Abstract. Direct measurements of OH and HO 2 over a tropical rainforest were made for the first time during the GABRIEL campaign in October 2005, deploying the custom-built HORUS instrument (HydrOxyl Radical measurement Unit based on fluorescence Spectroscopy), adapted to fly in a Learjet wingpod. Biogenic hydrocarbon emissions were expected to strongly reduce the OH and HO 2 mixing ratios as the air is transported from the ocean over the forest. However, surprisingly high mixing ratios of both OH and HO 2 were encountered in the boundary layer over the rainforest.The HORUS instrumentation and calibration methods are described in detail and the measurement results obtained are discussed. The extensive dataset collected during GABRIEL, including measurements of many other trace gases and photolysis frequencies, has been used to quantify the main sources and sinks of OH. Comparison of these measurementderived formation and loss rates of OH indicates strong previously overlooked recycling of OH in the boundary layer over the tropical rainforest, occurring in chorus with isoprene emission.

As a major source region of the hydroxyl radical OH, the Tropics largely control the oxidation capacity of the atmosphere on a global scale. However, emissions of hydrocarbons from the tropical rainforest that react rapidly with OH can potentially deplete the amount of OH and thereby reduce the oxidation capacity. The airborne GABRIEL field campaign in equatorial South America (Suriname) in October 2005 investigated the influence of the tropical rainforest on the HO_x budget (HO_x = OH + HO₂). The first observations of OH and HO₂ over a tropical rainforest are compared to steady state concentrations calculated with the atmospheric chemistry box model MECCA. The important precursors and sinks for HO_x chemistry, measured during the campaign, are used as constraining parameters for the simulation of OH and HO₂. Significant underestimations of HO_x are found by the model over land during the afternoon, with mean ratios of observation to model of 12.2 &plusmn; 3.5 and 4.1 &plusmn; 1.4 for OH and HO₂, respectively. The discrepancy between measurements and simulation results is correlated to the abundance of isoprene. While for low isoprene mixing ratios (above ocean or at altitudes >3 km), observation and simulation agree fairly well, for mixing ratios >200 pptV (<3 km over the rainforest) the model tends to underestimate the HO_x observations as a function of isoprene. <br></br> Box model simulations have been performed with the condensed chemical mechanism of MECCA and with the detailed isoprene reaction scheme of MCM, resulting in similar results for HO_x concentrations. Simulations with constrained HO₂ concentrations show that the conversion from HO₂ to OH in the model is too low. However, by neglecting the isoprene chemistry in the model, observations and simulations agree much better. An OH source similar to the strength of the OH sink via isoprene chemistry is needed in the model to resolve the discrepancy. A possible explanation is that the oxidation of isoprene by OH not only dominates the removal of OH but also produces it in a similar amount. Several additional reactions which directly produce OH have been implemented into the box model, suggesting that upper limits in producing OH are still not able to reproduce the observations (improvement by factors of ≈2.4 and ≈2 for OH and HO₂, respectively). We determine that OH has to be recycled to 94% instead of the simulated 38% to match the observations, which is most likely to happen in the isoprene degradation process, otherwise additional sources are required

After formation of CH 4 its isotopic signature may be modified by secondary, isotope-fractionating processes. The most important of these processes is aerobic CH 4 oxidation by meth-8251

Abstract. During SPURT (Spurenstofftransport in derTropopausenregion, trace gas transport in the tropopause region) we performed measurements of a wide range of trace gases with different lifetimes and sink/source characteristics in the northern hemispheric upper troposphere (UT) and lowermost stratosphere (LMS). A large number of in-situ instruments were deployed on board a Learjet 35A, flying at altitudes up to 13.7 km, at times reaching to nearly 380 K potential temperature. Eight measurement campaigns (consisting of a total of 36 flights), distributed over all seasons and typically covering latitudes between 35 • N and 75 • N in the European longitude sector (10 • W-20 • E), were performed. Here we present an overview of the project, describing the instrumentation, the encountered meteorological situations during the campaigns and the data set available from SPURT. Measurements were obtained for N 2 O, CH 4 , CO, CO 2 , CFC12, H 2 , SF 6 , NO, NO y , O 3 and H 2 O. We illustrate the strength of this new data set by showing mean distributions of the mixing ratios of selected trace gases, using a potential temperature-equivalent latitude coordinate system. The observations reveal that the LMS is most stratospheric in character during spring, with the highest mixing ratios of O 3 and NO y and the lowest mixing ratios of N 2 O and SF 6 . The lowest mixing ratios of NO y and O 3 are observed during autumn, together with the highest mixing ratios of N 2 O and SF 6 indicating a strong tropospheric influence. For H 2 O, however, the maximum concentrations in the LMS are found during summer, suggesting unique (temperatureCorrespondence to: A. Engel (an.engel@meteor.uni-frankfurt.de) and convection-controlled) conditions for this molecule during transport across the tropopause. The SPURT data set is presently the most accurate and complete data set for many trace species in the LMS, and its main value is the simultaneous measurement of a suite of trace gases having different lifetimes and physical-chemical histories. It is thus very well suited for studies of atmospheric transport, for model validation, and for investigations of seasonal changes in the UT/LMS, as demonstrated in accompanying and elsewhere published studies.

Abstract. Atmospheric concentrations of nitrous acid (HONO), one of the major precursors of the hydroxyl radical (OH) in the troposphere, significantly exceed the values predicted by the assumption of a photostationary state (PSS) during daytime. Therefore, additional sources of HONO were intensively investigated in the last decades. This study presents budget calculations of HONO based on simultaneous measurements of all relevant species, including HONO and OH at two different measurement heights, i.e. 1 m above the ground and about 2 to 3 m above the canopy (24 m above the ground), conducted in a boreal forest environment. We observed mean HONO concentrations of about 6.5 × 10 8 molecules cm −3 (26 ppt) during daytime, more than 20 times higher than expected from the PSS of 0.2 × 10 8 molecules cm −3 (1 ppt). To close the budgets at both heights, a strong additional source term during daytime is required. This unidentified source is at its maximum at noon (up to 1.1 × 10 6 molecules cm −3 s −1 , 160 ppt h −1 ) and in general up to 2.3 times stronger above the canopy than close to the ground. The insignificance of known gas phase reactions and other processes like dry deposition or advection compared to the photolytic decomposition of HONO at this measurement site was an ideal prerequisite to study possible correlations of this unknown term to proposed HONO sources. But neither the proposed emissions from soils nor the proposed photolysis of adsorbed HNO 3 contributed substantially to the unknown source. However, the unknown source was found to be perfectly correlated to the unbalanced photolytic loss of HONO.

We describe here a sensitive quantum cascade laser absorption spectrometer (QCLAS) employed for aircraft based measurements during the GABRIEL 2005 and HOOVER 2006 and 2007 campaigns. This 3-channel instrument measures CO, HCHO, CH4 and N2O using a 64-m path double corner cube White cell. Performance of the instrument was examined for the four species and precisions for CO, N2O and CH4 were measured in the field to be 0.5, 0.5 and 0.7% respectively (2σ). The 1σ detection limit for HCHO was ∼500 pptv for a 2 s average, while signal averaging of the HCHO over a 2 min time interval resulted in a 150 pptv detection limit with a duty cycle of 60%.

Abstract. We present an evaluation of sources, sinks and turbulent transport of nitrogen oxides, ozone and volatile organic compounds (VOC) in the boundary layer over French Guyana and Suriname during the October 2005 GABRIEL campaign by simulating observations with a single-column chemistry and climate model (SCM) along a zonal transect. Simulated concentrations of O 3 and NO as well as NO 2 photolysis rates over the forest agree well with observations when a small soil-biogenic NO emission flux was applied. This suggests that the photochemical conditions observed during GABRIEL reflect a pristine tropical low-NO x regime. The SCM uses a compensation point approach to simulate nocturnal deposition and daytime emissions of acetone and methanol and produces daytime boundary layer mixing ratios in reasonable agreement with observations. The area average isoprene emission flux, inferred from the observed isoprene mixing ratios and boundary layer height, is about half the flux simulated with commonly applied emission algorithms. The SCM nevertheless simulates too high isoprene mixing ratios, whereas hydroxyl concentrations are strongly underestimated compared to observations, which can at least partly explain the discrepancy. Furthermore, the model substantially overestimates the isoprene oxidation products methlyl vinyl ketone (MVK) and methacrolein (MACR) partlyCorrespondence to: L. Ganzeveld laurens.ganzeveld@wur.nl due to a simulated nocturnal increase due to isoprene oxidation. This increase is most prominent in the residual layer whereas in the nocturnal inversion layer we simulate a decrease in MVK and MACR mixing ratios, assuming efficient removal of MVK and MACR. Entrainment of residual layer air masses, which are enhanced in MVK and MACR and other isoprene oxidation products, into the growing boundary layer poses an additional sink for OH which is thus not available for isoprene oxidation. Based on these findings, we suggest pursuing measurements of the tropical residual layer chemistry with a focus on the nocturnal depletion of isoprene and its oxidation products.

Abstract. Representative values of the atmospheric NO 2 photolysis frequency j (NO 2 ) are required for the adequate calculation and interpretation of NO and NO 2 concentrations and exchange fluxes near the surface. Direct measurements of j (NO 2 ) at ground level are often not available in field studies. In most cases, modeling approaches involving complex radiative transfer calculations are used to estimate j (NO 2 ) and other photolysis frequencies for air chemistry studies. However, important input parameters for accurate modeling are often missing, most importantly with regard to the radiative effects of clouds. On the other hand, solar global irradiance ("global radiation", G) is nowadays measured as a standard parameter in most field experiments and in many meteorological observation networks around the world. Previous studies mainly reported linear relationships between j (NO 2 ) and G. We have measured j (NO 2 ) using spectro-or filter radiometers and G using pyranometers side-by-side at several field sites. Our results cover a solar zenith angle range of 0-90 • , and are based on nine field campaigns in temperate, subtropical and tropical environments during the period 1994-2008. We show that a second-order polynomial function (intercept = 0): and B 2 = (−4.84 ± 0.31) × 10 −9 W −2 m 4 s −1 can be used to estimate ground-level j (NO 2 ) directly from G, independent of solar zenith angle under all atmospheric conditions. The absolute j (NO 2 ) residual of the empirical function is ±6 × 10 −4 s −1 (2σ ). The relationship is valid for sites below 800 m a.s.l. and with low surface albedo (α < 0.2). It is not valid in high mountains, above snow or ice and sandy or dry soil surfaces.