Abstract
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.
Highlights
Atmospheric visibility (AV) is a measure of the light extinction caused by atmospheric aerosols and is considered a good indicator of air pollution
The spectral and correlation results suggested that, with the expected effects of terrestrial and meteorological conditions on atmospheric visibility (AV), long-term AV variations can be related to the solar activity and associated cosmic ray (CR) modulations
The mean visibility appears to increase from 5.4 km in 1995 to a maximum of 6.2 km in 1999
Summary
Atmospheric visibility (AV) is a measure of the light extinction caused by atmospheric aerosols and is considered a good indicator of air pollution. 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. The characterization of the long-term AV trend over the central region of the Arabian Peninsula has never been reported
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