Atmospheric brown clouds (ABCs) are widespread hazy accumulation of particles and gases dominant with absorbing nature impacting the air quality, hydrology and climate. The Indo-Gangetic plain (IGP) being an ABCs hotspots is prone to extreme pollution episodes due to anthropogenic activities. However, there is a scarcity of studies that specifically compare episodes of absorbing aerosols (ABCs) and comparatively non-absorbing aerosols (non-ABCs) along with their respective radiative impacts over the IGP. Here, we have used ground-based remote-sensors’ retrieved datasets from AErosol RObotic NETwork (AERONET) to study spatio-temporal variation of ABCs’ frequency and associated radiative properties across the IGP region during 2001–2019. The shortwave aerosol radiative effect (ARE) during ABC and non-ABC episodes is estimated using Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model. Our results show significant spatial variability in the frequency of ABC episodes across the IGP stations with ∼17 ± 5%, ∼19 ± 11% and ∼18 ± 13% average occurrence during winter, pre-monsoon and post-monsoon seasons, respectively. ARE estimations show enhanced surface dimming (ARE) and atmospheric warming (ARE) during ABC episodes as compared to non-ABC episodes. A general increasing trend for ARE is observed as we move from west to east across the IGP due to the meteorological and geographical variations in the region. Our results show day-time averaged ARE varying between −142.46 ± 25.13 and −56.66 ± 6.88 W/m while ARE ranging from 58.07 ± 2.58 to 132.69 ± 7.43 W/m for ABC episodes. Though ABC episodes show lower aerosol loading as compared to non-ABC episodes in our study, we found higher atmospheric heating during ABC episodes mainly due to distinct chemical composition (lower single scattering albedo) of absorbing aerosols. The upper bound of the difference between atmospheric heating rate during ABC and non-ABC episodes is found to be 0.48 ± 0.27 K/day.

中文翻译：

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印度恒河平原上空大气棕色云的辐射效应

大气棕色云 (ABC) 是颗粒和气体的广泛的模糊积累，主要吸收自然影响空气质量、水文和气候。印度恒河平原（IGP）是ABCs热点地区，很容易因人类活动而发生极端污染事件。然而，缺乏专门比较吸收性气溶胶（ABC）和相对非吸收性气溶胶（非 ABC）的事件以及它们各自对 IGP 的辐射影响的研究。在这里，我们使用地面遥感器从 AErosol RObotic NETwork (AERONET) 检索的数据集来研究 2001 年至 2019 年整个 IGP 区域 ABC 频率和相关辐射特性的时空变化。使用圣巴巴拉 DISORT 大气辐射传输 (SBDART) 模型估算 ABC 和非 ABC 事件期间的短波气溶胶辐射效应 (ARE)。我们的结果显示，IGP 站 ABC 发生频率存在显着的空间变异，在冬季、季风前和季风后季节平均发生率分别为~17 ± 5%、~19 ± 11% 和~18 ± 13%。ARE 估计显示，与非 ABC 事件相比，ABC 事件期间地表变暗 (ARE) 和大气变暖 (ARE) 增强。由于该地区的气象和地理变化，当我们从西向东穿过 IGP 时，观察到 ARE 总体呈增加趋势。我们的结果显示，白天平均 ARE 在 -142.46 ± 25.13 和 -56.66 ± 6.88 W/m 之间变化，而 ABC 事件的 ARE 范围在 58.07 ± 2.58 到 132.69 ± 7.43 W/m 之间。尽管在我们的研究中，与非 ABC 事件相比，ABC 事件显示出较低的气溶胶负荷，但我们发现 ABC 事件期间大气加热较高，这主要是由于吸收气溶胶的不同化学成分（较低的单散射反照率）。发现 ABC 和非 ABC 事件期间大气加热速率差异的上限为 0.48 ± 0.27 K/天。