-
Finland experienced extraordinary smoke episodes in 2006. The smoke was measured at the three SMEAR measurement network stations in Finland after it had been transported several hundreds of kilometers from burning areas in Eastern Europe. A trajectory method combining MODIS fire detections and HYSPLIT trajectories enabled us to separate the effect of biomass burning smoke from the measured concentrations and also study the changes in the smoke during its transport. The long-range transported smoke included at least NO<sub>x</sub>, SO<sub>2</sub>, CO<sub>2</sub>, CO, black carbon and fine aerosol particles, peaking at 100 to 200 nm size. The most reliable smoke markers were CO and SO<sub>2</sub>, especially when combined with particle data, for which black carbon or the condensation sink are very effective parameters separating the smoke episodes from no-smoke episodes. Signs of fresh secondary particles was observed based on the particle number size distribution data. While transported from south to north, particles grew in size, even after transport of tens of hours and several hundreds of kilometres. No new aerosol particle formation events were observed at the measurement sites during the smoke periods.
-
A significant desert dust deposition event occurred on Mt. Elbrus, Caucasus Mountains, Russia on 5 May 2009, where the deposited dust later appeared as a brown layer in the snow pack. An examination of dust transportation history and analysis of chemical and physical properties of the deposited dust were used to develop a new approach for high-resolution "provenancing" of dust deposition events recorded in snow pack using multiple independent techniques. A combination of SEVIRI red-green-blue composite imagery, MODIS atmospheric optical depth fields derived using the Deep Blue algorithm, air mass trajectories derived with HYSPLIT model and analysis of meteorological data enabled identification of dust source regions with high temporal (hours) and spatial (ca. 100 km) resolution. Dust, deposited on 5 May 2009, originated in the foothills of the Djebel Akhdar in eastern Libya where dust sources were activated by the intrusion of cold air from the Mediterranean Sea and Saharan low pressure system and transported to the Caucasus along the eastern Mediterranean coast, Syria and Turkey. Particles with an average diameter below 8 μm accounted for 90% of the measured particles in the sample with a mean of 3.58 μm, median 2.48 μm. The chemical signature of this long-travelled dust was significantly different from the locally-produced dust and close to that of soils collected in a palaeolake in the source region, in concentrations of hematite. Potential addition of dust from a secondary source in northern Mesopotamia introduced uncertainty in the "provenancing" of dust from this event. Nevertheless, the approach adopted here enables other dust horizons in the snowpack to be linked to specific dust transport events recorded in remote sensing and meteorological data archives.
-
Large volcanic eruptions impact significantly on climate and lead to ozone depletion due to injection of particles and gases into the stratosphere where their residence times are long. In this the composition of volcanic aerosol is an important but inadequately studied factor. Samples of volcanically influenced aerosol were collected following the Kasatochi (Alaska), Sarychev (Russia) and also during the Eyjafjallajökull (Iceland) eruptions in the period 2008–2010. Sampling was conducted by the CARIBIC platform during regular flights at an altitude of 10–12 km as well as during dedicated flights through the volcanic clouds from the eruption of Eyjafjallajökull in spring 2010. Elemental concentrations of the collected aerosol were obtained by accelerator-based analysis. Aerosol from the Eyjafjallajökull volcanic clouds was identified by high concentrations of sulphur and elements pointing to crustal origin, and confirmed by trajectory analysis. Signatures of volcanic influence were also used to detect volcanic aerosol in stratospheric samples collected following the Sarychev and Kasatochi eruptions. In total it was possible to identify 17 relevant samples collected between 1 and more than 100 days following the eruptions studied. The volcanically influenced aerosol mainly consisted of ash, sulphate and included a carbonaceous component. Samples collected in the volcanic cloud from Eyjafjallajökull were dominated by the ash and sulphate component (∼45% each) while samples collected in the tropopause region and LMS mainly consisted of sulphate (50–77%) and carbon (21–43%). These fractions were increasing/decreasing with the age of the aerosol. Because of the long observation period, it was possible to analyze the evolution of the relationship between the ash and sulphate components of the volcanic aerosol. From this analysis the residence time (1/e) of sulphur dioxide in the studied volcanic cloud was estimated to be 45 ± 22 days.
-
Complementary approaches have been taken to better understand the sources and their spatial distribution for secondary inorganic (nitrate and sulphate) and secondary organic aerosol sampled at a rural site (Harwell) in the southern United Kingdom. A concentration field map method was applied to 1581 daily samples of chloride, nitrate and sulphate from 2006 to 2010, and 982 samples for organic carbon and elemental carbon from 2007 to 2010. This revealed a rather similar pattern of sources for nitrate, sulphate and secondary organic carbon within western/central Europe, which in the case of nitrate, sulphate, organic carbon and secondary organic carbon, correlated significantly with EMEP emissions maps of NO<sub>x</sub>, SO<sub>2</sub>, and VOC respectively. A slightly more southerly source emphasis for secondary organic carbon may reflect the contribution of biogenic sources. Trajectory clusters confirm this pattern of behaviour with a major contribution from mainland European sources. Similar behaviours of, on the one hand, sulphate and organic carbon and, on the other hand, EC and nitrate showed that the former are more subject to regional influence than the latter in agreement with the slower atmospheric formation of sulphate and secondary organic aerosol than for nitrate, and the local/mesoscale influences upon primary EC. However, careful analysis of back trajectories and Concentration Field Maps indicates a strong contribution of mainland European sites to EC concentrations at Harwell. In a separate study, measurements of sulphate, nitrate, elemental and organic carbon were made in 100 simultaneously collected samples at Harwell and at a suburban site in Birmingham (UK). This showed a significant correlation in concentrations between the two sites for all of the secondary constituents, further indicating secondary organic aerosol to be a regional pollutant behaving similarly to sulphate and nitrate.
-
Volcanic SO<sub>2</sub> column amount and injection height retrieved from the Ozone Monitoring Instrument (OMI) with the Extended Iterative Spectral Fitting (EISF) technique are used to initialize a global chemistry transport model (GEOS-Chem) to simulate the atmospheric transport and lifecycle of volcanic SO<sub>2</sub> and sulfate aerosol from the 2008 Kasatochi eruption, and to subsequently estimate the direct shortwave, top-of-the-atmosphere radiative forcing of the volcanic sulfate aerosol. Analysis shows that the integrated use of OMI SO<sub>2</sub> plume height in GEOS-Chem yields: (a) good agreement of the temporal evolution of 3-D volcanic sulfate distributions between model simulations and satellite observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar with Orthogonal Polarisation (CALIOP), and (b) an e-folding time for volcanic SO<sub>2</sub> that is consistent with OMI measurements, reflecting SO<sub>2</sub> oxidation in the upper troposphere and stratosphere is reliably represented in the model. However, a consistent (~25%) low bias is found in the GEOS-Chem simulated SO<sub>2</sub> burden, and is likely due to a high (~20%) bias of cloud liquid water amount (as compared to the MODIS cloud product) and the resultant stronger SO<sub>2</sub> oxidation in the GEOS meteorological data during the first week after eruption when part of SO<sub>2</sub> underwent aqueous-phase oxidation in clouds. Radiative transfer calculations show that the forcing by Kasatochi volcanic sulfate aerosol becomes negligible 6 months after the eruption, but its global average over the first month is −1.3 Wm<sup>−2</sup>, with the majority of the forcing-influenced region located north of 20° N, and with daily peak values up to −2 Wm<sup>−2</sup> on days 16–17. Sensitivity experiments show that every 2 km decrease of SO<sub>2</sub> injection height in the GEOS-Chem simulations will result in a ~25 % decrease in volcanic sulfate forcing; similar sensitivity but opposite sign also holds for a 0.03 μm increase of geometric radius of the volcanic aerosol particles. Both sensitivities highlight the need to characterize the SO<sub>2</sub> plume height and aerosol particle size from space. While more research efforts are warranted, this study is among the first to assimilate both satellite-based SO<sub>2</sub> plume height and amount into a chemical transport model for an improved simulation of volcanic SO<sub>2</sub> and sulfate transport.
-
Physicochemical properties of aerosol were investigated by analyzing the inorganic water soluble content in PM<sub>2.5</sub> samples collected in the eastern part of the Po Valley (Italy). In this area the EU limits for many air pollutants are frequently exceeded as a consequence of local sources and regional-scale transport of secondary inorganic aerosol precursors. Nine PM<sub>2.5</sub>-bound major inorganic ions (F<sup>−</sup>, Cl<sup>−</sup>, NO<sub>3</sub><sup>−</sup>, SO<sub>4</sub><sup>2−</sup>, Na<sup>+</sup>, NH<sub>4</sub><sup>+</sup>, K<sup>+</sup>, Mg<sup>2+</sup>, Ca<sup>2+</sup>) were monitored over one year in three sites categorized as semi-rural background, urban background and industrial. The acidic properties of the PM<sub>2.5</sub> were studied by applying the recently developed E-AIM thermodynamic model 4 (Extended Aerosol Thermodynamics Model). The experimental data were also examined in relation to the levels of gaseous precursors of secondary inorganic aerosol (SO<sub>2</sub>, NO<sub>x</sub>, NO, NO<sub>2</sub>) and on the basis of some environmental conditions having an effect on the secondary aerosols generation processes. A chemometric procedure using cluster analysis on experimental [NH<sub>4</sub><sup>+</sup>]/[SO<sub>4</sub><sup>2−</sup>] molar ratio and NO<sub>3</sub><sup>−</sup> concentration has been applied to determine the conditions needed for ammonium nitrate formation in different chemical environments. Finally, some considerations on the secondary inorganic aerosol formation and the most relevant weather conditions concerning the sulfate-nitrate-ammonium system were also discussed. The obtained results and discussion can help in understanding the secondary aerosol formation dynamics in the Po Valley, which is one of the most critical regions for air pollution in southern Europe.
-
Secondary organic aerosols (SOA) exert a significant influence on ambient air quality and regional climate. Recent field, laboratorial and modeling studies have confirmed that in-cloud processes contribute to a large fraction of SOA production with large space-time heterogeneity. This study evaluates the key factors that govern the production of cloud-process SOA (SOA<sub>cld</sub>) on a global scale based on the GFDL coupled chemistry-climate model AM3 in which full cloud chemistry is employed. The association between SOA<sub>cld</sub> production rate and six factors (i.e., liquid water content (LWC), total carbon chemical loss rate (TC<sub>loss</sub>), temperature, VOC/NO<sub>x</sub>, OH, and O<sub>3</sub>) is examined. We find that LWC alone determines the spatial pattern of SOA<sub>cld</sub> production, particularly over the tropical, subtropical and temperate forest regions, and is strongly correlated with SOA<sub>cld</sub> production. TC<sub>loss</sub> ranks the second and mainly represents the seasonal variability of vegetation growth. Other individual factors are essentially uncorrelated spatiotemporally to SOA<sub>cld</sub> production. We find that the rate of SOA<sub>cld</sub> production is simultaneously determined by both LWC and TC<sub>loss</sub>, but responds linearly to LWC and nonlinearly (or concavely) to TC<sub>loss</sub>. A parameterization based on LWC and TC<sub>loss</sub> can capture well the spatial and temporal variability of the process-based SOA<sub>cld</sub> formation (<i>R</i><sup>2</sup> = 0.5) and can be easily applied to global three dimensional models to represent the SOA production from cloud processes.
-
We report on the AeroCom Phase II direct aerosol effect (DAE) experiment where 16 detailed global aerosol models have been used to simulate the changes in the aerosol distribution over the industrial era. All 16 models have estimated the radiative forcing (RF) of the anthropogenic DAE, and have taken into account anthropogenic sulphate, black carbon (BC) and organic aerosols (OA) from fossil fuel, biofuel, and biomass burning emissions. In addition several models have simulated the DAE of anthropogenic nitrate and anthropogenic influenced secondary organic aerosols (SOA). The model simulated all-sky RF of the DAE from total anthropogenic aerosols has a range from −0.58 to −0.02 Wm<sup>−2</sup>, with a mean of −0.27 Wm<sup>−2</sup> for the 16 models. Several models did not include nitrate or SOA and modifying the estimate by accounting for this with information from the other AeroCom models reduces the range and slightly strengthens the mean. Modifying the model estimates for missing aerosol components and for the time period 1750 to 2010 results in a mean RF for the DAE of −0.35 Wm<sup>−2</sup>. Compared to AeroCom Phase I (Schulz et al., 2006) we find very similar spreads in both total DAE and aerosol component RF. However, the RF of the total DAE is stronger negative and RF from BC from fossil fuel and biofuel emissions are stronger positive in the present study than in the previous AeroCom study. We find a tendency for models having a strong (positive) BC RF to also have strong (negative) sulphate or OA RF. This relationship leads to smaller uncertainty in the total RF of the DAE compared to the RF of the sum of the individual aerosol components. The spread in results for the individual aerosol components is substantial, and can be divided into diversities in burden, mass extinction coefficient (MEC), and normalized RF with respect to AOD. We find that these three factors give similar contributions to the spread in results.
-
This paper presents a step in the development of a top-down method to complement the bottom-up inventories of halocarbon emissions in California using high frequency observations, forward simulations and inverse methods. The Scripps Institution of Oceanography high-frequency atmospheric halocarbons measurement sites are located along the California coast and therefore the evaluation of transport in the chosen Weather Research Forecast (WRF) model at these sites is crucial for inverse modeling. The performance of the transport model has been investigated by comparing the wind direction and speed and temperature at four locations using aircraft weather reports as well at all METAR weather stations in our domain for hourly variations. Different planetary boundary layer (PBL) schemes, horizontal resolutions (achieved through nesting) and two meteorological datasets have been tested. Finally, simulated concentration of an inert tracer has been briefly investigated. All the PBL schemes present similar results that generally agree with observations, except in summer when the model sea breeze is too strong. At the coarse 12 km resolution, using ERA-interim (ECMWF Re-Analysis) as initial and boundary conditions leads to improvements compared to using the North American Model (NAM) dataset. Adding higher resolution nests also improves the match with the observations. However, no further improvement is observed from increasing the nest resolution from 4 km to 0.8 km. Once optimized, the model is able to reproduce tracer measurements during typical winter California large-scale events (Santa Ana). Furthermore, with the WRF/CHEM chemistry module and the European Database for Global Atmospheric Research (EDGAR) version 4.1 emissions for HFC-134a, we find that using a simple emission scaling factor is not sufficient to infer emissions, which highlights the need for more complex inversions.
-
Stratospheric ozone profiles are retrieved for the period 2002–2009 from SCIAMACHY measurements of limb-scattered solar radiation in the Hartley and Chappuis absorption bands of ozone. This data set is used to determine the chemical ozone losses in both the Arctic and Antarctic polar vortices by averaging the ozone in the vortex at a given potential temperature. The chemical ozone losses at isentropic levels between 450 K and 600 K are derived from the difference between observed ozone abundances and the ozone modelled taking diabatic cooling into account, but no chemical ozone loss. Chemical ozone losses of up to 30–40% between mid-January and the end of March inside the Arctic polar vortex are reported. Strong inter-annual variability of the Arctic ozone loss is observed, with the cold winters 2004/2005 and 2006/2007 showing chemical ozone losses inside the polar vortex at 475 K, where 1.7 ppmv and 1.4 ppmv of ozone were removed, respectively, over the period from 22 January to beginning of April and 0.9 ppmv and 1.2 ppmv, respectively, during February. For the winters of 2007/2008 and 2002/2003, ozone losses of about 0.8 ppmv and 0.4 ppmv, respectively are estimated at the 475 K isentropic level for the period from 22 January to beginning of April. Essentially no ozone losses were diagnosed for the relatively warm winters of 2003/2004 and 2005/2006. The maximum ozone loss in the SCIAMACHY data set was found in 2007 at the 600 K level and amounted to about 2.1 ppmv for the period between 22 January and the end of April. Enhanced losses close to this altitude were found in all investigated Arctic springs, in contrast to Antarctic spring. The inter-annual variability of ozone losses and PSC occurrence rates observed during Arctic spring is consistent with the known QBO effects on the Arctic polar vortex, with exception of the unusual Arctic winter 2008/2009. <br><br> The maximum total ozone mass loss of about 25 million tons was found in the cold Arctic winter of 2004/2005 inside the polar vortex between the 450 K and 600 K isentropic levels from mid-January until the middle of March. <br><br> The Antarctic vortex averaged ozone loss as well as the size of the polar vortex do not vary much from year to year. The total ozone mass loss inside the Antarctic polar vortex between the 450 K and 600 K isentropic levels is about 50–60 million tons and the vortex volume for this altitude range varies between about 150 and 300 km<sup>3</sup> for the period between mid-August and mid-November of every year studied, except for 2002. In 2002 a mid-winter major stratospheric warming occurred in the second half of September and the ozone mass loss was only about half of the value in the other years. However, inside the polar vortex we find chemical ozone losses at the 475 K isentropic level that are similar to those in all other years studied. At this isentropic level ozone losses of 70–90% between mid-August and mid-November or about 2.5 ppmv are observed every year. At isentropic levels above 500 K the chemical ozone losses were found to be larger in 2002 than in all other years studied. <br><br> Comparisons of the vertical variation of ozone losses derived from SCIAMACHY observations with several independent techniques for the Arctic winter 2004/2005 show that the SCIAMACHY results fall in the middle of the range of previously published results for this winter. For other winters in both hemispheres – for which comparisons with other studies were possible – the SCIAMACHY results are consistent with the range of previously published results.
