Columnar modelling of nucleation burst evolution in the convective boundary layer – first results from a feasibility study BR Part III: Preliminary results on physicochemical model performance using two "clean air mass" reference scenarios

摘要

In Paper I of four papers, a revised columnar high-order modelto investigate gas-aerosol-turbulence interactions in theconvective boundary layer (CBL) was proposed. In Paper II, themodel capability to predict first-, second-and third-order moments of meteorological variables in the CBL wasdemonstrated using available observational data. In the present Paper III, thehigh-order modelling concept is extendedto sulphur and ammonia chemistry as well as to aerosol dynamics. Based onthe previous CBL simulation, a feasibility study is performed usingtwo "clean air mass" scenarios with an emission source at theground but low aerosol background concentration. Such scenarios synopticallycorrespond to the advection of fresh post-frontal air in ananthropogenically influenced region. The aim is to evaluate thetime-height evolution ofultrafine condensation nuclei (UCNs)and to elucidate the interactions betweenmeteorological and physicochemical variables in a CBL column. The scenariosdiffer in the treatment ofnew particle formation (NPF), whereashomogeneous nucleation according to the classical nucleation theory (CNT)is considered. The first scenario considers nucleationof a binary system consisting of water vapour andsulphuric acid (H₂SO₄) vapour,the second one nucleation of a ternary system additionally involvingammonia (NH₃). Here, the two synthetic scenariosare discussed in detail, whereasspecial attention is payed to the role of turbulence in the formationof the typical UCN burst behaviour, that can often be observed inthe surface layer. The intercomparison of the two scenariosreveals large differences in the evolution of the UCN number concentrationin the surface layer as well as in thetime-height cross-sections of first-order moments anddouble correlation terms. Although in both cases the occurrence ofNPF bursts could be simulated,the burst characteristics and genesis of the bursts are completelydifferent. It is demonstrated, that observations from the surface layer aloneare not conclusive to elucidate the origin of newly formedparticles. This is also true with respect to the interpretation ofbox modelling studies. The binary and ternary NPF bursts observedin the surface layer differ with respect to burstamplitude and phase. New particles simulated in the binary scenarioare formed in the forenoon in the upper part of thegrowing CBL, followed by turbulence-induced top-down transport. Hence, withrespect to the burst observation site in the surface layer,new particles are formed ex situ. In opposite to this, the ternary casereveals a much more complex pattern. Here, NPF is initiated in theearly morning hours in the surface layer, when temperature (T) is low andrelative humidity (RH), sulphur dioxide (SO₂) and NH₃concentrations are high, hencenew particles are formed in situ. Shortly after that, ex situ NPF inthe free troposphere sets in, followed by entrainment and top-downdiffusion of newly formed particles into the surface layer. Altogether, theseprocesses mainly contribute to the formation of a strongburst in the morning hours in the ternary scenario. While the time-heightcross-section of the binary nucleation rateresembles a "blob"-like evolution pattern, the ternary oneresembles a "sucking tube"-like pattern. The time-height cross-sectionsof the flux pattern and double correlationscould be plausibly interpreted in terms of CBL turbulence andentrainment/detrainment processesboth in the binary and in the ternary case. Although the presentapproach is a pure conceptual one, it shows the feasibilityto simulate gas-aerosol-turbulence interactions in the CBL. Prior to a dedicatedverification/validation study,further attempts are necessary to consider a more advanceddescription of the formation and activation ofthermodynamically stable clustersaccording to modern concepts proposed by Kulmala et al. (2000), Kulmala (2003) and Kulmala et al.(2004a).