3.2. Temporal analysis of SSA, D(λ) and RF during 2004-2016 period at NNTK-1
Figure 2 shows SSA values for both spectral ranges (Bbr and Ecr) for the pixel corresponding to NNTK-1. After filtering, 312 days (20% of the study period) with snow and clear-sky conditions were found. SSA exhibits a similar behavior for Bbr (
Figure 2a) and the Ecr (
Figure 2b), with minimum values during May that increase as the austral winter progresses. The distribution of values until the third quartile is similar for both spectral ranges, ranging between 0.12-0.35 in May, 0.25-0.67 in June, 0.50-0.70 in July, and 0.55-0.75 in August. Monthly mean values (shown as white boxes in
Figure 2) are similar, with a maximum variation of 0.02 for the month of May.
To quantify the influence of LAPs on the radiative forcing, we need to compute D (Equation 5) for both spectral ranges. D values are obtained using
SSAfresh simulated by SNICAR during the austral winter between 2004 and 2016 considering clear-sky and the other conditions outlined in
Section 2.3. We obtained daily mean values of 0.863±0.007 for the Bbr range and 0.933±0.007 for the Ecr (see
Figure 3). .
Figure 3 shows estimates of D for the entire study period. For each month, all DEcr values (
Figure 3b) are higher than DBbr (
Figure 3a), including those below quartile 3 and the monthly mean values. D
Ecr values within quartile 1 and 3 (above q1 and below q3) are approximately 0.07 units higher for the month of July, whereas for the other months (May, June, and August) values in the Ecr range are 0.04 units higher than Bbr. The monthly mean values of D (white boxes) in the Ecr are larger by 0.05 units for May and August (during 2004 to 2016 period) compared to those for Bbr. The monthly mean D
Ecr values were also higher than D
Bbr for June and July, with differences of 0.07 and 0.08 units respectively. We note that the differences between D
Ecr and D
Bbr increase if the analysis is made considering the daily mean values. The values of D for the Ecr were generally higher (as much as 0.25 units) than in the Bbr, with the exception of only 60 days within the 2004-2016 period (
Figure 4).
Figure 5 shows results of DIRF for the Bbr and Ecr, which are consistent with our previous results of SSA and D. DIRF reaches the higher monthly mean values (white boxes) in May, with 8.02 and 0.27 MJ m
-2 d
-1 for the Bbr and Ecr, respectively, and decreasing to 2.82 (Bbr) and 0.12 MJ m
-2 d
-1 (Ecr) in July. This scenario follows the trend observed for D between May and July (
Figure 3), however, monthly mean values of DIRF in August increased to 3.58 (Bbr) and 0.13 (Ecr) MJ m
-2 d
-1, whereas D continued decreasing. This difference is explained by the fact that ISR values reach minimum values during June and July, and then starts to increase (
Figure A4).
In
Figure 5 DIRF values reaching 13.21 MJ m-2 d-1 in the Bbr and 0.42 MJ m-2 d-1 in the Ecr. DIRF
Ecr values are within the range reported by Rowe et al. 2019 at the same location. We note that although these authors had used a different spectral range to analyze BC effects in snow (650-700nm), the DIRF values are of the same order of magnitude (0.1-2.3 MJ m
-2 d
-1). We found a maximum Instantaneous RF in the Bbr of 571.54 W m
-2 (observed on August 29th, 2004) which is attributed to the melting of most of the snow at the corresponding pixel (snow cover value of 0.23). The instantaneous radiative forcing found in other regions around the world such as the Himalaya or across the United States, using the 350-850 nm wavelength range, are comparable with the values obtained here for the Bbr (e.g., [
28]).
To compare the DIRF values between the Ecr and Bbr spectral ranges, which have different magnitudes of received radiation (3% and 99%, as shown in
Section 2.3.2), we normalize the values with respect to Bbr. This normalization allows for the determination of the percentage impact on Ecr in terms of the DIRF (DIRF
Ecr/Bcr), which can then be compared to the normalized radiation received in the Ecr range (ISR
Ecr/Bbr). From the monthly mean values in
Figure 4, the normalized DIRF
Ecr/Bcr values are 3.42%, 3.74%, 4.22%, and 3.74% for the months of May to August, respectively. Similarly, the normalized ISR
Ecr/Bbr values are 3.17%, 3.22%, 3.23% and 3.17% for the same month, respectively. As observed, there is a difference between the ISR
Ecr/Bb and DIRF
Ecr/Bcr ratios for each of the months evaluated, with higher ratios observed for DIRF
Ecr/Bcr. This suggests that the variations in SSA in the Ecr are higher, and that this increase may be attributed to the presence of LAPs as these particles have a greater impact on the albedo at wavelengths between 841-876 nm (Ecr) than in the range of 300-2500 nm (Bbr), regardless of the total amount of received radiation [
15,
63,
64].
With the purpose of following daily changes in SSA, we selected consecutive days without liquid precipitations and with equal or less snow cover after the initial day to make sure that local snowfall at Portillo did not occur (see
Table 1).
Figure 6 shows the variations in spectral SSA (for MODIS bands 1-4) for the periods that satisfy the former conditions throughout our study period. As expected, we observe that SSA decreases with time (from one day to the next) for most cases, which is consistent with the aging of snow and the size increase of snow grains that occur as time progresses after the storm [
9,
65]. For some of the selected periods, such as June 28-29th 2004 (
Figure 6a), August 15-16th 2007 (
Figure 6b), July 2-th 2016 (
Figure 6g), and June 6-7th 2016 (
Figure 6m), the spectral SSA values showed an evident decrease in the Ecr (841-876 nm) and the closest MODIS band (620-670 nm), larger than in the 459-479 nm and 545-565 nm bands. However, for the majority of days shown in
Figure 6. the decrease in spectral SSA did not show much variation between bands (e.g.,
Figure 6c–e,h,l,m). Although there was one period where a slight increase in SSA was observed (
Figure 6d), these data show a systematic decrease in spectral SSA for the visible wavelengths (MODIS bands 1-4) and particularly for the Ecr, which is usually associated to the presence of BC [
19,
66]. We note that the other bands in the visible wavelengths may be associated to the presence of mineral dust [
39].
Table 1 shows daily SSA variations for MODIS bands 1,3 and 4, and for the Bbr and Ecr spectral ranges, as well as daily D and DIRF values for the Bbr and Ecr ranges. In general, we observe that differences between SSA for bands 1,3,4 and D (Ecr and Bbr) is approximately -0.3, with a larger difference observed for second day (see
Figure 6). The maximum daily rates of albedo decrease for the Bbr and Ecr were observed in August 2013 and August 2007, respectively. These reductions in albedo seem to be much higher than values reported by previous studies in the central Andes area [
32,
33,
34]. Cereceda-Balic et al. 2018 reported a reduction of 0.08 units per day in the broadband albedo (SSA
Bbr) resulting from the deposition of BC from vehicle emissions in the Portillo location. We attribute these differences to: i) differences in contamination sources associated to the periods and location considered in the studies, and ii) the fact that the remote sensing measurements consist of only one measurement, either at 10am or 2pm, whereas the in-situ study used continuous data from a radiometer [
32].
The results presented in
Table 1 also show the differences in radiative forcing (DIRF) for the two spectral ranges of interest (Ecr and Bcr) for two consecutive days. On the second day, DIRF
Bbr shows a 60% increase compared with the average radiative forcing of the evaluated days, whereas DIRF
Ecr exhibited an increase of approximately 50%. When examining DIRF
Ecr/Bbr, the radiative forcing in the Ecr band represents on average a 4.46% of the radiative forcing in the Bbr band, with values reaching a 12.21% (August 18
th, 2013). On the other hand, the incident solar radiation ratio ISR
Ecr/Bbr is on average 3.18% for the days considered in
Table 1, with values that reach 3.21% for June 6-7
th, 2016, and 3.14% for August 18
th, 2013. The above confirms previous results indicating that radiative forcing has a greater percentage impact on the narrow Ecr band than on the much broader Bcr band.