Understanding the evolution of brown carbon (BrC) in the atmosphere is essential for investigating its climate effects. This study deployed a novel in‐situ BrC continuous observation system to firstly measure water‐soluble BrC absorption (AbsWS‐BrC) in an offshore island over the Bohai Sea in the winter of 2020. The AbsWS‐BrC abundance before the cold wave (BCW) was more than twice higher than that after the cold wave (ACW). This was mainly ascribed to the substantially suppressed formation of secondary WS‐BrC (WS‐BrCsec). Diurnal patterns of AbsWS‐BrC exhibited nighttime peaks, which derived from enhanced primary emissions and strong aqueous‐phase processes. Photochemical processes bleached daytime WS‐BrC during BCW, while daytime peaks of WS‐BrCsec emerged during ACW due to the weakened photobleaching effect. Statistical analysis indicated ambient temperature and relative humidity as well as total oxidized nitrogen (NO2 + NO3−) and reduced nitrogen (NH4+ + NH3) were the dominant factors promoting WS‐BrCsec. Moderate aerosol pH (>2.5) also facilitated the formation of WS‐BrCsec while no obvious dependences of WS‐BrCsec on gas‐particle partitioning of ammonia, O3, and sulfur precursors were found. At the molecular level, BrC chromophores with the identified compositions, carbon oxidation state, O/C, H/C, and absorption spectra were compared between the two periods. Characteristics of water‐soluble CHO‐ and CHON‐BrC were more of secondary origin during BCW, while primary emissions contributed significantly to BrC chromophores during ACW. This study highlights the advantage of high resolution BrC measurement in probing its dynamic evolution and influencing factors.

Wind erosion is a common but critical geophysical process, especially in arid and semi-arid regions. It is found that the emission flux of dust with a geometric diameter up to 20 μm (PM 20 ) is approximately 5,000 Tg yr −1 (Kok et al., 2021). Dust aerosols have crucial impacts on human health, weather, climate, and biogeochemical cycles. Dust aerosols inhaled by humans can negatively affect the respiratory system and lungs (Brunekreef & Holgate, 2002;Choudhury et al., 1997). In addition, dust aerosols can affect climate through changing radiative forcing and impacting cloud formation by acting as cloud condensation nuclei or ice nuclei (Miller & Tegen, 1998;Ramanathan et al., 2001). Moreover, the deposition of dust aerosols in ocean and land systems is an important way to transport nutrients, such as iron-containing compounds (Jickells, 2005;Okin et al., 2004).Wind erosion occurs when the wind speed exceeds a threshold velocity and forces soil particles to move. Large soil particles would strike the surface and collide with each other, and then shake off smaller particles which can be lifted up by wind and form dust aerosols (Y. P. Shao, 2008). For soil particles to be lifted up into the air, the forces associated with the wind shear near the surface or the collision forces among moving soil particles must overcome the gravitational force and cohesive forces, including wet-bonding forces, van der Waals forces and electrostatic forces between particle surfaces (Y. P. Shao, 2008). It is also interesting to note that dust aerosols usually carry electrical charges. Soils are generally electrically neutral. However, charges are separated, transferred, or redistributed when soil particles colliding with each other, which results in a net charge transfer between these particles (Harrison et al., 2010;Kok & Renno, 2009;Wei & Gu, 2015). It is observed that soil particles are indeed charged in a wind tunnel (Zheng et al., 2004). And electric field generated by suspended charged dust aerosols could, in turn, affects dust emission under various relative humidity (RH) conditions (Esposito et al., 2016).

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Abstract. Dust is a major source of atmospheric aerosols. Its chemical composition is often assumed to be similar to the parent soil. However, this assumption has not been rigorously verified. Here, we generated dust aerosols from soils to determine if there is particle size-dependent selectivity of heavy metals in the dust generation. Mn, Cd, Pb and other heavy metals were found to be highly enriched in fine (PM2.5) dust aerosols, which can be up to ~6.5-fold. To calculate the contributions of dust to atmospheric heavy metals, regional air quality models usually use the dust chemical profiles from the US EPA’s SPECIATE database, which does not capture the correct size-dependent selectivity of heavy metals in dust aerosols. Our air quality modeling for China demonstrates that the calculated contribution of fine dust aerosols to atmospheric heavy metals, as well as their cancer risks, could have significant errors without using proper dust profiles.