Abstract
Abstract. The size of aerosol particles has fundamental effects on their chemistry and radiative effects. We explore those effects using aerosol size and composition data in the lowermost stratosphere along with calculations of light scattering. In the size range between about 0.1 and 1.0 µm diameter (accumulation mode), there are at least two modes of particles in the lowermost stratosphere. The larger mode consists mostly of particles produced in the stratosphere, and the smaller mode consists mostly of particles transported from the troposphere. The stratospheric mode is similar in the Northern and Southern Hemisphere, whereas the tropospheric mode is much more abundant in the Northern Hemisphere. The purity of sulfuric acid particles in the stratospheric mode shows that there is limited production of secondary organic aerosol in the stratosphere, especially in the Southern Hemisphere. Out of eight sets of flights sampling the lowermost stratosphere (four seasons and two hemispheres) there were three with large injections of specific materials: volcanic, biomass burning, or dust. The stratospheric and tropospheric modes have very different roles for radiative effects on climate and for heterogeneous chemistry. Because the larger particles are more efficient at scattering light, most of the radiative effect in the lowermost stratosphere is due to stratospheric particles. In contrast, the tropospheric particles can have more surface area, at least in the Northern Hemisphere. The surface area of tropospheric particles could have significant implications for heterogeneous chemistry because these particles, which are partially neutralized and contain organics, do not correspond to the substances used for laboratory studies of stratospheric heterogeneous chemistry. We then extend the analysis of size-dependent properties to particles injected into the stratosphere, either intentionally or from volcanoes. There is no single size that will simultaneously maximize the climate impact relative to the injected mass, infrared heating, potential for heterogeneous chemistry, and undesired changes in direct sunlight. In addition, light absorption in the far ultraviolet is identified as an issue requiring more study for both the existing and potentially modified stratosphere.
Highlights
Stratospheric particles have been studied for over 60 years (Junge and Manson, 1961; reviewed by Kremser et al, 2016)
We extend the previous results to show that the mixed organic–sulfate particles from the troposphere are generally smaller than both types of sulfuric acid particles
Out of eight samplings of the lowermost stratosphere, three exhibited much higher aerosol concentrations for three different reasons: a volcanic eruption, biomass burning aerosol, and transport of dust and other near-surface particles. None of these events were targeted during the flights. Such variations in the stratospheric aerosol layer are important for both heterogeneous chemistry (Solomon et al, 1996) and climate (Solomon et al, 2011)
Summary
Stratospheric particles have been studied for over 60 years (Junge and Manson, 1961; reviewed by Kremser et al, 2016). The stratospheric aerosol layer has a maximum in mixing ratio between about 20 and 25 km altitude. The larger air density at lower altitudes means that the majority of the mass of stratospheric aerosol is in the lowermost stratosphere, below the maximum in mixing ratio (Yu et al, 2016). Various trends have been reported for the background stratospheric aerosol at times not influenced by major volcanic eruptions. Deshler et al (2006) concluded there was little long-term change in background stratospheric aerosol from 1971 to 2004. Hofmann et al (2009) found an increasing trend after 2000, and Friberg et al (2014) found an increasing trend from 1999 to 2008.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
