Assessing the Effect of Aerosol Optical Depth on Solar Radiation Components over Iraq (2010-2022)

Abstract

Aerosols play an important role in modifying the components of solar radiation at the Earth's surface, especially in dry environments with high dust activity, such as Iraq. This study evaluates the impact of aerosol optical depth (AOD at 550 nm) on solar radiation components (DNI, DHI, and GHI) over the period 2010–2022, using MERRA-2 reanalysis data for aerosols and ERA5 data for solar radiation. The analysis includes daily, monthly, and spatial assessments, in addition to a comparison between clear-sky conditions and actual atmospheric conditions. Temporal analysis revealed a statistically significant downward trend in AOD according to the Mann–Kendall test (p = 0.002), while global horizontal irradiance (GHI) did not exhibit a comparable temporal trend, suggesting a compensatory mechanism between direct and diffuse radiation components. At the national daily scale, a statistically significant inverse relationship was identified between AOD and direct normal irradiance (DNI), with a slope of −326.99 W m⁻² per unit AOD (R² = 0.28, p < 0.001) and a relative sensitivity of −157% compared to its overall mean value (207.77 W m⁻²). In contrast, diffuse horizontal irradiance (DHI) showed a statistically significant positive relationship with AOD, with a slope of +123.68 W m⁻² per unit AOD (R² = 0.38, p < 0.001) and a relative sensitivity of +149% relative to its mean (82.74 W m⁻²). For GHI, although the regression slope was positive (≈ +22.51 W m⁻² per unit AOD), the explanatory power was extremely weak (R² ≈ 0.002), indicating that the relationship is statistically detectable but physically limited, primarily reflecting energy redistribution rather than a net radiative gain or loss. The aerosol impact was found to be more pronounced under clear-sky conditions compared to all-sky conditions, confirming the masking role of clouds in modulating aerosol–radiation interactions. The maps of spatial radiative sensitivity revealed clear geographic variability, with stronger negative sensitivities concentrated over central and southern Iraq. Seasonal analysis showed that the highest radiative sensitivity occurred during winter (DJF: −2.99 kWh m⁻² per unit AOD, p = 0.00019) and autumn (SON: −2.24 kWh m⁻², p < 0.001), whereas spring exhibited weaker and statistically insignificant responses. These findings demonstrate that aerosols substantially redistribute solar energy over Iraq and directly influence the estimation of solar energy resources, particularly for photovoltaic resource assessment, highlighting the necessity of incorporating aerosol effects into regional solar energy modeling frameworks.