Ari Asmi

Indoor air measurement campaign in Helsinki, Finland 1999 – the effect of outdoor air pollution on indoor air

Abstract Wintertime indoor and outdoor particle size distributions were studied in an office building near Helsinki downtown by measuring the particle size distributions with two similar differential mobility particle sizer systems (DMPS). Measurements were made simultaneously at two places; on the rooftop of a building (30 m above the ground level) in front of a ventilation system corresponding to outdoor concentration, and an office room (first floor). The ventilation rate was also continuously monitored. Indoor particle concentrations were observed to vary from 500 to 10 4 cm −3 with a high dependence on the outdoor concentrations. This indicates that in this scenario, indoor particles are mainly of outdoor origin. Effects of ventilation rates on indoor air particle concentrations and several inorganic gases were studied. We found that ventilation had a strong influence on indoor particle and gas concentrations. I/O (indoor/outdoor) ratio in different in different particle size classes.

Global observations of aerosol‐cloud‐precipitation‐climate interactions

Cloud drop condensation nuclei (CCN) and ice nuclei (IN) particles determine to a large extent cloud microstructure and, consequently, cloud albedo and the dynamic response of clouds to aerosol-induced changes to precipitation. This can modify the reflected solar radiation and the thermal radiation emitted to space. Measurements of tropospheric CCN and IN over large areas have not been possible and can be only roughly approximated from satellite-sensor-based estimates of optical properties of aerosols. Our lack of ability to measure both CCN and cloud updrafts precludes disentangling the effects of meteorology from those of aerosols and represents the largest component in our uncertainty in anthropogenic climate forcing. Ways to improve the retrieval accuracy include multiangle and multipolarimetric passive measurements of the optical signal and multispectral lidar polarimetric measurements. Indirect methods include proxies of trace gases, as retrieved by hyperspectral sensors. Perhaps the most promising emerging direction is retrieving the CCN properties by simultaneously retrieving convective cloud drop number concentrations and updraft speeds, which amounts to using clouds as natural CCN chambers. These satellite observations have to be constrained by in situ observations of aerosol-cloud-precipitation-climate (ACPC) interactions, which in turn constrain a hierarchy of model simulations of ACPC. Since the essence of a general circulation model is an accurate quantification of the energy and mass fluxes in all forms between the surface, atmosphere and outer space, a route to progress is proposed here in the form of a series of box flux closure experiments in the various climate regimes. A roadmap is provided for quantifying the ACPC interactions and thereby reducing the uncertainty in anthropogenic climate forcing.

Indoor air aerosol model: the effect of outdoor air, filtration and ventilation on indoor concentrations

A simple model has been developed to predict the indoor air concentration, indoor surface accumulation and the connection between the outdoor and indoor air concentrations. The model is valid for systems where the chemical reactions are slow or the aerosol size distribution is pretty constant, for example for the number concentration of radioactive aerosols (particularly the total activity) or fungal spores. In the model, several factors such as filtration, ventilation, deposition, re-emission, outdoor concentration and indoor sources are included. When simulating radioactive clouds the model results show that there is always a time period after which the radioactivity indoors is higher than outdoors. If the outdoor aerosol particles have a periodic behaviour, also the indoor aerosols behave periodically but smoother and with some delay. Indoor sources are shown to be able to increase indoor concentrations continuously.

Interaction between SO2 and submicron atmospheric particles

In the atmosphere, oxidation of sulfur dioxide (SO2) to sulfate may occur in the gas phase, in cloud or fog droplets, or in the aerosol phase on the surface or inside aerosol particles. While aerosol phase reactions have been studied in the case of supermicron sea-salt and crustal particles, very few investigations regarding submicron particles are available. In this paper, the importance of aerosol phase sulfate production to the dynamics of submicron particle populations was examined. The investigation was based on model simulations and theoretical evaluations regarding potential SO2 oxidation reactions. None of the relatively well-quantified aqueous phase reactions was rapid enough to make small nuclei grow to cloud condensation nuclei (CCN) size within the particle lifetime in the lower troposphere. This is consistent with the few observations showing that the smallest atmospheric particles are enriched in organics rather than sulfate. The amount of submicron particulate matter could be enhanced significantly by certain aerosol phase reactions, but this is likely to require a particle population having a pH close to 7. Aerosol phase reactions could partly explain the apparently too low SO2-to-sulfate conversion rates predicted by several chemical transport models over polluted regions. In addition to the bulk aerosol phase, SO2-to-sulfate conversion might involve physical adsorption of SO2 or a compound reacting with it by the particle surface, or it could take place in a liquid surface layer that usually covers atmospheric particles. Reactions involving physical adsorption seem to have negligible influence on the dynamics of submicron atmospheric particle populations. Aerosol phase reactions worth future investigation are those occurring in particle surface layers and those occurring in cloud interstitial particles.

Novel insights on new particle formation derived from a pan-european observing system

The formation of new atmospheric particles involves an initial step forming stable clusters less than a nanometre in size (<~1 nm), followed by growth into quasi-stable aerosol particles a few nanometres (~1–10 nm) and larger (>~10 nm). Although at times, the same species can be responsible for both processes, it is thought that more generally each step comprises differing chemical contributors. Here, we present a novel analysis of measurements from a unique multi-station ground-based observing system which reveals new insights into continental-scale patterns associated with new particle formation. Statistical cluster analysis of this unique 2-year multi-station dataset comprising size distribution and chemical composition reveals that across Europe, there are different major seasonal trends depending on geographical location, concomitant with diversity in nucleating species while it seems that the growth phase is dominated by organic aerosol formation. The diversity and seasonality of these events requires an advanced observing system to elucidate the key processes and species driving particle formation, along with detecting continental scale changes in aerosol formation into the future.

Substantial large-scale feedbacks between natural aerosols and climate

The terrestrial biosphere is an important source of natural aerosol. Natural aerosol sources alter climate, but are also strongly controlled by climate, leading to the potential for natural aerosol–climate feedbacks. Here we use a global aerosol model to make an assessment of terrestrial natural aerosol–climate feedbacks, constrained by observations of aerosol number. We find that warmer-than-average temperatures are associated with higher-than-average number concentrations of large (>100 nm diameter) particles, particularly during the summer. This relationship is well reproduced by the model and is driven by both meteorological variability and variability in natural aerosol from biogenic and landscape fire sources. We find that the calculated extratropical annual mean aerosol radiative effect (both direct and indirect) is negatively related to the observed global temperature anomaly, and is driven by a positive relationship between temperature and the emission of natural aerosol. The extratropical aerosol–climate feedback is estimated to be −0.14 W m−2 K−1 for landscape fire aerosol, greater than the −0.03 W m−2 K−1 estimated for biogenic secondary organic aerosol. These feedbacks are comparable in magnitude to other biogeochemical feedbacks, highlighting the need for natural aerosol feedbacks to be included in climate simulations.Extratropical feedbacks between climate and aerosols from landscape fire and biogenic secondary organic aerosols are significant, according to a global aerosol model that is constrained by observations.

Long-term size-segregated cloud condensation nuclei counter (CCNc) measurements in a boreal environment and the implications for aerosol-cloud interactions

Ambient aerosol CCN and hygroscopic properties were measured with a size-segregated CCNc in a boreal environment of Southern Finland at the SMEAR II station since February 2009. The overall median critical diameter Dc for CCN activation is reported at 75 nm, exhibiting a clear maximum in February and a minimum in July. The overall median aerosol hygroscopicity parameter κ is reported at 0.22, indicating that ambient aerosol in Hyytiala is less hygroscopic than the global continental and European continental averages. It is, however, more hygroscopic than ambient aerosol in an Amazon rainforest, the European high alpine site or the mountainous forest. The low hygroscopicity in the boreal forest is attributed to a large organic fraction present in the aerosol mass comparative to other locations within Europe. Aerosol mass spectrometer (AMS) data were used to demonstrate a positive correlation between κ and sulphate and ammonia, and a negative correlation between κ and the organic mass fraction. No distingui...

Climatic implications of the Brazilian biofuel transition

A global climate model is being run to study the consequences of Brazil transitioning from fossil to biofuels over the next few decades. Assuming all other factors constant, preliminary results suggest little to no climatic relevance.

Number Size Distributions of Submicron Particles in Europe

The aerosol particle number size distribution is a key component in aerosol indirect climate effects, and is also a key factor on potential nanoparticle health effects. This chapter will give background on particle number size distributions, their monitoring and on potential climate and health effects of submicron aerosol particles. The main interest is on the current variability and concentration levels in European background air.

Effect of Nucleation and Secondary Organic Aerosol Formation on Cloud Droplet Number Concentrations

The global general circulation model ECHAM5 is used together with HAM aerosol module to investigate the effect of the nucleation scheme on cloud droplet number concentrations. It is shown that nucleation can have a significant role on indirect aerosol effect. Also an efficient SOA formation scheme is intro- duced, and results are compared with original ECHAM5-HAM.