AEROSOL VARIABILITY OBSERVED WITH RPAS

Abstract. In July 2018, the University of Oklahoma deployed three CopterSonde remotely piloted aircraft systems (RPASs) to take measurements of the evolving thermodynamic and kinematic state of the atmospheric boundary layer (ABL) over complex terrain in the San Luis Valley, Colorado. A total of 180 flights were completed over 5 d, with teams operating simultaneously at two different sites in the northern half of the valley. A total of 2 d of operations focused on convection initiation studies, 1 d focused on ABL diurnal transition studies, 1 d focused on internal comparison flights, and the last day of operations focused on cold air drainage flows. The data from these coordinated flights provide insight into the horizontal heterogeneity of the atmospheric state over complex terrain. This dataset, along with others collected by other universities and institutions as a part of the LAPSE-RATE campaign, have been submitted to Zenodo (Greene et al., 2020) for free and open access (https://doi.org/10.5281/zenodo.3737087).

Effect of a cold, dry air incursion on atmospheric boundary layer processes over a high-altitude lake in the Tibetan Plateau

Low-cost sensors for measuring particulate matter (PM) offer the ability to understand human exposure to air pollution at spatiotemporal scales that have previously been impractical. However, such low-cost PM sensors tend to be poorly characterized, and their measurements of mass concentration can be subject to considerable error. Recent studies have investigated how individual factors can contribute to this error, but these studies are largely based on empirical comparisons and generally do not examine the role of multiple factors simultaneously. Here, we present a new physics-based framework and open-source software package (opcsim) for evaluating the ability of low-cost optical particle sensors (optical particle counters and nephelometers) to accurately characterize the size distribution and/or mass loading of aerosol particles. This framework, which uses Mie theory to calculate the response of a given sensor to a given particle population, is used to estimate the fractional error in mass loading for different sensor types given variations in relative humidity, aerosol optical properties, and the underlying particle size distribution. Results indicate that such error, which can be substantial, is dependent on the sensor technology (nephelometer vs. optical particle counter), the specific parameters of the individual sensor, and differences between the aerosol used to calibrate the sensor and the aerosol being measured. We conclude with a summary of likely sources of error for different sensor types, environmental conditions, and particle classes and offer general recommendations for the choice of calibrant under different measurement scenarios.

Seasonal and diurnal variations in aerosol concentrations at a high-altitude site on the northern boundary of Qinghai-Xizang Plateau

Positive Matrix Factorization (PMF) is a powerful method for the apportionment of aerosol sources. Traditionally applied to chemical datasets, it has more recently been extended to physical datasets, focusing predominantly on size distributions of ultra-fine particles (Hopke et al., 2022). In this study, aerosol physical properties (particle size distributions, from ultrafine to coarse mode, and spectral aerosol absorption) were used as input to PMF (EPA 5.0) for identifying emission sources of two distinct sites, and results compared with those from chemical PMF. Located at the two extremes of the Italian territory, the sites of Aosta and Lecce represent two markedly different environments. The first lies in the mountainous area of the northwestern Alps, the second in a flat area at the southeastern edge of the peninsula, in a typical Mediterranean context. In previous studies, PMF of aerosol chemical composition was performed using chemical speciation of PM over the period 2019-2021 (Aosta, e.g. Diemoz et al., 2019) and 2017 (Lecce, Giannossa et al., 2022).  The PMF input dataset at the urban-background site in Aosta included aerosol size distributions measured by an Optical Particle Counter (OPC) across the 0.18–18 µm range and wavelength-dependent aerosol absorption from an AE33-aethalometer. At the urban-background observatory in Lecce, an aerosol size distribution range (0.02–10 µm) was available using both a Scanning Mobility Particle Sizer (SMPS) and an OPC, complemented by single-wavelength aerosol absorption (Multi-Angle Absorption Photometer, MAAP) providing eBC concentrations. Ancillary data included meteorological parameters and trace-gases concentration. Although with differences in the physical properties used, at both sites the physical PMF allowed the identification of 6 aerosol sources. In Aosta, the aerosol sources included fossil fuel combustion, biomass burning, secondary droplet mode, secondary condensation, dust, and coarse particles. In Lecce, the sources were nucleation (thanks to the additional use of the SMPS), traffic emissions, secondary nitrate, secondary sulfate, regional transport (sea spray and dust), and local resuspension (coarse particles). The work will present a comparison of the two PMF approaches (Chemistry- and Physics-based PMF) and advantages and limits of the different physical input datasets used at the two sites. For example, at the Lecce site, the information on ultrafine and fine particle distribution well captured features of the nucleation and traffic factors, while the absence of wavelength-dependent absorption coefficients limited the ability to distinguish biomass burning from fossil fuel sources, which was key for Aosta. Overall, the use of physical aerosol data as input to PMF proved to be an effective method for source apportionment and could usefully complement the chemical-PMF analysis. In fact, this approach offers significant advantages, such as the capability for quasi real-time monitoring and the relative ease of instruments use and data analysis compared to the ‘traditional’ chemical analysis of PM. The authors acknowledge the MUR for funding the research through the CIR01_00015-PER-ACTRIS-IT.  Diémoz, H.et al., https://doi.org/10.5194/acp-19-10129-2019, 2019.  Giannossa, L. et al., http://doi.10.1016/j.jenvman.2022.115752, 2022. Hopke, P. K. et al., https://doi.org/10.1016/j.scitotenv.2022.153104, 2022. 

Number‐size distributions of free tropospheric aerosol particles of 9–300‐nm diameter were observed at Mt. Norikura (36.1°N, 137.5°E, 2770 m a.s.l.) in central Japan in August and September 2002. We observed two distinct air masses transported from over the Pacific Ocean and continental Asia. Size distributions of aerosol particles were analyzed in terms of their relation to the air mass transportation pathway, cumulative precipitation amount in the air mass for the last 24 h, and local precipitation rate. Particle concentrations of the accumulation mode range (100–300 nm) decreased by an order of magnitude when cumulative precipitation amount increased from 0 to 20 mm. The cumulative precipitation amount is suggested as an important factor to modify the size distribution by decreasing particle concentration. Considering data for air masses without experience of precipitation scavenging before arrival, concentrations of accumulation mode particles were almost identical (approximately 400 cm−3) in air masses transported from over the Pacific Ocean and continental Asia. Concentration of Aitken mode particles was high (890 cm−3 as a median value) in the air mass subsiding from over the subtropical Pacific Ocean (> 4 km a.s.l.). Based on the relationship with the controlling factors described above, this study provides number‐size distribution parameters of free tropospheric aerosol particles in the west Pacific area.

Abstract. Southern West Africa (SWA) is an African pollution hotspot but a relatively poorly sampled region of the world. We present an overview of in situ aerosol optical measurements collected over SWA in June and July 2016 as part as of the DACCIWA (Dynamics-Aerosol-Chemistry-Clouds Interactions in West Africa) airborne campaign. The aircraft sampled a wide range of air masses, including anthropogenic pollution plumes emitted from the coastal cities, long-range transported biomass burning plumes from central and southern Africa and dust plumes from the Sahara and Sahel region, as well as mixtures of these plumes. The specific objective of this work is to characterize the regional variability of the vertical distribution of aerosol particles and their spectral optical properties (single scattering albedo: SSA, asymmetry parameter, extinction mass efficiency, scattering Ångström exponent and absorption Ångström exponent: AAE). The first findings indicate that aerosol optical properties in the planetary boundary layer were dominated by a widespread and persistent biomass burning loading from the Southern Hemisphere. Despite a strong increase in aerosol number concentration in air masses downwind of urban conglomerations, spectral SSA were comparable to the background and showed signatures of the absorption characteristics of biomass burning aerosols. In the free troposphere, moderately to strongly absorbing aerosol layers, dominated by either dust or biomass burning particles, occurred occasionally. In aerosol layers dominated by mineral dust particles, SSA varied from 0.81 to 0.92 at 550 nm depending on the variable proportion of anthropogenic pollution particles externally mixed with the dust. For the layers dominated by biomass burning particles, aerosol particles were significantly more light absorbing than those previously measured in other areas (e.g. Amazonia, North America), with SSA ranging from 0.71 to 0.77 at 550 nm. The variability of SSA was mainly controlled by variations in aerosol composition rather than in aerosol size distribution. Correspondingly, values of AAE ranged from 0.9 to 1.1, suggesting that lens-coated black carbon particles were the dominant absorber in the visible range for these biomass burning aerosols. Comparison with the literature shows a consistent picture of increasing absorption enhancement of biomass burning aerosol from emission to remote location and underscores that the evolution of SSA occurred a long time after emission. The results presented here build a fundamental basis of knowledge about the aerosol optical properties observed over SWA during the monsoon season and can be used in climate modelling studies and satellite retrievals. In particular and regarding the very high absorbing properties of biomass burning aerosols over SWA, our findings suggest that considering the effect of internal mixing on absorption properties of black carbon particles in climate models should help better assess the direct and semi-direct radiative effects of biomass burning particles.

Summary: The structure and evolution of Titan’s planetary boundary layer (PBL) are investigated through large eddy simulation (LES) modeling.  The PBL is the interface between the surface and the free atmosphere through which energy, mass, and momentum are exchanged.  Results from LES studies forced by diurnally-varying radiation are presented.  The development of three distinct PBL layers are noted: 1) a near-surface layer dominated by frictional dissipation; 2) a mixed-layer of near neutral stability; and 3) an entrainment layer capping the top of the PBL.  The three layers are similar in character to what is often observed in the Earth’s convective PBL. The depth and evolution of these layers is strongly affected by the initial thermal conditions, the surface thermophysical properties, and soil moisture.Background and Motivation:  The PBL is the lowest atmospheric layer and is critical to the exchange of mass, momentum, and energy between the surface and the atmosphere. Thus, the structure, evolution, and dynamics of Titan’s PBL is relevant to understanding numerous physical processes on Titan, including but not limited to:Volatile evaporation from damp regolith and lakes, and contributions to the global volatile cycle; Dunes and other aeolian formations, and waves upon lakes driven by turbulent surface wind stress; Conversion of surface insolation to atmospheric heating through turbulent fluxes; In addition, missions that descend through and operate on the surface or in the atmosphere have in interest in predictions of the turbulent structure and evolution of the PBL in both the horizontal and vertical.  These predictions are relevant to the engineering design and operational planning of Dragonfly, and the predictions can be largely verified through the measurements that will be made by Dragonfly when it explores Titan.The sounding from the Huygens probe provides the only direct, vertically resolved measurement of the structure of the PBL at a single moment in time. How the observed structures develop and evolve remain uncertain, and the turbulent exchange processes are challenging to constrain from the single profile. LES techniques provide a mechanism for understanding the observed structure and dynamics of the PBL.Methodology: The PBL structure and evolution are investigated through LES using a version of the mesoscale Titan Weather, Research, and Forecasting (mtWRF) model (Rafkin and Soto, 2020). mtWRF has been undergoing rapid development with the addition of radiative transfer (Chatain et al., 2022), topography (Moisan et al., 2024), and realistic 3-D lake geometries (Chatain et al., 2024). In this study, we add the capability for soil moisture and prognostic latent heat fluxes and we configure mtWRF with LES grid spacing of 10 to 40 m.  The baseline simulation uses a dry regolith (no latent heat flux). Simulations with the addition of a moist surface are compared against the dry scenario. Since LES of Titan is a mostly unexplored area of research, the simulations are conducted in 2-D in order to speed analysis and discovery.  The simulations will be extended to 3-D once the PBL dynamics are better understood in 2-D.The model is homogeneously initialized with a Huygens HASI sounding (Fulchignoni et al., 2002) over a vertical domain of 6 km and a horizontal domain of 40 km with 20 m spacing.  Starting at ~0945 local time (LT) when the HASI sounding is valid, the model is integrated until sunrise the following day.  Restarting from that point, the model is integrated again until sunset and the results of the daytime PBL evolution are analyzed.Results: In the baseline, dry regolith case, the daytime boundary layer grows to a depth of ~1 km.  The lowest ~100 m are slightly superadiabatic with the bulk of the PBL showing a near neutral profile (Fig. 1). The top of the PBL exhibits an entrainment layer where the free atmosphere above the PBL is mixed downward. The peak eddy heat flux is found just above the shallow superadiabatic layer and then decreases rapidly toward zero (Fig. 1). Narrow, thermally buoyant plumes with perturbations of ~0.1 K over ambient  are evident (Fig. 2). The maximum variance near the surface is explained by the plumes that originate in this layer.  The maximum aloft is the result of gravity waves triggered by the convection below and which are trapped in the stable region at 1 km.  The subgrid-scale turbulent kinetic energy (TKE) is similar to the thermal perturbations but shows a maximum value in the near-surface layer with a secondary maximum in the middle of the PBL (Fig. 3).  The 1 km level has yet another local maximum of TKE, but is subdued compared to the thermal variance (Fig. 2).  

A set of 152 vertical profiles of aerosol number concentration and size distribution with diameter ranging from 0.12 to 3.0 μm observed by the airborne optical spectrometer probe in Beijing, China, between February 2005 and September 2006 is analysed and discussed. The statistic of aerosol number concentration (Na) reveals a high aerosol number density in this region with average surface level number concentration (N0) of about 6600 cm-3 (0.12–3.0 μm). The average vertical profile of Na approximately satisfies an exponential decline function with a scale height of 1419 m. The Na profiles are influenced by the structures of planetary boundary layer (PBL) significantly and two typical types of Na profile under different conditions of PBL are presented and parametrized in this study. The observations of aerosol size distribution show that, in most cases the aerosol size distributions are not very sensitive to altitude, with effective radii ranging from 0.16 to 0.28 μm. Comparison between aircraft-derived aerosol optical depth (AOD) and Moderate Resolution Imaging Spectroradiometer-derived AOD shows good agreement. The Mie model calculations suggest that the surface level number concentration, the PBL height and the structure of PBL can influence the AOD significantly.

The optical properties of sea ice exhibit considerable spatial, temporal, and spectral variability. During a field experiment at Barrow, Alaska, we examined the horizontal variability of spectral albedo and transmittance as well as the vertical variability of in‐ice radiance. Temporal changes were monitored under cold conditions in April and during the onset of melt in June. Physical properties, including ice structure and concentrations of particulate and dissolved material, were measured to provide a context for understanding the observed temporal, horizontal, vertical, and spectral variability in optical properties. For snow‐covered first‐year ice in April, wavelength‐integrated (300–3000 nm) albedos were high (0.8) and spatially uniform, but there was considerable variability in transmittance. Transmittance at 440 nm ranged by more than a factor of 2 over horizontal distances of only 25 m, owing primarily to differences in snow depth, although spectral variations in transmittance indicate that absorbing organic materials in the ice column contribute significantly to the horizontal variability. Peak values of transmittance in April were 1% near 500 nm, decreasing at both longer and shorter wavelengths. At the onset of melt in June, the ice surface rapidly evolved into a variegated mixture of melting snow, bare ice, and melt ponds. Albedos were much lower and exhibited considerable spatial variability, ranging from 0.2 to 0.5 over distances of a few meters concomitant with the variation in surface characteristics. Transmission increased over the spring transition as surface characteristics evolved to decrease albedo and as inice structure was altered by heating to reduce attenuation within the ice. The exception to this trend occurred over a period of a few days when an algal bloom developed on the underside of the ice and transmission was significantly reduced. Variability in the in‐ice spectral radiance values was observed between nearby sites in both first‐year and multiyear ice. While the radiance measurements are strongly dependent on the incident solar radiance, under similar solar conditions there was an observed shift in the peak of the maximum in the spectral radiance from 460 nm in clean ice to between 500 and 550 nm in ice that contained particulates in the surface layer. More impressive spectral shifts were found in an old melt pond that had accumulated particles at its base. Not only was there a strong shift in the spectral nature of the radiance as a function of horizontal distance, but there also existed large changes vertically within the ice. The vertical variability in the radiance attenuation coefficient was spatially coherent with variations in both the physical structure of the ice, especially grain size, and the concentrations of particulate and dissolved materials entrapped in the ice. Not surprisingly, the short‐lived algal layer on the underside of the ice resulted in changes in the radiance attenuation coefficient from approximately 1 m−1 in the interior ice to approximately 40 m−1 within that layer.

Abstract. Volatile organic compounds (VOCs) play crucial roles in regulating the formation of tropospheric ozone. However, limited knowledge on the interactions between vertical VOC variations and photochemical ozone formation in the planetary boundary layer (PBL) has hindered effective ozone control strategies, especially in large cities. In this study, we investigated the vertical changes in concentrations, compositions, and key driving factors of a large suite of VOCs using online gradient measurements taken from a 325 m tall tower in urban Beijing. The impacts of these vertical VOC variations on photochemical ozone formation were also analyzed using box model simulations. Our results indicate that VOCs exhibited distinct vertical variation patterns due to their differences in sources and chemical reactivities, along with the diurnal evolution of the PBL. During daytime, reactive VOCs (e.g., hydrocarbons) are rapidly oxidized as they mix upward, accompanied by the formation and accumulation of oxygenated VOCs (OVOCs) in the middle and upper layers. In addition, the photochemical formation of ozone responds positively to changes in both NOx and VOCs. As a result, the production rate of ozone declines with height due to the simultaneous decreases in concentrations of reactive VOCs and NOx but remains high in the middle and upper layers. The strong production of ozone aloft is primarily driven by the presence of high OVOC concentrations. Therefore, careful consideration should be given to the vertical variations in both photochemical ozone production rates and formation regimes in the whole PBL when developing regional ozone control strategies.

The conversion of tropical rainforest to agriculturally used land is a spreading process throughout Indonesia and South-East Asia. Besides the effects on the biological diversity and the hydrological functions of a forest, this also has an impact on the turbulent exchange processes between vegetation and atmosphere, the radiative properties of the surface and therefore on atmospheric boundary layer and local climate. Within the framework of the research project STORMA "Stability of rain forest margins" (SFB 552, University Goettingen, financed by the German Research Foundation), the carbon dioxide, energy and water fluxes between a Cocoa plantation and the atmospheric boundary layer were investigated using the Eddy-Covariance (EC) technique. Simultaneous measurements of meteorological variables and the components of the radiation budget were conducted to investigate the dependencies of the turbulent exchange processes on canopy and atmospheric boundary layer conditions. A SVAT model was used to compute the heat exchange between canopy and atmosphere, to conduct a plausibility test to the measured fluxes, and to investigate the component fluxes. Measurements were conducted for 15 months.The sensible heat flux showed an unexpected large contribution to the total turbulent heat transport compared to investigations of natural tropical rainforests, and equaled the latent heat flux. This resulted in an averaged day-time Bowen ratio of approximately 1. The seasonal course of Bowen ratio coincided with that of precipitation. The comparison to measurements above undisturbed rainforest indicates a significantly different atmospheric boundary layer above the Cocoa plantation. This work assesses the hypothesis that net exchange of carbon dioxide and partitioning of energy fluxes between surface layer of the atmosphere and vegetation differs fundamentally from that of primary rainforests. The conclusion is that conversion of rainforest to land-use types as Cocoa plantations leads to enhanced heating of the convective atmospheric boundary layer, and to a significant difference in net uptake of carbon by the vegetation.

At the lowest point along the tourist route in the Postojna Cave, the activity concentration of radon ((222)Rn) short-lived decay products and number concentration and size distribution of background aerosol particles in the size range of 10-1100 nm were measured. In the warm yearly season, aerosol concentration was low (52 cm(-3)) with 21 % particles smaller than 50 nm, while in the cold season, it was higher (1238 cm(-3)) with 8 % of <50 nm particles. Radon activity concentrations were 4489 and 1108 Bq m(-3), and fractions of unattached radon decay products were 0.62 and 0.13, respectively.

Vertical distributions of aerosols under different weather conditions: Analysis of in-situ aircraft measurements in Beijing, China

Abstract. ​​​​​​​Land–atmosphere coupling (LAC) has long been studied, focusing on land surface and atmospheric boundary layer processes. However, the influence of humidity in the lower troposphere (LT), especially that above the planetary boundary layer (PBL), on LAC remains largely unexplored. In this study, we use radiosonde observations from the US Southern Great Plains (SGP) site and an entrained parcel buoyancy model to investigate the impact of LT humidity on LAC there during the warm season (May–September). We quantify the effect of LT humidity on convective buoyancy by measuring the difference between the 2–4 km vertically integrated buoyancy with the influence of background LT humidity and that without it. Our results show that, under dry soil conditions, anomalously high LT humidity is necessary to produce the buoyancy profiles required for afternoon precipitation events (APEs). These APEs under dry soil moisture cannot be explained by commonly used local LAC indices such as the convective triggering potential and low-level humidity index (CTP / HILow), which do not account for the influence of the LT humidity. On the other hand, consideration of LT humidity is unnecessary to explain APEs under wet soil moisture conditions, suggesting that the boundary layer moisture alone could be sufficient to generate the required buoyancy profiles. These findings highlight the need to consider the impact of LT humidity, which is often decoupled from the humidity near the surface and is largely controlled by moisture transport, in understanding land–atmospheric feedbacks under dry soil conditions, especially during droughts or dry spells over the SGP.