The complex origin and spatial distribution of non-pure sulfate particles (NSPs) in the stratosphere

Abstract. While droplets with pure mixtures of water and sulfuric acid are the main component of stratospheric aerosols, field measurements performed for more than 30 years have shown that non-sulfate materials, thereafter referred to by us as NSP (for Non-pure Sulfate Particles, not considering frozen material) are also present. Such materials, which are released from both the Earth through volcanic eruptions, pollution or biomass burning, and from space through interplanetary dust and micrometeoroids, present a wide variety of composition and shape, with sizes ranging from several nm to several hundreds of μm. No single instrumental technique, from ground, from airplanes, under balloons and onboard satellites using remote-sensing and in-situ instruments. can provide alone a global view of the stratospheric NSPs, which exhibit a strong variability in terms of spatial distribution and composition. To better understand the origin of the NSPs, we have performed new field measurements from mid- 2013 with the Light Optical Aerosol Counter (LOAC) instrument during 135 flights carried out under weather balloons at various latitudes and up to altitudes of 35 km. Coupled with previous measurements obtained with the Tropospheric and Stratospheric Aerosols Counter (STAC) under stratospheric balloons in the 2004–2011 period, the LOAC measurements show the presence of stratospheric layers presenting enhanced-concentrations associated with NSPs, with a bimodal vertical repartition centered by 17 and 30 km altitude. Also, large particles are detected, with sizes up to several tens of μm, with decreasing concentrations with increasing altitudes. Such observations, which are not correlated with meteor shower events, could be due to dynamical and photophoretic effects lifting and sustaining particles mainly coming from the Earth. When combining all the detections in the stratosphere from different methods of measurements, we may conclude that the concentrations and the vertical distributions of NSPs are highly variable and do not match the estimated concentrations of material in space at Earth orbit. The paper ends by highlighting some open questions on these stratospheric materials and presents some possible new strategies for frequent measurements, to confirm that NSPs are indeed mainly of terrestrial origin, and to better circumvent the NSPs impact on stratospheric chemistry and on the Earth’s climate.