ACPD - Latest Articles

Long-term monitoring of atmospheric total gaseous mercury (TGM) at a remote site in Mt. Changbai area, northeastern China

2012-02-07T00:00:00+01:00

Long-term monitoring of atmospheric total gaseous mercury (TGM) at a remote site in Mt. Changbai area, northeastern China

Atmospheric Chemistry and Physics Discussions, 12, 4417-4446, 2012

Author(s): X. W. Fu, X. Feng, L. H. Shang, S. F. Wang, and H. Zhang

Total gaseous mercury (TGM) was continuously monitored at a remote site (CBS) in the Mt. Changbai area, northeastern China biennially from 24 October 2008 to 31 October 2010. The overall mean TGM concentration was 1.60 ± 0.51 ng m⁻³, which is lower than those reported from remote sites in eastern, southwestern and western China, indicating a relatively low regional anthropogenic mercury (Hg) emission intensity in northeastern China. Measurements at a site in the vicinity (~1.2 km) of the CBS station during August 2005 and July 2006 showed a significantly higher mean TGM concentration of 3.58 ± 1.78 ng m⁻³. The divergent result was partially attributed to fluctuations in the regional surface wind system and moreover an effect of local emission sources. The temporal variation of TGM at CBS was obviously influenced by regional sources as well as long-range transported Hg. Regional sources, frequently contributing to episodical high TGM concentrations, were pinpointed as a large iron mining district in northern North Korea and two large power plants and urban areas to the southwest of the sampling site. Source areas in Beijing, Tianjin, southern Liaoning, Hebei, northwestern Shanxi and northwestern Shandong were found to contribute to elevated TGM observations at CBS via long-range transport. The diurnal pattern of TGM at CBS was mainly regulated by regional sources, likely as well as intrusion of air masses from the free troposphere during summer season. There are no discernible seasonal pattern of TGM at CBS, which mainly showed links with the patterns of regional air movements and long-range transport.

Ice nuclei in marine air: bioparticles or dust?

2012-02-07T00:00:00+01:00

Ice nuclei in marine air: bioparticles or dust?

Atmospheric Chemistry and Physics Discussions, 12, 4373-4416, 2012

Author(s): S. M. Burrows, C. Hoose, U. Pöschl, and M. G. Lawrence

Ice nuclei can influence the properties of clouds and precipitation, but their sources and distribution in the atmosphere are still not well known. Particularly little attention has been paid to IN sources in marine environments, although anecdotal evidence suggests that IN populations in remote marine regions may be dominated by biological particles associated with sea spray. In this exploratory model study, we aim to bring attention to this long-neglected topic and identify promising target regions for future field campaigns. We assess the likely global distribution of marine biological ice nuclei using a combination of historical observations, satellite data and model output. By comparing simulated marine biological IN distributions and dust IN distributions, we predict strong regional differences in the importance of marine biological IN relative to dust IN. Our analysis suggests that marine biological IN are most likely to play a dominant role in determining IN concentrations over the Southern Ocean, so future field campaigns aimed at investigating marine biological IN should target that region. Climate-related changes in the abundance and emission of biological marine IN could affect marine cloud properties, thereby introducing previously unconsidered feedbacks that influence the hydrological cycle and the Earth's energy balance. Furthermore, marine biological IN may be an important aspect to consider in proposals for marine cloud brightening by artificial sea spray production.

AERONET and ESR sun direct products comparison performed on Cimel CE318 and Prede POM01 solar radiometers

2012-02-07T00:00:00+01:00

AERONET and ESR sun direct products comparison performed on Cimel CE318 and Prede POM01 solar radiometers

Atmospheric Chemistry and Physics Discussions, 12, 4341-4371, 2012

Author(s): V. Estellés, M. Campanelli, T. J. Smyth, M. P. Utrillas, and J. A. Martínez-Lozano

The European Skynet Radiometers network (EuroSkyRad or ESR) has been recently established as a research network of European Prede POM sun – sky radiometers. Moreover, ESR is federated with SKYNET (SKYrad NETwork), an international network mostly present in East Asia. In contrast to SKYNET, the European network also integrates users of the Cimel CE318 sunphotometer. Keeping instrumental duality in mind, a set of open source algorithms has been developed consisting of two modules for: (1) the retrieval of direct sun products from the sun extinction measurements; and (2) the inversion of the sky radiance to derive aerosol optical properties. In this study we evaluate the ESR direct sun products (spectral aerosol optical depth, Angström wavelength exponent and columnar content of water vapour) in comparison with the AERosol RObotic NETwork (AERONET) products. Specifically, we have applied the ESR algorithm to a Cimel CE318 and Prede POM01L simultaneously for a 4 yr database measured at the Burjassot site (Valencia, Spain), and compared the resultant products with the AERONET direct sun retrievals obtained with the same Cimel CE318 instrument. The comparison show that aerosol optical depth differences are mostly within the nominal uncertainty of 0.003 for a standard calibration instrument, and fall within the nominal AERONET uncertainty of 0.01–0.02 for a field instrument. Therefore, we present an open source code that can be used for both radiometers and whose results are comparable to those of AERONET and SKYNET.

Quantifying retrieval uncertainties in the CM-SAF cloud physical property algorithm with simulated SEVIRI observations

2012-02-07T00:00:00+01:00

Quantifying retrieval uncertainties in the CM-SAF cloud physical property algorithm with simulated SEVIRI observations

Atmospheric Chemistry and Physics Discussions, 12, 4311-4340, 2012

Author(s): B. J. Jonkheid, R. A. Roebeling, and E. van Meijgaard

The uncertainties in the cloud physical properties derived from satellite observations make it difficult to interpret model evaluation studies. In this paper, the uncertainties in the cloud water path (CWP) retrievals derived with the cloud physical properties retrieval algorithm (CPP) of the climate monitoring satellite application facility (CM-SAF) are investigated. To this end, a numerical simulator of MSG-SEVIRI observations was developed that calculates the reflectances at 0.64 and 1.63 μm for a wide range of cloud parameters, satellite viewing geometries and surface albedos. These reflectances are used as input to CPP, and the retrieved values of CWP are compared to the original input of the simulator.

It is shown that the CWP retrievals are very sensitive to the assumptions made in the CPP code. The CWP retrieval errors are generally small for unbroken single-phase clouds with COT >10, with retrieval errors of ~3% for liquid water clouds to ~10% for ice clouds. When both liquid water and ice clouds are present in a pixel, the CWP retrieval errors increase dramatically; depending on the cloud, this can lead to uncertainties of 40–80%. CWP retrievals also become more uncertain when the cloud does not cover the entire pixel, leading to errors of ~50% for cloud fractions of 0.75 and even larger errors for smaller cloud fractions. Thus, the satellite retrieval of cloud physical properties of broken clouds and multi-phase clouds is complicated by inherent difficulties, and the proper interpretation of such retrievals requires extra care.

An observation-based approach to identify local natural dust events from routine aerosol ground monitoring

2012-02-07T00:00:00+01:00

An observation-based approach to identify local natural dust events from routine aerosol ground monitoring

Atmospheric Chemistry and Physics Discussions, 12, 4279-4310, 2012

Author(s): D. Q. Tong, M. Dan, T. Wang, and P. Lee

Dust is a major component of atmospheric aerosols in many parts of the world. Although there exist many routine aerosol monitoring networks, it is often difficult to obtain dust records from these networks, because these monitors are either deployed far away from dust active regions (most likely collocated with dense population) or contaminated by anthropogenic sources and other natural sources, such as wildfires and vegetation detritus. Here we propose a new approach to identify local dust events relying solely on aerosol mass and composition from general-purpose aerosol measurements. Through analyzing the chemical and physical characteristics of aerosol observations during satellite-detected dust episodes, we select five indicators to be used to identify local dust records: (1) high PM₁₀ concentrations; (2) low PM_2.5/PM₁₀ ratio; (3) higher concentrations and percentage of crustal elements; (4) lower percentage of anthropogenic pollutants; and (5) low enrichment factors of anthropogenic elements. After establishing these identification criteria, we conduct hierarchical cluster analysis for all validated aerosol measurement data over 68 IMPROVE sites in the Western United States. A total of 182 local dust events were identified over 30 of the 68 locations from 2000 to 2007. These locations are either close to the four US Deserts, namely the Great Basin Desert, the Mojave Desert, the Sonoran Desert, and the Chihuahuan Desert, or in the high wind power region (Colorado). During the eight-year study period, the total number of dust events displays an interesting four-year activity cycle (one in 2000–2003 and the other in 2004–2007). The years of 2003, 2002 and 2007 are the three most active dust periods, with 46, 31 and 24 recorded dust events, respectively, while the years of 2000, 2004 and 2005 are the calmest periods, all with single digit dust records. Among these deserts, the Chihuahua Desert (59 cases) and the Sonoran Desert (62 cases) are by far the most active source regions. In general, the Chihuahua Desert dominates dust activities in the first half of the eight-year period while the Sonoran Desert in the second half. The monthly frequency of dust events shows a peak from March to July and a second peak in autumn from September to November. The large quantity of dust events occurring in summertime also suggests the prevailing impact of windblown dust across the year. This seasonal variation is consistent with previous model simulations over the United States.

Determinants and predictability of global wildfire emissions

2012-02-06T00:00:00+01:00

Determinants and predictability of global wildfire emissions

Atmospheric Chemistry and Physics Discussions, 12, 4243-4278, 2012

Author(s): W. Knorr, V. Lehsten, and A. Arneth

Biomass burning is one of the largest sources of atmospheric trace gases and aerosols globally. These emissions have a major impact on the radiative balance of the atmosphere and on air quality, and are thus of significant scientific and societal interest. Several datasets have been developed that quantify those emissions on a global grid and offered to the atmospheric modelling community. However, no study has yet attempted to systematically quantify the dependence of the inferred pyrogenic emissions on underlying assumptions and input data. Such a sensitivity study is needed for understanding how well we can currently model those emissions and what the factors are that contribute to uncertainties in those emissions estimates.

Here, we combine various satellite-derived burned area products, a terrestrial ecosystem model to simulate fuel loads and the effect of fire on ecosystem dynamics, a model of fuel combustion, and various emission models that relate combusted biomass to the emission of various trace gases and aerosols. We vary one key parameter of a simple fuel combustion model, the emission model, and the burned area product and assess its impact on the computed emissions fields and their uncertainties. We find that choice of burned area data set has by far the largest impact on interannual variability of simulated emissions. For total global emissions, burned area and combustion completeness have the largest impact on emissions for most species.

We conclude that reliable information on burned area is key for accurately modelling spatial and interannual variations of wildfire emissions, but uncertainties about the combustion process have a similar impact on the magnitude of global emission estimates. The results are important for chemical transport modelling studies, and for simulations of biomass burning impacts on the atmosphere under future climate change scenarios.

Anthropogenic changes in the surface all-sky UV-B radiation through 1850–2005 simulated by an Earth system model

2012-02-06T00:00:00+01:00

Anthropogenic changes in the surface all-sky UV-B radiation through 1850–2005 simulated by an Earth system model

Atmospheric Chemistry and Physics Discussions, 12, 4221-4242, 2012

Author(s): S. Watanabe, T. Takemura, K. Sudo, T. Yokohata, and H. Kawase

The historical anthropogenic change in the surface all-sky UV-B (solar ultraviolet: 280–315 nm) radiation through 1850–2005 is evaluated using an Earth system model. Responses of UV-B dose to anthropogenic changes in ozone and aerosols are separately evaluated using a series of historical simulations including/excluding these changes. Increases in these air pollutants cause reductions in UV-B transmittance, which occur gradually/rapidly before/after 1950 in and downwind of industrial and deforestation regions. Furthermore, changes in ozone transport in the lower stratosphere, which is induced by increasing greenhouse gas concentrations, increase ozone concentration in the extratropical upper troposphere and lower stratosphere. These transient changes work to decrease the amount of UV-B reaching the Earth's surface, counteracting the well-known effect increasing UV-B due to stratospheric ozone depletion, which developed rapidly after ca. 1980. As a consequence, the surface all-sky UV-B radiation change between 1850 and 2000 is negative in the tropics and NH extratropics and positive in the SH extratropics. Comparing the contributions of ozone and aerosol changes to the UV-B change, the transient change in ozone absorption of UV-B mainly determines the total change in the surface all-sky UV-B radiation at most locations. On the other hand, the aerosol direct and indirect effects on UV-B play an equally important role to that of ozone in the NH mid-latitudes and tropics. A typical example is East Asia (25° N–60° N and 120° E–150° E), where the effect of aerosols (ca. 70%) dominates the total UV-B change.

Climatological perspectives of air transport from atmospheric boundary layer to tropopause layer over Asian monsoon regions during boreal summer inferred from Lagrangian approach

2012-02-06T00:00:00+01:00

Climatological perspectives of air transport from atmospheric boundary layer to tropopause layer over Asian monsoon regions during boreal summer inferred from Lagrangian approach

Atmospheric Chemistry and Physics Discussions, 12, 4185-4219, 2012

Author(s): B. Chen, X. D. Xu, S. Yang, and T. L. Zhao

The Asian Summer Monsoon (ASM) region has been recognized as a key region that plays a vital role in troposphere-to-stratosphere transport (TST), which can significantly impact the budget of global atmospheric constituents and climate change. However, the details of transport from the boundary layer (BL) to tropopause layer (TL) over this region, particularly from a climatological perspective, remains an issue of uncertainty. In this study, we present the climatological properties of BL-to-TL transport over the ASM region during boreal summer season (June-July-August) from 2001 to 2009. A comprehensive tracking analysis is conducted based on a large ensemble of TST-trajectories departing from the atmospheric BL and arriving at TL. Driven by the winds fields from the NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) Global Forecast System, all TST-trajectories are selected from the high resolution datasets generated by the Lagrangian particle transport model FLEXPART using a domain-filling technique. Three key atmospheric boundary layer sources for BL-to-TL transport are identified with their contributions: (i) 38% from the region between tropical Western Pacific region and South China Seas (WP), (ii) 21% from Bay of Bengal and South Asian subcontinent (BOB), and (iii) 12% from the Tibetan Plateau, which includes the South Slope of the Himalayas (TIB). Controlled by the different patterns of atmospheric circulation, the air masses originating from these three source regions are transported along the different tracks into the TL. The spatial distributions of these three source regions remain similarly from year to year. The timescales of transport from BL to TL by the large-scale ascents range from 1 to 7 weeks, contributing up to 60–70% of the overall TST; whereas the transport governed by the deep convection overshooting becomes faster, with timescales of 1–2 days and contributions of 20–30%. These results provide clear policy implications for the control of very short lived substances, especially for the source regions over the Indian subcontinent with increasing populations and developing industries.

Atmospheric histories and growth trends of C₄F₁₀, C₅F₁₂, C₆F₁₄, C₇F₁₆ and C₈F₁₈

2012-02-03T00:00:00+01:00

Atmospheric histories and growth trends of C₄F₁₀, C₅F₁₂, C₆F₁₄, C₇F₁₆ and C₈F₁₈

Atmospheric Chemistry and Physics Discussions, 12, 4165-4184, 2012

Author(s): D. J. Ivy, T. Arnold, C. M. Harth, L. P. Steele, J. Mühle, M. Rigby, P. K. Salameh, M. Leist, P. B. Krummel, P. J. Fraser, R. F. Weiss, and R. G. Prinn

The first atmospheric observations and trends are presented for the high molecular weight perfluorocarbons (PFCs): decafluorobutane (C₄F₁₀), dodecafluoropentane (C₅F₁₂), tetradecafluorohexane (C₆F₁₄), hexadecafluoroheptane (C₇F₁₆) and octadecafluorooctane (C₈F₁₈). Their atmospheric histories are based on measurements of 38 Northern Hemisphere and 46 Southern Hemisphere archived air samples collected between 1973 to 2011 using the Advanced Global Atmospheric Gases Experiment (AGAGE) "Medusa" preconcentration gas chromatography-mass spectrometry systems. A new calibration scale was prepared for each PFC, with estimated accuracies of 6.8% for C₄F₁₀, 7.8% for C₅F₁₂, 4.0% for C₆F₁₄, 6.6% for C₇F₁₆ and 7.9% for C₈F₁₈. Based on our observations the 2011 globally averaged dry air mole fractions of these heavy PFCs are: 0.18 parts-per-trillion (ppt, i.e., parts per 10¹²) for C₄F₁₀, 0.12 ppt for C₅F₁₂, 0.28 ppt for C₆F₁₄, 0.12 ppt for C₇F₁₆ and 0.09 ppt for C₈F₁₈. These atmospheric mole fractions combine to contribute to a global average radiative forcing of 0.35 mW m⁻², which is 3.6% of the total PFC radiative forcing. The globally averaged mean atmospheric growth rates of these PFCs during 1973–2011 are 4.58 parts per quadrillion (ppq, i.e., parts per 10¹⁵) per year (yr) for C₄F₁₀, 3.29 ppq yr⁻¹ for C₅F₁₂, 7.50 ppq yr⁻¹ for C₆F₁₄, 3.19 ppq yr⁻¹ for C₇F₁₆ and 2.51 ppq yr⁻¹ for C₈F₁₈. The growth rates of the heavy perfluorocarbons were largest in the early 1990s for C₄F₁₀ and C₅F₁₂ and in the mid-to-late 1990s for C₆F₁₄, C₇F₁₆ and C₈F₁₈. The more recent slow down in the growth rates of the high molecular weight PFCs suggests that emissions are declining as compared to the 1980s and 1990s. Nevertheless continued monitoring of these potent, extremely long-lived greenhouse gases is necessary to verify that global PFC emissions continue to decline.

Central Arctic atmospheric summer conditions during the Arctic Summer Cloud Ocean Study (ASCOS): contrasting to previous expeditions

2012-02-03T00:00:00+01:00

Central Arctic atmospheric summer conditions during the Arctic Summer Cloud Ocean Study (ASCOS): contrasting to previous expeditions

Atmospheric Chemistry and Physics Discussions, 12, 4101-4164, 2012

Author(s): M. Tjernström, C. E. Birch, I. M. Brooks, M. D. Shupe, P. O. G. Persson, J. Sedlar, T. Mauritsen, C. Leck, J. Paatero, M. Szczodrak, and C. R. Wheeler

Understanding the rapidly changing climate in the Arctic is limited by a lack of understanding of underlying strong feedback mechanisms that are specific to the Arctic. Progress in this field can only be obtained by process-level observations; this is the motivation for intensive ice-breaker-based campaigns such as that described in this paper: the Arctic Summer Cloud-Ocean Study (ASCOS). However, detailed field observations also have to be put in the context of the larger-scale meteorology, and short field campaigns have to be analysed within the context of the underlying climate state and temporal anomalies from this.

To aid in the analysis of other parameters or processes observed during this campaign, this paper provides an overview of the synoptic-scale meteorology and its climatic anomaly during the ASCOS field deployment. It also provides a statistical analysis of key features during the campaign, such as some key meteorological variables, the vertical structure of the lower troposphere and clouds, and energy fluxes at the surface. In order to assess the representativity of the ASCOS results, we also compare these features to similar observations obtained during three earlier summer experiments in the Arctic Ocean, the AOE-96, SHEBA and AOE-2001 expeditions.

We find that these expeditions share many key features of the summertime lower troposphere. Taking ASCOS and the previous expeditions together, a common picture emerges with a large amount of low-level cloud in a well-mixed shallow boundary layer, capped by a weak to moderately strong inversion where moisture, and sometimes also cloud top, penetrate into the lower parts of the inversion. Much of the boundary-layer mixing is due to cloud-top cooling and subsequent buoyant overturning of the cloud. The cloud layer may, or may not, be connected with surface processes depending on the depths of the cloud and surface-based boundary layers and on the relative strengths of surface-shear and cloud-buoyancy turbulence generation. The latter also implies a connection between the cloud layer and the free troposphere through entrainment at cloud top.

Distributions, long term trends and emissions of four perfluorocarbons in remote parts of the atmosphere and firn air

2012-02-03T00:00:00+01:00

Distributions, long term trends and emissions of four perfluorocarbons in remote parts of the atmosphere and firn air

Atmospheric Chemistry and Physics Discussions, 12, 4073-4100, 2012

Author(s): J. C. Laube, C. Hogan, M. J. Newland, F. S. Mani, P. J. Fraser, C. A. M. Brenninkmeijer, P. Martinerie, D. E. Oram, T. Röckmann, J. Schwander, E. Witrant, G. P. Mills, C. E. Reeves, and W. T. Sturges

We report the first data set of atmospheric abundances for the following four perfluoroalkanes: n-decafluorobutane (n-C₄F₁₀), n-dodecafluoropentane (n-C₅F₁₂), n-tetradecafluorohexane (n-C₆F₁₄) and n-hexadecafluoroheptane (n-C₇F₁₆). All four compounds could be detected and quantified in air samples from remote locations in the Southern Hemisphere (at Cape Grim, Tasmania, archived samples dating back to 1978) and the upper troposphere (a passenger aircraft flying from Germany to South Africa). Further observations originate from air samples extracted from deep firn in Greenland and allow trends of atmospheric abundances in the earlier 20th century to be inferred. All four compounds were not present in the atmosphere prior to the 1960s. n-C₄F₁₀ and n-C₅F₁₂ were also measured in samples collected in the stratosphere confirming their very long atmospheric lifetimes. Emissions were inferred from these observations and found to be comparable with emissions from the EDGAR database for n-C₆F₁₄. However, emissions of n-C₄F₁₀, n-C₅F₁₂ and n-C₇F₁₆ were found to differ by up to five orders of magnitude. Although the abundances of the four perfluorocarbons reported here are currently small (less than 0.3 ppt, parts per trillion) they have strong Global Warming Potentials several thousand times higher than carbon dioxide and continue to increase in the atmosphere. The sum of their cumulative emissions reached 325 mt (million metric tonnes) CO₂ equivalent at the end of 2009.

Retrieval of aerosol optical depth over land based on a time series technique using MSG/SERIVI data

2012-02-03T00:00:00+01:00

Retrieval of aerosol optical depth over land based on a time series technique using MSG/SERIVI data

Atmospheric Chemistry and Physics Discussions, 12, 4031-4071, 2012

Author(s): L. Mei, Y. Xue, G. de Leeuw, T. Holzer-Popp, J. Guang, Y. Li, L. Yang, H. Xu, X. Xu, C. Li, Y. Wang, C. Wu, T. Hou, X. He, J. Liu, J. Dong, and Z. Chen

A novel approach for the joint retrieval of aerosol optical depth (AOD) and surface reflectance, using Meteosat Second Generation – Spinning Enhanced Visible and Infrared Imagers (MSG/SEVIRI) observations in two solar channels, is presented. The retrieval is based on a time series (TS) technique, which makes use of the two visible bands at 0.6 μm and 0.8 μm in three orderly scan times (15 min interval between two scans) to retrieve the AOD over land. Using the radiative transfer equation for plane-parallel atmospheres two coupled differential equations for the upward and downward fluxes are derived. The boundary conditions for the upward and downward fluxes at the top and at the bottom of the atmosphere are used in these equations to provide an analytic solution for the surface reflectance. To derive these fluxes, the aerosol single scattering albedo (SSA) and asymmetry factor are required to provide a solution. These are provided from a set of six pre-defined aerosol types with the SSA and asymmetry factor (g). We assume one aerosol type for a grid of 1° × 1° and the surface reflectance changes little between two consequent scans. A k approximation was used in the inversion to find the best solution of atmospheric properties and surface reflectance. The algorithm makes use of numerical minimisation routines to obtain the optimal solution of atmospheric properties and surface reflectance by selection of the most suitable aerosol type from pre-defined sets. Also, it is assumed that the surface reflectance is little influenced by aerosol scattering at 1.6 μm and therefore the ratio of surface reflectances in the solar band for two consequent scans can be well-approximated by the ratio of the reflectances at 1.6 μm. A further assumption is that the surface reflectance varies only slightly over a period of 30 min.

A detailed analysis of the retrieval results show that it is suitable for AOD retrieval over land. Six Aerosol Robotic Network (AERONET) sites with different surface types were used for detailed analysis and 42 other AERONET sites were used for validation. From 445 collocations representing stable and homogeneous aerosol type, we found that >75% of MSG-retrieved AOD values compared to AERONET observed values with an error envelope of ±0.05 ± 0.15τ and a high correlation (R > 0.86). The AOD datasets derived using the TS method with SEVIRI data was also compared with collocated AOD products derived from the NASA TERRA and AQUA MODIS data using the dark dense vegetation (DDV) method and the Deep Blue algorithms. Using the TS method, AOD could be retrieved for more pixels than with the NASA Deep Blue algorithm. The AOD values derived compare favourably.

MAX-DOAS measurements of NO₂, HCHO and CHOCHO at a rural site in Southern China

2012-02-03T00:00:00+01:00

MAX-DOAS measurements of NO₂, HCHO and CHOCHO at a rural site in Southern China

Atmospheric Chemistry and Physics Discussions, 12, 3983-4029, 2012

Author(s): X. Li, T. Brauers, A. Hofzumahaus, K. Lu, Y. P. Li, M. Shao, T. Wagner, and A. Wahner

We performed MAX-DOAS measurements during the PRIDE-PRD2006 campaign in the Pearl River Delta region (PRD), China, for 4 weeks in July 2006 at a site located 60 km north of Guangzhou. The vertical distributions of NO₂, HCHO, and CHOCHO were independently retrieved by an automated iteration method. The MAX-DOAS measured NO₂ mixing ratios showed reasonable agreement with the simultaneous, ground based in-situ data. While the tropospheric NO₂ vertical column densities (VCDs) observed by OMI on board EOS-Aura satellite agreed with those by MAX-DOAS, the 3-D chemical transport model CMAQ overestimated the NO₂ VCDs as well as the surface concentrations by about 40%. From this observation, a reduction of NO_X emission strength in CMAQ seems to be necessary in order to well reproduce the NO₂ observations. The average mixing ratios of HCHO and CHOCHO were 12 ppb and 1.6 ppb, respectively, substantially higher than in other rural or semirural environments. The high ratio of 0.135 between CHOCHO and HCHO corresponds to the high VOCs reactivity and high HO_X turnover rate consistent with other observations during the campaign.

Tropospheric methanol observations from space: retrieval evaluation and constraints on the seasonality of biogenic emissions

2012-02-03T00:00:00+01:00

Tropospheric methanol observations from space: retrieval evaluation and constraints on the seasonality of biogenic emissions

Atmospheric Chemistry and Physics Discussions, 12, 3941-3982, 2012

Author(s): K. C. Wells, D. B. Millet, L. Hu, K. E. Cady-Pereira, Y. Xiao, M. W. Shephard, C. L. Clerbaux, L. Clarisse, P.-F. Coheur, E. C. Apel, J. de Gouw, C. Warneke, H. B. Singh, A. H. Goldstein, and B. C. Sive

Methanol retrievals from nadir-viewing space-based sensors offer powerful new information for quantifying methanol emissions on a global scale. Here we apply an ensemble of aircraft observations over North America to evaluate new methanol measurements from the Tropospheric Emission Spectrometer (TES) on the Aura satellite, and combine the TES data with observations from the Infrared Atmospheric Sounding Interferometer (IASI) on the MetOp-A satellite to investigate the seasonality of methanol emissions from northern midlatitude ecosystems. Using the GEOS-Chem chemical transport model as an intercomparison platform, we find that the TES retrieval performs well when the degrees of freedom for signal (DOFS) are above 0.5, in which case the model : TES regressions are generally consistent with the model : aircraft comparisons. Including retrievals with DOFS below 0.5 degrades the comparisons, as these are excessively influenced by the a priori. The comparisons suggest DOFS > 0.5 as a minimum threshold for interpreting retrievals of trace gases with a weak tropospheric signal. We analyze one full year of satellite observations and find that GEOS-Chem, driven with MEGANv2.1 biogenic emissions, underestimates observed methanol concentrations throughout the midlatitudes in springtime, with the timing of the seasonal peak in model emissions 1–2 months too late. We attribute this discrepancy to an underestimate of emissions from new leaves in MEGAN, and apply the satellite data to better quantify the seasonal change in methanol emissions for midlatitude ecosystems. The derived parameters (relative emission factors of 11.0, 1.0, 0.05 and 8.6 for new, growing, mature, and old leaves, respectively, plus a leaf area index activity factor of 0.75 for expanding canopies with leaf area index < 2.0) provide a more realistic simulation of seasonal methanol concentrations in midlatitudes on the basis of IASI, TES, and ground-based measurements.

Projections of mid-century summer air-quality for North America: effects of changes in climate and precursor emissions

2012-02-03T00:00:00+01:00

Projections of mid-century summer air-quality for North America: effects of changes in climate and precursor emissions

Atmospheric Chemistry and Physics Discussions, 12, 3875-3940, 2012

Author(s): J. Kelly, P. A. Makar, and D. A. Plummer

Ten year simulations of North American current and future air-quality were carried out using a regional air-quality model driven by a regional climate model, in turn driven by a general circulation model. Three separate summer scenarios were performed: a scenario representing the years 1997 to 2006, and two SRES A2 climate scenarios for the years 2041 to 2050. The first future climate scenario makes use of 2002 anthropogenic precursor emissions, and the second applied emissions scaling factors derived from the IPCC Representative Concentration Pathway 6 (RCP 6) scenario to estimate emissions for 2050 from existing 2020 projections. Ten-year averages of ozone and PM_2.5 at North American monitoring network stations were used to evaluate the model's current chemical climatology. The model was found to have a similar performance for ozone as when driven by an operational weather forecast model. The PM_2.5 predictions had larger negative biases, likely resulting from the absence of rainwater evaporation, and from sub-regional negative biases in the surface temperature fields, in the version of the climate model used here.

The differences between the two future climate simulations and the current climate simulation were used to predict the changes to air-quality that might be expected in a future warmer climate, if anthropogenic precursor emissions remain constant at their current levels, versus if the RCP 6 emissions controls were adopted. Metrics of concentration, human health, and ecosystem damage were compared for the simulations. The scenario with future climate and current anthropogenic emissions resulted in worse air-quality than for current conditions – that is, the effect of climate-change alone, all other factors being similar, would be a worsening of air-quality. These effects are spatially inhomogeneous, with the magnitude and sign of the changes varying with region. The scenario with future climate and RCP 6 emissions for 2050 resulted in an improved air-quality, with decreases in key pollutant concentrations, in acute human mortality associated with air-pollution, and in sulphur and ozone deposition to the ecosystem. The positive outcomes of the RCP 6 emissions reductions were found to be of greater magnitude than the negative outcomes of climate change alone. The RCP 6 scenario however resulted in an increase in the deposition of nitrogen, as a result of increased ammonia emissions expected in that scenario, compared to current ammonia emissions levels.

The results of the study raise the possibility that simultaneous reductions of greenhouse gases and air pollution precursors may further reduce air pollution levels, with the added benefits of an immediate reduction in the impacts of air pollution on human and ecosystem health. Further scenarios to investigate this possibility are therefore recommended.

The EMEP MSC-W chemical transport model – Part 1: Model description

2012-02-02T00:00:00+01:00

The EMEP MSC-W chemical transport model – Part 1: Model description

Atmospheric Chemistry and Physics Discussions, 12, 3781-3874, 2012

Author(s): D. Simpson, A. Benedictow, H. Berge, R. Bergström, L. D. Emberson, H. Fagerli, G. D. Hayman, M. Gauss, J. E. Jonson, M. E. Jenkin, A. Nyíri, C. Richter, V. S. Semeena, S. Tsyro, J.-P. Tuovinen, Á. Valdebenito, and P. Wind

The Meteorological Synthesizing Centre-West (MSC-W) of the European Monitoring and Evaluation Programme (EMEP) has been performing model calculations in support of the Convention on Long Range Transboundary Air Pollution (CLRTAP) for more than 30 yr. The EMEP MSC-W chemical transport model is still one of the key tools within European air pollution policy assessments.

Traditionally, the EMEP model has covered all of Europe with a resolution of about 50 × 50 km², and extending vertically from ground level to the tropopause (100 hPa). The model has undergone substantial development in recent years, and is now applied on scales ranging from local (ca. 5 km grid size) to global (with 1 degree resolution). The model is used to simulate photo-oxidants and both inorganic and organic aerosols.

In 2008 the EMEP model was released for the first time as public domain code, along with all required input data for model runs for one year. Since then, many changes have been made to the model physics, and input data. The second release of the EMEP MSC-W model became available in mid 2011, and a new release is targeted for early 2012. This publication is intended to document this third release of the EMEP MSC-W model. The model formulations are given, along with details of input data-sets which are used, and brief background on some of the choices made in the formulation are presented. The model code itself is available at www.emep.int, along with the data required to run for a full year over Europe.

Lessons learnt from the first EMEP intensive measurement periods

2012-02-02T00:00:00+01:00

Lessons learnt from the first EMEP intensive measurement periods

Atmospheric Chemistry and Physics Discussions, 12, 3731-3780, 2012

Author(s): W. Aas, S. Tsyro, E. Bieber, R. Bergström, D. Ceburnis, T. Ellermann, H. Fagerli, M. Frölich, R. Gehrig, U. Makkonen, E. Nemitz, R. Otjes, N. Perez, C. Perrino, A. S. H. Prévôt, J.-P. Putaud, D. Simpson, G. Spindler, M. Vana, and K. E. Yttri

The first EMEP intensive measurement periods were held in June 2006 and January 2007. The measurements aimed to characterize the aerosol chemical compositions, including the gas/aerosol partitioning of inorganic compounds. The measurement program during these periods included daily or hourly measurements of the secondary inorganic components, with additional measurements of elemental- and organic carbon (EC and OC) and mineral dust in PM₁, PM_2.5 and PM₁₀. These measurements have provided extended knowledge regarding the composition of particulate matter and the temporal and spatial variability of PM, as well as an extended database for the assessment of chemical transport models. This paper summarise the first experiences of making use of measurements from the first EMEP intensive measurement periods along with EMEP model results from the updated model version to characterise aerosol composition. We investigated how the PM chemical composition varies between the summer and the winter month and geographically.

The observation and model data are in general agreement regarding the main features of PM₁₀ and PM_2.5 composition and the relative contribution of different components, though the EMEP model tends to slightly underestimate PM₁₀ and PM_2.5 compared to measurements. The intensive measurement data has identified areas where improvements are needed. In particular, the model description of formation of coarse nitrate on sea salt and dust particles requires further attention. Hourly concurrent measurements of gaseous and particulate components for the first time facilitated testing of modelled diurnal variability of the gas/aerosol partitioning of nitrogen species. In general, the modelled diurnal cycles of nitrate and ammonium aerosols are in good agreement with the measurements. As expected, the diurnal variability of ammonia is not very well captured, but this will probably improve if the EMEP model is coupled to a dynamic, mechanistic ammonia emission module. The largest underestimations of aerosol mass are seen in Italy during winter, which to a large extent may be explained by an underestimation of residential wood burning source. It should be noted that both primary and secondary OC has been included in the calculations for the first time, showing promising results. Mineral dust is important, especially in southern Europe, and the model seems to capture the dust episodes well. The lack of measurements of mineral dust hampers the possibility for model evaluation for this highly uncertain PM component.

There are also lessons learnt regarding improved measurements for future intensive periods. There is a need for increased comparability between the measurements at different sites. For the nitrogen compounds it is clear that more measurements using artefact free methods based on continuous measurement methods and/or denuders are needed. For EC/OC, a reference methodology (both in field and laboratory) was lacking during these periods giving problems with comparability, though presently measurement protocols have been established and these should be followed by the Parties to the EMEP Protocol. For measurements with no defined protocols, it might be a good solution to use centralised laboratories to ensure comparability across the network. To cope with the introduction of these new measurements new reporting guidelines have been developed to ensure that all proper information about the methodologies and data quality is given.

Regional air-quality forecasting for the Pacific Northwest using MOPITT/TERRA assimilated carbon monoxide MOZART-4 forecasts as a near real-time boundary condition

2012-02-02T00:00:00+01:00

Regional air-quality forecasting for the Pacific Northwest using MOPITT/TERRA assimilated carbon monoxide MOZART-4 forecasts as a near real-time boundary condition

Atmospheric Chemistry and Physics Discussions, 12, 3695-3730, 2012

Author(s): F. L. Herron-Thorpe, G. H. Mount, L. K. Emmons, B. K. Lamb, S. H. Chung, and J. K. Vaughan

Results from a regional air quality forecast model, AIRPACT-3, were compared to AIRS carbon monoxide column densities for the spring of 2010 over the Pacific Northwest. AIRPACT-3 column densities showed high correlation (R>0.9) but were significantly biased (~25 %) with significant under-predictions for spring months with significant transport from Asia. The AIRPACT-3 CO bias relative to AIRS was eliminated by incorporating dynamic boundary conditions derived from NCAR's MOZART forecasts with assimilated MOPITT carbon monoxide. Changes in ozone-related boundary conditions derived from MOZART forecasts are also discussed and found to affect background levels by ±10 ppb but not found to significantly affect peak ozone surface concentrations.

Biogenic influence on cloud microphysics over the global ocean

2012-02-02T00:00:00+01:00

Biogenic influence on cloud microphysics over the global ocean

Atmospheric Chemistry and Physics Discussions, 12, 3655-3694, 2012

Author(s): A. Lana, R. Simó, S. M. Vallina, and J. Dachs

Aerosols have a large potential to influence climate through their effects on the microphysics and optical properties of clouds and, hence, on the Earth's radiation budget. Aerosol-cloud interactions have been intensively studied in polluted air, but the possibility that the marine biosphere plays a role in regulating cloud brightness in the pristine oceanic atmosphere remains largely unexplored. We used 9 yr of global satellite data and ocean climatologies to derive parameterizations of (a) production fluxes of sulfur aerosols formed by the oxidation of the biogenic gas dimethylsulfide emitted from the sea surface; (b) production fluxes of secondary organic aerosols from biogenic organic volatiles; (c) emission fluxes of biogenic primary organic aerosols ejected by wind action on sea surface; and (d) emission fluxes of sea salt also lifted by the wind upon bubble bursting. Series of global weekly estimates of these fluxes were correlated to series of cloud droplet effective radius data derived from satellite (MODIS). Similar analyses were conducted in more detail at 6 locations spread among polluted and clean regions of the oceanic atmosphere. The outcome of the statistical analysis was that negative correlation was common at mid and high latitude for sulfur and organic secondary aerosols, indicating both might be important in seeding cloud droplet activation. Conversely, primary aerosols (organic and sea salt) showed more variable, non-significant or positive correlations, indicating that, despite contributing to large shares of the marine aerosol mass, they are not major drivers of the variability of cloud microphysics. Uncertainties and synergisms are discussed, and recommendations of research needs are given.

Signals of El Niño Modoki in the tropical tropopause layer and stratosphere

2012-02-02T00:00:00+01:00

Signals of El Niño Modoki in the tropical tropopause layer and stratosphere

Atmospheric Chemistry and Physics Discussions, 12, 3619-3653, 2012

Author(s): F. Xie, J. Li, W. Tian, and J. Feng

The effects of El Niño Modoki events on the tropical tropopause layer (TTL) and on the stratosphere were investigated using European Center for Medium Range Weather Forecasting (ECMWF) reanalysis data, satellite observations from the Aura satellite Microwave Limb Sounder (MLS), oceanic El Niño indices, and general climate model outputs. El Niño Modoki events tend to depress convective activities in the western and eastern Pacific but enhance convective activities in the central and northern Pacific. Consequently, during Modoki events, negative water vapor anomalies occur in the western and eastern Pacific upper troposphere, whereas there are positive anomalies in the central and northern Pacific upper troposphere. The spatial patterns of the outgoing longwave radiation (OLR) and upper tropospheric water vapor anomalies exhibit a tripolar form. The empirical orthogonal function (EOF) analysis of the OLR and upper tropospheric water vapor anomalies reveals that canonical El Niño events are associated with the leading mode of the EOF, while El Niño Modoki events correspond to the second mode. El Niño Modoki activities tend to moisten the lower and middle stratosphere, but dry the upper stratosphere. It was also found that the canonical El Niño signal can overlay linearly on the QBO signal in the stratosphere, whereas the interaction between the El Niño Modoki and QBO signals is non-linear. Because of these non-linear interactions, El Niño Modoki events have a reverse effect on high latitudes stratosphere, as compared with the effects of typical Modoki events, i.e. the northern polar vortex is stronger and colder but the southern polar vortex is weaker and warmer during El Niño Modoki events. However, simulations suggest that canonical El Niño and El Niño Modoki activities actually have the same influence on high latitudes stratosphere, in the absence of interactions between QBO and ENSO signals. The present results also reveal that canonical El Niño events have a greater impact on the high-latitude Northern Hemisphere stratosphere than on the high-latitude Southern Hemisphere stratosphere. However, El Niño Modoki events can more profoundly influence the high-latitude Southern Hemisphere stratosphere than the high-latitude Northern Hemisphere stratosphere.

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