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[1] A global chemical transport model is used in conjunction with measurements from surface stations to study the importance of biomass burning and meteorology in driving Arctic carbon monoxide (CO) interannual variability (IAV). Simulations with yearly varying fire emissions capture 66%–93% of CO IAV and a simulation with yearly varying meteorology but fixed fire emissions captures 0–25%, showing that biomass burning variability is the dominant driver of surface CO IAV. Observed CO anomalies are found to be significantly correlated with El Nino (0.58 < r < 0.64, 99% confidence level (CL)) and results indicate that this is due to ENSO's influence on fire emissions. Boreal Alaska, Canada and north-east Siberia are found to contribute 59% to total Arctic fire CO and 67% to Arctic fire CO IAV. Analysis of meteorological fire drivers in these regions suggests that ENSO affects winter/spring precipitation, driving the Arctic/ENSO relationship.

Vegetation affects precipitation patterns by mediating moisture, energy and trace-gas fluxes between the surface and atmosphere. When forests are replaced by pasture or crops, evapotranspiration of moisture from soil and vegetation is often diminished, leading to reduced atmospheric humidity and potentially suppressing precipitation. Climate models predict that large-scale tropical deforestation causes reduced regional precipitation, although the magnitude of the effect is model and resolution dependent. In contrast, observational studies have linked deforestation to increased precipitation locally but have been unable to explore the impact of large-scale deforestation. Here we use satellite remote-sensing data of tropical precipitation and vegetation, combined with simulated atmospheric transport patterns, to assess the pan-tropical effect of forests on tropical rainfall. We find that for more than 60 per cent of the tropical land surface (latitudes 30 degrees south to 30 degrees north), air that has passed over extensive vegetation in the preceding few days produces at least twice as much rain as air that has passed over little vegetation. We demonstrate that this empirical correlation is consistent with evapotranspiration maintaining atmospheric moisture in air that passes over extensive vegetation. We combine these empirical relationships with current trends of Amazonian deforestation to estimate reductions of 12 and 21 per cent in wet-season and dry-season precipitation respectively across the Amazon basin by 2050, due to less-efficient moisture recycling. Our observation-based results complement similar estimates from climate models, in which the physical mechanisms and feedbacks at work could be explored in more detail.

[1] Iodine containing species have been measured in the particle phase (PM2.5) and the gas phase during a ship campaign between January and February 2007 in the South Atlantic marine boundary layer (MBL). Methyl iodide (CH3I) in the gas phase and soluble iodine species, i.e. iodide, iodate and an unidentified organic iodine species (UOI), in PM2.5 were measured. Temporal variations of gaseous and particulate iodine species were investigated. The exposure of the sampled air masses to phytoplankton along the back-trajectories was studied using a lagrangian transport model and satellite observations of oceanic chlorophyll-a concentration. Significant correlations were found between the concentrations of iodine species and the average chlorophyll exposure along back-trajectories, indicating an emission from phytoplankton in the open ocean. Good correlations were found when the ship approached or crossed the active biological region in the West-southern Atlantic. These results strongly suggest a connection between iodine species and the phytoplankton in the ocean. This study was extended to examine the relation of the iodine measurements to satellite derived phytoplankton distributions. We show that specific phytoplankton species may have impact on different iodine containing species.

During the POLARCAT summer campaign in 2008, two episodes (2g-5 July and 7g-10 July 2008) occurred where low-pressure systems traveled from Siberia across the Arctic Ocean towards the North Pole. The two cyclones had extensive smoke plumes from Siberian forest fires and anthropogenic sources in East Asia embedded in their associated air masses, creating an excellent opportunity to use satellite and aircraft observations to validate the performance of atmospheric transport models in the Arctic, which is a challenging model domain due to numerical and other complications. Here we compare transport simulations of carbon monoxide (CO) from the Lagrangian transport model FLEXPART and the Eulerian chemical transport model TOMCAT with retrievals of total column CO from the IASI passive infrared sensor onboard the MetOp-A satellite. The main aspect of the comparison is how realistic horizontal and vertical structures are represented in the model simulations. Analysis of CALIPSO lidar curtains and in situ aircraft measurements provide further independent reference points to assess how reliable the model simulations are and what the main limitations are. The horizontal structure of mid-latitude pollution plumes agrees well between the IASI total column CO and the model simulations. However, finer-scale structures are too quickly diffused in the Eulerian model. Applying the IASI averaging kernels to the model data is essential for a meaningful comparison. Using aircraft data as a reference suggests that the satellite data are biased high, while TOMCAT is biased low. FLEXPART fits the aircraft data rather well, but due to added background concentrations the simulation is not independent from observations. The multi-data, multi-model approach allows separating the influences of meteorological fields, model realisation, and grid type on the plume structure. In addition to the very good agreement between simulated and observed total column CO fields, the results also highlight the difficulty to identify a data set that most realistically represents the actual pollution state of the Arctic atmosphere. © 2011 Adis Data Information BV. All rights reserved., SCOPUS: ar.j, info:eu-repo/semantics/published

Assessments of the relationships between crop productivity and climate change rely upon a combination of modelling and measurement. As part of this review, this relationship is discussed in the context of crop and climate simulation. Methods for linking these two types of models are reviewed, with a primary focus on large-area crop modelling techniques. Recent progress in simulating the impacts of climate change on crops is presented, and the application of these methods to the exploration of adaptation options is discussed. Specific advances include ensemble simulations and improved understanding of biophysical processes. Finally, the challenges associated with impacts and adaptation research are discussed. It is argued that the generation of knowledge for policy and adaptation should be based not only on syntheses of published studies, but also on a more synergistic and holistic research framework that includes: (i) reliable quantification of uncertainty; (ii) techniques for combining diverse modelling approaches and observations that focus on fundamental processes; and (iii) judicious choice and calibration of models, including simulation at appropriate levels of complexity that accounts for the principal drivers of crop productivity, which may well include both biophysical and socio-economic factors. It is argued that such a framework will lead to reliable methods for linking simulation to real-world adaptation options, thus making practical use of the huge global effort to understand and predict climate change.

[1] Significant concentrations of organic carbon (OC) aerosol are observed at three oceanic surface sites (Amsterdam Island, Azores and Mace Head). Two global chemical transport models (CTMs) underpredict OC concentrations at these sites (normalised mean bias of −67% and −58%). During periods of high biological activity monthly mean concentrations are underpredicted by a factor of 5–20. At Amsterdam Island and Mace Head, observed OC correlates well (R2 = 0.61–0.77) with back-trajectory weighted chlorophyll-a, suggesting an oceanic OC source driven by biological activity. We use a combination of remote sensed chlorophyll-a, back trajectories and observed OC to derive an empirical relation between chlorophyll-a and the total oceanic OC emission flux. Using the GEOS-chem CTM we show a global oceanic OC emission, from primary and secondary sources, of ∼8 Tg/year matches observations. This emission is comparable in magnitude to the fossil fuel OC source and increases the simulated global OC burden by 20%.

Environmental context. Laboratory incubation experiments and shipboard measurements in the Southern Atlantic Ocean have provided the first evidence for marine production of monoterpenes. Nine marine phytoplankton monocultures were investigated using a GC-MS equipped with an enantiomerically-selective column and found to emit monoterpenes including (–)-/(+)-pinene, limonene and p-ocimene, all of which were previously thought to be exclusively of terrestrial origin. Maximum levels of 100–200 pptv total monoterpenes were encountered when the ship crossed an active phytoplankton bloom. Abstract. Laboratory incubation experiments and shipboard measurements on the Southern Atlantic Ocean have provided the first evidence for marine production of monoterpenes. Nine marine phytoplankton monocultures were investigated using a GC-MS equipped with an enantiomerically-selective column and found to emit at rates, expressed as nmol C10H16 (monoterpene) g [chlorophyll a]–1 day–1, from 0.3 nmol g [chlorophyll a]–1 day–1 for Skeletonema costatum and Emiliania huxleyi to 225.9 nmol g [chlorophyll a]–1 day–1 for Dunaliella tertiolecta. Nine monoterpenes were identified in the sample and not in the control, namely: (–)-/(+)-pinene, myrcene, (+)-camphene, (–)-sabinene, (+)-3-carene, (–)-pinene, (–)-limonene and p-ocimene. In addition, shipboard measurements of monoterpenes in air were made in January–March 2007, over the South Atlantic Ocean. Monoterpenes were detected in marine air sufficiently far from land as to exclude influence from terrestrial sources. Maximum levels of 100–200 pptv total monoterpenes were encountered when the ship crossed an active phytoplankton bloom, whereas in low chlorophyll regions monoterpenes were mostly below detection limit.

We evaluate the Meteo-France global chemistry transport 3D model MOCAGE (MOdele de Chimie Atmospherique a Grande Echelle) using the important set of aircraft measurements collected during the ICARRT/ITOP experiment. This experiment took place between US and Europe during summer 2004 (July 15-August 15). Four aircraft were involved in this experiment providing a wealth of chemical data in a large area including the North East of US and western Europe. The model outputs are compared to the following species of which concentration is measured by the aircraft: OH, H2O2, CO, NO, NO2, PAN, HNO3, isoprene, ethane, HCHO and O3. Moreover, to complete this evaluation at larger scale, we used also satellite data such as SCIAMACHY NO2 and MOPITT CO. Interestingly, the comprehensive dataset allowed us to evaluate separately the model representation of emissions, transport and chemical processes. Using a daily emission source of biomass burning, we obtain a very good agreement for CO while the evaluation of NO2 points out incertainties resulting from inaccurate ratio of emission factors of NOx/CO. Moreover, the chemical behavior of O3 is satisfactory as discussed in the paper.

A case of long-range transport of a biomass burning plume from Alaska to Europe is analyzed using a Lagrangian approach. This plume was sampled several times in the free troposphere over North America, the North Atlantic and Europe by 3 different aircraft during the IGAC Lagrangian 2K4 experiment which was part of the ICARTT/ITOP measurement intensive in summer 2004. Measurements in the plume showed enhanced values of CO, VOCs and NOy, mainly in form of PAN. Observed O3 levels increased by 17 ppbv over 5 days. A photochemical trajectory model, CiTTyCAT, is used to examine processes responsible for the chemical evolution of the plume. The model was initialized with upwind data, and compared with downwind measurements. The influence of high aerosol loading on photolysis rates in the plume is investigated using in-situ aerosol measurements in the plume and lidar retrievals of optical depth as input into a photolysis code (Fast-J), run in the model. Significant impacts on photochemistry are found with a decrease of 18 percent in O3 production and 24 percent in O3 destruction over 5 days when including aerosols. The plume is found to be chemically active with large O3 increases attributed primarily to PAN decomposition during descent of the plume towards Europe. The predicted O3 changes are very dependent on temperature changes during transport, and also, on water vapor levels in the lower troposphere which can lead to O3 destruction. Simulation of mixing/dilution was necessary to reproduce observed pollutants level in the plume. Mixing was simulated using background concentrations from measurements in air masses in close proximity to the plume, and mixing timescales (averaging 6.25 days) were derived from CO changes. Observed and simulated O3/CO correlations in the plume are also compared in order to evaluate the photochemistry in the model. Observed slopes changed from negative to positive over 5 days. This change, which can be attributed largely to photochemistry, is well reproduced by multiple model runs even if slope values are slightly underestimated suggesting small underestimation of photochemical processes. The possible impact of this biomass burning plume on O3 levels in the Europe boundary layer is also examined by running the model for a further 5 days, and comparing with data collected at surface sites, such as Jungfraujoch, which showed small O3 increases and elevated CO levels. The model predicts significant changes in O3 over this 10 days due to photochemistry but the signal is largely lost due to the effects of dilution. However, measurement in several others BB plumes over Europe show that O3 impact of Alaskan fires can be punctually significant over Europe.

[1] Bayesian inference has been used to determine rigorous estimates of hydroxyl radical concentrations () and air mass dilution rates (K) averaged following air masses between linked observations of nonmethane hydrocarbons (NMHCs) spanning the North Atlantic during the Intercontinental Transport and Chemical Transformation (ITCT)-Lagrangian-2K4 experiment. The Bayesian technique obtains a refined (posterior) distribution of a parameter given data related to the parameter through a model and prior beliefs about the parameter distribution. Here, the model describes hydrocarbon loss through OH reaction and mixing with a background concentration at rate K. The Lagrangian experiment provides direct observations of hydrocarbons at two time points, removing assumptions regarding composition or sources upstream of a single observation. The estimates are sharpened by using many hydrocarbons with different reactivities and accounting for their variability and measurement uncertainty. A novel technique is used to construct prior background distributions of many species, described by variation of a single parameter α. This exploits the high correlation of species, related by the first principal component of many NMHC samples. The Bayesian method obtains posterior estimates of , K and α following each air mass. Median values are typically between 0.5 and 2.0 × 106 molecules cm−3, but are elevated to between 2.5 and 3.5 × 106 molecules cm−3, in low-level pollution. A comparison of estimates from absolute NMHC concentrations and NMHC ratios assuming zero background (the “photochemical clock” method) shows similar distributions but reveals systematic high bias in the estimates from ratios. Estimates of K are ∼0.1 day−1 but show more sensitivity to the prior distribution assumed.

The atmospheric composition of the central North Atlantic region has been sampled using the FAAM BAe146 instrumented aircraft during the Intercontinental Transport of Ozone and Precursors (ITOP) campaign, part of the wider International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). This paper presents an overview of the ITOP campaign. Between late July and early August 2004, twelve flights comprising 72 hours of measurement were made in a region from approximately 20 to 40°W and 33 to 47°N centered on Faial Island, Azores, ranging in altitude from 50 to 9000 m. The vertical profiles of O3 and CO are consistent with previous observations made in this region during 1997 and our knowledge of the seasonal cycles within the region. A cluster analysis technique is used to partition the data set into air mass types with distinct chemical signatures. Six clusters provide a suitable balance between cluster generality and specificity. The clusters are labeled as biomass burning, low level outflow, upper level outflow, moist lower troposphere, marine and upper troposphere. During this summer, boreal forest fire emissions from Alaska and northern Canada were found to provide a major perturbation of tropospheric composition in CO, PAN, organic compounds and aerosol. Anthropogenic influenced air from the continental boundary layer of the USA was clearly observed running above the marine boundary layer right across the mid-Atlantic, retaining high pollution levels in VOCs and sulfate aerosol. Upper level outflow events were found to have far lower sulfate aerosol, resulting from washout on ascent, but much higher PAN associated with the colder temperatures. Lagrangian links with flights of other aircraft over the USA and Europe show that such signatures are maintained many days downwind of emission regions. Some other features of the data set are highlighted, including the strong perturbations to many VOCs and OVOCs in this remote region.

The ozone variability observed by tropospheric ozone lidars during the ESCOMPTE campaign is analyzed by means of a hybrid-Lagrangian modeling study. Transport processes responsible for the formation of ozone-rich layers are identified using a semi-Lagrangian analysis of mesoscale simulations to identify the planetary boundary layer (PBL) footprint in the free troposphere. High ozone concentrations are related to polluted air masses exported from the Iberian PBL. The chemical composition of air masses coming from the PBL and transported in the free troposphere is evaluated using a Lagrangian chemistry model. The initial concentrations are provided by a model of chemistry and transport. Different scenarios are tested for the initial conditions and for the impact of mixing with background air in order to perform a quantitative comparison with the lidar observations. For this meteorological situation, the characteristic mixing time is of the order of 2 to 5 days depending on the initial conditions. Ozone is produced in the free troposphere within most air masses exported from the Iberian PBL at an average rate of 0.2 ppbv h-1, with a maximum ozone production of 0.4 ppbv h-1. Transport processes from the PBL are responsible for an increase of 13.3 ppbv of ozone concentrations in the free troposphere compared to background levels; about 45% of this increase is attributed to in situ production during the transport rather than direct export of ozone.

[1] We have used the TOMCAT three-dimensional chemical transport model (CTM) to investigate the impact of recent laboratory measurements of the temperature dependence of the acetone photolysis quantum yield on global tropospheric chemistry. The new acetone quantum yields cause a significant decrease in the calculated acetone loss rate in troposphere. The annual global mean photolysis loss of acetone is reduced by a factor of ∼2, making OH oxidation the dominant acetone sink. Photolysis rates decrease by between ∼80 and 90% in the cold upper troposphere (UT). The atmospheric lifetime of acetone increases from 22 to 35 days, with an increase in the global burden from 2.6 to 4.1 Tg. This is maintained through a global source strength of 42.5 Tg/yr, which is approximately half of that inferred by some previous model studies. Comparisons of modeled and observed acetone profiles from the remote tropical Pacific demonstrate much improved agreement with the new quantum yields, with a reduction in the model bias relative to aircraft observations from −50 to −17%. With the new quantum yields, modeled peroxyacetylnitrate (PAN) decreases in the UT and throughout the Northern Hemisphere. PAN increases are modeled in Southern Hemisphere, as the increases in acetone outweigh the slower rate of peroxyacetyl production. The new quantum yields reduce the model HOx(=OH + HO2) throughout the troposphere. The locations of largest changes to HOx and the OH:HO2 ratio, caused by changes in NOx, mean the impact on model global OH is small (−0.5%). The net effect of using the new quantum yields on tropospheric ozone is also small; the model predicts a maximum 1% decrease in the Northern Hemisphere lower troposphere.

[1] We have used a photochemical box model to investigate the effect of temperature-dependent quantum yields of acetone on the chemistry in the upper troposphere. The T-dependent quantum yields produce a photolysis (J) rate up to factor 3–10 slower than room temperature values. This reduced J rate significantly reduces the contribution of acetone to the mid-latitude and tropical UT HOx budgets by factors of 4 and 2 respectively, when constrained with location-specific aircraft observations. The calculated lifetime of acetone in the UT increases from ∼75 to ∼250 days at mid-latitudes and from ∼10 to ∼20 days in the tropics, for the conditions used here. This will significantly affect studies aimed at balancing the atmospheric acetone budget. Overall, these new quantum yields have large implications for our understanding of the role of acetone in the chemistry of the UT.

[1] The photodissociation of acetone has been studied over the wavelength (λ) range 279–327.5 nm as a function of temperature (T) and pressure (p) using a spectroscopic method to monitor the acetyl (CH3CO) radical fragment. Above 310 nm the quantum yield (QY) is substantially smaller than previous measurements, and decreases with T. The QYs for production of CH3CO + CH3 and CH3 + CH3 + CO have been parameterised as a function of λ, p and T and used to calculate the altitude dependence of the photolysis frequency. In the upper troposphere (UT) the acetone photolysis lifetime is a factor of 2.5–10 longer, dependent upon latitude and season, than if the previously recommended QYs are used.

[1] Can airborne observations from infrequent flights be used to infer the budget of ozone in the upper troposphere with any degree of certainty or representativeness? Fluctuations in ozone mixing ratio observed along flights are dominated by flying between air masses with distinct origins, rather than the recent chemical transformation that has occurred within those air masses. Reverse domain filling trajectories arriving on a high-resolution three-dimensional grid (RDF3D) can simulate air mass structure accurately by coloring arrival grid points with specific humidity (q) from the origin of each trajectory. Typical displacement errors in tracer filaments are only about 30 km, but the associated phase errors greatly reduce the correlations between airborne observations of long-lived chemicals and their model simulations. However, the comparison can be vastly improved if equivalent potential temperature (θe) and specific humidity are used as coordinates to label air masses. Both properties are approximately conserved following unsaturated air and serve as good markers of air masses even if the air is saturated or mixing takes place. Ozone simulations from a Lagrangian model are evaluated against observations in thermodynamic coordinates, factoring out many of the transport phase errors. The proportion of the atmosphere occupied by different chemical air masses is estimated by using RDF3D trajectories to simulate the distributions of q and θe and then assuming that chemical composition is homogeneous within air masses, each with characteristic (q, θe). Mass density in thermodynamic coordinates is used to weight the modeled ozone transformation and error in concentration (calculated along flight tracks) to estimate photochemically produced ozone throughout a volume encompassing the flights.

[1] The vertical distribution of 21 C2–C7 nonmethane hydrocarbons (NMHCs) has been determined in planetary boundary layer (PBL) and free tropospheric (FT) air over central Europe under a range of meteorological conditions. High-frequency whole air sampling was conducted on board the U.K. Meteorological Office C-130 Hercules aircraft during the European Export of Precursors and Ozone by Long-Range Transport (EXPORT) experiment in August 2000. When vertical transport by large-scale flow or convection was weak, the expected large concentration gradient between the PBL and FT was observed for all short and medium lifetime hydrocarbons (e.g., average iso-butane, PBL 100 pptV, FT 6 pptV). During periods of strong convective activity associated with the passage of a cold front, a rapid uplift of reactive carbon from the boundary layer to the mid free troposphere was observed. Using changing ratios of hydrocarbons with different atmospheric lifetimes, a timescale for transport during this event was determined. Hydrocarbon lifetime measurements suggest that in certain regions of the system, it is convective transport embedded within the cold front rather than larger-scale advection along the warm conveyor belt that is dominant in transporting ozone precursors into the free troposphere.

Smoke haze, caused by vegetation and peat fires in Southeast Asia, is of major concern because of its adverse impact on regional air quality. We apply two different methods (a chemical transport model and a Lagrangian atmospheric transport model) to identify the locations of fires contributing to the increased mass concentration of particulate matter with diameters less than 2.5 μm (PM[subscript 2.5]) in Singapore over the period 2004–09. We find that fires in southern Sumatra account for the greatest percentage of the total fire enhancement to PM[subscript 2.5] concentrations in Singapore (42–62%), with fires in central Sumatra and Kalimantan contributing 21–35% and 14–15%, respectively. Furthermore, we find that fires in these regions also increase PM[subscript 2.5] concentrations in other major cities across Southeast Asia. Our results suggest that acting to reduce fires in southern and central Sumatra (specifically in the eastern parts of the provinces of Jambi, South Sumatra, Lampung and Riau) and southwest Kalimantan (the southern extent of the provinces of West, Central and South Kalimantan) would have the greatest benefit to particulate air quality in Singapore and more widely across Southeast Asia., Natural Environment Research Council (Great Britain) (NE/J009822/1), Natural Environment Research Council (Great Britain) (NE/J014257/1)

Nature 489, 282–285 (2012); doi:10.1038/nature11390 A. M. Makarieva, V. G. Gorshkov, D. Sheil, A. D. Nobre and P. Bunyard pointed out that the units in our Fig. 3b were confusing. To clarify, the legend should read: “b, Same as Fig. 2b, but for cumulative continental surface evaporation (ΣET; here plotted per day of atmospheric transport) as a function of ΣLAI.

International audience; Air pollution in the Arctic caused by local emission sources is a challenge that is important but often overlooked. Local Arctic air pollution can be severe and significantly exceed air quality standards, impairing public health and affecting ecosystems. Specifically in the wintertime, pollution can accumulate under inversion layers. However, neither the contributing emission sources are well identified and quantified nor the relevant atmospheric mechanisms forming pollution are well understood. In the summer, boreal forest fires cause high levels of atmospheric pollution. Despite the often high exposure to air pollution, there are neither specific epidemiological nor toxicological health impact studies in the Arctic. Hence, effects on the local population are difficult to estimate at present. Socioeconomic development of the Arctic is already occurring and expected to be significant in the future. Arctic destination shipping is likely to increase with the development of natural resource extraction, and tourism might expand. Such development will not only lead to growth in the population living in the Arctic but will likely increase emission types and magnitudes. Present‐day inventories show a large spread in the amount and location of emissions representing a significant source of uncertainty in model predictions that often deviate significantly from observations. This is a challenge for modeling studies that aim to assess the impacts of within Arctic air pollution. Prognoses for the future are hence even more difficult, given the additional uncertainty of estimating emissions based on future Arctic economic development scenarios.

The Pan-Eurasian Experiment (PEEX) is a new multidisciplinary, global change research initiative focusing on understanding biosphere-ocean-cryosphere-climate interactions and feedbacks in Arctic and boreal regions in the Northern Eurasian geographical domain. PEEX operates in an integrative way and it aims at solving the major scientific and society relevant questions in many scales using tools from natural and social sciences and economics. The research agenda identifies the most urgent large scale research questions and topics of the land-atmosphere-aquatic-anthropogenic systems and interactions and feedbacks between the systems for the next decades. Furthermore PEEX actively develops and designs a coordinated and coherent ground station network from Europe via Siberia to China and the coastal line of the Arctic Ocean together with a PEEX-modeling platform. PEEX launches a program for educating the next generation of multidisciplinary researcher and technical experts. This expedites the utilization of the new scientific knowledge for producing a more reliable climate change scenarios in regional and global scales, and enables mitigation and adaptation planning of the Northern societies. PEEX gathers together leading European, Russian and Chinese research groups. With a bottom-up approach, over 40 institutes and universities have contributed the PEEX Science Plan from 18 countries. In 2014 the PEEX community prepared Science Plan and initiated conceptual design of the PEEX land-atmosphere observation network and modeling platform. Here we present the PEEX approach as a whole with the specific attention to research agenda and preliminary design of the PEEX research infrastructure.

15 pages, 2 figures, The cryosphere, which comprises a large portion of Earth’s surface, is rapidly changing as a consequence of global climate change. Ice, snow, and frozen ground in the polar and alpine regions of the planet are known to directly impact atmospheric composition, which for example is observed in the large influence of ice and snow on polar boundary layer chemistry. Atmospheric inputs to the cryosphere, including aerosols, nutrients, and contaminants, are also changing in the anthropocene thus driving cryosphere-atmosphere feedbacks whose understanding is crucial for understanding future climate. Here, we present the Cryosphere and ATmospheric Chemistry initiative (CATCH) which is focused on developing new multidisciplinary research approaches studying interactions of chemistry, biology, and physics within the coupled cryosphere – atmosphere system and their sensitivity to environmental change. We identify four key science areas: (1) micro-scale processes in snow and ice, (2) the coupled cryosphere-atmosphere system, (3) cryospheric change and feedbacks, and (4) improved decisions and stakeholder engagement. To pursue these goals CATCH will foster an international, multidisciplinary research community, shed light on new research needs, support the acquisition of new knowledge, train the next generation of leading scientists, and establish interactions between the science community and society, CATCH is sponsored by the International Global Atmospheric Chemistry (IGAC) project igacproject.org, the International Surface Ocean – Lower Atmosphere Study (SOLAS) project solas-int.org, and the International Arctic Science Committee (IASC, iasc.info). Support for CATCH activities has been provided by the French Chantier Arctique Project Pollution in the Arctic System (PARCS). This work was supported by the H2020 ERA-PLANET (689443) iCUPE project

Hydroxyl radical (OH) is the main daytime oxidant in the troposphere and determines the atmospheric lifetimes of many compounds. We use aircraft measurements of O, HO, NO, and other species from the Convective Transport of Active Species in the Tropics (CONTRAST) field campaign, which occurred in the tropical western Pacific (TWP) during January–February 2014, to constrain a photochemical box model and estimate concentrations of OH throughout the troposphere. We find that tropospheric column OH (OH) inferred from CONTRAST observations is 12 to 40% higher than found in chemical transport models (CTMs), including CAM-chem-SD run with 2014 meteorology as well as eight models that participated in POLMIP (2008 meteorology). Part of this discrepancy is due to a clear-sky sampling bias that affects CONTRAST observations; accounting for this bias and also for a small difference in chemical mechanism results in our empirically based value of OH being 0 to 20% larger than found within global models. While these global models simulate observed O reasonably well, they underestimate NO (NO + NO) by a factor of 2, resulting in OH ~30% lower than box model simulations constrained by observed NO. Underestimations by CTMs of observed CHCHO throughout the troposphere and of HCHO in the upper troposphere further contribute to differences between our constrained estimates of OH and those calculated by CTMs. Finally, our calculations do not support the prior suggestion of the existence of a tropospheric OH minimum in the TWP, because during January–February 2014 observed levels of O and NO were considerably larger than previously reported values in the TWP.

We have combined the first satellite maps of the global distribution of phytoplankton functional type and new measurements of phytoplankton-specific isoprene productivities, with available remote marine isoprene observations and a global model, to evaluate our understanding of the marine isoprene source and its impacts on organic aerosol abundances. Using satellite products to scale up data on phytoplankton-specific isoprene productivity to the global oceans, we infer a mean "bottom-up" oceanic isoprene emission of 0.31±0.08 (1σ) Tg/yr. By minimising the mean bias between the model and isoprene observations in the marine atmosphere remote from the continents, we produce a "top-down" oceanic isoprene source estimate of 1.9 Tg/yr. We suggest our reliance on limited atmospheric isoprene data, difficulties in simulating in-situ isoprene production rates in laboratory phytoplankton cultures, and limited knowledge of isoprene production mechanisms across the broad range of phytoplankton communities in the oceans under different environmental conditions as contributors to this difference between the two estimates. Inclusion of secondary organic aerosol (SOA) production from oceanic isoprene in the model with a 2% yield produces small contributions (0.01–1.4%) to observed organic carbon (OC) aerosol mass at three remote marine sites in the Northern and Southern Hemispheres. Based on these findings we suggest an insignificant role for isoprene in modulating remote marine aerosol abundances, giving further support to a recently postulated primary OC source in the remote marine atmosphere.