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Atmospheric radiocarbon (<sup>14</sup>C) represents an important observational constraint on emissions of fossil-fuel derived carbon into the atmosphere due to the absence of <sup>14</sup>C in fossil fuel reservoirs. The high sensitivity and precision that accelerator mass spectrometry (AMS) affords in atmospheric <sup>14</sup>C analysis has greatly increased the potential for using such measurements to evaluate bottom-up emissions inventories of fossil fuel CO<sub>2</sub> (CO<sub>2</sub>ff), as well as those for other co-emitted species. Here we use observations of <sup>14</sup>CO<sub>2</sub> and a series of primary hydrocarbons and combustion tracers from discrete air samples collected between June 2009 and September 2010 at the National Oceanic and Atmospheric Administration Boulder Atmospheric Observatory (BAO; Lat: 40.050° N, Lon: 105.004° W) to derive emission ratios of each species with respect to CO<sub>2</sub>ff. The BAO tower is situated at the boundary of the Denver metropolitan area to the south and a large industrial and agricultural region to the north and east, making it an ideal location to study the contrasting mix of emissions from the activities in each region. The species considered in this analysis are carbon monoxide (CO), methane (CH<sub>4</sub>), acetylene (C<sub>2</sub>H<sub>2</sub>), benzene (C<sub>6</sub>H<sub>6</sub>), and C<sub>3</sub>–C<sub>5</sub> alkanes. We estimate emissions for a subset of these species by using the Vulcan high resolution CO<sub>2</sub>ff emission data product as a reference. We find that CO is overestimated in the 2008 National Emissions Inventory (NEI08) by a factor of ~2. A close evaluation of the inventory suggests that the ratio of CO emitted per unit fuel burned from on-road gasoline vehicles is likely over-estimated by a factor of 2.5. Using a wind-directional analysis of the data, we find enhanced concentrations of CH<sub>4</sub>, relative to CO<sub>2</sub>ff, in air influenced by emissions to the north and east of the BAO tower when compared to air influenced by emissions in the Denver metro region to the south. Along with enhanced CH<sub>4</sub>, the strongest enhancements of the C<sub>3</sub>–C<sub>5</sub> alkanes are also found in the north and east wind sector, suggesting that both the alkane and CH<sub>4</sub> enhancements are sourced from oil and gas fields located to the northeast, though it was not possible to rule out the contribution of non oil and gas CH<sub>4</sub> sources.
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Simulations of sulfuric acid concentration and new particle formation are performed by using the zero-dimensional version of the model MALTE (Model to predict new Aerosol formation in the Lower TropospherE) and measurements from the Campaign of Air Quality Research in Beijing and Surrounding areas (CAREBeijing) in 2008. Chemical reactions from the Master Chemical Mechanism version 3.2 (MCM v3.2) are used in the model. High correlation (slope = 0.72, <i>R</i> = 0.74) between the modelled and observed sulfuric acid concentrations is found during daytime (06:00–18:00). The aerosol dynamics are simulated by the University of Helsinki Multicomponent Aerosol (UHMA) model including several nucleation mechanisms. The results indicate that the model is able to predict the on- and offset of new particle formation in an urban atmosphere in China. In addition, the number concentrations of newly formed particles in kinetic-type nucleation including homogenous homomolecular (<i>J=K</i>[H<sub>2</sub>SO<sub>4</sub>]<sup>2</sup>) and homogenous heteromolecular nucleation involving organic vapours (<i>J=K</i><sub>het</sub>[H<sub>2</sub>SO<sub>4</sub>][Org]) are in satisfactory agreement with the observations. However, the specific organic compounds that possibly participate in the nucleation process should be investigated in further studies. For the particle growth, only a small fraction of the oxidized total organics condense onto the particles in polluted environments. Meanwhile, the OH and O<sub>3</sub> oxidation mechanism contribute 5.5% and 94.5% to the volume concentration of small particles, indicating the particle growth is more controlled by the precursor gases and their oxidation by O<sub>3</sub>.
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The secondary organic aerosol (SOA) formation from four C<sub>12</sub> alkanes (<i>n</i>-dodecane, 2-methylundecane, hexylcyclohexane, and cyclododecane) is studied in the Caltech Environmental Chamber under low-NO<sub>x</sub> conditions, in which the principal fate of the peroxy radical formed in the initial OH reaction is reaction with HO<sub>2</sub>. Simultaneous gas- and particle-phase measurements elucidate the effect of alkane structure on the chemical mechanisms underlying SOA growth. Reaction of branched structures leads to fragmentation and more volatile products, while cyclic structures are subject to faster oxidation and lead to less volatile products. Product identifications reveal that particle-phase reactions involving peroxyhemiacetal formation from several multifunctional hydroperoxide species are key components of initial SOA growth in all four systems. The continued chemical evolution of the particle-phase is structure-dependent, with 2-methylundecane SOA formation exhibiting the least extent of chemical processing and cyclododecane SOA achieving sustained growth with the greatest variety of chemical pathways. The extent of chemical development is not necessarily reflected in the oxygen to carbon (O : C) ratio of the aerosol as cyclododecane achieves the lowest O : C, just above 0.2, by the end of the experiment and hexylcyclohexane the highest, approaching 0.35.
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There has been growing interest in the vertical structure of the recent Arctic warming. We investigated temperatures at the surface, 925, 700, 500 and 300 hPa levels in the Arctic (north of 70° N) using observations and four reanalyses: ERA-Interim, CFSR, MERRA and NCEP II. For the period 1979–2011, the layers at 500 hPa and below show a warming trend in all seasons in all the chosen reanalyses and observations. Restricting the analysis to the 1998–2011 period, however, all the reanalyses show a cooling trend in the Arctic-mean 500 hPa temperature in autumn, and this also applies to both observations and the reanalyses when restricting the analysis to the locations with available IGRA radiosoundings. During this period, the surface observations mainly representing land areas surrounding the Arctic Ocean reveal no summertime trend, in contrast with the reanalyses whether restricted to the locations of the available surface observations or not. <br><br> In evaluating the reanalyses with observations, we find that the reanalyses agree better with each other at the available IGRA sounding locations than for the Arctic average, perhaps because the sounding observations were assimilated into reanalyses. Conversely, using the reanalysis data only from locations matching available surface (air) temperature observations does not improve the agreement between the reanalyses. At 925 hPa, CFSR deviates from the other three reanalyses, especially in summer after 2000, and it also deviates more from the IGRA radiosoundings than the other reanalyses do. The CFSR error in summer <i>T</i><sub>925</sub> is due mainly to underestimations in the Canadian-Atlantic sector between 120° W and 0°. The other reanalyses also have negative biases in this longitude band.
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Electrified aerosols have been observed in the lower troposphere and in the mesosphere, but have never been detected in the stratosphere and upper troposphere. We present measurements of aerosols obtained during a balloon flight to an altitude of ~ 24 km. The measurements were performed with an improved version of the Stratospheric and Tropospheric Aerosol Counter (STAC) aerosol counter dedicated to the search for charged aerosols. It is found that most of the aerosols are charged in the upper troposphere for altitudes below 10 km and in the stratosphere for altitudes above 20 km. Conversely, the aerosols seem to be uncharged between 10 km and 20 km. Model calculations are used to quantify the electrification of the aerosols with a stratospheric aerosol-ion model. The percentages of charged aerosols obtained with model calculations are in excellent agreement with the observations below 10 km and above 20 km. However, the model cannot reproduce the absence of electrification found in the lower stratosphere, as the processes leading to neutralisation in this altitude range are unknown. The presence of sporadic transient layers of electrified aerosol in the upper troposphere and in the stratosphere could have significant implications for sprite formation.
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In terms of atmospheric impact, the volcanic eruption of Mt. Pinatubo (1991) is the best characterized large eruption on record. We investigate here the model-derived stratospheric warming following the Pinatubo eruption as derived from SAGE II extinction data including recent improvements in the processing algorithm. This method, termed SAGE_4λ, makes use of the four wavelengths (385, 452, 525 and 1024 nm) of the SAGE II data when available, and uses a data-filling procedure in the opacity-induced "gap" regions. Using SAGE_4λ, we derived aerosol size distributions that properly reproduce extinction coefficients also at much longer wavelengths. This provides a good basis for calculating the absorption of terrestrial infrared radiation and the resulting stratospheric heating. However, we also show that the use of this data set in a global chemistry–climate model (CCM) still leads to stronger aerosol-induced stratospheric heating than observed, with temperatures partly even higher than the already too high values found by many models in recent general circulation model (GCM) and CCM intercomparisons. This suggests that the overestimation of the stratospheric warming after the Pinatubo eruption may not be ascribed to an insufficient observational database but instead to using outdated data sets, to deficiencies in the implementation of the forcing data, or to radiative or dynamical model artifacts. Conversely, the SAGE_4λ approach reduces the infrared absorption in the tropical tropopause region, resulting in a significantly better agreement with the post-volcanic temperature record at these altitudes.
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The North Atlantic Oscillation (NAO) controls a large amount of the European climate variability with asymmetric impacts in both time and space. Here we investigate how the NAO impact on the local atmospheric processes (disregarding the NAO impact on the large inter-continental transport mechanisms) influences the levels of various aerosol species using simulated data under constant emissions, which are fixed to the 2005 levels in order to avoid anthropogenic-induced signals. In particular, we analyze interannual variations at the seasonal timescale and focus on the ground-level. The results show that positive NAO phases favor increased aerosol concentrations in southern (northern) regions during winter (summer), while negative NAO phases enhance them in northern (southern) regions during winter (summer). The underlying processes are clearly related to the NAO impact on precipitation and wind, as they act to clean the atmosphere through removal and dispersion processes, and to the NAO impact on the radiation balance (i.e., cloudiness) as it affects the biogenic emitting activity and on the oxidative capacity of the atmosphere. Differences for all the species studied (natural inert, secondary inorganic and organic aerosols) are up to 5 μg m<sup>−3</sup>, reaching 10 and 20 μg m<sup>−3</sup> for PM<sub>10</sub> and PM<sub>2.5</sub> respectively, which represents variations of about 20–40% in their mean levels between opposite NAO phases.
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Mineral dusts are well known to be efficient ice nuclei, where the source of this efficiency has typically been attributed to the presence of clay minerals such as illite and kaolinite. However, the ice nucleating abilities of the more minor mineralogical components have not been as extensively examined. As a result, the deposition ice nucleation abilities of 24 atmospherically relevant mineral samples have been studied, using a continuous flow diffusion chamber at −40.0 ± 0.3 °C and particles size-selected at 200 nm. By focussing on using the same experimental procedure for all experiments, a relative ranking of the ice nucleating abilities of the samples was achieved. In addition, the ice nucleation behaviour of the pure minerals is compared to that of complex mixtures, such as Arizona Test Dust (ATD) and Mojave Desert Dust (MDD), and to lead iodide, which has been previously proposed for cloud seeding. Lead iodide was the most efficient ice nucleus (IN), requiring a critical relative humidity with respect to ice (RH<sub>i</sub>) of 122.0 ± 2.0% to activate 0.1% of the particles. MDD (RH<sub>i</sub>) 126.3 ± 3.4%) and ATD (RH<sub>i</sub> 129.5 ± 5.1%) have lower but comparable activity. From a set of clay minerals (kaolinite, illite, montmorillonite), non-clay minerals (e.g. hematite, magnetite, calcite, cerussite, quartz), and feldspar minerals (orthoclase, plagioclase) present in the atmospheric dusts, it was found that the feldspar minerals (particularly orthoclase) and some clays (particularly kaolinite) were the most efficient ice nuclei. Orthoclase and plagioclase were found to have critical RH<sub>i</sub> values of 127.1 ± 6.3% and 136.2 ± 1.3%, respectively. The presence of feldspars (specifically orthoclase) may play a significant role in the IN behaviour of mineral dusts despite their lower percentage in composition relative to clay minerals.
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Field measurements of atmospheric peroxides were obtained during the summer on two consecutive years over urban Beijing, which highlighted the impacts of aerosols on the chemistry of peroxide compounds and hydroperoxyl radicals (HO<sub>2</sub>). The major peroxides were determined to be hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>), methyl hydroperoxide (MHP), and peroxyacetic acid (PAA). A negative correlation was found between H<sub>2</sub>O<sub>2</sub> and PAA in rainwater, providing evidence for a conversion between H<sub>2</sub>O<sub>2</sub> and PAA in the aqueous phase. A standard gas phase chemistry model based on the NCAR Master Mechanism provided a good reproduction of the observed H<sub>2</sub>O<sub>2</sub> profile on non-haze days but greatly overpredicted the H<sub>2</sub>O<sub>2</sub> level on haze days. We attribute this overprediction to the reactive uptake of HO<sub>2</sub> by the aerosols, since there was greatly enhanced aerosol loading and aerosol liquid water content on haze days. The discrepancy between the observed and modeled H<sub>2</sub>O<sub>2</sub> can be diminished by adding to the model a newly proposed transition metal ion catalytic mechanism of HO<sub>2</sub> in aqueous aerosols. This confirms the importance of the aerosol uptake of HO<sub>2</sub> and the subsequent aqueous phase reactions in the reduction of H<sub>2</sub>O<sub>2</sub>. The closure of HO<sub>2</sub> and H<sub>2</sub>O<sub>2</sub> between the gas and aerosol phases suggests that the aerosols do not have a net reactive uptake of H<sub>2</sub>O<sub>2</sub>, because the conversion of HO<sub>2</sub> to H<sub>2</sub>O<sub>2</sub> on aerosols compensates for the H<sub>2</sub>O<sub>2</sub> loss. Laboratory studies for the aerosol uptake of H<sub>2</sub>O<sub>2</sub> in the presence of HO<sub>2</sub> are urgently required to better understand the aerosol uptake of H<sub>2</sub>O<sub>2</sub> in the real atmosphere.
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It has become possible to retrieve the global, long-term trends of trace gases that are important to atmospheric chemistry, climate, and air quality from satellite data records that span more than a decade. However, many of the satellite remote sensing techniques produce measurements that have variable sensitivity to the vertical profiles of atmospheric gases. In the case of constrained retrievals like optimal estimation, this leads to a varying amount of a priori information in the retrieval and is represented by an averaging kernel (AK). In this study, we investigate to what extent the estimation of trends from retrieved data can be biased by temporal changes of averaging kernels used in the retrieval algorithm. In particular, the surface carbon monoxide data retrieved from the Measurements Of Pollution In The Troposphere (MOPITT) instrument from 2001 to 2010 were analyzed. As a practical example based on the MOPITT data, we show that if the true atmospheric mixing ratio is continuously 50% higher or lower than the a priori state, the temporal change of the averaging kernel at the surface level gives rise to an artificial trend in retrieved surface carbon monoxide, ranging from −10.71 to +13.21 ppbv yr<sup>−1</sup> (−5.68 to +8.84 % yr<sup>−1</sup>) depending on location. Therefore, in the case of surface (or near-surface level) CO derived from MOPITT, the AKs trends multiplied by the difference between true and a priori states must be quantified in order to estimate trend biases.
