Mixing Of Air Masses Research Articles

Number size distributions and seasonality of submicron particles in Europe 2008–2009

Abstract. Two years of harmonized aerosol number size distribution data from 24 European field monitoring sites have been analysed. The results give a comprehensive overview of the European near surface aerosol particle number concentrations and number size distributions between 30 and 500 nm of dry particle diameter. Spatial and temporal distribution of aerosols in the particle sizes most important for climate applications are presented. We also analyse the annual, weekly and diurnal cycles of the aerosol number concentrations, provide log-normal fitting parameters for median number size distributions, and give guidance notes for data users. Emphasis is placed on the usability of results within the aerosol modelling community. We also show that the aerosol number concentrations of Aitken and accumulation mode particles (with 100 nm dry diameter as a cut-off between modes) are related, although there is significant variation in the ratios of the modal number concentrations. Different aerosol and station types are distinguished from this data and this methodology has potential for further categorization of stations aerosol number size distribution types. The European submicron aerosol was divided into characteristic types: Central European aerosol, characterized by single mode median size distributions, unimodal number concentration histograms and low variability in CCN-sized aerosol number concentrations; Nordic aerosol with low number concentrations, although showing pronounced seasonal variation of especially Aitken mode particles; Mountain sites (altitude over 1000 m a.s.l.) with a strong seasonal cycle in aerosol number concentrations, high variability, and very low median number concentrations. Southern and Western European regions had fewer stations, which decreases the regional coverage of these results. Aerosol number concentrations over the Britain and Ireland had very high variance and there are indications of mixed air masses from several source regions; the Mediterranean aerosol exhibit high seasonality, and a strong accumulation mode in the summer. The greatest concentrations were observed at the Ispra station in Northern Italy with high accumulation mode number concentrations in the winter. The aerosol number concentrations at the Arctic station Zeppelin in Ny-Å lesund in Svalbard have also a strong seasonal cycle, with greater concentrations of accumulation mode particles in winter, and dominating summer Aitken mode indicating more recently formed particles. Observed particles did not show any statistically significant regional work-week or weekday related variation in number concentrations studied. Analysis products are made for open-access to the research community, available in a freely accessible internet site. The results give to the modelling community a reliable, easy-to-use and freely available comparison dataset of aerosol size distributions.

Analysis of <sup>13</sup>C and <sup>18</sup>O isotope data of CO<sub>2</sub> in CARIBIC aircraft samples as tracers of upper troposphere/lower stratosphere mixing and the global carbon cycle

Abstract. The project CARIBIC (http://caribic-atmospheric.com) aims to study atmospheric chemistry and transport by regularly measuring many compounds in the free troposphere and the upper troposphere/lowermost stratosphere (UT/LMS) by using passenger aircraft. Here we present CO2 concentrations and isotope results, and analyze the data together with supporting trace gas data. 509 CARIBIC-2 samples (highest precision and accuracy δ13C(CO2) and δ18O(CO2) data) from June 2007 until March 2009, together with CARIBIC-1 samples (flights between November 1999 and April 2002, 350 samples in total, 270 for NH, mostly δ13C(CO2) data) give a fairly extensive, unique data set for the NH free troposphere and the UT/LMS region. Total uncertainty of the data is the same as reported for the global monitoring program by NOAA-ESRL. To compare data from different years a de-trending is applied. In the UT/LMS region δ13C(CO2), δ18O(CO2) and CO2 are found to correlate well with stratospheric tracers, in particular N2O; δ18O(CO2) appears to be a useful, hitherto unused, tracer of atmospheric transport in the UT/LMS region and also inter-hemispheric mixing. By filtering out the LMS data (based on N2O distributions), the isotope variations for the free and upper troposphere are obtained. These variations have only small latitudinal gradients, if any, and are in good agreement with the data of selected NOAA stations in NH tropics. Correlations between δ13C(CO2) and CO2 are observed both within single flight(s) covering long distances and during certain seasons. The overall variability in de-trended δ13C(CO2) and CO2 for CARIBIC-1 and CARIBIC-2 are similar and are generally in agreement, which underscores agreement between high and low resolution sampling. Based on all correlations, we infer that the CO2 distribution in the NH troposphere along CARIBIC flight routes is chiefly regulated by uplift and pole-wards transport of tropical air up to approximately 50° N. The main reason for variability of signals in the troposphere (which is larger for the higher resolution sampling during CARIBIC-2) is mixing of different tropospheric air masses affected by different CO2 sources and sinks. The effect of stratospheric flux appears to be limited. All in all it is demonstrated that CARIBIC produced new important and reliable data sets for little explored regions of the atmosphere. A logical next step will be global scale modeling of 13C and especially 18O, which is linked to the hydrological cycle.

Concentrations and size distributions of aerosol particles at Maitri during the passage of cyclonic storms revolving around the continent of Antarctica

The number size distributions of aerosol particles in the size‐range of 0.003–20 μm diameter have been measured at Maitri (70° 45′ 52″S, 11° 44′ 03″E, 117m above mean sea level), Antarctica, when two circumpolar cyclonic storms passed close to the station during February, 2005. As a storm approaches toward Maitri, concentration of coarse particles increases by about an order of magnitude and the number size‐distribution frequently shows a coarse mode at ∼ 2 μm, a broad Aitken mode from 0.04 to 0.1 μm and, occasionally, a nucleation mode at 0.018 μm diameter. When the storm is going away from Maitri, in addition to the coarse mode at ∼ 2 μm and a peak at 0.08 μm diameter, a nucleation mode frequently appears at < 0.01 μm diameter and the mode existing at 0.018 μm diameter shifts to 0.02−0.04 μm diameter. Particles in the range of 0.008–0.03 μm diameter grow at the rate of 0.2−0.6 nm h−1 in the case of Storm I, but no appreciable growth is observed in the case of Storm II. The peak at 0.02–0.04 μm is often so dominant that it envelops the peak at 0.08 μm diameter. Results are interpreted in terms of the mixing of continental and oceanic air masses with the subsidence associated with the storm. The nucleation mode at 0.01 μm diameter has been associated with the new particles formed in the outflow at the top of clouds and the coarse mode at 2 μm diameters with the re‐suspension of particles from the surface.

An aircraft‐based upper troposphere lower stratosphere O3, CO, and H2O climatology for the Northern Hemisphere

We present a climatology of O3, CO, and H2O for the upper troposphere and lower stratosphere (UTLS), based on a large collection of high‐resolution research aircraft data taken between 1995 and 2008. To group aircraft observations with sparse horizontal coverage, the UTLS is divided into three regimes: the tropics, subtropics, and the polar region. These regimes are defined using a set of simple criteria based on tropopause height and multiple tropopause conditions. Tropopause‐referenced tracer profiles and tracer‐tracer correlations show distinct characteristics for each regime, which reflect the underlying transport processes. The UTLS climatology derived here shows many features of earlier climatologies. In addition, mixed air masses in the subtropics, identified by O3‐CO correlations, show two characteristic modes in the tracer‐tracer space that are a result of mixed air masses in layers above and below the tropopause (TP). A thin layer of mixed air (1–2 km around the tropopause) is identified for all regions and seasons, where tracer gradients across the TP are largest. The most pronounced influence of mixing between the tropical transition layer and the subtropics was found in spring and summer in the region above 380 K potential temperature. The vertical extent of mixed air masses between UT and LS reaches up to 5 km above the TP. The tracer correlations and distributions in the UTLS derived here can serve as a reference for model and satellite data evaluation.

Airborne observations of the effect of a cold front on the aerosol particle size distribution and new particle formation

Airborne measurements of condensation nuclei (CN), aerosol particles (APs) and sulphur dioxide (SO2) were made near a cold front to examine new particle formation and the effects of the front on CN and SO2 concentrations and the AP size distribution and concentration. Measurements were made in the boundary layer (BL) and free troposphere (FT) both preceding and following the passage of the front. Statistical analyses of CN and AP concentrations in the air masses around the front show that new particle formation was prevalent in the pre-frontal and post-frontal FT, however the post-frontal FT contained higher concentrations of ultrafine condensation nuclei (UCN). Mixing diagrams of conserved thermodynamic quantities, total water content and wet equivalent potential temperature were constructed for horizontal aircraft transects in the pre-frontal and post-frontal FT regions; they show that many of the fluctuations in UCN concentrations can be explained by mixing of air masses with differing initial concentrations of UCN. However, regions of enhanced UCN were found to be associated with mixing between air masses of distinct thermodynamic properties, suggesting that mixing between air masses with large gradients in temperature and relative humidity may have promoted new particle formation. Furthermore, analysis of fluctuations in UCN concentrations during vertical soundings show that low pre-existing AP surface area was not a necessary requirement for the production of new particles but, rather, turbulent mixing was a major mechanism for new particle production. Observations also suggest that species other than SO2 were required for new particle production to occur. Copyright © 2010 Royal Meteorological Society

A simplified description of the evolution of organic aerosol composition in the atmosphere

Organic aerosol (OA) in the atmosphere consists of a multitude of organic species which are either directly emitted or the products of a variety of chemical reactions. This complexity challenges our ability to explicitly characterize the chemical composition of these particles. We find that the bulk composition of OA from a variety of environments (laboratory and field) occupies a narrow range in the space of a Van Krevelen diagram (H:C versus O:C), characterized by a slope of ∼−1. The data show that atmospheric aging, involving processes such as volatilization, oxidation, mixing of air masses or condensation of further products, is consistent with movement along this line, producing a more oxidized aerosol. This finding has implications for our understanding of the evolution of atmospheric OA and representation of these processes in models.

Relationship between photochemical ozone production and NOx oxidation in Houston, Texas

An instrumented aircraft was used to study anthropogenic emissions and subsequent ozone and reactive nitrogen photochemistry in the continental boundary layer downwind of Houston, Texas. Measurements of ozone, carbon monoxide, NOx, and NOx oxidation products were conducted from the NOAA WP‐3 aircraft during the 2006 Texas Air Quality Study under a variety of meteorological conditions. Sixty‐five crosswind transects of plumes from Houston urban and industrial areas performed on 10 daytime flights from 13 September to 6 October 2006 are examined. Coincident measurements of NOx and its oxidation products show that NOx was oxidized predominately to nitric acid and peroxy acyl nitrates on time scales of a few hours. The observed relationships between O3 and NOx oxidation products are affected by both photochemistry and mixing of different air masses. On four flights, background pollutant mixing ratios were constant and CO to NOy enhancement ratios in downwind plume transects remained at the emission ratio. The enhancement ratio of O3 to NOx oxidation products was also nearly constant and could be used to derive ozone production efficiency (OPE) in plumes downwind from the Houston area. On the other flights, variable mixing of regionally polluted background air with plumes caused CO to NOy and O3 to NOy − NOx enhancement ratios to increase as plumes were transported. In such cases, enhancement ratios do not solely reflect plume processing, and OPE could not be determined. The OPE averages 5.9 ± 1.2 in coalesced plumes from urban and petrochemical industrial sources in Houston, with higher values in isolated plumes downwind from petrochemical facilities located along the Houston ship channel.

A system for high‐quality CO2 isotope analyses of air samples collected by the CARIBIC Airbus A340‐600

In 2007, JRC-IRMM began a series of atmospheric CO2 isotope measurements, with the focus on understanding instrumental effects, corrections as well as metrological aspects. The calibration approach at JRC-IRMM is based on use of a plain CO2 sample (working reference CO2) as a calibration carrier and CO2-air mixtures (in high-pressure cylinders) to determine the method-related correction under actual analytical conditions (another calibration carrier, in the same form as the samples). Although this approach differs from that in other laboratories, it does give a direct link to the primary reference NBS-19-CO2. It also helps to investigate the magnitude and nature for each of the instrumental corrections and allows for the quantification of the uncertainty introduced. Critical tests were focused on the instrumental corrections. It was confirmed that the use of non-symmetrical capillary crimping (an approach used here to deal with small samples) systematically modifies delta13C(CO2) and delta18O(CO2), with a clear dependence on the amount of extracted CO2. However, the calibration of CO2-air mixtures required the use of the symmetrical dual-inlet mode. As a proof of our approach, we found that delta13C(CO2) on extracts from mixtures agreed (within 0.010 per thousand) with values obtained from the 'mother' CO2 used for the mixtures. It was further found that very low levels of hydrocarbons in the pumping systems and the isotope ratio mass spectrometry (IRMS) instrument itself were critical. The m/z 46 values (consequently the calculated delta18O(CO2) values) are affected by several other effects with traces of air co-trapped with frozen CO2 being the most critical. A careful cryo-distillation of the extracted CO2 is recommended. After extensive testing, optimisation, and routine automated use, the system was found to give precise data on air samples that can be traced with confidence to the primary standards. The typical total combined uncertainty in delta13C(CO2) and delta18O(CO2) on the VPDB-CO2 scale, estimated on runs of CO2-air mixtures, is +/-0.040 per thousand and 0.060 per thousand (2-sigma values). Inter-comparison with MPI-BGC resulted in a scale discrepancy of a similar magnitude. Although the reason(s) for this discrepancy still need to be understood, this basically confirms the approach of using specifically prepared CO2-air mixtures as a calibration carrier, in order to achieve scale unification among laboratories. As important practical application and as a critical test, JRC-IRMM took part in the passenger aircraft-based global monitoring project CARIBIC (http://www.caribic-atmospheric.com). In this way, reliable CO2 isotope data for the tropopause region and the free troposphere were obtained. From June 2007 to January 2009, approximately 500 CARIBIC air samples have been analysed. Some flights demonstrated a compact correlation of both delta13C(CO2) and delta18O(CO2) with respect to CO2 concentration, demonstrating mixing of tropospheric and stratospheric air masses. These excellent correlations provide an independent, realistic data quality check.

Atmospheric 210Pb and 7Be in ambient aerosols over low‐ and high‐altitude sites in semiarid region: Temporal variability and transport processes

Time series measurements of 210Pb and 7Be made on ambient aerosols, collected from a low‐altitude urban site (Ahmedabad, 23.0°N, 72.6°E, 49 m asl) and a high‐altitude station (Mt. Abu, 24.6°N, 72.7°E, 1680 m asl) located in a high‐dust semiarid region of western India, reveal characteristic pattern of temporal variability consistent over three years (2000–2002). The relatively high concentrations of 210Pb (>1 mBq m−3) during wintertime (November–February) are dominated by continental air masses from the northeast; whereas lower values during the summer months (April–May) and southwest monsoon season (June–August) are associated with (southwesterly) maritime air. The concentration of 210Pb in individual rain events at Ahmedabad, collected during the southwest monsoon for the same three‐year period, also exhibits large variability (range of 3–367 mBq L−1; volume‐weighted mean of 74 mBq L−1). This first set of data on the simultaneous measurements in rain and aerosols from a semiarid region is useful in deriving scavenging ratio (SR = 290) of 210Pb. On the basis of data for the three consecutive years (2000–2002), we find that both dry and wet deposition fluxes of 210Pb center around 2–4 mBq cm−2 a−1, suggesting relative dominance of dry deposition in a semiarid region. When used in conjunction with 210Pb, 7Be provides information on the vertical mixing of air masses. During the dry season (January–May and September–December), abundances of 7Be and 210Pb (mBq m−3) in ambient aerosols over Ahmedabad (7Be, 1.9 ± 0.1 to 6.0 ± 0.3; 210Pb, 0.32 ± 0.03 to 1.9 ± 0.2) and Mt. Abu (7Be, 3.8 ± 0.2 to 7.6 ± 0.3; 210Pb, 0.39 ± 0.05 to 1.8 ± 0.2) do not show any covariance, suggesting their usefulness as independent tracers of air masses and pollutants transport. Assuming literature‐based constant 222Rn flux and measured abundance of 210Pb in aerosols, a model‐based approach has provided a simple way to ascertain residence time of tropospheric aerosols, varying from ∼5 days during the dry season and ∼2 days in the wet season. These results attempt to fill a major existing gap for the south Asian region under Global Atmospheric Watch program.

Transport and mixing of chemical air masses in idealized baroclinic life cycles

The transport, mixing, and three‐dimensional evolution of chemically distinct air masses within growing baroclinic waves are studied in idealized, high‐resolution, life cycle experiments using suitably initialized passive tracers, contrasting the two well‐known life cycle paradigms, distinguished by predominantly anticyclonic (LC1) or cyclonic (LC2) flow at upper levels. Stratosphere‐troposphere exchange differs significantly between the two life cycles. Specifically, transport from the stratosphere into the troposphere is significantly larger for LC2 (typically by 50%), due to the presence of large and deep cyclonic vortices that create a wider surf zone than for LC1. In contrast, the transport of tropospheric air into the stratosphere is nearly identical between the two life cycles. The mass of boundary layer air uplifted into the free troposphere is similar for both life cycles, but much more is directly injected into the stratosphere in the case of LC1 (fourfold, approximately). However, the total mixing of boundary layer with stratospheric air is larger for LC2, owing to the presence of the deep cyclonic vortices that entrain and mix both boundary layer air from the surface and stratospheric air from the upper levels. For LC1, boundary layer and stratospheric air are brought together by smaller cyclonic structures that develop on the poleward side of the jet in the lower part of the middleworld, resulting in correspondingly weaker mixing. As both the El Niño‐Southern Oscillation and the North Atlantic Oscillation are correlated with the relative frequency of life cycle behaviors, corresponding changes in chemical transport and mixing are to be expected.

Comparison of monthly and daily isotopic composition of precipitation in the coastal area of Slovenia†

The stable isotopic composition (δ2H and δ18O) of short-term (daily) precipitation collected from October 2002 to September 2003 at two stations in a coastal, karstic area in south-western Slovenia was investigated. In addition, monthly composite samples were collected and analysed for comparison with amount-weighted monthly means. The δ2H and δ18O values obtained show a wide range and reflect seasonal climatic variations. Deuterium excess and local meteoric water lines (LMWLs) were determined and cumulative frequency analysis and coincidence tests were performed. The statistical coincidence test showed that the LMWLs calculated from monthly data for Portorož and Kozina are coincident, but the LMWLs calculated from daily precipitation events are not. This difference could be explained by the greater variance of the isotopic composition of daily precipitation in comparison to monthly composite samples and also to the influence of evaporation during events below<1 mm at Portorož during the extremely dry and warm spring–summer season of 2003. Finally, synoptic maps and backward trajectories of a selected precipitation event showed that changes of isotopic composition are related to mixing of air masses originating from the continent and Mediterranean cyclogenesis. †Revised version of a paper presented at the 27th Annual Meeting of the German Association for Stable Isotope Research (GASIR), October, 4–6, 2006, Freiberg, Germany.

Chemical behavior of the tropopause observed during the Stratosphere‐Troposphere Analyses of Regional Transport experiment

During the Stratosphere‐Troposphere Analyses of Regional Transport (START) experiment in December 2005, the behavior of the extratropical tropopause was examined under a variety of dynamical conditions. Using in situ measurements of ozone and water vapor, on board the new NSF/NCAR research aircraft Gulfstream V, and data from large‐scale meteorological analyses, we address issues of the tropopause definitions and sharpness. Comparisons of the data from two flights show that the sharpness of chemical transitions across the tropopause varies with the sharpness of the static stability change across the tropopause. Using tracer correlations, air masses of mixed stratospheric and tropospheric characteristics are identified. The mixed air mass does not form a uniform mixing layer near the tropopause, but rather shows strong spatial variation. A depth of mixed air (∼5 km in vertical distribution) is found on the cyclonic side of the polar jet, where the thermal gradient is weak and significant separation occurs between the thermal and the dynamical tropopause. Away from the jet or on the anticyclonic side of the jet, where the stability gradient is strong, the chemical transition across the tropopause was much more abrupt and shows minimum mixing. In both cases (either significant or minimal mixing), the thermal tropopause is shown to be approximately at the center of the mixing layer, and the altitude relative to the thermal tropopause is found to be an effective coordinate for locating the chemical transition. To further understand the role of the thermal and dynamical tropopause as a chemical transport boundary, tracer correlations are used to examine the chemical characteristics, and the trajectory calculations are used to infer the fate of the air mass between the thermal and dynamic tropopauses in the region of significant separation. The tracer correlation analysis shows that the air mass in this region is a mixture of stratospheric and tropospheric air but predominantly of tropospheric characteristics. Trajectory model calculations show that a significant fraction of the air parcels in this region ended in the mid to lower troposphere, which suggest the irreversible nature of the observed stratospheric intrusion.

Temporal variations in ozone concentrations derived from Principal Component Analysis

The application of principal components and cluster analysis to vertical ozone concentration profiles in Tsukuba, Japan, has been explored. Average monthly profiles and profiles of the ratio between standard deviation and the absolute ozone concentration (SDPR) of 1 km data were calculated from the original ozone concentration data. Mean (first) and gradient (second) components explained more than 80% of the variation in both the 0–6 km tropospheric and 11–20 km troposphere–stratosphere (interspheric) layers. The principal components analysis not only reproduced the expected inverse relationship between mean ozone concentration and tropopause height (r2 = 0.41) and that in the tropospheric layer this is larger in spring and summer, but also yielded new information as follows. The larger gradient component score in summer for the interspheric layer points to the seasonal variation of the troposphere–stratosphere exchange. The minimum SDPR was at about 3 km in the tropospheric layer and the maximum was at about 17 km in the interspheric layer. The tropospheric SDPR mean component score was larger in summer, possibly reflecting the mixing of Pacific maritime air masses with urban air masses. The cluster analysis of the monthly ozone profiles for the 1970s and 2000s revealed different patterns for winter and summer. The month of May was part of the winter pattern in the 1970s but part of the summer pattern during the 2000s. This statistically detected change likely reflects the influence of global warming. Thus, these two statistical analysis techniques can be powerful tools for identifying features of ozone concentration profiles.

Source identification analysis for the airborne bacteria and fungi using a biomarker approach

Our recent studies have reported the feasibility of employing the 3-hydoxy fatty acids (3-OH FAs) and ergosterol as biomarkers to determine the loading of the airborne endotoxin from the Gram-negative bacteria and fungal biomass in atmospheric aerosols, respectively [Lee, A.K.Y., Chan, C.K., Fang, K., Lau, A.P.S., 2004. The 3-hydroxy fatty acids as biomarkers for quantification and characterization of endotoxins and Gram-negative bacteria in atmospheric aerosols in Hong Kong. Atmospheric Environment 38, 6807–6317; Lau, A.P.S., Lee, A.K.Y., Chan, C.K., Fang, K., 2006. Ergosterol as a biomarker for the quantification of the fungal biomass in atmospheric aerosols. Atmospheric Environment 40, 249–259]. These quantified biomarkers do not, however, provide information on their sources. In this study, the year-long dataset of the endotoxin and ergosterol measured in Hong Kong was integrated with the common water-soluble inorganic ions for source identification through the principal component analysis (PCA) and backward air mass trajectory analysis. In the coarse particles (PM 2.5–10), the bacterial endotoxin is loaded in the same factor group with Ca 2+ and accounted for about 20% of the total variance of the PCA. This implies the crustal origin for the airborne bacterial assemblage. The fungal ergosterol in the coarse particles (PM 2.5–10) had by itself loaded in a factor group of 10.8% of the total variance in one of the sampling sites with large area of natural vegetative coverage. This suggests the single entity nature of the fungal spores and their independent emission to the ambient air upon maturation of their vegetative growth. In the fine particles (<PM 2.5), the endotoxin and ergosterol associated closely with K + and accounted for 34–38% of the total variance in the two sites studied. The K +/Na + ratio is indicative of the possible sources of K +, which in turn, provides information on the sources of the associated endotoxin and ergosterol. High K +/Na + ratios (>2.0) in the fine particles of the continental air masses imply the microbial source from activities related to biomass burning and industries from the north. The low K +/Na + ratios (1.2–1.4) in the fine particles of the mixed air masses suggest microbial sources from the local and regional indoor environment through kitchen emissions and the re-suspension of the road dust due to vehicular exhausts.

Model Study on the Transport and Mixing of Dust Aerosols and Pollutants during an Asian Dust Storm in March 2002

The transport and mixing of dust aerosols and pollutants in East Asia during March 18 to 22, 2002 was studied using the nested air quality prediction model system (NAQPMS). Dust was primarily generated in the Gobi desert on 19 March and then swept across several areas of East Asia. The model results were verified with observations of surface weather, TSP/PM10, SO2 and lidar data. The model simulated the right timing and strength of dust events, capturing most of the variation features in dust and SO2. Numerical results showed that the dust aerosols were mainly transported in two layers and mixed with pollutants in different ways. Some of the dust kicked up in the source region was uplifted to a higher layer (200-2000 m layer) and transported downwind faster than dust of the lower level. This lower-level dust was of greater concentration. The dust arriving at the upper layer began to drop and mixed well with pollutants in the atmosphere during ”the first period”. During ”the second period”, pollutants were diluted by the dust air mass that was transported along the lower layer. The remaining pollutants mixed well with dust aerosols during this period. The mixed air mass of the higher layer (1500 m) eventually reached the Northwestern Pacific. A large amount of clouds in the upper layers potentially led to an increase in sulfate mass on the surface of dust particles.

Icelandic Coastal Sea Surface Temperature Records Constructed: Putting the Pulse on Air–Sea–Climate Interactions in the Northern North Atlantic. Part I: Comparison with HadISST1 Open-Ocean Surface Temperatures and Preliminary Analysis of Long-Term Patterns and Anomalies of SSTs around Iceland

Abstract A new comprehensive record of long-term Icelandic sea surface temperature measurements, which have been updated and filled in with reference to air temperature records, is presented. The new SST series reveal important features of the variability of climate in Iceland and the northern North Atlantic. This study documents site histories and possible resulting inconsistencies and biases, for example, changes in observing sites and instruments. A new 119-yr continuous time series for north Iceland SST is presented, which should prove particularly useful for investigating air–sea ice interactions around northern Iceland. As this is the only part of the country to be regularly engulfed by winter and/or spring sea ice, it is therefore highly sensitive to climatic change. The coastal series correlate well overall with independent Hadley Centre Sea Ice and SST dataset version 1 (HadISST1) series from the adjacent open ocean (mean r = 0.59), although correlations are generally higher in summer than winter and for south and east Iceland compared with the west and north. The seasonal temperature range is generally twice as large at the coastal sites because of differential effects of radiation, melting, mixing, and advection of warmer or colder air or water masses, as well as spatial resolution differences and smoothing in HadISST1. The long-term climatological averages and graphs for the 10 SST stations and/or their composites reveal decadal variations and trends that are generally similar to Icelandic air temperature records: a cold late-nineteenth-century, rapid warming around the 1920s, an overall warm peak circa 1940, cooling until an “icy” period circa 1970, followed by warming. Regional differences between sites include relatively greater (lesser) long-term variations for the eastern and southern (western and northern) Icelandic coasts, suggesting greater variability and influence of ocean current advection in the southeast. Moreover, Vestmannaeyjar SST data reveal that the late-nineteenth-century cold period in the ocean was not confined to the cold currents off north and east Iceland but also affected the south coast markedly. The Stykkishólmur, Iceland, SST record is relatively noisy and shows very little decadal variation, which may largely be due to fjord ice in cold winters suppressing low temperatures. It is anticipated that researchers may find these Icelandic SST series of practical use as a historic measure of air–sea–climate interactions around Iceland.

Measurements of N2O5 near Fairbanks, Alaska

We report measurements of N2O5, CO, and O3 at a subarctic site near Fairbanks, Alaska (65°N), during two periods of approximately 2 weeks each, one in December 2002 and the other in February 2004. N2O5 mixing ratios range from below the instrument detection limit of 2 parts per trillion by volume (pptv) up to approximately 250 pptv. Ozone and carbon monoxide measurements help distinguish several different air mass types: “clean” (near background CO and ozone), “titrated” (ozone titrated to near zero and elevated CO), and “mixed” (intermediate ozone and some CO elevation). Observed N2O5 mixing ratios are near zero for air masses that are either clean or titrated. Times with measurable N2O5 are almost always associated with mixed air masses. Measurements of CO and the CO:NOx emissions ratio in Fairbanks are used to estimate NO2 mixing ratios and calculate steady state lifetimes for N2O5. The lifetime estimates vary from subminute to 4 hours, with a median for the February study of 10 min and a median for the December study of 4 min. These lifetimes are quite short compared to the lifetime of NO2 with respect to reaction with ozone and formation of NO3, suggesting that the rate limiting step of NOx oxidation in the nighttime atmosphere is the formation of NO3, not the losses of NO3 and N2O5. These data show sinks of N2O5 (presumably mostly heterogeneous hydrolysis) are an efficient mechanism of NOx removal near the surface in the dark, high‐latitude winter.

Modelling the budget of middle atmospheric water vapour isotopes

Abstract. A one-dimensional chemistry model is applied to study the stable hydrogen (D) and stable oxygen isotope (17O, 18O) composition of water vapour in stratosphere and mesosphere. In the troposphere, this isotope composition is determined by "physical'' fractionation effects, that are phase changes (e.g. during cloud formation), diffusion processes (e.g. during evaporation from the ocean), and mixing of air masses. Due to these processes water vapour entering the stratosphere first shows isotope depletions in D/H relative to ocean water, which are ~5 times of those in 18O/16O, and secondly is mass-dependently fractionated (MDF), i.e. changes in the isotope ratio 17O/16O are ~0.52 times of those of 18O/16O. In contrast, in the stratosphere and mesosphere "chemical'' fractionation mechanisms, that are the production of HO due to the oxidation of methane, re-cycling of H2O via the HOx family, and isotope exchange reactions considerably enhance the isotope ratios in the water vapour imported from the troposphere. The model reasonably predicts overall enhancements of the stable isotope ratios in H2O by up to ~25% for D/H, ~8.5% for 17O/16O, and ~14% for 18O/16O in the mesosphere relative to the tropopause values. The 17O/16O and 18O/16O ratios in H2O are shown to be a measure of the relative fractions of HOx that receive the O atom either from the reservoirs O2 or O3. Throughout the middle atmosphere, MDF O2 is the major donator of oxygen atoms incorporated in OH and HO2 and thus in H2O. In the stratosphere the known mass-independent fractionation (MIF) signal in O3 is in a first step transferred to the NOx family and only in a second step to HOx and H2O. In contrast to CO2, O(1D) only plays a minor role in this MIF transfer. The major uncertainty in our calculation arises from poorly quantified isotope exchange reaction rate coefficients and kinetic isotope fractionation factors.

Characteristics and origin of lowermost stratospheric aerosol at northern midlatitudes under volcanically quiescent conditions based on CARIBIC observations

Characteristics and origin of the aerosol in the lowermost stratosphere at northern midlatitudes were studied using measurements from a passenger aircraft (Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container, or CARIBIC). Aerosol samples were collected during 60 intercontinental flights during 1999–2002 and analyzed for elemental composition with particle‐induced X‐ray emission (PIXE). Concurrent measurements of trace gases were used to interpret the aerosol measurements. It was found that particulate sulfur concentration increased steadily in the potential vorticity (PV) region of 2–7 PVU, whereas particulate potassium and iron showed no such dependence. The variability in concentration of the latter two elements was mainly connected with season, similar to their variation in the upper troposphere, whereas PV dominated the particulate sulfur variability. An ozone‐based model was developed to quantitatively determine the mixing of stratospheric and tropospheric air masses. A significant dependence on PV was found, and the stratospheric fraction of the air peaked during spring. It was found that the particulate sulfur concentration was strongly dependent on the origin of the air masses. The concentration increased by a factor of 3 over the lowermost stratosphere. A discontinuity in the concentration over the tropopause indicated particle formation from sulfur dioxide transported across the tropopause. The concentration at the top of the lowermost stratosphere was used to estimate that the particulate sulfur production in the stratosphere is 0.066 Tg S/yr with approximately half of the amount transported across the top of the lowermost stratosphere originating in carbonyl sulfide.

Aerosol characteristics of air masses in northern Europe: Influences of location, transport, sinks, and sources

Synoptic‐scale air masses at different stations were classified following a definition based on Berliner Wetterkarte. This air mass classification has been related to 1 year of aerosol number size distributions measurements performed at four different stations extending from Aspvreten in Sweden (58.8°N) to Pallas in northern Finland (68°N). The air mass classification describes both class of air mass, based on the origin of the air mass, and character of air in terms of marine, mixed, and continental air masses. The aerosol size distribution properties were evaluated in relation to the air masses. Emphasis was put on the differences between marine, mixed, and continental character air masses. It is shown that continental air masses exceed marine and mixed character air masses both in number and mass concentration. Different classes of air masses were further associated with different aerosol size distribution properties. It is also shown that although serving as a somewhat good qualifier for the aerosol at individual stations, the air mass classification cannot be used to estimate the aerosol burden over large geographical areas. Instead, a sharp gradient was shown to exist between different stations, although aerosol properties were observed in equal air masses according to the definition by Berliner Wetterkarte. This gradient manifests as a south‐northerly decrease in aerosol total number and volume, indicating that the aerosol properties including the aerosol size distribution are less conservative than the thermodynamic properties (e.g., pseudo‐potential temperature and humidity profiles) that characterize the different air masses. Further, using a pseudo‐Lagrangian approach, the aerosol turnover time was estimated for different sized aerosols in air moving from south to north (i.e., depletion of aerosols in air arriving from the continent). Turnover time of Aitken particles was found to be in the range of 1–2 days, while accumulation mode turnover time was estimated to be in the order of 2–3 days.