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A comprehensive characterisation of atmospheric aerosols in the subtropical eastern North Atlantic has been carried out using long-term ground-based Aerosol Robotic NETwork (AERONET) photometric observations over the period 2005–2020 from a unique network made up of four stations strategically located from sea level to 3555 m on the island of Tenerife. This site can be considered a sentinel for the passage of airmasses going to Europe from Africa, and therefore the aerosol characterisation performed here adds important information for analysing their evolution during their path toward Northern Europe. Two of these stations (Santa Cruz de Tenerife – SCO – at sea level and La Laguna – LLO – at 580 m a.s.l.) are located within the marine atmospheric boundary layer (MABL), and the other two (Izaña – IZO – at 2373 m a.s.l. and Teide Peak – TPO – at 3555 m a.s.l.) are high mountain stations within the free troposphere (FT). Monthly climatology of the aerosol optical depth (AOD), Ångström exponent (AE), aerosol concentration, size distribution and aerosol optical properties has been obtained for the MABL and FT. Measurements that are quite consistent across the four sites have been used to categorise the main atmospheric scenarios, and these measurements confirm an alternation between predominant background conditions and predominant dust-loaded Saharan air mass conditions caused by seasonal dust transport over the subtropical North Atlantic. Background conditions prevail in the MABL and FT for most of the year, while dust-laden conditions dominate in July and August. The MABL under background conditions appears as a well-mixed layer with a low aerosol concentration (the volume concentration, VolCon, ranges from 0.02 ± 0.01 to 0.04 ± 0.02 µm3 µm−2), a predominance of coarse-mode marine aerosols (the effective radius, Reff, changes from 1.60 ± 0.19 to 1.91 ± 0.34 µm), and a volume contribution of the fine-mode fraction Vf/Vt . The clean FT is characterised by remarkably low aerosol loading and a predominant impact of fine-mode aerosols throughout the year (Vf/Vt has a maximum value of 0.93 ± 0.13), with an average Reff of 0.16 ± 0.02 µm. However, under dust-laden conditions and mainly in summer, we observe a predominance of coarse-mode aerosols with maximum VolCon values of 0.26 ± 0.23 µm3 µm−2 for the MABL and 0.16 ± 0.12 (0.06 ± 0.05) µm3 µm−2 for IZO (TPO), and a similar and quite consistent fine-mode fraction of 0.12 ± 0.03 in the vertical within the MABL and FT. Similarities in micro-physical and optical intensive aerosol properties confirm that the Saharan Air Layer (SAL) is a well-mixed layer in terms of the particulate composition. An estimation of the difference in the aerosol loading in the 1 km layer between IZO and TPO (in terms of VolCon and AOD) is performed in this study, and this shows that aerosol loading at IZO is double that at TPO, but they have similar fine-mode fractions, effective radii and intensive optical properties. The long-term trend analysis at SCO shows a significant negative trend in the fine-mode AOD between 2005 and 2020 (−1.8 ± 0.5) × 10−5 yr−1, which might be linked to the large reduction in oil-refining SO2 emissions from the SCO refinery in 2012.

Numerical weather prediction systems depend on Hyperspectral Infrared Sounder (HIS) data, yet the impacts of dust-contaminated HIS radiances on weather forecasts has not been quantified. To determine the impact of dust aerosol on HIS radiance assimilation, we use a modified radiance assimilation system employing a one-dimensional variational assimilation system (1DVAR) developed under the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Numerical Weather Prediction–Satellite Application Facility (NWP-SAF) project, which uses the Radiative Transfer for TOVS (RTTOV). Dust aerosol impacts on analyzed temperature and moisture fields are quantified using synthetic HIS observations from rawinsonde, Micropulse Lidar Network (MPLNET), and Aerosol Robotic Network (AERONET). Specifically, a unit dust aerosol optical depth (AOD) contamination at 550 nm can introduce larger than 2.4 and 8.6 K peak biases in analyzed temperature and dewpoint, respectively, over our test domain. We hypothesize that aerosol observations, or even possibly forecasts from aerosol predication models, may be used operationally to mitigate dust induced temperature and moisture analysis biases through forward radiative transfer modeling.

At the end of 2015, a CO2/CH4/CO cavity ring-down spectrometer (CRDS) was installed at the Izaña Global Atmosphere Watch (GAW) station (Tenerife, Spain) to improve the Izaña Greenhouse Gases GAW Measurement Programme, and to guarantee the renewal of the instrumentation and the long-term maintenance of this program. We present the results of the CRDS acceptance tests, the raw data processing scheme applied, and the response functions used. Also, the calibration results, the implemented water vapor correction, the target gas injection statistics, the ambient measurements performed from December 2015 to July 2017, and their comparison with other continuous in situ measurements are described. The agreement with other in situ continuous measurements is good most of the time for CO2 and CH4, but for CO it is just outside the GAW 2 ppb objective. It seems the disagreement is not produced by significant drifts in the CRDS CO World Meteorological Organization (WMO) tertiary standards. The more relevant contributions of the present article are (1) determination of linear relationships between flow rate, CRDS inlet pressure, and CRDS outlet valve aperture; (2) determination of a slight CO2 correction that takes into account changes in the inlet pressure/flow rate (as well as its stability over the years), and attributing it to the existence of a small spatial inhomogeneity in the pressure field inside the CRDS cavity due to the gas dynamics; (3) drift rate determination for the pressure and temperature sensors located inside the CRDS cavity from the CO2 and CH4 response function drift trends; (4) the determination of the H2O correction for CO has been performed using raw spectral peak data instead of the raw CO provided by the CRDS and using a running mean to smooth random noise in a long water-droplet test (12 h) before performing the least square fit; and (5) the existence of a small H2O dependence in the CRDS flow and of a small spatial inhomogeneity in the temperature field inside the CRDS cavity are pointed out and their origin discussed.

Extracting globally representative trend information from lower tropospheric ozone observations is extremely difficult due to the highly variable distribution and interannual variability of ozone, and the ongoing shift of ozone precursor emissions from high latitudes to low latitudes. Here we report surface ozone trends at 27 globally distributed remote locations (20 in the Northern Hemisphere, 7 in the Southern Hemisphere), focusing on continuous time series that extend from the present back to at least 1995. While these sites are only representative of less than 25% of the global surface area, this analysis provides a range of regional long-term ozone trends for the evaluation of global chemistry-climate models. Trends are based on monthly mean ozone anomalies, and all sites have at least 20 years of data, which improves the likelihood that a robust trend value is due to changes in ozone precursor emissions and/or forced climate change rather than naturally occurring climate variability. Since 1995, the Northern Hemisphere sites are nearly evenly split between positive and negative ozone trends, while 5 of 7 Southern Hemisphere sites have positive trends. Positive trends are in the range of 0.5–2 ppbv decade–1, with ozone increasing at Mauna Loa by roughly 50% since the late 1950s. Two high elevation Alpine sites, discussed by previous assessments, exhibit decreasing ozone trends in contrast to the positive trend observed by IAGOS commercial aircraft in the European lower free-troposphere. The Alpine sites frequently sample polluted European boundary layer air, especially in summer, and can only be representative of lower free tropospheric ozone if the data are carefully filtered to avoid boundary layer air. The highly variable ozone trends at these 27 surface sites are not necessarily indicative of free tropospheric trends, which have been overwhelmingly positive since the mid-1990s, as shown by recent studies of ozonesonde and aircraft observations. The IAGOS program acknowledges the European Commission for its support of the MOZAIC project (1994–2003) the preparatory phase of IAGOS (2005–2013) and IGAS (2013–2016).

Air pollution has many adverse effects on health and is associated with an increased risk of mortality. Desert dust outbreaks contribute directly to air pollution by increasing particulate matter concentrations. We investigated the influence of desert dust outbreaks on air quality in Santa Cruz de Tenerife, a city located in the dust export pathway off the west coast of North Africa, using air-quality observations from a six-year period (2012&ndash, 2017). During winter intense dust outbreaks PM 10 mean (24-h) concentrations increased from 14 &mu, g m - 3 to 98 &mu, g m - 3 , on average, and PM 2 . 5 mean (24-h) concentrations increased from 6 &mu, g m - 3 to 32 &mu, g m - 3 . Increases were less during summer outbreaks, with a tripling of PM 10 and PM 2 . 5 daily mean concentrations. We found that desert dust outbreaks reduced the height of the marine boundary layer in our study area by &gt, 45%, on average, in summer and by &sim, 25%, on average, in winter. This thinning of the marine boundary layer was associated with an increase of local anthropogenic pollution during dust outbreaks. NO 2 and NO mean concentrations more than doubled and even larger relative increases in black carbon were observed during the more intense summer dust outbreaks, increases also occurred during the winter outbreaks but were less than in summer. This has public health implications, local anthropogenic emissions need to be reduced even further in areas that are impacted by desert dust outbreaks to reduce adverse health effects.

It was previously shown that during August the export of Saharan dust to the Atlantic was strongly affected by the difference of the 700-hPa geopotential height anomaly between the subtropics and the tropics over North Africa, which was termed the North African Dipole Intensity (NAFDI). In this work a more comprehensive analysis of the NAFDI is performed, focusing on the entire summer dust season (June–September), and examining the interactions between the mid-latitude Rossby waves (MLRWs) and NAFDI. Widespread and notable aerosol optical depth (AOD) monthly anomalies are found for each NAFDI-phase over the dust corridors off the Sahara, indicating that NAFDI presents intra-seasonal variability and drives dust transport over both the Mediterranean basin and the North Atlantic. Those summer months with the same NAFDI-phase show similar AOD-anomaly patterns. Variations in NAFDI-phase also control the displacement of the Saharan Heat Low (SHL) westwards or eastwards through horizontal advection of temperature over Morocco-Western Sahara or eastern Algeria-Western Libya, respectively. The connection between the SHL and the NAFDI is quantified statistically by introducing two new daily indexes that account for their respective phases (NAFDI daily index -NAFDIDI-, and SHL longitudinal shift index -SHLLSI-) and explained physically using the energy equation of the atmospheric dynamics. The Pearson's correlation coefficient between the one–day-lag SHLLSI and the NAFDIDI for an extended summer season (1980–2013) is 0.78. A positive NAFDI is associated with the West-phase of the SHL, dust sources intensification on central Algeria, and positive AOD anomalies over this region and the Subtropical North Atlantic. A negative NAFDI is associated with the East-phase of the SHL, and positive AOD anomalies over central-eastern Sahara and the central-western Mediterranean Sea. The results point out that the phase changes of NAFDI at intra-seasonal time scale are conducted by those MLRWs that penetrate deeply into the low troposphere. This work is part of the research activities developed by the WMO SDS-WAS Regional Centre for Northern Africa, Middle East and Europe, held by AEMET and BSC-CNS. This study also contributes to Copernicus Atmosphere Monitoring Service (CAMS). Our acknowledgment to ECMWF for providing MACC-dust reanalysis. The authors wish to thank NCEP/NCAR Reanalysis Project. We acknowledge NASA LADS for providing MODIS data. The University of Granada (Spain) and its “Physics and Space Sciences” PhD Programme are acknowledged by A. J. Gómez-Peláez and E. Cuevas. AEROATLAN project (CGL2015-17 66229-P), co-funded by the Ministry of Economy and Competitiveness of Spain and the European Regional Development Fund contributed to this study. Sara Basart acknowledges the CICYT project (CGL2013-46736). 3.708 JCR (2017) Q1, 54/241 Environmental Sciences, 16/86 Meteorology and Atmospheric Sciences UEC

A new zenith-looking narrow-band radiometer based system (ZEN), conceived for dust aerosol optical depth (AOD) monitoring, is presented in this paper. The ZEN system comprises a new radiometer (ZEN-R41) and a methodology for AOD retrieval (ZEN-LUT). ZEN-R41 has been designed to be stand alone and without moving parts, making it a low-cost and robust instrument with low maintenance, appropriate for deployment in remote and unpopulated desert areas. The ZEN-LUT method is based on the comparison of the measured zenith sky radiance (ZSR) with a look-up table (LUT) of computed ZSRs. The LUT is generated with the LibRadtran radiative transfer code. The sensitivity study proved that the ZEN-LUT method is appropriate for inferring AOD from ZSR measurements with an AOD standard uncertainty up to 0.06 for AOD500 nm ∼ 0.5 and up to 0.15 for AOD500 nm ∼ 1.0, considering instrumental errors of 5 %. The validation of the ZEN-LUT technique was performed using data from AErosol RObotic NETwork (AERONET) Cimel Electronique 318 photometers (CE318). A comparison between AOD obtained by applying the ZEN-LUT method on ZSRs (inferred from CE318 diffuse-sky measurements) and AOD provided by AERONET (derived from CE318 direct-sun measurements) was carried out at three sites characterized by a regular presence of desert mineral dust aerosols: Izaña and Santa Cruz in the Canary Islands and Tamanrasset in Algeria. The results show a coefficient of determination (R2) ranging from 0.99 to 0.97, and root mean square errors (RMSE) ranging from 0.010 at Izaña to 0.032 at Tamanrasset. The comparison of ZSR values from ZEN-R41 and the CE318 showed absolute relative mean bias (RMB) R2 higher than 0.97.

Cirrus (Ci) cloud properties can change significantly from place to place over the globe as a result of weather processes, reflecting their likely different radiative and climate implications. In this work Cirrus clouds (Ci) features observed in late autumn/early winter season at both subtropical and polar latitudes are examined and compared to CALIPSO/CALIOP observations. Lidar measurements were carried out in three stations: Sao Paulo (MSP, Brazil) and Tenerife (SCO, Canary Islands, Spain), as subtropical sites, and the polar Belgrano II base (BEL, Argentina) in the Antarctic continent. The backscattering ratio (BSR) profiles and the top and base heights of the Ci layers together to their Cirrus Cloud Optical Depth (CCOD) and Lidar Ratio (LR) for Ci clouds were derived. In addition, temperatures at the top and base boundaries of the Ci clouds were also obtained from local radiosoundings to verify pure ice Ci clouds occurrence using a given temperature top threshold ( 70 km far), inferring the irregular extension and inhomogeneity of the Ci clouds over each study area. These considerations can be useful for assimilation of the Ci features into climate models and evaluation of future space-borne lidar observations of Ci clouds, especially for the future ESA/Copernicus-Sentinel and ESA/EarthCARE missions.

The Baseline Surface Radiation Network (BSRN) was implemented by the World Climate Research Programme (WCRP) starting observations with nine stations in 1992, under the auspices of the World Meteorological Organization (WMO). Currently, 59 BSRN stations submit their data to the WCRP. One of these stations is the Izaña station (station IZA, no. 61) that enrolled in this network in 2009. This is a high-mountain station located in Tenerife (Canary Islands, Spain, at 28.3∘ N, 16.5∘ W; 2373 m a.s.l.) and is a representative site of the subtropical North Atlantic free troposphere. It contributes with basic-BSRN radiation measurements, such as global shortwave radiation (SWD), direct radiation (DIR), diffuse radiation (DIF) and longwave downward radiation (LWD), and extended-BSRN measurements, including ultraviolet ranges (UV-A and UV-B), shortwave upward radiation (SWU) and longwave upward radiation (LWU), and other ancillary measurements, such as vertical profiles of temperature, humidity and wind obtained from radiosonde profiles (WMO station no. 60018) and total column ozone from the Brewer spectrophotometer. The IZA measurements present high-quality standards since more than 98 % of the data are within the limits recommended by the BSRN. There is an excellent agreement in the comparison between SWD, DIR and DIF (instantaneous and daily) measurements with simulations obtained with the LibRadtran radiative transfer model. The root mean square error (RMSE) for SWD is 2.28 % for instantaneous values and 1.58 % for daily values, while the RMSE for DIR is 2.00 % for instantaneous values and 2.07 % for daily values. IZA is a unique station that provides very accurate solar radiation data in very contrasting scenarios: most of the time under pristine sky conditions and periodically under the effects of the Saharan air layer characterized by a high content of mineral dust. A detailed description of the BSRN program at IZA, including quality control and quality assurance activities, is given in this work.

International audience; The authors regret that there is an error in equation 12. The right equation for κλ, as is defined in Barreto et al. (The new Sun-sky-lunar Cimel CE318-T multiband photometer - a comprehensive performance evaluation, Atmos. Meas. Tech., 9, 631-654, 10.5194/amt-9-631-2016, 2016), is the reversed form of equation 12. That is: κλ = V0,λ*G/E0,λ.

Fil: Adame, José Antonio. Instituto Nacional de Técnica Aeroespacial; España Fil: Cupeiro, Manuel. Servicio Meteorológico Nacional; Argentina Fil: Yela, Margarita. Instituto Nacional de Técnica Aeroespacial; España Fil: Cuevas, Emilio. Agencia Estatal de Metorología; España Fil: Carbajal, Gerardo. Servicio Meteorológico Nacional; Argentina Fil: Carbajal, Gerardo. Pontificia Universidad Católica Argentina. Facultad de Ingeniería y Ciencias Agrarias; Argentina Abstract: Five years (2010-2014) of hourly surface measurements of ozone (O3) and carbon monoxide (CO) at the GAW-WMO station in Ushuaia (Argentina) were analysed and characterised. A meteorological study of the region was carried out using in situ observations and meteorological fields from the ECMWF (European Centre for Medium-Range Weather Forecast) global meteorological model. Atmospheric transport was investigated with the air mass trajectories computed with HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory) model using ERA-Interim meteorological fields. Airflows primarily arise from the W-SW (South Pacific Ocean) which are associated with an almost permanent low pressure system. Collected winds from the South (Antarctic Peninsula and Weddell Sea), polar easterlies, occur less frequently. The hourly averages of O3 and CO were 20±7 ppb and 71±45 ppb, respectively, typical values in remote environments. A clear seasonal pattern was obtained for O3, with a monthly peak in winter of 25 ppb and minimum in summer of 12 ppb. Similar behaviour was found for CO, with 93 and 48 ppb maximum and minimum values, respectively. Both species show a weak daily cycle with an amplitude of 2-4 ppb for O3 and 13-20 ppb for CO. Peaks in O3 and CO in the cold season could be associated with low photochemical activity, fewer destructive processes and transport of these species from the South Pacific. O3 vertical behaviour was analysed using 139 O3 soundings at Ushuaia in the period studied. The seasonal patterns and levels of the O3 profiles from the surface up to 5 km are similar to measurements on surface and thus it can be assumed that the O3 measured on the surface could be representative of the low-mid troposphere. To investigate the spatial distribution of O3 and CO in this region and the spatial representativeness of the Ushuaia measurements, daily observations from the AIRS (Atmospheric Infrared Sounder) instrument on board the AQUA satellite were used. The results obtained show that the O3 levels in Ushuaia could be representative of a wide region in the Pacific South and that the CO levels are representative of a continental region around the observatory.

Based at Izaña Observatory (~2400 m a.s.l. in Tenerife), we performed 1-h resolution measurements of elemental composition of dust in the Saharan Air Layer (SAL) and studied the variability of the ratios of these elements to aluminium (elemental ratios). In a period (~1 week) of continuous dust presence (50–200 μg/m), we observed rapid variations of dust composition; some elemental ratios changed by a factor 2 in a few (5–8) hours. The lowest variability (Normalized Variability Range, %) was found for Si/Al (9%) and Fe/Al (9%), followed by the ratios of K, Ti, Mg, Mn, Ca and Sr to Al (20–80%), and the highest for S/Al, Na/Al and Cl/Al (110–160%) and a number of trace metals (Cr, Cu, Ni, Zn, Zr) and Br (>200%). This variability was induced by the alternating impacts of three of the large North African dust sources: NE Algeria (rich in evaporite minerals bearing Ca, S, Sr, K and Mg and in illite mineral), Western Sahara to Bechar region (containing Na, S and Cl rich Yermosol soils) and SW Sahara – Western Sahel (rich in illite and hematite). We traced the variability in large-scale meteorology using the so-called North African Dipole Intensity (NAFDI: strength of the subtropical Saharan high -Morocco-to the monsoon tropical low -Nigeria-). The mobilization of dust from the different sources was associated with westward propagating Harmattan pulses linked to the change of phase of NAFDI (- to +), the associated westward shifts of the Saharan Heat Low and convective monsoon inflow. We found a correlation between dust composition in the SAL and NAFDI: moderate NAFDI values (0 to +2.5) were associated with Ca, K, Na, Mg and S rich dust linked to dust sources in NE Algeria, whereas higher NAFDI values (+2.5 to +4) were linked to Fe rich dust (Ca, Na and S depleted) linked to dust sources in SW Sahara – Western Sahel. The results of this study also show that some trace elements (Br, Cr, Ni, Zn and Zr) are influenced by industrial emissions into North Africa., This study was performed within the project VARDUST-SAL (PGC2018-099166-BI00), funded by the Ministry of Science, Research and Innovation of Spain, the Research State Agency of Spain and the European Regional Development Fund (ERDF). JLD is awarded with a posdoc contract Agustín de Bethencourt, funded by the Program Fomento de Transferencia del Conocimiento, funded by the Cabildo de Tenerife. A stage of JLD at the INFN was funded by the Universidad de La Laguna

We present two years of in situ measurements of water vapour (H2O) and its isotopologue ratio (δD, the standardized ratio between H216O and HD16O), made at two remote mountain sites on Tenerife in the subtropical North Atlantic. We show that the data – if measured during night-time – are well representative for the lower/middle free troposphere. We use the measured H2O-δD pairs, together with dust measurements and back trajectory modelling for analysing the moisture pathways to this region. We can identify four principally different transport pathways. The air mass transport from high altitudes and high latitudes shows two different scenarios. The first scenario brings dry air masses to the stations, as the result of condensation events occurring at low temperatures. The second scenario brings humid air masses to the stations, due to cross-isentropic mixing with lower-level and more humid air during transport since last condensation (LC). The third pathway is transportation from lower latitudes and lower altitudes, whereby we can identify rain re-evaporation as an occasional source of moisture. The fourth pathway is linked to the African continent, where during summer, dry convection processes over the Sahara very effectively inject humidity from the boundary layer to higher altitudes. This so-called Saharan Air Layer (SAL) is then advected westward over the Atlantic and contributes to moisten the free troposphere. We demonstrate that the different pathways leave distinct fingerprints on the measured H2O-δD pairs.

This paper presents the new photometer CE318-T, able to perform daytime and night-time photometric measurements using the sun and the moon as light source. Therefore, this new device permits a complete cycle of diurnal aerosol and water vapour measurements valuable to enhance atmospheric monitoring to be extracted. In this study we have found significantly higher precision of triplets when comparing the CE318-T master instrument and the Cimel AErosol RObotic NETwork (AERONET) master (CE318-AERONET) triplets as a result of the new CE318-T tracking system. Regarding the instrument calibration, two new methodologies to transfer the calibration from a reference instrument using only daytime measurements (Sun Ratio and Sun-Moon gain factor techniques) are presented and discussed. These methods allow the reduction of the previous complexities inherent to nocturnal calibration. A quantitative estimation of CE318-T AOD uncertainty by means of error propagation theory during daytime revealed AOD uncertainties (uDAOD) for Langley-calibrated instruments similar to the expected values for other reference instruments (0.002–0.009). We have also found uDAOD values similar to the values reported in sun photometry for field instruments ( ∼ 0.015). In the case of the night-time period, the CE318-T-estimated standard combined uncertainty (uNAOD) is dependent not only on the calibration technique but also on illumination conditions and the instrumental noise. These values range from 0.011–0.018 for Lunar Langley-calibrated instruments to 0.012–0.021 for instruments calibrated using the Sun Ratio technique. In the case of moon-calibrated instruments using the Sun-Moon gain factor method and sun-calibrated using the Langley technique, we found uNAOD ranging from 0.016 to 0.017 (up to 0.019 in 440 nm channel), not dependent on any lunar irradiance model.A subsequent performance evaluation including CE318-T and collocated measurements from independent reference instruments has served to assess the CE318-T performance as well as to confirm its estimated uncertainty. Daytime AOD evaluation, performed at Izaña station from March to June 2014, encompassed measurements from a reference CE318-T, a CE318-AERONET master instrument, a Precision Filter Radiometer (PFR) and a Precision Spectroradiometer (PSR) prototype, reporting low AOD discrepancies between the four instruments (up to 0.006). The nocturnal AOD evaluation was performed using CE318-T- and star-photometer-collocated measurements and also by means of a day/night coherence transition test using the CE318-T master instrument and the CE318 daytime data from the CE318-AERONET master instrument. Results showed low discrepancies with the star photometer at 870 and 500 nm channels ( ≤ 0.013) and differences with AERONET daytime data (1 h after and before sunset and sunrise) in agreement with the estimated uNAOD values at all illumination conditions in the case of channels within the visible spectral range, and only for high moon's illumination conditions in the case of near-infrared channels.Precipitable water vapour (PWV) validation showed a good agreement between CE318-T and Global Navigation Satellite System (GNSS) PWV values for all illumination conditions, within the expected precision for sun photometry.Finally, two case studies have been included to highlight the ability of the new CE318-T to capture the diurnal cycle of aerosols and water vapour as well as short-term atmospheric variations, critical for climate studies.

A 1-year inter-comparison of classical and modern radiation and sunshine duration (SD) instruments has been performed at Izaña Atmospheric Observatory (IZO) located in Tenerife (Canary Islands, Spain) starting on 17 July 2014. We compare daily global solar radiation (GSRH) records measured with a Kipp & Zonen CM-21 pyranometer, taken in the framework of the Baseline Surface Radiation Network, with those measured with a multifilter rotating shadowband radiometer (MFRSR), a bimetallic pyranometer (PYR) and GSRH estimated from sunshine duration performed by a Campbell–Stokes sunshine recorder (CS) and a Kipp & Zonen sunshine duration sensor (CSD). Given that the BSRN GSRH records passed strict quality controls (based on principles of physical limits and comparison with the LibRadtran model), they have been used as reference in the inter-comparison study. We obtain an overall root mean square error (RMSE) of ∼ 0.9 MJm−2 (4 %) for PYR and MFRSR GSRH, 1.9 (7 %) and 1.2 MJm−2 (5 %) for CS and CSD GSRH, respectively. Factors such as temperature, relative humidity (RH) and the solar zenith angle (SZA) have been shown to moderately affect the GSRH observations. As an application of the methodology developed in this work, we have re-evaluated the GSRH data time series obtained at IZO with two PYRs between 1977 and 1991. Their high consistency and temporal stability have been proved by comparing with GSRH estimates obtained from SD observations. These results demonstrate that (1) the continuous-basis inter-comparison of different GSRH techniques offers important diagnostics for identifying inconsistencies between GSRH data records, and (2) the GSRH measurements performed with classical and more simple instruments are consistent with more modern techniques and, thus, valid to recover GSRH data time series and complete worldwide distributed GSRH data. The inter-comparison and quality assessment of these different techniques have allowed us to obtain a complete and consistent long-term global solar radiation series (1977–2015) at Izaña.

In November 2005 an extratropical storm named Delta affected the Canary Islands (Spain). The high sustained wind and intense gusts experienced caused significant damage. A numerical sensitivity study of Delta was conducted using the Weather Research & Forecasting Model (WRF-ARW). A total of 27 simulations were performed. Non-hydrostatic and hydrostatic experiments were designed taking into account physical parameterizations and geometrical factors (size and position of the outer domain, definition or not of nested grids, horizontal resolution and number of vertical levels). The Factor Separation Method was applied in order to identify the major model sensitivity parameters under this unusual meteorological situation. Results associated to percentage changes relatives to a control run simulation demonstrated that boundary layer and surface layer schemes, horizontal resolutions, hydrostaticity option and nesting grid activation were the model configuration parameters with the greatest impact on the 48 h maximum 10 m horizontal wind speed solution.

This study simultaneously examines wind speed trends at the land?ocean interface, and below?above the trade-wind inversion layer in the Canary Islands and the surrounding Eastern North Atlantic Ocean: a key region for quantifying the variability of trade-winds and its response to large-scale atmospheric circulation changes. Two homogenized data sources are used: (1) observed wind speed from nine land-based stations (1981?2014), including one mountain weather station (Izaña) located above the trade-wind inversion layer; and (2) simulated wind speed from two atmospheric hindcasts over ocean (i.e., SeaWind I at 30 km for 1948?2014; and SeaWind II at 15 km for 1989?2014). The results revealed a widespread significant negative trend of trade-winds over ocean for 1948?2014, whereas no significant trends were detected for 1989?2014. For this recent period wind speed over land and ocean displayed the same multi-decadal variability and a distinct seasonal trend pattern with a strengthening (late spring and summer; significant in May and August) and weakening (winter?spring?autumn; significant in April and September) of trade-winds. Above the inversion layer at Izaña, we found a predominance of significant positive trends, indicating a decoupled variability and opposite wind speed trends when compared to those reported in boundary layer. The analysis of the Trade Wind Index (TWI), the North Atlantic Oscillation Index (NAOI) and the Eastern Atlantic Index (EAI) demonstrated significant correlations with the wind speed variability, revealing that the correlation patterns of the three indices showed a spatio-temporal complementarity in shaping wind speed trends across the Eastern North Atlantic. C. A. -M. has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 703733 (STILLING project). This research was also supported by the Research Projects: Swedish BECC, MERGE, VR (2014–5320), PCIN-2015-220, CGL2014-52135-C03-01 and Red de variabilidad y cambio climático RECLIM (CGL2014-517221-REDT). M.M is indebted to the Spanish Government for funding through the “Ramón y Cajal” program and supported by Grant PORTIO (BIA2015-70644-R)

Three years of multi-axis differential optical absorption spectroscopy (MAXDOAS) measurements (2011–2013) have been used for estimating the NO2 mixing ratio along a horizontal line of sight from the high mountain subtropical observatory of Izaña, at 2370 m a.s.l. (NDACC station, 28.3° N, 16.5° W). The method is based on horizontal path calculation from the O2–O2 collisional complex at the 477 nm absorption band which is measured simultaneously to the NO2 column density, and is applicable under low aerosol-loading conditions. The MAXDOAS technique, applied in horizontal mode in the free troposphere, minimizes the impact of the NO2 contamination resulting from the arrival of marine boundary layer (MBL) air masses from thermally forced upwelling breeze during middle hours of the day. Comparisons with in situ observations show that during most of the measuring period, the MAXDOAS is insensitive or very slightly sensitive to the upwelling breeze. Exceptions are found for pollution events during southern wind conditions. On these occasions, evidence of fast, efficient and irreversible transport from the surface to the free troposphere is found. Background NO2 volume mixing ratio (vmr), representative of the remote free troposphere, is in the range of 20–45 pptv. The observed seasonal evolution shows an annual wave where the peak is in phase with the solar radiation. Model simulations with the chemistry–climate CAM-Chem model are in good agreement with the NO2 measurements, and are used to further investigate the possible drivers of the NO2 seasonality observed at Izaña.

The stability of the lower troposphere along the east side of the sub-tropical North Atlantic is analyzed and characterized using upper air meteorological long-term records at the Canary Islands (Tenerife), Madeira (Madeira) and Azores (Terceira) archipelagos. The most remarkable characteristic is the strong stratification observed in the lower troposphere, with a strengthening of stability centred at levels near 900 and 800 hPa in a significant percentage of soundings (ranging from 17 % in Azores to 33 % in Guimar, Canary Islands). We show that this double structure is associated with the top of the marine boundary layer (MBL) and the trade-wind inversion (TWI) respectively. The top of the MBL coincides with the base of the first temperature inversion ( $$\approx $$ 900 hPa) where a sharp change in water vapour mixing ratio is observed. A second temperature inversion is found near 800 hPa, which is characterized by a large directional wind shear just above the inversion layer, tied to the TWI. We find that seasonal and latitudinal variations of the height and strength of both temperature inversions are driven by large-scale subsiding air from the upper troposphere associated with the descent branch of the Hadley cell. Increased general subsidence in summertime enhances stability in the lower troposphere, more markedly in the southern stations, where the inversion-layer heights are found at lower levels enhancing the main features of these two temperature inversions. A simple conceptual model that explains the lower tropospheric inversion enhancement by subsidence is proposed.

In this work, we have carried out a comparison study between measured and modelled direct and global ultraviolet (UV) (300–400 nm) spectral irradiances and the determination of the surface effective albedo at the high altitude Izana Observatory [IZO; 2400 m above sea level (a.s.l.)] located in Tenerife (Canary Islands, Spain). The spectral measurements were performed with a Bentham spectroradiometer during the Quality Assurance of Solar Ultraviolet Spectral Irradiance Measurements (QASUME) comparison campaign in June 2005. The simulations were obtained with the LibRadtran radiative transfer model. The model input parameters, such as total ozone and aerosol optical depth (AOD), were measured at IZO. The comparison between measured and modelled direct solar radiation component was made at 0.5 nm spectral resolution, showing excellent agreement, with differences below 5% for solar zenith angle (SZA)

In the present work, atmospheric mineral dust from a MACC-II short reanalysis run for 2 years (2007–2008) has been evaluated over northern Africa and the Middle East using satellite aerosol products (from MISR, MODIS and OMI satellite sensors), ground-based AERONET data, in situ PM10 concentrations from AMMA, and extinction vertical profiles from two ground-based lidars and CALIOP satellite-based lidar. The MACC-II aerosol optical depth (AOD) spatial and temporal (seasonal and interannual) variability shows good agreement with those provided by satellite sensors. The capability of the model to reproduce the AOD, Ångström exponent (AE) and dust optical depth (DOD) from daily to seasonal time-scale is quantified over 26 AERONET stations located in eight geographically distinct regions by using statistical parameters. Overall DOD seasonal variation is fairly well simulated by MACC-II in all regions, although the correlation is significantly higher in dust transport regions than in dust source regions. The ability of MACC-II in reproducing dust vertical profiles has been assessed by comparing seasonal averaged extinction vertical profiles simulated by MACC-II under dust conditions with corresponding extinction profiles obtained with lidar instruments at M'Bour and Santa Cruz de Tenerife, and with CALIOP. We find a good agreement in dust layers structures and averaged extinction vertical profiles between MACC-II, the lidars and CALIOP above the marine boundary layer from 1 to 6 km. Surface dust daily mean concentrations from MACC-II reanalysis has been evaluated with daily averaged PM10 at three monitoring stations of the Sahelian Dust Transect. MACC-II correctly reproduces daily to interannual surface dust concentration variability, although it underestimates daily and monthly means all year long, especially in winter and early spring (dry season). MACC-II reproduces well the dust variability recorded along the station transect which reflects the variability in dust emission by different Saharan sources, but fails in reproducing the sporadic and very strong dust events associated to mesoscale convective systems during the wet season.

We validate the Ozone Monitoring Instrument (OMI) Ozone Profile (PROFOZ) product from October 2004 through December 2014 retrieved by the Smithsonian Astrophysical Observatory (SAO) algorithm against ozonesonde observations. We also evaluate the effects of OMI row anomaly (RA) on the retrieval by dividing the dataset into before and after the occurrence of serious OMI RA, i.e., pre-RA (2004–2008) and post-RA (2009–2014). The retrieval shows good agreement with ozonesondes in the tropics and midlatitudes and for pressure ∼ 50 hPa after applying OMI averaging kernels to ozonesonde data. The MBs of the stratospheric ozone column (SOC, the ozone column from the tropopause pressure to the ozonesonde burst pressure) are within 2 % with SDs of ∼ 50 hPa. The SOC MBs increase up to 3 % with SDs as great as 6 % and the TOC SDs increase up to 30 %. The comparison generally degrades at larger solar zenith angles (SZA) due to weaker signals and additional sources of error, leading to worse performance at high latitudes and during the midlatitude winter. Agreement also degrades with increasing cloudiness for pressure > ∼ 100 hPa and varies with cross-track position, especially with large MBs and SDs at extreme off-nadir positions. In the tropics and midlatitudes, the post-RA comparison is considerably worse with larger SDs reaching 2 % in the stratosphere and 8 % in the troposphere and up to 6 % in TOC. There are systematic differences that vary with latitude compared to the pre-RA comparison. The retrieval comparison demonstrates good long-term stability during the pre-RA period but exhibits a statistically significant trend of 0.14–0.7 % year−1 for pressure

A new approximation is proposed to estimate O3 and NO2 mixing ratios in the northern subtropical free troposphere (FT). The proposed method uses O4 slant column densities (SCDs) at horizontal and near-zenith geometries to estimate a station-level differential path. The modified geometrical approach (MGA) is a simple method that takes advantage of a very long horizontal path to retrieve mixing ratios in the range of a few pptv. The methodology is presented, and the possible limitations are discussed. Multi-axis differential optical absorption spectroscopy (MAX-DOAS) high-mountain measurements recorded at the Izaña observatory (28° 18' N, 16° 29' W) are used in this study. The results show that under low aerosol loading, O3 and NO2 mixing ratios can be retrieved even at very low concentrations. The obtained mixing ratios are compared with those provided by in situ instrumentation at the observatory. The MGA reproduces the O3 mixing ratio measured by the in situ instrumentation with a difference of 28%. The different air masses scanned by each instrument are identified as a cause of the discrepancy between the O3 observed by MAX-DOAS and the in situ measurements. The NO2 is in the range of 20–40 ppt, which is below the detection limit of the in situ instrumentation, but it is in agreement with measurements from previous studies for similar conditions.

Ozone (O3) reduction caused by dust particles is a challenging topic that has not been sufficiently investigated. Previous studies have provided limited quantification of O3, mainly based on case studies or short-term campaigns. In this paper, 157 O3 vertical profiles from ozone soundings launched from the Canary Islands over 13 summers are analyzed. Using aerosol optical depth as a discrimination tool, the data are classified into two groups: clean atmospheric conditions and dusty conditions under the presence of the Saharan Air Layer (SAL). The results show that ozone mixing ratios on SAL days are systematically lower than those during clean conditions, with a difference of as much as 35%. An analysis of independent total ozone columns from ground-based and satellite spectrometers confirm the observed O3 reductions. In addition, a vertical redistribution of the total water content takes place during these events. A comparison of the vertical shape of the O3 reductions with the vertical distribution of dust loading and precipitable water vapor shows that although O3 reductions occur in parts of the layer where the dust is present, a better correlation is found with the total amount of water present in the air. This finding suggests that the latter component plays a role in the observed O3 reduction within the SAL.

[1] This paper presents a comparative study of shortwave downward radiation (SDR) measurements and simulations, obtained with the radiative transfer model LibRadtran, at the Baseline Surface Radiation Network (BSRN) site of Izana Atmospheric Observatory (IZA, Spain). The analysis is based on cloud-free days between March 2009 and August 2012 (386 days), including aerosol-free and Saharan mostly pure mineral dust conditions and comparing the day-to-day, annual, and interannual variability. The observed agreement between simulations and measurements is excellent: the variance of daily measurements overall agrees within 99% with the variance of daily simulations, and the mean bias (simulations−measurements) is −0.30±0.24 MJm−2(−1.1±0.9%) for global, −0.16±0.34 MJm−2(−0.4±0.9%) for direct, and +0.02±0.25 MJm−2(+0.9±9.2%) for diffuse SDR. Furthermore, the diurnally averaged aerosol radiative forcing (ΔDF) and radiative forcing efficiency (ΔDFeff) due to Saharan mostly pure mineral dust events has been computed at Izana Observatory. The mean ΔDF values are −7±1, −96±5, and 44±2 Wm−2 for global, direct, and diffuse BSRN SDR, respectively (mean aerosol optical depth, AOD, at 500 nm of 0.18±0.01), whereas the mean ΔDFeffvalues are −59±6, −495±11, and 230±8 Wm−2 per unit of AOD at 500 nm for global, direct, and diffuse BSRN SDR, respectively. These values highlight the importance of scattering processes for mineral dust aerosols: the ratio between ΔDF and the corresponding SDR without aerosols is ~2.5% for diffuse SDR versus 0.2% for direct SDR. This illustrates the significant potential of mineral dust particles to cool the Earth-atmosphere system.

In this paper we present the preliminary results of atmospheric column-integrated precipitable water vapor (PWV) obtained with a new Lunar Cimel photometer (LC) at the high mountain Izaña Observatory in the period July–August 2011. We have compared quasi-simultaneous nocturnal PWV from LC with PWV from a Global Positioning System (GPS) receiver and nighttime radiosondes (RS92). LC data have been calibrated using the Lunar Langley method (LLM). We complemented this comparative study using quasi-simultaneous daytime PWV from Cimel AERONET (CA), GPS and RS92. Comparison of daytime PWV from CA shows differences between GPS and RS92 up to 0.18 cm. Two different filters, with and approximate bandwidth of 10 nm and central wavelengths at 938 nm (Filter#1) and 937 nm (Filter#2), were mounted onto the LC. Filter#1 is currently used in operational AERONET sun photometers. PWV obtained with LC-Filter#1 showed an overestimation above 0.18 and 0.25 cm compared to GPS and RS92, respectively, and root-mean-square errors (RMSEs) up to 0.27 cm and 0.24 cm, respectively. Filter#2, with a reduced out-of-band radiation, showed very low differences compared with the same references (≤ 0.05 cm) and RMSE values ≤ 0.08 cm in the case of GPS precise orbits. These results demonstrate the ability of the new lunar photometer to obtain accurate and continuous PWV measurements at night, and the remarkable influence of the filter's transmissivity response to PWV determination at nighttime. The use of enhanced bandpass filters in lunar photometry, which is affected by more important inaccuracies than sun photometry, is necessary to infer PWV with similar precision to AERONET.

Longer-term (i.e., 20–40 years) tropospheric ozone (O3) time series obtained from surface and ozonesonde observations have been analyzed to assess possible changes with time through 2010. The time series have been selected to reflect relatively broad geographic regions and where possible minimize local scale influences, generally avoiding sites close to larger urban areas. Several approaches have been used to describe the changes with time, including application of a time series model, running 15-year trends, and changes in the distribution by month in the O3 mixing ratio. Changes have been investigated utilizing monthly averages, as well as exposure metrics that focus on specific parts of the distribution of hourly average concentrations (e.g., low-, mid-, and high-level concentration ranges). Many of the longer time series (∼30 years) in mid-latitudes of the Northern Hemisphere, including those in Japan, show a pattern of significant increase in the earlier portion of the record, with a flattening over the last 10–15 years. It is uncertain if the flattening of the O3 change over Japan reflects the impact of O3 transported from continental East Asia in light of reported O3 increases in China. In the Canadian Arctic, declines from the beginning of the ozonesonde record in 1980 have mostly rebounded with little overall change over the period of record. The limited data in the tropical Pacific suggest very little change over the entire record. In the southern hemisphere subtropics and mid-latitudes, the significant increase observed in the early part of the record has leveled off in the most recent decade. At the South Pole, a decline observed during the first half of the 35-year record has reversed, and O3 has recovered to levels similar to the beginning of the record. Our understanding of the causes of the longer-term changes is limited, although it appears that in the mid-latitudes of the northern hemisphere, controls on O3 precursors have likely been a factor in the leveling off or decline from earlier O3 increases.

An analysis of the 22-yr ozone (O3) series (1988–2009) at the subtropical high mountain Izaña~station (IZO; 2373 m a.s.l.), representative of free troposphere (FT) conditions, is presented. Diurnal and seasonal O3 variations as well as the O3 trend (0.19 ± 0.05 % yr−1 or 0.09 ppbv yr−1), are assessed. A climatology of O3 transport pathways using backward trajectories shows that higher O3 values are associated with air masses travelling above 4 km altitude from North America and North Atlantic Ocean, while low O3 is transported from the Saharan continental boundary layer (CBL). O3 data have been compared with PM10, 210Pb, 7Be, potential vorticity (PV) and carbon monoxide (CO). A clear negative logarithmic relationship was observed between PM10 and surface O3 for all seasons. A similar relationship was found between O3 and 210Pb. The highest daily O3 values (90th percentile) are observed in spring and in the first half of summer time. A positive correlation between O3 and PV, and between O3 and 7Be is found throughout the year, indicating that relatively high surface O3 values at IZO originate from the middle and upper troposphere. We find a good correlation between O3 and CO in winter, supporting the hypothesis of long-range transport of photochemically generated O3 from North America. Aged air masses, in combination with sporadic inputs from the upper troposphere, are observed in spring, summer and autumn. In summer time high O3 values seem to be the result of stratosphere-to-troposphere (STT) exchange processes in regions neighbouring the Canary Islands. Since 1995–1996, the North Atlantic Oscillation has changed from a predominantly high positive phase to alternating between negative, neutral or positive phases. This change results in an increased flow of the westerlies in the mid-latitude and subtropical North Atlantic, thus favouring the transport of O3 and its precursors from North America, and a higher frequency of storms over North Atlantic, with a likely higher incidence of STT processes in mid-latitudes. These processes lead to an increase of tropospheric O3 in the subtropical North Atlantic region after 1996 that has been reflected in surface O3 records at IZO.

A study of the vertical mass impact of Saharan dust intrusions is presented in this work. Simultaneous ground-based remote-sensing and airborne in-situ measurements performed during the AMISOC-TNF campaign over the Tenerife area (Canary Islands) in summertime from 01 July to 11 August 2013 were used for that purpose. A particular dusty (DD) case, associated to a progressively arriving dust intrusion lasting for two days on 31 July (weak incidence) and 01 August (strong incidence), is especially investigated. AERONET AOD and AEx values were ranging, respectively, from 0.2 to 1.4 and 0.35 to 0.05 along these two days. Vertical particle size distributions within fine and coarse modes (0.16–2.8 μm range) were obtained from aircraft aerosol spectrometer measurements. Extinction profiles and Lidar Ratio (LR) values were derived from MPLNET/Micro Pulse Lidar observations. MAXDOAS measurements were also used to retrieve the height-resolved aerosol extinction for evaluation purposes in comparison to Lidar-derived profiles. The synergy between Lidar observations and airborne measurements is established in terms of the Mass Extinction Efficiency (MEE) to calculate the vertical mass concentration of Saharan dust particles. Both the optical and microphysical profilings show dust particles mostly confined in a layer of 4.3 km thickness from 1.7 to 6 km height. LR ranged between 50 and 55 sr, typical values for Saharan dust particles. In addition, this 2-day dust event mostly affected the Free Troposphere (FT), being less intense in the Boundary Layer (BL). In particular, rather high Total Mass Concentrations (TMC) were found on the stronger DD day (01 August 2013): 124, 70 and 21 μg m −3 were estimated, respectively, at FT and BL altitudes and on the near-surface level. This dust impact was enhanced due to the increase of large particles affecting the FT, but also the BL, likely due to their gravitational settling. However, the use of an assumed averaged MEE value can be especially critical for estimating the mass concentration of particular layers. Moreover, the potential of MAXDOAS retrieval for aerosol extinction profiling is also evidenced by showing a relatively good agreement with the Lidar-derived extinction profiles, once a particular smoothing procedure is applied to Lidar measurements.

Iodine monoxide (IO) differential slant column densities (DSCD) have been retrieved from a new multi-axis differential optical absorption spectroscopy (MAX-DOAS) instrument deployed at the Izaña subtropical observatory as part of the Network for the Detection of Atmospheric Composition Change (NDACC) programme. The station is located at 2370 m a.s.l., well above the trade wind inversion that limits the top of the marine boundary layer, and hence is representative of the free troposphere. We report daily observations from May to August 2010 at different viewing angles. During this period, the spectral signature of IO was unequivocally detected on every day of measurement. A mean IO DSCD of 1.52×1013 molecules cm−2 was observed at the 5° instrument elevation angle (IEA) on clear days using a single zenith reference for the reported period, with a day-to-day variability of 33% at one standard deviation. Based on the simulation of the DSCDs using radiative transfer calculations with five different hypothesized IO profiles, the IO mixing ratio is estimated to range between 0.2 and 0.4 pptv in the free troposphere. Episodes of Saharan dust outbreaks were also observed, with large increases in the DSCDs at higher IEA, suggesting an enhancement of IO inside the dust cloud.

Temperature variation is studied at different altitudes and orientation on the island of Tenerife, according to the trends in the mean, maximum and minimum at 21 meteorological stations. Reference series are obtained by sectors, along with a representative overall series for Tenerife, in which temperature shows a statistically significant growth trend of 0.09 ± 0.04°C/decade since 1944. Night-time temperatures have risen most (0.17°C ± 0.04°C/decade), while by day they have been more stable. Consequently, the diurnal temperature range between day and night has narrowed. By regions, warming has been much more intense in the high mountains than the other sectors below the inversion layer between 600 and 1,400 m altitude, and progressively milder towards the coast. The temperature rise on the windward (north-northeast) slopes is greater than on the leeward side and could be related to the increase in cloudiness on the northern side. The general warming of the island is less than in continental areas at between 24 and 44oN, being closer to the sea surface temperature in the same area. This is probably explained largely by the insular conditions. In fact warming is more evident in the high mountains (0.14 ± 0.07°C/decade), where the tempering effect of the ocean and the impact of changes in the stratocumulus is weaker, being similar to the mean continental values in the northern hemisphere.

High altitude mountaintop observatories provide the opportunity to study aerosol properties in the free troposphere without the added expense and difficulty of making airborne measurements. Climatologies for free tropospheric aerosol radiative properties in cloud-free air, including light scattering, light absorption, light extinction, single scattering albedo, Angstrom exponent, hemispheric backscatter fraction and radiative forcing efficiency, from twelve high altitude (2.2–5.1 km) measurement platforms are presented at low relative humidity and at standard temperature and pressure. These climatologies utilize data from ten mountaintop observatories in the 20–50oN latitude band: Mauna Loa, USA; Lulin Mountain, Taiwan; Nepal Climate Observatory — Pyramid; Izana, Spain; Mount Waliguan, China; Beo Moussala, Bulgaria; Mount Bachelor, USA; Monte Cimone, Italy; Jungfraujoch, Switzerland; Whistler Mountain, Canada. Results are also included from two multi-year, in-situ aerosol vertical profiling programs: Southern Great Plains, USA and Bondville, USA. The amount of light absorption and scattering observed at these high altitude sites either peaks in the spring or it has a broad spring to summer enhancement. The seasonal variation of the aerosol single scattering albedo, backscatter fraction and Angstrom exponent changes from site to site but the timing can be related to aerosol sources and transport processes known to impact the individual sites. The seasonal variation of in-situ aerosol light extinction from these high altitude measurements is in excellent agreement with extinction values derived from CALIPSO lidar measurements. Analysis of the systematic variability among in-situ aerosol properties shows that these relationships can be used to infer aerosol types. In particular, the relationship between single scattering albedo and Angstrom exponent can indicate the presence of dust aerosol. Radiative forcing efficiency (RFE = aerosol forcing/aerosol optical depth) is used to assess the importance of single scattering albedo and backscatter fraction on aerosol forcing by eliminating aerosol amount (i.e., aerosol optical depth) from the calculation. Variability in monthly cycles of RFE corresponds with changes in single scattering albedo and hemispheric backscatter fraction. Utilizing site-specific, climatological values of single scattering albedo and backscatter fraction to calculate RFE results in departures from the monthly median values of RFE typically in the range 10–30%. The greatest discrepancy occurs for months with low aerosol loading where the observed variability of single scattering albedo and backscatter fraction is the greatest. At most sites the radiative forcing efficiency at low aerosol loading (light scattering −1 ) is slightly less negative (more warming) than at higher aerosol loading.

Synoptic geopotential height anomalies patterns favouring African dust outbreaks into the marine boundary layer (MBL) of the subtropical Eastern North Atlantic Region (SENAR) were objectively identified. The proportion of the total variance explained by each of these patterns was also calculated. Dust intrusions into the MBL of the SENAR were identified using total suspended particles (TSP) data at a rural background station in Tenerife Island (El Rio station, ER). Geopotential height anomalies at 1,000, 850, 700 and 500 hPa, respectively, in days of African dust intrusion in the period 1998–2003 were grouped in monthly sets. Two different but complementary methods (K-means and Principal Components) were applied to daily geopotential height anomalies for each month and for each pressure level in case of African dust intrusion. Three principal geopotential height anomalies patterns were found. Type I consist on a high-pressure system over Europe that affects North Africa, occasionally giving rise to a ridge. The Canary Islands are in the south-west flank of this high-pressure system. This pattern is dominant throughout the whole year. Type II and type III patterns consist on a low located to the northeast and southeast of the Canary Islands, respectively, coupled with a high over the Mediterranean basin and/or North Africa. Two case analyses are presented, as well as a systematic validation of the meteorological pattern classification for all dust intrusions detected at ER station within the period 2004–2007.

African dust intrusions in the marine mixing layer of the Eastern North Atlantic subtropical region (23.5°N to 35°N) are favoured in winter when the eastern edge of the Azores High covers Southwestern Europe and North Africa. In situ ground pressure observations and reanalysis from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and European Centre for Medium-Range Weather Forecasts (ECMWF) indicate that the Azores High has strengthened and shifted eastward in winter over the last three decades. This is evidenced by the increase over time of the Madrid–Tenerife Index which is defined as the geopotential height anomaly difference at 1000 mb between Tenerife (28.5°N; 16.3°W) and Madrid (40.5°N; 3.5°W) in winter and of the African Index which is defined as the residence time over Africa of air mass trajectories entering the subtropical Eastern North Atlantic Ocean. Barcelona Supercomputing Center/Dust Regional Atmospheric Model (BSC-DREAM) dust regional model simulations from 1958 to 2006 were performed, assuming that the soil characteristics of dust sources remained unchanged over time. Simulated winter dust concentration levels are well correlated (0.67) with the available background observations for the 1998–2004 period. The model results show a two-fold increase in winter dust concentrations over the 1980–2006 period with respect to the 1958–1979 period, corresponding to the strengthening and eastward shift of the Azores High. DOI: 10.1111/j.1600-0889.2010.00524.x

Emissions of sulphur hexafluoride (SF6), one of the strongest greenhouse gases on a per molecule basis, are targeted to be collectively reduced under the Kyoto Protocol. Because of its long atmospheric lifetime (estimated as 800 to 3200 years), the accumulation of SF6 in the atmosphere is a direct measure of its global emissions. Examination of our extended data set of globally distributed high-precision SF6 observations shows an increase in SF6 abundance from near zero in the 1970s to a global mean of 6.7 ppt by the end of 2008. In-depth evaluation of our long-term data records shows that the global source of SF6 decreased after 1995, most likely due to SF6 emission reductions in industrialised countries, but increased again after 1998. By subtracting those emissions reported by Annex I countries to the United Nations Framework Convention of Climatic Change (UNFCCC) from our observation-inferred SF6 source leaves a surprisingly large gap of more than 70–80% of non-reported SF6 emissions in the last decade. This suggests a strong under-estimation of emissions in Annex I countries and underlines the urgent need for independent atmospheric verification of greenhouse gases emissions accounting.

We provide an atmospheric aerosol characterization for North Africa, Northeastern Atlantic, Mediterranean and Middle East based on the analysis of quality-assured direct-sun observations of 39 stations of the AErosol RObotic NETwork (AERONET) which include at least an annual cycle within the 1994–2007 period. We extensively test and apply the recently introduced graphical method of Gobbi and co-authors to track and discriminate different aerosol types and quantify the contribution of mineral dust. The method relies on the combined analysis of the Ångström exponent (α) and its spectral curvature δα. Plotting data in these coordinates allows to infer aerosol fine mode radius (Rf) and fractional contribution (η) to total Aerosol Optical Depth (AOD) and separate AOD growth due to fine-mode aerosol humidification and/or coagulation from AOD growth due to the increase in coarse particles or cloud contamination. Our results confirm the robustness of this graphical method. Large mineral dust is found to be the most important constituent in Northern Africa and Middle East. Under specific meteorological conditions, its transport to Southern Europe is observed from spring to autumn and decreasing with latitude. We observe "pure Saharan dust" conditions to show AOD>0.7 (ranging up to 5), α1.5 and δα~−0.2 corresponding to η>70% and Rf~0.13 μm. Here, dust mixed with fine pollution aerosols shifts the observations to the region α

We compare the currently most precise, ground-based total O3 measurement techniques: Brewer and FTIR. We give an overview of the similarities and the differences between the measurements and the retrieval approaches of both experiments. We compare coincident measurements performed at the Atmospheric Observatory of Izaña from 2005 to 2007 and demonstrate that, if the properties of the instruments are well characterised, the scatter between both experiments is as small as 0.5%. This is in agreement with the theoretical predictions and confirms empirically that both techniques are able to monitor total O3 amounts with a precision of better than 0.4%. However, we found systematic differences between both techniques of around 4.5%, which we think are mainly due to discrepancies between the applied UV and infrared spectroscopic parameters.

Tenerife (Canary Islands) is often affected by Saharan mineral dust outbreaks. These events result in high PM10 concentrations in ambient air, well above the limiting values adopted by the European Union (Directive EU/1999/30). To comply with the EU Air Quality Directive, a quantitative proxy for the characterisation of high PM10 events is required. Furthermore, this proxy must be capable of differentiating natural sources (as defined by the Directive), such as Saharan mineral dust outbreaks, from other anthropogenic sources. In this study, we have analysed the time series of 137Cs, 40K and PM10 recorded at the island of Tenerife (2000–2006) in the Marine Boundary Layer (MBL), to test the possible usefulness of the two mentioned radiotracers as markers of Saharan mineral dust events. The results of the analysis showed that this is indeed the case for 137Cs and that there are, at least, two different sources of 40K concentrations in the collected atmospheric aerosol filters. While the detection of 137Cs in atmospheric aerosol filters could be directly associated with the arrival of low altitude ( 2000 m.a.s.l) dust intrusions. Moreover, two different mechanisms of aerosol loading in the MBL were, also, identified with the mentioned radiotracers. Good correlations (R2 > 0.6) were found between the three parameters when all the compiled data was considered. The correlation values increased to nearly 0.8 when only those samples that had 137Cs above detection limits were considered. The highest concentrations of 137Cs, 40K and PM10 were recorded in March 2004 in connection with a very intense low altitude Saharan dust intrusion. The results of this study, also, indicated that the 137Cs/40K ratio could be potentially useful to differentiate low from high altitude dust intrusions at this site.

The physical characterisation of metrics representative of ambient air particle concentration is becoming a topic of great interest for urban air quality monitoring and human exposure assessment. In this article, the influence of sea breeze circulation and primary road traffic emissions on the relationship between the urban aerosol number (N3, particles >3 nm), black carbon

Temporal series of atmospheric PM 1 and PM 10 matter (particulate matter with aerodynamic diameter below 1 and 10 μm) as well as 40 K and 7 Be concentrations in aerosol filters, collected in the Island of Tenerife from 30 June 2003 until 17 January 2005, were analysed here to: (1) study the variability of 7 Be in the atmosphere at this site and (2) to identify the origin of high 7 Be events and check their possible connection with the transport of Saharan dust to Tenerife. A complex relationship was observed between PM 10 matter and 7 Be concentrations in the measured aerosol filters. Due to this fact, the analysed atmospheric events had to be grouped in: (I) high 7 Be and high PM 10 matter events; (II) low 7 Be and high PM 10 matter events; and (III) high 7 Be and low PM 10 matter events. The low 7 Be and low PM 10 matter events were not considered in this study. The results indicated that four times during the study period significant concentrations of 7 Be were observed with the arrival of suspended material from the African continent. An enhanced scavenging effect of the 7 Be available in the aerosol transport zone by the re-suspended material is believed to have caused these detected increments in local atmospheric concentrations. Seventy-two hours forecasts created with the Dust Regional Atmospheric model (DREAM model) for these events, also, seem to support this hypothesis.

On 28–29 November 2005 an extratropical storm affected the Canary Islands causing significant damage related to high average wind speeds and intense gusts over some islands of the archipelago. Delta was the twenty-sixth tropical or subtropical storm of the 2005 Atlantic hurricane season. It represents an unusual meteorological phenomenon for that region, and its impacts were underestimated by the different operational meteorological forecasts during the previous days of the arrival of the low near Canary Islands. The aim of this study is to reproduce the local effects of the flow that were observed over the Canary Islands during the travel of the Delta storm near the region using high-resolution mesoscale meteorological simulations. The Advanced Research Weather Research & Forecasting Model (WRF-ARW) is applied at 9, 3 and 1 km horizontal resolution using ECMWF forecasts as initial and boundary conditions. The high-resolution simulation will outline the main features that contributed to the high wind speeds observed in the archipelago. Variations in vertical static stability, vertical windshear and the intense synoptic winds of the southwestern part of Delta with a warm core at 850 hPa were the main characteristics that contributed to the development and amplification of intense gravity waves while the large-scale flow interacted with the complex topography of the islands.

Daily NO2 vertical column density (VCD) has been routinely measured by zenith sky spectroscopy at the subtropical station of Izaña (28° N, 16° W) since 1993 in the framework of the Network for the Detection of Atmospheric Composition Change (NDACC). Based on 14 years of data the first low latitude NO2 VCD climatology has been established and the main characteristics from short timescales of one day to interannual variability are presented. Instrumental descriptions and different sources of errors are described in detail. The observed diurnal cycle follows that expected by gas-phase NOx chemistry, as can be shown by the good agreement with a vertically integrated chemical box model, and is modulated by solar radiation. The seasonal evolution departs from the phase of the hours of daylight, indicating the signature of upper stratospheric temperature changes. From the data record (1993–2006) no significant long-term trends in NO2 VCD can be inferred. Comparison of the ground-based data sets with nadir-viewing satellite spectrometers shows excellent agreement for SCIAMACHY with differences between both datasets of 1.1%. GOME displays unrealistic features with the largest discrepancies during summer. The ground-based data are compared with long-term output of the SLIMCAT 3-D chemical transport model (CTM). The basic model, forced by ECMWF (ERA-40) analyses, captures the observed NO2 annual cycle but significantly underestimates the spring/summer maximum (by 12% at sunset and up to 25% at sunrise). In a model run which uses assimilation of satellite CH4 profiles to constrain the model long-lived tracers the agreement is significantly improved. This improvement in modelled column NO2 is due to better modelled NOy profiles and points to transport errors in the ECMWF ERA-40 reanalyses.

In an urban site affected by fresh vehicle exhaust emissions, the ambient air number concentrations of particles coarser than 3 nm (N) was split into two components, N = N 1 + N 2 . This was done using a method based on the high correlation between black-carbon (BC) and number (N) concentrations which is typically observed in ambient air and is the result of vehicle exhaust emissions. The component N1 accounts for “those aerosol components directly emitted in the particle phase” and “those components nucleating immediately after emission”. The component N2 accounts for the new particle formation enhancements during the “dilution and cooling of the vehicle exhaust” and is also influenced by “in situ new particle formation in ambient air”. The contribution of N1 to N exhibits a maximum of 55% during the morning rush hours (07:00–08:00). The contribution of N2 to N exhibits a daily evolution with a broad maximum during daylight (as solar radiation intensity), while for about 7 h (11:00–17:00) the N2 contribution to N is about 70%. During some “afternoon N2 events”, N2 contributions exceeded 90%. Enhancements in the new particle formation processes may increase the N/BC concentrations ratio in one order of magnitude, from 4.82 × 10 6 particles/ng BC to 47 × 10 6 particles/ng BC and during some events up to 97 × 10 6 particles/ng BC. The results show evidence of the high potential of the vehicle exhausts and of the urban atmosphere to trigger new particle formation if the ambient air conditions are favourable. The method used in this study is useful in assessing future changes in the number to BC relationship due to forthcoming regulations in the vehicle exhaust emissions.

Samples from two strong homogeneous dust plumes from the Saharan desert reaching Izana (Tenerife, Spain) in July and August 2005 were taken with a miniature impactor system and filter samplers. Size, aspect ratio and chemical composition of more than 22,000 individual particles were studied by scanning electron microscopy. The mineralogical phase composition of about 200 particles was investigated by transmission electron microscopy. In addition, the aerosol size distribution was measured with an optical particle spectrometer. In all samples, the aerosol was dominated by mineral dust with an average composition (by volume) of 64% silicates, 6% quartz, 5% calcium-rich particles, 14% sulfates, 1% hematite, 1% soot and 9% other carbonaceous material. Sulfate was found predominantly as coating on other particles with an average thickness of approximately 60 nm. The aerosol calcium content is correlated with the calcite concentrations of soils in the source region, highest values were observed for northern and central Algeria and Morocco. The average aspect ratio of the particles was 1.64. The distributions of the aspect ratios are parameterized by log-normal functions for modeling purpose. Single-scattering albedo (0.95) and asymmetry factor (0.74–0.81) was measured by polar aerosol photometry on filter samples using a light source resembling the solar spectrum. The apparent soot content of the sample (1 vol%) was determined by the same technique. From the mineralogical data, an average complex refractive index of 1.59–9×10 −3 i for visible light was derived. The imaginary part of the complex refractive index decreases with increasing particle size from −2.5×10 −2 i to −3 i, reflecting the decreasing hematite and soot contents. The imaginary part derived from optical measurements was −7×10 −3 i.

Six years (1998–2003) of measurements of ambient air concentrations of total suspended particulate (TSP) measured at a rural background monitoring station in Tenerife (Canary Islands), the El Rio station (ER, 28°08′35″N, 16°39′20″W, 500 m a.s.l.) were studied. African dust outbreaks were objectively identified using a new quantitative tool, called the African Index. This index indicates the percentage of time that an air mass remained over an African region at one of three possible height intervals of the lower troposphere. After identifying these episodes, a study of the background TSP levels at the ER station and of direct and indirect (those which cause vertical deposition of dust) African air mass intrusion impacts was performed. Taking into account both direct and indirect episodes, a total of 322 days of African dust intrusion were objectively identified (a mean of 54 episodes per year) in the period 1998–2003, some of them caused by “transition episodes” or “return African air masses”. A subjective method confirmed that 256 of these days were caused by direct impacts of African dust on the ER station. A mean TSP value of 21.6 μg m−3 was found at the station during this period. All the episodes occurred when the TSP concentration was >28.5 μg m−3. The TSP background (∼14 μg m−3) can be assumed to be representative of the MBL of the Eastern North Atlantic subtropical region. The highest number of dust gravitational settlement (or indirect) episodes occurs in summer, but the highest contribution of these episodes to the TSP levels is in March with a monthly mean TSP contribution of up to 30.5 μg m−3.

The origin of the daily exceedances of 50 μg PM10 m−3 (daily limit value or DLV of the EU air quality directive) and of an arbitrary daily value (DV) 35 μg PM2.5 m−3 recorded in 2001–2003 in 13 regional background stations of the Iberian Peninsula were interpreted. This was carried out by means of back-trajectory analysis, available PM model outputs, satellite data and meteorological maps. This allows the detection of high PM episodes on a regional scale and the study of their seasonal and geographical variability. The number of exceedances of the PM10 DLV ranged in 2001–2003 from 6 to 41 depending on the monitoring site. For the selected PM2.5 DV, the range of daily exceedances was 0–10 in the study period. The majority of the PM10 (>70% in most stations) and PM2.5 (17–55% in most stations) exceedances in regional background monitoring stations are caused by African dust outbreaks. These exceedances were less frequent in winter than in summer due to: (a) the frequent long range transport of dust in the warm seasons over Iberia, (b) the re-suspension associated with convective atmospheric dynamics, and (c) the relative low rainfall favouring re-suspension and high residence time of PM. Moreover, a regional contribution of secondary aerosols derived from the efficient photochemical transformation of gaseous precursors may coincide with African transport in summer. Episodes with lack of advective conditions caused 2–29% and 20–50% of the PM10 and PM2.5 exceedances. These occurred mainly in summer due to poor renovation of air masses, increased convective re-suspension, dispersion of pollutants towards rural areas and regional re-circulation and aging of air masses which result in the proliferation of secondary inorganic species. Long-range transport of PM from continental Europe caused exceedances (9–40% and 18–38% of the PM10 and PM2.5 exceedances, respectively), only in northern Iberia because, as the European air masses evolve towards the south, the pollutants suffer dispersion/dilution. Local exceedances are associated with the advection of the clean Atlantic air masses, which cannot increase PM levels to a great extent without the influence of a local source of PM. The proportion of local exceedances of PM10 and PM2.5 ranged 6–33% and 17–40%, respectively.

We have studied the relationship between the long-term interannual variability in large-scale meteorology in western North Africa – the largest and most active dust source worldwide – and Saharan dust export in summer, when enhanced dust mobilization in the hyper-arid Sahara results in maximum dust impacts throughout the North Atlantic. We address this issue by analyzing 28 years (1987–2014) of summer averaged dust concentrations at the high-altitude Izaña observatory (~ 2400 m a.s.l.) on Tenerife, and satellite and meteorological reanalysis data. The summer meteorological scenario in North Africa (aloft 850 hPa) is characterized by a high over the the subtropical Sahara and a low over the tropics linked to the monsoon. We measured the variability of this high–low dipole-like pattern in terms of the North African dipole intensity (NAFDI): the difference of geopotential height anomalies averaged over the subtropics (30–32° N, Morocco) and the tropics (10–13° N, Bamako region) close to the Atlantic coast (at 5–8° W). We focused on the 700 hPa standard level due to dust export off the coast of North Africa tending to occur between 1 and 5 km a.s.l. Variability in the NAFDI is associated with displacements of the North African anticyclone over the Sahara and this has implications for wind and dust export. The correlations we found between the 1987–2014 summer mean of NAFDI with dust at Izaña, satellite dust observations and meteorological re-analysis data indicate that increases in the NAFDI (i) result in higher wind speeds at the north of the Inter-Tropical Convergence Zone that are associated with enhanced dust export over the subtropical North Atlantic, (ii) influence the long-term variability of the size distribution of exported dust particles (increasing the load of coarse dust) and (iii) are associated with enhanced rains in the tropical and northern shifts of the tropical rain band that may affect the southern Sahel. Interannual variability in NAFDI is also connected to spatial distribution of dust over the North Atlantic; high NAFDI summers are associated with major dust export (linked to winds) in the subtropics and minor dust loads in the tropics (linked to higher rainfall), and vice versa. The evolution of the summer NAFDI values since 1950 to the present day shows connections to climatic variability (through the Sahelian drought, ENSO (El Niño–Southern Oscillation) and winds) that have implications for dust export paths. Efforts to anticipate how dust export may evolve in future decades will require a better understanding of how the large-scale meteorological systems represented by the NAFD will evolve. The Izaña GAW program is funded by AEMET and by the Minister of Economy and Competitiveness of Spain (POLLINDUST, CGL2011-26259) J. M. Prospero’s research is supported by NSF grant AGS-0962256 5.114 JCR (2015) Q1, 6/84 Meteorology and atmospheric sciences UEC

A strong African dust outbreak episode affecting the Canary Islands from 28 to 31/07/02 has been characterised at the Izana Observatory (IZO), located in the free troposphere (FT), and at Sta. Cruz de Tenerife (SCO), in the Marine Boundary Layer (MBL). The Saharan air mass intruded above the trade wind inversion layer resulting in daily mean PM levels of up to 616, 312, 98 and 26 μg m−3 of TSP, PM10, PM2.5 and PM1, respectively, at IZO. As demonstrated by the vertical sounding profiles, the MBL is compressed during the Saharan intrusion, favouring the deposition of coarse dust particles to the MBL and giving rise to daily levels of up to 85, 52 and 30 μg m−3 of PM10, PM2.5 and PM1, respectively, at SCO. A complete chemical and mineralogical characterisation has been performed for TSP and PM2.5 collected simultaneously at both sites. Levels of mineral elements increased during the Saharan episode at both sampling sites. In addition, at the MBL, the levels of the secondary inorganic aerosols registered during the Saharan episode were considerably higher than levels recorded during non-Saharan episodes. The partial formation of secondary coarse Ca and/or Na sulphates and nitrates by the reaction of gaseous pollutants (or derived oxidised phases), of a major local origin, with the natural aerosols has been deduced. These reactions may be favoured by the high concentration of coarse mineral and marine aerosols particles measured at SCO, the high relative humidity (RH) measured, as well as the strong compression of the MBL during the Saharan episode.

The influence of two intensive low-altitude atmospheric-dust intrusions on the activity levels of 137Cs and 40K as well as atmospheric particle matter (PM10) concentrations in the lower atmosphere of the Canary Islands are analysed here. These two events took place at the beginning of January 2002 and March 2004, respectively. 3D atmospheric back-trajectories indicated that the main source of dust material involved in the considered atmospheric intrusions came from NW Africa. A consequence of these dust intrusions was the major increase of PM10 concentrations in the lower atmosphere. Both episodes were characterised by having weekly averages of PM10 concentration surpassing 150 μg m−3, higher than the daily PM10 limit value established by the EC/1999/30 directive for PM10 from 2005. Similarly, during these two events, both 137Cs and 40K activities increased by a factor of 6 and 13 as well as 13 and 14, respectively, over the basal values calculated for each radionuclide and time period (0.59±0.02 and 0.88±0.07 μBq m−3 as well as 12±6 and 24±8 μBq m−3).

In this study we analysed two time series of surface ozone recorded in Tenerife: one in a marine environment (Aguere, in the marine boundary layer at 580 m.a.s.l.) and other in the free troposphere (Izana, above the trade wind inversion layer at 2367 m.a.s.l.). We performed a systematic 4-year analysis of ozone, backtrajectories and synoptic charts. The transport pathways of ozone for different O3 percentile (Pth) ranges were determined. At both sites, low ozone levels ( 90 Pth) occur from March to June, and are mainly associated with a subsiding transport pathway from the western side of lows developing over Western/Central Europe and the North Atlantic mid-latitudes. At Izana, high ozone events (>90 Pth) occur from April to August, and are associated with subsidence at the western edge of lows over the North Atlantic (very frequently at the W or SW of the Iberian Peninsula). The results of this study highlight the importance of the downward transport of ozone. High ozone events under meteorological scenarios classically associated with stratospheric intrusions have been recorded every year at the two sites. However, with our results we cannot ultimately determine if these high ozone events are caused by downward transport from the upper troposphere/lower stratosphere, or by lifting of continental ozone pollution and subsequent subsidence. The occurrence of high ozone events (>90 Pth) at Izana and not in Aguere during summer (when the trade wind inversion located between these sites reaches its strongest stage) is an evidence of the importance of the ozone downward supply from mid-upper troposphere in this region.