Particle number concentrations and size distributions in the stratosphere: implications of nucleation mechanisms and particle microphysics

[摘要] While formation and growth of particles in the troposphere have been extensively studied in the past two decades, very limited efforts havebeen devoted to understanding these in the stratosphere. Here we use bothCosmics Leaving OUtdoor Droplets (CLOUD) laboratory measurements taken undervery low temperatures (205–223 K) and Atmospheric Tomography Mission (ATom) in situ observations of particle number size distributions (PNSDs) down to 3 nm to constrain nucleation mechanisms and to evaluatemodel-simulated particle size distributions in the lowermost stratosphere (LMS). We show that the binary homogenous nucleation (BHN) scheme used in most of the existing stratospheric aerosol injection (a proposed method of solarradiation modification) modeling studies overpredicts the nucleation rates by3–4 orders of magnitude (when compared to CLOUD data) and particle numberconcentrations in the background LMS by a factor ∼  2–4 (whencompared to ATom data). Based on a recently developed kinetic nucleationmodel, which gives rates of both ion-mediated nucleation (IMN) and BHN atlow temperatures in good agreement with CLOUD measurements, both BHN and IMNoccur in the stratosphere. However, IMN rates are generally more than 1order of magnitude higher than BHN rates and thus dominate nucleation in thebackground stratosphere. In the Southern Hemisphere (SH) LMS with minimuminfluence of anthropogenic emissions, our analysis shows that ATom-measuredPNSDs generally have four apparent modes. The model captures reasonably wellthe two modes (Aitken mode and the first accumulation mode) with the highestnumber concentrations and size-dependent standard deviations. However,the model misses an apparent second accumulation mode peaking around300–400 nm, which is in the size range important for aerosol directradiative forcing. The bimodal structure of accumulation mode particles hasalso been observed in the stratosphere well above tropopause and in thevolcano-perturbed stratosphere. We suggest that this bimodal structure maybe caused by the effect of charges on coagulation and growth, which is notyet considered in any existing models and may be important in thestratosphere due to high ionization rates and the long lifetime of aerosols.Considering the importance of accurate PNSDs for projecting a realisticradiation forcing response to stratospheric aerosol injection (SAI), it isessential to understand and incorporate such potentially important processesin SAI model simulations and to carry out further research to find out whatother processes the present models might have missed.

[学科分类] 医学（综合）