4.1. Analysis of the WRF skill to simulate the wind field
Figures 5-9 present the frequency distribution diagrams of the wind data extracted from the simulations with the WRF model and from the data measured in the respective surface meteorological stations.
At the Galeão station, the most frequently observed wind directions were East and Southeast (Figure 5), associated with the afternoon-occurring sea breeze and bay breeze, as highlighted in Pimentel et al. (2014a). The results obtained with the WRF indicate that the model represents the prevailing wind directions at a frequency like those observed at the Galeão station (Figure 5). This result is above expectations since the representation of the wind at the SBGL station is difficult because it is located on an island within Guanabara Bay (Figure 2), where the breeze acts in different directions in a short distance, following the irregular shoreline. Regarding the wind speed, the observed winds in all directions are more intense than the simulated, (Figure 5), in general, and this difference in intensity occurs in the order of one class, that is, approximately 1.5 m.s−1, which is excellent considering the scales of uncertainties proposed by Clifford (2011) (Table 2). For calm winds, the model results represent a percentage (4.86%) close to that observed (6.96%).
At the Campo dos Afonsos station (Figure 6), there are mainly observed winds from the south, related to the sea breeze which enters over the continent and is channeled by the Tijuca and Pedra Branca massifs (Pimentel et al. 2014a). The incidence of light winds with around 14% of calm winds, which usually occur during the night, early morning, and morning periods (Pimentel et al. 2014a), can also be highlighted. The results obtained with the WRF demonstrate that the model was able to reproduce wind patterns like those registered at the SBAF station (Figure 6). It is possible to recognize the ability of the model in the reproduction of the variability of wind directions, indicating its sensibility in simulating the coupling of processes in several scales: micro, mesoscale, and synoptic, which result in the wind regime in this region. It is also noteworthy that the frequency of the prevailing winds, the winds from the south, are like those recorded at the station, around 20% (Figure 6). Likewise, the simulated frequency of the calm winds indicates the optimal accuracy of the model (Figure 6). Regarding the wind speed, subjectively, it can be said that the results are consistent with the observed data indicating excellent performance according to criteria given by Clifford (2011) and thus being better than the results modeled for the SBGL station.
According to Pimentel et al. (2014a) the Santos Dumont (SBRJ) station (Figure 7), situated at the entrance of the Guanabara bay, suffers a strong influence from the land and sea breezes regime, where the directions South (sea breeze) and North (land breeze) are predominant. However, since the observational data available for this study were registered only during the period of 6 a.m. to 11 p.m. local time, it was not possible to observe the action of the land breeze (North winds) in the same proportion as the sea breeze (Figure 7). In the simulated results, the predominant winds are southerly winds as well as the observed data, but 10% less often (Figure 7). This difference between the observed and modeled frequency is balanced by the southeast winds, which are modeled at a frequency of approximately 10% higher than those observed (Figure 7). From this, it is assumed that the model can represent the sea breeze phenomenon at the same frequency as that observed. It is noteworthy that the second most frequent phenomenon at the station was also modeled by WRF, that is, the land breeze associated with the north winds (Figure 7). Still, about this site, the intensity of the most frequent winds varied between 3.6 m.s−1 and 5.7 m.s−1 for both observed and modeled results (Figure 7). As well as the results obtained for SBGL and SBAF, the performance of WRF to model periods of calm winds on the SBRJ site is excellent, with only about a 1% difference between those registered at the station.
For Jacarepaguá station, which is located to the South of the Campo dos Afonsos station and at 2 kilometers away from the shoreline, meteorological observations were realized only between 6 a.m. and 10 p.m. local time. In this station, it is possible to observe mainly the South direction, related to the sea breeze, and North direction, related to the land breeze (Figure 8). The occurrence of low-speed winds and the high percent of calm winds (10.15%) in the period of the study should be highlighted (Figure 8). Although it does not present the highest frequency specifically for the southern winds, it appears that the model represents the phenomenon of the sea breeze at a frequency like that observed, if assuming the southeast winds as one of the possible consequent directions of this phenomenon. It should also be noted that the northern winds associated with the land breeze are represented by the model at a frequency like the observation (Figure 8). Note that the WRF model simulates more intense winds than those observed, as well as verified for the SBGL station, and this average difference in intensity is about 1.5 m.s−1. These are excellent results according to the criteria of Clifford (2011). However, it is noted that the model does not present good accuracy in representing calm winds for this site, i.e., 3.9% modeled against 10.15% observed, the worst result among all sites.
The data observed at the Santa Cruz (SBSC) station, located on the West portion of the RJMR, near the Sepetiba bay, is presented in Figure 9. It can be noted that Southwest and Northeast winds are predominant due to the action of the land-sea and bay breeze circulations due to the presence of the Sepetiba bay and the Atlantic Ocean, besides the 8% of registered calm winds (Figure 9). Regarding the simulations, it is highlighted that the results demonstrate that the model could represent the phenomenon of the bay breeze associated with southwest winds. However, the present simulated results did not represent the wind direction associated with land breezes (northeastern winds) as good as the bay breeze (southwest winds) (Figure 9). The adequate simulation of the bay breeze represents a gain compared to the results from Paiva et al. (2014), who mentioned that the model fails to detect when the wind turns completely from the Southwest direction, even using numerical grids with higher horizontal resolution (300 m).
On the other hand, the high frequency of the north winds simulated by the WRF may be a model response to the present phenomenon since the meridional component of the wind has an important weight in the composition of the northeast winds. Other possibilities raised here are an excessive weight given by the model to the flow of synoptic-scale that produces winds from the north in this region, or even a flow-induced by mountain breezes to the north of this region (Aragão et al. 2017). Regarding the wind speed, both observed and modeled indicate that there are two most frequent classes, 2.1 to 3.6 m.s−1 and 3.6 to 5.7 m.s−1(Figure 9). It is also verified that the model represents only about 50% of the cases of calms observed in the SBSC station.
In Table 3, the statistical indexes obtained for wind direction and speed are presented. As a most notable result, the bias index indicates a systematic trend of the model for all evaluated sites: a counterclockwise deviation of -4° to -12° about the wind direction observations. This trend of deviation in the wind direction agrees with the differences showed in the wind roses, where the model results always tend to deviate counterclockwise from observations, as demonstrated in the results for SBRJ, SBJR, and SBSC stations (Figures 7-9), that is, the three stations most influenced by the land-sea breeze. This may be evidence that the model has difficulties in representing mesoscale systems such as breezes, which in turn, maybe due to the excess weight given to the synoptic forces, or even the poor representation of the land-sea temperature gradient, which was remarked by Dragaud et al., (2019) for the coastal region of Rio de Janeiro state. Considering the bias and RMSE indexes the wind speed results are better in general than those demonstrated in previous literature by Hanna and Yang (2001), Zhang et al. (2011), Paiva et al. (2014), and Yver (2013), even considering the FDDA technique applied in Hanna & Yang (2001) and the RJMR ultra-high-resolution surface databases used by Paiva et al. (2014). The values for bias evidence a slight overestimation of the simulated winds. According to Jiménez and Dudhia (2012b), this pattern of stronger simulated winds is frequently located over plains and valleys in complex terrain regions. Based on the criteria established by Clifford (2011) (Table 2), it can be verified in Table 3 that during the five months where the WRF model skill was evaluated its simulated results were considered excellent. The only exception was the RMSE index for SBSC (Table 3), considered a good result according to Clifford (2011). Since the results obtained through the WRF model demonstrate satisfactory performance and even superior to studies with ultra-high resolution and FDDA, it is assumed that the uncertainties from the atmospheric model are acceptable for air quality modeling.
Table 3
Statistical indexes for surface winds simulated with the WRF.
Stations
|
Wind speed (m.s−1)
|
Wind direction (°)
|
bias
|
MAE
|
RMSE
|
bias
|
MAE
|
RMSE
|
SBGL
|
-1.00
|
1.46
|
1.85
|
-10.27
|
56.52
|
73.77
|
SBRJ
|
0.25
|
1.36
|
1.80
|
-4.32
|
52.01
|
71.65
|
SBAF
|
0.07
|
1.15
|
1.51
|
-7.42
|
67.40
|
85.08
|
SBJR
|
0.96
|
1.34
|
1.69
|
-12.57
|
53.74
|
73.93
|
SBSC
|
0.32
|
1.61
|
2.10
|
-12.47
|
60.79
|
80.09
|
4.2. Evaluation of the SO2 hourly average concentrations results by using CONFIG1, CONFIG2, and CONFIG3
In Table 4, the statistical results for the simulated concentrations with CONFIG1, CONFIG2, and CONFIG3, at each AQ monitoring stations are shown. The analysis related to the NMSE index demonstrates that the CONFIG3 performs better in all AQ stations and the CONFIG1 presents the worst results except for the São Cristóvão station. It should also be noted that the NMSE values obtained for CONFIG3 are of the same order of magnitude or lower than those obtained by Cui et al. (2011), a study developed for the region of the Gan Jiang River, in China, whose values varied between 3 and 6. Systematically, all values for the bias index are positive, with CONFIG2 results being the worst of the three settings strategies. Although the bias indicates that the simulations overestimated the concentrations observed, the FOEX index demonstrates that the number of overestimated events (hours) is not always more frequent than the underestimated events. In this case, the results obtained for the Jardim Primavera station stand out, in which the underestimated events for CONFIG1 and CONFIG3 occur in 69% and 65% of the cases. In addition to this station, CONFIG1 also underestimated the concentrations observed at São Cristóvão station in 55% of cases. Still discussing FOEX, the results indicate that CONFIG3 presents the best balance between the number of underestimated and overestimated events.
Regarding the FA2 and FA5 indexes, in general, the results presented for CONFIG3 are also better, while the results for CONFIG2 are the worst. Despite important recent advances in atmospheric models, it is noted that the isolated use of simulated results in AQ models may not be the best strategy. The values of FA2 and FA5 attained through CONFIG3 suggest a better representation of the simulated pollutant dispersion than those obtained by Cui et al. (2011). Its maximum values were 27% and 61% for FA2 and FA5, respectively, which reinforces the model’s inclination to better results with this setting strategy. The standard deviation analysis indicates that, for all air quality monitoring stations, the magnitude of the simulated index is higher than that observed with the air quality monitoring data, with greater discrepancy on the Jardim Primavera and São Bento stations.
The results for the R index (Table 4) and the daily cycle of SO2 (Fig. 10) demonstrate that the simulations are outdated in relation to that observed for all air quality monitoring stations, and there is a significant overestimation of the SO2 concentration for the stations of São Cristóvão, São Bento and Jardim Primavera. It is not surprising the R index values close to 0, since the present evaluation is audacious in the sense of trying to model a highly urban environment with infinite casualties, unlike well-controlled experiments. However, the decision to present the correlation result is to encourage future studies and more investments in the data used in non-steady-state dispersion models. To be successful in modeling the variability of pollutant concentrations in the atmosphere, two main factors must be well represented: (1) emissions and, (2) advective and turbulent transport.
The first and perhaps the most important factor that impacts the correlation index is associated with the better representation of temporal variability of the emission sources. Although SO2 vehicular emissions are small compared to fixed sources, and there is an hourly function for these emissions in the model, it is still an average representation, which does not include casual traffic jams, traffic diversions, and the difference between weekdays and weekends. Then, it’s not surprising that there are lags between the maximum and minimum values and discrepancies in the intensity between the simulated and observed hourly concentration, as shown in Figure 10. As for industrial sources, probably they are not responsible for the poor correlation found, since the oil refineries are the main sources of SO2 in RJMR and operate at constant load (INEA 2010), with exceptions for eventual maintenance.
The second factor is related to pollutant transport. Based on WRF results, the advective transport was satisfactorily represented in the present study. The turbulent transport and the atmospheric stability were not evaluated in this study, but as the representation of vehicle emissions, they can be sources of uncertainty. However, the investigation of these factors deserves further specific study using other data sources (e.g., SODAR, micrometeorological stations, etc.). Improvements in the meteorological monitoring network could also bring gains in transport representation. The use of meteorological data with higher temporal resolution would allow the simulation of the transient effects of greater frequency (< 1 h), and more meteorological stations could accurately represent the spatial inhomogeneities of the area.
Table 4
Statistical results for CALPUFF’s SO2 hourly concentrations.
AQ Stations
|
Std. dev. (µg.m−3)
|
Runs
|
Std. dev. (µg.m−3)
|
NMSE
|
Bias (µg.m−3)
|
FOEX (%)
|
FA2 (%)
|
FA5 (%)
|
R
|
Copacabana
|
2.52
|
CONFIG1
|
4.63
|
2.33
|
1.61
|
1.00
|
39.0
|
77.0
|
0.16
|
CONFIG2
|
4.72
|
2.02
|
1.86
|
3.00
|
38.0
|
76.0
|
0.17
|
CONFIG3
|
4.02
|
1.79
|
1.25
|
0.00
|
41.0
|
80.0
|
0.20
|
São Cristóvão
|
6.46
|
CONFIG1
|
8.68
|
3.19
|
1.31
|
-5.00
|
40.0
|
75.0
|
0.10
|
CONFIG2
|
19.92
|
5.49
|
13.85
|
16.00
|
33.0
|
66.0
|
0.05
|
CONFIG3
|
9.08
|
3.15
|
3.20
|
5.00
|
43.0
|
79.0
|
0.08
|
Tijuca
|
3.54
|
CONFIG1
|
5.07
|
2.35
|
0.68
|
-9.00
|
28.0
|
62.0
|
0.18
|
CONFIG2
|
5.78
|
2.09
|
2.40
|
5.00
|
33.0
|
68.0
|
0.18
|
CONFIG3
|
5.11
|
1.84
|
1.77
|
4.00
|
35.0
|
70.0
|
0.20
|
Jardim Primavera
|
3.01
|
CONFIG1
|
10.54
|
8.70
|
2.68
|
-19.00
|
33.0
|
69.0
|
0.05
|
CONFIG2
|
16.69
|
8.18
|
10.31
|
6.00
|
32.0
|
68.0
|
0.00
|
CONFIG3
|
9.38
|
5.44
|
2.21
|
-15.00
|
33.0
|
73.0
|
0.06
|
São Bento
|
5.35
|
CONFIG1
|
13.78
|
7.59
|
6.71
|
6.00
|
32.0
|
68.0
|
0.08
|
CONFIG2
|
17.23
|
6.31
|
11.35
|
13.00
|
27.0
|
57.0
|
0.11
|
CONFIG3
|
15.14
|
5.16
|
8.44
|
8.00
|
32.0
|
66.0
|
0.16
|
The bias of hourly wind speed and hourly SO2 concentrations by observed wind speed categories (Figure 11) reveals important information that can contribute to a better understanding about the model errors and consequently indicate possible paths to be taken in future studies. According to Figure 11 left, it is observed that the smallest errors in wind intensity occur between 0.5 - 5.7 m·s−1, whereas high deviations occur for the calm winds and speeds above 5.7 m·s−1. It is noteworthy that no significant differences were found between the three configurations evaluated. Note that for the wind class with the smallest bias (2.1 - 3.6 m·s−1) (Figure 11, left), the smallest deviations for SO2 concentration also occur, mainly for CONFIG1 and CONFIG3 (Figure 11, right). Despite the similar bias values regarding the wind speed among the three configurations (Figure 11, left), CONFIG2 presents the biggest discrepancies for SO2 concentration (Figure 11, right). These analyzes highlight the importance of a meteorological monitoring network representative for wind and micrometeorological conditions.
4.3 Outlook for regulatory purposes
For regulatory purposes, the air quality standards in Brazil for the sulfur dioxide pollutant are established for 24-hour (125 µg.m−3) and annual sampling times (40 µg.m−3). Thus, in this section the results are analyzed considering the comparison between the simulated and observed average concentrations of 24 h, for the entire period of analysis, aiming to establish the performance of the CAQMS for practical applications.
In general, from the perspective of the evaluation of meteorological configurations among the simulations for regulatory purposes (Table 5 and Figure 12), a similar pattern of results was obtained for average hourly concentrations (Table 4). The best performances were obtained for configurations that used observed meteorological data (CONFIG1 and CONFIG3), with a slight superiority for CONFIG3. However, it is notable that the results for 24-hour averages (Table 5) are significantly better than those presented for hourly averages (Table 4). This corroborates with the previous argument that the difficulty of representing meteorological and emissions variations on minutes to hours scales impacts the results of the simulations.
As can be seen in Table 5 and Figure 12, the simulation results overestimate the average concentrations over the entire period at all air quality monitoring stations. It is evident in Figure 12 that the simulated concentrations rarely underestimate the observed concentrations. This result is reinforced by the overall positive bias index, whose all values are positive. Another information presented that is of paramount importance for regulatory studies is the model's ability to represent the maximum concentrations. As shown in Table 5, all maximum concentrations simulated overestimated those observed, reinforcing the tendency for overestimations. These errors follow the concentration levels of the stations, that is, the higher the concentration level at the station, the greater the associated error. Despite the models showing a positive bias, the simulated concentrations did not exceed the national air quality standard, in agreement with what was observed (Table 5).
Table 5
Statistical results for CALPUFF’s daily averaged SO2 concentrations (µg.m−3).
AQ Stations
|
Mean/Max Obs. (µg.m−3)
|
Runs
|
Mean/Max Sim. (µg.m−3)
|
NMSE
|
bias (µg.m−3)
|
FOEX (%)
|
FA2 (%)
|
FA5 (%)
|
R
|
Copacabana
|
3.83 / 14.08
|
CONFIG1
|
5.41 / 17.28
|
0.76
|
1.54
|
14.0
|
58.0
|
95.0
|
0.09
|
CONFIG2
|
5.69 / 16.73
|
0.59
|
1.83
|
18.0
|
61.0
|
96.0
|
0.31
|
CONFIG3
|
5.06 / 16.89
|
0.52
|
1.21
|
11.0
|
69.0
|
97.0
|
0.30
|
São Cristóvão
|
8.36 / 25.16
|
CONFIG1
|
9.49 / 36.09
|
0.79
|
1.09
|
-2.0
|
56.0
|
90.0
|
0.03
|
CONFIG2
|
21.64 / 61.71
|
1.44
|
13.24
|
44.0
|
40.0
|
79.o
|
0.16
|
CONFIG3
|
11.27 / 37.53
|
0.72
|
2.67
|
9.0
|
64.0
|
93.0
|
0.03
|
Tijuca
|
4.45 / 14.55
|
CONFIG1
|
5.12 / 19.82
|
0.68
|
0.73
|
4.0
|
53.0
|
96.0
|
0.31
|
CONFIG2
|
6.86 / 17.74
|
0.56
|
2.46
|
26.0
|
60.0
|
93.0
|
0.41
|
CONFIG3
|
6.27 / 22.11
|
0.50
|
1.82
|
23.0
|
64.0
|
96.0
|
0.45
|
Jardim Primavera
|
6.47 / 21.39
|
CONFIG1
|
9.32 / 53.37
|
1.78
|
2.87
|
-5.0
|
47.0
|
88.0
|
0.14
|
CONFIG2
|
16.85 / 63.42
|
2.41
|
10.49
|
30.0
|
39.0
|
80.0
|
-0.03
|
CONFIG3
|
8.84 / 44.15
|
1.43
|
2.43
|
-3.0
|
47.0
|
88.0
|
0.06
|
São Bento
|
6.19 / 27.56
|
CONFIG1
|
12.84 / 55.08
|
1.78
|
6.67
|
25.0
|
47.0
|
80.0
|
0.13
|
CONFIG2
|
17.72 / 50.10
|
1.96
|
11.45
|
36.0
|
32.0
|
71.0
|
0.23
|
CONFIG3
|
14.74 / 45.12
|
1.45
|
8.45
|
33.0
|
42.0
|
81.0
|
0.36
|
Figure 13 reinforces what has already been shown in Figure 12. In the study region, SO2 levels remain predominantly below 6 µg.m-3 in 24 hours (~ 65%), that is, in the lowest frequency classes of the graph (Figure 13). As for the bias of the simulations by frequency class, in general, the model errors more towards extreme concentrations, that is, tending to zero and >12 µg m−3 (Figure 13). However, it should be noted that CONFIG3 has the smallest errors for the extremes, while CONFIG1 has the best performance for the intermediate classes. In all classes, CONFIG2 has the worst performance with errors twice as large as CONFIG1 and CONFIG3 in most concentration frequency classes.