Catherine E. Scott

Oxidation products of biogenic emissions contribute to nucleation of atmospheric particles.

Out of the Air New-particle formation from gaseous precursors in the atmosphere is a complex and poorly understood process with importance in atmospheric chemistry and climate. Laboratory studies have had trouble reproducing the particle formation rates that must occur in the natural world. Riccobono et al. (p. 717) used the CLOUD (Cosmics Leaving Outdoor Droplets) chamber at CERN to recreate a realistic atmospheric environment. Sulfuric acid and oxidized organic vapors in typical natural concentrations caused particle nucleation at similar rates to those observed in the lower atmosphere. Experiments in the CLOUD chamber at CERN reproduce particle nucleation rates observed in the lower atmosphere. Atmospheric new-particle formation affects climate and is one of the least understood atmospheric aerosol processes. The complexity and variability of the atmosphere has hindered elucidation of the fundamental mechanism of new-particle formation from gaseous precursors. We show, in experiments performed with the CLOUD (Cosmics Leaving Outdoor Droplets) chamber at CERN, that sulfuric acid and oxidized organic vapors at atmospheric concentrations reproduce particle nucleation rates observed in the lower atmosphere. The experiments reveal a nucleation mechanism involving the formation of clusters containing sulfuric acid and oxidized organic molecules from the very first step. Inclusion of this mechanism in a global aerosol model yields a photochemically and biologically driven seasonal cycle of particle concentrations in the continental boundary layer, in good agreement with observations.

Ion-induced nucleation of pure biogenic particles

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.

The Seeds of Lotus japonicus Lines Transformed with Sense, Antisense, and Sense/Antisense Galactomannan Galactosyltransferase Constructs Have Structurally Altered Galactomannans in Their Endosperm Cell Walls

Galactomannan biosynthesis in legume seed endosperms involves two Golgi membrane-bound glycosyltransferases, mannan synthase and galactomannan galactosyltransferase (GMGT). GMGT specificity is an important factor regulating the distribution and amount of (1→6)-α-galactose (Gal) substitution of the (1→4)-β-linked mannan backbone. The model legume Lotus japonicus is shown now to have endospermic seeds with endosperm cell walls that contain a high-Gal galactomannan (mannose [Man]/Gal = 1.2-1.3). Galactomannan biosynthesis in developing L. japonicus endosperms has been mapped, and a cDNA encoding a functional GMGT has been obtained from L. japonicus endosperms during galactomannan deposition. L. japonicus has been transformed with sense, antisense, and sense/antisense (“hairpin loop”) constructs of the GMGT cDNA. Some of the sense, antisense, and sense/antisense transgenic lines exhibited galactomannans with altered (higher) Man/Gal values in their (T1 generation) seeds, at frequencies that were consistent with posttranscriptional silencing of GMGT. For T1 generation individuals, transgene inheritance was correlated with galactomannan composition and amount in the endosperm. All the azygous individuals had unchanged galactomannans, whereas those that had inherited a GMGT transgene exhibited a range of Man/Gal values, up to about 6 in some lines. For Man/Gal values up to 4, the results were consistent with lowered Gal substitution of a constant amount of mannan backbone. Further lowering of Gal substitution was accompanied by a slight decrease in the amount of mannan backbone. Microsomal membranes prepared from the developing T2 generation endosperms of transgenic lines showed reduced GMGT activity relative to mannan synthase. The results demonstrate structural modification of a plant cell wall polysaccharide by designed regulation of a Golgi-bound glycosyltransferase.

Reduced anthropogenic aerosol radiative forcing caused by biogenic new particle formation

Significance A mechanism for the formation of atmospheric aerosols via the gas to particle conversion of highly oxidized organic molecules is found to be the dominant aerosol formation process in the preindustrial boundary layer over land. The inclusion of this process in a global aerosol model raises baseline preindustrial aerosol concentrations and could lead to a reduction of 27% in estimates of anthropogenic aerosol radiative forcing. 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.

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.

Role of Organics in Particle Nucleation: From the Lab to Global Model

The role of oxidized organic compounds in the process of new particle formation in the atmosphere is poorly known. Here we used the ultraclean and most sophisticated CLOUD chamber to investigate systematically particle formation in the presence of sulfuric acid and oxidized organics. We varied independently the concentrations of both of these components. In addition, nucleation was observed without and in the presence of ionic compounds. From the results a new parameterized description of nucleation was derived for global climate model simulations.

Enhanced global primary production by biogenic aerosol via diffuse radiation fertilization

Terrestrial vegetation releases large quantities of plant volatiles into the atmosphere that can then oxidize to form secondary organic aerosol. These particles affect plant productivity through the diffuse radiation fertilization effect by altering the balance between direct and diffuse radiation reaching the Earth’s surface. Here, using a suite of models describing relevant coupled components of the Earth system, we quantify the impacts of biogenic secondary organic aerosol on plant photosynthesis through this fertilization effect. We show that this leads to a net primary productivity enhancement of 1.23 Pg C yr−1 (range 0.76–1.61 Pg C yr−1 due to uncertainty in biogenic secondary organic aerosol formation). Notably, this productivity enhancement is twice the mass of biogenic volatile organic compound emissions (and ~30 times larger than the mass of carbon in biogenic secondary organic aerosol) causing it. Hence, our simulations indicate that there is a strong positive ecosystem feedback between biogenic volatile organic compound emissions and plant productivity through plant-canopy light-use efficiency. We estimate a gain of 1.07 in global biogenic volatile organic compound emissions resulting from this feedback.Biogenic aerosols produced by terrestrial vegetation substantially enhance global primary productivity of plants, according to integrated model analyses.

Reassessment of pre-industrial fire emissions strongly affects anthropogenic aerosol forcing

Uncertainty in pre-industrial natural aerosol emissions is a major component of the overall uncertainty in the radiative forcing of climate. Improved characterisation of natural emissions and their radiative effects can therefore increase the accuracy of global climate model projections. Here we show that revised assumptions about pre-industrial fire activity result in significantly increased aerosol concentrations in the pre-industrial atmosphere. Revised global model simulations predict a 35% reduction in the calculated global mean cloud albedo forcing over the Industrial Era (1750–2000 CE) compared to estimates using emissions data from the Sixth Coupled Model Intercomparison Project. An estimated upper limit to pre-industrial fire emissions results in a much greater (91%) reduction in forcing. When compared to 26 other uncertain parameters or inputs in our model, pre-industrial fire emissions are by far the single largest source of uncertainty in pre-industrial aerosol concentrations, and hence in our understanding of the magnitude of the historical radiative forcing due to anthropogenic aerosol emissions.Several lines of evidence suggest that fire activity was much greater in the preindustrial era than currently assumed in climate models. Here the authors show that greater emission of aerosols from fires leads to a substantial reduction in the magnitude of aerosol radiative forcing over the Industrial Era.

The biogeochemical impacts of forests and the implications for climate change mitigation.

Vegetation emits biogenic volatile organic compounds (BVOCs) into the atmosphere which, once oxidised, may partition into the particle-phase, forming secondary organic aerosol (SOA). In this thesis, the climatic impacts of biogenic SOA are quantified, using a detailed global aerosol microphysics model, and the sensitivity of these radiative effects to the representation of various atmospheric processes is examined. By altering the size, composition and number of particles in the atmosphere, the presence of biogenic SOA very likely has a negative radiative effect on the climate (i.e. a cooling), via both the direct radiative effect (DRE) and first aerosol indirect effect (AIE). The DRE from biogenic SOA is sensitive to the large uncertainty in the amount of biogenic SOA being produced in the atmosphere (estimated to be between 12 and 1870 Tg(SOA) a-1). The presence of biogenic SOA increases the global annual mean concentration of particles with the potential to form cloud droplets (i.e. cloud condensation nuclei; CCN). Consequently, biogenic SOA exerts a global annual mean first AIE of between +0.03 W m-2 and -0.77 W m-2. Most of the range in the first AIE due to biogenic SOA can be attributed to uncertainty regarding the role of biogenic oxidation products in the very early stages of atmospheric new particle formation (i.e. nucleation). The most negative first AIEs (up to -0.77 W m-2) are simulated when BVOC oxidation products do participate in the very early stages of new particle formation; an approach which best captures the observed seasonal cycle in particle concentrations across the continental northern hemisphere. At the outside of the uncertainty range examined here, the DRE and first AIE due to biogenic SOA are almost half the strength (in terms of absolute magnitude) of the estimated net anthropogenic radiative forcing from 1750 to 2005 of +1.6 W m-2. The sign of the first AIE due to biogenic SOA is also sensitive to assumptions regarding the volatility of biogenic oxidation products and the manner in which their addition to the existing aerosol distribution is modelled. Taking a kinetic approach, in which SOA is partitioned according to particle surface area, gives a negative first AIE due to the role of secondary organics in the growth of newly formed particles. However, taking a thermodynamic equilibrium approach, in which SOA is added in proportion to existing organic mass, gives a positive first AIE because the growth of newly formed particles is suppressed in the presence of larger particles (i.e. due to the enhanced condensation sink). As a result, the thermodynamic approach is not able to capture the observed growth of new particles in the atmosphere and may not be suitable when examining processes that depend strongly on changes to ultrafine particle number, such as the first AIE. The negative radiative effects of biogenic SOA have implications for the climatic impact of forests and any changes to their distribution. The contribution of the first AIE, due to changes in the production of biogenic SOA, is quantified here using simplified deforestation scenarios. Globally, the replacement of forests with grass results in a first AIE of +0.26 W m-2 due to a 91% reduction in biogenic SOA production. This increases the total warming effect of deforestation, from the combined changes to carbon dioxide concentration and surface albedo, by 21%. Regionally, the strongest first AIE (+0.12 W m-2) comes from tropical (20°N – 20°S) deforestation which reduces global SOA production by 73%. The largest AIE per change in SOA comes from simulated temperate (20°N – 50°N and 20°S – 50°S) deforestation, which reduces global SOA production by only 15%, but leads to strong warming over remote ocean regions with high cloud fraction. This work suggests that present-day tropical deforestation is warming the climate more than previously thought, confirming that a reduction in deforestation should be a priority for climate change mitigation. These results also suggest that present-day afforestation in temperate regions may be exerting more of a cooling than would be attributed to CO2 sequestration alone, warranting further investigation.

Conclusions, Implications and Further Work

This thesis has explored the role of biogenic secondary organic aerosol (SOA) in the present-day, and pre-industrial, atmosphere. Chapters 3– 5 examined the behaviour of SOA, and the radiative impact of its presence, whilst Chap. 6 focussed on the climatic significance of forest-derived SOA.