Shortwave Aerosol Radiative Forcing Research Articles

Meteorological, atmospheric and climatic perturbations during major dust storms over Indo-Gangetic Basin

Abstract During the pre-monsoon season (April–June), the Indo-Gangetic Basin (IGB) suffers from frequent and intense dust storms originated from the arid and desert regions of southwest Asia (Iran, Afghanistan), Arabia and Thar desert blanketing IGB and Himalayan foothills. The present study examines the columnar and vertical aerosol characteristics and estimates the shortwave (0.25–4.0 μm) aerosol radiative forcing (ARF) and atmospheric heating rates over Kanpur, central IGB, during three intense dust-storm events in the pre-monsoon season of 2010. MODIS images, meteorological and AERONET observations clearly show that all the dust storms either originated from the Thar desert or transported over, under favorable meteorological conditions (low pressure and strong surface winds) affecting nearly the whole IGB and modifying the aerosol loading and characteristics (Angstrom exponent, single scattering albedo, size distribution and refractive index). CALIPSO observations reveal the presence of high-altitude (up to 3–5 km) dust plumes that strongly modify the vertical aerosol profile and are transported over Himalayan foothills with serious climate implications (atmospheric warming, enhanced melting of glaciers). Shortwave ARF calculations over Kanpur using SBDART model show large negative forcing values at the surface (−93.27, −101.60 and −66.71 W m −2 ) during the intense dusty days, associated with planetary (top of atmosphere) cooling (−18.16, −40.95, −29.58 W m −2 ) and significant atmospheric heating (75.11, 60.65, 37.13 W m −2 ), which is translated to average heating rates of 1.57, 1.41 and 0.78 K day −1 , respectively in the lower atmosphere (below ∼3.5 km). The ARF estimates are in satisfactory agreement with the AERONET ARF retrievals over Kanpur.

Changes in aerosol optical properties due to dust storms in the Middle East and Southwest Asia

Super dust storms occurred over the Middle East and southwest Asia on March 2012. These storms reduced the air quality over the Gulf Region, Iraq, Iran, and Pakistan. Airports were shut down due to poor visibility, schools were closed, and hundreds of people were hospitalized with respiratory problems. In order to better understand the effects of such dust storms we have analyzed aerosol optical and radiative properties during this event using data from the Moderate Resolution Imaging Spectroradiometer and the Aerosol Robotic Network. Maximum aerosol optical depth (AOD) values occurred on the18th of March in Kuwait, Bahrain, Qatar, and Saudi Arabia, where values of 4.9, 4.4, 4.3, and 4.9 were recorded, respectively. In Oman, the Arabian Sea, and Iran, maximum AOD values occurred on the 19th of March, reaching 4.5, 5, and 5, respectively. The dust storm then spread across Pakistan, passing through Multan, Faisalabad, and Lahore where maximum AOD values of 2.1, 2.6, and 2.7, respectively, were attained on the 20th of March. The maximum aerosol volume size distributions (VSDs) in Lahore occurred on dusty days and minimum VSDs on non-dusty days. The VSD, single scattering albedo, refractive index, and asymmetry parameter values on dusty days suggested that dust aerosols were predominant over anthropogenic aerosols in these urban environments. The shortwave aerosol radiative forcing (ARF) values (on both dusty and non-dusty days) ranged between −50Wm−2 and −194Wm−2 (average: −114±40Wm−2) at the earth's surface, and between −31Wm−2 and −105Wm−2 (average: −58±25Wm−2) at the top of the atmosphere (TOA). The longwave aerosol ARF values ranged between +6Wm−2 and +20Wm−2 (average: +12±4Wm−2) at the earth's surface, and between +7Wm−2 and +30Wm−2 (average: +16±7Wm−2) at the TOA. Longwave radiations therefore produced significant warming, both at the TOA and at the earth's surface.

Impact of aerosol on surface reaching solar irradiance over Mohal in the northwestern Himalaya, India

The present study, for the first time during 2007, is focused to examine the impact of aerosols on surface reaching solar irradiance over Mohal (31.9°N, 77.12°E, 1154m amsl) in the northwestern part of the Indian Himalaya. The study also aims to estimate shortwave aerosol radiative forcing (SWARF) and its effect on regional climate. The multi-wavelength solar radiometer (MWR) is used to measure aerosol optical depth (AOD) over a wider spectrum, i.e. ultraviolet, visible and near-infrared. The AOD is obtained by analyzing the data from MWR following the Langley technique. The radiative transfer model is used along with Optical Properties of Aerosols and Clouds model to estimate the SWARF. Aerosol shows a great efficiency to reduce substantial fraction of energy from the surface reaching direct solar beam, i.e. 154Wm−2μm−1 per unit AOD at 0.5μm. The SWARF at the surface, top of the atmosphere and the atmosphere is estimated to be −18.5±1.7, +0.6±3.7 and +19.1±3.1Wm−2, respectively. The large SWARF at the surface stood during the summer (April–July), while small during the monsoon (August–September). Moderate SWARF is obtained in the autumn (October–November) and winter (December–March). The study estimates a notable extinction in incoming solar radiation relatively with lower atmospheric heating from 0.41 to 0.73Kday−1. The potential effect of aerosol is found relatively higher on high aerosol loading days. On these days, the lower atmospheric heating increases by a factor 1.8 (during dust events) and 1.7 (during biomass burning). This study concludes that aerosols produce significant reduction in incoming solar radiation with substantial increase in lower atmospheric heating, leading to a remarkable effect on the atmospheric stability. In addition, as a subject of future interest, the present study has also important implications on the atmospheric circulation and regional climate.

Aerosol climatology in the northwestern Indian Himalaya: a study based on the radiative properties of aerosol

The present study aims to estimate the shortwave aerosol radiative forcing (SWARF) and their effect on regional climate over Mohal (31.9°N, 77.12°E) in the northwestern Indian Himalaya. The radiative properties of aerosols are measured using multiwavelength radiometer. These measurements are carried out during 2006–2010. Recent studies have shown that the continuously increasing level of aerosols along the elevated slopes of the Indian Himalaya results in many climatic implications. The major climatic effect of aerosol has changed the nature of SWARF, which depends on the single-scattering albedo (SSA). This study reports that the values of SSA are low during winter, which indicates the significant contribution of absorbing aerosols. The large values of SSA was observed during the premonsoon, which signifies notable contribution of scattering aerosols. The largest SWARF at the surface was estimated during the premonsoon, while the smallest was during the monsoon. This study reports that the burden of absorbing aerosols over the Indian region is higher, which results in notable solar heating over India.

Assessment of smoke shortwave radiative forcing using empirical angular distribution models

The Clouds and the Earth's Radiant Energy System (CERES) data has been used by several studies to calculate the top of atmosphere (TOA) shortwave aerosol radiative forcing (SWARF) of biomass burning aerosols over land. However, the current CERES angular distribution models that are used to convert measured TOA radiances to fluxes are not characterized by aerosols. Using our newly developed empirical angular models for smoke aerosols we calculate the SWARF over South America for eight years (2000–2008) during the biomass burning season. Our results indicate that when compared to our new angular distribution model-derived values, the instantaneous SWARF is underestimated by the CERES data by nearly 3.3Wm−2. Our studies indicate that it is feasible to develop angular models using empirical methods that can then be used to reduce uncertainties in aerosol radiative forcing calculations. More importantly, empirically-based methods for calculating radiative forcing can serve as a benchmark for modeling studies.

Temporal variation of aerosol optical depth and associated shortwave radiative forcing over a coastal site along the west coast of India

Optical characterization of aerosol was performed by assessing the columnar aerosol optical depth (AOD) and angstrom wavelength exponent (α) using data from the Microtops II Sunphotometer. The data were collected on cloud free days over Goa, a coastal site along the west coast of India, from January to December 2008. Along with the composite aerosol, the black carbon (BC) mass concentration from the Aethalometer was also analyzed. The AOD0.500μm and angstrom wavelength exponent (α) were in the range of 0.26 to 0.7 and 0.52 to 1.33, respectively, indicative of a significant seasonal shift in aerosol characteristics during the study period. The monthly mean AOD0.500μm exhibited a bi-modal distribution, with a primary peak in April (0.7) and a secondary peak in October (0.54), whereas the minimum of 0.26 was observed in May. The monthly mean BC mass concentration varied between 0.31μg/m3 and 4.5μg/m3, and the single scattering albedo (SSA), estimated using the OPAC model, ranged from 0.87 to 0.97. Modeled aerosol optical properties were used to estimate the direct aerosol shortwave radiative forcing (DASRF) in the wavelength range 0.25μm4.0μm. The monthly mean forcing at the surface, at the top of the atmosphere (TOA) and in the atmosphere varied between −14.1Wm−2 and −35.6Wm−2, −6.7Wm−2 and −13.4Wm−2 and 5.5Wm−2 to 22.5Wm−2, respectively. These results indicate that the annual SSA cycle in the atmosphere is regulated by BC (absorbing aerosol), resulting in a positive forcing; however, the surface forcing was governed by the natural aerosol scattering, which yielded a negative forcing. These two conditions neutralized, resulting in a negative forcing at the TOA that remains nearly constant throughout the year.

Effect of spectrally varying albedo of vegetation surfaces on shortwave radiation fluxes and aerosol direct radiative forcing

Abstract. This study develops an algorithm for representing detailed spectral features of vegetation albedo based on Moderate Resolution Imaging Spectrometer (MODIS) observations at 7 discrete channels, referred to as the MODIS Enhanced Vegetation Albedo (MEVA) algorithm. The MEVA algorithm empirically fills spectral gaps around the vegetation red edge near 0.7 μm and vegetation water absorption features at 1.48 and 1.92 μm which cannot be adequately captured by the MODIS 7 channels. We then assess the effects of applying MEVA in comparison to four other traditional approaches to calculate solar fluxes and aerosol direct radiative forcing (DRF) at the top of atmosphere (TOA) based on the MODIS discrete reflectance bands. By comparing the DRF results obtained through the MEVA method with the results obtained through the other four traditional approaches, we show that filling the spectral gap of the MODIS measurements around 0.7 μm based on the general spectral behavior of healthy green vegetation leads to significant improvement in the instantaneous aerosol DRF at TOA (up to 3.02 W m−2 difference or 48% fraction of the aerosol DRF, −6.28 W m−2, calculated for high spectral resolution surface reflectance from 0.3 to 2.5 μm for deciduous vegetation surface). The corrections of the spectral gaps in the vegetation spectrum in the near infrared, again missed by the MODIS reflectances, also contributes to improving TOA DRF calculations but to a much lower extent (less than 0.27 W m−2, or about 4% of the instantaneous DRF). Compared to traditional approaches, MEVA also improves the accuracy of the outgoing solar flux between 0.3 to 2.5 μm at TOA by over 60 W m−2 (for aspen 3 surface) and aerosol DRF by over 10 W m−2 (for dry grass). Specifically, for Amazon vegetation types, MEVA can improve the accuracy of daily averaged aerosol radiative forcing in the spectral range of 0.3 to 2.5 μm at equator at the equinox by 3.7 W m−2. These improvements indicate that MEVA can contribute to regional climate studies over vegetated areas and can help to improve remote sensing-based studies of climate processes and climate change.

Sources and radiative effects of wintertime black carbon aerosols in an urban atmosphere in east India

We carried out an analysis of black carbon (BC) surface mass concentration, its radiative effects, and sources of origin in an urban atmosphere in east India, during winter season, through ground-based measurements and application of modelling tools. BC surface mass concentration exhibited diurnal variation with their higher values and a larger variability during evening to early morning hours than during daytime (1100–1600h, Local Time, LT) hours. Daytime mean surface BC mass concentration and BC mass fraction in total aerosol (size range 0.23–20μm) and in submicronic aerosol (size range 0.23–1μm) during the study period, corresponding to the well-mixed atmospheric layer were 11μgm−3, 3–10%, and 9–16% respectively. The mean BC optical depth (BC-AOD) and BC-AOD fraction at 0.5μm were estimated in an optical model as 0.11 and 13% respectively. Mean shortwave aerosol radiative forcing due to BC at top-of-atmosphere (TOA) during the study period was found to be +0.94Wm−2, which is about 59% the global mean radiative forcing due to carbon-dioxide gases. Estimates from BC simulations in a general circulation model showed BC surface concentration and BC optical depth in east India are primarily attributed to emissions from biofuel and fossil fuel combustion. Most of BC surface concentration (95%) and BC optical depth (60%) are contributed by emissions arising from the Indo-Gangetic plain (IGP) but there is a significant influence to BC columnar loading through elevated transport channels attributed mainly to emissions from open biomass burning from distant regions outside IGP.

Radiative effects of aerosols over Indo-Gangetic plain: environmental (urban vs. rural) and seasonal variations

Aerosol radiative effects over two environmentally distinct locations, Kanpur (urban site) and Gandhi College (rural location) in the Indo-Gangetic plain (IGP), a regional aerosol hot spot, utilizing the measured optical and physical characteristics of aerosols, an aerosol optical properties model and a radiative transfer model, are examined. Shortwave aerosol radiative forcing (ARF) at the top of the atmosphere (TOA) is < -12 W m( - 2) over Kanpur and Gandhi College. ARF at the surface is ≥ -30 W m( - 2). Atmospheric warming is maximum during premonsoon (>30 W m( - 2)). Shortwave atmospheric heating due to aerosols is >0.4 K/day over IGP and peaks during premonsoon at >0.6 K/day due to lower single scattering albedo (SSA) and higher surface albedo. TOA forcing is always less negative over Kanpur when compared to Gandhi College due to lower surface albedo except in postmonsoon owing to higher SSA. This happens as TOA forcing depends on SSA and surface albedo in addition to aerosol optical depth. The magnitude of longwave forcing and atmospheric cooling in an absolute sense is significantly small and contributes only about 20% or less to the net (shortwave + longwave) forcing. Aerosol radiative effects over these two locations, despite differences in aerosol characteristics, are similar, thus confirming that aerosols and their radiative influence get transported due to circulation. ARF over Kanpur and Gandhi College is an order of magnitude higher when compared to greenhouse gas forcing. A large reduction in surface reaching solar irradiance accompanied by large atmospheric warming can have implications on precipitation and hydrological cycle, and these aerosol radiative effects should be included while performing regional-scale aerosol climate assessments.

Determination of Aerosol Characteristics and Direct Radiative Forcing at Pune

Simultaneous measurements of aerosol optical depth and incoming global solar flux were carried out with a MICROTOPS-II Sunphotometer and Eppley Precision Spectral Pyranometer over Nowrosjee Wadia College (NWC, Pune) as the nodal centre, and at Pune University (PU) and IUCAA Girwali Observatory (IGO), in a field campaign mode during December 2010-May 2011. Data was analyzed to determine the surface short-wave (SW) aerosol direct radiative forcing (ADRF, ΔF per unit 0.1 AOD) and to study the characteristics of the aerosols. The results indicate that ADRF shows significant day-to-day variability and co-varies with AOD. The cause of the day-to-day variation in ADRF is an anticorrelation between ADRF/AOD, and differences in the daily maximum minus minimum RH and temperature. At NWC, ADRF ranges between -37.7 W/m^2 (highest) and -5.9 W/m^2 (lowest). For 500 nm, ADRF takes values in the range -17.3 ± 7.1 W/m^2 to -54.2 ± 5.5 W/m^2 at PU, whereas the corresponding values at IGO are -15.1 ± 2.1 W/m^2 and -36.6 ± 6.4 W/m^2. Monthly ADRF is at the minimum level in winter and maximum during the pre-monsoon period. The magnitude of AOD shows significant diurnal variability. In winter, the mean AOD diurnal percentage departure at 500 nm is positive in the morning and negative during the afternoon, and this is reversed in the pre-monsoon period. The diurnal cycle of AOD is related to the prevalent meteorological conditions, surface-based nocturnal temperature inversion in the atmospheric boundary layer (ABL), and influx of aerosols from different source regions.

The June 2007 Saharan dust event in the central Mediterranean: Observations and radiative effects in marine, urban, and sub-urban environments

Abstract A desert dust episode in June 2007 and its radiative effects on the energy budget have been studied at three Italian stations (Rome, Lecce and Lampedusa) with the aim of investigating the interactions with different conditions and aerosol types over the Mediterranean. The three sites are representative for urban (Rome), sub-urban/rural (Lecce), and marine (Lampedusa) environment, respectively in the central Mediterranean region. Measured ground-based column-averaged aerosol optical properties and aerosol extinction profiles were used to initialize the MODTRAN4 radiative transfer model. The radiative transfer model was used to estimate the shortwave aerosol radiative forcing ( ARF ) and its forcing efficiency ( FE ) at two different solar zenith angles (20° and 60°) in the 280–2800 nm spectral range. The goal was to investigate the role of different aerosol types in the atmospheric boundary layer on the radiative budget during a dust event. During the event the aerosol optical depth was moderately high and similar at the three stations, with a maximum value of about 0.6. The Angstrom exponent was found to increase with the distance from the source (0.21, 0.36, and 0.43 at Lampedusa, Rome, and Lecce, respectively). Differences in the aerosol optical properties were observed, also depending on the aerosol type assumed in the boundary layer. The estimated direct aerosol forcing appears to depend on the changes in aerosol properties and to the surface albedo. The results show that the desert dust produces a cooling effect at both surface (largest ARF of −224 W m −2 at 20° solar zenith angle at Rome) and top of the atmosphere (largest ARF of −19 W m −2 at 20° solar zenith angle at Lecce). The cooling is largest in the rural and smallest in the marine environment. The surface forcing efficiency appears to be strongly affected by the aerosol absorption in the BL. Large differences exist between our results and the FE determinations by AERONET, derived considering a single layer with homogeneous optical properties and prescribed vertical distribution. The FE deviations are around 20, 60, and 40% at the surface, TOA, and in the atmosphere, respectively. These results suggest that the detailed description of the vertical distribution of the aerosol properties is needed for an accurate determination of its radiative effects.

Aerosol ageing in an urban plume – implication for climate

Abstract. The climate effects downwind of an urban area resulting from gaseous and particulate emissions within the city are as yet inadequately quantified. The aim of this work was to estimate these effects for Malmö city in southern Sweden (population 280 000). The chemical and physical particle properties were simulated with a model for Aerosol Dynamics, gas phase CHEMistry and radiative transfer calculations (ADCHEM) following the trajectory movement from upwind of Malmö, through the urban background environment and finally tens and hundreds of kilometers downwind of Malmö. The model results were evaluated using measurements of the particle number size distribution and chemical composition. The total particle number concentration 50 km (~ 3 h) downwind, in the center of the Malmö plume, is about 3700 cm−3 of which the Malmö contribution is roughly 30%. Condensation of nitric acid, ammonium and to a smaller extent oxidized organic compounds formed from the emissions in Malmö increases the secondary aerosol formation with a maximum of 0.7–0.8 μg m−3 6 to 18 h downwind of Malmö. The secondary mass contribution dominates over the primary soot contribution from Malmö already 3 to 4 h downwind of the emission sources and contributes to an enhanced total surface direct or indirect aerosol shortwave radiative forcing in the center of the urban plume ranging from −0.3 to −3.3 W m−2 depending on the distance from Malmö, and the specific cloud properties.

Assessment of Aerosol Radiative Impact over Oceanic Regions Adjacent to Indian Subcontinent Using Multisatellite Analysis

Using data from Ozone Monitoring Instrument (OMI) and Moderate Resolution Imaging Spectroradiometer (MODIS) instruments, we have retrieved regional distribution of aerosol column single scattering albedo (parameter indicative of the relative dominance of aerosol absorption and scattering effects), a most important, but least understood aerosol property in assessing its climate impact. Consequently we provide improved assessment of short wave aerosol radiative forcing (ARF) (on both regional and seasonal scales) estimates over this region. Large gradients in north‐south ARF were observed as a consequence of gradients in single scattering albedo as well as aerosol optical depth. The highest ARF (–37 W m−2 at the surface) was observed over the northern Arabian Sea during June to August period (JJA). In general, ARF was higher over northern Bay of Bengal (NBoB) during winter and premonsoon period, whereas the ARF was higher over northern Arabian Sea (NAS) during the monsoon and postmonsoon period. The largest forcing observed over NAS during JJA is the consequence of large amounts of desert dust transported from the west Asian dust sources. High as well as seasonally invariant aerosol single scattering albedos (~0.98) were observed over the southern Indian Ocean region far from continents. The ARF estimates based on direct measurements made at a remote island location, Minicoy (8.3∘N, 73∘E) in the southern Arabian Sea are in good agreement with the estimates made following multisatellite analysis.

Surface shortwave aerosol radiative forcing during the Atmospheric Radiation Measurement Mobile Facility deployment in Niamey, Niger

The Atmospheric Radiation Measurement (ARM) Program's Mobile Facility (AMF) was deployed to Niamey, Niger, during 2006. Niamey, which is located in sub‐Saharan Africa, is affected by both dust and biomass burning emissions. Column aerosol optical properties were derived from multifilter rotating shadowband radiometer, measurements and the vertical distribution of aerosol extinction was derived from a micropulse lidar during the two observed dry seasons (January–April and October–December). Mean aerosol optical depth (AOD) and single scattering albedo (SSA) at 500 nm during January–April were 0.53 ± 0.4 and 0.94 ± 0.05, while during October–December mean AOD and SSA were 0.33 ± 0.25 and 0.99 ± 0.01. Aerosol extinction profiles peaked near 500 m during the January–April period and near 100 m during the October–December period. Broadband shortwave surface fluxes and heating rate profiles were calculated using retrieved aerosol properties. Comparisons for noncloudy periods indicated that the remote sensing retrievals provided a reasonable estimation of the aerosol optical properties, with mean differences between calculated and observed fluxes of less than 5 W m−2 and RMS differences less than 25 W m−2. Sensitivity tests showed that the observed fluxes could be matched with variations of &lt;10% in the inputs to the radiative transfer model. The calculated 24‐h averaged SW instantaneous surface aerosol radiative forcing (ARF) was −21.1 ± 14.3 W m−2 and was estimated to account for 80% of the total radiative forcing at the surface. The ARF was larger during January–April (−28.5 ± 13.5 W m−2) than October–December (−11.9 ± 8.9 W m−2).

A Multisensor satellite‐based assessment of biomass burning aerosol radiative impact over Amazonia

Using spatially and temporally collocated multispectral, multiangle and broadband data sets from the Terra satellite, the role of biomass burning (BB) smoke particles on cloud‐free top of atmosphere (TOA) direct shortwave aerosol radiative forcing (SWARF) is examined. A 5‐year analysis during the peak biomass burning months of August and September is presented over South America (0°–20°S and 45°W–65°W). Our results indicate that over 5 years, the TOA diurnally averaged SWARF (DSWARF) from the Clouds and the Earth's Radiant Energy System (CERES) scanner ranges between −5.2 Wm−2 and −9.4 Wm−2 with a mean value of −7.6 Wm−2 and an estimated uncertainty of ±1.4 Wm−2. The corresponding Multi Angle Spectroradiometer (MISR) aerosol optical thickness (AOT at 0.558 μm) ranged from 0.15 to 0.36 with a mean value of 0.24. The estimated mean TOA aerosol radiative forcing efficiency (Eτ) is −44.2 Wm−2τ−1 and is in good agreement with previous studies. We also examined the beta versions of the MISR data products such as the angstrom exponent (AE) and fraction of AOT in different size bins to assess the role of BB aerosol particle properties on SWARF. Our analysis indicates that the MISR retrieved 5 year mean AE is 1.54. Contribution to total AOT from small, medium and large particles is 66%, 16% and 18% respectively. This is the first multiyear assessment of SWARF for biomass burning aerosol particles using satellite observations alone and should serve as a useful constraint for numerical modeling simulations that estimate SWARF.

Satellite‐derived aerosol radiative forcing from the 2004 British Columbia wildfires

The British Columbia wildfires of 2004 was one of the largest wildfire events in the last ten years in Canada. Both the shortwave and longwave smoke aerosol radiative forcing at the top‐of‐atmosphere (TOA) are investigated using data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Clouds and the Earth's Radiant Energy System (CERES) instruments. Relationships between the radiative forcing fluxes (?F) and wildfire aerosol optical thickness (AOT) at 0.55 μm (τ0.55) are deduced for both noontime instantaneous forcing and diurnally averaged forcing. The noontime averaged instantaneous shortwave and longwave smoke aerosol radiative forcing at the TOA are 45.8±27.5 W m−2 and ‐12.6±6.9 W m−2, respectively for a selected study area between 62°N and 68°N in latitude and 125°W and 145°W in longitude over three mainly clear‐sky days (23–25 June). The derived diurnally averaged smoke aerosol shortwave radiative forcing is 19.9±12.1 W m−2 for a mean τ0.55 of 1.88±0.71 over the same time period. The derived ?F‐τ relationship can be implemented in the radiation scheme used in regional climate models to assess the effect of wildfire aerosols.

Surface shortwave radiative forcing of different aerosol types in the central Mediterranean

Ground based measurements of aerosol optical depth, τ, and shortwave irradiance at the Mediterranean island of Lampedusa during 2003 and 2004 were used to estimate the surface aerosol shortwave radiative forcing. The shortwave forcing efficiency (FE) was derived at various solar zenith angles, θ, as the derivative of the shortwave irradiance with respect to τ. Values of FE for different classes of particles, namely desert dust, DD, biomass burning/industrial aerosols, BU, and for the whole dataset are derived. At the summer solstice the daily average FE is −86.4 W/m2 for DD, −70.5 W/m2 for BU, and −94.0 W/m2 for the whole dataset. The daily aerosol forcing of DD is much larger than for the other aerosol classes due to the combination of larger forcing efficiency and largest optical depths. The estimated average daily forcing at the summer solstice and equinox for DD is −30 and −24 W/m2, respectively.

Temporal heterogeneity in aerosol characteristics and the resulting radiative impacts at a tropical coastal station – Part 2: Direct short wave radiative forcing

Abstract. Seasonal distinctiveness in the microphysical and optical properties of columnar and near-surface (in the well mixed region) aerosols, associated with changes in the prevailing synoptic conditions, were delineated based on extensive (spread over 4 years) and collocated measurements at the tropical coastal location, Trivandrum (8.55° N; 76.97° E, 3 m a.m.s.l.), and the results were summarized in Part 1 of this two-part paper. In Part 2, we use these properties to develop empirical seasonal aerosol models, which represent the observed features fairly accurately, separately for winter monsoon season (WMS, December through March), inter-monsoon season (IMS, April and May), summer monsoon season (SMS, June through September) and post monsoon season (PMS, October and November). The models indicate a significant transformation in the aerosol environment from an anthropogenic-dominance in WMS to a natural-dominance in SMS. The modeled aerosol properties are used for estimating the direct, short wave aerosol radiative forcing, under clear-sky conditions. Our estimates show large seasonal changes. Under clear sky conditions, the daily averaged short-wave TOA forcing changes from its highest values during WMS, to the lowest values in SMS; this seasonal change being brought-in mainly by the reduction in the abundance and the mass fraction (to the composite) of black carbon aerosols and of accumulation mode aerosols. The resulting atmospheric forcing varies from the highest, (47 to 53 W m−2) in WMS to the lowest (22 to 26 W m−2) in SMS.

Seasonal changes in aerosol characteristics over Arabian Sea and their consequence on aerosol short-wave radiative forcing: Results from ARMEX field campaign

Extensive and collocated measurements of several aerosol parameters were made over the eastern Arabian Sea, during the inter-monsoon and summer monsoon seasons of 2003 as a part of the Arabian Sea Monsoon Experiment (ARMEX). Associated with the seasonal changes in the synoptic wind fields from northeasterly/easterly to westerly/northwesterly, the aerosol characteristics and columnar optical depth show large variations. Consequently, the atmospheric forcing is found to increase from March to April and then to decrease consistently towards June. However, the magnitude of the forcing efficiency of aerosols continuously decreases from winter to summer. Such temporal changes in radiative forcing need to be accounted for in reducing the uncertainty in aerosol climate impact.

Aerosol Characterization and Direct Radiative Forcing Assessment over the Ocean. Part II: Application to Test Cases and Validation

Abstract A method based on the synergistic use of low earth orbit and geostationary earth orbit satellite data for aerosol-type characterization and aerosol optical thickness (AOT: τa) retrieval and monitoring over the ocean is presented in Part I of this paper. The method is now applied to a strong dust outbreak over the Atlantic Ocean in June 1997 and to two other relevant transport events of biomass burning and desert dust aerosol that occurred in 2000 over the Atlantic and Indian Oceans, respectively. The retrievals of the aerosol optical properties are checked against retrievals from sun and sky radiance measurements from the ground-based Aerosol Robotic Network (AERONET). The single-scattering albedo values obtained from AERONET are always within the error bars presented for Global Ozone Monitoring Experiment (GOME) retrievals, resulting in differences lower than 0.041. The retrieved AOT values are compared with the independent space–time-collocated measurements from the AERONET, as well as to the satellite aerosol official products of the Polarization and Directionality of the Earth Reflectances (POLDER) and the Moderate Resolution Imaging Spectroradiometer (MODIS). A first estimate of the AOT accuracy derived from comparisons with AERONET data leads to ±0.02 ± 0.22τa when all AOT values are retained or to ±0.02 ± 0.16τa for aerosol transport events (AOT &amp;gt; 0.4). The upwelling flux at the top of the atmosphere (TOA) was computed with radiative transfer calculations and used to estimate the TOA direct shortwave aerosol radiative forcing; a comparison with space–time-collocated measurements from the Clouds and the Earth's Radiant Energy System (CERES) TOA flux product was also done. It was found that more than 90% of the values differ from CERES fluxes by less than ±15%.