Abstract
Aerosol concentrations over Asia play a key role in modulating the Indian summer monsoon (ISM) rainfall. Lockdown measures imposed to prevent the spread of the COVID-19 pandemic led to ...substantial reductions in observed Asian aerosol loadings. Here, we use bottom-up estimates of anthropogenic emissions based on national mobility data from Google and Apple, along with simulations from the ECHAM6-HAMMOZ state-of-the-art aerosol-chemistry-climate model to investigate the impact of the reduced aerosol and gases pollution loadings on the ISM. We show that the decrease in anthropogenic emissions led to a 4 W m
−2
increase in surface solar radiation over parts of South Asia, which resulted in a strengthening of the ISM. Simultaneously, while natural emission parameterizations are kept the same in all our simulations, the anthropogenic emission reduction led to changes in the atmospheric circulation, causing accumulation of dust over the Tibetan plateau (TP) during the pre-monsoon and monsoon seasons. This accumulated dust has intensified the warm core over the TP that reinforced the intensification of the Hadley circulation. The associated cross-equatorial moisture influx over the Indian landmass led to an enhanced amount of rainfall by 4% (0.2 mm d
−1
) over the Indian landmass and 5%–15% (0.8–3 mm d
−1
) over central India. These estimates may vary under the influence of large-scale coupled atmosphere–ocean oscillations (e.g. El Nino Southern Oscillation, Indian Ocean Dipole). Our study indicates that the reduced anthropogenic emissions caused by the unprecedented COVID-19 restrictions had a favourable effect on the hydrological cycle over South Asia, which has been facing water scarcity during the past decades. This emphasizes the need for stringent measures to limit future anthropogenic emissions in South Asia for protecting one of the world’s most densely populated regions.
The spatial distribution and properties of submicron organic aerosol (OA)
are among the key sources of uncertainty in our understanding of aerosol
effects on climate. Uncertainties are particularly ...large over remote regions
of the free troposphere and Southern Ocean, where very few data have been
available and where OA predictions from AeroCom Phase II global models span 2 to 3 orders of magnitude, greatly exceeding the model spread over
source regions. The (nearly) pole-to-pole vertical distribution of
non-refractory aerosols was measured with an aerosol mass spectrometer
onboard the NASA DC-8 aircraft as part of the Atmospheric Tomography (ATom)
mission during the Northern Hemisphere summer (August 2016) and winter
(February 2017). This study presents the first extensive characterization of
OA mass concentrations and their level of oxidation in the remote
atmosphere. OA and sulfate are the major contributors by mass to submicron
aerosols in the remote troposphere, together with sea salt in the marine
boundary layer. Sulfate was dominant in the lower stratosphere. OA
concentrations have a strong seasonal and zonal variability, with the
highest levels measured in the lower troposphere in the summer and over the
regions influenced by biomass burning from Africa (up to 10 µg sm−3). Lower concentrations (∼0.1–0.3 µg sm−3)
are observed in the northern middle and high latitudes and very low
concentrations (<0.1 µg sm−3) in the southern middle and
high latitudes. The ATom dataset is used to evaluate predictions of eight
current global chemistry models that implement a variety of commonly used
representations of OA sources and chemistry, as well as of the AeroCom-II
ensemble. The current model ensemble captures the average vertical and
spatial distribution of measured OA concentrations, and the spread of the
individual models remains within a factor of 5. These results are
significantly improved over the AeroCom-II model ensemble, which shows large
overestimations over these regions. However, some of the improved agreement
with observations occurs for the wrong reasons, as models have the tendency
to greatly overestimate the primary OA fraction and underestimate the
secondary fraction. Measured OA in the remote free troposphere is highly
oxygenated, with organic aerosol to organic carbon (OA ∕ OC) ratios of
∼2.2–2.8, and is 30 %–60 % more oxygenated than in current
models, which can lead to significant errors in OA concentrations. The
model–measurement comparisons presented here support the concept of a more
dynamic OA system as proposed by Hodzic et al. (2016), with enhanced removal
of primary OA and a stronger production of secondary OA in global models
needed to provide better agreement with observations.
We introduce and evaluate aerosol simulations with the global aerosol–climate model ECHAM6.3–HAM2.3, which is the aerosol component of the fully coupled aerosol–chemistry–climate model ECHAM–HAMMOZ. ...Both the host atmospheric climate model ECHAM6.3 and the aerosol model HAM2.3 were updated from previous versions. The updated version of the HAM aerosol model contains improved parameterizations of aerosol processes such as cloud activation, as well as updated emission fields for anthropogenic aerosol species and modifications in the online computation of sea salt and mineral dust aerosol emissions. Aerosol results from nudged and free-running simulations for the 10-year period 2003 to 2012 are compared to various measurements of aerosol properties. While there are regional deviations between the model and observations, the model performs well overall in terms of aerosol optical thickness, but may underestimate coarse-mode aerosol concentrations to some extent so that the modeled particles are smaller than indicated by the observations. Sulfate aerosol measurements in the US and Europe are reproduced well by the model, while carbonaceous aerosol species are biased low. Both mineral dust and sea salt aerosol concentrations are improved compared to previous versions of ECHAM–HAM. The evaluation of the simulated aerosol distributions serves as a basis for the suitability of the model for simulating aerosol–climate interactions in a changing climate.
Radiative effects of absorbing black carbon and mineral dust aerosols are estimated from global aerosol climate model simulations with fixed sea surface temperatures as a boundary condition. ...Semi-direct effects are approximated as the residual between the total direct radiative effect and the instantaneous direct radiative effect of the simulated absorbing aerosol species. No distinction is made for aerosols from natural and anthropogenic sources. Results for global average are highly uncertain due to high model variability, but consistent with previous estimates. The global average results for black carbon aerosol semi-direct effects are small due to cancellation of regionally positive or negative effects, and may be positive or negative overall, depending on the model setup. The presence of mineral dust aerosol above dark surfaces and below a layer containing black carbon aerosol may enhance the reflectivity and act to enhance the positive radiative effect of black carbon aerosol. When mineral dust is absent the semi-direct effect at the top-of-atmosphere of black carbon aerosol from both anthropogenic and natural sources is −0.03 Wm−2, while averaging to +0.09 Wm−2 if dust is included.
THE ARCTIC CLOUD PUZZLE Wendisch, Manfred; Macke, Andreas; Ehrlich, André ...
Bulletin of the American Meteorological Society,
05/2019, Letnik:
100, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Clouds play an important role in Arctic amplification. This term represents the recently observed enhanced warming of the Arctic relative to the global increase of near-surface air temperature. ...However, there are still important knowledge gaps regarding the interplay between Arctic clouds and aerosol particles, and surface properties, as well as turbulent and radiative fluxes that inhibit accurate model simulations of clouds in the Arctic climate system. In an attempt to resolve this so-called Arctic cloud puzzle, two comprehensive and closely coordinated field studies were conducted: the Arctic Cloud Observations Using Airborne Measurements during Polar Day (ACLOUD) aircraft campaign and the Physical Feedbacks of Arctic Boundary Layer, Sea Ice, Cloud and Aerosol (PASCAL) ice breaker expedition. Both observational studies were performed in the framework of the German Arctic Amplification: Climate Relevant Atmospheric and Surface Processes, and Feedback Mechanisms (AC) project. They took place in the vicinity of Svalbard, Norway, in May and June 2017. ACLOUD and PASCAL explored four pieces of the Arctic cloud puzzle: cloud properties, aerosol impact on clouds, atmospheric radiation, and turbulent dynamical processes. The two instrumented Polar 5 and Polar 6 aircraft; the icebreaker Research Vessel (R/V) Polarstern; an ice floe camp including an instrumented tethered balloon; and the permanent ground-based measurement station at Ny-Ålesund, Svalbard, were employed to observe Arctic low- and mid-level mixed-phase clouds and to investigate related atmospheric and surface processes. The Polar 5 aircraft served as a remote sensing observatory examining the clouds from above by downward-looking sensors; the Polar 6 aircraft operated as a flying in situ measurement laboratory sampling inside and below the clouds. Most of the collocated Polar 5/6 flights were conducted either above the R/V Polarstern or over the Ny-Ålesund station, both of which monitored the clouds from below using similar but upward-looking remote sensing techniques as the Polar 5 aircraft. Several of the flights were carried out underneath collocated satellite tracks. The paper motivates the scientific objectives of the ACLOUD/PASCAL observations and describes the measured quantities, retrieved parameters, and the applied complementary instrumentation. Furthermore, it discusses selected measurement results and poses critical research questions to be answered in future papers analyzing the data from the two field campaigns.
During haze periods in the North China Plain, extremely high NO concentrations have been observed, commonly exceeding 1 ppbv, preventing the classical gas-phase H2O2 formation through HO2 ...recombination. Surprisingly, H2O2 mixing ratios of about 1 ppbv were observed repeatedly in winter 2017. Combined field observations and chamber experiments reveal a photochemical in-particle formation of H2O2, driven by transition metal ions (TMIs) and humic-like substances (HULIS). In chamber experiments, steady-state H2O2 mixing ratios of 116 ± 83 pptv were observed upon the irradiation of TMI- and HULIS-containing particles. Correspondingly, H2O2 formation rates of about 0.2 ppbv h–1 during the initial irradiation periods are consistent with the H2O2 rates observed in the field. A novel chemical mechanism was developed explaining the in-particle H2O2 formation through a sequence of elementary photochemical reactions involving HULIS and TMIs. Dedicated box model studies of measurement periods with relative humidity >50% and PM2.5 ≥ 75 μg m–3 agree with the observed H2O2 concentrations and time courses. The modeling results suggest about 90% of the particulate sulfate to be produced from the SO2 reaction with OH and HSO3 – oxidation by H2O2. Overall, under high pollution, the H2O2-caused sulfate formation rate is above 250 ng m–3 h–1, contributing to the sulfate formation by more than 70%.
Aerosol particles can contribute to the Arctic amplification (AA) by direct and indirect radiative effects.
Specifically, black carbon (BC) in the atmosphere, and when deposited on snow and sea ice, ...has a positive warming effect on the top-of-atmosphere (TOA) radiation balance during the polar day.
Current climate models, however, are still struggling to reproduce Arctic aerosol conditions.
We present an evaluation study with the global aerosol-climate model ECHAM6.3-HAM2.3 to examine emission-related uncertainties in the BC distribution and the direct radiative effect of BC.
The model results are comprehensively compared against the latest ground and airborne aerosol observations for the period 2005–2017, with a focus on BC.
Four different setups of air pollution emissions are tested.
The simulations in general match well with the observed amount and temporal variability in near-surface BC in the Arctic.
Using actual daily instead of fixed biomass burning emissions is crucial for reproducing individual pollution events but has only a small influence on the seasonal cycle of BC.
Compared with commonly used fixed anthropogenic emissions for the year 2000, an up-to-date inventory with transient air pollution emissions results in up to a 30 % higher annual BC burden locally.
This causes a higher annual mean all-sky net direct radiative effect of BC of over 0.1 W m−2 at the top of the atmosphere over the Arctic region (60–90∘ N), being locally more than 0.2 W m−2 over the eastern Arctic Ocean.
We estimate BC in the Arctic as leading to an annual net gain of 0.5 W m−2 averaged over the Arctic region but to a local gain of up to 0.8 W m−2 by the direct radiative effect of atmospheric BC plus the effect by the BC-in-snow albedo reduction.
Long-range transport is identified as one of the main sources of uncertainties for ECHAM6.3-HAM2.3, leading to an overestimation of BC in atmospheric layers above 500 hPa, especially in summer.
This is related to a misrepresentation in wet removal in one identified case at least, which was observed during the ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) summer aircraft campaign.
Overall, the current model version has significantly improved since previous intercomparison studies and now performs better than the multi-model average in the Aerosol Comparisons between Observation and Models (AEROCOM) initiative in terms of the spatial and temporal distribution of Arctic BC.
The magnitude of solar radiative effects (cooling or warming) of black carbon (BC) particles embedded in the Arctic atmosphere and surface snow layer was explored on the basis of case studies. For ...this purpose, combined atmospheric and snow radiative transfer simulations were performed for cloudless and cloudy conditions on the basis of BC mass concentrations measured in pristine early summer and more polluted early spring conditions. The area of interest is the remote sea-ice-covered Arctic Ocean in the vicinity of Spitsbergen, northern Greenland, and northern Alaska typically not affected by local pollution. To account for the radiative interactions between the black-carbon-containing snow surface layer and the atmosphere, an atmospheric and snow radiative transfer model were coupled iteratively.
For pristine summer conditions (no atmospheric BC, minimum solar zenith angles of 55∘) and a representative BC particle mass concentration of 5 ng g−1 in the surface snow layer, a positive daily mean solar radiative forcing of +0.2 W m−2 was calculated for the surface radiative budget. A higher load of atmospheric BC representing early springtime conditions results in a slightly negative mean radiative forcing at the surface of about −0.05 W m−2, even when the low BC mass concentration measured in the pristine early summer conditions was embedded in the surface snow layer. The total net surface radiative forcing combining the effects of BC embedded in the atmosphere and in the snow layer strongly depends on the snow optical properties (snow specific surface area and snow density).
For the conditions over the Arctic Ocean analyzed in the simulations, it was found that the atmospheric heating rate by water vapor or clouds is 1 to 2 orders of magnitude larger than that by atmospheric BC. Similarly, the daily mean total heating rate (6 K d−1) within a snowpack due to absorption by the ice was more than 1 order of magnitude larger than that of atmospheric BC (0.2 K d−1).
Also, it was shown that the cooling by atmospheric BC of the near-surface air and the warming effect by BC embedded in snow are reduced in the presence of clouds.
The radiative effect of mineral dust and black carbon aerosol are investigated with aerosolclimate model simulations with fixed sea surface temperatures as boundary condition. The semi-direct effects ...of the absorbing aerosol are assessed as the residual between the total direct radiative effect and the instantaneous direct radiative effect of the aerosol species. For Central Asia the presence of mineral dust aerosol below a black carbon aerosol layer enhances the positive radiative effect of the black carbon aerosol.
In Central Asia, climate and air quality are largely affected by local and long-travelled mineral dust. For the last century, the area has experienced severe land-use changes and water exploitation ...producing new dust sources. Today global warming causes rapid shrinking of mountain glaciers with yet unknow consequences for dust and its climate effects. Despite the importance for a growing population, only little is known about sources, transport pathways and properties of Central Asian dust. A transport study with a global aerosol-climate model is undertaken to investigate the life cycle of mineral dust in Central Asia for the period of a remote-sensing campaign in Tajikistan in 2015–2016. An initial evaluation with sun photometer measurements shows reasonable agreement for the average amount of dust, but a significant weakness of the model in reproducing the seasonality of local dust with maximum activity in summer. Source apportionment reveals a major contribution from Arabia throughout the year in accordance with observations. In the model, local sources mainly contribute in spring and autumn while summer-time dust production is underestimated. The results underline the importance of considering long-range transport and, locally, a detailed representation of atmospheric dynamics and surface characteristics for modelling dust in Central Asia.
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