Submicron Aerosol Particles Research Articles

Direct observation of Arctic Sea salt aerosol production from blowing snow and modeling over a changing sea ice environment

In the polar regions, there is significant model bias in the number concentrations and seasonality of sea salt aerosol (SSA) due to the lack of understanding of aerosol sources associated with sea ice, which is hampering accurate climate forecasts at high latitudes. Recently, SSA originating from the sublimation of blowing snow has been directly observed to be an important source of aerosol particles in the Antarctic during winter/spring, validating a mechanism proposed a decade ago. Here, we report in situ observations of coarse aerosol production (particle diameter 0.5–20.0 µm) dominated by sea salt from blowing snow above sea ice during winter/spring in the Central Arctic during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition from October 2019 to September 2020. Blowing snow conditions occurred 20–40% of the time during each of the months from December 2019 to April 2020, with a total of 26 blowing snow events. During blowing snow periods, coarse aerosol number concentrations increased often by an order of magnitude compared to no-blowing snow periods. Mass fractions of sodium chloride in sub-micron aerosol (particle diameter 0.01–0.625 µm) available during December 2019 and 10 m wind speed showed a significant correlation (R = 0.61, P < 0.05), indicating that much of the aerosol observed during storms is sea salt released by sublimating blowing snow. We use these observations to refine the current model parameterization by considering the spatial and temporal variability of atmospheric and sea ice conditions. Snow particle size distributions and snow salinities are expressed as a function of wind speed and snowpack depth, respectively, which can be easily implemented into climate models. Validation of the snow particle size distribution parameterization with previous polar winter observations showed agreement in the Arctic (N-ICE2015 cruise, March 2015) above the threshold for drift and blowing snow, but a negative bias in the Antarctic (Weddell Sea, June to August 2013). Updating the blowing snow mechanism in the chemical transport model p-TOMCAT with wind-dependent snow particle size distributions results in 14% more SSA produced and a slightly better correlation with MOSAiC observations of coarse aerosol (R = 0.28). Significant increases in aerosol number concentration due to blowing snow sublimation are calculated by as much as 70 cm−3 during the Antarctic winter and 50 cm−3 during the Arctic winter compared to a baseline simulation with no blowing snow. Thus, taking into account SSA from blowing snow above sea ice will be important to improve model predictions of polar aerosol and climate.

Development of improved spray system with effective electrical electrodes for aerosol removal: An experimental study in UTARTS facility

Safe reactor decommissioning, especially for damaged Fukushima Daiichi (1F) nuclear power plants, is vital for environmental safety. Key challenges include remotely cleaning radiation hotspots and cutting fuel debris within the damaged primary containment vessel. However, submicron radioactive Aerosol Particles (APs) can be generated, thus necessitating effective aerosol control and removal to avoid radioactive environmental pollution and reduce radiation exposure risks during 1F decommissioning. Flue gases containing submicron APs that result in environmental pollution can also generated from other industrial works, e.g., coal, mining and chemical sectors. Conventional water spray is difficult to scavenge these small APs. Although previous studies showed the effectiveness of charged droplets on accelerating aerosol removal, the charging configuration is also important to scavenging performance. Hence, this study performs aerosol scavenging experiments in our UTARTS facility with varying induction electrode designs. Experimental results show the saturation of scavenging efficiency at high voltage and indicate the importance of charging polarity. Moreover, proper configurations of electrode position, geometry and material are studied and discussed. Our findings can be beneficial for the improvement of spray system for aerosol removal to mitigate radioactivity release and minimize contaminated water production and have implications for gas purification in various environmental and chemical industries.

Experimental phase diagram and its temporal evolution for submicron 2-methylglutaric acid and ammonium sulfate aerosol particles.

Liquid-liquid phase separation (LLPS) in aerosol particles is important for the climate system due to its potential to impact heterogeneous chemistry, cloud condensation nuclei, and new particle growth. Our group and others have shown a lower separation relative humidity for submicron particles, but whether the suppression is due to thermodynamics or kinetics is unclear. Herein, we characterize the experimental LLPS phase diagram of submicron 2-methylglutaric acid and ammonium sulfate aerosol particles and compare it to that of supermicron-sized particles. Surprisingly, as the equilibration time of submicron-sized aerosol particles was increased from 20 min to 60 min, the experimental phase diagram converges with the results for supermicron-sized particles. Our findings indicate that nucleation kinetics are responsible for the observed lower separation relative humidities in submicron aerosol particles. Therefore, experiments and models that investigate atmospheric processes of organic aerosol particles may need to consider the temporal evolution of aerosol LLPS.

Marine carbohydrates in Arctic aerosol particles and fog – diversity of oceanic sources and atmospheric transformations

Abstract. Carbohydrates, originating from marine microorganisms, enter the atmosphere as part of sea spray aerosol (SSA) and can influence fog and cloud microphysics as cloud condensation nuclei (CCN) or ice-nucleating particles (INP). Particularly in the remote Arctic region, significant knowledge gaps persist about the sources, the sea-to-air transfer mechanisms, atmospheric concentrations, and processing of this substantial organic group. In this ship-based field study conducted from May to July 2017 in the Fram Strait, Barents Sea, and central Arctic Ocean, we investigated the sea-to-air transfer of marine combined carbohydrates (CCHO) from concerted measurements of the bulk seawater, the sea surface microlayer (SML), aerosol particles and fog. Our results reveal a wide range of CCHO concentrations in seawater (22–1070 µg L−1), with notable variations among different sea-ice-related sea surface compartments. Enrichment factors in the sea surface microlayer (SML) relative to bulk water exhibited variability in both dissolved (0.4–16) and particulate (0.4–49) phases, with the highest values in the marginal ice zone (MIZ) and aged melt ponds. In the atmosphere, CCHO was detected in super- and submicron aerosol particles (CCHOaer,super: 0.07–2.1 ng m−3; CCHOaer,sub: 0.26–4.4 ng m−3) and fog water (CCHOfog,liquid: 18–22 000 µg L−1; CCHOfog,atmos: 3–4300 ng m−3). Enrichment factors for sea–air transfer varied based on assumed oceanic emission sources. Furthermore, we observed rapid atmospheric aging of CCHO, indicating both biological/enzymatic processes and abiotic degradation. This study highlights the diverse marine emission sources in the Arctic Ocean and the atmospheric processes shaping the chemical composition of aerosol particles and fog.

Characteristics of aerosol at the research base “Ice Cape Baranova” in 2018–2023

Atmospheric aerosol plays an important role in the processes of radiative transfers and mass exchange by different substances in the “continent–atmosphere–ocean” system. In this paper we discuss the results of a five-year measurement cycle of the atmospheric aerosol characteristics at the polar station “Ice base Cape Baranov”, located on the Bolshevik Island (the Severnaya Zemlya Archipelago). The set of the characteristics analyzed includes: the aerosol optical depth (AOD) of the atmosphere; the ground concentration of aerosol particles in the radius range of 0.15–5microns; the content of the absorbing substance (soot) in the aerosol in the equivalent of elemental black carbon. The average values of the aerosol characteristics for the general measurement period (from April 2018 to May 2023) were: volumes of submicron and coarse aerosol particles 0.43 and 0.46 μm3/cm3, respectively; mass concentration of black carbon — 45.8 ng/m3; AOT of the atmosphere at a wavelength of 0.5 µm — 0.08; Angstrom selectivity exponent — 1.67. The average annual variation of aerosol and black carbon concentrations is characterized by a maximum in winter (January–March) and a minimum in summer (June–August). A comparison is made against the data from analogous measurements of aerosol characteristics at the polar station in Barentsburg (the Spitsbergen Archipelago) and against the data from model calculations, i. e., MERRA-2 reanalysis. A distinctive feature of the data in the Cape Baranov area is the low content of coarse aerosol — 1.7 less than in Barentsburg. There is agreement with the annual variation of black carbon concentrations at other polar stations, but the opposite nature of the seasonal variability of model (MERRA-2) concentrations: low values in winter and high values in summer. It is shown that the average spectral AODs of the atmosphere at the “Cape Baranov” are intermediate values between the data from polar stations in NyÅlesund and Barentsburg.

Salting out, non-ideality and synergism enhance surfactant efficiency in atmospheric aerosols

In Earth’s atmosphere, the surface tension of sub-micron aerosol particles is suspected to affect their efficiency in becoming cloud droplets. But this quantity cannot be measured directly and is inferred from the chemical compounds present in aerosols. Amphiphilic surfactants have been evidenced in aerosols but experimental information on the surface properties of their mixtures with other aerosol components is lacking. This work explores experimentally the surface properties of aqueous mixtures of amphiphilic surfactants (SDS, Brij35, TritonX100, TritonX114, and CTAC) with inorganic salts (NaCl, (NH4)2SO4) and soluble organic acids (oxalic and glutaric acid) using pendant droplet tensiometry. Contrary to what could be expected, inorganic salts and organic acids systematically enhanced the efficiency of the surfactants rather than reduced it, by further lowering the surface tension and, in some cases, the CMC. Furthermore, all the mixtures studied were strongly non-ideal, some even displaying some synergism, thus demonstrating that the common assumption of ideality for aerosol mixtures is not valid. The molecular interactions between the mixture components were either in the bulk (salting out), in the mixed surface monolayer (synergy on the surface tension) or in the micelles (synergy on the CMC) and need to be included when describing such aerosol mixtures.

High-efficiency carbon-coated steel wool filter for controlling cooking-induced oil smoke

Cooking oil smoke (COS) contains many harmful substances, such as particulate matter, formaldehyde, and phenyl esters. Currently, commercial COS treatment equipment is expensive and requires a large space. Furthermore, a large amount of agricultural waste is generated and is mainly burned onsite, producing large amounts of greenhouse gases and air pollutants. This waste could be reused as a precursor for biochar and activated carbon. Therefore, this research used saccharification and catalytic hydrothermal carbonization to process rice straw and produce compact carbon-based filters (steel wool-C) for removing cooking-induced pollutants. Scanning electron microscopy indicated that carbon layers were coated on the steel wool. The Brunauer-Emmett-Teller surface area of the carbon filter was 71.595 m2/g, 43 times larger than that of steel wool. The steel wool filter removed 28.9%–45.4% of submicron aerosol particles. Adding a negative air ionizer (NAI) to the filter system enhanced the particle removal efficiency by 10%–25%. The removal efficiency of total volatile organic compounds was 27.3%–37.1% with the steel wool filter, but 57.2%–74.2% with the carbon-containing steel wool filter, and the NAI improved the removal efficiency by approximately 1%–5%. The aldehyde removal efficiency of the carbon filter with NAI was 59.0%–72.0%. Conclusively, the compact steel wool-C and NAI device could be promising COS treatment equipment for households and small eateries.

Amino acids, carbohydrates, and lipids in the tropical oligotrophic Atlantic Ocean: sea-to-air transfer and atmospheric in situ formation

Abstract. This study examines carbohydrates, amino acids, and lipids as important contributors to organic carbon (OC) in the tropical Atlantic Ocean at the Cape Verde Atmospheric Observatory (CVAO). The above compounds were measured in both surface seawater and in ambient sub-micron aerosol particles to investigate their sea-to-air transfer, including their enrichment in the sea surface microlayer (SML), potential atmospheric in situ formation or degradation, and their oceanic contribution to the ambient marine aerosol particles. In bulk seawater and the SML, similar distributions among species were found for the lipids and carbohydrates with moderate SML enrichments (enrichment factors EFSML = 1.3 ± 0.2 and 1.1 ± 0.5 respectively). In contrast, the amino acids exhibited a higher enrichment in the SML with an average EFSML of 2.3 ± 0.4, although they are less surface-active than lipids. The same compounds studied in the seawater were found on the ambient sub-micron aerosol particles, whereas the lipids' enrichment was more pronounced (EFaer.=1.6×105) compared to the amino acids and carbohydrates (EFaer.=1.5×103 and 1.3×103 respectively), likely due to their high surface activity and/or the lipophilic character. Detailed molecular analysis of the seawater and aerosol particles revealed changes in the relative abundance of the individual organic compounds. They were most pronounced for the amino acids and are likely related to an in situ atmospheric processing by biotic and/or abiotic reactions. On average, 49 % of the OC on the aerosol particles (=∧97 ng m−3) could be attributed to the specific components or component groups investigated in this study. The majority (43 %) was composed of lipids. Carbohydrates and amino acids made up less than 1 % of the OC. This shows that carbohydrates, at least when resolved via molecular measurements of single sugars, do not comprise a very large fraction of OC on marine aerosol particles, in contrast to other studies. However, carbohydrate-like compounds are also present in the high lipid fraction (e.g. as glycolipids), but their chemical composition could not be revealed by the measurements performed here. Previously determined OC components at the CVAO, specifically amines, oxalic acid, and carbonyls, comprised an OC fraction of around 6 %. Since the identified compounds constituted about 50 % of the OC and belong to the rather short-lived biogenic material probably originating from the surface ocean, a pronounced coupling between ocean and atmosphere was indicated for this oligotrophic region. The remaining, non-identified OC fraction might in part contain recalcitrant OC; however, this fraction does not constitute the vast majority of OC in the aerosol particles investigated here.

The Effect of a Combined Electrofilter Power System on the Efficiency of Capturing Submicron Aerosol Particles

The Effect of a Combined Electrofilter Power System on the Efficiency of Capturing Submicron Aerosol Particles

Numerical simulation of aerosol generation by cutting fuel debris and aerosol removal by spray droplets during Fukushima decommissioning

The proposed plan for decommissioning the damaged Fukushima Daiichi nuclear reactors involves cutting the melted and re-solidified fuel debris into small pieces for retrieval from the reactor buildings. However, the cutting process generates submicron aerosol particles that need to be removed from the containment vessel before they escape into the environment. The existing spray system inside the containment vessel can be used to remove these aerosol particles using various collection mechanisms. To simulate both the aerosol generation and removal processes, a new numerical model has been developed using the open-source Computational Fluid Dynamics code OpenFOAM. The simulation results indicate that the rate of aerosol generation does not affect the efficiency of aerosol removal, and larger particles can be removed faster due to stronger inertial impaction. The newly developed numerical model has also been used to simulate aerosol generation and removal in the real-size containment vessel of Fukushima Daiichi nuclear reactor unit 3. The simulation results provide information on the spatial distribution of aerosol particles under different multiple-nozzle layouts, which can be used to optimize the design of future multiple-nozzle spray systems for the real Fukushima decommissioning.

Size Dependence of Liquid–Liquid Phase Separation by in Situ Study of Flowing Submicron Aerosol Particles

Liquid-liquid phase separation (LLPS) of atmospheric particles impacts a range of atmospheric processes. Driven by thermodynamics, LLPS occurs in mixed organic-inorganic particles when high inorganic salt concentrations exclude organic compounds, which develop into a separate phase. The effect of particle size on the thermodynamic and kinetic drivers of LLPS, however, remains incompletely understood. Here, the size dependence was studied for the separation relative humidity (SRH) of LLPS. Submicron organic-inorganic aerosol particles of ammonium sulfate mixed with 1,2,6-hexanetriol and polyethylene glycol (PEG) were studied. In a flow configuration, upstream size selection was coupled to a downstream fluorescence aerosol flow tube (F-AFT) at 293 ± 1 K. For both mixed particle types, the SRH values for submicron particle diameters of 260-410 nm agreed with previous measurements reported in the literature for supermicron particles. For smaller particles, the SRH values decreased by approximately 5% RH for diameters of 130-260 nm for PEG-sulfate particles and of 70-190 nm for hexanetriol-sulfate particles. From these observations, the nucleation rate in the hexanetriol-sulfate system was constrained, implying an activation barrier to nucleation of +1.4 to +2.0 × 10-19 J at 70% RH and 293 K. Quantifying the activation barrier is an approach for predicting size-dependent LLPS in the atmosphere.

Features of the mechanism of aerosol fractionation in solid hydrometeors

The object of the study is the dispersed fractions of an aerosol substance in the snow cover. At the sampling sites located in the middle taiga zone within the Mezen-Vychegoda Plain on a high terrace in the Sysola river valley, 3 km west of the city of Syktyvkar, fresh snow and surface frost were sampled. The analysis of the ratios in the surface hoarfrost of subdispersed fractions of the aerosol substance was carried out in conjunction with the granulometric analysis of freshly fallen snow. Studies using the method of dynamic light scattering of the granulometric composition of an aerosol substance in freshly fallen snow and surface hoarfrost formed between snowfalls showed that a bimodal distribution of particles is recorded in all snow samples. At the same time, the distribution of submicron aerosol particles in frost samples is already characterized by the presence of three modes. Also, with an increase in the duration of the period between snowfalls in hoarfrost samples, a redistribution of particles between fine and medium fractions is observed in the direction of a significant increase in particles in the fine fraction, but the volume concentration of particles of the large fraction changes slightly. It is assumed that the identified circumstance is associated with the action of capillary forces and the adhesion of dry precipitation during the crystal formation of hoarfrost (it is proposed to call this phenomenon "frosty condensation"). A similar effect was also observed in the West Siberian southern taiga and the coastal tundra of the Lower Pechora region.

Collection of Submicron Aerosol Particles by Filters Composed of Nanofibers

Collection of Submicron Aerosol Particles by Filters Composed of Nanofibers

Measurement report: Aerosol vertical profiles over the western North Atlantic Ocean during the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES)

Abstract. The NASA North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) ship and aircraft field campaign deployed to the western subarctic Atlantic between the years 2015 and 2018. One of the primary goals of NAAMES is to improve the understanding of aerosol–cloud interaction (ACI) over the Atlantic Ocean under different seasonal regimes. ACIs currently represent the largest source of uncertainty in global climate models. During three NAAMES field campaigns (NAAMES-1 in November 2015, NAAMES-2 in May 2016, and NAAMES-3 in September 2017), multiple 10 h science flights were conducted using the NASA C-130 aircraft to measure marine boundary layer aerosol and cloud properties. The standard flight pattern includes vertical spirals where the C-130 transitioned from high altitude to low altitude (and vice versa), collecting in situ measurements of aerosols, trace gases, clouds, and meteorological parameters as a function of altitude. We examine the data collected from 37 spirals during the three NAAMES field campaigns, and we present a comprehensive characterization of the vertical profiles of aerosol properties under different synoptic conditions and aerosol regimes. The vertical distribution of submicron aerosol particles exhibited strong seasonal variation, as well as elevated intra-seasonal variability depending on emission sources and aerosol processes in the atmospheric column. Pristine marine conditions and new particle formation were prevalent in the wintertime (NAAMES-1) due to low biogenic emissions from the surface ocean and reduced continental influence. Higher concentrations of submicron aerosol particles were observed in the spring (NAAMES-2) due to strong phytoplankton activity and the arrival of long-range-transported continental plumes in the free troposphere with subsequent entrainment into the marine boundary layer. Biomass burning from boreal wildfires was the main source of aerosol particles in the region during the late summer (NAAMES-3) in both the marine boundary layer and free troposphere.

The effect of the combined electrofilter power system on the efficiency of capturing submicron aerosol particles

The effect of the combined electrofilter power system on the efficiency of capturing submicron aerosol particles

Extensive urban air pollution footprint evidenced by submicron organic aerosols molecular composition

Transport and transformation of urban air pollutants are among the major factors driving the changes in the atmospheric composition in the downwind rural/remote areas of a megacity. Here, we assess the impacts of urban air pollution in a subtropical forest through characterization of the organic markers in submicron aerosol particles. The aerosol samples were collected and analyzed using TD-PTR-ToF-MS, where 163 ions were detected. The concentration of these extracted ions accounts for 83% of the mass of submicron organic aerosols, which are accordingly characterized by a median formula of C7H10O2. Molecular speciation indicates that urban and biomass burning pollution contributed substantially to the budget of organic aerosols, which were enhanced particularly by the liquid water content and acidity of the aerosols. Our results evidence that the footprint of urban air pollution was extended to its downwind forested areas and caused changes in the concentration and composition of submicron aerosols.

Indoor-outdoor relationship of submicron particulate matter in mechanically ventilated building: Chemical composition, sources and infiltration factor

To evaluate the impact of outdoor particulate pollution on indoor air quality, the chemical composition and sources of submicron aerosol particles (PM1) were studied indoors and outdoors. Measurements were carried out during the heating season from October 15, 2020, to February 8, 2021, at the Center for Physical Sciences and Technologies in Vilnius, Lithuania. Online measurements of PM1 chemical composition were performed using an Aerosol Chemical Speciation Monitor (ACSM (organics, sulfate, and nitrate)) and an Aethalometer (equivalent black carbon, BC). In parallel with the online measurements, filter-based elemental composition and 14C analysis of PM1 were performed using a Particle-Induced broad-beam X-ray Emission (PIXE) and a Single Stage Accelerated Mass Spectrometer (SSAMS), respectively. The source apportionment results showed a dominant contribution of biomass burning to the total carbonaceous aerosol particles, including primary (30%) and secondary (40%) fractions. According to the enrichment factors, the main source of trace elements was road dust resuspension (30%), while anthropogenic emissions accounted for only 13% of trace elements. The infiltration factor (Finf) of all studied PM1 constituents was low (Finf∼0.03). This result indicates that the three-stage building filter system (G4-F7-F9) provides high protection against particle pollution of different origins and significantly reduces indoor exposure to PM1. The changed chemical composition of indoor PM1 can be attributed to species-specific evaporation and some minor indoor sources.

Gas-Phase Peroxyl Radical Recombination Reactions: AComputational Study of Formation and Decomposition of Tetroxides.

The recombination (“dimerization”) of peroxyl radicals (RO2•) is one of the pathways suggested in the literature for the formation of peroxides (ROOR′, often referred to as dimers or accretion products in the literature) in the atmosphere. It is generally accepted that these dimers play a major role in the first steps of the formation of submicron aerosol particles. However, the precise reaction pathways and energetics of RO2• + R′O2• reactions are still unknown. In this work, we have studied the formation of tetroxide intermediates (RO4R′): their formation from two peroxyl radicals and their decomposition to triplet molecular oxygen (3O2) and a triplet pair of alkoxyl radicals (RO•). We demonstrate this mechanism for several atmospherically relevant primary and secondary peroxyl radicals. The potential energy surface corresponds to an overall singlet state. The subsequent reaction channels of the alkoxyl radicals include, but are not limited to, their dimerization into ROOR′. Our work considers the multiconfigurational character of the tetroxides and the intermediate phases of the reaction, leading to reliable mechanistic insights for the formation and decomposition of the tetroxides. Despite substantial uncertainties in the computed energetics, our results demonstrate that the barrier heights along the reaction path are invariably small for these systems. This suggests that the reaction mechanism, previously validated at a multireference level only for methyl peroxyl radicals, is a plausible pathway for the formation of aerosol-relevant larger peroxides in the atmosphere.

Measurement report: On the difference in aerosol hygroscopicity between high and low relative humidity conditions in the North China Plain

Abstract. Atmospheric processes, including both primary emissions and secondary formation, may exert complex effects on aerosol hygroscopicity, which is of significant importance in understanding and quantifying the effect of aerosols on climate and human health. In order to explore the influence of local emissions and secondary formation processes on aerosol hygroscopicity, we investigated the hygroscopic properties of submicron aerosol particles at a rural site in the North China Plain (NCP) in winter 2018. This was conducted by simultaneous measurements of aerosol hygroscopicity and chemical composition, using a custom-built hygroscopic tandem differential mobility analyzer (HTDMA) and a capture-vaporizer time-of-flight aerosol chemical speciation monitor (CV-ToF-ACSM). The hygroscopicity results showed that the particles during the entire campaign were mainly externally mixed, with a more hygroscopic (MH) mode and a less hygroscopic (LH) mode. The mean hygroscopicity parameter (κmean) values derived from hygroscopicity measurements for particles at 60, 100, 150, and 200 nm were 0.16, 0.18, 0.16, and 0.15, respectively. During this study, we classified two distinct episodes with different relative humidity (RH) and temperature (T) conditions, indicative of different primary emissions and secondary formation processes. It was observed that aerosols at all measured sizes were more hygroscopic under the high-RH (HRH) episode than those under the low-RH (LRH) episode. During the LRH, κ decreased with increasing particle size, which may be explained by the enhanced domestic heating at low temperature, causing large emissions of non-hygroscopic or less hygroscopic primary aerosols. This is particularly obvious for 200 nm particles, with a dominant number fraction (>50 %) of LH mode particles. Using O:C-dependent hygroscopic parameters of secondary organic compounds (κSOA), closure analysis between the HTDMA-measured κ and the ACSM-derived κ was carried out. The results showed that κSOA under the LRH episode was less sensitive to the changes in organic oxidation level, while κSOA under HRH had a relatively stronger dependency on the organic O:C ratio. This feature suggests that the different sources and aerosol evolution processes, partly resulting from the variation in atmospheric RH and T conditions, may lead to significant changes in aerosol chemical composition, which will further influence their corresponding physical properties.

Five-satellite-sensor study of the rapid decline of wildfire smoke in the stratosphere

Abstract. Smoke from western North American wildfires reached the stratosphere in large amounts in August 2017. Limb-oriented satellite-based sensors are commonly used for studies of wildfire aerosol injected into the stratosphere (OMPS-LP (Ozone Mapping and Profiler Suite Limb Profiler) and SAGE III/ISS (Stratospheric Aerosol and Gas Experiment III on the International Space Station)). We find that these methods are inadequate for studies of the first 1–2 months after such a strong fire event due to event termination (“saturation”). The nadir-viewing lidar CALIOP (Cloud-Aerosol LIdar with Orthogonal Polarization) is less affected due to shorter path in the smoke; furthermore, it provides a means to develop a method to correct for strong attenuation of the signal. After the initial phase, the aerosol optical depth (AOD) from OMPS-LP and CALIOP show very good agreement above the 380 K isentrope, whereas OMPS-LP tends to produce higher AOD than CALIOP in the lowermost stratosphere (LMS), probably due to reduced sensitivity at altitudes below 17 km. Time series from CALIOP of attenuation-corrected stratospheric AOD of wildfire smoke show an exponential decline during the first month after the fire, which coincides with highly significant changes in the wildfire aerosol optical properties. The AOD decline is verified by the evolution of the smoke layer composition, comparing the aerosol scattering ratio (CALIOP) to the water vapor concentration from MLS (Microwave Limb Sounder). Initially the stratospheric wildfire smoke AOD is comparable with the most important volcanic eruptions during the last 25 years. Wildfire aerosol declines much faster, 80 %–90 % of the AOD is removed with a half-life of approximately 10 d. We hypothesize that this dramatic decline is caused by photolytic loss. This process is rarely observed in the atmosphere. However, in the stratosphere this process can be studied with practically no influence from wet deposition, in contrast to the troposphere where this is the main removal path of submicron aerosol particles. Despite the loss, the aerosol particles from wildfire smoke in the stratosphere are relevant for the climate.