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Atmospheric deposition is a source of potentially bioavailable iron (Fe) and thus can partially control biological productivity in large parts of the ocean. However, the explanation of observed high aerosol Fe solubility compared to that in soil particles is still controversial, as several hypotheses have been proposed to explain this observation. Here, a statistical analysis of aerosol Fe solubility estimated from four models and observations compiled from multiple field campaigns suggests that pyrogenic aerosols are the main sources of aerosols with high Fe solubility at low concentration. Additionally, we find that field data over the Southern Ocean display a much wider range in aerosol Fe solubility compared to the models, which indicate an underestimation of labile Fe concentrations by a factor of 15. These findings suggest that pyrogenic Fe-containing aerosols are important sources of atmospheric bioavailable Fe to the open ocean and crucial for predicting anthropogenic perturbations to marine productivity.

Integrated measurements of aerosol, radiation, cloud, and turbulent transport in the planetary boundary layer (PBL) are essential for understanding and modeling climate and air quality. Here, we developed a new technique for the identification of convective turbulent regions and deriving the vertical distribution of aerosol turbulent mass fluxes within PBL. The algorithm uses retrievals from coherent Doppler lidars and a high spectral resolution lidar. The technique was applied to study particle mass fluxes over 2 months (November–December 2020) during the campaign conducted at the DOE Atmospheric Radiation Measurement Southern Great Plains (SGP) site in Lamont, Oklahoma. The algorithm developed here is capable of continuously deriving vertically resolved (curtains) aerosol mass fluxes. Our data analysis shows that at the site, the 30‐min averaged fluxes at 135 m above the surface were mainly positive (upward) at ∼1 μg m−2 s−1, suggesting that the surface is the primary source of the particle mass supplied to the boundary layer at the SGP site. Analyses of the individual case studies have revealed that not all the derived fluxes can be linked to surface emissions. Both positive and negative values in a range of ±5 μg m−2 s−1 can be caused by convective thermals interacting between the residual layer and the mixed layer and by rotation of the horizontal wind with the height. Large erroneous negative fluxes can also be caused by drizzling/precipitating clouds. We anticipate that the application of the current technique will lead to a more realistic representation of aerosol mass budgets and bidirectional mixing rates. Plain Language Summary: Fine particles suspended in the atmosphere can influence the air quality and climate. Therefore, improved quantification of production mechanisms and removal of atmospheric particles is a topic of interest for both air quality and climate sciences. Some particles are produced through wind‐driven processes and are emitted directly into the atmosphere, whereas others are formed in the atmosphere through complex processes involving nucleation or condensation of vapor species. Researchers have been using flux measurements to characterize sources and sinks of atmospheric particles. However, most of the flux measurements are conducted with the instruments mounted on low‐level masts. Such measurements cannot be used for the characterization of particle fluxes from complex inhomogeneous terrains. Here we apply ground‐based remote sensing techniques to infer vertical profiles of particle mass fluxes. The analysis was performed using remote sensing equipment at the DOE Atmospheric Radiation Measurement Southern Great Plains site in Lamont, Oklahoma. Our study revealed that the majority of the aerosol mass fluxes at the site were driven by mineral dust, mobilized from the surface by strong winds characteristic to the site. We expect our study will contribute to an improved understanding of aerosol transport in the atmosphere. Key Points: The particle linear depolarization ratio can be used for capturing the growth of a convective mixing layer after sunriseThe aerosol mass fluxes at the Southern Great Plains site were positive, suggesting surface‐emitted dust to be a major source of aerosolKnowledge of the planetary boundary layer structure and the position of clouds is necessary for proper interpretations of aerosol flux data [ABSTRACT FROM AUTHOR]

Abstract. This work reports on the current status of the global modeling of iron (Fe)deposition fluxes and atmospheric concentrations and the analyses of thedifferences between models, as well as between models and observations. Atotal of four global 3-D chemistry transport (CTMs) and general circulation(GCMs) models participated in this intercomparison, in the framework ofthe United Nations Joint Group of Experts on the Scientific Aspects of MarineEnvironmental Protection (GESAMP) Working Group 38, “The Atmospheric Inputof Chemicals to the Ocean”. The global total Fe (TFe) emission strength inthe models is equal to ∼72 Tg Fe yr−1 (38–134 Tg Fe yr−1)from mineral dust sources and around 2.1 Tg Fe yr−1 (1.8–2.7 Tg Fe yr−1)from combustion processes (the sum of anthropogeniccombustion/biomass burning and wildfires). The mean global labile Fe (LFe)source strength in the models, considering both the primary emissions and theatmospheric processing, is calculated to be 0.7 (±0.3) Tg Fe yr−1,accounting for both mineral dust and combustion aerosols. Themean global deposition fluxes into the global ocean are estimated to be in the rangeof 10–30 and 0.2–0.4 Tg Fe yr−1 for TFe and LFe, respectively,which roughly corresponds to a respective 15 and 0.3 Tg Fe yr−1 for the multi-model ensemble model mean. The model intercomparison analysis indicates that the representation of theatmospheric Fe cycle varies among models, in terms of both the magnitude ofnatural and combustion Fe emissions as well as the complexity of atmosphericprocessing parameterizations of Fe-containing aerosols. The model comparisonwith aerosol Fe observations over oceanic regions indicates that most modelsoverestimate surface level TFe mass concentrations near dust sourceregions and tend to underestimate the low concentrations observed in remoteocean regions. All models are able to simulate the tendency of higher Feconcentrations near and downwind from the dust source regions, with the meannormalized bias for the Northern Hemisphere (∼14), largerthan that of the Southern Hemisphere (∼2.4) for the ensemble modelmean. This model intercomparison and model–observation comparison studyreveals two critical issues in LFe simulations that require furtherexploration: (1) the Fe-containing aerosol size distribution and (2) therelative contribution of dust and combustion sources of Fe to labile Fe inatmospheric aerosols over the remote oceanic regions.

Quantifying surface–atmosphere exchange rates of particles is important for understanding the role of suspended particulate matter in radiative transfer, clouds, precipitation, and climate change. Emissions of coarse-mode particles with a diameter greater than 0.5 µm provide giant cloud condensation nuclei and ice nuclei. These emissions are critical for understanding the evolution of cloud microphysical properties yet remain poorly understood. Here we introduce a new method that uses lidar retrievals of the elastic backscatter and Doppler velocity to obtain surface number emissions of particles with a diameter greater than 0.53 µm. The technique is applied to study particle number fluxes over a 2-month period from 1 June to 10 August 2022 during the TRACER campaign at an urban site near Houston, TX, USA. We found that all the observed fluxes were positive (upwards), indicating particle emission from the surface. The fluxes followed a diurnal pattern and peaked near noon local time. Flux intensity varied through the 2 months with multi-day periods of strong fluxes and multi-day periods of weak fluxes. Emission particle number fluxes peaked near ∼ 100 cm -2 s -1. The daily averaged emission fluxes correlated with friction velocity and were anticorrelated with surface relative humidity. The emission flux can be parameterized as F= 3000 u*4 , where u* is the friction velocity in m s -1 and the emission flux F is in cm -2 s -1. The u* dependence is consistent with emission from wind-driven erosion. Estimated values for the mass flux are in the lower range of literature values from non-urban sites. These results demonstrate that urban environments may play an important role in supplying coarse-mode particles to the boundary layer. We anticipate that quantification of these emissions will help constrain aerosol–cloud interaction models that use prognostic aerosol schemes. [ABSTRACT FROM AUTHOR]

Fine particulate matter (i.e., PM2.5) has gained extensive attention owing to its adverse effects. The impacts of PM2.5 may vary in time and space due to the spatiotemporal variations of PM2.5 number size distribution and chemical compositions. This research analyzed the latest PM2.5 chemical compositions measurements with an aim to better understand the dynamic changes of PM2.5 in response to emission reductions due to the new regulations. The particulate measurements from the Southeastern Aerosol Research and Characterization (SEARCH) network between 2001 and 2016 were analyzed for the spatiotemporal variations of PM2.5 and inorganic PM2.5 (iPM2.5 = SO42− + NH4+ + NO3−) chemical compositions in the Southeastern United States (U.S.). It was discovered that PM2.5 and iPM2.5 mass concentrations exhibited significant downward trends in 2001–2016. Both PM2.5 and iPM2.5 mass concentrations were higher at urban and inland sites than rural/suburban and coastal sites. The higher iPM2.5 concentrations at agricultural sites were attributed to the influences of ammonia (NH3) emissions from animal feeding operations (AFOs). The iPM2.5 was the dominant contributor to PM2.5 in 2001–2016 at the coastal sites, whereas organic carbon matter (OCM) was the major contributor to PM2.5 after 2011 at the inland sites. Our data analysis suggests that significant decrease of PM2.5 concentrations is attributed to the reductions in nitrogen oxides (NOx) and sulfur dioxide (SO2) emissions in 2001–2016. Findings from this research provide insights into the development of effective PM2.5 control strategies and assessment of air pollutants exposure. [ABSTRACT FROM AUTHOR]

This work reports on the current status of global modelling of iron (Fe) deposition fluxes and atmospheric concentrations and analyses of the differences between models, as well as between models and observations. A total of four global 3-D chemistry-transport (CTMs) and general circulation (GCMs) models have participated in this intercomparison, in the framework of the United Nations Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) Working Group 38, "The Atmospheric Input of Chemicals to the Ocean". The global total Fe (TFe) emissions strength in the models is equal to ~ 72 Tg-Fe yr-1 (38-134 Tg-Fe yr-1) from mineral dust sources and around 2.1 Tg-Fe yr-1 (1.8-2.7 Tg-Fe yr-1) from combustion processes (sum of anthropogenic combustion/biomass burning and wildfires). The mean global labile Fe (LFe) source strength in the models, considering both the primary emissions and the atmospheric processing, is calculated to be 0.7 (±0.3) Tg-Fe yr-1, accounting for mineral dust and combustion aerosols together. The multi model ensemble global TFe and LFe deposition fluxes into the global ocean are calculated to be ~ 15 Tg-Fe yr-1 and ~ 0.3 Tg-Fe yr-1, respectively. The model intercomparison analysis indicates that the representation of the atmospheric Fe cycle varies among models, in terms of both the magnitude of natural and combustion Fe emissions as well as the complexity of atmospheric processing parametrizations of Fe-containing aerosols. The model comparison with aerosol Fe observations over oceanic regions indicate that most models overestimate surface level TFe mass concentrations near the dust source regions and tend to underestimate the low concentrations observed in remote ocean regions. All models are able to simulate the tendency of higher Fe loading near and downwind from the dust source regions, with the mean normalized bias for the Northern Hemisphere (~ 14), larger than the Southern Hemisphere (~ 2.4) for the ensemble model mean. This model intercomparison and model--observation comparison study reveals two critical issues in LFe simulations that require further exploration: 1) the Fe-containing aerosol size distribution and 2) the relative contribution of dust and combustion sources of Fe to labile Fe in atmospheric aerosols over the remote oceanic regions. [ABSTRACT FROM AUTHOR]