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Interannual Relationship between Haze Days in December-January and Satellite-Based Leaf Area Index in August-September over Central North China

Haze pollution in central North China has become a hot topic in recent decades due to its serious environmental and health effects. In this work, the interannual relationship between haze days in December–January (DJ_HD) and leaf area index in August–September (AS_LAI) over central North China, along with the possible physical mechanisms involved, are investigated. The relationship varies in different periods, being significant during 1982–2000 (P1) but insignificant during 2001–2014 (P2). During P1, there is an in-phase relationship between AS_LAI and surface evaporation, and an out-of-phase relationship between AS_LAI and surface albedo in August–September. The surface evaporation and albedo anomalies persist to October–November and are associated with lower top-layer volumetric soil water, upward sensible heat flux and downward latent heat flux anomalies in October–November, which act as the bridge in the relationship between AS_LAI and DJ_HD. Both the volumetric soil water and heat fluxes anomalies persist to December–January and correspond to atmospheric circulations similar to the weakened East Asian winter monsoon pattern, which is the dominant system for winter haze events. Thus, the ventilation conditions in December–January are favorable for the accumulation of haze particles. However, during P2, the relationships are not significant between AS_LAI and volumetric soil water or surface soil temperature during October–January. Meanwhile, the East Asian winter monsoon is likely strengthened and tends to be more significantly affected by factors including Arctic sea ice, Arctic Oscillation, etc. Therefore, the effects of AS_LAI on the monsoon may become insignificant and, in turn, the relationship between AS_LAI and DJ_HD becomes insignificant during P2.