Atmospheric aerosols and their effect on clouds are thought to be important for anthropogenic radiative forcing of the climate, yet remain poorly understood. Globally, around half of cloud ...condensation nuclei originate from nucleation of atmospheric vapours. It is thought that sulfuric acid is essential to initiate most particle formation in the atmosphere, and that ions have a relatively minor role. Some laboratory studies, however, have reported organic particle formation without the intentional addition of sulfuric acid, although contamination could not be excluded. Here we present evidence for the formation of aerosol particles from highly oxidized biogenic vapours in the absence of sulfuric acid in a large chamber under atmospheric conditions. The highly oxygenated molecules (HOMs) are produced by ozonolysis of α-pinene. We find that ions from Galactic cosmic rays increase the nucleation rate by one to two orders of magnitude compared with neutral nucleation. Our experimental findings are supported by quantum chemical calculations of the cluster binding energies of representative HOMs. Ion-induced nucleation of pure organic particles constitutes a potentially widespread source of aerosol particles in terrestrial environments with low sulfuric acid pollution.
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.
Droughts have become more severe and recurrent over the Indian sub-continent during the second half of the twentieth century, leading to more severe hydro-climatic and socio-economic impacts over one ...of the most densely populated parts of the world. So far, droughts have mostly been connected to circulation changes concomitant with the abnormal warming over the Pacific Ocean, prevalently known as "El Niño". Here, exploiting observational data sets and a series of dedicated sensitivity experiments, we show that the severity of droughts during El Niño is amplified (17%) by changes in aerosols. The model experiments simulate the transport of boundary layer aerosols from South Asian countries to higher altitudes (12-18 km) where they form the Asian Tropopause Aerosol Layer (ATAL) (~ 60-120°E, 20-40°N). During El Niño, the anomalous overturning circulation from the East Asian region further enriches the thickness of aerosol layers in the ATAL over the northern part of South Asia. The anomalous aerosol loading in the ATAL reduces insolation over the monsoon region, thereby exacerbating the severity of drought by further weakening the monsoon circulation. Future increases in industrial emissions from both East and South Asia will lead to a wider and thicker elevated aerosol layer in the upper troposphere, potentially amplifying the severity of droughts.
Tropospheric ozone concentrations are sensitive to natural
emissions of precursor compounds. In contrast to existing assumptions,
recent evidence indicates that terrestrial vegetation emissions in ...the
pre-industrial era were larger than in the present day. We use a chemical
transport model and a radiative transfer model to show that revised
inventories of pre-industrial fire and biogenic emissions lead to an
increase in simulated pre-industrial ozone concentrations, decreasing the
estimated pre-industrial to present-day tropospheric ozone radiative forcing
by up to 34 % (0.38 to 0.25 W m−2). We find that this change
is sensitive to employing biomass burning and biogenic emissions inventories
based on matching vegetation patterns, as the co-location of emission sources
enhances the effect on ozone formation. Our forcing estimates are at the
lower end of existing uncertainty range estimates (0.2–0.6 W m−2),
without accounting for other sources of uncertainty. Thus, future work
should focus on reassessing the uncertainty range of tropospheric ozone
radiative forcing.
The interannual variability of the greenhouse gases methane
(CH4) and tropospheric ozone (O3) is largely driven by natural variations in global emissions and meteorology. The El Niño–Southern
...Oscillation (ENSO) is known to influence fire occurrence, wetland emission
and atmospheric circulation, affecting sources and sinks of CH4 and
tropospheric O3, but there are still important uncertainties associated
with the exact mechanism and magnitude of this effect. Here we use a
modelling approach to investigate how fires and meteorology control the
interannual variability of global carbon monoxide (CO), CH4 and O3
concentrations, particularly during large El Niño events. Using a
three-dimensional chemical transport model (TOMCAT) coupled to a
sophisticated aerosol microphysics scheme (GLOMAP) we simulate changes to
CO, hydroxyl radical (OH) and O3 for the period 1997–2014. We then use
an offline radiative transfer model to quantify the climate impact of
changes to atmospheric composition as a result of specific drivers. During the El Niño event of 1997–1998, there were increased emissions
from biomass burning globally, causing global CO concentrations to increase
by more than 40 %. This resulted in decreased global mass-weighted
tropospheric OH concentrations of up to 9 % and a consequent 4 %
increase in the CH4 atmospheric lifetime. The change in CH4
lifetime led to a 7.5 ppb yr−1 increase in the global mean CH4
growth rate in 1998. Therefore, biomass burning emission of CO could account
for 72 % of the total effect of fire emissions on CH4 growth rate in
1998. Our simulations indicate that variations in fire emissions and meteorology
associated with El Niño have opposing impacts on tropospheric O3
burden. El Niño-related changes in atmospheric transport and humidity
decrease global tropospheric O3 concentrations leading to a −0.03 W m−2 change in the O3 radiative effect (RE). However, enhanced fire emission of precursors such as nitrogen oxides (NOx) and CO
increase O3 and lead to an O3 RE of 0.03 W m−2. While globally
the two mechanisms nearly cancel out, causing only a small change in global
mean O3 RE, the regional changes are large – up to −0.33 W m−2 with potentially important consequences for atmospheric heating and
dynamics.
Most socioeconomic pathways compatible with the aims of the Paris Agreement include large changes to land use and land cover. The associated vegetation changes can interact with the atmosphere and ...climate through numerous mechanisms. One of these is emissions of biogenic volatile organic compounds (BVOCs), which may lead to the formation of secondary organic aerosols (SOAs) and atmospheric chemistry changes. Here, we use a modeling framework to explore potential future global and regional changes in SOA and tropospheric ozone following idealized, large-scale vegetation perturbations, and their resulting radiative forcing (RF). Guided by projections in low-warming scenarios, we modify crop and forest cover, separately, and in concurrence with changes in anthropogenic emissions and CO2 level. We estimate that increasing global forest cover by 30% gives a 37% higher global SOA burden, with a resulting forcing of −0.13 W m−2. The effect on tropospheric ozone is relatively small. Large SOA burden changes of up to 48% are simulated for South America and Sub-Saharan Africa. Conversely, increasing crop cover at the expense of tropical forest, yields similar changes but of opposite sign. The magnitude of these changes is strongly affected by the concurrent evolution of anthropogenic emissions. Our land cover perturbations are representative of energy crop expansion and afforestation, two key mitigation measures in 1.5 °C compatible scenarios. Our results hence indicate that depending on the role of these two in the underlying mitigation strategies, scenarios with similar long-term global temperature levels could lead to opposite effects on SOA. Combined with the complexity of factors that control SOA, this highlights the importance of including BVOC effects in further studies and assessments of climate and air quality mitigation involving the land surface.
Convective transport plays a key role in aerosol enhancement in the upper
troposphere and lower stratosphere (UTLS) over the Asian monsoon region
where low-level convective instability persists ...throughout the year. We use
the state-of-the-art ECHAM6–HAMMOZ global chemistry–climate model to
investigate the seasonal transport of anthropogenic Asian sulfate aerosols
and their impact on the UTLS. Sensitivity simulations for SO2 emission
perturbation over India (48 % increase) and China (70 % decrease) are
performed based on the Ozone Monitoring Instrument (OMI) satellite-observed
trend, rising over India by ∼4.8 % per year and decreasing
over China by ∼7.0 % per year during 2006–2017. The
enhanced Indian emissions result in an increase in aerosol optical depth
(AOD) loading in the UTLS by 0.61 to 4.17 % over India. These aerosols
are transported to the Arctic during all seasons by the lower branch of the
Brewer–Dobson circulation enhancing AOD by 0.017 % to 4.8 %.
Interestingly, a reduction in SO2 emission over China inhibits the
transport of Indian sulfate aerosols to the Arctic in summer-monsoon and
post-monsoon seasons due to subsidence over northern India. The region of
sulfate aerosol enhancement shows significant warming in the UTLS over northern India, south China (0.2±0.15 to 0.8±0.72 K) and the Arctic
(∼1±0.62 to 1.6±1.07 K). The estimated seasonal
mean direct radiative forcing at the top of the atmosphere (TOA) induced by
the increase in Indian SO2 emission is −0.2 to −1.5 W m−2 over northern India. The Chinese SO2 emission reduction
leads to a positive radiative forcing of ∼0.6 to 6 W m−2 over China. The decrease in vertical velocity and
the associated enhanced stability of the upper troposphere in response to
increased Indian SO2 emissions will likely decrease rainfall over
India.
Abstract The large-scale convection during the Asian summer monsoon plays an important role in the rapid transport of boundary layer aerosols into the Asian summer monsoon anticyclone. Here, using ...the state-of-the-art ECHAM6–HAMMOZ aerosol-chemistry-climate model, we show that these aerosols are further transported to the Arctic along isentropic surfaces by the Brewer-Dobson-Circulation (BDC) during the monsoon season. Our model simulations show that East and South Asian anthropogenic emissions contribute significantly to the aerosol transported to the Arctic, which causes a higher negative net aerosol radiative forcing at the surface (dimming) of −0.09 ± 0.02 Wm −2 and −0.07 ± 0.02 Wm −2 , respectively. Over the Arctic, the East Asian anthropogenic aerosols that include large amounts of sulfate cause a seasonal mean net radiative forcing at the top of the atmosphere (TOA) of −0.003 ± 0.001Wm −2 and a surface cooling of −0.56 K while the black carbon dominated aerosol from South Asia shows a positive TOA forcing of +0.004 ± 0.001Wm −2 with an only minor surface cooling of −0.043 K. Overall, the long-range transport of South Asian aerosols results in a notably warming throughout the atmospheric column but minimal temperature response at the Arctic surface. Conversely, East Asian aerosols cool the troposphere and heat the lower stratosphere in the Arctic. The Asian aerosol thus plays an ambivalent role, with the East Asian sources in particular having the potential to counteract the rapid rise in Arctic temperatures and the associated melting of snow and ice.
Regional patterns of aerosol radiative forcing are important for understanding climate change on decadal time scales. Uncertainty in aerosol forcing is likely to vary regionally and seasonally ...because of the short aerosol lifetime and heterogeneous emissions. Here the sensitivity of regional aerosol cloud albedo effect (CAE) forcing to 31 aerosol process parameters and emission fluxes is quantified between 1978 and 2008. The effects of parametric uncertainties on calculations of the balance of incoming and outgoing radiation are found to be spatially and temporally dependent. Regional uncertainty contributions of opposite sign cancel in global-mean forcing calculations, masking the regional importance of some parameters. Parameters that contribute little to uncertainty in Earth’s global energy balance during recent decades make significant contributions to regional forcing variance. Aerosol forcing sensitivities are quantified within 11 climatically important regions, where surface temperatures are thought to influence large-scale climate effects. Substantial simulated uncertainty in CAE forcing in the eastern Pacific leaves open the possibility that apparent shifts in the mean ENSO state may result from a forced aerosol signal on multidecadal time scales. A likely negative aerosol CAE forcing in the tropical North Atlantic calls into question the relationship between Northern Hemisphere aerosol emission reductions and CAE forcing of sea surface temperatures in the main Atlantic hurricane development region on decadal time scales. Simulated CAE forcing uncertainty is large in the North Pacific, suggesting that the role of the CAE in altering Pacific tropical storm frequency and intensity is also highly uncertain.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Cirrus clouds play an important role in the radiation budget of the Earth; nonetheless, the radiative effect of ultra-thin cirrus clouds in the tropopause region and in the lowermost stratosphere ...remains poorly constrained. These clouds have a small vertical extent and optical depth and are frequently neither observed even by sensitive sensors nor considered in climate model simulations. In addition, their short-wave (cooling) and long-wave (warming) radiative effects are often in approximate balance, and their net effect strongly depends on the shape and size of the cirrus particles. However, the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere instrument (CRISTA-2) allows ultra-thin cirrus clouds to be detected. Here we use CRISTA-2 observations in summer 1997 in the Northern Hemisphere midlatitudes together with the Suite Of Community RAdiative Transfer codes based on Edwards and Slingo (SOCRATES) radiative transfer model to calculate the radiative effect of observed ultra-thin cirrus. Using sensitivity simulations with different ice effective particle size and shape, we provide an estimate of the uncertainty in the radiative effect of ultra-thin cirrus in the extratropical lowermost stratosphere and tropopause region during summer and – by extrapolation of the summer results – for winter. Cloud top height and ice water content are based on CRISTA-2 measurements, while the cloud vertical thickness was predefined to be 0.5 or 2 km. Our results indicate that if the ice crystals of these thin cirrus clouds are assumed to be spherical, their net cloud radiative effect is generally positive (warming). In contrast, assuming aggregates or a hexagonal shape, their net radiative effect is generally negative (cooling) during summer months and very likely positive (warming) during winter. The radiative effect is in the order of ±(0.1–0.01) W m−2 for a realistic global cloud coverage of 10 %, similar to the magnitude of the contrail cirrus radiative forcing (of ∼ 0.1 W m−2). The radiative effect is also dependent on the cloud vertical extent and consequently the optically thickness and effective radius of the particle size distribution (e.g. effective radius increase from 5 to 30 µm results in a factor ∼ 6 smaller long- and short-wave effects, respectively). The properties of ultra-thin cirrus clouds in the lowermost stratosphere and tropopause region need to be better observed, and ultra-thin cirrus clouds need to be evaluated in climate model simulations.
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