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Abstract. The eddy covariance method was applied for the first time to estimate fluxes of OH and HO 2 together with fluxes of isoprene, the sum of methyl vinyl ketone (MVK) and methacrolein (MACR) and the sum of monoterpenes above a mixed deciduous forest. Highly sensitive measurements of OH and HO 2 were performed by laser induced fluorescence (LIF), and biogenic volatile organic compounds (BVOCs) were measured by Proton-Transfer-Reaction Mass Spectrometry (PTR-MS) at a time resolution of 5 s, each. Wind speed was measured by a sonic anemometer at 10 Hz. The one-day feasibility study was conducted at a total height of 37 m, about 7 m above forest canopy, during the ECHO (Emission and CHemical transformation of biogenic volatile Organic compounds) intensive field study in July 2003. The daytime measurements yielded statistically significant OH fluxes directed downward into the direction of the canopy and HO 2 fluxes mainly upward out of the canopy. This hints towards a significant local chemical sink of OH by reactions with BVOCs, other organic and inorganic compounds and conversion of OH to HO 2 above the canopy. For OH the measured flux is locally balanced by chemical sources and sinks and direct transport of OH plays no important role for the local chemical OH budget at the measurement height, as expected from the short OH lifetime (<1 s). For HO 2 the chemical lifetime (20 s) is in the range of the turbulent transport time for transfer between the top of the canopy and the Correspondence to: R. Dlugi (rdlugi@gmx.de) measuring point. In this case, the radical balance is significantly influenced by both chemistry and transport processes. In addition, the highly time-resolved trace gas measurements were used to calculate the intensity of segregation of OH and BVOCs, demonstrating that the effective reaction rate of isoprene and OH was slowed down as much as 15% due to inhomogeneous mixing of the reactants. The paper describes the results, the applied methods and provides a detailed analysis of possible systematic errors of the covariance products.

Abstract. The input of nitrogen (N) to ecosystems has increased dramatically over the past decades. While total (wet + dry) N deposition has been extensively determined in temperate regions, only very few data sets of N wet deposition exist for tropical ecosystems, and moreover, reliable experimental information about N dry deposition in tropical environments is lacking. In this study we estimate dry and wet deposition of inorganic N for a remote pasture site in the Amazon Basin based on in-situ measurements. The measurements covered the late dry (biomass burning) season, a transition period and the onset of the wet season (clean conditions) (12 September to 14 November 2002) and were a part of the LBA-SMOCC (Large-Scale Biosphere-Atmosphere Experiment in Amazonia -Smoke, Aerosols, Clouds, Rainfall, and Climate) 2002 campaign. Ammonia (NH 3 ), nitric acid (HNO 3 ), nitrous acid (HONO), nitrogen dioxide (NO 2 ), nitric oxide (NO), ozone (O 3 ), aerosol ammonium (NH + 4 ) and aerosol nitrate (NO − 3 ) were measured in real-time, accompanied by simultaneous meteorological measurements. Dry deposition fluxes of NO 2 and HNO 3 are inferred using the "big leaf multiple resistance approach" and particle deposition fluxes are derived using an established empirical parameterization. Bi-directional surface-atmosphere exchange fluxes of NH 3 and HONO are estimated by applying a "canopy compensation point model". N dry and wet deposition is dominated by NH 3 and NH + 4 , which is largely the consequence of biomass burning during the dry season. TheCorrespondence to: I. Trebs (ivonne@mpch-mainz.mpg.de) grass surface appeared to have a strong potential for daytime NH 3 emission, owing to high canopy compensation points, which are related to high surface temperatures and to direct NH 3 emissions from cattle excreta. NO 2 also significantly accounted for N dry deposition, whereas HNO 3 , HONO and N-containing aerosol species were only minor contributors. Ignoring NH 3 emission from the vegetation surface, the annual net N deposition rate is estimated to be about −11 kgN ha −1 yr −1 . If on the other hand, surface-atmosphere exchange of NH 3 is considered to be bi-directional, the annual net N budget at the pasture site is estimated to range from −2.15 to −4.25 kgN ha −1 yr −1 .

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Results from numerical investigations regarding the exchange of HNO3, NH3, and NH4NO3 between the atmosphere and the biosphere are presented. The investigations were performed with a modified inferential method which is based on the generally accepted micrometeorological ideas of the transfer of momentum, sensible heat and matter near the Earth's surface and the chemical reactions among these nitrogen compounds. This modified inferential method calculates the micrometeorological quantities (such as the friction velocity and the fluxes of sensible and latent heat), the height-invariant fluxes of the composed chemically conservative trace species with 'group' concentrations c(1)= (HNO3) + (NH4NO3) (total nitrate), c(2)=(NH3) + (NH4NO3) (total ammonia), and c(3) = (HNO3) - (NH3) as well as the fluxes of the 'individual' nitrogen compounds. The parameterization of the fluxes is based on the flux-gradient relationships in the turbulent region of the atmospheric surface layer. The modified i nferential method requires only the data of wind velocity, temperature, humidity and concentrations (HNO3, NH3, and NH4NO3) measured at a reference height by stations of a monitoring network

An inhomogeneous mixing of reactants causes a reduction of their chemical removal compared to the homogeneously mixed case in turbulent atmospheric flows. This can be described by the intensity of segregation I S being the covariance of the mixing ratios of two species divided by the product of their means. Both terms appear in the balance equation of the mixing ratio and are discussed for the reaction between isoprene and OH for data of the field study ECHO 2003 above a deciduous forest. For most of these data, I S is negatively correlated with the fraction of mean OH mixing ratio reacting with isoprene. I S is also negatively correlated with the isoprene standard deviation. Both findings agree with model results discussed by Patton et al. (2001) and others. The correlation coefficient between OH and isoprene and, therefore, I S increases with increasing mean reaction rate. In addition, the balance equation of the covariance between isoprene and OH is applied as the theoretical framework for the analysis of the same field data. The storage term is small, and, therefore, a diagnostic equation for this covariance can be derived. The chemical reaction term R ij is dominated by the variance of isoprene times the quotient of mixing ratios of OH and isoprene. Based on these findings a new diagnostic equation for I S is formulated. Comparing different terms of this equation, I S and R ij show a relation also to the normalised isoprene standard deviation. It is shown that not only chemistry but also turbulent and convective mixing and advection -considered in a residual term -influence I S . Despite this finding, a detection of the influence of coherent eddy transport above the forest according to Katul et al. (1997) on I S fails, but a relation to the turbulent and advective transport of isoprene variance is determined.The largest values of I S are found for most unstable conditions with increasing buoyant production, confirming qualitatively model predictions by Ouwersloot et al. (2011).

In order to provide information about photolysis frequencies in the aqueous phase for chemical transport models including wet chemistry a parameterization which can be added to gaseous-phase photolysis models was developed. The actinic fluxes inside cloud droplets are calculated on the basis of rigorous Mie theory taking into account the effect of dissolved particulate aerosol material and 10 representative cloud droplet size distributions. The results show that the actinic flux inside cloud droplets are on the average more than twice as large as compared to the interstitial air. The newly developed parameterization has been applied together with the model STAR (System for Transfer of Atmospheric Radiation). Apart from the parameters influencing gas-phase photolysis frequencies the radiation quantities inside the cloud droplets and therefore the photolysis frequencies in the aqueous phase depend on the droplet size distribution, the mixing ratio of dry aerosol particulate material to c loud droplet water, and the amount of light absorbing material in the droplets. In-droplet radical source strengths have been calculated for the most important photolytic sources of OH and SO(4- )

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