Summertime total OH reactivity measurements from boreal forest during HUMPPA-COPEC 2010

Ambient total OH reactivity was measured at the Finnish boreal forest station SMEAR II in Hyyti¨al¨a (Latitude 61510 N; Longitude 24170 E) in July and August 2010 using the Comparative Reactivity Method (CRM). The CRM – total OH reactivity method – is a direct, in-situ determination of the total loss rate of hydroxyl radicals (OH) caused by all reactive species in air. During the intensive field campaign HUMPPA-COPEC 2010 (Hyyti¨al¨a United Measurements of Photochemistry and Particles in Air – Comprehensive Organic Precursor Emission and Concentration study) the total OH reactivity was monitored both inside (18 m) and directly above the forest canopy (24 m) for the first time. The comparison between these two total OH reactivity measurements, absolute values and the temporal variation have been analyzed here. Stable boundary layer conditions during night and turbulent mixing in the daytime induced low and high short-term variability, respectively. The impact on total OH reactivity from biogenic emissions and associated photochemical products was measured under “normal” and “stressed” (i.e. prolonged high temperature) conditions. The advection of biomass burning emissions to the site caused a marked change in the total OH reactivity vertical profile. By comparing the OH reactivity contribution from individually measured compounds and the directly measured total OH reactivity, the size of any unaccounted for “missing” sink can be deduced for various atmospheric influences. For “normal” boreal conditions a missing OH reactivity of 58 %, whereas for “stressed” boreal conditions a missing OH reactivity of 89% was determined. Various sources of not quantified OH reactive species are proposed as possible explanation for the high missing OH reactivity

Characterization of a boreal convective boundary layer and its impact on atmospheric chemistry during HUMPPA-COPEC-2010

We studied the atmospheric boundary layer (ABL) dynamics and the impact on atmospheric chemistry during the HUMPPA-COPEC-2010 campaign. We used vertical profiles of potential temperature and specific moisture, obtained from 132 radio soundings, to determine the main boundary layer characteristics during the campaign. We propose a classification according to several main ABL prototypes. Further, we performed a case study of a single day, focusing on the convective boundary layer, to analyse the influence of the dynamics on the chemical evolution of the ABL. We used a mixed layer model, initialized and constrained by observations. In particular, we investigated the role of large scale atmospheric dynamics (subsidence and advection) on the ABL development and the evolution of chemical species concentrations. We find that, if the large scale forcings are taken into account, the ABL dynamics are represented satisfactorily. Subsequently, we studied the impact of mixing with a residual layer aloft during the morning transition on atmospheric chemistry. The time evolution of NOx and O3 concentrations, including morning peaks, can be explained and accurately simulated by incorporating the transition of the ABL dynamics from night to day. We demonstrate the importance of the ABL height evolution for the representation of atmospheric chemistry. Our findings underscore the need to couple the dynamics and chemistry at different spatial scales (from turbulence to mesoscale) in chemistry-transport models and in the interpretation of observational data

Atmospheric benzenoid emissions from plants rival those from fossil fuels

Despite the known biochemical production of a range of aromatic compounds by plants and the presence of benzenoids in floral scents, the emissions of only a few benzenoid compounds have been reported from the biosphere to the atmosphere. Here, using evidence from measurements at aircraft, ecosystem, tree, branch and leaf scales, with complementary isotopic labeling experiments, we show that vegetation (leaves, flowers, and phytoplankton) emits a wide variety of benzenoid compounds to the atmosphere at substantial rates. Controlled environment experiments show that plants are able to alter their metabolism to produce and release many benzenoids under stress conditions. The functions of these compounds remain unclear but may be related to chemical communication and protection against stress. We estimate the total global secondary organic aerosol potential from biogenic benzenoids to be similar to that from anthropogenic benzenoids (~10 Tg y-1), pointing to the importance of these natural emissions in atmospheric physics and chemistry

The response of atmospheric chemistry to dynamical boundary layer processes associated to temporal transitions and surface heterogeneity

Here, the mechanism behind the collapse of turbulence in the evening is investigated as a precursor to the onset of the very stable boundary layer. We study how the atmospheric boundary layer dynamics impact atmospheric chemistry, guided and constrained by observations taken above the boreal forest during the HUMPPA-COPEC-2010 campaign. Based on the vertical profiles of potential temperature and specific moisture, obtained from 132 radio soundings, the vertical stratification is determined. Data is then classified according to different prototypes of the atmospheric boundary layer. By selecting a singular day that is characterized by a convective boundary layer and using a mixed layer model, the main dynamic contributions that influence atmospheric chemistry are determined. We will present how the evolution of the boundary layer height affects the concentrations of atmospheric chemical species. Our findings show the importance of an adequate knowledge of this evolution and, consequently, the need to account for large scale dynamical forcings (subsidence, advection) in order to represent atmospheric chemistry. Extra attention is directed at investigating the impact of temporal (morning) transitions and surface heterogeneity. More specifically, we investigate the impact of mixing with a residual layer aloft during the morning transition on atmospheric chemistry. Specific observed features in the time evolutions of the NOx and O3 concentrations, like morning concentration peaks, can be explained and represented by adequately incorporating the transition of the boundary layer dynamics from nocturnal to diurnal conditions. We complete the analysis by studying the effect of surface heterogeneity and the efficiency of turbulent mixing on the chemical reactivity using a Large Eddy Simulation model. We find that under heterogeneous surface forcings boundary layers become deeper, thereby affecting the dilution capacity of the boundary layer. We will also show that local instantaneous virtual vertical profiles of temperature and chemical species concentrations obtained from the Large Eddy Simulation model deviate more from area and time averaged profiles for heterogeneous surface conditions. In addition, the influence of non-uniform turbulent mixing on the chemical reactivity in the boundary layer is studied under homogeneous and heterogeneous surface conditions. We will present a sensitivity study how this effect, quantified by the intensity of segregation, depends on the surface (e.g., length scale of heterogeneity, differences in emissions) and dynamical conditions. We find that in order to represent atmospheric chemistry in a numerical model, dynamical and chemical effects should be resolved simultaneously