污染影响下的辐射估算及演变规律

Anthropogenic aerosols are increasingly released into the atmosphere by human activities, which in turn gradually modify the energy flows in the climate system. A significant decline in surface solar radiation (SSR) was observed worldwide since the 1950s (termed “global dimming”), which then transitioned to increase since the late 1980s (“global brightening”). These findings are based on radiation measurements on the ground, which are also of key importance for the evaluation of the surface radiation budgets derived from remote sensing and from climate models. However, the spatial and temporal coverage with these measurements is not satisfactory for an adequate representation of these flux fields. Sunshine duration (SD) with wider and longer observations has therefore been broadly used to estimate SSR. SD is measured when direct solar radiation (DSR) is of sufficient intensity (120 W m-2) to activate the recorder. Under polluted conditions, pollution driven declines in SSR may be ignored in the periods not recorded in SD due to insufficient DSR, which may eventually result in overestimating SSR from SD. This project, therefore, aims at revealing the magnitude and mechanism of pollution driven changes in SSR at diurnal cycle. Based on this, Angström-Prescott (A-P) model will be improved by incorporating a pollution-related dependency to infer an accurate estimation of SSR from SD under various pollution levels. With the adjusted A-P model, solar radiation climatologies as well as their temporal variations will be established from the widespread SD observations and air pollution information. This allows an unprecedented assessment of surface radiative fluxes and their changes particularly also in regions with no direct radiation measurements, and will improve our understanding of the roles of solar radiation and air pollution in climate change, hydrological cycle and plant growth.

大气中增多的污染物/气溶胶正在影响着地球的能量收支。自上世纪50年代以来，到达地表的太阳辐射经历了由下降到上升的演变趋势，即著名的“全球变暗/变亮现象”。这一重要发现基于地面观测数据，是评估遥感观测和模型模拟手段的基础。由于辐射的地面观测具有一定的时空局限性，因此日照时数常被用来估算辐射。在污染的情况下，利用日照时数估算辐射会忽略不被日照计记录的时段（直接辐射强度<120 W m-2）污染引发的辐射的下降，从而高估辐射量。因此本研究致力于揭示一天内各时段污染驱动辐射变化的规律及物理机制，并依此添加与污染有关的系数公式校正Angström-Prescott模型以实现不同污染情况下利用日照时数对太阳辐射的准确估算。借此可实现对辐射数据的时空拓展，并在包括无辐射直接地面观测地区建立辐射的气候学特征及时空变化规律，这对理解污染和辐射在气候变化、水循环及作物生长中的作用具有重要的科学意义和应用价值。

人类活动正在通过排放气溶胶影响着大气环境及地球能量收支平衡。本研究通过对比全国172对城市及临近县日照时数的变化情况发现，我国城市的光照变暗速率是乡村的1.5倍。与城市化发展相关的大气污染驱动的光照变暗主要发生在80年代前，随后我国太阳辐射趋于轻微回升。类似的现象现象广泛发生在全球大部分地区，形成著名的“全球变暗/变亮现象”。这一现象的发现主要基于地面观测数据，是评估遥感产品及模型模拟结果的基础。然而，我国太阳辐射的地面观测数据的时间序列在1990-1993年存在异常突增。本研究分析造成这一突变的原因1/3是由于51个台站在这一时期的废弃或增设，另外2/3则是来自7个台站辐射的自然波动及16个站辐射非自然因素波动。在排除来自16个台站的非自然因素影响及51个不完整数据序列后，本研究重新建立了我国太阳辐射在全球变暗及变亮时期时空变化趋势。此外，地面观测数据具有观测台站数量少且空间分布不均匀的局限性。而遥感产品具有广泛的地域覆盖和高的空间分辨率可有效弥补地面观测数据的局限性。本研究通过评估CM SAF CLARA-A2和SARAH-E太阳辐射遥感产品的准确性和稳定性，提出了改善太阳辐射遥感产品在我国估算精度的方法及方向，其中正确表征我国气溶胶的气候学特征及变化尤为重要。遥感产品通过提高估算精度将可以在更广泛的空间范围再评估我国辐射的变暗/变亮现象，这对于太阳能利用、水资源调配及作物生产等均具有重要的科学意义和应用价值。