**1. Introduction**

Surface albedo is represented by the ratio of reflected shortwave solar radiation in all directions from the Earth's surface to the total incoming solar radiation [1]. Albedo is an indicator that characterizes the reflective ability of the Earth's surface via solar radiation and determines the allocation of radiative energy between the Earth's surface and the atmosphere [2–4], making it an imperative parameter that also affects the Earth's climate [5]. The increase in albedo can reduce the absorption of solar radiation at the Earth's surface; this lowers the surface temperature and has an equivalent effect on the reduction in CO2 emissions, which mitigates greenhouse effects [6–8]. In recent years, there have been many studies focusing on the effects of variations in surface albedo on climate change in both global and local areas [9–13], and some have suggested that the effects that variations in albedo have on climate change are comparable to those of fossil fuel combustion [14,15]. Therefore, identifying changes in albedo is of grea<sup>t</sup> significance for further exploring climate change.

The influential factors and causes for change in albedo have been extensively studied. Surface albedo was found to decrease as the surface irregularity increases, and albedo increases with an increasing solar zenith angle, leading to a minimum albedo at noon during diurnal variation [16]. Soil moisture was also found to be an important factor. Some research has revealed that surface albedo decreased with an increase in soil moisture, indicating a typical exponential relationship between them [17]. Furthermore, much research [18–20] has revealed that meteorological factors, such as aerosol optical depth, temperature, rainfall and snowfall et al., also contribute to changes in surface albedo. Reflectivity measurements from 61 real-world surfaces in Dana's study indicated that albedo varies with surface roughness, as well as viewing and illumination directions [21]. Roughness has been well studied by many researchers [22–26], and their results showed that the increase of roughness will make the surface albedo decrease, which was explained by the fact that surfaces with greater roughness or irregularity offer more spaces and cracks where the incident light is trapped [27]. The aforementioned studies mostly focused on a single factor that might affect surface albedo. However, surface albedo is often affected by multiple factors in reality, which complicates the reasons for variations in albedo.

Although the changes in global land surface albedo have been widely studied, the impact of urbanization that human activities induce on albedo is not well understood. As we all know, urbanization is one of the most important aspects of human activities in the terrestrial ecosystem [28,29], and it has a significant impact on regional climate change [30,31]. Climate change in urban areas has received substantial public attention, especially regarding urban heat islands (UHIs) [32,33]; there have been many comprehensive studies on the UHI, including its morphological structure [34,35] and change process [36–38]. As a parameter that affects the distribution of solar radiation, surface albedo in urban areas influences surface temperatures in cities to some extent. However, due to the coexisting influences of land cover changes, industrial pollutants, aerosols, and vegetation growth during urbanization [39], changes in urban surface albedo have various uncertainties. Therefore, it is still very difficult to quantitatively analyze the processes of energy distribution and conversion in urban areas. Although existing studies have shown that the decrease in surface albedo is one of the most important causes of urban warming [8,40], the main factor affecting changes in albedo before and after urbanization is still unknown.

Therefore, this study quantitatively calculates the contributions of vegetation and urbanization to surface albedo in the Jing-Jin-Ji region and distinguishes the main driving factors behind its spatiotemporal changes during the most rapid population growth period (2001–2011). Based on the Moderate Resolution Imaging Spectroradiometer (MODIS) global land surface albedo product [41], we used a shift linear regression method [42] to detect the breakpoint year in the albedo time series, and we also analyzed the temporal and spatial patterns of albedo in the Jing-Jin-Ji region. With the nighttime light data from the U.S Air Force Defense Meteorological Satellites Program Operational Linescan System (DMSP/OLS), we used the Digital Number (DN) to represent the intensity of urbanization [43,44]. Combined with the MODIS product of the vegetation index data [45], we quantitatively calculated the contributions of urbanization and vegetation to variations in albedo via a partial derivative-based calculation method to determine the main controlling factors. The aim of this paper is to determine the contributions of urbanization and vegetation to albedo, which influence the urban climate, to provide a case basis for developing urban energy conservation programs.

#### **2. Study Area and Data**
