Long-Term Visibility Trends in the Riyadh Megacity, Central Arabian Peninsula and Their Possible Link to Solar Activity

In this study, atmospheric visibility (AV) data from Riyadh, Saudi Arabia (24.91˚N, 46.41˚E, 760 m), for the period 1976-2011 were utilized to investigate the interannual, monthly, and seasonal AV variations and trends. The magnitudes of these trends were characterized and tested using mann-kendall (MK) rank statistics at different significance levels. No significant trend in AV was observed during the 36-year period. However, a significant increase in the annual mean AV by 0.24 km per year for the period between 1976 and 1999 was found. For the period 1999-2011, AV decreased significantly by 0.16 km per year. The potential effects of air temperature and relative humidity on AV were investigated. While these two variables could explain the observed trend of AV over some periods, they failed to do so for the whole study period. To search for extraterrestrial causes for long-term AV variations, correlation analyses between the time series of cosmic ray (CR) data (measured by NM and muon detector) and solar activity (represented by sunspot number) and AV were conducted and showed that these two variables are able to explain the AV variations for the whole study period. Additionally, power spectra analyses were conducted to investigate periodicities in the AV time series. Several significant periodicities, such as 9.8, 5.2, 2.2, 1.7, and 1.3 years were recognized. The obtained periodicities were similar to those reported by several investigators and found in solar, interplanetary, and CR parameters. The spectral and correlation results suggested that, with the expected effects of terrestrial and meteorological conditions on AV, long-term AV variations can also be related to the solar activity and associated CR modulations.


Introduction
Atmospheric visibility (AV) is a measure of the light extinction caused by atmospheric aerosols and is considered a good indicator of air pollution. Depending on the prevailing atmospheric conditions, visibility may vary widely, from a few meters at some severely polluted sites to a few hundred kilometers within nonpolluted, pristine atmospheres e.g., (Lee, 1983;Malm, et al., 1996;Lee et al., 2005;Jaswal et al., 2013). Because of the adverse effects of air pollution on human lives, visibility has been a major concern in everyday life, mainly in the aviation industry, surface traffic, and climate change studies and climatology e.g., (Deng et al., 2011;Pui et al., 2014).
AV trends and the impact of air pollution on them have attracted scientists from all over the world, and many studies have been conducted to evaluate visibility trends at the local, regional, continental, and global scale (Miller et al., 1972;Sloane, 1982;Lee, 1994;Cheng et al., 1997;Doyle & Dorling, 2002;Ghim et al., 2005;Tsai, 2005;Molnar et al., 2008;Chang et al., 2009;Zhao et al., 2011;Sabetghadam et al., 2012;Balarabe et al., 2015). These include investigating the link between atmospheric composition and visibility. These investigations demonstrate that visibility is markedly influenced by the size, chemical composition, and concentration of airborne particles (Appel et al., 1985;Seinfeld & Pandis, 1998;Baumer et al., 2008;Tiwari et al., 2011;Lee et al., 2017). Other studies have attempted to link AV variations to changes in meteorological parameters (Dayan & Levy, 2004;Lee, 1990). While temperature (T), relative humidity (RH), wind speed (ws), and wind direction (wd) do not directly affect clear-sky visibility, they can influence the sources and sinks of trace gases and aerosol particles in the atmosphere. However, the characterization of the long-term AV trend over the central region of the Arabian Peninsula has never been reported.
In this study, datasets covering the period 1976-2011 were utilized to investigate the seasonal and long-term visibility variations for the last 36 years in Riyadh, Central Arabian Peninsula.
These results provide for the first time a better understanding of long-term visibility characteristics. The magnitudes of annual visibility trends are studied and statistically tested using Mann-Kendall rank statistics at different significance levels. Detailed results of this work will be presented here.
The first part of this study presents the long-term and seasonal trends of the AV in Riyadh during the last few decades. The correlation of long-term visibility trends with air temperature and RH was also examined. The second part will investigate the possible association between long-term variations in AV and cosmic rays (CRs).

Data and Methods
The study area of Riyadh lies in the central region of the Arabian Peninsula (24˚43'N, 46˚40'E, 764 m a.s.l.). Riyadh is the capital of Saudi Arabia and also its largest city; its population was 7 million according to the 2017 census. This as it is surrounded by industrial areas and major traffic routes and is experiencing a dramatic urbanization that has led to increasingly noticeable air pollution, which has become a serious environmental and health issue in recent years.
The prevalent arid conditions and continentality at this site are responsible for large seasonal temperature differences, with cool winters and extremely hot summers. Local aerosol sources are mainly the heavy traffic on the major roads of the city along with the resuspension of material on the ground, especially during the warm season when reduced rainfall and dry terrain increase the contribution of local mineral dust (Maghrabi et al., 2011).
AV and meteorological data, including temperature and RH, were obtained from the national climatic data center (NCDC) for the past 36 years .
The NCDC subjects the data to extensive automated quality control to correctly decode as much synoptic data as possible and eliminate many of the random errors found in the original data (available at https://www.ncdc.noaa.gov/). The data are comparable to those obtained from the ministry of environmental protection (MEP) of Saudi Arabia.
To avoid the effect of natural factors contributing to low visibility such as rain and fog, the observation days included only visibility data with RH < 90% for analysis because in a humid environment, the scattering cross section of a hygroscopic particle could increase by five or more times than in a dry environment (Malm & Day, 2001).
Linear interpolation procedures were followed to replace the missing data to complete the series. These interpolations were kept at a reasonable level to preserve the nature of the information contained in the data. With these considerations, 13, 540 days became available for analysis.
The mann-kendall (MK) test and Sen's slope estimator identified the presence of AV trends in Riyadh. These are nonparametric statistical tests that are widely used for analyzing trends in climatologic time series e.g., (Yue & Wang, 2004;Maghrabi, 2019).
The MK test can be applied to a time series i x ranked from 1, 2, , 1 i n = −  and j x ranked from 1, 2, , The Kendall test statistic S can be computed as where x j and x j are data values at times j and i, j > i, respectively, and sgn(x j − x i ) is the signum function. S is assumed to be asymptotically normal, with the expectation value E(S) = 0 for a sample size of n ≥ 8 and variance as follows (Yue & Here, q is the number of tied groups, and t p is the number of data values in group p. The values of S and V(S) are used to compute the test statistic Z as follows: The standardized MK test statistic Z follows the standard normal distribution with a mean of zero and a variance of one and is used to measure the significance of the trend. If Z is greater than Z crit (Z crit is the (100 × (1 − α/2))) th percentile of the standard normal distribution with a chosen significance level α (e.g., 5% with Z 0.025 = ±1.96), then the null hypothesis is invalid, implying that the trend is significant.
Meanwhile, Sen's measure is a nonparametric slope estimation method. According to (Sen, 1986), if a linear trend exists in a time series, then the slope (change per unit time) can be estimated using Equation (5): where C is a constant and Q is the slope and can be calculated from The median of N values of Q is represented as Sen's slope estimator, which is given as and is computed for all N pairs of time steps (i, j). The Q med sign indicates data trend (increase/decrease over time), while its value indicates the steepness of the trend.

General Visibility Trends
The variation in yearly mean visibility values over Riyadh from 1976 to 2011 is in Figure 1. The AV in Riyadh for the available data ranges between 7.2 km and 0.4 km with a mean of 5.68 ± 0.81 km. The obtained visibility at this site is lower compared with the average visibilities reported at several sites around the world.

Seasonal and Monthly Trends
To investigate the seasonal variation, we divided the data into four seasonal groups: Winter included December, January, and February (DJF); spring in-

Meteorological Effects on Atmospheric Visibility
Several sources may explain the long-term variations in AV during the whole study period or during certain periods within this time range. These include the influence of meteorological conditions, including temperature; RH; large-scale variations; dust storms from local, regional, and global sources; and climate change (Al Senafi & Anis, 2015;Mahowald et al., 2007;Miller et al., 2008). Moreover, additional direct or indirect causes include anthropogenic emissions from human and industrial activities as well as the increase in population and associate activities (Appel, et al., 1985;Malm & Kreidenweis, 1997;Tsai et al., 2007;Zhao et al., 2011). Malm & Kreidenweis, 1997; Mahowald et al., 2007;Miller et al., 2008).
In this subsection, the effect of meteorological variables on the AV in Riyadh, mainly RH and air temperature, will be investigated.
There are two significant periods in which the behaviors of the two variables present remarkable changes (Figure 3). These are between 1976-1990and 1991

Discussion
It is well-known that throughout the 1970s and 1980s, the massive inflow of revenue from high oil prices allowed the kingdom to increase its infrastructure  projects, leading to an increase in population and urbanization, which affect the air quality in Riyadh (Knauerhase, 1974). However, it was evident that during these periods the atmospheric AV showed an increasing trend, which is inconsistent with expectations. The results presented in the previous subsection indicated that meteorological conditions may explain some of the trends in observed visibilities during some periods but not all of them. Meanwhile, the effect of atmospheric aerosols can overwhelm meteorological effects at some periods and vice versa. This was clear during the last six years of the study period, when the effects of mega-projects established by the Saudi government and more frequent dust storms contributed to AV reduction in Riyadh. However, this cannot explain the trends in other years.
Therefore, considering long-term variability, the idea to search for extraterrestrial causes came to our attention (Maghrabi, 2019;Maghrabi & Kudela, 2019).  (Clette & Lefèvre, 2016). It can be seen that, apart from the absence of relationships during limited periods, AV is anticorrelated (correlated) with CRs (SSN) in most of the periods (Figure 4).
While there are spreads in the relationships between AV and the two variables, evidently AV is generally correlated with SSN and negatively with CR ( Figure 5). Correlation analyses were conducted and showed significant (p < 0.05) nonzero relationships between AV and the two variables. This result is consistent with the previous finding reported by (Maghrabi & Kudela, 2019), which established a positive relationship between CR and atmospheric aerosols. Based on the obtained results, significant correlations between time series of CR data measured by the NM and sunspot number and the AV were found. Consequently, using data that cover several solar cycles are recommended to further confirm the finding of this study.
Furthermore, power spectral analyses using the Fourier transform (FT) technique were conducted to search for periodicities in the AV time series data. The mean monthly values of the AV data between 1976 and 2011 were used for this purpose. The procedures followed in calculating the power spectra presented in this study are similar to those reported in e.g., (Kudela & Mavromichalaki, 2010;Maghrabi & Kudela, 2019;Maghrabi et al., 2021). Figure 6 shows a Lomb periodogram for all frequencies showing the main peaks of the visibility data (95% confidence level). The spectrum presents several significant peaks with different amplitudes. The most obvious peaks are 9.8 years, 5.2 years, and 1 year. The one-year peak is expected to be, in one way or another, of terrestrial origin and mainly associated with seasonal variations in meteorological parameters. However, the other peaks are expected to be of extraterrestrial origin.
Because of the effect of high peaks (mainly the one-year peak), which masks the presence of other periodicities in the AV data, two low-frequency filters were applied to the visibility data spectrum (Kay, 1988;Maghrabi, 2019;Maghrabi & Kudela, 2019;Maghrabi et al., 2021). These filters were between 0.02 and 0.05 month −1 (Figure 7(a)) and between 0.051 and 0.077 month −1 (Figure 7(b)).
It is well-known that solar activity and associated geomagnetic disturbances affect the propagation of high-energy CR. During high solar activity (high SSN), the number of CRs is modulated (decrease) because of the increase in magnetic activity, which affects the rate of CRs at the top of the atmosphere. This modulation in the flux of the primary CRs affects the level of atmospheric ionization because CR particles are one of the main sources of atmospheric ionization. This will result in changes in the rate of aerosol formation (Kristjánsson et al., 2002;Kristjánsson et al., 2004;Marsh & Svensmark, 2000;Svensmark & Friis-Christensen, 1997;Maghrabi & Kudela, 2019;Maghrabi et al., 2021). Since atmospheric aerosols are one of the major factors affecting AV through scattering and absorption, solar activity and associated CR modulation can be considered to affect AV directly or indirectly (Maghrabi & Kudela, 2019).
The results presented in this study are preliminary, and it is recommended that more detailed investigations be conducted.

Conclusion
AV data for the period 1976-2011 in Saudi Arabia were used to investigate interannual, monthly, and seasonal variations. Nonparametric MK tests were performed to detect AV magnitude and trends. No significant trends in AV were observed during the 36 years. However, between 1976 and 1999, both significant increases and decreases in AV between 2000 and 2011 were found. The possible effects of meteorological parameters, mainly relative humidity and air temperature, on AV during the study period were investigated. While these two variables could explain the visibility trend over some years, they failed to explain it in most of the others.
The possible effects of solar activity and the associated CR modulation on the long-term trend of visibility were examined as well. This was done using regression analyses and power spectral analyses. Correlation analyses showed a nonzero positive and significant correlation between the AV time series and CR (from Oulu NM) and a negative and significant correlation with solar activity (SSN).
Power spectra analyses using the FT technique were performed for the period 1976-2011 and revealed several long, mid-and short-term periodicities.
These include 9.8 years, 5.2 years, 2.2 years, 1.7 years, 1.3 years, and 1-year periods, the last one mainly of terrestrial origin (seasonal variation). The long-and mid-term periodicities are similar to those reported by several investigators and found in solar, interplanetary, and CR parameters. Therefore, we suggest that solar signals may directly or indirectly affect long-term variations in AV.
Correlation and power spectral analyses indicate possible mutual relations between both CR intensity and solar activity and variations in AV observed on the ground at specific positions analyzed here. The possible reason for this relationship was the fact that solar activity modulates CR, which affects the level of atmospheric ionization and aerosol formation. Since aerosols scatter and absorb light, the physical mechanism of the relationship can be considered.
Further confirmations are recommended. It is important to note that the effect of local, regional, and global sources of atmospheric aerosols and meteorological effects cannot be ignored when studying the effect of atmospheric aerosols and visibilities over certain periods.

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.