Index Geophysics

Items

• The impact of the chemical production of methyl nitrate from the NO + CH3O2 reaction on the global distributions of alkyl nitrates, nitrogen oxides and tropospheric ozone: a global modelling study

  The formation, abundance and distribution of organic nitrates are relevant for determining the production efficiency and resident mixing ratios of tropospheric ozone (O₃) on both regional and global scales. Here we investigate the effect of applying the recently measured direct chemical production of methyl nitrate (CH₃ONO₂) during NO_x recycling involving the methyl-peroxy radical on the global tropospheric distribution of CH₃ONO₂ and the perturbations introduced towards tropospheric NO_x and O₃ using the TM5 global chemistry transport model. By comparisons against numerous observations, we show that the global surface distribution of CH₃ONO₂ can be largely explained by introducing the chemical production mechanism using a branching ratio of 0.3%, when assuming a direct oceanic emission source of ~0.15 Tg N yr⁻¹. On a global scale, the chemical production of CH₃ONO₂ converts 1 Tg N yr⁻¹ from nitrogen oxide for this branching ratio. The resident mixing ratios of CH₃ONO₂ are found to be highly sensitive to the dry deposition velocity that is prescribed, where more than 50% of the direct oceanic emission is lost near the source regions, thereby mitigating the subsequent effects due to long-range and convective transport out of the source region. For the higher alkyl nitrates (RONO₂) we find improvements in the simulated distribution near the surface in the tropics (10° S–10° N) when introducing direct oceanic emissions equal to ~0.17 Tg N yr⁻¹ . In terms of the vertical profile of CH₃ONO₂, there are persistent overestimations in the free troposphere and underestimations in the upper troposphere across a wide range of latitudes and longitudes when compared against data from measurement campaigns. This suggests either a missing transport pathway or source/sink term, although measurements show significant variability in resident mixing ratios at high altitudes at global scale. For the vertical profile of RONO₂, TM5 performs better at tropical latitudes than at mid-latitudes, with similar features in the comparisons to those for CH₃ONO₂. Comparisons of CH₃ONO₂ with a wide range of surface measurements shows that further constraints are necessary regarding the variability in the deposition terms for different land surfaces in order to improve on the comparisons presented here. For total reactive nitrogen (NO_y) ~20% originates from alkyl nitrates in the tropics and subtropics, where the introduction of both direct oceanic emissions and the chemical formation mechanism of CH₃ONO₂ only makes a ~5% contribution to the total alkyl nitrate content in the upper troposphere when compared with aircraft observations. We find that the increases in tropospheric O₃ that occur due oxidation of CH₃ONO₂ originating from direct oceanic emission is negated when accounting for the chemical formation of CH₃ONO₂, meaning that the impact of such oceanic emissions on atmospheric lifetimes becomes marginal when a branching ratio of 0.3% is adopted.

• High-resolution (375 m) cloud microstructure as seen from the NPP/VIIRS satellite imager

  VIIRS (Visible Infrared Imaging Radiometer Suite), onboard the Suomi NPP (National Polar-orbiting Partnership) satellite, has an improved resolution of 750 m with respect to the 1000 m of the Moderate Resolution Imaging Spectroradiometer for the channels that allow retrieving cloud microphysical parameters such as cloud drop effective radius (<i>r</i>_e). VIIRS also has an imager with five channels of double resolution of 375 m, which was not designed for retrieving cloud products. A methodology for a high-resolution retrieval of <i>r</i>_e and microphysical presentation of the cloud field based on the VIIRS imager was developed and evaluated with respect to MODIS in this study. The tripled microphysical resolution with respect to MODIS allows obtaining new insights for cloud–aerosol interactions, especially at the smallest cloud scales, because the VIIRS imager can resolve the small convective elements that are sub-pixel for MODIS cloud products. Examples are given for new insights into ship tracks in marine stratocumulus, pollution tracks from point and diffused sources in stratocumulus and cumulus clouds over land, deep tropical convection in pristine air mass over ocean and land, tropical clouds that develop in smoke from forest fires and in heavy pollution haze over densely populated regions in southeastern Asia, and for pyro-cumulonimbus clouds. <br> It is found that the VIIRS imager provides more robust physical interpretation and refined information for cloud and aerosol microphysics as compared to MODIS, especially in the initial stage of cloud formation. VIIRS is found to identify significantly more fully cloudy pixels when small boundary layer convective elements are present. This, in turn, allows for a better quantification of cloud–aerosol interactions and impacts on precipitation-forming processes.

• Ozone weekend effects in the Beijing–Tianjin–Hebei metropolitan area, China

  The ozone weekend effect (OWE) was first investigated in the metropolitan area of Beijing–Tianjin–Hebei (BTH), China, using in situ measurements from the Atmospheric Environment Monitoring Network from July 2009 to August 2011. The results indicate that there is an obvious weekly periodical variation in the surface ozone concentration. There is a lower ozone concentration from Wednesday to Friday (weekday) and a higher concentration from Saturday to Monday (weekend) at all the locations of the study. NO_x also displays a weekly cycle, with the maximum level occurring on weekdays and the minimum level on weekends, especially later on Sunday night and early Monday morning. This pattern may be responsible for the higher concentration of ozone on weekends. Additionally, the vertical variations in O₃ and NO_x from the 8 m, 47 m, 120 m and 280 m observation platforms on the 325 m Beijing meteorological tower displayed obvious weekly cycles that corresponded to the surface results. <br> A smaller decrease in volatile organic compounds (VOCs; using CO as a proxy) and much lower NO_x concentrations on the weekend may lead to higher VOC / NO_x ratio, which can enhance the ozone production efficiency in VOC-limited regime areas. Additionally, a clear weekly cycle in the fine aerosol concentration was observed, with maximum values occurring on weekdays and minimum values occurring on weekends. Higher concentrations of aerosol on weekdays can reduce the UV radiation flux by scattering or absorbing, which leads to a decrease in the ozone production efficiency. A significant weekly cycle in UV radiation, consistent with the aerosol concentration, was discovered at the Beijing meteorological tower site (BJT), validating the assumption. A comprehensive understanding of the ozone weekend effect in the BTH area can provide deep insights into controlling photochemical pollution.

• WRF-Chem simulations of a typical pre-monsoon dust storm in northern India: influences on aerosol optical properties and radiation budget

  The impact of a typical pre-monsoon season (April–June) dust storm event on the regional aerosol optical properties and radiation budget in northern India is analyzed. The dust storm event lasted from 17 to 22 April 2010 and the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) estimated total dust emissions of 7.5 Tg over the model domain. Both in situ (AERONET – Aerosol Robotic Network) and satellite observations show significant increase (> 50%) in local to regional scale aerosol optical depth (AOD) and decrease (> 70%) in the Ångström exponent (&alpha;) during this period. Amongst the AERONET sites in this region, Kanpur was influenced the most, where the AOD reached up to 2.1 and the α decreased to −0.09 during the dust storm period. The WRF-Chem model reproduced the spatial and temporal distributions of dust plumes and aerosol optical properties but generally underestimated the AOD. The average MODIS and WRF-Chem AOD (550 nm) values in a subregion (70–80° E, 25–30° N) affected the most by the dust storm are estimated as 0.80 ± 0.30 and 0.68 ± 0.28, respectively. Model results show that dust particles cool the surface and the top of the atmosphere, but warm the atmosphere itself. The radiative perturbation due to dust aerosols averaged over the subregion is estimated as −2.9 ± 3.1 W m⁻² at the top of the atmosphere, 5.1 ± 3.3 W m⁻² in the atmosphere and −8.0 ± 3.3 W m⁻² at the surface. The simulated instantaneous cooling under the dust plume was much higher and reached −227 and −70 W m⁻² at the surface and the top of the atmosphere, respectively. The impact of these radiative perturbations on the surface energy budget is estimated to be small on a regional scale but significant locally.

• Investigating PAH relative reactivity using congener profiles, quinone measurements and back trajectories

  Vapour and particle-associated concentrations of 15 polycyclic aromatic hydrocarbons (PAH) and 11 PAH quinones have been measured in winter and summer campaigns at the rural site, Weybourne in eastern England. Concentrations of individual PAH are relatively smaller than average concentrations measured previously at urban sites in the UK. The concentrations of PAH of the air masses originating from southern England and mainland UK are significantly larger than those from Eastern Europe and the North Atlantic, while quinone to parent PAH ratios show an inverse behaviour, being highest in the more aged North Atlantic polar air masses. While concentrations of 1,2-naphthoquinone decline from winter to summer, those of 1,4-naphthoquinone and anthraquinone increase suggesting a photochemical formation pathway. A comparison of congener concentration profiles measured at Weybourne with those from an urban source area (Birmingham) reveals differential losses at the rural site, especially evident in fluoranthene : pyrene ratios and consistent with the known rates of vapour phase reactions of 3 and 4 ring compounds with hydroxyl radical. The ratios of quinones to their parent PAH at Weybourne are greater than those in the urban source area indicating either more rapid loss processes for PAH, or formation of quinones during advection of the air mass, or probably both.

• Fast photolysis of carbonyl nitrates from isoprene

  Photolysis is shown to be a major sink for isoprene-derived carbonyl nitrates, which constitute an important component of the total organic nitrate pool over vegetated areas. Empirical evidence from published laboratory studies on the absorption cross sections and photolysis rates of α-nitrooxy ketones suggests that the presence of the nitrate group (i) greatly enhances the absorption cross sections and (ii) facilitates dissociation to a point that the photolysis quantum yield is close to unity, with O–NO₂ dissociation as a likely major channel. On this basis, we provide new recommendations for estimating the cross sections and photolysis rates of carbonyl nitrates. The newly estimated photo rates are validated using a chemical box model against measured temporal profiles of carbonyl nitrates in an isoprene oxidation experiment by Paulot et al. (2009). The comparisons for ethanal nitrate and for the sum of methacrolein- and methyl vinyl ketone nitrates strongly supports our assumptions of large cross-section enhancements and a near-unit quantum yield for these compounds. These findings have significant atmospheric implications: the photorates of key carbonyl nitrates from isoprene are estimated to be typically between ~ 3 and 20 times higher than their sink due to reaction with OH in relevant atmospheric conditions. Moreover, since the reaction is expected to release NO₂, photolysis is especially effective in depleting the total organic nitrate pool.

• Space-based observations of fire NOx emission coefficients: a global biome-scale comparison

  Biomass burning represents both a significant and highly variable source of NO_x to the atmosphere. This variability stems from both the episodic nature of fires, and from fire conditions such as the modified combustion efficiency of the fire, the nitrogen content of the fuel and possibly other factors that have not been identified or evaluated by comparison with observations. Satellite instruments offer an opportunity to observe emissions from wildfires, providing a large suite of measurements which allow us to study mean behavior and variability on the regional scale in a statistically rigorous manner. Here we use space-based measurements of fire radiative power from the Moderate Resolution Imaging Spectroradiometer in combination with NO₂ tropospheric column densities from the Ozone Monitoring Instrument to measure mean emission coefficients (ECs in g NO MJ^&minus;1) from fires for global biomes, and across a wide range of smaller-scale ecoregions, defined as spatially-distinct clusters of fires with similar fuel type. Mean ECs for all biomes fall between 0.250–0.362 g NO MJ⁻¹, a range that is smaller than found in previous studies of biome-scale emission factors. The majority of ecoregion ECs fall within or near this range, implying that under most conditions, mean fire emissions of NO_x per unit energy are similar between different regions regardless of fuel type or spatial variability. In contrast to these similarities, we find that about 24% of individual ecoregion ECs deviate significantly (with 95% confidence) from the mean EC for the associated biome, and a similar number of ecoregion ECs falls outside the range of all mean biome ECs, implying that there are some regions where fuel type-specific global emission parameterizations fail to capture local fire NO_x emissions.

• Drift-corrected trends and periodic variations in MIPAS IMK/IAA ozone measurements

  Drifts, trends and periodic variations were calculated from monthly zonally averaged ozone profiles. The ozone profiles were derived from level-1b data of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) by means of the scientific level-2 processor run by the Karlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research (IMK). All trend and drift analyses were performed using a multilinear parametric trend model which includes a linear term, several harmonics with period lengths from 3 to 24 months and the quasi-biennial oscillation (QBO). Drifts at 2-sigma significance level were mainly negative for ozone relative to Aura MLS and Odin OSIRIS and negative or near zero for most of the comparisons to lidar measurements. Lidar stations used here include those at Hohenpeissenberg (47.8° N, 11.0° E), Lauder (45.0° S, 169.7° E), Mauna Loa (19.5° N, 155.6° W), Observatoire Haute Provence (43.9° N, 5.7° E) and Table Mountain (34.4° N, 117.7° W). Drifts against the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) were found to be mostly insignificant. The assessed MIPAS ozone trends cover the time period of July 2002 to April 2012 and range from −0.56 ppmv decade⁻¹ to +0.48 ppmv decade⁻¹ (−0.52 ppmv decade⁻¹ to +0.47 ppmv decade⁻¹ when displayed on pressure coordinates) depending on altitude/pressure and latitude. From the empirical drift analyses we conclude that the real ozone trends might be slightly more positive/less negative than those calculated from the MIPAS data, by conceding the possibility of MIPAS having a very small (approximately within −0.3 ppmv decade⁻¹) negative drift for ozone. This leads to drift-corrected trends of −0.41 ppmv decade⁻¹ to +0.55 ppmv decade⁻¹ (−0.38 ppmv decade⁻¹ to +0.53 ppmv decade⁻¹ when displayed on pressure coordinates) for the time period covered by MIPAS Envisat measurements, with very few negative and large areas of positive trends at mid-latitudes for both hemispheres around and above 30 km (~10 hPa). Negative trends are found in the tropics around 25 and 35 km (~25 and 5 hPa), while an area of positive trends is located right above the tropical tropopause. These findings are in good agreement with the recent literature. Differences of the trends compared with the recent literature could be explained by a possible shift of the subtropical mixing barriers. Results for the altitude–latitude distribution of amplitudes of the quasi-biennial, annual and the semi-annual oscillation are overall in very good agreement with recent findings.

• Mass spectrometry of refractory black carbon particles from six sources: carbon-cluster and oxygenated ions

  We discuss the major mass spectral features of different types of refractory carbonaceous particles, ionized after laser vaporization with an Aerodyne high-resolution soot-particle aerosol mass spectrometer (SP-AMS). The SP-AMS was operated with a switchable 1064 nm laser and a 600 °C thermal vaporizer, yielding respective measurements of the refractory and non-refractory particle components. Six samples were investigated, all of which were composed primarily of refractory material: fuel-rich and fuel-lean propane/air diffusion-flame combustion particles; graphite-spark-generated particles; a commercial fullerene-enriched soot; Regal Black, a commercial carbon black; and nascent aircraft-turbine combustion particles. <br> All samples exhibited a spectrum of carbon-cluster ions C_xⁿ⁺ in their refractory mass spectrum. Smaller clusters (<i>x</i> < 6) were found to dominate the C_xⁿ⁺ distribution. For fullerene soot, fuel-rich-flame particles and spark-generated particles, significant C_xⁿ⁺ clusters at <i>x</i> &gg; 6 were present, with significant contributions from multiply charged ions (<i>n</i> > 1). In all six cases, the ions C₁⁺ and C₃⁺ contributed over 60% to the total C₁<x<16⁺ intensity. Furthermore, the ratio of these major ions C₁⁺ / C₃⁺ could be used to predict whether significant C_x^<i>n</i>+ signals with <i>x</i> > 5 were present. When such signals were present, C₁⁺ / C₃⁺ was close to 1. When absent, C₁⁺ / C₃⁺ was < 0.8. This ratio may therefore serve as a proxy to distinguish between the two types of spectra in atmospheric SP-AMS measurements. <br> Significant refractory oxygenated ions such as CO⁺ and CO₂⁺ were also observed for all samples. We discuss these signals in detail for Regal Black, and describe their formation via decomposition of oxygenated moieties incorporated into the refractory carbon structure. These species may be of importance in atmospheric processes such as water uptake and heterogeneous chemistry. <br> If atmospherically stable, these oxidized species may be useful for distinguishing between different combustion sources. If unstable, they may provide a means to estimate the atmospheric age of an rBC sample. Future studies should attempt to establish which of these scenarios is more realistic.

• Dimension of aircraft exhaust plumes at cruise conditions: effect of wake vortices

  The dispersion of aircraft emissions during the vortex phase is studied using a 3-D LES model with Lagrangian particle tracking. The simulations start with a fully rolled-up vortex pair of a type B747/A340 airplane and the tracer centred around the vortex cores. The tracer dilution and plume extent is studied for a variety of ambient and aircraft parameters until aircraft-induced effects have ceased. For typical upper tropospheric conditions, the impact of stratification is more dominant compared to turbulence intensity or vertical wind shear. Moreover, the sensitivity to the initial tracer distribution was found to be weak. Along the transverse direction, the tracer concentrations can be well approximated by a Gaussian distribution, along the vertical a superposition of three Gaussian distributions is adequate. For the studied parameter range, the vertical plume expansion ranges from 400 m to 550 m and cross-sectional area from 4.0 × 10⁴ m² to 6 × 10⁴ m² after six minutes. For validation, selected simulations were compared to an alternative LES model and to in-situ NO-measurements.