WMO Aerosol Bulletin No.1 2013 | 1

AerosolWMO

BulletinIntegrated observations of atmospheric aerosols

Prepared by GAW Aerosol SAGNo. 1 – June 2013

Important sources of global aerosols are depicted here (from left to right): industry, wild fires, desert dust, sea salt.

The WMO-GAW Integrated Aerosol Observing System

Airborne particles (“aerosols”) affect many aspects of human health and the environment. Aerosols are linked to chronic respiratory and acute cardio-vascular problems, as well as visibility reduction, acid rain, and urban smog. Furthermore, aerosols influence Earth‘s climate both directly, by scattering and absorbing sunlight, and indirectly, by altering the reflectivity of clouds; changes in cloud lifetimes andprecipitation are additional indirect effects.

Aerosol-related changes in Earth‘s radiative energy balance, called “radiative forcing”, are smaller in magnitude than the radiative forcing from long-lived greenhouse gases (LLGHG) on a global scale, but can be significantly larger in regions with high aerosol concentrations. Aerosol forcing differs from the LLGHG forcing in several crucial respects: it is much more variable in space and time, it depends on the size and composition of the aerosol particles, it generally has the opposite sign, and it is much more uncertain. A major goal of the Global Atmosphere Watch (GAW) Programme at WMO is to ensure long-term measurements of atmospheric constituents in order to detect and explain trends. With respect to aerosols, the objective of GAW is to determine the spatio-temporal distribution of aerosol properties

related to climate forcing and air quality up to multi-decadal time scales. Aerosol measurements by GAW stations are an integrated observing system. The Scientific Advisory Group (SAG) for aerosols recommends that GAW stations measure these aerosol variables continuously:

Multi-wavelength aerosol optical depth

Mass concentration in two size fractions (fine, coarse)

Mass concentration of major chemical components in two size fractions

Light scattering, hemispheric backscattering, and absorption coefficients at various wavelengths

Aerosol number concentration and size distribution

Cloud condensation nuclei number concentration at various super-saturations

Vertical distribution of aerosol backscattering

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GAW in situ measurements

Monitoring of aerosol properties is an essential part of the GAW in-situ network. Given the short life-time of aerosol particles and their complex nature, it is also one of the less trivial components of the atmospheric system to monitor. The relevant variables required for source identification (tracers/chemical composition) are not identical to those needed for deriving radiative forcing on climate or for air quality studies. The SAG recommends a list of the essential aerosol variables for which knowledge of the long-term patterns and trends is necessary to validate past and present emission inventories, constrain and test climate-chemistry models and improve our knowledge of global to regional cycling of atmospheric constituents. The number of stations contributing to GAW has been increasing in recent years along with a general improvement of data quality and standardization. The GAW in-situ station network providing aerosol observations at the World Data Centre for Aerosols (WDCA) http://www.gaw-wdca.org is quite large with more than 40 regional stations and 20 contributing stations in addition to 28 global stations. Two new global stations with substantial aerosol component were included in 2011 : Nepal Climate Observatory-Pyramid (Nepal) and Monte Cimone (Italy).

The new regional GAW observatory Chacaltaya in Bolivia at an altitude of 5200m started providing information in December 2011. The initiative led by the Universidad Mayor de San Andres (UMSA-LFA) received support from research groups in Europe (France, Germany, Italy, Switzerland and Sweden) and the USA to develop a fully integrated gas/aerosol station. The station is the highest point of the WMO/GAW network. Additional information on the station measurements and contact details are available at http://www.chacaltaya.edu.bo/index.php.

Seasonal trends of SeaWiFS AOD anomaly from January 1998 to December 2010. Dots indicate significance at 95% confidence level. Units are AOD/year. Gray patterns indicate regions with insufficient sampling for trend analysis due to coverage of clouds and snow/ice. (from Hsu, et al. ACP, 2012)

The sparse coverage over some regions of the globe remains, however, problematic with large areas not covered by any observations, not only over the oceans, but also over the continents. Lack of long-term records is also still problematic and is a major limitation for deriving statistically relevant trends.

While studies on CO2 cycles can benefit from observation records longer than 50 years, continuous measurement of aerosol parameters spanning over more than 15 years remain an exception. Considering the short life-time of aerosol particles and the high natural variability often observed, this remains a major limitation for deriving statistically significant trends and, for example, for assessing effectiveness of emission abatement strategies.

GAW - one stop-shop on satellite data

A new activity in GAW aerosol is to support the link with satellite data as a complementary source of information. To facilitate easy access to satellite data a one stop shop at http://wdc.dlr.de/data_products/AEROSOLS/ was implemented at the GAW World Data Centre for Remote Sensing of the Atmosphere (WDC-RSAT). It provides first a list of satellite aerosol products that have at least continental and one-year coverage, provides online access to data and documentation including validation. Secondly, for each product a standardized overview page is provided, which contains the main product and algorithm characteristics and the direct links to products and documentation.

This activity is of bi-directional benefit between the satellite and GAW communities. Satellite product validation can benefit from in situ data to assess information on aerosol properties. The GAW community should see further use of their high quality in situ measurements. Jointly, satellite observations, ground measurements and models are complementary, and will support climate research.

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GALION distribution of stations as available through the cooperation between existing networks.

GALION lidar network

A strong effort to determine the spatio-temporal distribution of aerosol properties related to climate forcing and air quality up to multidecadal time scales is in progress within the GAW aerosol programme where the establishment of a global lidar observation network has been considered of strategic importance. Lidar (LIght Detection And Ranging) techniques represent an optimal tool for range resolved data of atmospheric aerosol. Lidar observations of the spatial distribution of aerosols allow an understanding of their variation in time, their optical and physical properties, and their influence on cloud formation. The lidar information contributes to determining sources, transport and sinks of pollution, to observation and interpretation of long-term trends, and to predicting climate change.

It is the mission of the GAW Aerosol LIdar Observation Network (GALION) to organize the observational capability for the 4-dimensional distribution of key aerosol parameters at global scale. The specific objective of GALION is to provide the vertical component of this distribution through advanced laser remote sensing in a network of ground-based stations globally distributed. The observed aerosol properties include the identification of aerosol layers, profiles of directly measured optical properties (backscatter and extinction coefficients at selected wavelengths, lidar ratio, Ångström coefficients, particle depolarization ratios) and indirectly inferred properties (e.g., profiles of light-absorption and single-scattering albedo), aerosol type (e.g., dust, maritime, fire smoke, urban haze), and microphysical properties (e.g., volume and surface concentrations, size distribution parameters, refractive index).

Observations are made with sufficient coverage, resolution, and accuracy to establish a comprehensive aerosol climatology, to evaluate model performance, to assist and complement space-borne observations, and to provide input to forecast models of “chemical weather”.

GALION is based on the cooperation between existing lidar networks:

the American Lidar Network (ALINE), Latin America;

Asian Dust and Aerosol Lidar Observation Network (AD-Net), East Asia;

Commonwealth of Independent States LIdar NETwork (CIS-LINET), Belarus, Russia and Kyrgyz Republic;

the Canadian Operational Research Aerosol Lidar Network (CORALNet), Canada;

the European Aerosol Research LIdar NETwork (EARLINET), Europe;

Network for the Detection of Atmospheric Composition Change (NDACC), global stratosphere;

REALM/CREST, Eastern North America;

the MicroPulse Lidar NETwork (MPLNET), global.

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GAW Aerosol Optical Depth (AOD)

In March 2004, during a WMO workshop on Aerosol Optical Depth (AOD) held at Davos, Switzerland, 90 stations having a data record of more than 4 years were identified. About half of them were associated with the AERONET federation of networks; the remaining sites were affiliated with members of WMO. Their hemispheric distribution roughly corresponded to the continental landmasses. As of 2011, the number of long-term sites has increased to 250, predominantly located in the northern hemisphere, with AERONET contributing 2/3 of them. In May 2007, WMO EC-59 has approved a recommendation by CIMO-XIV, that the World Optical Depth Research and Calibration Centre

(WORCC) should be recognized as primary reference for AOD measurements. Several International Filter Radiometer Comparisons held at WORCC since 2000 have repeatedly demonstrated that AOD measurements taken by representative instruments from different networks are well within the GAW specification of 0.015 optical depths. By end of 2009, the World Data Centre for Aerosols (WDCA), hosted by the Joint Research Centre of the European Commission (JRC) in Italy, has archived quality assured AOD hourly mean data from 16 GAW stations from 1987 to 2007. Since 2010 WDCA was relocated to the Norwegian Institute of Air Research (NILU) in Norway where AOD observations are now also available in quasi real time (24h) from the WDCA for registered users

Locations of the GAW-PFR baseline network of AOD. Twelve additional stations in Scandinavia, Central Europe and Polar regions are not shown here

Questions and Answers

About the Aerosol Bulletin This Bulletin is the first in a series and provides general information on the aerosol component of GAW.

Future issues will focus on specific components or applications of GAW aerosol measurements.

Data availability Ground-based in-situ , lidar and AOD data from http://www.gaw-wdca.org/, http://ebas.nilu.no/and satellite data from http://wdc.dlr.de/data_products/AEROSOLS/

About measurement procedures

Follow the measurement guidelines and standard operating procedures, available at ftp://ftp.wmo.int/Documents/PublicWeb/arep/gaw/gaw153.pdf and http://www.wmo.int/pages/prog/arep/gaw/documents/FINAL_GAW_200_web.pdf.

About GAW Information about GAW is available at http://www.wmo.int/pages/prog/arep/gaw/gaw_home_en.html, the GAW Station Information System (GAWSIS) is at http://gaw.empa.ch/gawsis/ and SAG-Aerosol at www.wmo-gaw-sag-aerosol.org

Contact Aerosol SAG, Email: sag-aero@tropos.de