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Abstract This study evaluates a popular density current propagation speed equation using a large, novel set of radiosonde and dropsonde observations. Data from pairs of sondes launched inside and outside of cold pools along with the theoretical density current propagation speed equation are used to calculate sonde-based propagation speeds. Radar-/satellite-based propagation speeds, assumed to be the truth, are calculated by manually tracking the propagation of cold pools and correcting for advection due to the background wind. Several results arise from the comparisons of the theoretical sonde-based speeds with the radar-/satellite-based speeds. First, sonde-based and radar-based propagation speeds are strongly correlated for U.S. High Plains cold pools, suggesting the density current propagation speed equation is appropriate for use in midlatitude continental environments. Second, cold pool Froude numbers found in this study are in agreement with previous studies. Third, sonde-based propagation speeds are insensitive to how cold pool depth is defined since the preponderance of negative buoyancy is near the surface in cold pools. Fourth, assuming an infinite channel depth and assuming an incompressible atmosphere when deriving the density current propagation speed equation can increase sonde-based propagation speeds by up to 20% and 11%, respectively. Finally, sonde-based propagation speeds can vary by ∼300% based on where and when the sondes were launched, suggesting submesoscale variability could be a major influence on cold pool propagation. 

Abstract Ragweed pollen is a prevalent allergen in late summer and autumn, worsening seasonal allergic rhinitis and asthma symptoms. In the atmosphere, pollen can osmotically rupture to produce sub-pollen particles (SPP). Because of their smaller size, SPP can penetrate deeper into the respiratory tract than intact pollen grains and may trigger severe cases of asthma. Here we characterize airborne SPP forming from rupturing giant ragweed ( Ambrosia trifida ) pollen for the first time, using scanning electron microscopy and single-particle fluorescence spectroscopy. SPP ranged in diameter from 20 nm to 6.5 μm. Most SPP are capable of penetrating into the lower respiratory tract, with 82% of SPP < 1.0 μm, and are potential cloud condensation nuclei, with 50% of SPP < 0.20 μm. To support predictions of the health and environmental effects of SPP, we have developed a quantitative method to estimate the number of SPP generated per pollen grain ( $${n}_{\mathrm{f}}$$ n f ) based upon the principle of mass conservation. We estimate that one giant ragweed pollen grain generates 1400 SPP across the observed size range. The new measurements and method presented herein support more accurate predictions of SPP occurrence, concentration, and air quality impacts that can help to reduce the health burden of allergic airway diseases. Graphic abstract Rupturing ragweed pollen releasing cellular components (right), viewed by an inverted light microscope. 

Abstract. Understanding the impact of sea spray aerosol (SSA) on theclimate and atmosphere requires quantitative knowledge of their chemicalcomposition and mixing states. Furthermore, single-particle measurements areneeded to accurately represent large particle-to-particle variability. Toquantify the mixing state, the organic volume fraction (OVF), defined as therelative organic volume with respect to the total particle volume, ismeasured after generating and collecting aerosol particles, often usingdeposition impactors. In this process, the aerosol streams are either driedor kept wet prior to impacting on solid substrates. However, the atmosphericcommunity has yet to establish how dry versus wet aerosol depositioninfluences the impacted particle morphologies and mixing states. Here, weapply complementary offline single-particle atomic force microscopy (AFM)and bulk ensemble high-performance liquid chromatography (HPLC) techniquesto assess the effects of dry and wet deposition modes on thesubstrate-deposited aerosol particles' mixing states. Glucose and NaClbinary mixtures that form core–shell particle morphologies were studied asmodel systems, and the mixing states were quantified by measuring the OVF ofindividual particles using AFM and compared to the ensemble measured byHPLC. Dry-deposited single-particle OVF data positively deviated from thebulk HPLC data by up to 60&thinsp;%, which was attributed to significantspreading of the NaCl core upon impaction with the solid substrate. This ledto underestimation of the core volume. This problem was circumvented by (a) performing wet deposition and thus bypassing the effects of the solid corespreading upon impaction and (b) performing a hydration–dehydration cycle ondry-deposited particles to restructure the deformed NaCl core. Bothapproaches produced single-particle OVF values that converge well with thebulk and expected OVF values, validating the methodology. These findingsillustrate the importance of awareness in how conventional particledeposition methods may significantly alter the impacted particlemorphologies and their mixing states. 

We report on the sensitivity of enhanced ozone (O3) production, observed during lake breeze circulation along the coastline of Lake Michigan, to the concentrations of nitrogen oxides (NOx = NO + NO2) and volatile organic compounds (VOCs). We assess the sensitivity of O3 production to NOx and VOC on a high O3 day during the Lake Michigan Ozone Study 2017 (LMOS 2017) using an observationally-constrained chemical box model that implements the Master Chemical Mechanism (MCM v3.3.1) and recent emission inventories for NOx and VOCs. The MCM model is coupled to a backward air mass trajectory analysis from a ground supersite in Zion, IL where an extensive series of measurements of O3 precursors and their oxidation products, including hydrogen peroxide (H2O2), nitric acid (HNO3), and particulate nitrates (NO3-) serve as model constraints. We evaluate the chemical evolution of the Chicago-Gary urban plume as it advects over Lake Michigan and demonstrate how modeled indicators of VOC- vs. NOx- sensitive regimes can be constrained by measurements at the trajectory endpoint. Using the modeled ratio of the instantaneous H2O2 and HNO3 production rates (PH2O2 / PHNO3), we suggest that O3 production over the urban source region is strongly VOC-sensitive and progresses towards a more NOx-sensitive regime as the plume advects north along the Lake Michigan coastline on this day. We also demonstrate that ground-based measurements of the mean concentration ratio of H2O2 to HNO3 describe the sensitivity of O3 production to VOC and NOx as the integral of chemical production along the plume path.