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The atmospheric refractive index consists of both real and imaginary parts. The intensity of refractive index fluctuations is generally expressed as the refractive index structure parameter, with the real part reflecting the strength of atmospheric turbulence and the imaginary part reflecting absorption in the light path. A large aperture scintillometer (LAS) is often used to measure the structure parameter of the real part of the atmospheric refractive index, from which the sensible and latent heat fluxes can further be obtained, whereas the influence of the imaginary part is ignored or considered noise. In this theoretical analysis study, the relationship between logarithmic light intensity variance and the atmospheric refractive index structure parameter (ARISP), as well as that between the logarithmic light intensity structure function and the ARISP, is derived. Additionally, a simple expression for the imaginary part of the ARISP is obtained which can be conveniently used to determine the imaginary part of the ARISP from LAS measurements. Moreover, these relationships provide a new method for estimating the outer scale of turbulence. Light propagation experiments were performed in the urban surface layer, from which the imaginary part of the ARISP was calculated. The experimental results showed good agreement with the presented theory. The results also suggest that the imaginary part of the ARISP exhibits a different diurnal variation from that of the real part. For the wavelength of light used (0.62 μm), the variation of the imaginary part of the ARISP is related to both the turbulent transport process and the spatial distribution characteristics of aerosols.
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The multimodel ensemble exercise performed within the HTAP project context (Fiore et al., 2009) is used here as an example of how a <i>pre-inspection</i>, diagnosis and selection of an ensemble, can produce more reliable results. The procedure is contrasted with the often-used practice of simply averaging model simulations, assuming different models produce independent results, and using the diversity of simulation as an illusory estimate of model uncertainty. It is further and more importantly demonstrated how conclusions can drastically change when future emission scenarios are analysed using an un-inspected ensemble. The HTAP multimodel ensemble analysis is only taken as an example of a widespread and common practice in air quality modelling.
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Concentrations of cloud condensation nuclei (CCN) were measured throughout an expedition by icebreaker around the central Arctic Ocean, including a 3 week ice drift operation at 87° N, from 3 August to 9 September 2008. In agreement with previous observations in the area and season, median daily CCN concentrations at 0.2% water vapour supersaturation (SS) were typically in the range of 15 to 30 cm<sup>−3</sup>, but concentrations varied by 2 to 3 orders of magnitude over the expedition and were occasionally below 1 cm<sup>−3</sup>. The CCN concentrations were highest near the ice edge and fell by a factor of 3 in the first 48 h of transport from the open sea into the pack ice region. For longer transport times they increased again, indicating a local source over the pack ice, suggested to be polymer gels, via drops injected into the air by bubbles bursting on open leads. We inferred the properties of the unexplained non-water soluble aerosol fraction that was necessary for reproducing the observed concentrations of CCN. This was made possible by assuming Köhler theory and simulating the cloud nucleation process using a Lagrangian adiabatic air parcel model that solves the kinetic formulation for condensation of water on size resolved aerosol particles. We propose that the portion of the internally/externally mixed water insoluble particles was larger in the corresponding smaller aerosol size ranges. These particles were physically and chemically behaving as polymer gels: the interaction of the hydrophilic and hydrophobic entities on the structures of polymer gels during cloud droplet activation would at first only show a partial wetting character and only weak hygroscopic growth. Given time, a high CCN activation efficiency is achieved, which is promoted by the hydrophilicity or surface-active properties of the gels. Thus the result in this study argues that the behaviour of the high Arctic aerosol in CCN-counters operating at water vapour SSs > 0.4% (high relative humidities) may not be properly explained by conventional Köhler theory.
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Volcanoes are an important source of inorganic halogen species into the atmosphere. Chemical processing of these species generates oxidised, highly reactive, halogen species which catalyse considerable O<sub>3</sub> destruction within volcanic plumes. A campaign of ground-based in situ O<sub>3</sub>, SO<sub>2</sub> and meteorology measurements was undertaken at the summit of Mount Etna volcano in July/August 2012. At the same time, spectroscopic measurements were made of BrO and SO<sub>2</sub> columns in the plume downwind. <br><br> Depletions of ozone were seen at all in-plume measurement locations, with average O<sub>3</sub> depletions ranging from 11–35 nmol mol<sup>−1</sup> (15–45%). Atmospheric processing times of the plume were estimated to be between 1 and 4 min. A 1-D numerical model of early plume evolution was also used. It was found that in the early plume O<sub>3</sub> was destroyed at an approximately constant rate relative to an inert plume tracer. This is ascribed to reactive halogen chemistry, and the data suggests the majority of the reactive halogen that destroys O<sub>3</sub> in the early plume is generated within the crater, including a substantial proportion generated in a high-temperature "effective source region" immediately after emission. The model could approximately reproduce the main measured features of the ozone chemistry. Model results show a strong dependence of the near-vent bromine chemistry on the presence or absence of volcanic NO<sub>x</sub> emissions and suggest that near-vent ozone measurements can be used as a qualitative indicator of NO<sub>x</sub> emission.
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Chemical and dynamical processes lead to the formation of aerosol layers in the upper planetary boundary layer (PBL) and above it. Through vertical mixing and entrainment into the PBL these layers may contribute to the ground-level particulate matter (PM); however, to date a quantitative assessment of such a contribution has not been carried out. This study investigates this aspect by combining chemical and physical aerosol measurements with WRF/Chem (Weather Research and Forecasting with Chemistry) model simulations. The observations were collected in the Milan urban area (northern Italy) during the summer of 2007. The period coincided with the passage of a meteorological perturbation that cleansed the lower atmosphere, followed by a high-pressure period favouring pollutant accumulation. Lidar observations revealed the formation of elevated aerosol layers and evidence of their entrainment into the PBL. We analysed the budget of ground-level PM<sub>2.5</sub> (particulate matter with an aerodynamic diameter less than 2.5 μm) with the help of the online meteorology–chemistry WRF/Chem model, focusing in particular on the contribution of upper-level processes. Our findings show that an important player in determining the upper-PBL aerosol layer is particulate nitrate, which may reach higher values in the upper PBL (up to 30% of the aerosol mass) than in the lower PBL. The nitrate formation process is predicted to be largely driven by the relative-humidity vertical profile, which may trigger efficient aqueous nitrate formation when exceeding the ammonium nitrate deliquescence point. Secondary PM<sub>2.5</sub> produced in the upper half of the PBL may contribute up to 7–8 μg m<sup>−3</sup> (or 25%) to ground-level concentrations on an hourly basis. The residual aerosol layer above the PBL is also found to potentially play a large role, which may occasionally contribute up to 10–12 μg m<sup>−3</sup> (or 40%) to hourly ground-level PM<sub>2.5</sub> concentrations during the morning hours. Although the results presented here refer to one relatively short period in one location, this study highlights the importance of considering the interplay between chemical and dynamical processes occurring within and above the PBL when interpreting ground-level aerosol observations.
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The Canadian Arctic has experienced decreasing sea ice extent and increasing shipping activity in recent decades. While there are economic incentives to develop resources in the north, there are environmental concerns that increasing marine traffic will contribute to declining air quality in northern communities. In an effort to characterize the relative impact of shipping on air quality in the north, two monitoring stations have been installed in Cape Dorset and Resolute, Nunavut, and have been operational since 1 June 2013. The impact of shipping and other sources of emissions on NO<sub>x</sub>, O<sub>3</sub>, SO<sub>2</sub>, BC, and PM<sub>2.5</sub> pollution have been characterized for the 2013 shipping season from 1 June to 1 November. In addition, a high-resolution Air Quality Health Index (AQHI) for both sites was computed. Shipping consistently increased O<sub>3</sub> mixing ratio and PM<sub>2.5</sub> concentration. The 90% confidence interval for mean difference in O<sub>3</sub> mixing ratio between ship- and no ship-influenced air masses were up to 4.6–4.7 ppb and 2.5–2.7 ppb for Cape Dorset and Resolute, respectively. The same intervals for PM<sub>2.5</sub> concentrations were up to 1.8–1.9 μg m<sup>−3</sup> and 0.5–0.6 μg m<sup>−3</sup>. Ship-influenced air masses consistently exhibited an increase of 0.1 to 0.3 in the high-resolution AQHI compared to no ship-influenced air masses. Trajectory cluster analysis in combination with ship traffic tracking provided an estimated range for percent ship contribution to NO<sub>x</sub>, O<sub>3</sub>, SO<sub>2</sub>, and PM<sub>2.5</sub> that were 12.9–17.5 %, 16.2–18.1 %, 16.9–18.3 %, and 19.5–31.7 % for Cape Dorset and 1.0–7.2 %, 2.9–4.8 %, 5.5–10.0 %, and 6.5–7.2 % for Resolute during the 2013 shipping season. Additional measurements in Resolute suggested that percent ship contribution to black carbon was 4.3–9.8 % and that black carbon constituted 1.3–9.7 % of total PM<sub>2.5</sub> mass in ship plumes. Continued air quality monitoring in the above sites for future shipping seasons will improve the statistics in our analysis and characterize repeating seasonal patterns in air quality due to shipping, local pollution, and long-range transport.
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Atmospheric methane (CH<sub>4</sub>) increased from ~900 ppb (parts per billion, or nanomoles per mole of dry air) in 1900 to ~1800 ppb in 2010 at a rate unprecedented in any observational records. However, the contributions of the various methane sources and sinks to the CH<sub>4</sub> increase are poorly understood. Here we use initial emissions from bottom-up inventories for anthropogenic sources, emissions from wetlands and rice paddies simulated by a~terrestrial biogeochemical model, and an atmospheric general circulation model (AGCM)-based chemistry-transport model (i.e. ACTM) to simulate atmospheric CH<sub>4</sub> concentrations for 1910–2010. The ACTM simulations are compared with the CH<sub>4</sub> concentration records reconstructed from Antarctic and Arctic ice cores and firn air samples, and from direct measurements since the 1980s at multiple sites around the globe. The differences between ACTM simulations and observed CH<sub>4</sub> concentrations are minimized to optimize the global total emissions using a mass balance calculation. During 1910–2010, the global total CH<sub>4</sub> emission doubled from ~290 to ~580 Tg yr<sup>−1</sup>. Compared to optimized emission, the bottom-up emission data set underestimates the rate of change of global total CH<sub>4</sub> emissions by ~30% during the high growth period of 1940–1990, while it overestimates by ~380% during the low growth period of 1990–2010. Further, using the CH<sub>4</sub> stable carbon isotopic data (δ<sup>13</sup>C), we attribute the emission increase during 1940–1990 primarily to enhancement of biomass burning. The total lifetime of CH<sub>4</sub> shortened from 9.4 yr during 1910–1919 to 9 yr during 2000–2009 by the combined effect of the increasing abundance of atomic chlorine radicals (Cl) and increases in average air temperature. We show that changes of CH<sub>4</sub> loss rate due to increased tropospheric air temperature and CH<sub>4</sub> loss due to Cl in the stratosphere are important sources of uncertainty to more accurately estimate the global CH<sub>4</sub> budget from δ<sup>13</sup>C observations.
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We suggest that some metrics for quantifying distances in phase space are based on linearized flows about unrealistic reference states and hence may not be applicable to atmospheric flows. A new approach of defining a norm-induced metric based on the total energy norm is proposed. The approach is based on the rigorous mathematics of normed vector spaces and the law of energy conservation in physics. It involves the innovative construction of the phase space so that energy (or a certain physical invariant) takes the form of a Euclidean norm. The metric can be applied to both linear and nonlinear flows and for small and large separations in phase space. The new metric is derived for models of various levels of sophistication: the 2-D barotropic model, the shallow-water model and the 3-D dry, compressible atmosphere in different vertical coordinates. Numerical calculations of the new metric are illustrated with analytic dynamical systems as well as with global reanalysis data. The differences from a commonly used metric and the potential for application in ensemble prediction, error growth analysis and predictability studies are discussed.
