Every year, from December to April, anthropogenic haze spreads over most of the North Indian Ocean, and South and Southeast Asia. The Indian Ocean Experiment (INDOEX) documented this Indo‐Asian haze at scales ranging from individual particles to its contribution to the regional climate forcing. This study integrates the multiplatform observations (satellites, aircraft, ships, surface stations, and balloons) with one‐ and four‐dimensional models to derive the regional aerosol forcing resulting from the direct, the semidirect and the two indirect effects. The haze particles consisted of several inorganic and carbonaceous species, including absorbing black carbon clusters, fly ash, and mineral dust. The most striking result was the large loading of aerosols over most of the South Asian region and the North Indian Ocean. The January to March 1999 visible optical depths were about 0.5 over most of the continent and reached values as large as 0.2 over the equatorial Indian ocean due to long‐range transport. The aerosol layer extended as high as 3 km. Black carbon contributed about 14% to the fine particle mass and 11% to the visible optical depth. The single‐scattering albedo estimated by several independent methods was consistently around 0.9 both inland and over the open ocean. Anthropogenic sources contributed as much as 80% (±10%) to the aerosol loading and the optical depth. The in situ data, which clearly support the existence of the first indirect effect (increased aerosol concentration producing more cloud drops with smaller effective radii), are used to develop a composite indirect effect scheme. The Indo‐Asian aerosols impact the radiative forcing through a complex set of heating (positive forcing) and cooling (negative forcing) processes. Clouds and black carbon emerge as the major players. The dominant factor, however, is the large negative forcing (‐20±4 W m⁻²) at the surface and the comparably large atmospheric heating. Regionally, the absorbing haze decreased the surface solar radiation by an amount comparable to 50% of the total ocean heat flux and nearly doubled the lower tropospheric solar heating. We demonstrate with a general circulation model how this additional heating significantly perturbs the tropical rainfall patterns and the hydrological cycle with implications to global climate.

Although black carbon (BC) is one of the key atmospheric particulate components driving climate change and air quality, there is no agreement on the terminology that considers all aspects of specific properties, definitions, measurement methods, and related uncertainties. As a result, there is much ambiguity in the scientific literature of measurements and numerical models that refer to BC with different names and based on different properties of the particles, with no clear definition of the terms. The authors present here a recommended terminology to clarify the terms used for BC in atmospheric research, with the goal of establishing unambiguous links between terms, targeted material properties and associated measurement techniques.

Methods for reducing and quantifying the uncertainties in aerosol optical properties measured with the TSI 3563 integrating nephelometer are presented. For nearly all applications, the recommended calibration gases are air and CO₂. By routinely characterizing the instrumental response to these gases, a diagnostic record of instrument performance can be created. This record can be used to improve measurement accuracy and quantify uncertainties due to instrumental noise and calibration drift. When measuring scattering by particles, size segregation upstream of the nephelometer at about 1 μm aerodynamic diameter greatly increases the information content of the data for two reasons: one stemming from the independence of coarse and fine particles in the atmosphere, and the second stemming from the size dependence of the nephelometer response. For many applications (e.g., extinction budget studies) it is important to correct nephelometer data for the effects of angular nonidealities. Correction factors appropriate to a broad range of sampling conditions are given herein and are shown to be constrained by the wavelength dependence of light scattering, as measured by the nephelometer. Finally, the nephelometer measurement is nondestructive, such that the sampled aerosol can be further analyzed downstream. Data from two nephelometers operated in series are used to evaluate this procedure. A small loss of super-μm particles (5–10%) is found, while the sub-μm data demonstrates measurement reproducibility within ± 1%.

Mobility particle size spectrometers often referred to as DMPS (Differential Mobility Particle Sizers) or SMPS (Scanning Mobility Particle Sizers) have found a wide range of applications in atmospheric aerosol research. However, comparability of measurements conducted world-wide is hampered by lack of generally accepted technical standards and guidelines with respect to the instrumental set-up, measurement mode, data evaluation as well as quality control. Technical standards were developed for a minimum requirement of mobility size spectrometry to perform long-term atmospheric aerosol measurements. Technical recommendations include continuous monitoring of flow rates, temperature, pressure, and relative humidity for the sheath and sample air in the differential mobility analyzer. We compared commercial and custom-made inversion routines to calculate the particle number size distributions from the measured electrical mobility distribution. All inversion routines are comparable within few per cent uncertainty for a given set of raw data. Furthermore, this work summarizes the results from several instrument intercomparison workshops conducted within the European infrastructure project EUSAAR (European Supersites for Atmospheric Aerosol Research) and ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure Network) to determine present uncertainties especially of custom-built mobility particle size spectrometers. Under controlled laboratory conditions, the particle number size distributions from 20 to 200 nm determined by mobility particle size spectrometers of different design are within an uncertainty range of around ±10% after correcting internal particle losses, while below and above this size range the discrepancies increased. For particles larger than 200 nm, the uncertainty range increased to 30%, which could not be explained. The network reference mobility spectrometers with identical design agreed within ±4% in the peak particle number concentration when all settings were done carefully. The consistency of these reference instruments to the total particle number concentration was demonstrated to be less than 5%. Additionally, a new data structure for particle number size distributions was introduced to store and disseminate the data at EMEP (European Monitoring and Evaluation Program). This structure contains three levels: raw data, processed data, and final particle size distributions. Importantly, we recommend reporting raw measurements including all relevant instrument parameters as well as a complete documentation on all data transformation and correction steps. These technical and data structure standards aim to enhance the quality of long-term size distribution measurements, their comparability between different networks and sites, and their transparency and traceability back to raw data.

Two extreme cases of aerosol optics from the Reno Aerosol Optics Experiment are used to develop a model-based calibration scheme for the 7-wavelength aethalometer. The cases include those of very white and very dark aerosol samples. The former allows for an assessment of the scattering offset associated with this filter-based method, with the wavelength-dependent scattering measured from a 3-wavelength nephelometer, and interpolated and extrapolated to the 7 wavelengths of the aethalometer. A photoacoustic instrument operating at 532 nm is used to evaluate the filter loading effect caused by aerosol light absorption. Multiple scattering theory is used to analytically obtain a filter-loading correction function. This theory shows that the exponential behavior of light absorption in the strong multiple scattering limit scales as the square root of the total absorption optical depth rather than linearly with optical depth as is commonly assumed with Beer's law. The multiple scattering model also provides a theoretical justification for subtracting a small fraction of aerosol light scattering away from measured apparent light absorption by the filter method. The model is tested against ambient measurements and is found to require coefficients that are situation specific. Several hypotheses are given for this specificity, and suggested methods for reducing it are discussed. Specific findings are as follows. Simultaneous aerosol light-scattering measurements are required for accurate interpretation of aethalometer data for high aerosol single-scattering albedo. Instantaneous errors of up to ±50% are possible for uncorrected data, depending on filter loading. The aethalometer overpredicts black carbon (BC) concentration on a fresh filter and underpredicts BC on a loaded filter. BC and photoacoustic light absorption can be tightly correlated if the data are averaged over the full range of filter loadings and the aerosol source is constant. Theory predicts that the Aethalometer response may be sensitive to filter face velocity, and hence flow rate, to the extent that particle penetration depth depends on face velocity.

Absorption photometers for real time application have been available since the 1980s, but the use of filter-based instruments to derive information on aerosol properties (absorption coefficient and black carbon, BC) is still a matter of debate. Several workshops have been conducted to investigate the performance of individual instruments over the intervening years. Two workshops with large sets of aerosol absorption photometers were conducted in 2005 and 2007. The data from these instruments were corrected using existing methods before further analysis. The inter-comparison shows a large variation between the responses to absorbing aerosol particles for different types of instruments. The unit to unit variability between instruments can be up to 30% for Particle Soot Absorption Photometers (PSAPs) and Aethalometers. Multi Angle Absorption Photometers (MAAPs) showed a variability of less than 5%. Reasons for the high variability were identified to be variations in sample flow and spot size. It was observed that different flow rates influence system performance with respect to response to absorption and instrumental noise. Measurements with non absorbing particles showed that the current corrections of a cross sensitivity to particle scattering are not sufficient. Remaining cross sensitivities were found to be a function of the total particle load on the filter. The large variation between the response to absorbing aerosol particles for different types of instruments indicates that current correction functions for absorption photometers are not adequate.