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Field observations show that individual aerosol particles are a complex mixture of a wide variety of species, reflecting different sources and physico-chemical transformations. The impacts of individual aerosol morphology and mixing characteristics on the Earth system are not yet fully understood. Here we present a sensitivity study on climate-relevant aerosols optical properties to various approximations. Based on aerosol samples collected in various geographical locations, we have observationally constrained size, morphology and mixing, and accordingly simulated, using the discrete dipole approximation model (DDSCAT), optical properties of three aerosols types: (1) bare black carbon (BC) aggregates, (2) bare mineral dust, and (3) an internal mixture of a BC aggregate laying on top of a mineral dust particle, also referred to as polluted dust. <br><br> DDSCAT predicts optical properties and their spectral dependence consistently with observations for all the studied cases. Predicted values of mass absorption, scattering and extinction coefficients (MAC, MSC, MEC) for bare BC show a weak dependence on the BC aggregate size, while the asymmetry parameter (<i>g</i>) shows the opposite behavior. The simulated optical properties of bare mineral dust present a large variability depending on the modeled dust shape, confirming the limited range of applicability of spheroids over different types and size of mineral dust aerosols, in agreement with previous modeling studies. The polluted dust cases show a strong decrease in MAC values with the increase in dust particle size (for the same BC size) and an increase of the single scattering albedo (SSA). Furthermore, particles with a radius between 180 and 300 nm are characterized by a decrease in SSA values compared to bare dust, in agreement with field observations. <br><br> This paper demonstrates that observationally constrained DDSCAT simulations allow one to better understand the variability of the measured aerosol optical properties in ambient air and to define benchmark biases due to different approximations in aerosol parametrization.
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Utilizing an aircraft-specific parameterization based on smog chamber data in the Community Multiscale Air Quality (CMAQ) model with the volatility basis set (VBS), we estimated contributions of non-traditional secondary organic aerosols (NTSOA) for aircraft emissions during landing and takeoff (LTO) activities at the Hartsfield–Jackson Atlanta International Airport. NTSOA, formed from the oxidation of semi-volatile and intermediate volatility organic compounds (S/IVOCs), is a heretofore unaccounted component of fine particulate matter (PM<sub>2.5</sub>) in most air quality models. We expanded a prerelease version of CMAQ with VBS implemented for the Carbon Bond 2005 (CB05) chemical mechanism to use the Statewide Air Pollution Research Center 2007 (SAPRC-07) chemical mechanism and added species representing aircraft S/IVOCs and corresponding NTSOA oxidation products. Results indicated that the maximum monthly average NTSOA contributions occurred at the airport and ranged from 2.4 ng m<sup>−3</sup> (34 % from idle and 66 % from non-idle aircraft activities) in January to 9.1 ng m<sup>−3</sup> (33 and 67 %) in July. This represents 1.7 % (of 140 ng m<sup>−3</sup>) in January and 7.4 % in July (of 122 ng m<sup>−3</sup>) of aircraft-attributable PM<sub>2.5</sub> compared to 41.0–42.0 % from elemental carbon and 42.8–58.0 % from inorganic aerosols. As a percentage of PM<sub>2.5</sub>, impacts were higher downwind of the airport, where NTSOA averaged 4.6–17.9 % of aircraft-attributable PM<sub>2.5</sub> and, considering alternative aging schemes, was as high as 24.0 % – thus indicating the increased contribution of aircraft-attributable SOA as a component of PM<sub>2.5</sub>. However, NTSOA contributions were generally low compared to smog chamber results, particularly at idle, due to the considerably lower ambient organic aerosol concentrations in CMAQ compared to those in the smog chamber experiments.
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A 2-year data set of measured CCN (cloud condensation nuclei) concentrations at 0.2 % supersaturation is combined with aerosol size distribution and aerosol composition data to probe the effects of aerosol number concentrations, size distribution and composition on CCN patterns. Data were collected over a period of 2 years (2012–2014) in central Tucson, Arizona: a significant urban area surrounded by a sparsely populated desert. Average CCN concentrations are typically lowest in spring (233 cm<sup>−3</sup>), highest in winter (430 cm<sup>−3</sup>) and have a secondary peak during the North American monsoon season (July to September; 372 cm<sup>−3</sup>). There is significant variability outside of seasonal patterns, with extreme concentrations (1 and 99 % levels) ranging from 56 to 1945 cm<sup>−3</sup> as measured during the winter, the season with highest variability. <br><br> Modeled CCN concentrations based on fixed chemical composition achieve better closure in winter, with size and number alone able to predict 82 % of the variance in CCN concentration. Changes in aerosol chemical composition are typically aligned with changes in size and aerosol number, such that hygroscopicity can be parameterized even though it is still variable. In summer, models based on fixed chemical composition explain at best only 41 % (pre-monsoon) and 36 % (monsoon) of the variance. This is attributed to the effects of secondary organic aerosol (SOA) production, the competition between new particle formation and condensational growth, the complex interaction of meteorology, regional and local emissions and multi-phase chemistry during the North American monsoon. Chemical composition is found to be an important factor for improving predictability in spring and on longer timescales in winter. <br><br> Parameterized models typically exhibit improved predictive skill when there are strong relationships between CCN concentrations and the prevailing meteorology and dominant aerosol physicochemical processes, suggesting that similar findings could be possible in other locations with comparable climates and geography.
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Sources of sulfur dioxide, sulfates, and organic sulfur compounds, such as fossil fuels, volcanic eruptions, and animal feeding operations, have attracted considerable attention. In this study, we collected particles carried by geogas flows ascending through soil, geogas flows above the soil that had passed through the soil, and geogas flows ascending through deep faults of concealed sulfide ore deposits, and analysed them using transmission electron microscopy. Numerous crystalline and amorphous sulfur-containing particles or particle aggregations were found in the ascending geogas flows. In addition to S, the particles contained O, Ca, K, Mg, Fe, Na, Pb, Hg, Cu, Zn, As, Ti, Sr, Ba, Si, etc. Such particles are usually a few to several hundred nanometres in diameter with either regular or irregular morphology. The sulfur-containing particles originated from deep-seated weathering or faulting products of concealed sulfide ore deposits. The particles suspended in the ascending geogas flow migrated through faults from deep-seated sources to the atmosphere. This is a previously unknown source of the atmospheric particles. This paper reports, for the first time, the emission of sulfur-containing particles into the atmosphere from concealed sulfide ore deposits. The climatic and ecological influences of these sulfur-containing particles and particle aggregations should be assessed.
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Observations of lifted temperature minimum (LTM) profiles in the nocturnal boundary layer were first reported in 1932. It was defined by the existence of a temperature minimum some centimetres above the ground. During the following decades, several research studies analysed this phenomenon verifying its existence and postulating different hypotheses about its origin. <br><br> The aim of this work is to study the existence and characteristics of LTM during the evening transition by using observations obtained during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) campaign. Data obtained from two masts instrumented with thermocouples and wind sensors at different heights close to the ground and a mast with radiometers are used to study the role of mechanical turbulence and radiation in LTM development. <br><br> The study shows that LTM can be detected under calm conditions during the day–night transition, several hours earlier than reported in previous work. These conditions are fulfilled under weak synoptic forcing when the local flow shifts associated with a mountain–plain circulation in relatively complex orography. Under these special conditions, turbulence becomes a crucial parameter in determining the ideal conditions for observing LTM. Additionally, LTM observed profiles are also related to a change in the atmospheric radiative characteristics under calm conditions.
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Studying the isotopic composition of water vapour in the lower stratosphere can reveal the driving mechanisms of changes in the stratospheric water vapour budget and therefore help to explain the trends and variations of stratospheric water vapour during recent decades. We equipped a global chemistry climate model with a description of the water isotopologue HDO, comprising its physical and chemical fractionation effects throughout the hydrological cycle. We use this model to improve our understanding of the processes which determine the patterns in the stratospheric water isotope composition and in the water vapour budget itself. The link between the water vapour budget and its isotopic composition in the tropical stratosphere is presented through their correlation in a simulated 21-year time series. The two quantities depend on the same processes; however, they are influenced with different strengths. A sensitivity experiment shows that fractionation effects during the oxidation of methane have a damping effect on the stratospheric tape recorder signal in the water isotope ratio. Moreover, the chemically produced high water isotope ratios overshadow the tape recorder in the upper stratosphere. Investigating the origin of the boreal-summer signal of isotopically enriched water vapour reveals that in-mixing of old stratospheric air from the extratropics and the intrusion of tropospheric water vapour into the stratosphere complement each other in order to create the stratospheric isotope ratio tape recorder signal. For this, the effect of ice lofting in monsoon systems is shown to play a crucial role. Furthermore, we describe a possible pathway of isotopically enriched water vapour through the tropopause into the tropical stratosphere.
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Aerosol chemical speciation monitor (ACSM) measurements were performed in Zurich, Switzerland, for 13 months (February 2011 through February 2012). Many previous studies using this or related instruments have utilized the fraction of organic mass measured at <i>m/z</i> 44 (<i>f</i><sub>44</sub>), which is typically dominated by the CO<sub>2</sub><sup>+</sup> ion and related to oxygenation, as an indicator of atmospheric aging. The current study demonstrates that during summer afternoons, when photochemical processes are most vigorous as indicated by high oxidant – OX (O<sub>3</sub> + NO<sub>2</sub>), <i>f</i><sub>44</sub> for ambient secondary organic aerosol (SOA) is not higher but is rather similar or lower than on days with low OX. On the other hand, <i>f</i><sub>43</sub> (less oxidized fragment) tends to increase. These changes are discussed in the <i>f</i><sub>44</sub> / <i>f</i><sub>43</sub> space frequently used to interpret ACSM and aerosol mass spectrometer (AMS) data. This is likely due to the formation of semi-volatile oxygenated aerosol produced from biogenic precursor gases, whose emissions increase with ambient temperature. <br><br> In addition, source apportionment analyses conducted on winter and summer data using positive matrix factorization (PMF) yield semi-volatile oxygenated organic aerosol (SV-OOA) factors that retain source-related chemical information. Winter SV-OOA is highly influenced by biomass burning, whereas summer SV-OOA is to a high degree produced from biogenic precursor gases. These sources contribute to substantial differences between the winter and summer <i>f</i><sub>44</sub> / <i>f</i><sub>43</sub> data, suggesting that PMF analysis of multi-season data employing only two OOA factors cannot capture the seasonal variability of OOA.
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Vertically resolved distributions of sulfur dioxide (SO<sub>2</sub>) with global coverage in the height region from the upper troposphere to ~20 km altitude have been derived from observations by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat for the period July 2002 to April 2012. Retrieved volume mixing ratio profiles representing single measurements are characterized by typical errors in the range of 70–100 pptv and by a vertical resolution ranging from 3 to 5 km. Comparison with observations by the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) revealed a slightly varying bias with altitude of −20 to 50 pptv for the MIPAS data set in case of volcanically enhanced concentrations. For background concentrations the comparison showed a systematic difference between the two major MIPAS observation periods. After debiasing, the difference could be reduced to biases within −10 to 20 pptv in the altitude range of 10–20 km with respect to ACE-FTS. Further comparisons of the debiased MIPAS data set with in situ measurements from various aircraft campaigns showed no obvious inconsistencies within a range of around ±50 pptv. The SO<sub>2</sub> emissions of more than 30 volcanic eruptions could be identified in the upper troposphere and lower stratosphere (UTLS). Emitted SO<sub>2</sub> masses and lifetimes within different altitude ranges in the UTLS have been derived for a large part of these eruptions. Masses are in most cases within estimations derived from other instruments. From three of the major eruptions within the MIPAS measurement period – Kasatochi in August 2008, Sarychev in June 2009 and Nabro in June 2011 – derived lifetimes of SO<sub>2</sub> for the altitude ranges 10–14, 14–18 and 18–22 km are 13.3 ± 2.1, 23.6 ± 1.2 and 32.3 ± 5.5 days respectively. By omitting periods with obvious volcanic influence we have derived background mixing ratio distributions of SO<sub>2</sub>. At 10 km altitude these indicate an annual cycle at northern mid- and high latitudes with maximum values in summer and an amplitude of about 30 pptv. At higher altitudes of about 16–18 km, enhanced mixing ratios of SO<sub>2</sub> can be found in the regions of the Asian and the North American monsoons in summer – a possible connection to an aerosol layer discovered by Vernier et al. (2011b) in that region.
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In the OOMPH (Ocean Organics Modifying Particles in both Hemispheres) project a ship measurement cruise took place in the late austral summer from 01 to 23 March 2007. The French research vessel <I>Marion Dufresne</I> sailed from Punta Arenas, Chile (70.85° W, 53.12° S), to Réunion island (55.36° E, 21.06° S) across the South Atlantic Ocean. In situ measurements of hydrogen peroxide, methylhydroperoxide and ozone were performed and are compared to simulations with the atmospheric chemistry global circulation model EMAC (ECHAM/MESSy Atmospheric Chemistry). The model generally reproduces the measured trace gas levels, but it underestimates hydrogen peroxide mixing ratios at high wind speeds, indicating too-strong dry deposition to the ocean surface. An interesting feature during the cruise is a strong increase of hydrogen peroxide, methylhydroperoxide and ozone shortly after midnight off the west coast of Africa due to an increase in the boundary layer height, leading to downward transport from the free troposphere, which is qualitatively reproduced by the model.
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This study investigates the effect of sea ice reduction on Arctic cloud cover in historical simulations with the coupled atmosphere–ocean general circulation model MIROC5. During simulated global warming since the 1970s, the Arctic sea ice extent has reduced substantially, particularly in September. This simulated reduction is consistent with satellite observation results. However, the Arctic cloud cover increases significantly during October at grids with significant reductions in sea ice because of the enhanced heat and moisture flux from the underlying ocean. Cloud fraction increases in the lower troposphere. However, the cloud fraction in the surface thin layers just above the ocean decreases despite the increased moisture because the surface air temperature rises strikingly in the thin layers and the relative humidity decreases. As the cloud cover increases, the cloud radiative effect in surface downward longwave radiation (DLR) increases by approximately 40–60 % compared to a change in clear-sky surface DLR. These results suggest that an increase in the Arctic cloud cover as a result of a reduction in sea ice could further melt the sea ice and enhance the feedback processes of the Arctic amplification in future projections.
