The radiative effect of ion-induced inorganic nucleation in the free troposphere

To determine the effect of cosmic rays on the Earth's climate via ion-induced nucleation, a parametrisation of inorganic nucleation was formulated based on experiments at the CERN CLOUD experiment. The parametrisation was implemented in the GLOMAP aerosol microphysics model and used to estimate the radiative effect of the change in ionisation experienced over an 11-year solar cycle

The impact of COVID-19 lockdown measures on the Indian summer monsoon

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

Reduced anthropogenic aerosol radiative forcing caused by biogenic new particle formation

The magnitude of aerosol radiative forcing caused by anthropogenic emissions depends on the baseline state of the atmosphere under pristine preindustrial conditions. Measurements show that particle formation in atmospheric conditions can occur solely from biogenic vapors. Here, we evaluate the potential effect of this source of particles on preindustrial cloud condensation nuclei (CCN) concentrations and aerosol-cloud radiative forcing over the industrial period. Model simulations show that the pure biogenic particle formation mechanism has a much larger relative effect on CCN concentrations in the preindustrial atmosphere than in the present atmosphere because of the lower aerosol concentrations. Consequently, preindustrial cloud albedo is increased more than under present day conditions, and therefore the cooling forcing of anthropogenic aerosols is reduced. The mechanism increases CCN concentrations by 20-100% over a large fraction of the preindustrial lower atmosphere, and the magnitude of annual global mean radiative forcing caused by changes of cloud albedo since 1750 is reduced by 0.22 W m-2 (27%) to -0.60 W m-2. Model uncertainties, relatively slow formation rates, and limited available ambient measurements make it difficult to establish the significance of a mechanism that has its dominant effect under preindustrial conditions. Our simulations predict more particle formation in the Amazon than is observed. However, the first observation of pure organic nucleation has now been reported for the free troposphere. Given the potentially significant effect on anthropogenic forcing, effort should be made to better understand such naturally driven aerosol processes

Estimating the climate impact of linear contrails using the UK Met Office climate model

The HadGEM2 global climate model is employed to investigate some of the linear contrail effects on climate. Our study parameterizes linear contrails as a thin layer of aerosol. We find that at 100 times the air traffic of year 2000, linear contrails would change the equilibrium global-mean temperature by +0.13 K, corresponding to a climate sensitivity of 0.3 K/(Wm(-2)) and a climate efficacy of 31% (significantly smaller than the only previously published estimate of 59%). Our model suggests that contrails cause a slight warming of the surface and, as noted by most global warming modelling studies, land areas are affected more than the oceans. Also, unlike the contrail coverage and radiative forcing, the contrail temperature change response is not geographically correlated with air traffic patterns. In terms of the contrail impact on precipitation, the main feature is the northern shift of the Inter-Tropical Convergence Zone. Finally, our model strongly indicates that the contrail impact on both the diurnal temperature range and regional climate is significantly smaller than some earlier studies suggested