ACPD - Latest Articles

A long-term satellite study of aerosol effects on convective clouds in Nordic background air

2013-05-24T00:00:00+02:00

A long-term satellite study of aerosol effects on convective clouds in Nordic background air

Atmospheric Chemistry and Physics Discussions, 13, 13853-13888, 2013

Author(s): M. K. Sporre, E. Swietlicki, P. Glantz, and M. Kulmala

Aerosol-cloud interactions constitute a~major uncertainty in future climate predictions. This study combines 10 yr of ground-based aerosol particle measurements from 2 Nordic background stations (Vavihill and Hyytiälä) with MODIS (Moderate Resolution Imaging Spectroradiometer) satellite data of convective clouds. The merged data are used to examine the indirect aerosol effects on convective clouds over the Nordic countries. From the satellite scenes, vertical profiles of cloud droplet effective radius (r_e) are created by plotting r_e against cloud top temperature. The profiles have been divided according to aerosol loading but also modeled meteorological parameters from the ECMWF (European Centre for Medium-Range Forecasts). Furthermore, weather radar data from the BALTEX (Baltic Sea Experiment) and ground based precipitation measurements from several ground-based meteorological measurement stations have been investigated to determine whether aerosols affect precipitation intensity and amount.

Higher aerosol number concentrations result in smaller r_e throughout the entire profiles at both stations. Profiles associated with no or little precipitation have smaller droplets than those associated with more precipitation. Furthermore, an increase in aerosol loadings results in a suppression of precipitation rates, when the vertical extent of the clouds has been taken into account. Clouds with greater vertical extent have the highest precipitation rates and are most sensitive to aerosol perturbations. Nevertheless, meteorological parameters such as the vertical extent of the clouds, the atmospheric instability and the relative humidity in the lower atmosphere affect the amount of precipitation that reaches the ground more than the aerosols do. The combination of these ground-based and remote sensing datasets provides a unique long-term study of the effects of aerosols on convective clouds over the Nordic countries.

Secondary organic aerosol formation and primary organic aerosol oxidation from biomass burning smoke in a flow reactor during FLAME-3

2013-05-23T00:00:00+02:00

Secondary organic aerosol formation and primary organic aerosol oxidation from biomass burning smoke in a flow reactor during FLAME-3

Atmospheric Chemistry and Physics Discussions, 13, 13799-13851, 2013

Author(s): A. M. Ortega, D. A. Day, M. J. Cubison, W. H. Brune, D. Bon, J. A. de Gouw, and J. L. Jimenez

We report the physical and chemical effects of photochemically aging dilute biomass-burning smoke. A potential aerosol mass "PAM" flow reactor was used with analysis by a high-resolution aerosol mass spectrometer and a proton-transfer reaction ion-trap mass spectrometer during the FLAME-3 campaign. Hydroxyl (OH) radical concentrations in the reactor reached up to ~ 1000 times average tropospheric levels, producing effective OH exposures equivalent to up to 5 days aging in the atmosphere. VOC observations show aromatics and terpenes decrease with aging, while formic acid and other unidentified oxidation products increase. Unidentified gas-phase oxidation products, previously observed in atmospheric and laboratory measurements, were observed here, including evidence of multiple generations of photochemistry. Substantial new organic aerosol (OA) mass ("net SOA"; secondary OA) was observed from aging biomass-burning smoke, resulting in an total OA average of 1.42 ± 0.36 times the initial primary OA (POA) after oxidation. This study confirms that the net SOA to POA ratio of biomass burning smoke is far lower on average than that observed for urban emissions. Although most fuels were very reproducible, significant differences were observed among the biomasses, with some fuels resulting in a doubling of the OA mass, while for others a very small increase or even a decrease was observed. Net SOA formation in the photochemical reactor increased with OH exposure (OH_exp), typically peaking around three days of equivalent atmospheric photochemical age (OH_exp ~ 3.9 × 10¹¹ molecules cm⁻³ s⁻¹), then leveling off at higher exposures. The amount of additional OA mass added from aging is positively correlated with initial POA concentration, but not with the total VOC concentration or the concentration of known SOA precursors. The mass of SOA formed often exceeds the mass of the known VOC precursors, indicating the likely importance of primary semivolatile/intermediate volatility species, and possibly of unidentified VOCs as SOA precursors in biomass burning smoke. Chemical transformations continue even after mass concentration stabilizes. Changes in the biomass-burning tracer f₆₀ ranged from substantially decreasing to remaining constant with increased aging. With increased OH_exp, oxidation was always detected (as indicated by f₄₄ and O/C). POA O/C ranged 0.15–0.5, while aged OA O/C reached up to 0.87. The rate of oxidation and maximum O/C achieved differs for each biomass and appears to increase with the initial O/C of the POA.

Water-soluble organic carbon over the Pearl River Delta region during fall–winter: spatial variations and source apportionment

2013-05-23T00:00:00+02:00

Water-soluble organic carbon over the Pearl River Delta region during fall–winter: spatial variations and source apportionment

Atmospheric Chemistry and Physics Discussions, 13, 13773-13798, 2013

Author(s): X. Ding, X.-M. Wang, Q.-F. He, X.-X. Fu, and B. Gao

Water-soluble organic carbon (WSOC) is a major component of carbonaceous aerosols. However, the detailed information of WSOC origins is still unclear. In the current study, fine particles (PM_2.5) were collected at one sub-urban and two rural sites in the Pearl River Delta (PRD) region, south China during fall–winter 2008 to measure WSOC and organic tracers of biomass burning (BB) and secondary organic aerosols (SOA) from isoprene, monoterpenes, β-caryophyllene, aromatics and 2-ring polycyclic aromatic hydrocarbons (PAHs). WSOC concentrations ranged from 7.63 to 11.5 μg C m⁻³ and accounted for 38.8–57.9% of organic carbon (OC). Both WSOC and water-insoluble organic carbon (WIOC) exhibited higher levels at the sub-urban site than the rural sites. Subtracting BB-derived WSOC (WSOC_BB) from measured WSOC, secondary OC (SOC) and primary OC (POC) were estimated that POC exhibited dominance over SOC and contributed 68–79% to OC. Significant correlation between WSOC and EC was observed, suggesting that BB could have important contributions to ambient WSOC in the PRD region during fall–winter. Organic tracers were applied to do source apportionment of WSOC, which further confirmed that BB was the dominant contributor, accounting for 42–47% of measured WSOC. SOC estimated by SOA tracers totally contributed 22–40% of WSOC, among which anthropogenic SOC (sum of aromatics and 2-ring PAHs, 18–25%) exhibited dominance over biogenic SOC (sum of isoprene, monoterpenes and β-caryophyllene, 4–15%). The unexplained WSOC (18–31%) showed a positive correlation with POC, indicating that this portion might be associated with POC aging.

Comparison of mixed layer heights from airborne high spectral resolution lidar, ground-based measurements, and the WRF-Chem model during CalNex and CARES

2013-05-23T00:00:00+02:00

Comparison of mixed layer heights from airborne high spectral resolution lidar, ground-based measurements, and the WRF-Chem model during CalNex and CARES

Atmospheric Chemistry and Physics Discussions, 13, 13721-13772, 2013

Author(s): A. J. Scarino, M. D. Obland, J. D. Fast, S. P. Burton, R. A. Ferrare, C. A. Hostetler, L. K. Berg, B. Lefer, C. Haman, J. W. Hair, R. R. Rogers, C. Butler, A. L. Cook, and D. B. Harper

The California Research at the Nexus of Air Quality and Climate Change (CalNex) and Carbonaceous Aerosol and Radiative Effects Study (CARES) field campaigns during May and June 2010 provided a data set appropriate for studying characteristics of the planetary boundary layer (PBL). The NASA Langley Research Center (LaRC) airborne High Spectral Resolution Lidar (HSRL) was deployed to California onboard the NASA LaRC B-200 aircraft to aid in characterizing aerosol properties during these two field campaigns. Measurements of aerosol extinction (532 nm), backscatter (532 and 1064 nm), and depolarization (532 and 1064 nm) profiles during 31 flights, many in coordination with other research aircraft and ground sites, constitute a diverse data set for use in characterizing the spatial and temporal distribution of aerosols, as well as the depth and variability of the daytime mixed layer (ML), which is a subset within the PBL. This work illustrates the temporal and spatial variability of the ML in the vicinity of Los Angeles and Sacramento, CA. ML heights derived from HSRL measurements are compared to PBL heights derived from radiosonde profiles, ML heights measured from ceilometers, and simulated PBL heights from the Weather Research and Forecasting Chemistry (WRF-Chem) community model. Comparisons between the HSRL ML heights and the radiosonde profiles in Sacramento result in a correlation coefficient value (R) of 0.93 (root-mean-square (RMS) difference of 157 m and bias difference (HSRL – radiosonde) of 57 m). HSRL ML heights compare well with those from the ceilometer in the LA Basin with an R of 0.89 (RMS difference of 108 m and bias difference (HSRL – Ceilometer) of −9.7 m) for distances of up to 30 km between the B-200 flight track and the ceilometer site. Simulated PBL heights from WRF-Chem were compared with those obtained from all flights for each campaign, producing an R of 0.58 (RMS difference of 604 m and a bias difference (WRF-Chem – HSRL) of −157 m) for CalNex and 0.59 (RMS difference of 689 m and a bias difference (WRF-Chem – HSRL) of 220 m) for CARES. Aerosol backscatter simulations are also available from WRF-Chem and are compared to those from HSRL to examine differences among the methods used to derive ML heights.

Evaluating evidence for Cl sources and oxidation chemistry in a coastal, urban environment

2013-05-23T00:00:00+02:00

Evaluating evidence for Cl sources and oxidation chemistry in a coastal, urban environment

Atmospheric Chemistry and Physics Discussions, 13, 13685-13720, 2013

Author(s): C. J. Young, R. A. Washenfelder, P. M. Edwards, D. D. Parrish, J. B. Gilman, W. C. Kuster, L. H. Mielke, H. D. Osthoff, C. Tsai, O. Pikelnaya, J. Stutz, P. R. Veres, J. M. Roberts, S. Griffith, S. Dusanter, P. S. Stevens, J. Flynn, N. Grossberg, B. Lefer, J. S. Holloway, J. Peischl, T. B. Ryerson, E. L. Atlas, D. R. Blake, and S. S. Brown

The role of chlorine atoms (Cl) in atmospheric oxidation was traditionally thought to be limited to the marine boundary layer, where they are produced through heterogeneous reactions involving sea salt. However, recent observation of photolytic Cl precursors (ClNO₂ and Cl₂) formed from anthropogenic pollution has expanded the potential importance of Cl to include coastal and continental urban areas. Measurements of ClNO₂ in Los Angeles during CalNex showed it to be an important primary (first generation) radical source. Ratios of volatile organic compounds (VOCs) have been proposed as a sensitive method to quantify Cl oxidation, but have shown little evidence for a significant role of Cl outside of the Arctic. We used a box model with the Master Chemical Mechanism (MCM v3.2) chemistry scheme, constrained by observations in Los Angeles, to examine the Cl-sensitivity of the most commonly used VOC ratios (i-butane, n-butane, and propane) as a function of NO_x and secondary radical production. Model results indicated these and faster reacting VOC tracer ratios could not detect the influence of Cl unless the sustained ratio of OH to Cl was below 200. However, the model results also show that secondary (second generation) OH production resulting from Cl oxidation of VOCs is strongly influenced by NO_x, and that this effect can obscure the importance of Cl as a primary oxidant. Calculated concentrations of Cl showed a maximum in mid-morning due to a photolytic source from ClNO₂ and loss primarily to reactions with VOCs. The OH to Cl ratio was below 200 for approximately three hours in the morning, but Cl oxidation was not evident from the measured ratios of VOCs. Instead, model simulations show that secondary OH production causes VOC ratios to follow the values expected for OH oxidation despite the significant input of primary Cl from ClNO₂ photolysis in the morning. Despite the prevalence of secondary OH as an oxidant in Los Angeles, Cl may play an important role in tropospheric chemistry. The reactivity of Cl in Los Angeles during CalNex was more than an order of magnitude larger than that of OH. In addition, because of its reactivity toward different classes of VOCs and its greater propensity to participate in chain propagation rather than sink reactions, Cl atoms have a different impact on regional atmospheric oxidation than do OH radicals.

Diagnosing the average spatio-temporal impact of convective systems – Part 1: A methodology for evaluating climate models

2013-05-23T00:00:00+02:00

Diagnosing the average spatio-temporal impact of convective systems – Part 1: A methodology for evaluating climate models

Atmospheric Chemistry and Physics Discussions, 13, 13653-13684, 2013

Author(s): M. S. Johnston, P. Eriksson, S. Eliasson, M. D. Zelinka, R. M. Forbes, and K. Wyser

A~method to determine the mean response of upper tropospheric water to localised deep convective (DC) events is improved and applied to the EC-Earth climate model. Following Zelinka and Hartmann (2009), several fields related to moist processes and radiation are composited with respect to local maxima in rain rate to determine their spatio-temporal evolution with deep convection in the central Pacific Ocean.

Major improvements to the above study are the isolation of DC events in time so as to prevent multiple sampling of the same event, and a revised definition of the mean background state that allows for better characterization of the DC-induced anomalies.

The DC events observed in this study propagate westward at ~ 4 m s⁻¹. Both the upper tropospheric relative humidity and outgoing longwave radiation are substantially perturbed over a broad horizontal extent during peak convection and for long periods of time. Cloud fraction anomaly increases throughout the upper troposphere, especially in the 200–250 hPa layer, reaching peak coverage following deep convection. Cloud ice water content anomaly confined to pressures greater than about 250 hPa and peaks near 450 hPa within a few hours of the DC event but remain enhanced following the DC event. Consistent with the large increase in upper tropospheric cloud ice, albedo increases dramatically and persists for sometime following the DC event.

Applying the method to the model demonstrates that it is able to capture the large-scale responses to DC events, most notably for outgoing longwave radiation, but there are a number of important differences. For example, the DC signature of upper tropospheric humidity consistently covers a broader horizontal area than what is observed. In addition, the DC events move eastward in the model, but westward in the observations, and exhibit an unrealistic 24 h repeat cycle. Moreover, the modeled upper tropospheric cloud fraction anomalies – despite being of comparable magnitude and exhibiting similar longevity – are confined to a thinner layer that is closer to the tropopause and peak earlier than in observations. Finally, the modeled ice water content anomalies at pressures greater than about 350 hPa are about twice as large as in the observations and do not persist as long after peak convection.

The Arctic Summer Cloud-Ocean Study (ASCOS): overview and experimental design

2013-05-23T00:00:00+02:00

The Arctic Summer Cloud-Ocean Study (ASCOS): overview and experimental design

Atmospheric Chemistry and Physics Discussions, 13, 13541-13652, 2013

Author(s): M. Tjernström, C. Leck, C. E. Birch, B. J. Brooks, I. M. Brooks, L. Bäcklin, R. Y.-W. Chang, E. Granath, M. Graus, A. Hansel, J. Heintzenberg, A. Held, A. Hind, S. de la Rosa, P. Johnston, J. Knulst, G. de Leeuw, L. Di Liberto, M. Martin, P. A. Matrai, T. Mauritsen, M. Müller, S. J. Norris, M. V. Orellana, D. A. Orsini, J. Paatero, P. O. G. Persson, Q. Gao, C. Rauschenberg, Z. Ristovski, J. Sedlar, M. D. Shupe, B. Sierau, A. Sirevaag, S. Sjogren, O. Stetzer, E. Swietlicki, M. Szczodrak, P. Vaattovaara, N. Wahlberg, M. Westberg, and C. R. Wheeler

The climate in the Arctic is changing faster than anywhere else on Earth. Poorly understood feedback processes relating to Arctic clouds and aerosol-cloud interactions contribute to a poor understanding of the present changes in the Arctic climate system, and also to a large spread in projections of future climate in the Arctic. The problem is exacerbated by the paucity of research-quality observations in the central Arctic. Improved formulations in climate models require such observations, which can only come from measurements in-situ in this difficult to reach region with logistically demanding environmental conditions.

The Arctic Summer Cloud-Ocean Study (ASCOS) was the most extensive central Arctic Ocean expedition with an atmospheric focus during the International Polar Year (IPY) 2007–2008. ASCOS focused on the study of the formation and life cycle of low-level Arctic clouds. ASCOS departed from Longyearbyen on Svalbard on 2 August and returned on 9 September 2008. In transit into and out of the pack ice, four short research stations were undertaken in the Fram Strait; two in open water and two in the marginal ice zone. After traversing the pack-ice northward an ice camp was set up on 12 August at 87°21' N 01°29' W and remained in operation through 1 September, drifting with the ice. During this time extensive measurements were taken of atmospheric gas and particle chemistry and physics, mesoscale and boundary-layer meteorology, marine biology and chemistry, and upper ocean physics.

ASCOS provides a unique interdisciplinary data set for development and testing of new hypotheses on cloud processes, their interactions with the sea ice and ocean and associated physical, chemical, and biological processes and interactions. For example, the first ever quantitative observation of bubbles in Arctic leads, combined with the unique discovery of marine organic material, polymer gels with an origin in the ocean, inside cloud droplets suggest the possibility of primary marine organically derived cloud condensation nuclei in Arctic stratocumulus clouds. Direct observations of surface fluxes of aerosols could, however, not explain observed variability in aerosol concentrations and the balance between local and remote aerosols sources remains open. Lack of CCN was at times a controlling factor in low-level cloud formation, and hence for the impact of clouds on the surface energy budget. ASCOS provided detailed measurements of the surface energy balance from late summer melt into the initial autumn freeze-up, and documented the effects of clouds and storms on the surface energy balance during this transition. In addition to such process-level studies, the unique, independent ASCOS data set can and is being used for validation of satellite retrievals, operational models, and reanalysis data sets.

Estimating neutral nanoparticle steady state size distribution and growth according to measurements of intermediate air ions

2013-05-23T00:00:00+02:00

Estimating neutral nanoparticle steady state size distribution and growth according to measurements of intermediate air ions

Atmospheric Chemistry and Physics Discussions, 13, 13519-13540, 2013

Author(s): H. Tammet, K. Komsaare, and U. Hõrrak

The concentration of nanometer aerosol particles in atmospheric air during quiet periods of new particle formation is low and direct measuring is difficult. We study what information about neutral particles can be drawn from measurements of intermediate ions, which are the electrically charged particles between 1.5–7.5 nm in diameter. If the coagulation sink of nanoparticles and the growth rate of charged particles are known, then the steady state equations allow us to calculate the size distribution of neutral nanoparticles. Variations in the trial value of the growth rate have a minor effect on the estimates of the concentrations and size distributions. There exists a value of the constant growth rate of charged nanoparticles that leads to a minimum deviation of the estimated growth rate of neutral nanoparticles from the growth rate of charged nanoparticles. Rough estimates of the growth rate and size distribution of neutral nanoparticles are derived despite the fact that the sample data of intermediate ion measurements is not accompanied by simultaneous measurements of the background aerosol and ionization rate. In the case of a near-median intermediate ion concentration of 21 ± 2 cm⁻³ in the urban air of a small town, the growth rate of nanoparticles is estimated to be about 2 nm h⁻¹, while the growth flux or apparent nucleation rate is about 0.5 cm⁻³ s⁻¹ at 3 nm and about 0.08 cm⁻³ s⁻¹ at 7 nm. The results suggest that the process of new particle formation is not interrupted during the quiet periods between events of intensive nucleation of atmospheric aerosols.

Net influence of an internally-generated QBO on modelled stratospheric climate and chemistry

2013-05-23T00:00:00+02:00

Net influence of an internally-generated QBO on modelled stratospheric climate and chemistry

Atmospheric Chemistry and Physics Discussions, 13, 13495-13518, 2013

Author(s): M. M. Hurwitz, L. D. Oman, P. A. Newman, and I.-S. Song

A Goddard Earth Observing System Chemistry–Climate Model (GEOSCCM) simulation with strong tropical non-orographic gravity wave drag (GWD) is compared to an otherwise identical simulation with near-zero tropical non-orographic GWD. The GEOSCCM generates a quasi-biennial oscillation (QBO) zonal wind signal in response to strong, quasi-realistic tropical GWD. The modelled QBO has a frequency and amplitude that closely resembles observations. In the annual mean, the modelled QBO improves the simulation of tropical zonal winds and enhances tropical and sub-tropical stratospheric variability. Also, inclusion of the QBO slows the meridional overturning circulation, resulting in a generally older stratospheric mean age-of-air. Slowing of the overturning circulation, changes in stratospheric temperature and enhanced sub-tropical mixing all affect the mean distributions of ozone, methane and nitrous oxide. Furthermore, the modelled QBO enhances polar stratospheric variability in winter. Because tropical zonal winds are easterly in the simulation without a QBO, there is a relative increase in tropical zonal winds in the simulation with a QBO. Extra-tropical differences between the simulations with and without a QBO thus reflect a bias toward the westerly phase of the QBO: a relative strengthening of the polar stratospheric jet, polar stratospheric cooling and a weak reduction in Arctic lower stratospheric ozone.

Cross-validation of inferred daytime airborne CO₂ urban-regional scale surface fluxes with eddy-covariance observations and emissions inventories in Greater London

2013-05-22T00:00:00+02:00

Cross-validation of inferred daytime airborne CO₂ urban-regional scale surface fluxes with eddy-covariance observations and emissions inventories in Greater London

Atmospheric Chemistry and Physics Discussions, 13, 13465-13493, 2013

Author(s): A. Font, C. S. B. Grimmond, J.-A. Morguí, S. Kotthaus, M. Priestman, and B. Barratt

Data obtained from eleven flight surveys on six days during October 2011 were used to characterize the urban CO₂ dome in Greater London (GL) and to calculate CO₂ fluxes at the city scale. Flights crossed GL along two transects (SW-NE and SSE-NNW) at an altitude of 360 m. Increments as high as 23 ppmv were measured. The maximum CO₂ mixing ratios were localized over GL under low wind speeds, whereas a displacement of the urban plume downwind from the centre of the urban area occurred during high wind speeds. The urban-regional surface CO₂ flux was calculated for four days by the Integrative Mass Boundary Layer (IMBL) method. The diurnal CO₂ flux in GL obtained from the aircraft observations ranged from 46 to 104 μmol CO₂ m⁻² s⁻¹ during the day time. The mean CO₂ fluxes estimated from the IMBL method were statistically similar to those observed by eddy-covariance systems located in central London and a spatially integrated emissions inventory for GL. This study provides an important cross-validation of two independent measurement-based methods to infer the contribution of urban areas to climate change in terms of CO₂ surface fluxes, both of which complement bottom-up emissions inventories. The uncertainties of fluxes estimated by the IMBL method are considered and the limits of implementation of atmospheric methods to infer city-scale fluxes are discussed.

Simulation of the dispersion of the Eyjafjallajökull plume over Europe with COSMO-ART in the operational mode

2013-05-22T00:00:00+02:00

Simulation of the dispersion of the Eyjafjallajökull plume over Europe with COSMO-ART in the operational mode

Atmospheric Chemistry and Physics Discussions, 13, 13439-13463, 2013

Author(s): H. Vogel, J. Förstner, B. Vogel, T. Hanisch, B. Mühr, U. Schättler, and T. Schad

An extended version of the German operational weather forecast model was used to simulate the ash dispersion during the eruption of the Eyjafjallajökull. Sensitivity runs show the ability of the model to simulate thin ash layers when an increased vertical resolution is used. Calibration of the model results with measured data allows for a quantitative forecast of the ash concentration. An independent comparison of the simulated number concentration of 3 μm particles and observations reveals nearly perfect agreement. However, this perfect agreement could only be reached after modification of the emissions. As an operational forecast was launched every six hours, a time-lagged ensemble was obtained. Hence, the probability of violation of a certain threshold can be calculated. This is valuable information for the forecasters advising the organizations responsible for the closing of the airspace.

Asymmetric and axisymmetric dynamics of tropical cyclones

2013-05-22T00:00:00+02:00

Asymmetric and axisymmetric dynamics of tropical cyclones

Atmospheric Chemistry and Physics Discussions, 13, 13323-13438, 2013

Author(s): J. Persing, M. T. Montgomery, J. C. McWilliams, and R. K. Smith

We present the results of idealized numerical experiments to examine the difference between tropical cyclone evolution in three-dimensional (3-D) and axisymmetric (AX) model configurations. We focus on the prototype problem for intensification, which considers the evolution of an initially unsaturated AX vortex in gradient-wind balance on an f-plane. Consistent with findings of previous work, the mature intensity in the 3-D model is reduced relative to that in the AX model. In contrast with previous interpretations invoking barotropic instability and related horizontal mixing processes as a mechanism detrimental to the spin-up process, the results indicate that 3-D eddy processes associated with vortical plume structures can assist the intensification process by contributing to a radial contraction of the maximum tangential velocity and to a vertical extension of tangential winds through the depth of the troposphere. These plumes contribute significantly also to the azimuthally-averaged heating rate and the corresponding azimuthal-mean overturning circulation.

The comparisons show that the resolved 3-D eddy momentum fluxes above the boundary layer exhibit counter-gradient characteristics and are generally not represented properly by the subgrid-scale parameterizations in the AX configuration. The resolved eddy fluxes act to support the contraction and intensification of the maximum tangential winds. The comparisons indicate fundamental differences between convective organization in the 3-D and AX configurations for meteorologically relevant forecast time scales. While the radial and vertical gradients of the system-scale angular rotation provide a hostile environment for deep convection in the 3-D model, with a corresponding tendency to strain the convective elements in the tangential direction, deep convection in the AX model does not suffer this tendency. Also, since during the 3-D intensification process the convection has not yet organized into annular rings, the azimuthally-averaged heating rate and radial gradient thereof is considerably less than that in the AX model. This lack of organization results broadly in a slower intensification rate in the 3-D model and leads ultimately to a weaker mature vortex after 12 days of model integration. While axisymmetric heating rates in the 3-D model are weaker than those in the AX model, local heating rates in the 3-D model exceed those in the AX model and at times the vortex in the 3-D model intensifies more rapidly than AX. Analyses of the 3-D model output do not support a recent hypothesis concerning the key role of small-scale vertical mixing processes in the upper-tropospheric outflow in controlling the intensification process.

In the 3-D model, surface drag plays a particularly important role in the intensification process for the prototype intensification problem on meteorologically relevant time scales by helping foster the organization of convection in azimuth. There is a radical difference in the behaviour of the 3-D and AX simulations when the surface drag is reduced or increased from realistic values. Borrowing from ideas developed in a recent paper, we give a partial explanation for this difference in behaviour.

Our results provide new qualitative and quantitative insight into the differences between the asymmetric and symmetric dynamics of tropical cyclones and would appear to have important consequences for the formulation of a fluid dynamical theory of tropical cyclone intensification and mature intensity. In particular, the results point to some fundamental limitations of strict axisymmetric theory and modelling for representing the azimuthally-averaged behaviour of tropical cyclones in three dimensions.

Air/sea DMS gas transfer in the North Atlantic: evidence for limited interfacial gas exchange at high wind speed

2013-05-21T00:00:00+02:00

Air/sea DMS gas transfer in the North Atlantic: evidence for limited interfacial gas exchange at high wind speed

Atmospheric Chemistry and Physics Discussions, 13, 13285-13322, 2013

Author(s): T. G. Bell, W. De Bruyn, S. D. Miller, B. Ward, K. Christensen, and E. S. Saltzman

Shipboard measurements of eddy covariance DMS air/sea fluxes and seawater concentration were carried out in the North Atlantic bloom region in June/July 2011. Gas transfer coefficients (k₆₆₀) show a linear dependence on mean horizontal wind speed at wind speeds up to 11 m s⁻¹. At higher wind speeds the relationship between k₆₆₀ and wind speed weakens. At high winds, measured DMS fluxes were lower than predicted based on the linear relationship between wind speed and interfacial stress extrapolated from low to intermediate wind speeds. In contrast, the transfer coefficient for sensible heat did not exhibit this effect. The apparent suppression of air/sea gas flux at higher wind speeds appears to be related to sea state, as determined from shipboard wave measurements. These observations are consistent with the idea that long waves suppress near surface water side turbulence, and decrease interfacial gas transfer. This effect may be more easily observed for DMS than for less soluble gases, such as CO₂, because the air/sea exchange of DMS is controlled by interfacial rather than bubble-mediated gas transfer under high wind speed conditions.

Quantifying tracer transport in the tropical lower stratosphere using WACCM

2013-05-21T00:00:00+02:00

Quantifying tracer transport in the tropical lower stratosphere using WACCM

Atmospheric Chemistry and Physics Discussions, 13, 13245-13283, 2013

Author(s): M. Abalos, W. J. Randel, D. E. Kinnison, and E. Serrano

The zonal mean transport of ozone and carbon monoxide (CO) near the tropical tropopause is investigated using the Whole-Atmosphere Community Climate Model version 4 (WACCM4). The variability in temperature, ozone and CO in the model shows good agreement with satellite and balloon observations. Modeled temperature and tracers exhibit large and closely coupled annual cycles in the tropical lower stratosphere, as in the observations. The thermodynamic and tracer budgets in the model are analyzed based on the Transformed Eulerian Mean (TEM) framework on log-pressure coordinates and also using the isentropic formulation. Results show that the coupled seasonal cycles are mainly forced by tropical upwelling over altitudes with large vertical tracer gradients, in agreement with previous observational studies. The model also allows explicit calculation of eddy transport terms, which make an important contribution to ozone tendencies in the tropical lower stratosphere. The character of the eddy fluxes changes with altitude. At higher levels (~2 km above the cold point tropopause), isentropic eddy transport occurs during winter and spring in each hemisphere in the sub-tropics, associated with transient Rossby waves acting on strong background latitudinal gradients. At lower altitudes, close to the tropical tropopause, there is a maximum in horizontal eddy transport during boreal summer associated with the Asian monsoon anticyclone. Sub-seasonal variability in ozone and CO, tied to fluctuations in temperature, is primarily driven by transient tropical upwelling. In isentropic coordinates, the overall tracer budgets are similar to the log-pressure results, highlighting cross-isentropic mean advection as the main term in the balance. However, in isentropic coordinates the tracer variability is largely reduced on both seasonal and sub-seasonal timescales, because the tracer and temperature fluctuations are highly correlated (as a response to upwelling).

Cloud and boundary layer interactions over the Arctic sea-ice in late summer

2013-05-17T00:00:00+02:00

Cloud and boundary layer interactions over the Arctic sea-ice in late summer

Atmospheric Chemistry and Physics Discussions, 13, 13191-13244, 2013

Author(s): M. D. Shupe, P. O. G. Persson, I. M. Brooks, M. Tjernström, J. Sedlar, T. Mauritsen, S. Sjogren, and C. Leck

Observations from the Arctic Summer Cloud Ocean Study (ASCOS), in the central Arctic sea-ice pack in late summer 2008, provide a detailed view of cloud-atmosphere-surface interactions and vertical mixing processes over the sea–ice environment. Measurements from a suite of ground-based remote sensors, near surface meteorological and aerosol instruments, and profiles from radiosondes and a helicopter are combined to characterize a week-long period dominated by low-level, mixed-phase, stratocumulus clouds. Detailed case studies and statistical analyses are used to develop a conceptual model for the cloud and atmosphere structure and their interactions in this environment.

Clouds were persistent during the period of study, having qualities that suggest they were sustained through a combination of advective influences and in-cloud processes, with little contribution from the surface. Radiative cooling near cloud top produced buoyancy-driven, turbulent eddies that contributed to cloud formation and created a cloud-driven mixed layer. The depth of this mixed layer was related to the amount of turbulence and condensed cloud water. Coupling of this cloud-driven mixed layer to the surface boundary layer was primarily determined by proximity. For 75% of the period of study, the primary stratocumulus cloud-driven mixed layer was decoupled from the surface and typically at a warmer potential temperature. Since the near-surface temperature was constrained by the ocean–ice mixture, warm temperatures aloft suggest that these air masses had not significantly interacted with the sea–ice surface. Instead, back trajectory analyses suggest that these warm airmasses advected into the central Arctic Basin from lower latitudes. Moisture and aerosol particles likely accompanied these airmasses, providing necessary support for cloud formation. On the occasions when cloud-surface coupling did occur, back trajectories indicated that these air masses advected at low levels, while mixing processes kept the mixed layer in equilibrium with the near-surface environment. Rather than contributing buoyancy forcing for the mixed-layer dynamics, the surface instead simply appeared to respond to the mixed-layer processes aloft. Clouds in these cases often contained slightly higher condensed water amounts, potentially due to additional moisture sources from below.

The global impact of the transport sectors on atmospheric aerosol: simulations for year 2000 emissions

2013-05-17T00:00:00+02:00

The global impact of the transport sectors on atmospheric aerosol: simulations for year 2000 emissions

Atmospheric Chemistry and Physics Discussions, 13, 13119-13189, 2013

Author(s): M. Righi, J. Hendricks, and R. Sausen

We use the EMAC-MADE global aerosol model to quantify the impact of transport emissions (land transport, shipping and aviation) on global aerosol. We consider a present-day (2000) scenario and the CMIP5 emission dataset developed in support of the IPCC Fifth Assessment Report. The model takes also into account particle number emissions, which are derived from mass emissions under different assumptions on the size distribution of particles emitted by the three transport sectors. Additional sensitivity experiments are performed to quantify the effects of the uncertainties behind such assumptions. The model simulations show that the impact of the transport sectors closely matches the emission patterns. Land transport is the most important source of black carbon pollution in USA, Europe and Arabian Peninsula. Shipping strongly contributes to aerosol sulfate concentrations along the most-traveled routes of the northern Atlantic and northern Pacific oceans, with a significant impact along the coastlines. The effect of aviation is mostly confined to the upper-troposphere (7–12 km), in the northern mid-latitudes, although significant effects are also simulated at the ground, due to the emissions from landing and take-off cycles. The transport-induced perturbations to particle number concentrations are very sensitive to the assumptions on the size distribution of emitted particles, with the largest uncertainties obtained for the land transport sector. The simulated climate impacts, due to aerosol direct and indirect effects, are strongest for the shipping sector, as a consequence of the large impact of sulfate aerosol on low marine clouds and their optical properties.

Long term in-situ observations of biomass burning aerosol at a high altitude station in Venezuela – sources, impacts and inter annual variability

2013-05-17T00:00:00+02:00

Long term in-situ observations of biomass burning aerosol at a high altitude station in Venezuela – sources, impacts and inter annual variability

Atmospheric Chemistry and Physics Discussions, 13, 13079-13117, 2013

Author(s): T. Hamburger, M. Matisāns, P. Tunved, J. Ström, S. Calderon, P. Hoffmann, G. Hochschild, J. Gross, T. Schmeissner, and R. Krejci

First long-term observations of South American biomass burning aerosol within the tropical lower free troposphere are presented. The observations were conducted between 2007 and 2009 at a high altitude station (4765 m a.s.l.) on the Pico Espejo, Venezuela. Sub-micron aerosol volume, number concentrations of primary particles and particle absorption were observed. Orographic lifting and shallow convection leads to a distinct diurnal cycle at the station. It enables measurements within the lower free troposphere during night time and observations of boundary layer air masses during day time and at their transitional regions. The seasonal cycle is defined by a wet rainy season and a dry biomass burning season. The particle load of biomass burning aerosol is dominated by fires in the Venezuelan savannah. Increases of aerosol concentrations could not be linked to long-range transport of biomass burning plumes from the Amazon basin or Africa due to effective wet scavenging of particles. Highest particle concentrations were observed within boundary layer air masses during the dry season. Ambient sub-micron aerosol volume reached 1.4 ± 1.3 μm³ cm⁻³, heated (300 °C) particle number concentrations 510 ± 420 cm⁻³ and the absorption coefficient 0.91 ± 1.2 Mm⁻¹. The respective concentrations were lowest within the lower free troposphere during the wet season and averaged at 0.19 ± 0.25 μm³ cm⁻³, 150 ± 94 cm⁻³ and 0.15 ± 0.26 Mm⁻¹. A decrease of particle concentrations during the dry seasons from 2007–2009 could be connected to a decrease in fire activity in the wider region of Venezuela using MODIS satellite observations. The variability of biomass burning is most likely linked to the El Niño-Southern Oscillation (ENSO). Low biomass burning activity in the Venezuelan savannah was observed to follow La Niña conditions, high biomass burning activity followed El Niño conditions.

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

2013-05-17T00:00:00+02:00

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

Atmospheric Chemistry and Physics Discussions, 13, 13045-13078, 2013

Author(s): Y. H. Wang, B. Hu, and Y. S. Wang

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 based on 24 h averaged value. There is a lower ozone concentration from Wednesday to Friday (weekday) and a higher concentration from Saturday to Monday (weekend) over the entire study area. NO_x also displays 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.

A smaller decrease in VOCs (a proxy for CO) 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-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 absorption or scattering, which leads to a decrease in the ozone production efficiency. A significant weekly cycle in UV radiation, in consistent with the aerosol concentration, was discovered at the BJT site, validating the assumption. A comprehensive understanding of the ozone weekend effect in the BTH area can provide deep insights into controlling photochemical pollution.

Reconstruction of Northern Hemisphere 1950–2010 atmospheric non-methane hydrocarbons

2013-05-15T00:00:00+02:00

Reconstruction of Northern Hemisphere 1950–2010 atmospheric non-methane hydrocarbons

Atmospheric Chemistry and Physics Discussions, 13, 12991-13043, 2013

Author(s): D. Helmig, V. Petrenko, P. Martinerie, E. Witrant, T. Röckmann, A. Zuiderweg, R. Holzinger, J. Hueber, C. Stephens, J. White, W. Sturges, A. Baker, T. Blunier, D. Etheridge, M. Rubino, and P. Tans

The short-chain non-methane hydrocarbons (NMHC) are mostly emitted into the atmosphere by anthropogenic processes. Recent studies have pointed out a tight linkage between the atmospheric mole fractions of the NMHC ethane to the atmospheric growth rate of methane. Consequently, atmospheric NMHC are valuable indicators for tracking changes in anthropogenic emissions, photochemical ozone production, and greenhouse gases. This study investigates the 1950–2010 Northern Hemisphere atmospheric C₂-C₅ NMHC ethane, propane, i-butane, n-butane, i-pentane, and n-pentane. Atmospheric mole fractions of these trace gases were constructed from (a) air samples of these trace gases from air samples extracted from three firn boreholes in 2008 and 2009 at the North Greenland Eemian Ice Drilling (NEEM) site using state of the art models of trace gas transport in firn, and by (b) considering eight years of ambient NMHC monitoring data from five Arctic sites within the NOAA Global Monitoring Division (GMD) Cooperative Air Sampling Network. Results indicate that these NMHC increased by ~ 40–120% after 1950, peaked around 1980 (with the exception of ethane, which peaked approximately 10 years earlier), and have since dramatically decreased to be now back close to 1950 levels. The earlier peak time of ethane versus the C₃-C₅ NMHC suggests that different processes and emissions mitigation measures contributed to the decline in these NMHC. The 60 yr record also illustrates notable increases in the ratios of the isomeric iso-/n-butane and iso-/n-pentane ratios. Comparison of the reconstructed NMHC histories with 1950–2000 volatile organic compounds (VOC) emissions data and with other recently published ethane trend analyses from ambient air Pacific transect data showed (a) better agreement with North America and Western Europe emissions than with total Northern Hemisphere emissions data, and (b) better agreement with other Greenland firn air data NMHC history reconstructions than with the Pacific region trends. These analyses emphasize that for NMHC, having atmospheric lifetimes on the order of < 2 months, the Greenland firn air records are primarily a representation of Western Europe and North America emission histories.

The balances of mixing ratios and segregation intensity: a case study from the field (ECHO 2003)

2013-05-15T00:00:00+02:00

The balances of mixing ratios and segregation intensity: a case study from the field (ECHO 2003)

Atmospheric Chemistry and Physics Discussions, 13, 12913-12989, 2013

Author(s): R. Dlugi, M. Berger, M. Zelger, A. Hofzumahaus, F. Rohrer, F. Holland, K. Lu, and G. Kramm

An inhomogeneous mixing of reactants causes a reduction of their chemical removal compared to the homogeneously mixed case in turbulent atmospheric flows. This can be described by the intensity of segregation I_S being the covariance of the mixing ratios of two species divided by the product of their means. Both terms appear in the balance equation of the mixing ratio and are discussed for the reaction between isoprene and OH for data of the field study ECHO 2003 above a deciduous forest. For most of these data, I_S is negatively correlated with the fraction of mean OH mixing ratio reacting with isoprene. I_S is also negatively correlated with the isoprene standard deviation. Both findings agree with model results discussed by Patton et al. (2001) and others. The correlation coefficient between OH and isoprene, and, therefore, I_S increases with increasing mean reaction rate. In addition, the balance equation of the covariance between isoprene and OH is applied for the analysis of the same field data. The storage term is small, and, therefore, a diagnostic equation for this covariance can be derived. The chemical reaction term R_ij is dominated by the variance of isoprene times the quotient of mixing ratios of OH and isoprene. In addition a diagnostic equation for I_S is formulated. Comparing different terms of this equation, I_S and R_ij show a relation also to the normalized isoprene standard deviation. It is shown that not only chemistry, but also turbulent and convective mixing and advection – considered in a residual term – influence I_S. Despite this finding, a detection of the influence of coherent eddy transport above the forest according to Katul et al. (1997) on I_S fails, but a relation with the turbulent transport of isoprene variance is determined. In addition, largest values of I_S are found for most unstable conditions with increasing buoyancy. These results are compared to model results by Ouwersloot et al. (2011).

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The global impact of the transport sectors on atmospheric aerosol: simulations for year 2000 emissions

Long term in-situ observations of biomass burning aerosol at a high altitude station in Venezuela – sources, impacts and inter annual variability

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

Reconstruction of Northern Hemisphere 1950–2010 atmospheric non-methane hydrocarbons

The balances of mixing ratios and segregation intensity: a case study from the field (ECHO 2003)

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