ABSTRACT

Recent studies have attempted to monitor global air quality with geostationary satellites. However, the monitoring accuracy is affected by a high solar zenith angle (SZA) during the day-to-night transition period, since the retrieval algorithms commonly use shortwave radiation. This study investigates the inaccuracy of background cloud retrievals for air quality at high SZAs that commonly use the O₂–O₂ absorption band at 477 nm. To this end, the differential optical absorption spectroscopy (DOAS) method with a radiative transfer model (RTM) simulations is used to quantify the amount of O₂–O₂ absorption by the cloud properties. The results show that the deviation of cloud retrievals from that retrieved at zero SZA increases exponentially with increasing SZA. The cloud deviation with increasing SZA can be larger for a smaller effective cloud fraction (ECF) and higher cloud centroid pressure (CCP). At an SZA of 80°, the ECF is deviated up to 0.06 (28%) and the CCP up to – 90 hPa (–13%). This cloud deviation originates from the reduction in the 477 nm reflectance in the DOAS calculation, which is primarily owing to the in-cloud absorption effect in addition to the out-of-cloud O₂–O₂ absorption effect. Therefore, minimizing these absorption effects in the DOAS method during day-to-night transition is crucial for improving cloud retrievals and, eventually, the overall reliability of air quality monitoring.

摘要

最近的研究试图用对地静止卫星监测全球空气质量。但是，由于检索算法通常使用短波辐射，因此在昼夜转换期间，监视精度会受到较高的太阳天顶角（SZA）的影响。这项研究调查了在高SZA（通常使用477 nm的O ₂ -O ₂吸收带）下，背景云反演对于空气质量的准确性。为此，使用具有辐射传递模型（RTM）模拟的差分光学吸收光谱（DOAS）方法来量化O ₂ -O ₂的量被云层吸收的特性。结果表明，随着SZA的增加，云检索的偏差与SZA为零时的检索偏差呈指数增长。对于较小的有效云分数（ECF）和较高的云质心压力（CCP），随着SZA的增加，云偏差可能会更大。当SZA为80°时，ECF偏差最大为0.06（28％），CCP偏差最大为– 90 hPa（–13％）。这种云偏差源自DOAS计算中477 nm反射率的降低，这主要归因于云外O ₂ -O ₂以及云内吸收效应吸收效果。因此，在白天到夜晚的过渡期间，在DOAS方法中最小化这些吸收效应对于改善云的获取以及最终改善空气质量监测的整体可靠性至关重要。