3.2.1. Measurements Comparing Two Time Intervals on Different Nights
As a first approach to analysing the SQM data, values were compared for two time intervals, one before midnight (from 21:00 to 22:00 UT) and another one after midnight (from 2:00 to 3:00 UT). This has been done to see if the changes in lighting and those associated with the decrease in activity are reflected in any way in the values of sky brightness in different filters, taking into account that part of the ornamental lighting is turned off in the second half of the night and that the intensity of the general lighting decreases after legal midnight.
Figure 5 shows the average values of sky brightness from the IAA headquarters in the two time intervals mentioned and at the valid nights, and the average of the B-V colour index which results from subtracting the measurements obtained with the two filters in these time intervals.
We have defined two groups of data based on the pandemic’s policies in Spain: from the days of 26 January and 19–26 February; and from the days of 15, 22, 23, 28, 29, 30 April and 17–22 May. The first group corresponds to pre-lockdown sky brightness and the second to sky brightness obtained at the time of lockdown. The first thing to note is that both in the brightness measurement and in the B-V index there is a clear difference between the first hours of the night and the last hours, especially before lockdown: from 21:00 to 22:00 UT the night sky of Granada is on average 0.54 mag/arcsec2 brighter than from 2:00 to 3:00 UT. Similarly, the average value of B-V is higher from 2:00 to 3:00 UT (B-V is 1.28 after midnight and 1.11 before midnight, so a 0.17 difference).
The variation in the colour index suggests that this is a consequence of the turning off of ornamental lighting (examples are the Alhambra Palace illumination and facade illuminations of several monuments) and private lighting (aka. cars, private outdoor lighting, commercial lighting and indoor lighting), as most of the lamps used for ornamental lighting and private outdoor lighting are metal halide lamps, or have been replaced by LED that produce white or blue-white light with a significant emission in blue, so that once they are switched off the records of the photometer with B filter are significantly higher due to the lower brightness in this band (see
Figure 6). This effect has been documented before in many other cities, like Berlin or Madrid [
33,
34,
35].
This difference between night hours also occurred during the lockdown, although to a lesser extent. During lockdown, the Granada sky between 21:00 and 22:00 UT was on average 0.24 mag/arcsec2 brighter than 2:00 to 3:00 UT, while in the B-V index there was a difference of 0.12.
If we compare the days before and during the lockdown, the Granada sky between 21:00 and 22:00 UT was 0.34 mag/arcsec2 darker after its declaration; the difference was 0.04 mag/arcsec2 from 3:00 to 4:00. For the B-V index the differences were greater in the first half of the night (0.17) than in the early morning hours (0.12).
3.2.2. Evolution of Measurements during the Night. Average Nights before and during Lockdown
Instead of analysing the average values in time periods before and after midnight, the average variation between 23:00 and 4:00 (legal time: UTC+1) of the brightness of the Granada sky and its colour index can be compared between February and April and May (legal time UTC+2). The average curve for the last week of February (
Figure 3, left brown line) represents a more or less progressive darkening over the course of the night from 18.2 mag/arcsec
2 to over 18.6 at 4:00, with a steeper slope until midnight and then smoothing out. There are three steps or “jumps” in the curve: the first (and greatest) occurs at about 23:00; the second occurs at 00:00 (midnight legal time) and is less noticeable than the other two; and the third one occurs at 2:00. If we look at the B-V curve (
Figure 3, right brown line), the 2:00 step appears while the others are not so clear. It also appears on the curve for the days following the lockdown (blue line, summer time: UTC+2). We can infer that at that time some important lighting with a considerable emission in the blue band is switched off. The other steps may be related to a decrease in the intensity of public lighting, and in B-V a slight increasing can also be seen at midnight. We can interpret this as a spectral power distribution slanting more toward long wavelengths (i.e., blue light is being removed from the zenith).
The curves for the days following the lockdown (blue lines) show a greater divergence from the previous period in the early hours of the night, with higher values both in darkness (between 18.4 and 18.6 mag/arcsec2) and in B-V (close to 1.3). In this case the legal midnight step is much more pronounced. If this is due to a decrease in the intensity of street lighting, it is interesting that before the lockdown this was not so clearly seen. The behaviour of the B-V graph can give some clues: before the lockdown it starts from values between 1.1 and 1.2 without exceeding 1.2 until after midnight, while in April it stays close to 1.3 until it reaches 1.4 in the second half of the night. In the first case, there is a greater brightness in filter B, which also decreases progressively as the night progresses, producing only a jump of some importance at 2:00.
3.2.3. Evolution of Air Pollution and Sky Brightness
Both particulate PM10 and nitrogen dioxide concentrations were highest in the hours before midnight of the days prior to lockdown, while the lowest values occurred in the early morning and during lockdown (
Figure 7 and
Figure 8,
Table 1). Similarly, the Granada sky was darker at zenith during the early morning hours on days of lockdown and, conversely, brighter during the first nighttime hours prior to lockdown. The differences are less if we compare the hours after 00:00. In the case of measurements obtained with the ASTMON device the differences are less significant. Only in the B band a darkening of 0.12 mag/arcsec
2 is observed comparing the first hours of the night before and after the declaration of the lockdown.
The strongest correlations occur between the concentration of PM10 particles and the brightness of the sky SQM without added filter (
) and SQM with filter V (
), where
is the Spearman correlation index (see
Table A1 and
Table A2 in
Appendix B). Also noteworthy is the correlation between nitrogen dioxide concentration and sky brightness in the B-band (
SQMB) (
), and with the B-V colour index (
). The variables most related to the hour of the night (aka. proxy of human activity) of measurement are those corresponding to the sky brightness in all filters. This effect would dominate (higher correlation) versus air pollution (particle concentration).
In the case of ASTMON device measurements, there are also correlations between sky brightness and air pollution variables, although they are weaker than those described above. The best correlations occur between the measurements obtained in the B band and the concentrations of NO2 () and PM10 (). The brightness measurements taken from the Sierra Nevada Observatory refer to a point located 20 degrees above the horizon in the direction of the city of Granada, and from a place located 2000 m above and at a distance of 20 km. Evidently, air mass and scattering play an important role, and with a radiative transfer model these correlations could be better explained. However, for this paper we have focused on the measures obtained within the city of Granada, as they present stronger correlations, especially for PM10 particle concentration.
Figure 9 presents the measurements of PM10 particle concentration versus the sky brightness value (SQM without added filter, with filter V and with filter B). The upper left hand area of each graph (darker sky and lower particle concentration) is mostly occupied by measurements taken after 00:00 (legal time) during lockdown, to which the lower NO
2 concentration values correspond. In contrast, the lower right zone (higher particle concentration and brighter sky) corresponds to hours before midnight on days prior to the declaration of the alarm state, and which are associated with higher NO
2 concentration values. In the hours before midnight and prior to lockdown there was more traffic and activity, but it is also necessary to take into account that the concentration of nitrogen dioxide can be increased by higher levels of artificial light [
37]. It should come as no surprise that the correlation between sky brightness and particle concentration is linear. The astronomical magnitude is logarithmic and the single scattering flux is proportional to
, where
is optical thickness, and
is proportional to the particle concentration [
38,
39,
40].
The linear fitting equations for measurements of sky brightness within Granada on different filters and the PM10 particle concentration values are:
SQM (no filter) | |
SQM (filter V) | |
SQM (filter B) | |
(: sky brightness in mag/arcsec; x: PM10 particle concentration in μg/m3) |
The correlation between sky brightness and particulate air pollution is also evident by performing a multivariate analysis, including time as a third dimension. Variations in urban lighting depend mainly on the time of night, and therefore can be expected to influence the sky brightness values. On the other hand, pollution levels depend on human activity, which varies throughout the night. Thus, we have calculated a model that estimates a value of sky brightness as a function of time (as a fraction of a Julian day) and particle concentration.
Table A6,
Table A7,
Table A8 and
Table A9 (
Appendix C) show the values of the multivariate models for sky brightness with SQM without filter and SQM with B filter, before and during lockdown.
Figure 10 and
Figure 11 show the models fitting for the SQM photometer without added filter and the SQM with filter B.
The multivariate linear fitting equations for measurements of sky brightness within Granada on different filters, PM10 particle concentration values and time are:
SQM (no filter), prior to lockdown: | |
SQM (no filter), during lockdown: | |
SQM (filter B), prior to lockdown: | |
SQM (filter B), during lockdown: | |
(: sky brightness in mag/arcsec; x: PM10 particle concentration in μg/m; t: time as a fraction of a Julian day. See Table A6, Table A7, Table A8 and Table A9 (Appendix C) for errors, residuals and F-statistic) |