions may be less intercepted by the canopy at Djougou.
Line 402: You mentioned temperature here but you never showed any temperature analysis.
Line 410: Are you sure the IASI trends are in units of molec cm-2 yr-1, not x10^15 molec cm-2 yr-1?
Line 460: This may explain the lack of correlation between IASI and ground data, but not necessarily the contrasting trends. If biomass burning was increasing over time (which supposedly would be captured better by IASI), the ground sites should show insignificant trends if they simply missed the fire plumes. However, since decreasing ground-based trends were seen at almost all sites, something else was happening. Perhaps it was the LAI that you mentioned, but we would need to see the season-to-season change in LAI at all sites to be sure.
Line 471: You used the word “more important” multiple times throughout the text when comparing the NH3 concentrations. The meaning of “important” can be misinterpreted by some. I suggest using descriptions such as “significantly higher” if you simply want to compare the values (e.g., ground based NH3 concentrations were significantly higher in dry savannas than wet savannas and forests).
Line 475: I’m not sure if this conclusion holds true just based on the surface concentrations. You did not actually conduct a source attribution to quantify how much NH3 is from soils, livestock versus biomass burning.
Minor comments
Line 65: I suggest changing to “~250 Mha of land area was burned”
Figure 1 caption: Remove “location” in “10 stations across Africa location”
Line 125: No need to use quotation marks around Laboratoire d’Aérologie (LAERO)
Line 163: The IASI version 3 datasets
Line 190: is limited
Line 238: when the weather is at its warmest and driest
Line 290: Should be Table 3, not Table 2
Line 395: wet and dry seasons
Line 460: so biomass burning alone could not explain
Citation: https://doi.org/10.5194/acp-2022-793-RC1 -
RC2: 'Comment on acp-2022-793', Anonymous Referee #2, 13 Apr 2023
The manuscript provides valuable insights into NH3 concentrations and trends in Africa using both ground-based observations and satellite measurements. However, the manuscript simply describes NH3 seasonality, IASI &INDAAF comparison, and NH3 trends without detailed interpretations. The goal and implications need to be clarified and further analyses need to be done to illustrate the results.
Major comments:
The introduction needs to be restructured to address the following questions:
(1) Why do we care about NH3 seasonality? Is NH3 seasonality highly uncertain?
(2) You only briefly mentioned biomass burning emissions without explaining the importance, e.g., biomass burning accounts for how much NH3 emissions in Africa? What is the impact on air quality and radiative forcing?
(3) Line 163 – 169 should be moved to the introduction.
IASI & INDAAF comparison:
(1) Is there any difference between the agreement for WS and DS? There may be fewer IASI measurements in WS, and hence worse agreement compared to DS.
(2) Figure 3, 5, and 7: add monthly temperature and number of IASI pixels.
(3) Table 2, 3, and 4: add IASI & INDAAF agreement.
(4) What are the implications of using IASI data in Africa? Can IASI represent the ground truth in all major African ecosystems?
Trend analysis:
(1) It is unfair to compare the trends for different time domains. Can you add a figure for 2008 – 2018 ground NH3 trends and comparison with IASI trends? I eyeballed Figure 4 and for Lamto the decrease from 1998 to 2008 is significant and that might have driven the long-term trends for 1998 – 2018.
(2) It would be interesting to see if the decreasing trends are slowing down or in transitions to increasing trends during the past decade.
(3) Add trend lines for Figure 2, 4, and 6.
(4) GFED only provides wildfire emissions. For the emission trends, use EDGAR or other inventories that provide total NH3 emissions.
(5) Any thoughts on the contribution from trends in meteorology and the partitioning between NH3 and NH4+?
Minor comments:
Line 41: (Bouwman et al., 2002a) delete brackets and the comma.
Line 48: PM2.5 subscript.
Line 68: wrong citation. Van Damme et al. 2018 does not focus on wildfire emissions.
Line 78: (Van Damme et al., 2021) delete brackets and the comma.
Line 79: change 2.3%.decade-1 to 2.3%·decade-1.
Line 124: (Ferm, 1991) delete brackets.
Line 175: NO2 data?
Line 333: change second largest to second largest terrestrial.
Line 388: NH3 subscript.
Line 451: change 0.56 to 0.56×1015.
Line 471: change more important to higher.
Line 740: duplicates for Whitburn et al. 2015a and 2015b.
Citation: https://doi.org/10.5194/acp-2022-793-RC2
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