NH3 surface concentrations
Europe
Figure 3 shows the warm season (April–September) mean NH3 surface
concentrations in 2013 and 2014. Figure 3a, c, e, g show the modelled
concentrations from LOTOS-EUROS (which we will refer to as the “modelled
concentrations”) and concentrations that are derived from IASI in
combination with LOTOS-EUROS (which we will refer to as “IASI-derived
concentrations”). The dots represent the corresponding measurements from the
EMEP stations. Figure 3b, d, f, h show the absolute differences between the
EMEP measurements and the modelled and IASI-derived concentrations. In
general, the pattern of the EMEP measurements and the modelled and
IASI-derived concentrations matches quite well. The majority of the EMEP
measurements agree with the modelled and IASI-derived concentrations to
-0.75 to +0.75 µg m-3. The sum of the absolute differences
between the warm season mean NH3 surface concentrations in a cubic
metre from EMEP and LOTOS-EUROS was 23.0 µg in 2013 and 32.5 µg in
2014. The sum of the absolute differences between the warm season mean
NH3 surface concentrations from EMEP and IASI was slightly lower:
22.6 µg in 2013 and 28.0 µg in 2014.
Comparison of the warm season (April–September) mean NH3
surface concentrations (µg m-3) from LOTOS-EUROS and derived from
IASI and the warm season mean NH3 surface concentrations measured by
the EMEP stations in 2013 (a, b, c, d) and 2014 (e, f, g, h). The absolute
differences between the two are shown in the right figures.
Comparison of the monthly mean (a, b, e, f) and warm season
(April–September) mean (c, d, g, h) NH3 surface concentrations
measured by the EMEP stations and the corresponding NH3 surface
concentrations from LOTOS-EUROS (blue dots) and inferred from IASI (orange
dots) in 2013 (top) and 2014 (bottom).
Figure 4 shows scatterplots of the monthly mean (Fig. 4a, b, e, f) and the warm season
mean (Fig. 4c, d, g, h) NH3 surface concentrations. The x axis represents
concentrations measured by the EMEP stations. The y axis represents either
the modelled concentrations (blue) or the IASI-derived concentrations
(orange). The monthly mean modelled concentrations and the EMEP measurements
show a reasonably strong linear relationship in 2013 (r=0.71). The
correlation between the two was weaker (r=0.39) in 2014. The correlation
between the IASI-derived concentrations and the EMEP measurements was
similar in 2013 (r=0.71) and was higher in 2014 (r=0.46). The warm
season mean IASI-derived concentrations and the EMEP measurements have a
slightly stronger correlation coefficient and an improved slope compared to
the modelled concentrations.
Mean of the NH3 surface concentrations at all EMEP locations
per month (green line) and the coinciding NH3 surface concentrations
from LOTOS-EUROS (blue line) and derived from IASI (orange line) in 2013
(a) and 2014 (b). The absolute differences between EMEP and LOTOS-EUROS are
shown in blue and the absolute differences between EMEP and IASI are shown
in orange.
Figure 5 shows the mean NH3 surface concentration of all EMEP stations
per month and the corresponding modelled and IASI-derived concentrations at
the same locations. The absolute differences per month are plotted in the
same figure in blue (LOTOS-EUROS vs. EMEP) and orange (IASI-derived vs. EMEP).
All concentration time profiles show a peak value in April, resulting from
spring fertilization. The LOTOS-EUROS time profile at the EMEP locations
decreases from April to May and starts to increase towards the end of the
year. The time profile of the EMEP stations follows the same pattern from
April to June but decreases towards the end of the year. The IASI-derived
time profile shows a decreasing pattern, except in August, where there is a
small peak. The IASI-derived time profile shows a relatively better
comparison with the EMEP measurements in April and July to September in 2013
and in April and September in 2014. The sum of the absolute differences of
the mean NH3 surface concentrations in a cubic metre at all EMEP
locations between LOTOS-EUROS and EMEP amounts to 3.1 µg in 2013 and
2.5 µg in 2014. The sum of the absolute differences between IASI and
EMEP was somewhat smaller in 2013, amounting to 1.7 µg, and somewhat
higher in 2014, amounting to 3.0 µg.
In summary, the majority of the IASI-derived concentrations showed a
slightly stronger correlation with the EMEP measurements than modelled
concentrations on a monthly basis. The correlation became more pronounced on
a seasonal basis (mean of April–September).
The Netherlands
Comparison with LML measurements
Figure 6 shows the warm season (April–September) mean NH3 surface
concentrations (µg m-3) in the Netherlands in 2013 and 2014. The
corresponding LML measurements are plotted on top of the modelled and
IASI-derived concentrations. LOTOS-EUROS seems to capture the general
pattern of the LML measurements fairly well in both 2013 and 2014. The sum
of the absolute differences between the warm season mean NH3 surface
concentrations in a cubic metre from LML and LOTOS-EUROS was 47.3 µg in
2013 and 44.8 µg in 2014. The sum of the absolute differences between
the warm season mean NH3 surface concentrations from LML and IASI was
slightly lower in 2013, namely 44.9 µg, and somewhat higher in 2014,
namely 48.5 µg.
Comparison of the warm season (April–September) mean NH3
surface concentration in 2013 (a, b, c, d) and in 2014 (e, f, g, h) from
LOTOS-EUROS and derived using IASI. The corresponding warm season mean
NH3 surface concentrations measured by the LML stations are plotted
on top of the left figures. The right figures depict the differences between
the two.
Figure 7 shows scatterplots of the monthly mean NH3 surface
concentrations (µg m-3). The x axis depicts the LML measured
concentrations. The y axis depicts the corresponding modelled and
IASI-derived concentrations. The modelled concentrations and the LML
measurements show a moderate linear relationship (r=0.39 in 2013, r=0.50 in 2014). The high NH3 concentration stations (Vredepeel and
Wekerom) are underestimated by LOTOS-EUROS. The other stations are closer to
the 1:1 line and appear to match quite well. The correlation
coefficient of the IASI-derived concentrations and the LML measurements is
r=0.39 in 2013 and r=0.53 in 2014. The IASI-derived concentrations also
underestimate the high-concentration LML stations (Vredepeel and Wekerom) in
both years. The majority of the low-concentration LML stations are
overestimated by the IASI-derived concentrations in 2013 and underestimated
by the IASI-derived concentrations in 2014. In general, both high and low
LML measurements were reproduced inadequately by the IASI-derived
concentrations. The elimination of the high-concentration stations (Vredepeel
and Wekerom) does not lead to a better comparison of the LML measurements to
the IASI-derived concentrations.
Comparison of the monthly mean NH3 surface concentrations
measured by the LML stations and the corresponding LOTOS-EUROS and
IASI-derived NH3 surface concentrations during the warm season
(April–September) of 2013 (top) and 2014 (bottom). The high-concentration
stations (Vredepeel and Wekerom) are eliminated from the right figures (c, d, g, h).
Table 3 gives a month-by-month comparison of the correlation coefficient,
the slope and the intercept of the monthly mean NH3 surface
concentrations of all LML stations vs. the corresponding modelled and
IASI-derived concentrations. In 5 out of 12 months, the IASI-derived
concentrations and the LML measurements have a better correlation
coefficient and slope compared to the modelled concentrations and the
LML measurements. The modelled concentrations are consistently lower than
the LML measurements.
In short, the IASI-derived concentrations do not show a better comparability
with the LML measurements compared to the modelled concentrations.
Month-by-month comparison of the correlation coefficient (r), slope
and intercept of the monthly mean NH3 surface concentrations of the LML
stations (x axis) and the coinciding monthly mean LOTOS-EUROS and
IASI-derived NH3 surface concentrations (y axis). The arrows
denote which of the two (LOTOS-EUROS or IASI) gives the most desirable
value. The arrows are attributed to either LOTOS-EUROS or IASI based
on the following criteria: highest r, slope closest to 1, intercept closest
to 0 and smallest RMSD.
Month
LOTOS-EUROS
IASI-derived
r
Slope
Intercept
RMSD
r
Slope
Intercept
RMSD
Apr 2013
0.57
0.39 ↑
4.12
7.78 ↑
0.57
0.36
0.01 ↑
10.80
May 2013
0.49 ↑
0.19 ↑
2.16 ↑
7.53
-0.21
-0.30
9.61
7.20 ↑
Jun 2013
0.38
0.19
1.73 ↑
8.58
0.44 ↑
0.45 ↑
1.74
6.80 ↑
Jul 2013
0.36
0.18
3.31 ↑
11.67
0.46 ↑
0.34 ↑
3.74
10.00 ↑
Aug 2013
0.49
0.23
3.82
10.10
0.86 ↑
0.35 ↑
3.63 ↑
7.93 ↑
LML
Sep 2013
0.27 ↑
0.33
4.28
5.79 ↑
0.04
0.65 ↑
0.38 ↑
7.31
Apr 2014
0.69 ↑
0.56 ↑
4.36
5.81 ↑
0.21
0.46
0.44 ↑
10.32
May 2014
0.39
0.29
1.90 ↑
6.35
0.76 ↑
0.72 ↑
-2.79
6.15 ↑
Jun 2014
0.63
0.20
2.31
9.65
0.85 ↑
0.66 ↑
-0.99 ↑
6.60 ↑
Jul 2014
0.70 ↑
0.19
2.27
10.53
0.68
0.29 ↑
1.22 ↑
10.19 ↑
Aug 2014
0.68 ↑
0.47 ↑
0.75
4.97 ↑
0.46
0.31
0.69 ↑
6.50
Sep 2014
0.55 ↑
0.33 ↑
4.84
8.20 ↑
0.04
0.27
1.49 ↑
11.59
Comparison with MAN measurements
Figure 8 shows the warm season mean NH3 surface concentrations in the
Netherlands in 2013 and 2014. The dots represent the corresponding MAN
measurements. The patterns of the MAN measurements are captured quite well
by the modelled concentrations, with low NH3 surface concentrations
near the coast and increasing values towards the east of the Netherlands.
The sum of the absolute differences between the warm season mean NH3
surface concentrations in a cubic metre from MAN and LOTOS-EUROS was
444.7 µg in 2013 and 494.3 µg in 2014. The sum of the absolute
differences between the warm season mean NH3 surface concentrations
from MAN and IASI was slightly higher in both years, amounting to
512.1 µg in 2013 and 513.6 µg in 2014.
Comparison of the warm season (April–September) mean NH3
surface concentration in 2013 (a, b, c, d) and in 2014 (e, f, g, h) from
LOTOS-EUROS and derived using IASI. The corresponding warm season mean
NH3 surface concentrations measured by the MAN stations are plotted
on top of the left figures. The right figures depict the differences between
the two.
Comparison of the monthly mean (a, b, e, f) and warm season
(April–September) mean (c, d, g, h) NH3 surface concentrations measured by
the MAN stations and the corresponding NH3 surface concentrations from
LOTOS-EUROS (blue dots) and inferred from IASI (orange dots) in 2013 (top)
and 2014 (bottom).
Mean of the NH3 surface concentrations at all MAN locations
per month (green line) and the coinciding NH3 surface concentrations
from LOTOS-EUROS (blue line) and IASI (orange line) in 2013 (a) and 2014
(b). The absolute differences between MAN and LOTOS-EUROS are shown in blue
and the absolute differences between MAN and IASI are shown in orange.
Figure 9 shows scatterplots of the monthly mean (Fig. 9a, b, e, f) and warm season mean
(Fig. 9c, d, g, h) NH3 surface concentrations. The x axis depicts the MAN
measurements. The y axis depicts the corresponding modelled or IASI-derived
concentrations. The modelled concentrations and the MAN measurements show a
moderate positive linear relationship (r=0.5 in 2013, r=0.46 in
2014). The correlation of the IASI-derived concentrations and the MAN
measurements is somewhat weaker in both years (r=0.40 in 2013, r=0.38 in 2014). The IASI-derived concentrations and the MAN measurements show
a similar to slightly stronger correlation (r=0.59 in 2013, r=0.54
in 2014) compared to the modelled concentrations and the MAN measurements
for the warm season (r=0.54 in 2013, r=0.54 in 2014).
Figure 10 shows the mean NH3 surface concentration of all MAN stations
per month and the corresponding modelled and IASI-derived concentrations at
the same locations. The absolute differences per month are plotted in blue
(LOTOS-EUROS vs. MAN) and orange (IASI-derived vs. MAN). The mean of all MAN
stations peaks in April in both years. In 2013, the mean of all MAN stations
increases from May on, peaks in July and then decreases towards the end
of the year. In 2014, there is an additional peak in July, followed by
another decrease.
The sum of the absolute differences of the mean NH3 surface
concentrations in a cubic metre at all MAN locations between LOTOS-EUROS and
MAN amounts to 7.2 µg in 2013 and 10.9 µg in 2014. The sum of the
absolute differences between IASI and MAN was somewhat larger in 2013,
amounting to 7.9 µg, but considerably smaller in 2014, amounting to 6.0 µg.
Month-by-month comparison of the correlation coefficient (r), slope
and intercept of the monthly mean NH3 surface concentrations of the MAN
stations (x axis) and the coinciding monthly mean LOTOS-EUROS and
IASI-derived NH3 surface concentrations (y axis). The arrows
denote which of the two (LOTOS-EUROS or IASI) gives the most desirable
values. The arrows are attributed to either LOTOS-EUROS or IASI based
on the following criteria: highest r, slope closest to 1, intercept closest
to 0 and smallest RMSD.
Month
LOTOS-EUROS
IASI-derived
r
Slope
Intercept
RMSD
r
Slope
Intercept
RMSD
Apr 2013
0.53 ↑
1.48
-1.41
4.33
0.46
1.05 ↑
-1.08 ↑
3.37 ↑
May 2013
0.48 ↑
0.92 ↑
0.30
1.95 ↑
0.44
1.69
0.04 ↑
3.94
Jun 2013
0.59
0.70 ↑
-0.06 ↑
2.66 ↑
0.59
1.42
-1.19
3.23
Jul 2013
0.48 ↑
0.71
0.94
3.32 ↑
0.44
1.15 ↑
-0.06 ↑
4.18
Aug 2013
0.49
0.89 ↑
1.67
3.37 ↑
0.49
1.15
1.11 ↑
4.03
MAN
Sep 2013
0.40 ↑
1.45 ↑
0.15 ↑
3.47 ↑
0.25
3.05
-7.48
6.09
Apr 2014
0.52 ↑
1.75
-2.80
5.66
0.35
0.98 ↑
-2.03 ↑
4.24 ↑
May 2014
0.39
0.80
-0.10 ↑
2.78 ↑
0.46 ↑
1.08 ↑
-2.12
3.17
Jun 2014
0.70
0.87 ↑
0.12 ↑
2.08 ↑
0.71 ↑
1.41
-1.44
2.74
Jul 2014
0.56
0.76
0.18 ↑
2.74 ↑
0.56
1.08 ↑
-1.79
3.13
Aug 2014
0.47
1.31 ↑
-0.57 ↑
2.44 ↑
0.47
1.50
-2.09
2.58
Sep 2014
0.28 ↑
1.22 ↑
3.42 ↑
6.03 ↑
0.12
1.87
-3.73
6.23
The absolute differences between the monthly mean NH3
surface concentrations modelled in LOTOS-EUROS (blue) and derived from IASI
(orange) and the monthly mean NH3 surface concentrations measured by
the MAN stations in the warm season (April–September) in 2013 (a) and 2014
(b), grouped as function of the MAN monthly mean NH3 surface
concentrations. The black line indicates the median, the edges of the boxes
indicate the 25th and the 75th percentiles (Q1 and Q2), the
whiskers indicate the full range of the absolute differences (Q1 - 1.5*IQR
and Q3 + 1.5*IQR), and the dots indicate the outliers values that lie
outside the whiskers.
Table 4 shows the correlation coefficient, the slope and the intercept of
the MAN measurements vs. the modelled and IASI-derived concentrations for
the warm months in 2013 and 2014. In 2013, the IASI-derived concentrations
show a weaker correlation with the MAN measurements than the modelled
concentrations in all months. Only in May and June in 2014, the IASI-derived
concentrations compared slightly better to the MAN measurements than the
modelled concentrations.
The data are grouped into different MAN NH3 surface concentration ranges
to test the performance of the modelled and IASI-derived concentrations as a
function of concentration level. Figure 11 shows the grouped absolute
differences between the monthly mean NH3 surface concentrations
measured by the MAN stations and the corresponding modelled (blue) and
IASI-derived (orange) concentrations. For low MAN concentration ranges
(0–10 µg m-3), the modelled concentrations agree fairly well with the MAN
measurements in both years. For higher MAN concentration ranges
(>10 µg m-3), the model seems to underestimate the
monthly mean NH3 surface concentrations. The IASI-derived
concentrations were relatively higher than the modelled concentrations for
all concentration levels in 2013. The opposite is true in 2014, where the
IASI-derived concentrations were relatively lower than the modelled
concentrations. We conclude that the differences between modelled and
IASI-derived concentrations in the Netherlands cannot be assigned to
specific concentration levels.
In summary, the comparison with the MAN measurements does also not show any
significant or consistent improvement in the IASI-derived concentrations
compared to the modelled concentrations.
The warm season (April to September) mean NH3 dry
deposition modelled in LOTOS-EUROS (a, c) and inferred from IASI (b, d) in
kg N ha-1 yr-1 in 2013 (a, b) and 2014 (c, d).
The absolute (a, b) and relative (c, d) differences in the warm
season (April to September) mean NH3 dry deposition modelled in
LOTOS-EUROS and inferred from IASI in 2013 (a, c) and 2014 (b, d).
The warm season (April to September) mean NH3 dry
deposition in the Netherlands modelled in LOTOS-EUROS (a, c) and inferred
from IASI (b, d) in kg N ha-1 yr-1 in 2013 (a, b)
and 2014 (c, d).
The absolute (a, b) and relative (c, d) differences in the warm
season (April to September) mean NH3 dry deposition in the
Netherlands modelled in LOTOS-EUROS and inferred from IASI in 2013 (a, c) and 2014 (b, d).
Summary of the comparison with in situ measurements
We compared the modelled and IASI-derived concentrations to measurements of
the European EMEP network. The IASI-derived concentrations showed in general
a slightly stronger correlation with the EMEP measurements than modelled
concentrations on a monthly basis. Moreover, the correlation became more
pronounced on a seasonal basis (mean of April–September). We then compared
the modelled and the IASI-derived concentrations to measurements of Dutch
MAN and LML networks. This comparison, on the other hand, did not show any
significant or consistent improvement in the IASI-derived concentrations
compared to the modelled concentrations.
In general, both the modelled and the IASI-derived concentrations seem to be
overestimated in emission areas. This could potentially be related to the
overpass time of the satellite. In high emission areas, the NH3
concentrations are more variable in time, and the IASI observations might
have an uncertain representativeness. Moreover, the measurements in high
emission areas are generally more uncertain with regard to their spatial
representativeness. Overall, these measurements can be more affected by
local rather than regional sources.
Generally, the modelled and the observed NH3 total columns match quite
well. This means that the LOTOS-EUROS model represents the spatial
distribution of NH3 rather well. There are some areas with large
discrepancies between the two where we see considerable deviations in the
modelled and the IASI-derived concentrations. Most of these areas, however,
cannot be validated against measurements, because of the lack of
measurements here. The changes in the comparison of the available
measurements with modelled vs. IASI-derived concentrations are therefore
relativity small. Based on the measurements we have, we conclude that we do
not see any significant improvement in the IASI-derived concentrations
compared to the modelled concentrations.
The differences between Europe and the Netherlands could be explained by the
location of the ground measurements. The majority of the European-scale
stations are located in background regions, with relatively well-mixed and
low NH3 concentrations. Most stations in the Netherlands, on the other
hand, are located in, or nearby, regions with relatively higher NH3
concentrations. As a result, the vertical profile shapes in LOTOS-EUROS in
the Netherlands are more complex and variable in time, as this region is
influenced by a constantly changing combination of transport, emission and
deposition. The use of an inadequate vertical profile to derive NH3
surface concentrations from IASI could lead to an erroneous redistribution
of the total amount of measured NH3, therewith worsening the
comparability with in situ measurements. On the contrary, the vertical
profile shapes in background regions are more stable and constant in time,
and therefore more likely to be described adequately by the LOTOS-EUROS model.
Side note on validation with in situ measurements
The differences between the in situ measurement and the modelled and
IASI-derived concentrations can partially be explained by their discrepancy
in terms of spatial representation, which limits their comparability to some
extent. The footprint of the in situ measurements is relatively small and
easily influenced by local factors, whereas the model and the satellite
provide us with a mean value over a much larger area. The two high-concentration stations of the LML network in the Netherlands, Vredepeel and
Wekerom, are, for instance, influenced by nearby emission sources which cannot
be resolved by regional models at the current resolution.
NH3 dry deposition flux
Europe
The monthly mean dry NH3 deposition flux has been computed for the warm
season (April to September) in 2013 and 2014. Figure 12 shows the warm
season mean dry NH3 deposition flux (kg N ha-1 yr-1).
Figure 12a, c show the original, modelled flux from LOTOS-EUROS (which will be
referred to as the “modelled flux”). Figure 12b, d show the modelled
flux adjusted by the IASI satellite observations (which will be referred to
as “IASI-derived flux”). The modelled fluxes were very similar in both
years. Figure 13 shows the absolute and relative differences between the
modelled and the IASI-derived flux. In 2013, the IASI-derived fluxes were
higher than the modelled fluxes in the Netherlands and Belgium. This depicts
that the IASI-observed NH3 total columns here were higher than the
modelled total columns in LOTOS-EUROS. The IASI-derived fluxes were higher
than the modelled fluxes in other areas such as Germany and large parts of
central Europe, mainly in Poland, Belarus and Romania. In 2014, the
IASI-derived fluxes were much higher than the modelled flux in parts of
central Europe, mainly in Poland and the Czech Republic, and in parts of the
United Kingdom, for instance, Northern Ireland. In both years, the IASI-derived
fluxes were much lower than modelled fluxes in Switzerland, the Po Valley in
Italy and the northern part of Turkey. Here, the IASI-observed NH3
total columns were thus consistently lower than the modelled total columns
in LOTOS-EUROS. Inadequate emission input data could explain the differences
at these locations. Another possible cause is incorrect modelling of the
atmospheric transport and/or stability of NH3 in LOTOS-EUROS.
The Netherlands
The modelled and IASI-derived fluxes in the Netherlands are shown in
Fig. 14. Figure 14 shows that the modelled fluxes were similar in both
years, whereas the IASI-derived flux varied quite a lot. The IASI-derived
flux was higher than the modelled flux in 2013 and lower than the modelled
flux in 2014. The IASI-observed NH3 total columns in the Netherlands
were thus in general somewhat higher than the modelled NH3 columns in
2013 and somewhat lower than the modelled NH3 columns in 2014.
The median change (%) in the terrestrial NH3 dry
deposition flux in 2014 in (kg N ha-1 yr-1) from LOTOS-EUROS
(a) and IASI-derived fluxes (b), resulting from different perturbations of model inputs
of LOTOS-EUROS. The orange lines indicate the 25th and the 75th quartiles.
The change (%) in the monthly mean IASI-derived NH3 dry
deposition flux resulting from different perturbations of the LOTOS-EUROS
model.
Figure 15 depicts the absolute and relative differences between the modelled
and IASI-derived fluxes. In 2013, the main differences occurred in the central
and northernmost parts of the Netherlands, where the IASI-derived fluxes were
clearly higher than the modelled ones. Furthermore, the IASI-derived fluxes
were higher than the modelled fluxes for the largest part of the Netherlands.
In 2014, the IASI-derived fluxes were lower than the modelled fluxes for the
largest part of the Netherlands, except for the centre and the northernmost part.
The relative standard deviation (%) of the warm season mean
output of all perturbed runs and the associated dry deposition estimate
inferred from IASI in 2014. Panel (a) shows the LOTOS-EUROS NH3 total
column concentration at overpass time, (b) the LOTOS-EUROS NH3 surface
concentration, (c) the NH3 dry deposition flux in LOTOS-EUROS and
(d) the resulting IASI-derived NH3 dry deposition flux.
Interannual differences
The interannual variations of the modelled and IASI-derived flux
differences (see Figs. 13 and 15) could be related to different
meteorological conditions. The annual global climate reports from the National Oceanic and Atmosphere Administration
(NOAA) show that the mean
temperatures in Europe were higher in 2014 than in 2013, especially in
western Europe. This might have had an effect on the actual emissions and
their variability, which is only limited taken into account by the model.
The annual precipitation in both years was near average for Europe as a
whole. However, if we zoom in to a more regional scale, we see that it was
much wetter than average during the warm season in nearly all parts of the
Balkan peninsula and Turkey (NOAA, 2014,
2015). Figure 13 shows
that the largest interannual variations on a European scale occur around
the Black Sea: in Ukraine but also in the eastern parts of the Balkan
peninsula and Turkey. Some of these regions thus coincide with regions that
experienced heavy rainfall in 2014 and might have affected emission and
deposition processes which are not taken into account by the model. This
suggests that meteorological effects might indeed influence our results.
However, the examined period of two warm seasons only is too short to draw a
conclusion.
LOTOS-EUROS sensitivity study
The results of the sensitivity runs are summarized in Figs. 16, 17 and 18.
Figure 16 shows the relative changes in the warm season mean terrestrial dry
NH3 deposition flux over Europe modelled in LOTOS-EUROS (Fig. 16a) and derived
from IASI (Fig. 16b) in 2014 for different model runs. The mean LOTOS-EUROS dry
NH3 deposition over the land cells in the modelling grid in 2014 was
1.76 kg N ha-1 yr-1. The mean IASI-derived dry NH3 deposition
flux was somewhat higher, namely 2.20 kg N ha-2 yr-1.
Variations in the MACC-III NH3 emissions caused the largest changes in
the modelled flux. The smallest change was obtained by variation of the wet
deposition scavenging coefficient Gscav. The variations in the dry
deposition velocity Vd led to the biggest changes in the IASI-derived
flux. The effect appears to be amplified compared to the effect on the
modelled flux. The effect of the MACC-III NH3 emissions is damped. On
the other hand, the effect of the MACC-III NOx and SO2 emissions
is also amplified. The signs of the changes in the IASI-derived flux have
flipped because of the changes in MACC-III NH3, MACC-III NOx and
SO2 and Gscav. The modelled flux is 1:1 sensitive to
emission changes in NH3, whereas for IASI-derived flux this is much
lower. The IASI-derived flux, in turn, changes 1:1 with the Vd.
The variations in the modelled flux are a result of daily and monthly
variations in emissions. The variations in the IASI-derived flux are also a
result of these variations, but on top of this they also include an effect of the
overpass time of the satellite.
Figure 17 shows the changes (%) of monthly mean IASI-derived fluxes in
2014 resulting from the different LOTOS-EUROS sensitivity runs. Note that
the effect of the runs with changes in wet deposition through variations of
the gas scavenging coefficient for NH3 is enlarged by a factor of 10. We
see that the changes with respect to the standard LOTOS-EUROS run are in
general constant over the months. The least variation is observed for the
runs with changed Vdry values, that all resulted in a change of
∼31 % per month. The runs with adjusted MACC-III emissions
of NH3 and emissions of NOx and SO2 led to largest changes
in May and the smallest changes in September. The maximum differences between
months are 9.5 % and 5.6 %, respectively, for the runs with adjusted
NH3 and the runs with adjusted NOx and SO2 values. The runs
with changed values of Gscav for NH3 seem to be affected most by
changing weather conditions, which resulted in the relatively largest
variation per month. However, because the changes in the IASI-derived flux
are small (-2.4 % to +1.7 %), we now continue to look at yearly changes.
Figure 18 shows the relative standard deviation (%) of all eight sensitivity
runs for Europe. Figure 18d shows the relative standard
deviation of the final IASI-derived flux. The relative standard deviation
varies from ∼20 % to ∼50 % throughout
Europe. The smallest variations can be seen in the southwestern and central
parts of Europe. The highest variations, of ∼40 %–50 %, are
mainly found in long-distance transport areas with low NH3
concentrations and deposition fluxes, such as Scandinavia, and in areas with
high aerosol precursor emissions, such as the Balkans.