Correlation analyses between downward longwave radiation particulate matters, aerosol optical depth, and metrological variables under non-dusty and cloudless conditions

Abstract

This study used downward longwave (LW) radiation measurements, air temperature (T), particulate matter (PM) concentrations of fine (PM 2.5) and coarse (PM10) particles, aerosol optical depth (AOD), wind speed (WS), and precipitable water vapor (PWV) data from Riyadh, an arid site in central Saudi Arabia, between 2014 and 2019 to characterize their variations and investigate the influence of these variables on the measured downward LW radiation under non-dusty, clear sky conditions. The LW radiation and air temperature, it was found, attain their maximum in summer and minimum in winter. Conversely, the PM mean concentrations and AOD showed their maximum in spring and minimum in winter. PWV features an increasing trend during spring, summer, and fall, whereas it features a decreasing trend during the winter. The monthly variation of WS shows low monthly values during fall and winter and considerably higher monthly mean variation during spring and summer. Apart from wind speed, which does not affect the LW radiation, correlation analyses demonstrated that the LW correlates positively with the remainder of the considered variables. The strength and degree of association between the LW radiation and these variables differ from one variable to another, with air temperature having the strongest correlation (correlation coefficient = 0.98) with the LW radiation, followed by the AOD (correlation coefficient = 0.69), with a correlation coefficient of 0.36 with the PWV. Similarly, the relationships between the LW radiation and both PM10 and PM2.5 had the same correlation coefficient of 0.32. The effect of the meteorological variables (mainly air temperature, relative humidity, and wind speed) on the PM concentrations was investigated using the regression analyses. The results showed a significant positive relationship with air temperature and wind speed and a negative relationship with RH.