bluuebunny/crossref_metadata_2025 huggingface dataset:
Photochemical Activity and Solar Ultraviolet Radiation (PAUR) Modulation Factors: An overview of the project
Authors: Zerefos Christos S. , Kourtidis Kostas A. , Melas Dimitris , Balis Dimitris , Zanis Prodromos , Katsaros Loukas , Mantis Homer T. , Repapis C. , Isaksen Ivar , Sundet Jostein , Herman Jay , Bhartia P. K. , Calpini Bertrand
Published: October 2002

The Photochemical Activity and Ultraviolet Radiation (PAUR) II project (a continuation of an earlier PAUR I project) had the purpose of studying the interrelationships between changes in total ozone, tropospheric aerosols, UV radiation and photochemical activity. As part of PAUR II project, a campaign took place in Greece and Italy during May–June 1999, with the participation of 15 European and 3 American research institutions. A variety of radiation and gaseous and aerosol atmospheric composition and optical characteristics measurements were made during the campaign. Radiative transfer models and three‐dimensional (3‐D) regional chemistry transport models (CTM) were applied and compared to the available data set of PAUR II. The present overview paper gives an introduction to the project and to the meteorological and environmental conditions that prevailed and outlines some results that are extensively described in the subsequent papers which form this special section. The modulation of the UVB field in the presence of different types of aerosols, its transmittance and role in the photochemistry of the particular eastern Mediterranean environment is overviewed. Using a 3‐D CTM, it is shown that even a 50% reduction in Greek anthropogenic emissions has only a small effect in reducing the ozone levels over the eastern Mediterranean in summer. The environmental conditions, which prevailed during the PAUR I and PAUR II campaigns, offered cases of background conditions over the Aegean Sea as well as conditions with Saharan dust episodes and extremes in total ozone.
Measurements and modeling of photolysis rates during the Photochemical Activity and Ultraviolet Radiation (PAUR) II campaign
Authors: Balis D. S. , Zerefos C. S. , Kourtidis K. , Bais A. F. , Hofzumahaus A. , Kraus A. , Schmitt R. , Blumthaler M. , Gobbi G. P.
Published: October 2002

In this paper we compare radiative transfer model calculations of the actinic flux in the UV spectral region with airborne measurements of the actinic flux, obtained during the Photochemical Activity and Ultraviolet Radiation (PAUR) campaign, which took place in the Aegean Sea, Greece, in June 1996, in order to assess the accuracy of the model in calculating photolysis rates, when the model input parameters are well defined from measurements. The model can simulate the total actinic flux (4π sr) in the UV‐A region with an accuracy of 5% for all altitudes (0.1–12 km) in the cloud free troposphere, while in the UV‐B the impact of the vertical distribution of ozone and aerosol can lead to differences, of about 5–10% at low altitudes up to 20% at higher altitudes. Next, the photolysis rates of J(O1D) and J(NO2) measured during the PAUR 2, in Crete, Greece (35.5°N, 23.8°E), in May 1999, at two altitudes (Gerani 30 m and Prases 1000 m), are compared with the respective model calculations in order to examine the effect of the alternating Sahara dust/maritime aerosol environments imposed to these photolysis rates, as well as to examine their differences due to the altitude difference. It is shown that high levels of tropospheric ozone and absorbing aerosols can cause a decrease in the photolysis rates of ozone near the surface, even under conditions of reduced total ozone content. This fact indicates that tropospheric ozone can be disproportionately important as a filter against UV‐B radiation when most scattering of the radiation by air molecules and dust occurs in the troposphere. This behavior is not simulated accurately using a radiative transfer model constrained by observations of ozone and aerosol optical depths. The differences of the of the photolysis rates between the two altitudes as determined by the model and by the measurements, differ significantly for the Sahara dust event, indicating that during this event in the boundary layer, there is a mixture of desert and nonabsorbing aerosols.
A case study on the possible link between surface ozone photochemistry and total ozone column during the PAUR II experiment at Crete: Comparison of observations with box model calculations
Authors: Zanis P. , Kourtidis K. , Rappenglueck B. , Zerefos C. , Melas D. , Balis D. , Schmitt R. , Rapsomanikis S. , Fabian P.
Published: October 2002

A variety of surface chemical measurements, including surface ozone, NO, NO2, NOx, peroxyacetyl nitrate, and nonmethane hydrocarbon, were carried out at a rural coastal site, Nopigia, Crete, and at a rural elevated site, Prases, Crete, within the framework of the Photochemical Activity and Ultraviolet Radiation (PAUR) II experiment, which took place in Greece and Italy in early summer (1–31 May 1999). The total ozone exhibited very large fluctuations (larger than 90 Dobson units, 1 DU = 2.69 × 1016molecules cm−2) during the campaign with maximum values of around 400 DU during 10 May and minimum values of <300 DU during 20 May, while the total aerosol content exhibited a reverse fluctuation as the W‐NW flow on 10 May changed to SW flow on 20 May, transporting air from the Sahara. This special atmospheric situation with such distinct differences in total ozone column and total aerosol content between these 2 days provides the opportunity, on the one hand, to investigate the induced changes on local ozone production rates and, on the other hand, to assess the local photochemistry for different atmospheric conditions at both a high elevated site and a low elevated site. Box model calculations constrained by the measurements show that changes of the total ozone column and hence ofJ(O1D) can influence net ozone production rates at Nopigia but not at Prases due mainly to the differences in the NOxlevels between the two sites. The observed surface ozone concentrations at Nopigia display larger diurnal variations and higher ozone levels on 20 May compared to the 10 May, whereas at Prases, no such differences are observed in agreement with the model results. The 20% change inJ(O1D) due to the change in total ozone column can produce ∼1.3 ppbv of ozone from 0600 to 1200 UT at Nopigia. However, the concurrent effect of increasing absorbing aerosol (Sahara dust) content on 20 May may mask part of the increase inJ(O1D) and offsets part of the increase in local ozone production at Nopigia.
Effects of desert dust and ozone on the ultraviolet irradiance at the Mediterranean island of Lampedusa during PAUR II
Authors: di Sarra A. , Cacciani M. , Chamard P. , Cornwall C. , DeLuisi J. J. , Di Iorio T. , Disterhoft P. , Fiocco G. , Fuá D. , Monteleone F.
Published: October 2002

Observations of the ultraviolet spectral irradiance, tropospheric aerosols, and ozone were carried out at Lampedusa (35.5°N, 12.6°E), during May and June 1999, in conjunction with the Photochemical Activity and Ultraviolet Radiation modulating factors II experiment. Lampedusa is a small Italian island in the Mediterranean, about 100 km off the west coast of Tunisia, North Africa. During the campaign, aerosol transport from Sahara, mainly controlled by the development of high‐pressure systems over North Africa, strongly affected the troposphere at Lampedusa. Aerosol optical depths at 415 nm ranged between 0.14 and 0.91. Dust aerosols with an optical depth of 0.5 at 415 nm produced a reduction of the measured irradiance at 350 nm by about 20%, at a solar zenith angle of 30°, and by about 25% at 60°. The occurrence of large aerosol optical depths was associated with low total ozone values, mainly due to the influence of surface pressure on the stratospheric vertical structure and transport. Thus the aerosol attenuation of the UV irradiance was large when low absorption by ozone occurred, and a large ozone absorption occurred when aerosol attenuation was small. Mostly because of their large variability, aerosols produce larger attenuation effects than ozone on the UV irradiance for wavelengths longer than 315 nm. By neglecting the influence of aerosols on the radiation amplification factors, calculation of the dependence of the UV irradiance on ozone can be significantly underestimated. For the erythemally weighted irradiance a value for the radiation amplification factor between −0.1 and 0.7, depending on the solar zenith angle, has been derived. A simple model to take into account the aerosol effect in the determination of the radiation amplification factor has been developed. By applying this model an erythemal radiation amplification factor between 0.8 and 1.25 is obtained. It is estimated that an aerosol layer of optical depth 0.5 at 415 nm produces a reduction of the erythemal irradiance as large as 25 and 27% at solar zenith angles of 30° and 60°, respectively. When conditions similar to those of Lampedusa occur, i.e., when large aerosol loads may be present, and for certain meteorological conditions, the effects of the aerosol on the surface ultraviolet irradiance cannot be neglected.
Regional levels of ozone in the troposphere over eastern Mediterranean
Authors: Kourtidis K. , Zerefos C. , Rapsomanikis S. , Simeonov V. , Balis D. , Perros P. E. , Thompson A. M. , Witte J. , Calpini B. , Sharobiem W. M. , Papayannis A. , Mihalopoulos N. , Drakou R.
Published: October 2002

During the last 5 years, information on the regional tropospheric ozone levels over the eastern Mediterranean has become available but was confined to measurements at ground level. Here available information is expanded with measurements at two more rural ground level sites spanning 8° latitude, ozonesonde ascents, lidar observations, ship cruises, and aircraft flights. During wintertime the ozone monthly values are 10–20 ppbv higher than values at other European sites, while during summertime the values are comparable. This results in regional ozone background levels in the area that are above the European Union (EU) 32 ppb/24 hours phytotoxicity limit during the entire year. Late spring lidar observations show that south and southwestern synoptic flows which are associated with Saharan dust events result in lower ozone above the planetary boundary layer (PBL) by 20–35 ppbv as compared to these during northerly flows, which transport air from continental Europe. These lidar observations along with ship measurements during July show that ozone is enhanced 1.5–2.2 times in the continental outflow, when compared to aged maritime air or air from the African continent. These results along with ozonesonde observations suggest that ozone abatement in the area is largely beyond the control of regional emissions and can be controlled only with emission reductions on a European scale. During September, measured background NO and NO2 levels between 3 and 4.5 km above sea level ranged from 1 to 221 pptv and from 102 to 580 pptv, respectively. The troposphere contains around 40–70 Dobson units of ozone during summer and around 20–30 Dobson units during winter.
Effect of periodic melting on geochemical and isotopic signals in an ice core from Lomonosovfonna, Svalbard
Authors: Pohjola V. A. , Moore J. C. , Isaksson E. , Jauhiainen T. , van de Wal R. S. W. , Martma T. , Meijer H. A. J. , Vaikmäe R.
Published: October 2002

We examine the quality of atmospherically deposited ion and isotope signals in an ice core taken from a periodically melting ice field, Lomonosovfonna in central Spitsbergen, Svalbard. The aim is to determine the degree to which the signals are altered by periodic melting of the ice. We use three diagnostics: (1) the relation between peak values in the ice chemical and isotopic record and ice facies type, (2) the number of apparent annual cycles in these records compared with independently determined number of years represented in the ice core, and (3) a statistical comparison of the isotopic record in the ice core and the isotope records from coastal stations from the same region. We find that during warm summers, as much as 50% of the annual accumulation may melt and percolate into the firn; in a median year this decreases to ∼25%. As a consequence of percolation, the most mobile acids show up to 50% higher concentrations in bubble‐poor ice facies compared with facies that are less affected by melt. Most of the other chemical species are less affected than the strong acids, and the stable water isotopes show little evidence of mobility. Annual or biannual cycles are detected in most parameters, and the water isotope record has a comparable statistical distribution to isotopic records from coastal stations. We conclude that ice cores from sites like Lomonosovfonna contain a useful environmental record, despite melt events and percolation and that most parameters preserve an annual, or in the worst cases, a biannual atmospheric signal.
Transport of ozone to the surface by convective downdrafts at night
Authors: Betts Alan K. , Gatti Luciana V. , Cordova Ana Maria , Silva Dias Maria A. F. , Fuentes Jose D.
Published: October 2002

During the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia wet season experiment, the near‐surface measurements of equivalent potential temperature and ozone at night, when background levels of ozone are low, clearly show that convective downdrafts rapidly transport air with higher ozone and lower equivalent potential temperature down to the surface from around 800 hPa. This largely unreported downward transport of ozone may play a significant role in the photochemistry of the atmosphere boundary layer and increase the surface deposition of ozone.
Trends in eastern U.S. sulfur air quality from the Atmospheric Integrated Research Monitoring Network
Authors: Hicks Bruce B. , Artz Richard S. , Meyers Tilden P. , Hosker Rayford P.
Published: October 2002

Surface air quality data obtained at long‐term stations operated by the Atmospheric Integrated Research Monitoring Network of the National Oceanic and Atmospheric Administration are combined with precipitation chemistry data from the National Atmospheric Deposition Program to construct indicators of air chemistry in the lower atmosphere starting in 1985/1986 and extending through 1997. The analysis is based on data from seven stations east of the Mississippi River. On the whole, levels of airborne sulfur reached a maximum in the late 1980s; since then, levels have been declining steadily at a rate of about 2.8% per annum, but with an increase to about 5% per annum for a period of the 1990s. There is no clear‐cut step function that could be attributed to the initiation of emissions trading and resulting SO2 emissions reductions in about 1995. Instead, the data appear to indicate that the new regulatory trading philosophies have continued the long‐term improvement in overall air quality that was already under way. The data from individual sites indicate that the benefits of emissions decreases are not necessarily evident at all locations. Improvements appear to be moderated to a considerable extent by local factors that may dominate in some situations. Overall, the air quality data correlate well with changes in emissions of sulfur as reported by the Environmental Protection Agency. The analysis shows that the percentage improvement in sulfur concentrations in the planetary boundary layer east of the Mississippi corresponds one to one to the percentage change in emissions from electric utilities (within the statistical uncertainties involved). Overall, the results support the hypothesis that regional air quality is governed by the largest point‐source emitters, rather than by the integral of all upwind sources.
Cointegration analysis of hemispheric temperature relations
Authors: Kaufmann Robert K. , Stern David I.
Published: October 2002

We use cointegration procedures that are designed to estimate and test relations among integrated time series to develop a model of the relation between surface temperature and the radiative forcing of solar irradiance, greenhouse gases, and tropospheric sulfates. We use this model to test some basic hypotheses regarding the relation between surface temperature and radiative forcing. We find that there is a statistically meaningful relation between surface temperature and changes in the radiative forcing associated with natural variability and human activity. We also find that hemispheric temperatures cannot be explained by hemispheric forcings alone: Hemispheric temperatures are linked. Differences in hemispheric temperatures are associated with differences in the hemispheric temperature effects of greenhouse gases, anthropogenic sulfur emissions, and solar irradiance. Estimates for the temperature sensitivity (ΔT2x) are consistent with the middle and lower range of values estimated by physically based models.
Hygroscopic properties of nucleation mode and Aitken mode particles during nucleation bursts and in background air on the west coast of Ireland
Authors: Väkevä M. , Hämeri K. , Aalto P. P.
Published: October 2002

The hygroscopic and cloud condensation nuclei (CCN) properties of submicrometer atmospheric aerosol particles were investigated using an Ultrafine Tandem Differential Mobility Analyzer (UF‐TDMA) and a CCN counter at the Mace Head Monitoring Station on the west coast of Ireland during the New Particle Formation and Fate in the Coastal Environment (PARFORCE) field campaign in September 1998 and June 1999. These measurements give indirect in situ information on the composition and state of mixing of the aerosol particles. The UF‐TDMA was used for monitoring of hygroscopic diameter growth factors of aerosol particles with dry mobility diameters 8–20 nm when taken from dry state to a controlled humid environment (RH 90%). The CCN counter was used to study the activation of aerosol particles when exposed to supersaturated conditions (dry diameters of 15–150 nm). It was seen that in clean marine air masses during the observed particle formation events, the newly formed nucleation mode particles (8 and 10 nm) most often had low growth factors (between 1.0 and 1.1) resulting from low solubility. This indicates that the nucleation mode particles consist of nonsoluble or weakly soluble species, possible accompanied by a small soluble part; however, when nucleation mode particles were observed outside the event periods, the growth factors were higher (about 1.3–1.4). In contrast, the 20 nm particles usually clearly belonged to the Aitken mode (based on number size distribution measurements) and had hygroscopic properties similar to some common salts (growth factors 1.4–1.5).
Aerosol optical depths and direct radiative forcing for INDOEX derived from AVHRR: Theory
Authors: Coakley James A. , Tahnk William R. , Jayaraman A. , Quinn Patricia K. , Devaux Claude , Tanré Didier
Published: October 2002

A scheme is presented for retrieving aerosol properties for ocean regions from reflected sunlight at both the visible and near infrared wavelengths measured by the NOAA advanced very high resolution radiometer (AVHRR). For the Indian Ocean Experiment (INDOEX), aerosols were presumed to be a mixture of a continental haze that had small particles, contained soot, and absorbed sunlight, and a marine haze that had large particles and absorbed practically no sunlight. Because of the difference in particle sizes, the two aerosols reflect sunlight differently at visible and near infrared wavelengths. Reflectances at visible and near infrared wavelengths were thus used to determine mixing fractions for the continental and marine aerosols and the optical depth of the aerosol mixture. The fractions and optical depths along with the optical properties of the aerosols were then used in radiative transfer calculations to estimate the diurnally averaged top of the atmosphere and surface aerosol direct radiative forcing for ocean regions. Comparison of retrieved optical depths at visible and near infrared wavelengths with surface measurements revealed that several different retrieval schemes employing a variety of aerosol types provided comparable levels of agreement, but none of the aerosol models or retrieval schemes produced ratios of the near infrared to visible optical depths that agreed with the ratios obtained with the surface measurements. In estimating the top of the atmosphere radiative forcing, errors in the retrieved optical depths were in some cases found to be partially compensated by the effect of the aerosol on the radiative flux. For example, different aerosol models led to retrieved optical depths that differed by as much as 60%, but the top of the atmosphere forcing obtained with the models differed by less than 35% for cloud‐free conditions. When aerosols absorb sunlight, there is no comparable compensation for the surface forcing. Cloud conditions contribute sizable uncertainties to estimates of the aerosol direct radiative forcing. For INDOEX, estimates of the aerosol direct radiative forcing for average cloud conditions were obtained by (1) setting the forcing to zero for all 1° × 1° latitude‐longitude boxes that contained any amount of upper‐level cloud; (2) ascribing to regions with upper‐level clouds the radiative forcing obtained for regions having only low‐level clouds and, (3) setting the forcing to zero for all regions containing upper‐level clouds and all portions of regions overcast by low‐level clouds. Relative differences in the extreme values for the top of the atmosphere aerosol direct radiative forcing were less than 50%, but for the surface, the relative differences of the extreme values reached 70%.
Aerosol optical depth and direct radiative forcing for INDOEX derived from AVHRR: Observations, January–March 1996–2000
Authors: Tahnk William R. , Coakley James A.
Published: October 2002

Visible and near infrared reflectances from NOAA‐14 Advanced Very High Resolution Radiometer (AVHRR) daytime passes are used to derive optical depths at 0.55 μm, an index of aerosol type, continental or marine, and the direct effect of the aerosol on the top of the atmosphere and surface solar radiative fluxes for the oceans in the Indian Ocean Experiment (INDOEX) region (30°S to 30°N and 50°–110°E) during the January–March 1996–2000 winter monsoons. Comparison of aerosol optical depth and radiative forcing in the Northern Hemisphere with those in the Southern Hemisphere suggests that the additional pollution sources augment the 0.55‐μm optical depth by, on average, 0.1 in the Northern Hemisphere. As a result of the aerosol, the region of the Indian Ocean in the Northern Hemisphere loses about 1.6 Wm−2 in reflected sunlight and the ocean surface loses about 5 Wm−2 during the months of the winter monsoon. Aerosol burdens and the aerosol direct radiative forcing are a relatively constant feature of the Northern Hemisphere, although the southeastern Arabian Sea experienced considerably larger aerosol burdens during the February–March 1999 INDOEX Intensive Field Phase (IFP) than in other years. Frequency distributions of the optical depth for 1° × 1° latitude‐longitude regions are well represented by gamma distribution functions. The day‐to‐day and year‐to‐year variability of the optical depth for such regions is correlated with the long‐term average optical depth. Interannual variability of the monthly mean optical depths for such regions is found to be as large as the day‐to‐day variability. Such large variability suggests that long‐term records of in situ observations will be required in order to assess the performance of models that generate climatologies of aerosol concentrations.
Aerosol optical properties during the Lindenberg Aerosol Characterization Experiment (LACE 98)
Authors: Bundke U. , Hänel G. , Horvath H. , Kaller W. , Seidl S. , Wex H. , Wiedensohler A. , Wiegner M. , Freudenthaler V.
Published: October 2002

The optical properties of atmospheric aerosol particles were measured close to ground level using different methods at Lindenberg/Falkenberg (Germany) during the Lindenberg Aerosol Characterization Experiment (LACE 98), 13 July 1998 to 14 August 1998 [Ansmann et al., 2002]. The experimental setup consisted of (a) an aerosol photometer, which measured a complete set of aerosol optical properties, such as extinction, scattering, and absorption coefficients, single scattering albedo, apparent complex refractive index, asymmetry parameter of the phase function of scattering, and apparent volume soot content, (b) an integrating plate, which measured the absorption coefficient, and (c) a nephelometer and (d) a particle soot absorption photometer (PSAP) for measurements of scattering and absorption coefficients. All these measurements were performed under dry conditions. A telephotometer and a horizontal lidar were used to determine the aerosol extinction coefficient under ambient conditions. This paper presents a closure study of the different measurement methods. An empirical correction function for the PSAP is introduced and compared with a semiempirical correction function previously introduced by Bond et al. [1999]. A detailed model of humidity effects on the extinction coefficient is presented using “coated sphere” calculations based on measured input values of the size distribution, the chemical composition, the growth factor at 90% relative humidity, the water‐soluble fraction of particulate materials, the temperature, and the relative humidity. Model calculations allowed an intercomparison of measured values of particles in the dry state and at ambient relative humidity and showed good agreement.
Aerosol states in the free troposphere at northern midlatitudes
Authors: Schröder F. , Kärcher B. , Fiebig M. , Petzold A.
Published: October 2002

A statistical analysis of in situ observations of aerosols in the midlatitude free troposphere and tropopause region in summer is presented. Vertical profiles of the number density and the associated variability of nuclei mode and larger aerosol particles are discussed. These data confirm that the upper troposphere is an efficient net source region of new particles. Aerosols are studied by analyzing size‐segregated data, and it is suggested that observed extreme cases of aerosol evolution include a nucleation state, an accumulation state, and a cloud‐processed state. This interpretation implies that aerosol sources (nucleation from the gas phase) are closely tied to aerosol sinks (cloud scavenging of aerosol particles). Processes that could modify the proposed aerosol life cycle are discussed. Number size and surface area distributions typical for the various aerosol states are examined. The dry aerosol surface area ranges from 1 to 20 μm2 cm−3 in the free troposphere and from 2 to 13 μm2 cm−3 in the tropopause region. Parameterizations of the aerosol size distributions and total surface area concentrations are provided to facilitate the use of the data in atmospheric models.
Complex refractive index of aerosols during LACE 98#x2010; as derived from the analysis of individual particles
Authors: Ebert, M. , Weinbruch, S. , Rausch, A. , Gorzawski, G. , Helas, G. , Hoffmann, P. , Wex H.
Published: October 2002

During the Lindenberger Aerosol Characterization Experiment (LACE 98), impactor sampling of aerosol particles in the size range of 0.1 to 25 μm was performed. The atmospheric concentrations of the elements sodium to lead (11 ≤ Z ≤ 83) were determined by total reflection X‐ray fluorescence analysis. Approximately 15,500 individual particles were examined by high‐resolution scanning electron microscopy and energy‐dispersive X‐ray microanalysis, and about 3800 particles were examined by transmission electron microscopy combined with energy‐dispersive X‐ray microanalysis. On the basis of morphology and chemical composition the particles were classified into 10 different groups: ammonium sulfates, calcium sulfates, sea salt, metal oxides/hydroxides, carbonates, silicates, soot, biological particles, carbon/sulfate mixed particles, and rest of carbon‐rich particles Crest. The phases present in the different particle groups were determined by selected area electron diffraction in the transmission electron microscope. In addition, the heterogeneous phase composition of agglomerates was studied in detail. On the basis of the size distribution and the relative abundance of the particle groups, the average and size‐resolved complex refractive index of the total aerosol were calculated. The real part of the average refractive index mainly depends on the abundance of metal oxide/hydroxide particles and varied between 1.52 and 1.57 on the different sampling days. The average imaginary part varied between 0.031 and 0.057 depending on the amounts of soot and carbon/sulfate mixed particles. The average complex refractive index deduced from the analysis of individual aerosol particles is in good agreement with the results of photometer measurements of dried filter samples.
Optical and microphysical characterization of biomass‐ burning and industrial‐pollution aerosols from‐ multiwavelength lidar and aircraft measurements
Authors: Wandinger, Ulla , Müller, Detlef , Böckmann, Christine , Althausen, Dietrich , Matthias, Volker , Bösenberg, Jens , Weiß, Volker , Fiebig, Markus , Wendisch, Manfred , Stohl, Andreas , Ansmann Albert
Published: October 2002

During the Lindenberg Aerosol Characterization Experiment (LACE 98) simultaneous measurements with ground‐based and airborne lidars and with two aircraft equipped with aerosol in situ instrumentation were performed. From the lidar measurements, particle backscatter coefficients at up to eight wavelengths between 320 and 1064 nm and particle extinction coefficients at 2–3 wavelengths between 292 and 532 nm were determined. Thus, for the first time, an extensive set of optical particle properties from several lidar platforms was available for the inversion into particle microphysical quantities. For this purpose, two different inversion algorithms were used, which provide particle effective radius, volume, surface‐area, and number concentrations, and complex refractive index. The single‐scattering albedo follows from Mie‐scattering calculations. The parameters were compared to the ones from airborne measurements of particle size distributions and absorption coefficients. Two measurement cases were selected. During the night of 9–10 August 1998 measurements were taken in a biomass‐burning aerosol layer in the free troposphere, which was characterized by a particle optical depth of about 0.1 at 550 nm. Excellent agreement between remote‐sensing and in situ measurements was found. In the center of this plume the effective radius was approximately 0.25 μm, and all methods showed rather high complex refractive indices, ranging from 1.56–1.66 in real part and from 0.05–0.07iin imaginary part. The single‐scattering albedo showed low values from 0.78–0.83 at 532 nm. The second case, taken on 11 August 1998, presents the typical conditions of a polluted boundary layer in central Europe. Optical depth was 0.35 at 550 nm, and particle effective radii were 0.1–0.2 μm. In contrast to the first case, imaginary parts of the refractive index were below 0.03i. Accordingly, the single‐scattering albedo ranged from 0.87–0.95.
Submicrometer aerosol particle size distribution and hygroscopic growth measured in the Amazon rain forest during the wet season
Authors: Zhou Jingchuan , Swietlicki Erik , Hansson Hans Christen , Artaxo Paulo
Published: October 2002

The number‐size distribution and hygroscopic growth of submicrometer aerosol particles were measured in central Amazonia during the first Cooperative LBA Airborne Regional Experiment (CLAIRE) wet season experiment in March–April 1998. This was the first time ever that these types of measurements were performed in the Amazon rain forest. A Differential Mobility Particle Sizer (DMPS) was used to measure aerosol number‐size distribution with diameters in the range 3–850 nm. The observed total number concentrations were frequently between 300 and 600 cm−3 with a mean value around 450 cm−3. Two aerosol particle modes (Aitken and accumulation mode) were always present. The average particle concentrations for those two modes were 239 and 177 cm−3, with geometric diameters of 68 and 151 nm, respectively. An ultrafine mode had a number concentration and a mean diameter of 92 cm−3 and 24 nm, respectively, and only occurred at 18% of the time, causing the size distribution to be trimodal instead of bimodal. The hygroscopic growth of aerosol particles was measured in situ with a Hygroscopic Tandem Differential Mobility Analyzer (H‐TDMA) at six dry particle diameters between 35 and 265 nm. In contrast to the bimodal hygroscopic behavior found in polluted continental environments, the hygroscopic properties of aerosol particles in the Amazon rain forest is essentially unimodal with average diameter growth factors of 1.16–1.32 from dry to 90% relative humidity (RH). Aerosol soluble volume fractions were, in general, between 0.14 and 0.27, estimated by assuming that only ammonium hydrogen sulphate interacted with water vapour.
Airborne measurements of NOx, tracer species, and small particles during the European Lightning Nitrogen Oxides Experiment
Authors: Huntrieser H. , Feigl C. , Schlager H. , Schröder F. , Gerbig C. , van Velthoven P. , Flatøy F. , Théry C. , Petzold A. , Höller H. , Schumann U.
Published: October 2002

Airborne in situ measurements of NO, NO2, NOy, CO, CO2, O3, J(NO2), and CN were performed in European thunderstorms during the field experiment EULINOX in July 1998. The measurements in the upper troposphere show enhanced NOx (= NO + NO2) concentrations within thunderstorms and their outflow at horizontal scales from 300 m to several 100 km. The maximum NO mixing ratio measured inside a thundercloud close to lightning (the aircraft was also hit by a small lightning strike) was 25 ppbv. A regional NOx enhancement of 0.5 ppbv over central Europe could be traced back to a thunderstorm event starting ∼24 hours earlier over Spain. The fractions of NOx in thunderclouds which are produced by lightning and convectively transported from the polluted boundary layer are determined by using CO2 and CO as tracers for boundary layer air. The analyses show that on average about 70% of the NOx increase measured in the anvil region was found to result from production by lightning and about 30% from NOx in the boundary layer. Thunderstorms are also strong sources of small particles. The peak CN concentrations measured within thunderstorm outflows (>30,000 particles STP cm−3) were distinctly higher than in the polluted boundary layer. The amount of NOx produced per thunderstorm and NO produced per lightning flash was estimated. The results imply that the annual mean NOx budget in the upper troposphere over Europe is dominated by aircraft emissions (0.1 TgN yr−1) in comparison to lightning production (∼0.03 TgN yr−1). On the global scale, NOx produced by lightning (mean 3 TgN yr−1) prevails over aircraft‐produced NOx (0.6 TgN yr−1).
Findings on H2O2/HNO3 as an indicator of ozone sensitivity in Baden‐Württemberg, Berlin‐Brandenburg, and the Po valley based on numerical simulations
Authors: Hammer, M.‐U. , Vogel, B. , Vogel H.
Published: October 2002

The usefulness of the indicator H2O2/HNO3 to separate NOx‐sensitive and VOC‐sensitive regions in the polluted planetary boundary layer on the regional scale is investigated using numerical simulations. The results of box model runs are applied to highlight individual definitions of the NOx‐VOC‐ozone sensitivity. In particular, different ways to determine the transition value of the indicators H2O2/HNO3 and NOy are explained. Three‐dimensional model runs are performed for two model domains in Germany and one domain in the Po valley (Italy). These domains differ significantly in the spatial distribution of the emissions, in the absolute values of the emissions, and in the composition of the VOC. The domains also differ in their meteorological conditions and hence in the involved transport processes. Nevertheless, the transition values of H2O2/HNO3 found for the individual domains differ from 0.2 by less than 15%. This transition value is applied to an emission reduction based on the assumption of all aromatic compound emissions being switched off. It shows that this transition value and the horizontal distribution of H2O2/HNO3 of the base case allow a forecast of those regions where ozone is affected by this emission reduction. This offers the possibility to apply the indicator H2O2/HNO3 and the transition value of 0.2 to observations.
A model prediction of the yield of cloud condensation nuclei from coastal nucleation events
Authors: Pirjola Liisa , O'Dowd Colin D. , Kulmala Markku
Published: October 2002

The formation and evolution of new particles during coastal nucleation events are examined using the aerosol dynamic and gas‐phase chemistry model AEROFOR2. Coastal regions are known to be a strong source of natural aerosol particles and are also strong sources of biogenic vapors which can condense onto aerosol particles, thus resulting in particle growth. A number of model simulations were performed to determine the instantaneous nucleation rate along with the source rate of a generic biogenic vapor leading to the observed particle size distributions which indicate the rapid appearance of ∼105–106 cm−3 nucleation mode particles in this environment. Model calculations suggest values of 3 × 105 cm−3 s−1 to 3 × 106 cm−3 s−1 for the instantaneous nucleation rate and a value of 5 × 107 cm−3 s−1 for the condensable vapor source rate in order to reproduce the observed concentrations. A significant fraction of these new particles survive to grow into cloud condensation nuclei (CCN) sizes for supersaturations typically encountered in boundary layer clouds during subsequent evolution over 3 days under clear‐sky conditions, thus contributing to the indirect radiative effect of aerosols. The amount of CCN is mainly affected by coagulation between particles and condensation of the biogenic vapor and, to a lesser extent, by condensation of sulphuric acid formed by DMS oxidation. In all simulated cases, an increase of more than 100% in CCN concentration, for supersaturations >0.35% was observed.
Aerosol‐radiation interaction in the cloudless atmosphere during LACE 98 1. Measured and calculated broadband solar and spectral surface insolations
Authors: Wendisch Manfred , Keil Andreas , Müller Dörthe , Wandinger Ulla , Wendling Peter , Stifter Armin , Petzold Andreas , Fiebig Markus , Wiegner Matthias , Freudenthaler Volker , Armbruster Wolfgang , von Hoyningen‐Huene Wolfgang , Leiterer Ulrich
Published: October 2002

Vertical profile measurements of aerosol particle size distributions and of meteorological parameters (obtained from aircraft, radiosondes, and lidar) are used as input to a spectral radiative transfer model to calculate broadband solar and spectral surface insolations. The calculated values are compared to measured ones gathered with broadband solar pyranometers and pyrheliometers, and a fixed‐grating photodiode array spectroradiometer with 512 spectral channels between 500 and 920 nm wavelength. The measurements were obtained during the joint field campaign Lindenberg Aerosol Characterization Experiment (LACE) 98 near Berlin/Germany in the summer of 1998. Two cases (days with high and low aerosol loading, respectively) are investigated in detail. Furthermore, a measurement‐based sensitivity analysis was carried out focusing on the influence of particle composition (complex refractive index) and of microphysical and humidity growth uncertainties on the calculated surface insolations. Assuming a spectral refractive index of ammonium sulfate for the aerosol particles, on average the global component of the broadband solar surface insolations is 11–20 W m−2(2–3%) greater than the measured values; the direct portion is 17–28 W m−2(4–5%) higher, and its diffuse component is 6–7 W m−2(4–10%) lower in comparison to the measurements. The measured and calculated spectral surface insolations (global portion) agree well in the central visible spectral region (500–600 nm wavelength). Toward larger wavelengths (near infrared) the calculated spectral surface insolations are increasingly higher than the measured ones.
A Lagrangian approach to studying Arctic polar stratospheric clouds using UARS MLS HNO3 and POAM II aerosol extinction measurements
Authors: Santee M. L. , Tabazadeh A. , Manney G. L. , Fromm M. D. , Bevilacqua R. M. , Waters J. W. , Jensen E. J.
Published: October 2002

We assess the viability of diagnosing polar stratospheric cloud (PSC) composition and denitrification using existing satellite measurements. A Lagrangian approach is used to track PSC evolution from formation through dissipation. Upper Atmosphere Research Satellite Microwave Limb Sounder observations of gas‐phase HNO3 and Polar Ozone and Aerosol Measurement II observations of aerosol extinction from the Arctic late winter of 1995/1996 are correlated and compared to results from PSC composition models along air parcel trajectories. This approach is successful in capturing the broad patterns of PSC development. That is, a strong correlation is found between low‐temperature, low‐HNO3, and high‐extinction points. In most cases the observed behavior along the Lagrangian paths falls within the range predicted by equilibrium liquid ternary aerosol and nitric acid trihydrate composition models. At no time along the trajectories (when there is data coverage) do the models display large changes in either HNO3 or aerosol extinction that are not reflected in the data. In general, however, there is a large degree of overlap in the comparisons between the models and the data. Unmeasured quantities and large uncertainties in both measurements and model calculations preclude conclusive determination of PSC composition or phase in most cases. In the majority of PSC events studied, gas‐phase HNO3 fully recovers to pre‐PSC abundances following cloud evaporation. We conclude that while severe denitrification (50% or greater) may have occurred in highly localized regions in the Arctic in 1996, it did not occur over spatial scales comparable to or larger than the MLS field of view (∼400 km × 200 km × 6 km), even though it was a relatively cold winter. Improved measurements from upcoming satellite missions, such as Earth Observing System Aura, will ameliorate many of the difficulties in diagnosing PSC composition and denitrification encountered in this study.
Hygroscopic properties and water‐soluble volume fraction of atmospheric particles in the diameter range from 50 nm to 3.8 μm during LACE 98
Authors: Busch B. , Kandler K. , Schütz L. , Neusüß C.
Published: October 2002

Hygroscopic properties of atmospheric aerosol particles in the Aitken, large, and giant particle range were studied during the Lindenberg Aerosol Characterization Experiment (LACE 98) in a rural area 80 km southeast of Berlin. The hygroscopic behavior of Aitken particles were determined in situ in four size classes (50, 100, 150, 250 nm) with a Hygroscopic Tandem Differential Mobility Analyzer for relative humidities (RH) of 60% and 90%. Measurements at 60% RH served as reference data used by other LACE 98 investigators for mass closure and radiative transfer calculations. In most cases, at 90% RH, the atmospheric particles could be classified into two groups (“more” and “less” hygroscopic fraction) with different hygroscopic properties, whereas this classification was not possible for the measurements at 60% RH. The measured average growth factors of the “more” hygroscopic fraction were found to be at 1.43, 1.49, 1.56, and 1.63 for particles with a size of 50, 100, 150, and 250 nm, respectively. Growth factors of the “less” hygroscopic mode were about 1.1 for all particle sizes. The water‐soluble volume fraction of the particles was estimated using a hygroscopic model, assuming that the particles consist of ammonium sulfate with an insoluble core. Chemical analysis of size‐segregated impactor samples confirmed this assumption. The corresponding estimated water‐soluble volume fractions for the “more” hygroscopic particles were about 0.47, 0.52, 0.59, and 0.68, whereas the estimated water‐soluble volume fractions of the “less” hygroscopic particles were in the order of 0.1. At 60% RH, the measured average growth factors were in the range of 1.15–1.22, the estimated water‐soluble volume fractions were in the range of 0.41–0.59. For large and giant aerosol particles, the determination of the soluble volume fraction was extended by direct measurements with the water‐soluble fraction of large and giant aerosol particles system. Quasi‐monodisperse samples of particles 0.4–3.8 μm in diameter were investigated for seven specific particle sizes. The particles measured within this size range show up to three classes of water‐soluble volume fractions: In some cases, a class of nearly insoluble particles corresponding to the less hygroscopic fraction in the Aitken range can be identified. A second class corresponding to the more hygroscopic fraction is found with a water‐soluble volume fraction of about 0.5–0.7. In addition to the Aitken range, a third class of particles with a soluble volume fraction of about 0.85 is usually found. Below 0.7 μm particle size, the third class of nearly complete water‐soluble particles is strongly dominant; above 0.7 μm, there is an equal distribution of all classes. No significant variation can be seen during the LACE 98 campaign in the large and giant particle size range.
Gas‐aerosol relationships of H2SO4, MSA, and OH: Observations in the coastal marine boundary layer at Mace Head, Ireland
Authors: Berresheim H. , Elste T. , Tremmel H. G. , Allen A. G. , Hansson H.‐C. , Rosman K. , Dal Maso M. , Mäkelä J. M. , Kulmala M. , O'Dowd C. D.
Published: October 2002

Atmospheric concentrations of gaseous sulfuric acid (H2SO4), methane sulfonic acid (MSA), and hydroxyl radicals (OH) were measured by chemical ionization mass spectrometry (CIMS) during the second New Particle Formation and Fate in the Coastal Environment (PARFORCE) campaign in June 1999 at Mace Head, Ireland. Overall median concentrations in marine background air were 1.5, 1.2, and 0.12 × 106 cm−3, respectively. H2SO4 was also present at night indicating significant contributions from nonphotochemical sources. A strong correlation was found between daytime OH and H2SO4 levels in clean marine air suggesting a fast local production of H2SO4 from sulfur precursor gases. Steady state balance calculations of ambient H2SO4 levels agreed with measured concentrations if either very low H2SO4 sticking coefficients (0.02–0.03) or sources in addition to the SO2 + OH reaction were assumed. Overall, variations in ambient H2SO4 levels showed no correlation with either the tidal cycle or ultrafine particle (UFP) concentrations. However, on particular days an anticorrelation between H2SO4 and UFP levels was occasionally observed providing evidence for the contribution of H2SO4 to new particle formation and/or particle growth. Gaseous MSA concentrations were inversely correlated with dew point temperature reflecting a highly sensitive gas‐particle partitioning equilibrium of this compound. The present observations seriously question the general use of MSA as a conservative tracer to infer the relative production yield of H2SO4 from dimethylsulfide (DMS) oxidation. MSA/H2SO4 concentration ratios typically ranged between 0.06 and 1.0 in marine air at ground level. Measured diel OH profiles showed a significant deviation from concurrent variations of the ozone photolysis frequency. They also showed up to 1 order of magnitude lower values compared to OH concentrations calculated with a simple photochemical box model. These differences were most pronounced during particle nucleation events occurring on sunny days around noon and at low tide. The present results suggest that both the oxidation capacity and the particle formation potential in the coastal boundary layer were significantly affected by reactions of unknown compounds prevailing in this type of environment.