Probing into the radiative mechanisms and drivers influencing urban heat-cold islands and urban pollution islands during heavy-haze events in a cold megacity of Northeast China.

Persistent-heavy-haze (PHH) events and extreme-cold temperatures are prevalent problems confronting the Northeastern part of China (especially Harbin) during the prolonged winter season, leading to two challenging phenomena known as the urban-heat/cold-island (UHI/UCI) and urban-pollution-island (UPI) intensities (UHII/UCII and UPII) defined as the differences between urban and rural temperature/air pollution. Although UHI and UPI have been explored, the mechanisms and novel drivers (urban-stilling-island-intensity (USII), urban-dryness-island-intensity (UDII), and urban-planetary-boundary-layer-height-intensity (UPBLHI) which are by-products of wind speed (WS), relative humidity, and planetary-boundary-layer-height (PBLH), respectively; with wind direction (WD)) influencing them are still underexplored in extreme-cold regions. Using the Weather Research and Forecasting (WRF) model coupled with Chemistry (WRF-Chem), we designed the mechanisms into non-radiative-effect (NRE), direct-radiative-effect (DRE), indirect-radiative-effect (IRE), with slope-and-shading-effect (SSE) during early-winter-heavy-haze-episode (EWHHE; December 2019) and peak-winter-persistent-heavy-haze-episode (PWPHHE; January 2020). We found that the spatial distributions of the mass concentration of pollutants (e.g., PM 2.5) dominated the downtown, with the IRE and SSE accounting for >200 μg/m3 in both episodes. In assessing the drivers impacting UHII and UPII, the USII favors the increase of UHII and UPII in the EWHHE but lowers the UCII in PWPHHE. Moreover, the UDII and UPBLHI were weaker in PWPHHE than the EWHHE, with NO 2 having the highest UDII at 7.05% (SSE), 7.00% (IRE and NRE), and 6.24% (DRE); hence, revealing that PHH lowered the PBLH. Additionally, the dominant south-to-north WD impacted the UHII and UPII. Generally, low USII, UDII, and UPBLHI with WD are major contributors to PHH in the cold region. Therefore, this work provides insight into drivers for other urban areas with severe air pollution and extreme cold problems. [Display omitted] • Novel drivers were introduced, which are by-products of meteorological conditions. • Spatial distribution of pollutants dominated the downtown, with the IRE and SSE impacting both episodes. • USII favors the increase of UHII and UPII in the EWHHE but lowers the UCII in PWPHHE. • NO 2 has the highest UDII at 7.05% (SSE), 7.00% (IRE and NRE), and 6.24% (DRE). • Low USII, UDII, and UPBLHI with WD are major contributors to PHH in the cold region. [ABSTRACT FROM AUTHOR]