Long Range Transport of Dust Aerosols

Continuing from our last blog post on the issue of aerosol radiative forcing (ARF) and the importance of its vertical distribution, l would like to bring to attention another important aspect associated with aerosols that affect global ARF. This aspect is the transport of aerosols which is very closely related to its vertical distribution. One of the largest sources of aerosols is desert. These desert dust aerosols have gained much attention due to the long range transport associated with them. The dust from these deserts is lifted higher in the atmosphere and ‘because of the fast large-scale transport in the upper troposphere, aerosols such as dust and black carbon, once they enter the upper troposphere (above ~8 km), can be transported around the earth in a latitudinal belt in a week or two’. ‘The impact of long range transport of dust and air pollution from their continental sources over oceanic regions is one of the outstanding problems in understanding regional and global climate change’ and in also determining the air quality of regions to which dust is advected to. Using Calipso and surface micropulse lidar, Huang et al., [2008] observe the long-range transport and vertical distribution of Asian dust aerosols during the PACific Dust EXperiment (PACDEX). ‘The MPL measurements were made at the Loess Plateau (35.95°N, 104.1°E) near the major dust source regions of the Taklamakan and Gobi Deserts’ in China during March-May 2007. They find that ‘Dust events were more frequent in the Taklamakan, where floating dust dominates, while more intensive, less frequent dust storms were more common in the Gobi region. The vertical distribution of the CALIPSO backscattering/depolarization ratios indicate that non-spherically shaped dust aerosols floated from near the ground to an altitude of approximately 9 km around the source regions. This suggests the possible long-range transport of entrained dust aerosols via upper tropospheric westerly jets. A very distinct large depolarization layer was also identified between 8 and 10 km over eastern China and the western Pacific Ocean corresponding to dust aerosols transported from the Taklamakan and Gobi areas, as confirmed by back trajectory analyses. The combination of these dust sources results in a two-layer or multilayered dust structure over eastern China and the western Pacific Ocean.’ We have seen in our last blog spot that ‘the vertical distribution of dust aerosols is another critical factor impacting the effects of dust on radiative forcing and climate [Claquin et al., 1998; Zhu et al., 2007; Forster et al., 2007].

References:

Huang, J., P. Minnis, B. Chen, Z. Huang, Z. Liu, Q. Zhao, Y. Yi, and J. Ayers (2008), Long-range Transport and Vertical Structure of Asian Dust from CALIPSO and Surface Measurements during PACDEX, J. Geophys. Res., doi:10.1029/2008JD010620, in press.

Claquin, T., M. Schulz, Y. J. Balkanski, and O. Boucher (1998), Uncertainties in assessing radiative forcing by mineral dust, Tellus, Ser. B, 50, 491–505.

Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn,G. Raga, M. Schulz and R. Van Dorland (2007), Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Zhu, A., V. Ramanathan, F. Li, and D. Kim (2007), Dust plumes over the Pacific, Indian, and Atlantic oceans: Climatology and radiative impact, J. Geophys. Res., 112, D16208, doi:10.1029/2007JD008427

Impact of Aerosol Vertical Distribution on their Radiative Effects

From over a decade of aerosol research, it is now a well established fact that atmospheric aerosols play a crucial role in altering the earth's radiation budget [IPCC 2007]. Toa large extent, the effect on aerosols on the top-of-atmosphere radiation balance has been addressed using both observations and global models. A close review of literature suggests the imperative need to address the vertical distribution of aerosols in the atmosphere [Gadhavi et al., 2006] and that is exactly what aerosol science is gearing towards. Recently, there have been many research studies that have begun to address this question. One interesting paper by Johnson et al., appeared in JGR last week that talks about this very issue.

This paper provides very good insights into this topic by using ground and aircraft based lidar observations during DABEX field campaign. 'In general, mineral dust was observed at low altitudes (up to 2 km), and a mixture of biomass burning aerosol and dust was observed at altitudes of 2–5 km.'

For clear sky conditions, when the observed low-level dust layer was included in a radiative transfer model, the absorption of solar radiation by the biomass burning aerosols increased by 10%.' This enhancement in absorption is due to reflection of solar radiation by dust aerosols in background up into the biomass burning aerosol layer above. This situation is analogous to presence absorbing aerosols over a bright background. Thus, depending on the distribution of aerosols and the type of aerosols present at different heights in the atmosphere, their radiative effects can be altered. This in-turn changes the differential heating of the atmosphere and hence the atmospheric stability that influences convective and turbulent motions and clouds [Ackerman et al., 2000]. This can be important for both TOA and surface radiation budget. The scenario is a little more complex when aerosols are above clouds. 'For example, the elevation of biomass burning aerosols above marine stratocumulus clouds during the Southern African Regional Science Initiative (SAFARI-2000) greatly enhanced their absorption of shortwave radiation. This led to a positive direct aerosol shortwave radiative effect over the Southern Atlantic, whereas the effect was negative in clear sky conditions [Keil and Haywood, 2003; Abel et al., 2005; Myhre et al., 2003a]'. Thus, the need to consider the treatment and appropriate representation of vertical distribution of aerosol species in the global models when estimating the impact of anthropogenic absorbing aerosols is crucial.

Please refer to the paper by Johnson et al (below) for all references in this blog-post:

Johnson, B. T., B. Heese, S. A. McFarlane, P. Chazette, A. Jones, and N. Bellouin (2008), Vertical distribution and radiative effects of mineral dust and biomass burning aerosol over West Africa during DABEX, J. Geophys. Res., 113, D00C12, doi:10.1029/2008JD009848

Principal components analysis (PCA) for source identification

Source identification for atmospheric aerosol is important for developing effective strategy to reduce their emissions. One of the methods for source identification is principal components analysis (PCA). My attention was drawn to this method recently first through a review article about methods and results for aerosol source apportionment over European region and then through the article “Identification of PM sources by principal component analysis (PCA) coupled with wind direction ”. Dr. Viana Rodríguez, Mª del Mar is the lead author on both the articles . She is a researcher at the Institute of Earth Sciences Jaume Almera, Spain.

Application of the principal components analysis method for aerosol is based on the foundation that each source has unique blending of various aerosol components. Variability of the components is strongly correlated among themselves when they are coming from same source when compared to a case where they are coming from heterogeneous sources. Mathematically, PCA seeks to determine matrices A and S in the equation C=A∙S, where column matrix C represents concentration of various particulate matter (PM) components, S is the source contribution and A is the source profiles.

The article Viana et al. (2006) is more about results obtained using PCA analysis rather than the method itself. I liked the article for its clear conclusions and bold figures. I think figure 1 will be useful for those who are interested in knowing typical combination of various aerosol sources for given total mass. Figure 4 will be very useful for the people of the town Llodio, for knowing where to look for reducing pollution. I have recreated images for quick look using data from the manuscript.

References:

Viana, M., et al., (In Press), Source apportionment of particulate matter in europe: A review of methods and results, Journal of Aerosol Science, Accepted Manuscript.

Viana, M., X. Querol, A. Alastuey, J. I. Gil, and M. Menéndez (2006, December),Identification of pm sources by principal component analysis (pca) coupled with wind direction data, Chemosphere 65 (11), 2411-2418.

Aerosol Radiative Effects over Global Land: A Satellite based Study

GRL published an article on estimation of aerosol radiative effects over global land using purely observations from Terra satellite on February 22, 2008. Although, I am one of the co-author in the paper but I would like to congrats Falguni Patadia, first author of the paper for her research, which is first attempt in certain ways. This paper presents the estimation of top of the atmosphere (TOA) short wave aerosol radiative effects over global land areas for each half degree by half degree grid point. The uniqueness of the study comes from the fact that it is purely observation based study, which does not involve any complex radiative transfer and/or climate model runs. Results of the study are encouraging and matches very well with other purely model or hybrid (model and observations) type of studies. This research used one year worth of satellite observations of TOA fluxes derived from CERES broadband instrument, MODIS high resolution cloud masks and MISR derived aerosol optical thickness data sets to perform the analysis. As we all know that, aerosol impacts in climate change studies in one of the most uncertain component and level of scientific understanding about this component is very low. This study is certainly a good start and will help to understand aerosol effects on earth-atmosphere radiation budget. For more details on the results and methodology, please refer the publication and if you do not have access to the article, we will be happy to share reprints with you.


Complete Reference:

Patadia, F., P. Gupta, and S. A. Christopher (2008), First observational estimates of global clear sky shortwave aerosol direct radiative effect over land, Geophys. Res. Lett., 35, L04810, doi:10.1029/2007GL032314

Is atmospheric aerosol an aerosol? comments on the article by Jaenicke

The title of a recent article "Is atmospheric aerosol an aerosol?" by Jaenicke caught my attention. This took me back in time, when I was just beginning my career in this field. The first definition I came across or rather assumed was: "anything solid or liquid suspended in the air is aerosol". According to this definition birds and aeroplanes were also aerosols! Thinking of birds and aeroplanes as aerosols wasn't intuitive hence I had to search for a more refined definition. A better one that I came across (well I don't remember from where) was: "any thing suspended in the air and doesn't have self-propelling mechanism is aerosol". This definition implied that mosquitoes are not aerosols but bacteria and virus are. However, this new definition didn't help me win an argument with my friend Neeraj (one of the authors of this blog) who held an opinion that water and ice clouds are aerosols as well, whereas I held the opinion that they are not. We concluded arguments by accepting that clouds are special cases of aerosols and if not explicitly mentioned, atmospheric aerosols mean liquid and solid particles suspended in the air and have aerodynamic diameter between 1e-3 and 1 µm.

Technically aerosols are defined as colloid of air and solid/liquid particles, where air is the dispersion medium and particles are in dispersed phase. The word aerosols brings-in naturally the interaction between particles and air. When particles are not in the air, for example particles collected on filter papers, they are no more aerosols. Hence aerosol is a state of particles rather than the particle itself. This is the reason why most pollution and chemistry related studies report them as particulate matter (PM) because these studies require collecting them on filter papers, while most climate related studies report particles as aerosols since particles are climate modulator as long they are in the air.

Coming back to Jaenicke's article, he starts with questioning the very definition of atmospheric aerosol as colloid. Schmauss and Wigand were probably the first to define atmosphere as colloid of air and particles. The word colloid itself was coined in the year 1875 by Gerber to describe a pseudosolution prepared by Selmi (reference in Jaenicke, 2008) . Jaenicke sees a reason to question this definition because colloid implies a stable state, homogeneity and monodispersed size distribution. Hardly any of these is true for atmospheric aerosols. On the other hand there are properties such as surface-to-volume ratio, interactions between nearby particles, multiple scattering, etc that support their definition as colloid. Jaenicke concludes that atmospheric aerosols can be considered colloid in dynamic equilibrium.

The crux of the paper is not the discussion on the definition of aerosol but modeling aerosol concentration in the atmosphere under the framework of dynamic equilibrium. Dynamic equilibrium by its nature results in highly variable aerosol concentration. Which in turn requires better temporal resolution for measuring them. In this discussion, Jaenicke highlights quite a few gaps in our knowledge about atmospheric aerosols.


References

Jaenicke, R. (2008). Is atmospheric aerosol an aerosol?-a look at sources and variability. Faraday Discuss 137, 235-243.


Art work: courtesy Malkaush

Satellite remote sensing of active fires: Impact of clouds

It has been long time since passive satellite observations in visible and thermal part of the spectrum have been used to monitor and quantify the biomass burning over global areas. Space sensors such as AVHRR, MODIS, and GOES are some of the examples, which continuously monitor vegetation fires under clear sky conditions. But, these sensors are limited to cloud free conditions and there could be large errors in estimation of fire activities due to lack of sampling under cloudy conditions. Remote Sensing of Environment published a research article entitled “Quantifying the impact of cloud obscuration on remote sensing of active fires in the Brazilian Amazon” by Wilfrid Schroeder and coauthors discuss the bias in remote sensing fire data due to possible cloud cover during fire activities over Brazilian Amazon.

The abstract read as “Vegetation fires remain as one of the most important processes governing land use and land cover change in tropical areas. The large area extent of fire prone areas associated with human activities makes satellite remote sensing of active fires a valuable tool to help monitor biomass burning in those regions. However, identification of active fire fronts under optically thick clouds is not possible through passive remote sensing, often resulting in omission errors. Previous analyses of fire activity either ignored the cloud obscuration problem or applied corrections based on the assumption that fire occurrence is not impacted by the presence of clouds. In this study we addressed the cloud obscuration problem in the Brazilian Amazon region using a pixel based probabilistic approach, using information on previous fire occurrence, precipitation and land use. We implemented the methodology using data from the geostationary GOES imager, covering the entire diurnal cycle of fire activity and cloud occurrence. Our assessment of the method indicated that the cloud adjustment reproduced the number of potential fires missed within 1.5% and 5% of the true fire counts on annual and monthly bases respectively. Spatially explicit comparison with high resolution burn scar maps in Acre state showed a reduction of omission error (from 58.3% to 43.7%) and only slight increase of commission error (from 6.4% to 8.8%) compared to uncorrected fire counts. A basin-wide analysis of corrected GOES fire counts during 2005 showed a mean cloud adjustment factor of approximately 11%, ranging from negligible adjustment in the central and western part of the Brazilian Amazon to as high as 50% in parts of Roraima, Para and Mato Grosso.”


For more details refer the original publication:

Wilfrid Schroeder, Ivan Csiszar and Jeffrey Morisette, Quantifying the impact of cloud obscuration on remote sensing of active fires in the Brazilian Amazon, Remote Sensing of Environment, Volume 112, Issue 2, 15 February 2008, Pages 456-470.

Air pollution in mega cities in China: A Review

Air pollution (both gas and particle) in Chinese mega cities is one of the biggest growing problems due to rapid increase in industrial activities in the area. A review article on air pollution in China entitled “Air pollution in mega cities in China” by Chan and Yao is published in January 2008 issue of Atmospheric Environment. This article provides through review of available literature on air pollution research in China and focused areas are Beijing, Shanghai, and the Pearl River Delta region.

Here, I am providing some highlights of the article. Please refer original article for more details.

"Air pollution has become one of the top environmental concerns in China. Currently, Beijing, Shanghai, and the Pearl River Delta region including Guangzhou, Shenzhen and Hong Kong, and their immediate vicinities are the most economically vibrant regions in China.”

“Air quality in most Chinese cities has improved despite the rapid growth of the economy…, however… He et al. (2001) and Ye et al. (2003) reported that PM2.5 concentrations in Beijing and Shanghai, the two largest cities in China, were about 10 times and six times the WHO guideline values, respectively.”

“From 1980 to 2005, the urban population in China increased from 19.6 to 40.5%. The number of cities increased to over 660, and more than 170 cities had over 1 million permanent residents (not including the migrant population) in 2004.”

“Much attention has been paid to reducing emissions, particularly vehicle emissions. Although the number of vehicles has increased by about 10% per year in these cities, NO2 and CO concentrations have not increased due to effective control measures… Particulate pollution is still severe, and it is the major air pollution problem in the mega cities.”

Complete Reference:

Chan, C. K. and X. Yao (2008, January), Air pollution in mega cities in china, Atmospheric Environment 42 (1), 1-42, doi:10.1016/j.atmosenv.2007.09.003

One Year of Aerosol Blog (about us)

Hello all Readers,

Today, aerosol blog completed successful one year. So we thought to write a blog about our blog. The blog was started last year in first week of January. Initially there were only two authors (Harish and Pawan) but soon Falguni, Neeraj and Dilip joined team, making it possible to post 42 posts in one year, almost one post per week. Here we take a look on what make us to start this blog, what content is dear to us and what is the future of this blog.

"Hey listen to this!"

It is common tendency of human being when we see something interesting we want to share it with others. Who is not aware of such a friendly shout "hey listen to this" with family members at home, with friends in hostel, when someone found interesting paragraph in news-paper or book. We five members of current team once upon a time where in Physical Research Laboratory and use to enjoy such a sharing. Later on we moved to different places but still use to shout through e-mail "Hey see this is interesting". Every year more than 7500 peer reviewed articles are published in the field of atmospheric science, quite a big fraction of it are published in the field quite close to our research work. We realized that it would be very useful if there would have been a guide to help short-list relevant articles. With these two intention we started blog, later on we realized that blog is quite promising medium for publication and can work as e-magazine.

Objectives

Our current objective is to provide pointer to information relevant to the research in the field of aerosols, clouds and climate change, be it journal article, book, web-site, biography, patent, database, conference or job. It is also part of objective to provide perspective why that piece of information is interesting. However as far as possible we want to focus on technical aspects of the research in this field rather than popular aspects. In future we want to transform our blog in e-magazine with its own articles, commentaries, digest and announcements.

Is this blog free?

This blog is free for viewing, but free is very deceptive word. We believe truly free viewing should be free of advertisements. What is the point in shortlisting articles if again one has to search them through jungle of ad links. We are able to provide it free because we are using resources available free of charge. As stated in earlier paragraph that we intend to make our blog an e-magazine. This may require some kind of investment such as server, software, domain name etc. But it will be our highest priority to run this blog free of charge and free of advertisement.

Who are the authors?

We are currently five authors who are responsible to update blog on regular basis. We have established credentials in this field and have articles published in peer reviewed journals. More information about authors can be found at their home-page listed below

Harish Gadhavi- http://www.atmos.umd.edu/~harish/
Post doctoral research fellow at University of Maryland, College Park

Pawan Gupta - http://www.nsstc.uah.edu/~gupta/
Graduate student at University of Alabama, Huntsville

Neeraj Rastogi - http://www.prl.res.in/~nrastogi
Post doctoral research fellow at University of Georgia, Atlanta

Falguni Gupta - http://www.nsstc.uah.edu/~falguni/
Graduate student at Universtiy of Alabama, Huntsville

Dilip Ganguly - http://www.prl.res.in/~dganguly/
Post doctoral research fellow at Princeton University, Princeton, NJ

We welcome contributions from anyone who wants to contribute or share a piece of information consistent with the theme of blog. Announcement regarding conferences, job are most welcome. Contact information can be found in the end of the blog

Copyright

We have unspecified status of copyright for our blog. Authors own the copyright individually as allows the law. If not specifically mentioned about restriction, feel free to use our blog for non-commercial purpose. We sometime use figures and texts from others as fair use policy allows, but copyright to those figures and texts belongs to originating source. We try our best to cite full reference for such a source. If we have adversely used copyrighted material, please bring to our notice and we will immediately remove it. Contact information can be found in the end of the blog

Please see the home-pages of authors to contact authors individually.

Thanks to Harish for preparing these notes!