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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 Recent studies have suggested that microbial aerosolization in wildfire smoke is an understudied source of microbes to the atmosphere. Wildfire smoke can travel thousands of kilometers from its source with the potential to facilitate the transport of microbes, including microbes that can have far‐reaching impacts on human or ecosystem health. However, the relevance of longer‐range detection of microbes in smoke plumes remains undetermined, as previous studies have mainly focused on analyses of bioaerosols collected adjacent to or directly above wildfires. Therefore, we investigated whether wildfire smoke estimated to originate >30 km from different wildfire sources would contain detectable levels of bacterial and fungal DNA at ground level, hypothesizing that smoke‐impacted air would harbor greater amounts and a distinct composition of microbes as compared to ambient air. We used cultivation‐independent approaches to analyze 150 filters collected over time from three sampling locations in the western United States, of which 34 filters were determined to capture wildfire smoke events. Contrary to our hypothesis, smoke‐impacted samples harbored lower amounts of microbial DNA. Likewise, there was a limited signal in the composition of the microbial assemblages detected in smoke‐affected samples as compared to ambient air, but we did find that changes in humidity were associated with temporal variation in the composition of the bacterial and fungal bioaerosols. With our study design, we were unable to detect a robust and distinct microbial signal in ground‐level smoke originating from distant wildfires. 

Abstract. Sea spray aerosols (SSAs) represent one of the most abundant aerosol types on a global scale and have been observed at all altitudes including the upper troposphere. SSA has been explored in recent years as a source of ice-nucleating particles (INPs) in cirrus clouds due to the ubiquity of cirrus clouds and the uncertainties in their radiative forcing. This study expands upon previous works on low-temperature ice nucleation of SSA by investigating the effects of atmospheric aging of SSA and the ice-nucleating activity of newly formed secondary marine aerosols (SMAs) using an oxidation flow reactor. Polydisperse aerosol distributions were generated from a marine aerosol reference tank (MART) filled with 120 L of real or artificial seawater and were dried to very low relative humidity to crystallize the salt constituents of SSA prior to their subsequent freezing, which was measured using a continuous flow diffusion chamber (CFDC). Results show that for primary SSA (pSSA), as well as aged SSA and SMA (aSSA+SMA) at temperatures >220 K, homogeneous conditions (92 %–97 % relative humidity with respect to water – RHw) were required to freeze 1 % of the particles. However, below 220 K, heterogeneous nucleation occurs for both pSSA and aSSA+SMA at much lower RHw, where up to 1 % of the aerosol population freezes between 75 % and 80 % RHw. Similarities between freezing behaviors of the pSSA and aSSA+SMA at all temperatures suggest that the contributions of condensed organics onto the pSSA or alteration of functional groups in pSSA via atmospheric aging did not hinder the major heterogeneous ice nucleation process at these cirrus temperatures, which have previously been shown to be dominated by the crystalline salts. Occurrence of a 1 % frozen fraction of SMA, generated in the absence of primary SSA, was observed at or near water saturation below 220 K, suggesting it is not an effective INP at cirrus temperatures, similar to findings in the literature on other organic aerosols. Thus, any SMA coatings on the pSSA would only decrease the ice nucleation behavior of pSSA if the organic components were able to significantly delay water uptake of the inorganic salts, and apparently this was not the case. Results from this study demonstrate the ability of lofted primary sea spray particles to remain an effective ice nucleator at cirrus temperatures, even after atmospheric aging has occurred over a period of days in the marine boundary layer prior to lofting. We were not able to address aging processes under upper-tropospheric conditions. 

Abstract Supercooled liquid clouds are ubiquitous over the Southern Ocean (SO), even to temperatures below −20°C, and comprise a large fraction of the marine boundary layer (MBL) clouds. Earth system models and reanalysis products have struggled to reproduce the observed cloud phase distribution and occurrence of cloud ice in the region. Recent simulations found the microphysical representation of ice nucleation and growth has a large impact on these properties, however, measurements of SO ice nucleating particles (INPs) to validate simulations are sparse. This study presents measurements of INPs from simultaneous aircraft and ship campaigns conducted over the SO in austral summer 2018, which include the first in situ observations in and above cloud in the region. Our results confirm recent observations that INP concentrations are uniformly lower than measurements made in the late 1960s. While INP concentrations below and above cloud are similar, higher ice nucleation efficiency above cloud supports model simulations that the dominant INP composition varies with height. Model parameterizations based solely on aerosol properties capture the mean relationship between INP concentration and temperature but not the observed variability, which is likely related to the only modest correlations observed between INPs and environmental or aerosol metrics. Including wind speed in addition to activation temperature in a marine INP parameterization reduces bias but does not explain the large range of observed INP concentrations. Direct and indirect inference of marine INP size suggests MBL INPs, at least during Austral summer, are dominated by particles with diameters smaller than 500 nm. 

Abstract Iron emissions from human activities, such as oil combustion and smelting, affect the Earth's climate and marine ecosystems. These emissions are difficult to quantify accurately due to a lack of observations, particularly in remote ocean regions. In this study, we used long‐term, near‐source observations in areas with a dominance of anthropogenic iron emissions in various parts of the world to better estimate the total amount of anthropogenic iron emissions. We also used a statistical source apportionment method to identify the anthropogenic components and their sub‐sources from bulk aerosol observations in the United States. We find that the estimates of anthropogenic iron emissions are within a factor of 3 in most regions compared to previous inventory estimates. Under‐ or overestimation varied by region and depended on the number of sites, interannual variability, and the statistical filter choice. Smelting‐related iron emissions are overestimated by a factor of 1.5 in East Asia compared to previous estimates. More long‐term iron observations and the consideration of the influence of dust and wildfires could help reduce the uncertainty in anthropogenic iron emissions estimates. 

Abstract. Rapid Arctic climate warming, amplified relative to lower-latitude regions, has led to permafrost thaw and associated thermokarst processes. Recent work has shown permafrost is a rich source of ice-nucleating particles (INPs) that can initiate ice formation in supercooled liquid clouds. Since the phase of Arctic clouds strongly affects the surface energy budget, especially over ice-laden surfaces, characterizing INP sources in this region is critical. For the first time, we provide a large-scale survey of potential INP sources in tundra terrain where thermokarst processes are active and relate to INPs in the air. Permafrost, seasonally thawed active layer, ice wedge, vegetation, water, and aerosol samples were collected near Utqiaġvik, Alaska, in late summer and analyzed for their INP contents. Permafrost was confirmed as a rich source of INPs that was enhanced near the coast. Sensitivity to heating revealed differences in INPs from similar sources, such as the permafrost and active layer. Water, vegetation, and ice wedge INPs had the highest heat-labile percentage. The aerosol likely contained a mixture of known and unsurveyed INP types that were inferred as biological. Arctic water bodies were shown to be potential important links of sources to the atmosphere in thermokarst regions. Therefore, a positive relationship found with total organic carbon considering all water bodies gives a mechanism for future parameterization as permafrost continues to thaw and drive regional landscape shifts. 

Oceans emit ice-nucleating particles (INPs) which freeze supercooled cloud droplets, modifying clouds. We added dead biomass of three phytoplankton to seawater. Each time, this stimulated INP production in the water and INP emissions in sea spray.