Assessing the consequences of including aerosol absorption in potential Stratospheric Aerosol Injection Climate Intervention Strategies

Theoretical Stratospheric Aerosol Intervention (SAI) strategies model the deliberate injection of aerosols or their precursors into the stratosphere thereby reflecting incident sunlight back to space and counterbalancing a fraction of the warming due to increased concentrations of greenhouse gases. This cooling mechanism is known to be relatively robust through analogues from explosive volcanic eruptions which have been documented to cool the climate of the Earth. However, a practical difficulty of SAI strategies is how to deliver the injection high enough to ensure dispersal of the aerosol within the stratosphere on a global scale. Recently, it has been suggested that including a small amount of absorbing material in a dedicated 10-day intensive deployment might enable aerosols or precursor gases to be injected at significantly lower, more technologically-feasible altitudes. The material then absorbs sunlight causing a localised heating and ‘lofting’ of the particles, enabling them to penetrate into the stratosphere. Such self-lofting has recently been observed following the intensive wildfires in 2019–2020 in south east Australia, where the resulting absorbing aerosol penetrated into the stratosphere and was monitored by satellite instrumentation for many months subsequent to emission. This study uses the fully coupled UKESM1 climate model simulations performed for the Geoengineering Model Intercomparison Project (GeoMIP) and new simulations where the aerosol optical properties have been adjusted to include a moderate degree of absorption. The results indicate that partially absorbing aerosols i) reduce the cooling efficiency per unit mass of aerosol injected, ii) increase deficits in global precipitation iii) delay the recovery of the stratospheric ozone hole, iv) disrupt the Quasi Biennial Oscillation when global mean temperatures are reduced by as little as 0.1 K, v) enhance the positive phase of the wintertime North Atlantic Oscillation which is associated with floods in Northern Europe and droughts in Southern Europe. While these results are dependent upon the exact details of the injection strategies and our simulations use ten times the ratio of black carbon to sulfate that is considered in the recent intensive deployment studies, they demonstrate some of the potential pitfalls of injecting an absorbing aerosol into the stratosphere to combat the global warming problem.

Optische Eigenschaften und Strahlungswechselwirkung von Aerosol unter wolkenfreien Bedingungen über Deutschland

<p>Der Strahlungseffekt aus Aerosol-Strahlungs-Wechselwirkung beeinflusst den Energiehaushalt am Erdboden und ist daher von Bedeutung für unser Verständnis des Klimasystems und von großem Interesse für den Bereich der erneuerbaren Energien. In dieser Studie wird der Strahlungseffekt unter wolkenfreien Bedingungen insbesondere im Hinblick auf saisonale und regionale Variationen für die Region Deutschland und das Jahr 2015 an der Erdoberfläche sowie im oberen Bereich der Atmosphäre mit zwei komplementären Ansätzen untersucht.</p> <p>Zum einen wird ein Ensemble von empirischen Modellen, die Aerosole explizit berücksichtigen, verwendet, um die optische Tiefe und die direkte Strahlungswirkung von Aerosol am Boden mithilfe einer Fehlerminimierung zu ermitteln. Hierbei werden Messungen der solaren Einstrahlung an 25 Stationen des Beobachtungsnetzes des Deutschen Wetterdienstes (DWD) als Datengrundlage verwendet. Unter Anwendung eines Erkennungsalgorithmus werden dabei wolkenlose Situationen identifiziert. Die verwendeten empirischen Modelle sind MMAC, MRM v6.1, METSTAT, ESRA, Heliosat-1, CEM und das vereinfachte Solis-Modell. Die zugrundeliegenden Annahmen über Aerosol- und atmosphärische Eigenschaften in diesen Modellen werden kritisch analysiert und diskutiert.</p> <p>Im zweiten Ansatz werden explizite Strahlungstransfersimulationen des Strahlungseffekts unter Verwendung der CAMS-Reanalyse genutzt. Die optischen Eigenschaften des Aerosols, welche als Eingangsgrößen aus der CAMS-Reanalyse bezogen werden (Optische Tiefe des Aerosols, Angström Exponent, Einzelstreualbedo und Asymmetrieparameter), werden zunächst mit den direkten Sonnen- und Inversionsprodukten von AERONET bewertet. Die größte Inkonsistenz wurde bei der Aerosol-Absorption festgestellt, die von der CAMS-Reanalyse um etwa 30 % überschätzt wird. Weiterhin wird die Sensitivität der Simulationen auf Unsicherheiten in den Eingangsgrößen untersucht, und damit die resultierende Unsicherheit im Strahlungseffekt abgeschätzt. Die Unsicherheit wird auf -1,5±7,7 Wm<sup>-2</sup> am Boden und auf 0,6±3,5 Wm<sup>-2</sup> am Oberrand der Atmosphäre geschätzt. Nach Korrektur von systematischen Abweichungen in der CAMS-Reanalyse hat Aerosol im Jahre 2015 einen mittleren abkühlenden Strahlungseffekt von -10,6 Wm<sup>-2</sup> am Boden und -6,5 Wm<sup>-2</sup> am Oberrand der Atmosphäre. Es resultiert ein wärmender Strahlungseffekt von 4,1 Wm<sup>-2</sup> der kompletten Atmosphäre.</p> <p>Die Ermittlung des Aerosol-Strahlungseffekts mit empirischen Modellen zeigt ein hohes Maß an Übereinstimmung mit den Strahlungstransfersimulationen. Der Jahreszyklus kann allerdings, durch die festen Aerosol-Charakterisierung in den Modellen, nicht reproduziert werden. Von der Auswahl der vorgestellten empirischen Modelle zeigen die Modelle ESRA und MRM v6.1 die größte Übereinstimmung.</p>

Combining POLDER-3 satellite observations and WRF-Chem numerical simulations to derive biomass burning aerosol properties over the southeast Atlantic region

Aerosol absorption is a key property to assess the radiative impacts of aerosols on climate at both global and regional scales. The aerosol physico-chemical and optical properties remain not sufficiently constrained in climate models, with difficulties to properly represent both the aerosol load and their absorption properties in clear and cloudy scenes, especially for absorbing biomass burning aerosols (BBA). In this study we focus on biomass burning (BB) particle plumes transported above clouds over the southeast Atlantic (SEA) region off the southwest coast of Africa, in order to improve the representation of their physico-chemical and absorption properties. The methodology is based on aerosol regional numerical simulations from the WRF-Chem coupled meteorology–chemistry model combined with a detailed inventory of BB emissions and various sets of innovative aerosol remote sensing observations, both in clear and cloudy skies from the POLDER-3/PARASOL space sensor. Current literature indicates that some organic aerosol compounds (OC), called brown carbon (BrOC), primarily emitted by biomass combustion absorb the ultraviolet-blue radiation more efficiently than pure black carbon (BC). We exploit this specificity by comparing the spectral dependence of the aerosol single scattering albedo (SSA) derived from the POLDER-3 satellite observations in the 443–1020 nm wavelength range with the SSA simulated for different proportions of BC, OC and BrOC at the source level, considering the homogeneous internal mixing state of particles. These numerical simulation experiments are based on two main constraints: maintaining a realistic aerosol optical depth both in clear and above cloudy scenes and a realistic BC/OC mass ratio. Modelling experiments are presented and discussed to link the chemical composition with the absorption properties of BBA and to provide estimates of the relative proportions of black, organic and brown carbon in the African BBA plumes transported over the SEA region for July 2008. The absorbing fraction of organic aerosols in the BBA plumes, i.e. BrOC, is estimated at 2 % to 3 %. The simulated mean SSA are 0.81 (565 nm) and 0.84 (550 nm) in clear and above cloudy scenes respectively, in good agreement with those retrieved by POLDER-3 (0.85±0.05 at 565 nm in clear sky and at 550 nm above clouds) for the studied period.

Strong control of effective radiative forcing by the spatial pattern of absorbing aerosol

Over the coming decades it is expected that the spatial pattern of anthropogenic aerosol will change dramatically and that the global composition of aerosols will become relatively more absorbing. However, despite this the climatic impact of the evolving spatial pattern of absorbing aerosol has received relatively little attention, in particular the impact of this pattern on global-mean effective radiative forcing. Here we use novel climate model experiments to show that the effective radiative forcing from absorbing aerosol varies strongly depending on their location, driven by rapid adjustments of clouds and circulation. Our experiments generate positive effective radiative forcing in response to aerosol absorption throughout the midlatitudes and most of the tropical regions and a strong ‘hot spot’ of negative effective radiative forcing in response to aerosol absorption over the Western tropical Pacific. We show that these diverse responses can be robustly attributed to changes in atmospheric dynamics and highlight the importance of this previously unknown ‘aerosol pattern effect’ for transient forcing from regional biomass-burning aerosol.

Aerosol Light Absorption at 1064 nm: Pollution Sources, Meteorological Parameters and Gas Pollutants in Qingdao Coastal Area, China

A two-month sampling campaign was carried out from 1 November to 30 December 2019, to investigate the light absorption of aerosols at coastal sites in Qingdao. The average values and standard deviations of the absorption coefficient (OAC) at λ = 1064 nm during the measurement period were 18.52 ± 13.31 Mm−1. Combined with the backward trajectory model, the aerosol absorption coefficient and gas pollution concentration of six possible air mass trajectories were obtained and calculated. The maximum absorption coefficient of local air masses was approximately 20.4 Mm−1 and anthropogenic pollution originated from mainly local sources in the Jiaozhou area. In our measurements at this site, the results also showed that there was a positive correlation between relative humidity (RH) and aerosol absorption. Without considering other factors, the size of aerosol particles grew with the increasing of RH, which changed the nonlinear relationship between the size and the absorption cross section of aerosol particles subsequently. In addition, the correlations between gas pollutants and OAC were calculated. The atmospheric environment is complex in sea–land intersection areas, especially in coastal cities. Analysis of various aerosol sources, meteorological conditions, and gas precursors enhances the study of aerosol optical absorption.

Aerosol absorption in global models from AeroCom phase III

Aerosol-induced absorption of shortwave radiation can modify the climate through local atmospheric heating, which affects lapse rates, precipitation, and cloud formation. Presently, the total amount of aerosol absorption is poorly constrained, and the main absorbing aerosol species (black carbon (BC), organic aerosols (OA), and mineral dust) are diversely quantified in global climate models. As part of the third phase of the Aerosol Comparisons between Observations and Models (AeroCom) intercomparison initiative (AeroCom phase III), we here document the distribution and magnitude of aerosol absorption in current global aerosol models and quantify the sources of intermodel spread, highlighting the difficulties of attributing absorption to different species. In total, 15 models have provided total present-day absorption at 550 nm (using year 2010 emissions), 11 of which have provided absorption per absorbing species. The multi-model global annual mean total absorption aerosol optical depth (AAOD) is 0.0054 (0.0020 to 0.0098; 550 nm), with the range given as the minimum and maximum model values. This is 28 % higher compared to the 0.0042 (0.0021 to 0.0076) multi-model mean in AeroCom phase II (using year 2000 emissions), but the difference is within 1 standard deviation, which, in this study, is 0.0023 (0.0019 in Phase II). Of the summed component AAOD, 60 % (range 36 %–84 %) is estimated to be due to BC, 31 % (12 %–49 %) is due to dust, and 11 % (0 %–24 %) is due to OA; however, the components are not independent in terms of their absorbing efficiency. In models with internal mixtures of absorbing aerosols, a major challenge is the lack of a common and simple method to attribute absorption to the different absorbing species. Therefore, when possible, the models with internally mixed aerosols in the present study have performed simulations using the same method for estimating absorption due to BC, OA, and dust, namely by removing it and comparing runs with and without the absorbing species. We discuss the challenges of attributing absorption to different species; we compare burden, refractive indices, and density; and we contrast models with internal mixing to models with external mixing. The model mean BC mass absorption coefficient (MAC) value is 10.1 (3.1 to 17.7) m2 g−1 (550 nm), and the model mean BC AAOD is 0.0030 (0.0007 to 0.0077). The difference in lifetime (and burden) in the models explains as much of the BC AAOD spread as the difference in BC MAC values. The difference in the spectral dependency between the models is striking. Several models have an absorption Ångstrøm exponent (AAE) close to 1, which likely is too low given current knowledge of spectral aerosol optical properties. Most models do not account for brown carbon and underestimate the spectral dependency for OA.

The Aerosol Module in the Community Radiative Transfer Model (v2.2 and v2.3): accounting for aerosol transmittance effects on the radiance observation operator

The Community Radiative Transfer Model (CRTM), a sensor-based radiative transfer model, has been used within the Gridpoint Statistical Interpolation (GSI) system for directly assimilating radiances from infrared and microwave sensors. We conducted numerical experiments to illustrate how including aerosol radiative effects in CRTM calculations changes the GSI analysis. Compared to the default aerosol-blind calculations, the aerosol influences reduced simulated brightness temperature (BT) in thermal window channels, particularly over dust-dominant regions. A case study is presented, which illustrates how failing to correct for aerosol transmittance effects leads to errors in meteorological analyses that assimilate radiances from satellite IR sensors. In particular, the case study shows that assimilating aerosol-affected BTs affects analyzed temperatures in the lower atmosphere significantly in several different regions of the globe. Consequently, a fully-cycled aerosol-aware experiment improves 1–5 day forecasts of wind, temperature, and geopotential height in the tropical troposphere and Northern Hemisphere stratosphere. Whilst both GSI and CRTM are well documented with online user guides, tutorials and code repositories, this article is intended to provide a joined-up documentation for aerosol absorption and scattering calculations in the CRTM and GSI. It also provides guidance for prospective users of the CRTM aerosol option and GSI aerosol-aware radiance assimilation. Scientific aspects of aerosol-affected BT in atmospheric data assimilation are briefly discussed.

Source-specific light absorption by carbonaceous components in the complex aerosol matrix from yearly filter-based measurements

Understanding the sources of light-absorbing organic (brown) carbon (BrC) and its interaction with black carbon (BC) and other non-refractory particulate matter (NR-PM) fractions is important for reducing uncertainties in the aerosol direct radiative forcing. In this study, we combine multiple filter-based techniques to achieve long-term, spectrally resolved, source- and species-specific atmospheric absorption closure. We determine the mass absorption efficiency (MAE) in dilute bulk solutions at 370 nm to be equal to 1.4 m2 g−1 for fresh biomass smoke, 0.7 m2 g−1 for winter-oxygenated organic aerosol (OA), and 0.13 m2 g−1 for other less absorbing OA. We apply Mie calculations to estimate the contributions of these fractions to total aerosol absorption. While enhanced absorption in the near-UV has been traditionally attributed to primary biomass smoke, here we show that anthropogenic oxygenated OA may be equally important for BrC absorption during winter, especially at an urban background site. We demonstrate that insoluble tar balls are negligible in residential biomass burning atmospheric samples of this study and thus could attribute the totality of the NR-PM absorption at shorter wavelengths to methanol-extractable BrC. As for BC, we show that the mass absorption cross-section (MAC) of this fraction is independent of its source, while we observe evidence for a filter-based lensing effect associated with the presence of NR-PM components. We find that bare BC has a MAC of 6.3 m2 g−1 at 660 nm and an absorption Ångström exponent of 0.93 ± 0.16, while in the presence of coatings its absorption is enhanced by a factor of ∼ 1.4. Based on Mie calculations of closure between observed and predicted total light absorption, we provide an indication for a suppression of the filter-based lensing effect by BrC. The total absorption reduction remains modest, ∼ 10 %–20 % at 370 nm, and is restricted to shorter wavelengths, where BrC absorption is significant. Overall, our results allow an assessment of the relative importance of the different aerosol fractions to the total absorption for aerosols from a wide range of sources and atmospheric ages. When integrated with the solar spectrum at 300–900 nm, bare BC is found to contribute around two-thirds of the solar radiation absorption by total carbonaceous aerosols, amplified by the filter-based lensing effect (with an interquartile range, IQR, of 8 %–27 %), while the IQR of the contributions by particulate BrC is 6 %–13 % (13 %–20 % at the rural site during winter). Future studies that will directly benefit from these results include (a) optical modelling aiming at understanding the absorption profiles of a complex aerosol composed of BrC, BC and lensing-inducing coatings; (b) source apportionment aiming at understanding the sources of BC and BrC from the aerosol absorption profiles; (c) global modelling aiming at quantifying the most important aerosol absorbers.

Absorption instruments inter-comparison campaign at the Arctic Pallas station

Aerosol light absorption was measured during a 1-month field campaign in June–July 2019 at the Pallas Global Atmospheric Watch (GAW) station in northern Finland. Very low aerosol concentrations prevailed during the campaign, which posed a challenge for the instruments' detection capabilities. The campaign provided a real-world test for different absorption measurement techniques supporting the goals of the European Metrology Programme for Innovation and Research (EMPIR) Black Carbon (BC) project in developing aerosol absorption standard and reference methods. In this study we compare the results from five filter-based absorption techniques – aethalometer models AE31 and AE33, a particle soot absorption photometer (PSAP), a multi-angle absorption photometer (MAAP), and a continuous soot monitoring system (COSMOS) – and from one indirect technique called extinction minus scattering (EMS). The ability of the filter-based techniques was shown to be adequate to measure aerosol light absorption coefficients down to around 0.01 Mm−1 levels when data were averaged to 1–2 h. The hourly averaged atmospheric absorption measured by the reference MAAP was 0.09 Mm−1 (at a wavelength of 637 nm). When data were averaged for >1 h, the filter-based methods agreed to around 40 %. COSMOS systematically measured the lowest absorption coefficient values, which was expected due to the sample pre-treatment in the COSMOS inlet. PSAP showed the best linear correlation with MAAP (slope=0.95, R2=0.78), followed by AE31 (slope=0.93). A scattering correction applied to PSAP data improved the data accuracy despite the added noise. However, at very high scattering values the correction led to an underestimation of the absorption. The AE31 data had the highest noise and the correlation with MAAP was the lowest (R2=0.65). Statistically the best linear correlations with MAAP were obtained for AE33 and COSMOS (R2 close to 1), but the biases at around the zero values led to slopes clearly below 1. The sample pre-treatment in the COSMOS instrument resulted in the lowest fitted slope. In contrast to the filter-based techniques, the indirect EMS method was not adequate to measure the low absorption values found at the Pallas site. The lowest absorption at which the EMS signal could be distinguished from the noise was >0.1 Mm−1 at 1–2 h averaging times. The mass absorption cross section (MAC) value measured at a range 0–0.3 Mm−1 was calculated using the MAAP and a single particle soot photometer (SP2), resulting in a MAC value of 16.0±5.7 m2 g−1. Overall, our results demonstrate the challenges encountered in the aerosol absorption measurements in pristine environments and provide some useful guidelines for instrument selection and measurement practices. We highlight the need for a calibrated transfer standard for better inter-comparability of the absorption results.