The interactions of multiscale meteorology and haze in the Chinese gigacity
Authors: Kokkonen Tom , Ciarelli Giancarlo , Xia Men , Liu Yongchun , Yan Chao , Nie Wei , Ding Aijun , Kerminen Veli-Matti , Petäjä Tuukka , Kulmala Markku 
Published: July 2024

Recently, Kulmala et al. (2021) proposed that in eastern China, a cluster of megacities could be classified as one huge continuous urban area &#8211; a gigacity. Already now about 10% of the global population is living in the Chinese Gigacity confined roughly by the lines Shanghai-Xian-Beijing. Gigacity region has huge areas of increased surface roughness and urban heat and pollution islands. These together can strongly influence the local scale meteorology (Ding et al., 2016; Kokkonen et al., 2019).There are essentially three different meteorological pollution regimes in the Chinese gigacity area having different driving processes of haze: 1) strong synoptic scale winds, leads to effective ventilation of the urban areas and transports local emissions to areas outside gigacity, frequent new particle formation (NPF) due to low condensation sink with sufficient amount of precursor gases available, 2) weak synoptic scale winds, local circulation with clear diurnal cycle is dominating (e.g., mountain-valley and land-sea breeze), local emissions with frequent NPF events together with weakening boundary layer dynamics (BLD) are deteriorating the air quality and slowly pushes the conditions towards the next regime, and 3) haze formed through a mixture of long-range transport, NPF, and local emissions, suppressed radiative driven local circulation (e.g., mountain-valley breeze) and BLD, stagnant conditions with no effective ventilation and the air quality is deteriorating further.Moreover, boundary layer dynamics &#8211; as well as meteorological conditions in general &#8211; are interlinked with the pollution regimes, e.g., changes in meteorological or pollution conditions might cause shifts in either way in different regimes described.We are utilizing continuous, comprehensive observations of atmospheric composition and fluxes from two flagship stations in the gigacity region: the SORPES in Nanjing (Ding et al., 2016) and the BUCT-AHL in Beijing (Liu et al., 2020). In addition, the national meteorological and air quality networks will enable spatial analyses together with reanalysis data.We are focusing especially on: 1) how boundary layer dynamics and local and synoptic scale meteorological conditions affect haze severity and vice versa, 2) the effect of gigacity heat and pollution islands and surface roughness on atmospheric circulation and precipitation. Our preliminary results have shown e.g. that in Beijing the radiative effect of haze on the BLH has a strong seasonal behaviour with a dependence on the surface heat fluxes. The effect was strongest during autumn, winter and spring months when the decrease of BLH was 40&#8211;56 % due to the haze.Ding et al., Geophys Res Lett, 43, 2873-2879, 2016.Kokkonen et al., Atmos Chem Phys, 19, 7001-7017, 2019.Kulmala et al., Atmos. Chem. Phys., 21, 8313-8322, 2021.Kulmala et al., Environ Sci Atmos, 2, 352-361, 2022.Liu et al., Big Earth Data, 4, 295-321, 2020.