Characteristics of fine particle matters at the top of Shanghai Tower

To investigate the physical and chemical processes of fine particle matters at mid-upper planetary boundary layer (PBL), we conducted one-year continuous measurements of fine particle matters (PM), chemical composition of non-refractory submicron aerosol (NR-PM₁) and some gas species (including sulfur dioxide, nitrogen oxides and ozone) at an opening observatory (~600 m) at the top of Shanghai Tower (SHT), which is the Chinese 1^st and World’s 2^nd highest building located in the typical financial central business district of Shanghai, China. This is the first report for the characteristics of fine particles based on continuous and sophisticated online measurements at the mid-upper level of urban PBL. The observed PM_2.5 and PM₁ mass concentrations at SHT were 25.5±17.7 and 17.3±11.7 μg m^-3 respectively. Organics, nitrate (NO₃) and sulfate (SO₄) occupied the first three leading contributions to NR-PM₁ at SHT, accounting for 35.8 %, 28.6 % and 20.8 % respectively. The lower PM_2.5 concentration was observed at SHT by 16.4 % compared with that near surface during the observation period. It was attributed to the decreased nighttime PM_2.5 concentrations (29.4 % lower than surface) at SHT in all seasons due to the complete isolations from both emissions and gas precursors near surface. However, daytime PM_2.5 concentrations at SHT were 12.4-35.1 % higher than those near surface from June to October, resulted from unexpected larger PM_2.5 levels during early to middle afternoon at SHT than surface. We suppose the significant chemical production of secondary aerosols existed in mid-upper PBL because strong solar irradiance, adequate gas precursors (e.g., NOx) and lower temperature were observed at SHT favorable for both photochemical production and gas-to-particle partitioning. This was further demonstrated by the significant increasing rate of oxygenated organic aerosols and NO₃ observed at SHT during 8:00-12:00 in spring (7.4 % h^-1 and 12.9 % h^-1), autumn (9.3 % h^-1 and 9.1 % h^-1) and summer (13.0 % h^-1 and 11.4 % h^-1), which cannot be fully explained by vertical mixing. It was noting that extremely high NO₃ was observed at SHT both in daytime and nighttime in winter, accounting for 37.2 % in NR-PM₁, suggesting the efficient pathway from heterogeneous and gas oxidated formation. Therefore, we highlight the priority of NOx reduction in Shanghai for the further improvement of air quality. This study reported greater daytime PM_2.5 concentrations at the height of 600 m in urban PBL compared with surface measurement, providing insight into their potential effects on local air quality, radiation forcing, and cloud/fog formations. We propose that the efficient production of secondary aerosol in mid-upper PBL should be cognized and explored more comprehensively by synergetic observations in future.