Solar ultraviolet B (UVB) radiation (280 - 315 nm) constitutes a small but biologically and ecologically critical portion of the solar spectrum reaching Earth’s surface. Unlike broadband global horizontal irradiance (G, ~290 - 3000 nm), UVB is highly sensitive to atmospheric attenuation by total column ozone (O₃), aerosols, clouds, and solar geometry. Biologically, UVB drives cutaneous vitamin D synthesis yet is also the primary cause of erythema, DNA damage, immunosuppression, nonmelanoma skin cancers, and ocular disorders such as cataracts and photokeratitis [1][2]. Ecologically, it influences phytoplankton productivity, terrestrial plant physiology, and biogeochemical cycles. In high-insolation regions, excessive exposure poses significant public health challenges. Long-term, detailed ground-based UVB measurements are therefore indispensable for exposure risk assessment, satellite validation, epidemiological modeling, and studies of climate-radiative interactions. Beyond human health and ecology, UVB radiation plays important roles in material degradation (e.g., photodegradation of polymers, paints, and photovoltaic module encapsulants), agricultural productivity through effects on plant growth and stress responses, and assessments of solar energy system durability under intense UV exposure. Surface UVB exhibits pronounced temporal and spatial variability. Analyses from well calibrated networks show that interannual variability typically ranges from 2% - 5% of the mean irradiance, while seasonal cycles are dominated by solar zenith angle (SZA) and day length. For instance, Hicke et al. (2008) examined 8 - 11 years of broadband UVB data from eight U.S. stations and reported interannual variability of 2% - 5% of the mean with no clear decadal trend [3]. Similar variability patterns have been documented in Europe, where aerosol and cloud effects often dominate observed trends [4]. Clouds are among the most variable and effective attenuators of UVB. A comprehensive review by Calbó et al. [5] demonstrated that UV attenuation under clouds is 15% - 45% weaker than for total solar radiation, owing to enhanced scattering at shorter wavelengths and occasional cloud edge reflections that can produce UVB enhancements of up to 20% even under broken cloud conditions. Overcast skies typically reduce UVB to 30% - 70% of clear sky values, with the exact reduction depending on cloud optical thickness, type, and SZA [5]-[7]. These observations support the use of the clearness index (KT = G/G₀) for sky condition classification (clear: KT ≥ 0.65; partly cloudy: 0.35 ≤ KT < 0.65; overcast: KT < 0.35) [8]. Aerosols, particularly mineral dust and urban particulates common in arid environments, further modulate UVB through scattering and absorption. In desert regions, high PM₁₀ and PM_2.5 loadings from dust storms can reduce UVB by 10% - 30% under otherwise clear conditions [9]. Radiation amplification factors (RAF) quantify these sensitivities: a 1% increase in aerosol optical depth typically reduces erythemal UV by 0.18% - 0.63%, whereas a 1% decrease in column ozone increases it by 0.67% - 1.74% [10]. Recent studies confirm that dust aerosols play a dominant role in shortwave and UVB attenuation across the Middle East and North Africa (MENA), with severe dust events causing reductions exceeding 300 W m⁻² [11][12]. Ground-based measurements remain essential despite satellite products (e.g., OMI, TOMS), which frequently overestimate surface UVB by 20% - 30% in dusty or polluted regions due to incomplete aerosol characterization [13]. High-quality, long-term in situ records with rigorous quality control are particularly scarce in the MENA region, where intense solar regimes coincide with extreme aerosol loading and rapid urbanization. Within Saudi Arabia, earlier investigations have documented the country’s exceptionally high solar resource and corresponding UV exposure. Al-Dhafiri et al. [14] reported 5 years (1983-1987) of hourly UV (290 - 380 nm) and global radiation data for Riyadh, documenting monthly mean UV/G ratios of approximately 2.9% with clear seasonal peaks in June-July [14]. AlAruri et al. [15] developed early empirical correlations between global and ultraviolet (290 - 385 nm) radiation in Kuwait and nearby Gulf sites. Recently, Maghrabi et al. [16] characterized UVA (315 - 400 nm) over 2015-2020 at the same central Riyadh station, noting strong dust storm suppression and establishing G based models under varying sky conditions. Khalil et al. [13] assessed UVB across four other Saudi cities (AlBaha, Abha, Jeddah, Taif) for 2003-2017, confirming pronounced seasonal cycles driven by SZA and KT [13]. The public health context in Saudi Arabia further underscores the societal relevance of precise UVB quantification. Despite abundant solar radiation, vitamin D deficiency remains paradoxically prevalent [17], attributed to cultural sun avoidance behaviours, traditional clothing, and indoor lifestyles. Concurrently, rising skin cancer awareness necessitates reliable UVB exposure estimates to guide photo protection guidelines (AlJasser et al., 2020) [18]. Empirical modelling of UVB from more widely measured broadband G has been pursued globally. Exponential or linear relations (e.g., UVB = a∙exp(b∙G) or multivariable forms incorporating SZA) routinely achieve R > 0.85 under all sky conditions, with improved skill when stratified by cloud regime [19]. The present study extends this approach by deriving sky specific models and performing residual analyses to isolate secondary ozone and PM₁₀ influences after removing dominant geometric and radiative controls. Such models also support broader applications, including evaluation of UV-induced material degradation in solar energy systems and prediction of ecological impacts in arid ecosystems.

Despite earlier work on UV radiation in Saudi Arabia and the wider MENA region [13]-[16], decadal (≥10-year), high-temporal-resolution broadband UVB datasets with documented quality control, recalibration history, and integrated analysis of sky conditions, aerosols, and ozone remain scarce in the central Arabian Peninsula. The present study addresses this gap with a unique 2015-2025 dataset collected at a single detailed station in Riyadh, Saudi Arabia. The main novelty lies in the length and quality of the Riyadh-specific record, the systematic combination of sky-condition stratification, simple physically interpretable empirical models, ANN comparison, and deseasonalised residual analysis of PM₁₀ and ozone under clear skies. These elements provide both a local climatology and practical estimation tools that have not previously been developed for this exact setting.

This study provides the first decadal climatology of daily UVB doses in central Saudi Arabia using ten years of high-quality, half-hourly ground-based measurements in Riyadh. The results reveal a pronounced seasonal cycle driven primarily by solar geometry, substantial interannual variability linked to changes in clear-sky frequency and aerosol loading, strong suppression of UVB under overcast conditions, and robust simple linear empirical models based on global horizontal irradiance (G) and cos(SZA). Residual analysis under clear skies further demonstrates that secondary atmospheric factors continue to modulate UVB even after the dominant effects of solar geometry and broadband radiation are removed. The seasonal cycle, with summer maxima of ~85 kJ∙m⁻²∙day⁻¹ in June-July and winter minima of 35 - 38 kJ∙m⁻²∙day⁻¹, is governed predominantly by astronomical factors. Higher solar elevation angles and shorter optical air mass in summer reduce atmospheric path length, thereby decreasing attenuation by ozone, Rayleigh scattering, and aerosols, while longer winter paths enhance attenuation. This astronomical control persists across all sky categories but is expressed most clearly under clear skies, consistent with observations in other arid regions where solar zenith angle and day length constitute the primary drivers [13][26][27]. Clouds and aerosols mainly modulate the magnitude of the received dose rather than the underlying seasonal envelope. Interannual variability is pronounced, with annual mean daily UVB doses ranging from 43.84 kJ∙m⁻²∙day⁻¹ in 2022 to 68.11 kJ∙m⁻²∙day⁻¹ in 2020. High-UVB years were associated with clear-sky fractions exceeding 83% and reduced aerosol loading during peak solar months, whereas low-UVB years showed increased cloudiness and dust activity. Even in years maintaining high clear-sky frequency (~88% in 2024), doses remained below average, indicating that non-cloud factors, particularly aerosol burden, play a significant role. This variability exceeds the 2% - 5% typically reported in mid-latitude networks [3][4] and is amplified in the MENA region by frequent dust events and fluctuations in atmospheric transparency [11][12]. Sky-condition stratification using the clearness index highlights the strong attenuating effect of clouds. Clear skies dominate the record (~85% of days) and deliver the highest doses (mean 63.65 kJ∙m⁻²∙day⁻¹), while overcast conditions suppress UVB to approximately 30% of clear-sky values (~70% reduction). This attenuation is weaker than for total solar radiation owing to enhanced scattering at shorter wavelengths and occasional cloud-edge enhancements [5]. The observed gradient is consistent with reports from arid environments in Egypt and the Negev Desert [26][27] and earlier Saudi studies [13]. The prevalence of clear skies is characteristic of high-insolation desert climates, yet occasional dust storms and cloud episodes drive the observed interannual extremes [9]. The simple linear empirical models using only G and cos(SZA) explain 74% - 75% of day-to-day variability with near-zero mean bias under both all-sky and clear-sky conditions. Attempts to incorporate additional variables (RH and T) produced only marginal improvements, confirming that G and solar geometry capture the dominant controls. The linear additive structure was preferred over multiplicative formulations for its superior physical interpretability, as it clearly separates geometric and radiative contributions from atmospheric attenuation effects. These models achieve skill comparable to more complex approaches reported elsewhere while offering greater simplicity and transparency, making them particularly suitable for operational use in aerosol-variable arid environments. An important question addressed in this work is how secondary atmospheric factors influence surface UVB under clear-sky conditions once the dominant effects of solar zenith angle and global horizontal irradiance are removed. To investigate this, residuals were calculated as observed minus predicted UVB using the clear-sky linear model. Monthly PM₁₀ data were obtained from ground-based beta attenuation monitors (BAM 1020, Met One Instruments) operated at the measurement site, while total column ozone (TOC) values were derived from the SBUV Version 8.6 Merged Ozone Data Sets [21] (e.g., Maghrabi et al. [20] for more details). Deseasonalized anomaly analysis was then applied to isolate genuine departures from the climatological seasonal cycle (Figure 8).

The analysis reveals that ozone exhibits the expected negative relationship with UVB residuals, consistent with its strong absorption cross-section below 320 nm. In contrast, PM₁₀ anomalies show a positive relationship with UVB residuals. In this arid environment, elevated coarse mineral dust often coincides with drier atmospheric conditions that reduce water-vapor absorption and limit thin-cloud formation, thereby acting as a proxy for enhanced UVB transmission rather than direct attenuation. This positive residual relationship contrasts with the typical negative (anti-correlated) raw relationship reported in the same region, where UVB decreases with increasing PM₁₀ due to scattering and absorption [20][28]. The reversal after removing SZA and G effects highlights the value of anomaly

Figure 8. Scatter plots showing the cleaned deseasonalized monthly anomaly relationships between UVB residual anomaly and (a) ozone anomaly, (b) PM₁₀ anomaly, (c) relative humidity (RH) anomaly, and (d) temperature (T) anomaly.

analysis in disentangling proxy effects from direct attenuation. Relative humidity shows a modest positive association with UVB residuals, likely reflecting reduced dust mobilization and lower coarse-particle loading under higher RH conditions. Temperature has limited independent explanatory power once G is accounted for, as its primary influence is already captured through broadband radiation. These findings confirm that, even under predominantly clear skies (~85% of days), aerosols (represented by PM₁₀), ozone, and to a lesser extent RH remain important modulators of surface UVB. The deseasonalized approach strengthens statistical confidence by removing shared seasonal structure and aligns with residual analyses in other arid and mid-latitude regions [4][20][29]. The high summer UVB levels and marked interannual variability have direct public-health relevance for Saudi Arabia, where vitamin D deficiency remains prevalent despite abundant sunshine, largely due to cultural sun-avoidance behaviours and indoor lifestyles (Alshahrani & Aljohani, 2013) [17]. The findings can inform targeted photoprotection guidelines and supplementation strategies, especially during low-UVB years, and support skin-cancer risk mapping (AlJasser et al., 2020) [18]. For solar-energy applications, the strong clear-sky dominance and aerosol modulation are relevant for assessing UV-induced degradation of photovoltaic materials. The developed models provide transparent, practical tools for historical reconstruction and resource assessments in data-limited arid settings. Limitations of the study include its single-station design and the empirical nature of the models, which do not incorporate real-time aerosol optical depth. Future research should extend the observational record to examine long-term trends, integrate direct AOD and ground-based ozone measurements for refined residual analysis, and develop erythemal or vitamin D-weighted UV indices.

The present study provides a decadal climatology of daily UVB doses for central Saudi Arabia based on ten years of half-hourly ground-based measurements at a single station in Riyadh. By integrating quality control following BSRN/WMO standards, clearness-index sky-condition classification, physically transparent empirical modelling, artificial neural network comparison, and clear-sky residual analysis, the work delivers new insights into the dominant and secondary controls on surface UVB radiation in a high-insolation, aerosol-variable desert environment.

The simple linear models based solely on global horizontal irradiance (G) and the cosine of the solar zenith angle (cos SZA) achieve good predictive skill (R² = 0.74 - 0.75) with negligible bias and outperform more complex nonlinear ANN approaches. Residual analysis demonstrates that, even under predominantly clear skies, PM₁₀ and total column ozone exert measurable influences on UVB variability. These findings help fill an important observational gap in the Arabian Peninsula and provide practical, transparent tools for estimating daily UVB doses in data-limited arid regions. The results have potential relevance for public-health policy, solar-energy yield assessments, and studies of UV-induced material degradation in desert environments.

We gratefully acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) and Imam Mohammad Bin Saud Islamic University. We also sincerely thank the anonymous reviewers for their valuable comments and constructive suggestions, which greatly improved the quality of this manuscript.

All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Abdullrahman Maghrabi, Latifa Shaber, Abdulah Aldosari, Mohammed Altilasi, and Ghada Khouqeer. The first draft of the manuscript was written by Abdullrahman Maghrabi, and all authors commented on it for its improvement. All authors read and approved the final manuscript.