Environmental conditions regulate the impact of plants on cloud formation

Abstract

The terrestrial vegetation emits large amounts of volatile organic compounds (VOC) into the atmosphere, which on oxidation produce secondary organic aerosol (SOA). By acting as cloud condensation nuclei (CCN), SOA influences cloud formation and climate. In a warming climate, changes in environmental factors can cause stresses to plants, inducing changes of the emitted VOC. These can modify particle size and composition. Here we report how induced emissions eventually affect CCN activity of SOA, a key parameter in cloud formation. For boreal forest tree species, insect infestation by aphids causes additional VOC emissions which modifies SOA composition thus hygroscopicity and CCN activity. Moderate heat increases the total amount of constitutive VOC, which has a minor effect on hygroscopicity, but affects CCN activity by increasing the particles' size. The coupling of plant stresses, VOC composition and CCN activity points to an important impact of induced plant emissions on cloud formation and climate.

Figure 1. Interactions of plant emissions and cloud formation.

The schematic shows the interactions of environmental conditions, plant volatile organic compound (VOC) emissions, secondary organic aerosol (SOA), cloud formation and climate. In unstressed conditions, plants emit constitutive VOC (black arrows on the left path), which on oxidation form SOA that act as cloud condensation nuclei (CCN) and can affect cloud formation and climate. Unfavourable environmental conditions (stresses) can induce VOC emissions (red arrows on the right path). Climatic changes and the resulting environmental conditions can affect the amount of constitutive VOC emissions and/or induce VOC emissions that modify the VOC composition. Such alterations in VOC emissions will be reflected in the particle size and/or particle composition. The latter determines the hygroscopicity parameter (κ) of the SOA, which is a measure of CCN activity at a given particle size. Both, particle size and κ, determine the CCN number concentration (N_CCN) (cf. Supplementary Fig. 5), and thus affect cloud formation and climate. +/− indicates the changes of parameters.

Figure 2. Volatile organic compounds composition and hygroscopicity parameter κ.

The ratio of monoterpene (MT) to sesquiterpene (SQT) emissions (ppbC/ppbC, yellow bar, left axis) from unstressed trees (monoterpene-dominated emissions) and biotically stressed (aphid infestation) boreal trees (sesquiterpene-dominated emissions) and the corresponding κ value (blue bar, right axis) of the resulting secondary organic aerosol (SOA) are shown. The emissions from unstressed trees are from pines here. The ‘intermediate' case was obtained for the emissions of stressed boreal trees in the dark, when fractions of monoterpenes and sesquiterpenes were between those of monoterpene-dominated and sesquiterpene-dominated cases. The emissions here were obtained at room temperature (22–25 °C). The error bars represent the standard deviations of measurements for κ and volatile organic compounds concentrations (see Supplementary Table 3 for detailed data).

Figure 3. Effect of heat on emission amounts and particle properties.

The mixing ratios of total volatile organic compounds (VOC) from plant emissions (black bar, note log axis), median diameter of aerosol particles (red bar) and hygroscopicity parameter κ (blue bar) is plotted as a function of plant temperature. The constitutive emissions from unstressed pine trees and induced emissions from stressed mixed boreal trees are shown. The error bars represent the s.d. of the measurement (the detailed number of measurements included in Supplementary Table 4). For constitutive emissions at 20 °C, κ data are not available (noted by *) because particle sizes were too small (median diameter 26 nm) for cloud condensation nuclei (CCN) activation measurements.

Figure 4. Impact of hygroscopicity parameter κ changes on cloud condensation nuclei concentrations.

Measured ambient particle number size distribution (black dotted line, left axis) and the derived accumulated cloud condensation nuclei (CCN) number size distribution (right axis) are shown. The particle size distribution was measured in a boreal forest near Jämijärvi, Finland in May 2013 at a period when organics dominated the total aerosol mass (>80%). The CCN concentration was obtained considering 0.2% supersaturation, a typical supersaturation in clouds, using κ of 0.15 (blue line) and 0.07 (red line) corresponding to the value for SOA from monoterpene-dominated and sesquiterpene-dominated emissions obtained in our plant chamber study shown above. Note the log x axis.