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Atmospheric carbonyl sulfide (OCS) variation from 1992–2004 by ground-based solar FTIR spectrometry

Analysis of ground-based high-resolution solar FTIR absorption spectra from four sites was performed to determine trends and variability in OCS columns over the period 1992–2004. The sites were Wollongong, Australia (34.45° S, 150.88° E), Lauder, New Zealand (45.0° S, 169.7° E), Arrival Heights, Antarctica (77.8° S, 166.6° E) and Mauna Loa, Hawaii (19.5° N, 155.6° W). Small but significant long-term trends of −0.18±0.02% yr-1 above Hawaii, −0.30±0.12% yr-1 above Wollongong and −0.29±0.14% yr-1 above Lauder, were seen. No significant trend was seen above Arrival Heights. A large peak-to-peak seasonal difference observed in 1996–1997 above Wollongong and reported earlier was confirmed, but not repeated in later years. This seasonal feature correlated with particularly high water vapour columns present during late summer and early autumn, and suggests a link to warm oceanic airmasses. Seasonal variation of approximately 6% per year is observed in the total column in other years for all four locations.
Atmospheric HULIS: How humic-like are they? A comprehensive and critical review

A class of organic molecules extracted from atmospheric aerosol particles and isolated from fog and cloud water has been termed HUmic-LIke Substances (HULIS) due to a certain resemblance to terrestrial and aquatic humic and fulvic acids. In light of the interest that this class of atmospheric compounds currently attracts, we comprehensively review HULIS properties, as well as laboratory and field investigations concerning their formation and characterization in atmospheric samples. While sharing some important features such as polyacidic nature, accumulating evidence suggests that atmospheric HULIS differ substantially from terrestrial and aquatic humic substances. Major differences between HULIS and humic substances, including smaller average molecular weight, lower aromatic moiety content, greater surface activity, better droplet activation ability, as well as others, are highlighted. Several alternatives are proposed that may explain such differences: (1) the possibility that mono- and di-carboxylic acids and mineral acids abundant in the atmosphere prevent the formation of large humic "supramolecular associations"; (2) that large humic macromolecules are destroyed in the atmosphere by UV radiation, O3, and OH- radicals; (3) that "HULIS" actually consists of a complex, unresolved mixture of relatively small molecules rather than macromolecular entities; and (4) that HULIS formed via abiotic and short-lived oxidative reaction pathways differ substantially from humic substances formed over long time periods via biologically-mediated reactions. It should also be recalled that the vast majority of studies of HULIS relate to the water soluble fraction, which would include only the fulvic acid fraction of humic substances, and exclude the humic acid (base-soluble) and humin (insoluble) fractions of humic substances. A significant effort towards adopting standard extraction and characterization methods is required to develop a better and meaningful comparison between different HULIS samples.
Phase transitions and hygroscopic growth of aerosol particles containing humic acid and mixtures of humic acid and ammonium sulphate

The phase transitions and hygroscopic growth of two humic acid aerosols (Aldrich sodium salt and Leonardite Standard (IHSS)) and their mixtures with ammonium sulphate have been investigated using a combination of two techniques, Fourier transform infra-red (FTIR) spectroscopy and tandem differential mobility analysis (TDMA). A growth factor of 1.16 at 85% relative humidity (RH) was found for the Aldrich humic acid which can be regarded as an upper limit for growth factors of humic-like substances (HULIS) found in atmospheric aerosol and is significantly smaller than that of typical atmospheric inorganics. We find that the humic acid aerosols exhibit water uptake over all relative humidities with no apparent phase changes, suggesting that these aerosols readily form supersaturated droplets. In the mixed particles, the humic acid component decreases the deliquescence relative humidity (DRH) and increases the efflorescence relative humidity (ERH) of the ammonium sulphate component, and there is some degree of water uptake prior to ammonium sulphate deliquescence. In addition, at low RH, the FTIR spectra show that the ammonium is present in a different chemical environment in the mixed aerosols than in crystalline ammonium sulphate, perhaps existing as a complex with the humic materials. The growth factors of the mixed aerosols are intermediate between those of the single-component aerosols and can be predicted assuming that the inorganic and organic fractions take up water independently.
Eddy covariance measurements and parameterisation of traffic related particle emissions in an urban environment

Urban aerosol sources are important due to the health effects of particles and their potential impact on climate. Our aim has been to quantify and parameterise the urban aerosol source number flux F (particles m−2 s−1), in order to help improve how this source is represented in air quality and climate models. We applied an aerosol eddy covariance flux system 118.0 m above the city of Stockholm. This allowed us to measure the aerosol number flux for particles with diameters >11 nm. Upward source fluxes dominated completely over deposition fluxes in the collected dataset. Therefore, the measured fluxes were regarded as a good approximation of the aerosol surface sources. Upward fluxes were parameterised using a traffic activity (TA) database, which is based on traffic intensity measurements. The footprint (area on the surface from which sources and sinks affect flux measurements, located at one point in space) of the eddy system covered road and building construction areas, forests and residential areas, as well as roads with high traffic density and smaller streets. We found pronounced diurnal cycles in the particle flux data, which were well correlated with the diurnal cycles in traffic activities, strongly supporting the conclusion that the major part of the aerosol fluxes was due to traffic emissions. The emission factor for the fleet mix in the measurement area EFfm=1.4±0.1×1014 veh−1 km−1 was deduced. This agrees fairly well with other studies, although this study has an advantage of representing the actual effective emission from a mixed vehicle fleet. Emission from other sources, not traffic related, account for a F0=15±18×106 m−2 s−1. The urban aerosol source flux can then be written as F=EFfmTA+F0. In a second attempt to find a parameterisation, the friction velocity U* normalised with the average friction velocity <IMG WIDTH="21" HEIGHT="36" ALIGN="MIDDLE" BORDER="0" src="https://www.atmos-chem-phys.net/6/769/2006/acp-6-769-img15.gif" ALT="$overline{U_ast}$"> has been included, F=EF <IMG WIDTH="136" HEIGHT="51" ALIGN="MIDDLE" BORDER="0" src="https://www.atmos-chem-phys.net/6/769/2006/acp-6-769-img16.gif" ALT="$_{fm }TAleft({frac{U_ast }{overline{U_ast}}}right)^{0.4}{+}F_{0}$">. This parameterisation results in a somewhat reduced emission factor, 1.3×1014 veh−1 km−1. When multiple linear regression have been used, two emission factors are found, one for light duty vehicles EFLDV=0.3±0.3×1014 veh−1 km−1 and one for heavy-duty vehicles, EFHDV=19.8±4.0×1014 veh−1 km−1, and F0=19±16×106 m−2 s−1. The results show that during weekdays ~70–80% of the emissions came from HDV.
Water activity and activation diameters from hygroscopicity data - Part II: Application to organic species

A method has been developed for using particle hygroscopicity measurements made with a humidified tandem differential mobility analyzer (HTDMA) to determine water activity as a function of solute weight percent. In Part I, the method was tested for particles composed of sodium chloride and ammonium sulfate. Here, we report results for several atmospherically-relevant organic species: glutaric acid, malonic acid, oxalic acid and levoglucosan. Predicted water activities for aqueous dicarboxylic acid solutions are quite similar in some cases to published estimates and the simplified predictions of Köhler theory, while in other cases substantial differences are found, which we attribute primarily to the semivolatile nature of these compounds that makes them difficult to study with the HTDMA. In contrast, estimates of water activity for levoglucosan solutions compare very well with recently-reported measurements and with published data for aqueous glucose and fructose solutions. For all studied species, the critical dry diameters active at supersaturations between 0.2 and 1% that are computed with the HTDMA-derived water activities are generally within the experimental error (~20%) estimated in previously-published direct measurements using cloud condensation nuclei counters. For individual compounds, the variations in reported solution water activity lead to uncertainties in critical dry diameters of 5-25%, not significantly larger than the uncertainty in the direct measurements. To explore the impact of these uncertainties on modeled aerosol-cloud interactions, we incorporate the variations in estimates of solution water activities into the description of hygroscopic growth of aerosol particles in an adiabatic parcel model and examine the impact on the predicted drop number concentrations. For the limited set of initial conditions examined here, we find that the uncertainties in critical dry diameters for individual species lead to 0-21% changes in drop number concentration, with the largest effects at high aerosol number concentrations and slow updraft velocities. Ammonium sulfate, malonic acid and glutaric acid have similar activation behavior, while glutaric acid and levoglucosan are somewhat less hygroscopic and lead to lower drop number concentrations; sodium chloride is the most easily activated compound. We explain these behaviors in terms of a parameter that represents compound hygroscopicity, and conclude that this parameter must vary by more than a factor of 2 to induce more than a 15% change in activated drop number concentrations. In agreement with earlier studies, our results suggest that the number concentration of activated drops is more sensitive to changes in the input aerosol size and number concentrations and the applied updraft velocity than to modest changes in the aerosol composition and hygroscopic properties.
Cluster activation theory as an explanation of the linear dependence between formation rate of 3nm particles and sulphuric acid concentration

According to atmospheric observations new particle formation seems to be a function of sulphuric acid concentration to the power from one to two. The nucleation theorem then predicts that the critical cluster contains one to two sulphuric acid molecules. However, existing nucleation theories predicts that the power is more (or equal) than 2. Here we present an activation theory, which can explain the observed slope. In cluster activation the clusters containing one sulphuric acid molecule will activate for further growth due to heterogeneous nucleation, heterogeneous chemical reactions including polymerization or activation of soluble clusters. In the activation process organic vapours are typically needed as condensing agents.
Water vapour profiles by ground-based FTIR spectroscopy: study for an optimised retrieval and its validation

The sensitivity of ground-based instruments measuring in the infrared with respect to tropospheric water vapour content is generally limited to the lower and middle troposphere. The large vertical gradients and variabilities avoid a better sensitivity for the upper troposphere/lower stratosphere (UT/LS) region. In this work an optimised retrieval is presented and it is demonstrated that compared to a commonly applied method, it improves the performance of the FTIR technique. The reasons for this improvement and the possible deficiencies of the method are discussed. Only by applying the method proposed here and using measurements performed at mountain observatories can water vapour variabilities in the UT/LS be detected in a self-consistent manner. The precision, expressed as noise to signal ratio, is estimated at 45%. In the middle and lower troposphere, precisions of 22% are achieved. These estimations are confirmed by a comparison of retrieval results based on real FTIR measurements with coinciding measurements of synoptical meteorological radiosondes.
Technical note: analytical estimation of the optimal parameters for the EOF retrievals of the IASI Level 2 Product Processing Facility and its application using AIRS and ECMWF data

The Empirical Orthogonal Function (EOF) retrieval technique consists of calculating the eigenvectors of the spectra to later perform a linear regression between these and the atmospheric states, this first step is known as training. At a later stage, known as performing the retrievals, atmospheric profiles are derived from measured atmospheric radiances. When EOF retrievals are trained with a statistically different data set than the one used for retrievals two basic problems arise: significant biases appear in the retrievals and differences between the covariances of the training data set and the measured data set degrade them. The retrieved profiles will show a bias with respect to the real profiles which comes from the combined effect of the mean difference between the training and the real spectra projected into the atmospheric state space and the mean difference between the training and the atmospheric profiles. The standard deviations of the difference between the retrieved profiles and the real ones show different behavior depending on whether the covariance of the training spectra is bigger, equal or smaller than the covariance of the measured spectra with which the retrievals are performed. The procedure to correct for these effects is shown both analytically and with a measured example. It consists of first calculating the average and standard deviation of the difference between real observed spectra and the calculated spectra obtained from the real atmospheric state and the radiative transfer model used to create the training spectra. In a later step, measured spectra must be bias corrected with this average before performing the retrievals and the linear regression of the training must be performed adding noise to the spectra corresponding to the aforementioned calculated standard deviation. This procedure is optimal in the sense that to improve the retrievals one must resort to using a different training data set or a different algorithm.
2-D reconstruction of atmospheric concentration peaks from horizontal long path DOAS tomographic measurements: parametrisation and geometry within a discrete approach

In this study, we theoretically investigate the reconstruction of 2-D cross sections through Gaussian concentration distributions, e.g. emission plumes, from long path DOAS measurements along a limited number of light paths. This is done systematically with respect to the extension of the up to four peaks and for six different measurement setups with 2-4 telescopes and 36 light paths each. We distinguish between cases with and without additional background concentrations. Our approach parametrises the unknown distribution by local piecewise constant or linear functions on a regular grid and solves the resulting discrete, linear system by a least squares minimum norm principle. We show that the linear parametrisation not only allows better representation of the distributions in terms of discretisation errors, but also better inversion of the system. We calculate area integrals of the concentration field (i.e. total emissions rates for non-vanishing perpendicular wind speed components) and show that reconstruction errors and reconstructed area integrals within the peaks for narrow distributions crucially depend on the resolution of the reconstruction grid. A recently suggested grid translation method for the piecewise constant basis functions, combining reconstructions from several shifted grids, is modified for the linear basis functions and proven to reduce overall reconstruction errors, but not the uncertainty of concentration integrals. We suggest a procedure to subtract additional background concentration fields before inversion. We find large differences in reconstruction quality between the geometries and conclude that, in general, for a constant number of light paths increasing the number of telescopes leads to better reconstruction results. It appears that geometries that give better results for negligible measurement errors and parts of the geometry that are better resolved are also less sensitive to increasing measurement errors.
Observation of unusual chlorine activation by ground-based infrared and microwave spectroscopy in the late Arctic winter 2000/01

During the Arctic winter of 2000/01, ground-based FTIR and millimetre-wave measurements revealed significant amounts of ClO over Kiruna after the final warming in February 2001. In fact, column amounts of ClO were still increased in March 2001 when temperatures were about 20K above the PSC (Polar Stratospheric Clouds) threshold. At these temperatures, chlorine activation due to heterogeneous processes on PSCs is not possible even in the presence of strong lee wave effects. In order to discuss possible reasons of this feature, time series of other chemical species will be presented and discussed, too. Measurements of HF and COF2 indicated that vortex air was still observed in mid-March 2001. Since the time series of HNO3 column amounts do not give any evidence of a denitrification later than 11 February, chlorine activation persisting for several weeks after the presence of PSCs due to denitrification is rather unlikely. The photolysis of ClONO2-rich air which had been formed at the end of February and beginning of March 2001 as well as chlorine activation due to the presence of an unusual aerosol layer are discussed as possible causes of the increased ClO column amounts after the final warming.