Clean the Air, Heat the Planet?

What are the possible effects on global warming of air pollutant emissions that are relatively short-lived in the atmosphere compared to carbon dioxide? Many aerosols such as the sulphate aerosol cool the atmosphere (a negative radiative forcing), whereas ozone and black carbon aerosol have a warming effect (a positive forcing). Pollution control could accelerate climate change if reduction of short-lived pollutants with negative forcing - especially sulphate aerosols - outweighs reduction of those with positive forcing [1].

The distribution of short-lived species is inherently uneven, and a small global mean radiative forcing may hide substantial variation arising from localized emissions, specific oxidation and deposition pathways. Direct and indirect interactions between climate change, land ecosystems, and chemistry can amplify or dampen the climate effects of air pollutants, but are poorly represented in models. The climate effects of reduced sulphate could be offset by possibly enhanced production of secondary organic aerosol if their biogenic precursor emissions are stimulated by warmer temperatures. However, such climate feedbacks may be diminished if secondary aerosol precursor emissions are inhibited by rising CO2. Furthermore, land use and land cover change may alter biogenic and pyrogenic emissions of short-lived species as strongly as, if not more than, climate change. These interactions are increasingly being studied but are not yet well understood.

Changes in aerosol burdens may alter local and regional cloud cover and precipitation, cloud cover and precipitation will also feed back on the photochemistry and rainout of short-lived species. These issues must be considered if aerosol emissions are to become part of climate policy. Climate sensitivity analysis suggests temperature increases well above IPCC estimates for all but the lowest estimates of net aerosol forcing [2]. Changes in the atmospheric aerosol load may thus lead to a strong greenhouse gas warming response [1].

  1. Arneth A, Unger N, Kulmala M, Andreae MO (2009) Atmospheric Science: Clean the Air, Heat the Planet? Science 326 (5953) 672–673, doi: 10.1126/science.
  2. Andreae MO, Jones CD, Cox PM (2005) Nature 435:1187 (2005).

Towards quantitative understanding of new particle formation

The backbone of The Finnish Center of Excellence activities is our efficient infrastructure, especially the network of comprehensive field stations like SMEARI-III and GAW stations. The data obtained in continuous measurements over 14 years can be used to make significant conceptual development, and forms links between the different disciplines in a unique manner.

Initial clusters of particle formation have been outside the measurement range until recently. By development of ion spectrometers and condensation particle counters (CPCs), we have reached sizes around 0.8 nm in mobility diameter for ions, and 1.3 nm for neutral clusters [1]. Therefore, it's possible to measure close to the size range, where nucleation actually occurs.

As a part of the global aerosol mass spectrometer community, we participated in developing a unified framework describing the evolution of organic aerosol (OA) in the atmosphere. Our experimental data from Hyytiälä showed that OA transforms into less volatile and more hygroscopic during aging in the atmosphere [2]. The data also revealed that during new particle formation events, the smallest particles activated for growth at clearly smaller sizes in water than in butanol vapour [1]. However, even at 2–4 nm, there were days when the particles seemed to be less hygroscopic than ammonium sulfate or sulfuric acid, which are often referred to as the most likely compounds present in atmospheric nucleation. This observation points to the presence of water-soluble organics, even at the very first steps on particle formation. The water-affinity of the particles decreased with increasing particle size, indicating that the vapours that participate in the first steps of the particle formation and growth are more hygroscopic than the vapours contributing to the later stages of the growth. This suggests that the relative role of less hygroscopic organics in atmospheric particle growth increases as a function of particle size.

These results have important global consequences, regarding both visibility degradation and the ability of the organic aerosol particles to act as cloud condensation nuclei.

  1. Riipinen, I., Manninen HE, Yli-Juuti T, Boy M, Sipilä M, Ehn M, Junninen H, Petäjä T, Kulmala M. (2009). Applying the Condensation Particle Counter Battery (CPCB) to study the water-affinity of freshly-formed 2–9 nm particles in boreal forest. Atmospheric Chemistry and Physics 9, 3317-3330, 2009.
  2. Jimenez, J., Canagaratna, M., Donahue, N., Prevot, A., Zhang, Q., Kroll, J., DeCarlo, P., Allan, J., Coe,H., Ng, N., Aiken, A., Docherty, K., Ulbrich, I., Grieshop, A., Robinson, A., Duplissy, J., Smith, J., Wilson, K., Lanz, V., Hueglin, C., Sun, Y., Tian, J., Laaksonen, A., Raatikainen, T., Rautiainen, J., Vaattovaara, P., Ehn, M., Kulmala, M., Tomlinson, J., Collins, D., Cubison, M., Dunlea, E., Huffman, J., Onasch, T., Alfarra, M., Williams, P., Bower, K., Kondo, Y., Schneider, J., Drewnick, F., Borrmann, S., Weimer, S., Demerjian, K., Salcedo, D., Cottrell, L., Griffin, R., Takami, A., Miyoshi, T., Hatakeyama, S., Shimono, A., Sun, J., Zhang, Y., Dzepina, K., Kimmel, J., Sueper, D., Jayne, J., Herndon, S., Trimborn, A., Williams, L., Wood, E., Middlebrook, A., Kolb, C., Baltensperger, U. and Worsnop, D. (2009). Evolution of organic aerosols in the atmosphere, Science, 326, 1525-1529, 2009.

Organic emissions contribute to tropospheric new particle formation

Currently, the controlling mechanism of tropospheric nanoparticle formation is still an open question. Field and laboratory measurements have clearly indicated a strong correlation between observed sulphuric acid – a product of SO2 oxidation – and nanoparticle concentrations and formation rates (e.g. [1]). On the other hand, observed seasonality and comparisons with plant VOC emission strengths show that aerosol formation is also correlated with biogenic organic oxidation. Laboratory studies with real plant emissions have shown a clear dependence of aerosol formation on the VOC emission strength and also the chemical mixture [2, 3], thereby ruling out the possibility that nanoparticle formation by nucleation would be completely independent of organic compounds.

It was found that the addition of isoprene suppressed aerosol number formation while having a negligible effect on the growth rate of fresh particles. The suppression effect could be parameterized by using a model that assumes that particle number formation is dependent on the oxidation of other plant VOC emissions by the OH radical, and isoprene acts as a OH scavenger in the system [3]. We have also shown that the spring period, when the recovery of photosynthesis occurs, is a surprisingly active particle production period in the boreal forest. Using a parameter describing the physiological state of forest trees during the spring recovery time, we showed that the intitial, spin-up period of photosynthesis leads to a significant increase in nanoparticle production [4].

  1. Riipinen, I., Sihto, S.-L., Kulmala, M., Arnold, F., Dal Maso, M., Birmili, W., Saarnio, W., Teinilä, K., Kerminen, V.-M., Laaksonen A., and Lehtinen, K. E.J.: Connections between atmospheric sulphuric acid and new particle formation during QUEST III - IV campaigns in Heidelberg and Hyytiälä Atmospheric Chemistry and Physics , 7, pp. 1899-1914, (2007).
  2. Mentel, Th. F., Wildt, J., Kiendler-Scharr, A., Kleist, E., Tillmann, R., Dal Maso, M., Fisseha, R., Hohaus, Th., Spahn, H., Uerlings, R. Wegener, R., Griffiths, P. T., Dinar, E., Rudich, Y., and Wahner, A. (2009) Photochemical production of aerosols from real plant emissions Atmos. Chem. Phys., 9 (13), 4387-4406, 2009.
  3. Kiendler-Scharr, Astrid, Jürgen Wildt, Miikka Dal Maso, Thorsten Hohaus, Einhard Kleist, Thomas F. Mentel, Ralf Tillmann, Ricarda Uerlings, Uli Schurr, and Andreas Wahner (2009): Isoprene emissions inhibit new particle formation in forests, Nature, 461, (7262): 381-384, 2009. DOI: 10.1038/nature08292.
  4. Dal Maso, M., Hari, P., and Kulmala, M. Spring recovery of photosynthesis and atmospheric particle formation, Boreal Env. Res., 14 (4): 711-721, 2009.