V.-M. Kerminen

A new atmospherically relevant oxidant of sulphur dioxide

Atmospheric oxidation is a key phenomenon that connects atmospheric chemistry with globally challenging environmental issues, such as climate change, stratospheric ozone loss, acidification of soils and water, and health effects of air quality. Ozone, the hydroxyl radical and the nitrate radical are generally considered to be the dominant oxidants that initiate the removal of trace gases, including pollutants, from the atmosphere. Here we present atmospheric observations from a boreal forest region in Finland, supported by laboratory experiments and theoretical considerations, that allow us to identify another compound, probably a stabilized Criegee intermediate (a carbonyl oxide with two free-radical sites) or its derivative, which has a significant capacity to oxidize sulphur dioxide and potentially other trace gases. This compound probably enhances the reactivity of the atmosphere, particularly with regard to the production of sulphuric acid, and consequently atmospheric aerosol formation. Our findings suggest that this new atmospherically relevant oxidation route is important relative to oxidation by the hydroxyl radical, at least at moderate concentrations of that radical. We also find that the oxidation chemistry of this compound seems to be tightly linked to the presence of alkenes of biogenic origin.

Chemistry of Atmospheric Nucleation: On the Recent Advances on Precursor Characterization and Atmospheric Cluster Composition in Connection with Atmospheric New Particle Formation

The recent development in measurement techniques and theoretical understanding has enabled us to study atmospheric vapor, cluster and nanoparticle concentrations, dynamics, and their connection to atmospheric nucleation. Here we present a summary of the chemistry of atmospheric clustering, growing nanoparticles, and their precursors. In this work, we focus particularly on atmospheric gas-to-particle conversion and recent progress in its understanding.

Enhanced haze pollution by black carbon in megacities in China

Aerosol-planetary boundary layer (PBL) interactions have been found to enhance air pollution in megacities in China. We show that black carbon (BC) aerosols play the key role in modifying the PBL meteorology and hence enhancing the haze pollution. With model simulations and data analysis from various field observations in December 2013, we demonstrate that BC induces heating in the PBL, particularly in the upper PBL, and the resulting decreased surface heat flux substantially depresses the development of PBL and consequently enhances the occurrences of extreme haze pollution episodes. We define this process as the “dome effect” of BC and suggest an urgent need for reducing BC emissions as an efficient way to mitigate the extreme haze pollution in megacities of China.

The natural aerosol over Northern Europe and its relation to anthropogenic emissions—implications of important climate feedbacks

Abstract We use a recently developed parametrization to estimate the regional particle field in the summer time troposphere over Scandinavia that would result if the forest were the only source of particles. The calculated field is compared with available observational data. It is concluded that the needle leaf forest above 58.N alone is capable of producing aerosol mass concentrations corresponding to 12.50% of todays values in the boundary layer over Scandinavia. We also demonstrate that the forest itself could produce up to 200 CCN per cubic centimetre on average over Scandinavia and further show that an increase in temperature by 5.8 .C compared to todays average temperature could increase this CCN population by 40%. The study shows that we are able to approximate the natural aerosol field resulting from biogenic emissions over the boreal forest in the northern hemispheric region. This information provide an important contribution in the evaluation of the climate effect caused by anthropogenic emissions of particles over the forest and also opens the possibility to better address the climate feedbacks believed to be associated with the boreal region.

Effect of aerosol size distribution changes on AOD, CCN and cloud droplet concentration: Case studies from Erfurt and Melpitz, Germany

[1] For the period of 1990 to 2000, atmospheric particulate mass concentrations have decreased in Central Europe. Simultaneously, the amount of shortwave radiation reaching the ground increased during clear sky conditions. The aerosol indirect effect has not been seen as clearly, as the radiation reaching the ground during overcast conditions has not increased as much as might be expected. Here we show that this may be caused by the condensation kinetics of water during cloud droplet formation. The decrease in the particulate mass led to a clear decrease in the number concentration of cloud condensation nuclei (CCN). However, in urban areas a relatively larger decrease in the number of particles in the upper end of the accumulation mode has led to slower condensation of water. As a result, a higher maximum supersaturation is reached during the cloud droplet formation. This compensates for the effect of decreased CCN concentrations. For example in Erfurt between 1991 and 1996, the aerosol properties changed so that aerosol optical depth decreased by 58% and CCN concentration decreased by 25 to 50%. These led to a 4 to 12% reduction in cloud droplet number concentration (CDNC) and less than a 2 Wm � 2 increase in shortwave radiation during overcast conditions. These results demonstrate that locally the aerosol direct effect can be much larger than the aerosol indirect effect. Furthermore, even though AOD appears to be a valid proxy for CCN, the correlation between AOD and CDNC is not straightforward and thus AOD cannot be used as a proxy for CDNC.

Production of neutral molecular clusters by controlled neutralization of mobility standards

ABSTRACT Measuring aerosols and molecular clusters below the 3 nm size limit is essential to increase our understanding of new particle formation. Instruments for the detection of sub-3 nm aerosols and clusters exist and need to be carefully calibrated and characterized. So far calibrations and laboratory tests have been carried out using mainly electrically charged aerosols, as they are easier to handle experimentally. However, the charging state of the cluster is an important variable to take into account. Furthermore, instrument characterization performed with charged aerosols could be biased, preventing a correct interpretation of data when electrically neutral sub-3 nm aerosols are involved. This article presents the first steps to generate electrically neutral molecular clusters as standards for calibration. We show two methods: One based on the neutralization of well-known molecular clusters (mobility standards) by ions generated in a switchable aerosol neutralizer. The second is based on the controlled neutralization of mobility standards with mobility standards of opposite polarity in a recombination cell. We highlight the challenges of these two techniques and, where possible, point out solutions. In addition, we give an outlook on the next steps toward generating well-defined neutral molecular clusters with a known chemical composition and concentration. Published with license by American Association for Aerosol Research