New particle formation and aerosol dynamics from polluted urban to pristine rainforest environments
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Abstract
Aerosol particles play a crucial role in cloud formation and the Earth's energy balance, with significant impacts on both climate and human health. This study explores new particle formation (NPF) and aerosol dynamics across three distinct environments: an urban area in Mainz, Germany; a pristine site in the Amazon rainforest (ATTO site, Brazil); and a polluted region in the North China Plain (Gucheng, China). The study employs state-of-the-art instruments to characterize aerosol properties, including particle and cluster number size distribution (PNSD), particle formation rates (J), growth rates (GR), and diurnal variations.
In Mainz, Germany, measurements were conducted between March and July 2022 using the Nano-Scanning Mobility Particle Sizer (Nano-SMPS), Particle Size Magnifier (PSM), and Neutral Cluster and Air Ion Spectrometer (NAIS) to assess NPF events and nucleation mode particle dynamics. The results showed that 11 days (8 %) out of 132 days were identified as NPF event days, which is lower than some megacities such as Beijing and comparable to other European cities such as Leipzig. During event periods, the formation rate (J≤1.5 nm) ranged from 0.3 to 11.3 cm-3 s-1, and the growth rate (particle size range 1.5-20 nm) ranged from 1.9 to 6.6 nm h-1. In addition, the condensation sink (CS) values ranged from (1.8-6.9) × 10-3 s-1. It was found that meteorological factors such as low relative humidity, low wind speed and strong solar radiation are favorable for the occurrence of NPF events. Meanwhile, the importance of precise measurement of aerosol PNSD is highlighted. The accurate measurement of particles within the sub-10 nm size range, where technical challenges are present due to low detection efficiency and high diffusion loss, was achieved through the regular calibration of the instrument, the optimization and shortening of sampling lines, and the precise correction of losses.
The Amazon Tall Tower Observatory (ATTO) site in the Amazon rainforest provides a unique opportunity to study aerosol dynamics in a pristine environment. As part of the Chemistry of the Atmosphere: Field Experiment in Brazil (CAFE-Brazil) campaign, particles and cluster ions with diameter smaller than 40 nm were continuously observed in the Amazonian boundary layer in December 2022 and January 2023. The median concentration of sub-3 nm particles during the observation period was 491 cm-3, accounting for 51 % of the total particle concentration, with a median of 969 cm-3. The median concentrations of naturally charged clusters (0.8-2 nm), intermediate (2-7 nm) and large (7-20 nm) ions were 624, 33 and 30 cm-3 respectively. Despite the presence of these sub-3 nm particles and ions, typical regional NPF events were not detected. However sub-3 nm particles showed a clear diurnal variation, with concentrations demonstrating a gradual increase from sunrise until midday, followed by a decline until midnight. Furthermore, on days with minimal pollution influence, daytime concentrations of sub-3 nm were observed to significantly exceed those recorded on polluted days. These daytime maximums are likely related to the photochemical formation of precursor vapors from different sources.
In Gucheng, China, aerosol measurements showed extremely high particle number concentrations (median 20800 cm-3) and mass concentrations (median 121 µg m-3), reflecting the severe pollution in this region. The campaign was conducted during the winter period from November to December 2018. Size-resolved PNSD indicate that Aitken mode particles are less sensitive to relative humidity (RH) than accumulation mode particles. Specifically, the log-normal fitted diameters for Aitken mode particles were 31 nm and 33 nm for high and low RH conditions, respectively, whereas the fitted diameters for accumulation mode particles were 125 nm and 120 nm, respectively. However, the chemical composition of the aerosols clearly varied with RH, with higher RH favoring the formation of secondary inorganic and organic matter (Sun et al., 2020b).
The findings from Mainz contribute to our knowledge of aerosol dynamics, offering insights into NPF processes in urban environments and their potential impact on air quality and climate modeling. The study in the Amazon rainforest provides valuable data on nanoparticle dynamics in pristine environments, and paving the way for future studies of gas-phase precursors to aerosol formation in the Amazon basin and similar regions. The findings in Gucheng highlight the complex interactions between aerosol dynamics, chemical composition and environmental factors in polluted regions. Overall, this study provides valuable insights into aerosol particle dynamics and NPF processes in urban, pristine and polluted environments. The ultimate goal was to improve our understanding of aerosol dynamics and NPF processes in different environments, with a focus on how natural and anthropogenic forcing shape particle size distributions, chemical composition, and diurnal variations. This knowledge will help to improve air quality models, better assess the impact of aerosols on climate change, more precise radiative forcing estimates, and support the development of more effective emission control policies, especially in regions with different levels of human impact.