[Show abstract][Hide abstract] ABSTRACT: Observations of atmospheric mercury at the Mace Head Atmospheric Research Station on the Atlantic coast of Ireland made from February 1996 to December 2013 are analyzed. Using meteorological analysis and a sophisticated Lagrangian dispersion model, the hourly averaged mercury concentrations were attributed to four different air mass types: baseline, local, European polluted, and sub-tropical maritime. Monthly median Hg concentrations of all types decreased over the analyzed period but the trend for sub-tropical maritime air masses was with −0.016 ± 0.002 ng m−3 yr−1 in absolute terms significantly smaller than the trends for all other classes which varied between −0.021 and −0.023 ng m−3 yr−1. The seasonal variation for sub-tropical maritime air masses is also shallower than for all other classes. This is most likely due to shallower seasonal variation of oxidant concentrations at lower latitudes. The north-south gradient of the trend is qualitatively consistent with the GEOS-Chem model predictions based on decrease of mercury concentrations in surface waters of the North Atlantic but the trends are smaller than predicted. Tests for temporal change of the trends indicate that the decreasing trends of mercury concentrations are leveling off for all air masses with possible exception of the sub-tropical maritime air mass. Quantitative assessment of the trend changes, however, will require a longer time series of the mercury measurements at Mace Head.
[Show abstract][Hide abstract] ABSTRACT: Cluster analysis of particle number size distributions from background sites across Europe is presented. This generated a total of nine clusters of particle size distributions which could be further combined into two main groups, namely: a south-to-north category (four clusters) and a west-to-east category (five clusters). The first group was identified as most frequently being detected inside and around northern Germany and neighbouring countries, showing clear evidence of local afternoon nucleation and growth events that could be linked to movement of air masses from south to north arriving ultimately at the Arctic contributing to Arctic haze. The second group of particle size spectra proved to have narrower size distributions and collectively showed a dependence of modal diameter upon the longitude of the site (west to east) at which they were most frequently detected. These clusters indicated regional nucleation (at the coastal sites) growing to larger modes further inland. The apparent growth rate of the modal diameter was around 0.6–0.9 nm* h^(−1)
Four specific air mass back-trajectories were successively taken as case studies to examine in real time the evolution of aerosol size distributions across Europe. While aerosol growth processes can be observed as aerosol traverses Europe, the processes are often obscured by the addition of aerosol by emissions en route. This study revealed that some of the 24 stations exhibit more complex behaviour than others, especially when impacted by local sources or a variety of different air masses. Overall, the aerosol size distribution clustering analysis greatly simplifies the complex data set and allows a description of aerosol aging processes, which reflects the longer-term average development of particle number size distributions as air masses advect across Europe.
[Show abstract][Hide abstract] ABSTRACT: Many of the next generation of global climate models will include aerosol schemes which explicitly simulate the microphysical processes that determine the particle size distribution. These models enable aerosol optical properties and cloud condensation nuclei (CCN) concentrations to be determined by fundamental aerosol processes, which should lead to a more physically based simulation of aerosol direct and indirect radiative forcings. This study examines the global variation in particle size distribution simulated by 12 global aerosol microphysics models to quantify model diversity and to identify any common biases against observations. Evaluation against size distribution measurements from a new European network of aerosol supersites shows that the mean model agrees quite well with the observations at many sites on the annual mean, but there are some seasonal biases common to many sites. In particular, at many of these European sites, the accumulation mode number concentration is biased low during winter and Aitken mode concentrations tend to be overestimated in winter and underestimated in summer. At high northern latitudes, the models strongly underpredict Aitken and accumulation particle concentrations compared to the measurements, consistent with previous studies that have highlighted the poor performance of global aerosol models in the Arctic. In the marine boundary layer, the models capture the observed meridional variation in the size distribution, which is dominated by the Aitken mode at high latitudes, with an increasing concentration of accumulation particles with decreasing latitude. Considering vertical profiles, the models reproduce the observed peak in total particle concentrations in the upper troposphere due to new particle formation, although modelled peak concentrations tend to be biased high over Europe. Overall, the multimodel-mean data set simulates the global variation of the particle size distribution with a good degree of skill, suggesting that most of the individual global aerosol microphysics models are performing well, although the large model diversity indicates that some models are in poor agreement with the observations. Further work is required to better constrain size-resolved primary and secondary particle number sources, and an improved understanding of nucleation and growth (e. g. the role of nitrate and secondary organics) will improve the fidelity of simulated particle size distributions.
[Show abstract][Hide abstract] ABSTRACT:  Primary-produced sea spray aerosol, typically comprising sea-salt, but also enriched with organic matter (OM) in biologically active oceanic regions, impacts the global radiative budget through contributions to aerosol optical depth. We show that sea spray light-scattering enhancement, f(RH), as a function of relative humidity (RH) is suppressed when enriched with OM. A new hygroscopic growth factor parameterization reveals a dual hygroscopicity state, flipping from high hygroscopicity and high f(RH) to low hygroscopicity and low f(RH) as the OM mixing volume percentage exceeds ~ 55% in sea spray. Under elevated wind speeds, this affects the Top-of-atmosphere direct radiative forcing (ΔF) by reducing the cooling contribution of sea spray by ~ 5.5 times compared to pure sea-salt spray. These results suggest a positive feedback coupling between the marine biosphere, sea spray aerosol, and the direct radiative budget.
[Show abstract][Hide abstract] ABSTRACT: Sixteen years (1994 – 2009) of ozone profiling by ozonesondes at Valentia Meteorological and Geophysical Observatory, Ireland (51.94° N, 10.23° W) along with a co-located MkIV Brewer spectrophotometer for the period 1993–2009 are analyzed. Simple and multiple linear regression methods are used to infer the recent trend, if any, in stratospheric column ozone over the station. The decadal trend from 1994 to 2010 is also calculated from the monthly mean data of Brewer and column ozone data derived from satellite observations. Both of these show a 1.5 % increase per decade during this period with an uncertainty of about ±0.25 %. Monthly mean data for March show a much stronger trend of ~ 4.8 % increase per decade for both ozonesonde and Brewer data. The ozone profile is divided between three vertical slots of 0–15 km, 15–26 km, and 26 km to the top of the atmosphere and a 11-year running average is calculated. Ozone values for the month of March only are observed to increase at each level with a maximum change of +9.2 ± 3.2 % per decade (between years 1994 and 2009) being observed in the vertical region from 15 to 26 km. In the tropospheric region from 0 to 15 km, the trend is positive but with a poor statistical significance. However, for the top level of above 26 km the trend is significantly positive at about 4 % per decade. The March integrated ozonesonde column ozone during this period is found to increase at a rate of ~6.6 % per decade compared with the Brewer and satellite positive trends of ~5 % per decade.
[Show abstract][Hide abstract] ABSTRACT: The atmospheric concentration of elemental carbon (EC) in Europe during the six-year period 2005-2010 has been simulated with the EMEP MSC-W model. The model bias compared to EC measurements was less than 20% for most of the examined sites. The model results suggest that fossil fuel combustion is the dominant source of EC in most of Europe but that there are important contributions also from residential wood burning during the cold seasons and, during certain episodes, also from open biomass burning (wildfires and agricultural fires). The modelled contributions from open biomass fires to ground level concentrations of EC were small at the sites included in the present study, <3% of the long-term average of EC in PM10. The modelling of this EC source is subject to many uncertainties, and it was likely underestimated for some episodes.
EC measurements and modelled EC were also compared to optical measurements of black carbon (BC). The relationships between EC and BC (as given by mass absorption cross section, MAC, values) differed widely between the sites, and the correlation between observed EC and BC is sometimes poor, making it difficult to compare results using the two techniques and limiting the comparability of BC measurements to model EC results.
A new bottom-up emission inventory for carbonaceous aerosol from residential wood combustion has been applied. For some countries the new inventory has substantially different EC emissions compared to earlier estimates. For northern Europe the most significant changes are much lower emissions in Norway and higher emissions in neighbouring Sweden and Finland. For Norway and Sweden, comparisons to source-apportionment data from winter campaigns indicate that the new inventory may improve model-calculated EC from wood burning.
Finally, three different model setups were tested with variable atmospheric lifetimes of EC in order to evaluate the model sensitivity to the assumptions regarding hygroscopicity and atmospheric ageing of EC. The standard ageing scheme leads to a rapid transformation of the emitted hydrophobic EC to hygroscopic particles, and generates similar results when assuming that all EC is aged at the point of emission. Assuming hydrophobic emissions and no ageing leads to higher EC concentrations. For the more remote sites, the observed EC concentration was in between the modelled EC using standard ageing and the scenario treating EC as hydrophobic. This could indicate too-rapid EC ageing in the model in relatively clean parts of the atmosphere.
[Show abstract][Hide abstract] ABSTRACT: Clean-air policies in developing countries have resulted in reduced
levels of anthropogenic atmospheric aerosol pollution. Reductions in
aerosol pollution is thought to result in a reduction in haze and cloud
layers, leading to an increase in the amount of solar radiation reaching
the surface, and ultimately, an increase in surface temperatures. There
have been many studies illustrating coherent relationships between
surface solar radiation and temperature however, a direct link between
aerosol emissions, concentrations, and surface radiation has not been
demonstrated to date. Here, we illustrate a coherence between the trends
of reducing anthropogenic aerosol emissions and concentrations, at the
interface between the North-East Atlantic and western-Europe, leading to
a staggering increase in surface solar radiation of the order of ~20%
over the last decade.
[Show abstract][Hide abstract] ABSTRACT: Primary-produced sea-spray is typically composed of sea-salt, but in
biologically-active regions, the spray can become enriched with organic
matter which reduces hygroscopicity of sea-spray, thereby having a
potential impact on aerosol scattering. This study shows that scattering
enhancement of marine aerosol, as a function of increasing relative
humidity, is reduced when enriched with organics whose results are used
to develop a new hygroscopicity growth-factor parameterization for
sea-spray enriched in organic matter. The parameterization reveals a
dual state which flips from high-hygroscopicity and high-scattering
enhancement to low-hygroscopicity and low-scattering enhancement as the
organic volume fraction increases from below ~ 0.55 to above ~ 0.55. In
terms of organic enrichment, the effect on Top of Atmosphere (TOA)
direct radiative forcing (ΔF) is to reduce the cooling
contribution of sea-spray by ~ 5.5 times compared to pure sea-salt
spray. The results presented here highlight a significant coupling
between the marine biosphere and the direct radiative budget through
alteration of sea-spray chemical composition, potentially leading to
accelerated global warming should biological activity increase with
future projected temperature increases.
[Show abstract][Hide abstract] ABSTRACT: Aerosol light scattering measurements were carried out at the Global
Atmosphere Watch (GAW) research station, Mace Head, on the west coast of
Ireland. Analysis of ten years (2001-2010) of the aerosol light
scattering coefficient (σscat) at 550nm reveals a strong seasonal
trend with σscat values during the winter season being
approximately three times that of the summer season. The σscat and
the aerosol light backscattering coefficient (σbscat) for the
winter (Low Biological Activity - LBA) and the summer (High Biological
Activity-HBA) are dependent on the square of wind speed (U). Scattering
properties for the LBA period are approximately twice those of the HBA
period. 70% of the observed LBA-HBA scattering difference can be
attributed to differences in refractive index alone, resulting from
organicmatter enrichment during the HBA period.
[Show abstract][Hide abstract] ABSTRACT: Dr. Thomas C. (Tom) O'Connor received his foundation in atmospheric
aerosols through his M. Sc. work at University College Dublin (with P.J.
Nolan) and then as research scholar with Leo W. Pollak at the Dublin
Institute of Advanced Studies. On moving to Galway in 1956, a
significant legacy was his choosing of a field station site at Mace Head
and his pioneering measurements there. He played a pivotal role in the
development and progression of the Mace Head Atmospheric Research
Station (www.macehead.org) for some 50 years. He passed away peacefully
in November 2012.
[Show abstract][Hide abstract] ABSTRACT: Long-range transported air pollution has markedly decreased in Ireland
and Europe over the last two decades. Yearly average nss-sulphate
concentration has dropped by 70% at Mace Head during the last decade
alone. Backward air mass trajectories indicated little contact with the
North American continent precluding significant transport of
anthropogenic particulate matter in an increasingly cleaner atmosphere.
Polluted easterly air masses from the European continent rarely reach as
far as the mid North Atlantic, making the eastern North Atlantic one the
cleanest regions in the Northern Hemisphere. Carbon isotope analysis 
has confirmed the above findings also for carbonaceous particulate
matter. The influence of natural biogenic components including biogenic
non-sea-salt sulphate and sea salt is having increasing impact on air
quality and cloudiness with significant climatic implications.
[Show abstract][Hide abstract] ABSTRACT: A regional climate model is used to assess changes in atmospheric ozone for the years 2030, 2050 and 2100 relative to 2006 brought about by changes in meteorology and emissions. The simulations are evaluated against ozone measurements for 2006, exhibiting good agreement between the model-predicted measurements and the measured annual cycles. Under the RCP6 emission scenario used in these simulations, average ozone mixing ratios are set to reduce by 2.0 ppb over domains encompassing Europe and the North East Atlantic between 2006 and 2100 with the most significant decrease occurring after 2050 due to the pattern in changing emissions. Peak reductions of more than 8 ppb are observed during summer time over mainland Europe by 2100. Model output was studied for three relevant sub-domains, namely the North East Atlantic, Ireland and Europe. The relative contribution of changes in both emissions and meteorology is assessed. Over the whole domain, changing emissions are predominantly responsible for changes in surface ozone; although over the North East Atlantic domain, the changing emissions do not perturb surface ozone trends and the decrease in 2100 levels is entirely attributable to changing meteorology.
[Show abstract][Hide abstract] ABSTRACT: We have analysed the trends of total aerosol parti-cle number concentrations (N) measured at long-term mea-surement stations involved either in the Global Atmosphere Watch (GAW) and/or EU infrastructure project ACTRIS. The sites are located in Europe, North America, Antarc-tica, and on Pacific Ocean islands. The majority of the sites showed clear decreasing trends both in the full-length time-series, and in the intra-site comparison period of 2001–2010, especially during the winter months. Several potential driv-ing processes for the observed trends were studied, and even though there are some similarities between N trends and air temperature changes, the most likely cause of many north-ern hemisphere trends was found to be decreases in the an-thropogenic emissions of primary particles, SO 2 or some co-emitted species. We could not find a consistent agreement between the trends of N and particle optical properties in the few stations with long timeseries of all of these properties. The trends of N and the proxies for cloud condensation nu-clei (CCN) were generally consistent in the few European stations where the measurements were available. This work provides a useful comparison analysis for modelling studies of trends in aerosol number concentrations.
Atmospheric Chemistry and Physics 02/2013; 13:1-22. · 5.05 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Currently many ground-based atmospheric stations include in-situ
measurements of aerosol physical and optical properties, resulting in
more than 20 long-term (>10 yr) aerosol measurement sites in the
Northern Hemisphere and Antarctica. Most of these sites are located at
remote locations and monitor the aerosol particle number concentration,
wavelength-dependent light scattering, backscattering, and absorption
coefficients. The existence of these multi-year datasets enables the
analysis of long-term trends of these aerosol parameters of the derived
light scattering Ångström exponent and backscatter fraction.
Since the aerosol variables are not normally distributed, three
different methods (the seasonal Mann-Kendall test associated with the
Sen's slope, the generalized least squares fit associated with an
autoregressive bootstrap algorithm for confidence intervals, and the
least-mean square fit applied to logarithms of the data) were applied to
detect the long-term trends and their magnitudes for each month. To
allow a comparison among measurement sites with varying length of data
records, trends on the most recent 10 and 15 yr periods were calculated.
No significant trends were found for the three continental European
sites. Statistically significant trends were found for the two European
marine sites but the signs of the trends varied with aerosol property
and location. Statistically significant decreasing trends for both
scattering and absorption coefficient were found for most North American
stations, although positive trends were found for a few desert and
high-altitude sites. No significant trends in scattering coefficient
were found for the Arctic or Antarctic stations, whereas the Arctic
station had a negative trend in absorption coefficient.
[Show abstract][Hide abstract] ABSTRACT: The Mace Head dataset comprises the longest existing time series of atmospheric mercury measurements with high time resolution in the temperate marine background atmosphere, starting in September 1995. For this study, the concentrations of total gaseous mercury in baseline air masses arriving at Mace Head, Ireland have been analyzed for possible trends in the atmospheric mercury background concentration over a 16-year period (i.e., 1996-2011). Statistical analyses have revealed a significant negative (downwards) trend of -0.027 +/-0.01 ng/m(3) yr(-1), or -1.4 to 1.8% per year. Furthermore, evidence of a seasonal cycle was found with somewhat higher concentrations during the winter time and somewhat lower concentrations during summer.
16th International Conference on Heavy Metals in the Environment (ICHMET2012), Roma; 01/2013
[Show abstract][Hide abstract] ABSTRACT: We have analysed the trends of total aerosol particle number
concentrations (N) measured at long-term measurement stations involved
either in the Global Atmosphere Watch (GAW) and/or EU infrastructure
project ACTRIS. The sites are located in Europe, North America,
Antarctica, and on Pacific Ocean islands. The majority of the sites
showed clear decreasing trends both in the full-length time-series, and
in the intra-site comparison period of 2001-2010, especially during the
winter months. Several potential driving processes for the observed
trends were studied, and even though there are some similarities between
N trends and air temperature changes, the most likely cause of many
Northern Hemisphere trends was found to be decreases in the
anthropogenic emissions of primary particles, SO2 or some
co-emitted species. We could not find a consistent agreement between the
trends of N and particle optical properties in the few stations with
long timeseries of all of these properties. The trends of N and the
proxies for cloud condensation nuclei (CCN) were generally consistent in
the few European stations where the measurements were available. This
work provides a useful comparison analysis for modelling studies of
trends in aerosol number concentrations.
[Show abstract][Hide abstract] ABSTRACT: Mobility particle size spectrometers often referred to as DMPS (Differential Mobility Particle Sizers) or SMPS (Scanning Mobility Particle Sizers) have found a wide range of applications in atmospheric aerosol research. However, comparability of measurements conducted world-wide is hampered by lack of generally accepted technical standards and guidelines with respect to the instrumental set-up, measurement mode, data evaluation as well as quality control. Technical standards were developed for a minimum requirement of mobility size spectrometry to perform long-term atmospheric aerosol measurements. Technical recommendations include continuous monitoring of flow rates, temperature, pressure, and relative humidity for the sheath and sample air in the differential mobility analyzer.
We compared commercial and custom-made inversion routines to calculate the particle number size distributions from the measured electrical mobility distribution. All inversion routines are comparable within few per cent uncertainty for a given set of raw data.
Furthermore, this work summarizes the results from several instrument intercomparison workshops conducted within the European infrastructure project EUSAAR (European Supersites for Atmospheric Aerosol Research) and ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure Network) to determine present uncertainties especially of custom-built mobility particle size spectrometers. Under controlled laboratory conditions, the particle number size distributions from 20 to 200 nm determined by mobility particle size spectrometers of different design are within an uncertainty range of around ±10% after correcting internal particle losses, while below and above this size range the discrepancies increased. For particles larger than 200 nm, the uncertainty range increased to 30%, which could not be explained. The network reference mobility spectrometers with identical design agreed within ±4% in the peak particle number concentration when all settings were done carefully. The consistency of these reference instruments to the total particle number concentration was demonstrated to be less than 5%.
Additionally, a new data structure for particle number size distributions was introduced to store and disseminate the data at EMEP (European Monitoring and Evaluation Program). This structure contains three levels: raw data, processed data, and final particle size distributions. Importantly, we recommend reporting raw measurements including all relevant instrument parameters as well as a complete documentation on all data transformation and correction steps. These technical and data structure standards aim to enhance the quality of long-term size distribution measurements, their comparability between different networks and sites, and their transparency and traceability back to raw data.