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The atmospheric oxidation of biogenic volatile organic compounds (BVOC) by OH radicals over tropical rainforests impacts local particle production and the lifetime of globally distributed chemically and radiatively active gases. For the pristine Amazon rainforest during the dry season, we empirically determined the diurnal OH radical variability at the forest-atmosphere interface region between 80 and 325 m from 07:00 to 15:00 LT using BVOC measurements. A dynamic time warping approach was applied showing that median averaged mixing times between 80 to 325 m decrease from 105 to 15 min over this time period. The inferred OH concentrations show evidence for an early morning OH peak (07:00–08:00 LT) and an OH maximum (14:00 LT) reaching 2.2 (0.2, 3.8) × 106 molecules cm−3 controlled by the coupling between BVOC emission fluxes, nocturnal NOx accumulation, convective turbulence, air chemistry and photolysis rates. The results were evaluated with a turbulence resolving transport (DALES), a regional scale (WRF-Chem) and a global (EMAC) atmospheric chemistry model. [ABSTRACT FROM AUTHOR]

We studied the effects of shading by shallow cumulus (shallow Cu) and the subsequent effect of inducing heterogeneous conditions at the surface on boundary-layer characteristics. We placed special emphasis on quantifying the changes in the characteristic length and time scales associated with thermals, shallow Cu and induced thermal circulation structures. A series of systematic numerical experiments, inspired by Amazonian thermodynamic conditions, was performed using a large-eddy simulation model coupled to a land-surface model. We used four different experiments to disentangle the effects of shallow Cu on the surface and the response of clouds to these surface changes. The experiments include a 'clear case', 'transparent clouds', 'shading clouds' and a case with a prescribed uniform domain and reduced surface heat flux. We also performed a sensitivity study on the effect of introducing a weak background flow. Length and time scales were calculated using autocorrelation and two-dimensional spectral analysis, and we found that shading controlled by shallow Cu locally lowers surface temperatures and consequently reduces the sensible and latent heat fluxes, thus inducing spatial and temporal variability in these fluxes. The length scale of this surface heterogeneity is not sufficiently large to generate circulations that are superimposed on the boundary-layer scale, but the heterogeneity does disturb boundary-layer dynamics and generates a flow opposite to the normal thermal circulation. Besides this effect, shallow Cu shading reduces turbulent kinetic energy and lowers the convective velocity scale, thus reducing the mass flux. This hampers the thermal lifetime, resulting in a decrease in the shallow Cu residence time (from 11 to 7 min). This reduction in lifetime, combined with a decrease in mass flux, leads to smaller clouds. This is partially compensated for by a decrease in thermal cell size due to a reduction in turbulent kinetic energy. As a result, inter-cloud distance is reduced, leading to a larger population of smaller clouds, while maintaining cloud cover similar to the non-shading clouds experiment. Introducing a $$1\,\hbox {m}\, \hbox {s}^{-1}$$ background wind speed increases the thermal size in the sub-cloud layer, but the diagnosed surface-cloud coupling, quantified by characteristic time and length scales, remains. [ABSTRACT FROM AUTHOR]

Based on the prognostic equations of mixed-layer theory assuming a zeroth order jump at the entrainment zone, analytical solutions for the boundary-layer height evolution are derived with different degrees of accuracy. First, an exact implicit expression for the boundary-layer height for a situation without moisture is analytically derived without assuming any additional relationships or specific initial conditions. It is shown that to expand the solution to include moisture, only minor approximations have to be made. Second, for relatively large boundary-layer heights, the implicit representation is simplified to an explicit function. Third, a hybrid expression is proposed as a reasonable representation for the boundary-layer height evolution during the entire day. Subsequently, the analysis is extended to present the evolution of any boundary-layer averaged scalar, either inert or under idealized chemistry, as an analytical function of time and boundary-layer height. Finally, the analytical solutions are evaluated. This evaluation includes a sensitivity analysis of the boundary-layer height for the entrainment ratio, the free tropospheric lapse rate of the potential temperature, the time-integrated surface flux and the initial boundary-layer height and potential temperature jump. [ABSTRACT FROM AUTHOR]

We investigated the impact of aerosol heat absorption on convective atmospheric boundary-layer (CBL) dynamics. Numerical experiments using a large-eddy simulation model enabled us to study the changes in the structure of a dry and shearless CBL in depth-equilibrium for different vertical profiles of aerosol heating rates. Our results indicated that aerosol heat absorption decreased the depth of the CBL due to a combination of factors: (i) surface shadowing, reducing the sensible heat flux at the surface and, (ii) the development of a deeper inversion layer, stabilizing the upper CBL depending on the vertical aerosol distribution. Steady-state analytical solutions for CBL depth and potential temperature jump, derived using zero-order mixed-layer theory, agreed well with the large-eddy simulations. An analysis of the entrainment zone heat budget showed that, although the entrainment flux was controlled by the reduction in surface flux, the entrainment zone became deeper and less stably stratified. Therefore, the vertical profile of the aerosol heating rate promoted changes in both the structure and evolution of the CBL. More specifically, when absorbing aerosols were present only at the top of the CBL, we found that stratification at lower levels was the mechanism responsible for a reduction in the vertical velocity and a steeper decay of the turbulent kinetic energy throughout the CBL. The increase in the depth of the inversion layer also modified the potential temperature variance. When aerosols were present we observed that the potential temperature variance became significant already around $$0.7z_i$$ (where $$z_i$$ is the CBL height) but less intense at the entrainment zone due to the smoother potential temperature vertical gradient. [ABSTRACT FROM AUTHOR]

The role of turbulence in the dispersion of atmospheric pollutants that react with linear (decay) and nonlinear (second-order) chemical reactions is examined. The most relevant processes that drive the reactivity of species emitted in a surface area or released by a point source are studied by deriving the dimensionless scaling numbers from equations for the atmospheric turbulent reacting flow. The first number is the ratio of the time scale of turbulence to the time scale of the chemical reaction, namely the Damköhler number. The second number is the ratio of the concentrations of the species present in the chemical transformations. In this paper, model results and experimental studies of turbulent reacting flows in the atmospheric boundary layer are presented to show the modifications and control exerted by turbulence on the atmospheric chemistry as a function of these numbers and processes. We also discuss how the chemical transformation is affected when species are in a state of chemical equilibrium. By studying the plume dispersion of a reactant, that decays with a simple chemical reaction, one can analyse the dependence of concentration fluctuations on the Damköhler number. The study is extended to plumes that react nonlinearly. In such reacting systems, the large gradients and segregation of species result in a significant reduction in the reaction rates. Because of this modification, the chemistry of species related to NOx and HOx can be very different from the chemistry in conditions where the species are uniformly mixed. The lack of complete observational evidence is hampering our understanding of these processes and our evaluation of numerical modelling results. Finally, we discuss briefly how to represent, in the form of a parameterization, the effect that turbulence can have on the reactivity of species emitted by a point source or an area source. [ABSTRACT FROM AUTHOR]