Shortwave Aerosol Radiative Forcing Research Articles

Towards an improved representation of carbonaceous aerosols over the Indian monsoon region in a regional climate model: RegCM

Abstract. Mitigation of carbonaceous aerosol emissions is expected to provide climate and health co-benefits. The accurate representation of carbonaceous aerosols in climate models is critical for reducing uncertainties in their climate feedback. In this regard, emission fluxes and aerosol life cycle processes are the two primary sources of uncertainties. Here, we demonstrate that the incorporation of a dynamic ageing scheme and emission estimates that are updated for the local sources improves the representation of carbonaceous aerosols over the Indian monsoon region in a regional climate model, RegCM, compared with its default configuration. The respective mean black carbon (BC) and organic carbon (OC) surface concentrations in 2010 are estimated to be 4.25 and 10.35 µg m−3 over the Indo-Gangetic Plain (IGP) in the augmented model. The BC column burden over the polluted IGP is found to be 2.47 mg m−2, 69.95 % higher than in the default model configuration and much closer to available observations. The anthropogenic aerosol optical depth (AOD) increases by more than 19 % over the IGP due to the model enhancement, also leading to a better agreement with observed AOD. The respective top-of-the-atmosphere, surface, and atmospheric anthropogenic aerosol short-wave radiative forcing are estimated at −0.3, −9.3, and 9.0 W m−2 over the IGP and −0.89, −5.33, and 4.44 W m−2 over Peninsular India (PI). Our results suggest that the combined effect of two modifications leads to maximum improvements in the model performance in regions where emissions play a dominant role.

Effects of Aerosol on Reference Crop Evapotranspiration: A Case Study in Henan Province, China

An increase in atmospheric pollution markedly affects the climatic environment. Aerosol is the main component of atmospheric pollutants and has a significant influence on the changes of reference crop evapotranspiration (ET0), while the effects of aerosol on ET0 are still unclear. In this study, the influence of aerosol on the changes in meteorological elements and ET0 in Henan Province was evaluated using online two-way coupling of WRF (Weather Research Forecast)–Chem. The results of the 30-day Online Two-way Coupling indicated that the WRF–Chem model accurately simulated the temporal and spatial variation of each meteorological element in Henan Province. Aerosol decreased the overall temperature in Henan Province by 0.036 °C, wind speed by 0.176 m s−1, and barometric pressure by 20 Pa, while the relative humidity increased by 1.39%. The effect of aerosol on meteorological elements led to the change in ET0. The extent of the effect of aerosol on ET0 was closely related to the aerosol concentration. The variation of ET0 ranged from −0.545 to 0.676 mm d−1 for a pollution condition and −0.309 to 0.380 mm d−1 for an excellent condition. The extent of the effect of aerosol on ET0 varied among regions, and the variation of ET0 showed distinct spatial patterns under different pollution levels. The varying degree of ET0 in the daytime (ET0-d) was greater than ET0-n (ET0 in the nighttime) regardless of the circumstances. Shortwave aerosol radiative forcing was the main cause of this phenomenon. For an excellent condition, aerosol showed positive regulation of ET0-d in 63% of the regions and of ET0-n in 88% of the regions. ET0-A (aerodynamic term of ET0) plays a dominant role in ET0 changes in most of Henan Province. However, as the pollution level increased, more urban ET0-R (radiation term of ET0) also began to dominate the ET0 changes. These results contribute to an in-depth understanding of the response of regional evapotranspiration to atmospheric pollutants and climate change.

Long-term measurements of aerosol optical properties and radiative forcing (2011-2017) over Central Amazonia

The Amazon region is one of the most pristine continental areas whose concentrations of atmospheric trace gases and aerosol particles are very low, mainly in the wet season. This study provides observational results of aerosol optical and radiative characteristics in situ as well as atmospheric columnar at a pristine forest in Central Amazonia. Spectral variation of the aerosol properties (aerosol optical depth [AOD], single scattering albedo [SSA], and asymmetry parameter [AP]) was evaluated using the AErosol RObotic NETwork (AERONET) data. The SSA values under natural atmospheric column conditions (AERONET) were compared to the SSA values calculated with in situ measurements. The values of shortwave aerosol radiative forcing (SWARF) on top of the atmosphere (TOA) and the surface (SUR) were estimated using the SBDART model and were validated with the AERONET values with regression analysis. SWARF had a high correlation to TOA (0.97) and SUR (0.92), including dry and wet seasons. Monthly, seasonal and annual mean values of SWARFTOA and SWARFSUR were negative while SWARFATM values were positive. SWARFTOA was –9.18 ± 2.80 W m–2 and SWARFSUR was –20.77 ± 5.04 W m–2 in the dry season, inducing a heating rate (HR) of 0.37 ± 0.13 K.day–1. This study showed that, for a long series of measurements, the effects caused by aerosols on the radiative flux in the pristine forest of Central Amazonia were of the order of SWARFTOA of –3.66 ± 1.59 W m–2 and SWARFSUR of –11.86 ± 2.35 W m–2 during the wet season

Composition Dependence of Stratospheric Aerosol Shortwave Radiative Forcing in Northern Midlatitudes

AbstractBoth observations and recent model results reveal a significant organic component in stratospheric aerosols. However, intrinsic optical properties (i.e., complex refractive index) of this organic component and the mixing state of organic and ubiquitous sulfuric acid components are quite uncertain. We examine the effects of different complex refractive indices and particle mixing states on shortwave radiative forcing (RF) of stratospheric aerosols in northern midlatitudes. In the absence of large volcanic sulfur emissions, our calculations show that organic components may have substantial impacts on stratospheric aerosol optical depth (AOD). Compared to pure sulfuric acid/water aerosol, organic‐containing aerosols could cause ±100% change in shortwave RF (for low AOD conditions) depending on the refractive index and mixing state. The range found here of shortwave RF results, for different scenarios of organic complex refractive index and mixing state, call for better understandings of chemical and transport processes determining aerosol optical properties in the stratosphere.

Effect of Vertical Profile of Aerosols on the Local Shortwave Radiative Forcing Estimation

In this work, the effect of the aerosol vertical distribution on the local shortwave aerosol radiative forcing is studied. We computed the radiative forcing at the top and bottom of the atmosphere between 0.2 and 4 microns using the libRadTran package and compared the results with those provided by AERONET (AErosol RObotic NETwork). Lidar measurements were employed to characterize the aerosol vertical profile, and collocated AERONET measurements provided aerosol optical parameters required to calculate its radiative forcing. A good correlation between the calculated radiative forcings and those provide by AERONET, with differences smaller than 1 W m−2 (15% of estimated radiative forcing), is obtained when a gaussian vertical aerosol profile is assumed. Notwithstanding, when a measured aerosol profile is inserted into the model, differences between radiative forcings can vary up to 6.54 W m−2 (15%), with a mean of differences = −0.74 ± 3.06 W m−2 at BOA and −3.69 W m−2 (13%), with a mean of differences = −0.27 ± 1.32 W m−2 at TOA due to multiple aerosol layers and aerosol types. These results indicate that accurate information about aerosol vertical distribution must be incorporated in the radiative forcing calculation in order to reduce its uncertainties.

Assessment of regional aerosol radiative effects under the SWAAMI campaign – Part 2: Clear-sky direct shortwave radiative forcing using multi-year assimilated data over the Indian subcontinent

Abstract. Clear-sky, direct shortwave aerosol radiative forcing (ARF) has been estimated over the Indian region, for the first time employing multi-year (2009–2013) gridded, assimilated aerosol products, as an important part of the South West Asian Aerosol Monsoon Interactions (SWAAMI) which is a joint Indo-UK research field campaign focused at understanding the variabilities in atmospheric aerosols and their interactions with the Indian summer monsoon. The aerosol datasets have been constructed following statistical assimilation of concurrent data from a dense network of ground-based observatories and multi-satellite products, as described in Part 1 of this two-part paper. The ARF, thus estimated, is assessed for its superiority or otherwise over other ARF estimates based on satellite-retrieved aerosol products, over the Indian region, by comparing the radiative fluxes (upward) at the top of the atmosphere (TOA) estimated using assimilated and satellite products with spatiotemporally matched radiative flux values provided by CERES (Clouds and Earth's Radiant Energy System) single-scan footprint (SSF) product. This clearly demonstrated improved accuracy of the forcing estimates using the assimilated vis-à-vis satellite-based aerosol datasets at regional, subregional and seasonal scales. The regional distribution of diurnally averaged ARF estimates has revealed (a) significant differences from similar estimates made using currently available satellite data, not only in terms of magnitude but also the sign of TOA forcing; (b) the largest magnitudes of surface cooling and atmospheric warming over the Indo-Gangetic Plain (IGP) and arid regions from north-western India; and (c) negative TOA forcing over most parts of the Indian region, except for three subregions – the IGP, north-western India and eastern parts of peninsular India where the TOA forcing changes to positive during pre-monsoon season. Aerosol-induced atmospheric warming rates, estimated using the assimilated data, demonstrate substantial spatial heterogeneities (∼0.2 to 2.0 K d−1) over the study domain with the IGP demonstrating relatively stronger atmospheric heating rates (∼0.6 to 2.0 K d−1). There exists a strong seasonality as well, with atmospheric warming being highest during pre-monsoon and lowest during winter seasons. It is to be noted that the present ARF estimates demonstrate substantially smaller uncertainties than their satellite counterparts, which is a natural consequence of reduced uncertainties in assimilated vis-à-vis satellite aerosol properties. The results demonstrate the potential application of the assimilated datasets and ARF estimates for improving accuracies of climate impact assessments at regional and subregional scales.

Simulation of long-term direct aerosol radiative forcing over the arctic within the framework of the iAREA project

This paper presents the climatology of aerosol optical properties and radiative forcing over the Arctic obtained within the framework of the iAREA (impact of absorbing aerosols on radiative forcing in the European Arctic) project. The presented data were obtained from the Navy Aerosol Analysis and Prediction System (NAAPS) and the Fu-Liou radiative transfer model. NAAPS was used to simulate particle concentration and aerosol optical depth (AOD) at 1 ° × 1 ° spatial resolution. Direct aerosol radiative forcing (ARF) was calculated for clear-sky and all-sky conditions based on NAAPS reanalysis (with AOD assimilation) and satellite observations of surface and cloud properties. Long-term data (2003–2015) from NAAPS show that anthropogenic and biogenic aerosol, as well as sea salt, make the most important contribution to total AOD (35 and 30%, respectively). However, smoke (15%) and mineral dust (20%) cannot be neglected, especially during spring and summer. Results of numerical simulations indicate mean shortwave (SW) ARF for the whole Arctic (>70.5oN) at the Earth's surface to be −4 W/m2 for clear-sky and −1.3 W/m2 for all-sky conditions, and at top of the atmosphere (TOA) −1.3 W/m2 and -0.4 W/m2, respectively. TOA ARF for anthropogenic and biogenic particles is only −0.1 W/m2 for clear-sky and almost zero for all-sky conditions. For smoke and dust particles, SW ARF is very similar for both Earth's surface and TOA, as well as for clear-sky and all-sky conditions. For sea salt, SW ARF is the same at the surface and at TOA: 0.6 W/m2 for clear-sky and −0.3 W/m2 for all-sky conditions, because of negligible solar absorption. Cloud cover reduces surface cooling (direct clear-sky SW ARF) by a factor of 40% and shifts TOA SW ARF towards positive values.

Contrasting variation in aerosol optical properties during dust episodes in the Middle East and Southwest Asia: Model results and ground measurement

Dust storms deteriorated air quality over the Gulf Region, Iraq, Iran, and Pakistan during the last decade. The purpose of this study is to investigate the changes in aerosol optical and radiative properties during a dust episode over the various locations in the Middle East and Southwest Asia using data from the MODerate resolution Imaging Spectroradiometer (MODIS) and the Aerosol Robotic Network (AERONET) during March, 2012. Maximum aerosol optical depth (AOD) values were found to be 2.18, 1.30, 4.33 and 1.80 over Lahore, Kanpur, Kaust, and Mezaira, respectively. The Volume Size Distributions, Single Scattering Albedo, Refractive Index, and Asymmetry parameter indicated that coarse mode aerosols were predominant relative to fine mode aerosols during the dust event. The average shortwave aerosol radiative forcing (ARF) values at the earth’s surface were found to be -96±45 W m-2, -86±22 W m-2, -77±51 W m-2, and -75±40 W m-2, over Lahore, Kanpur, Kaust and Mezaira, respectively. Likewise, the averaged ARF values over Lahore, Kanpur, Kaust and Mezaira at the top of the atmosphere (TOA) were found to be -45±25 W m-2, -27±9 W m-2, -41±29 W m-2, and -75±40 W m-2, respectively. The large differences between surface and TOA forcing produced significant heating within the atmosphere.

Aerosol chemistry, transport, and climatic implications during extreme biomass burning emissions over the Indo-Gangetic Plain

Abstract. The large-scale emissions of airborne particulates from burning of agricultural residues particularly over the upper Indo-Gangetic Plain (IGP) have often been associated with frequent formation of haze, adverse health impacts, and modification in aerosol climatology and thereby aerosol impact on regional climate. In this study, short-term variations in aerosol climatology during extreme biomass burning emissions over the IGP were investigated. Size-segregated particulate concentration was initially measured and submicron particles (PM1.1) were found to dominate particulate mass within the fine mode (PM2.1). Particulate-bound water-soluble ions were mainly secondary in nature and primarily composed of sulfate and nitrate. There was evidence of gaseous NH3 dominating neutralization of acidic aerosol species (SO42-) in submicron particles, in contrast to crustal-dominating neutralization in coarser particulates. Diurnal variation in black carbon (BC) mass ratio was primarily influenced by regional meteorology, while gradual increase in BC concentration was consistent with the increase in Delta-C, referring to biomass burning emissions. The influence of biomass burning emissions was established using specific organic (levoglucosan), inorganic (K+ and NH4+), and satellite-based (UV aerosol index, UVAI) tracers. Levoglucosan was the most abundant species within submicron particles (649±177 ng m−3), with a very high ratio (> 50) to other anhydrosugars, indicating exclusive emissions from burning of agriculture residues. Spatiotemporal distribution of aerosol and a few trace gases (CO and NO2) was evaluated using both spaceborne active and passive sensors. A significant increase in columnar aerosol loading (aerosol optical depth, AOD: 0.98) was evident, with the presence of absorbing aerosols (UVAI > 1.5) having low aerosol layer height (∼ 1.5 km). A strong intraseasonality in the aerosol cross-sectional altitudinal profile was even noted from CALIPSO, referring to the dominance of smoke and polluted continental aerosols across the IGP. A possible transport mechanism of biomass smoke was established using cluster analysis and concentration-weighted air mass back trajectories. Short-wave aerosol radiative forcing (ARF) was further simulated considering intraseasonality in aerosol properties, which resulted in a considerable increase in atmospheric ARF (135 W m−2) and heating rate (4.3 K day−1) during extreme biomass burning emissions compared to the non-dominating period (56 W m−2, 1.8 K day−1). Our analysis will be useful to improve understanding of short-term variation in aerosol chemistry over the IGP and to reduce uncertainties in regional aerosol–climate models.

Investigation of optical and radiative properties of aerosols during an intense dust storm: A regional climate modeling approach

The dynamical and optical properties of aerosols during an intense dust storm event over the Arabian Sea have been studied using Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) and space borne instruments such as MODIS, MISR, CALIPSO and CERES during the period 17 to 24 March, 2012. The model captures the spatio-temporal and vertical variations of meteorological and optical parameters, however an overestimation in simulated aerosol optical parameters are observed when compared to satellite retrievals. The correlation coefficients (R) between simulated and observed AOD from MODIS and MISR are found to be 0.54 and 0.32 respectively. Model simulated AOD on dusty days (20 and 21 March 2012) increased by 2–3 times compared to non-dusty days (17 and 24 March 2012) and the single scattering albedo (SSA) and the asymmetry parameter increased from 0.96 to 0.99 and from 0.56 to 0.66, respectively. The R between simulated shortwave (SW) radiation at top of the atmosphere (TOA) and TOA SW radiation obtained from CERES is found to be 0.43, however the model simulated SW radiation at the TOA showed an underestimation with respect to CERES. The shortwave aerosol radiative forcing (SWARF) during the event over surface and TOA are ∼ -19.3 and ∼ -14.2 Wm-2 respectively, which is about 2–5 times higher when compared to the respective forcing values during non-dust days. Estimated net radiative forcing was in the range of −13 to −21 Wm-2 at TOA and −12 to −20 Wm-2 at the surface. The heating rate during event days within the lower atmosphere near 850 hPa is found to 0.32 − 0.4 K day−1 and 0.18 − 0.22 K day−1 on dusty and non-dusty days, respectively. Results of this study may be useful for a better modeling of atmospheric aerosols and its optical and radiative properties over oceanic region.

Satellite‐retrieved direct radiative forcing of aerosols over North‐East India and adjoining areas: climatology and impact assessment

In order to understand the climatic implications of atmospheric aerosols, top of atmosphere (TOA) shortwave (SW, 0.3–5 µm) fluxes and aerosol optical depth (AOD) at 550 nm retrieved simultaneously by clouds and the earth's radiant energy system (CERES) and moderate resolution imaging spectroradiometer (MODIS) instruments, respectively, are analysed over North-East India and its adjoining areas for the period July 2002–December 2013. The aerosol-free TOA flux obtained by establishing the linear regression between CERES SW TOA fluxes and MODIS AODs exhibits strong seasonality with peak values in monsoon and minimum in winter. Same seasonality is captured by the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model, but with difference in absolute values. SBDART code is used to extend instantaneous radiative forcing estimates into 24-h averages. AOD over the North East India region with complex terrain shows altitudinal variation with maximum value at the lowest elevation site Dhaka and minimum value at the high-altitude locations Shillong and Aizwal. In general, strong seasonality in AOD is observed with a peak in pre-monsoon (March–May) and dip in post-monsoon (October–November) at all the locations. The direct instantaneous TOA shortwave aerosol radiative forcing (SWARF) shows maximum values in pre-monsoon over all the locations except at Guwahati, Banmauk, Aizawl, and Shillong. The lowest value of instantaneous SWARF is observed in post-monsoon except at Banmauk and Shillong. Climatologically TOA diurnally averaged SWARF varies between −6.95 W m−2 in Aizawl to −20.39 W m−2 in Shillong. In general, the TOA SW forcing efficiency is highest in monsoon at all the locations. The radiative forcing efficiency is found to be less negative when surface reflectance increases.

Heating rate profiles and radiative forcing due to a dust storm in the Western Mediterranean using satellite observations

We analyze the vertically-resolved radiative impact due to a dust storm in the Western Mediterranean. The dust plume travels around 3–5 km altitude and the aerosol optical depth derived by MODIS at 550 nm ranges from 0.33 to 0.52 at the overpass time (13:05 UT). The aerosol radiative forcing (ARF), forcing efficiency (FE) and heating rate profile (AHR) are determined throughout the dust trajectory in shortwave (SW) and longwave (LW) ranges. To do this, we integrate different satellite observations (CALIPSO and MODIS) and detailed radiative transfer modeling. The combined (SW + LW) effect of the dust event induces a net cooling in the studied region. On average, the FE at 22.4° solar zenith angle is −190.3 W m−2 and −38.1 W m−2, at surface and TOA, respectively. The corresponding LW/SW offset is 14% and 38% at surface and TOA, respectively. Our results at TOA are sensitive to the surface albedo in the SW and surface temperature in the LW. The absolute value of FE decrease (increase) in the SW (LW) with the surface albedo, resulting in an increasing LW/SW offset, up to 76%. The AHR profiles show a net warming within the dust layer, with a maximum value of 3.3 Kd-1. The ARF, FE and AHR are also highly sensitive to the dust optical properties in SW and LW. We evaluate this sensitivity by comparing the results obtained using two set of dust properties as input in our simulations: a) the prescribed dust model by Optical Properties of Aerosols and Clouds (OPAC) and; b) the dust optical properties derived from measurements of the size distribution and refractive index. Experimentally derived dust properties present larger SSA and asymmetry parameter in the SW than OPAC dust. Conversely, OPAC dust presents higher AOD in the LW range. These parameters drive the FE and AHR sensitivities in the SW and LW ranges, respectively. Therefore, when measured dust properties are used in our simulations: the ARF in the LW substantially reduces at surface and TOA (up to 57%); the absolute value of SW ARF is reduced by 15% at surface and an enhancement of 31% is observed at TOA; the AHR present less warming in the entire profile with deviations up to 53% within the dust layer, with respect to the results obtained using OPAC.

Satellite‐retrieved direct radiative forcing of aerosols over North‐East India and adjoining areas: climatology and impact assessment

ABSTRACTIn order to understand the climatic implications of atmospheric aerosols, top of atmosphere (TOA) shortwave (SW, 0.3–5 µm) fluxes and aerosol optical depth (AOD) at 550 nm retrieved simultaneously by clouds and the earth's radiant energy system (CERES) and moderate resolution imaging spectroradiometer (MODIS) instruments, respectively, are analysed over North‐East India and its adjoining areas for the period July 2002–December 2013. The aerosol‐free TOA flux obtained by establishing the linear regression between CERES SW TOA fluxes and MODIS AODs exhibits strong seasonality with peak values in monsoon and minimum in winter. Same seasonality is captured by the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model, but with difference in absolute values. SBDART code is used to extend instantaneous radiative forcing estimates into 24‐h averages. AOD over the North East India region with complex terrain shows altitudinal variation with maximum value at the lowest elevation site Dhaka and minimum value at the high‐altitude locations Shillong and Aizwal. In general, strong seasonality in AOD is observed with a peak in pre‐monsoon (March–May) and dip in post‐monsoon (October–November) at all the locations. The direct instantaneous TOA shortwave aerosol radiative forcing (SWARF) shows maximum values in pre‐monsoon over all the locations except at Guwahati, Banmauk, Aizawl, and Shillong. The lowest value of instantaneous SWARF is observed in post‐monsoon except at Banmauk and Shillong. Climatologically TOA diurnally averaged SWARF varies between −6.95 W m−2 in Aizawl to −20.39 W m−2 in Shillong. In general, the TOA SW forcing efficiency is highest in monsoon at all the locations. The radiative forcing efficiency is found to be less negative when surface reflectance increases.

Aerosol direct shortwave radiative forcing effect based on SBDART model in the Pearl River Delta, Guangdong (China)

Aerosols play an important role in the energy budget of the earth-atmosphere system. In this paper, we studied aerosol shortwave direct radiative forcing (DRF) effects in Pearl River Delta based on SBDART and a 'two-layer-single-wavelength' model. Simulation results indicated that the underlying surface type and solar zenith angle have significant impacts on aerosol radiative forcing. The comparison between aerosol radiative forcing effects on urban asphalt surface and vegetation shows cooling and warming effects of aerosol shortwave radiative forcing on urban asphalt are much more apparent than that on vegetation, implying aerosols over asphalt-predominated cities will impact the local climate. Then we estimated variations of average DRF and net radiation flux with solar zenith angle in the Pearl River Delta. DRF indicates warming at solar zenith angles of 0°, 20°, 40° and 60°, but cooling at 80°. Net radiation flux increases with a decrease in aerosol optical thickness (AOT) at low elevation, but with an increase in AOT above 5 km.

Long term characteristics of aerosols over Pune, central India - Effect on radiative forcing

Sun photometer (MICROTOPS II) and Sun-sky radiometer (POM-02, Prede) data for 2003-2013 period are used to study the optical and temporal characteristics of aerosols and their effect on radiative forcing over Pune, central India. The daily mean aerosol optical thickness at 500 nm (AOT500) and Angstrom exponent for the study period were 0.77 ± 0.22 0.92 ± 0.24 respectively. Short wave aerosol radiative forcing (ARF) computed for the year 2013 using Santa Barbara Discrete-ordinate Atmospheric Radiative Transfer (SBDART) model, showed ARF at surface (SUF) and top of the atmosphere (TOA) was negative indicating dominance of scattering type aerosols. AOT, single scattering albedo (SSA) and asymmetry parameter (ASP) derived from Sun sky radiometer showed more AOT and ASP and less SSA during April / May to August indicating prevalence of absorbing soil dust in this period. Atmospheric heating varied from +14.3 to 20.5 W/m2 and was more in monsoon (+ 35.4 W/m2). ARF at SUF was found to be a strong function of AOT500 (R2= 0.90). The radiative forcing efficiency inferred to scattering nature of aerosols at SUF (-66.2 W/m2/ AOT), TOA (- 0.2 W/m2 / AOT) and atmosphere (ATM) (+ 66.0 W/m2 /AOT). The atmospheric heating rates varied from 0.94 K/day in post monsoon to 2.78 K/day in monsoon. Monthly, seasonal and diurnal variability of AOT500 and with role of meteorological factors are also discussed.

Thirteen Years of Aerosol Radiative Forcing in Southwestern Iberian Peninsula

ABSTRACTNotable uncertainties in the aerosol impact on the Earth's radiative budget still remain, mainly at local scales. This study aims to accurately quantify the shortwave aerosol radiative forcing (ARF) and its efficiency (ARFE) at Evora (Portugal, Southwestern Europe). This is an interesting region affected by different aerosol types and sources. Towards this goal, ARF and ARFE are calculated for Evora during thirteen years (2003–2015), which is the longest period ever analyzed for this region. For this calculation, the most updated AERONET data have been used to simulate irradiance with the libRadtran code. Thus, 1676 daily values were analyzed, obtaining a mean ARF of –7.4 W m–2 at the Earth's surface (SURF) and –1.1 W m–2 at the top of the atmosphere (TOA), indicating a cooling of the Earth–atmosphere system. The difference in ARF between TOA and SURF indicates a mean net gain of 6.3 W m–2 for the atmosphere. While no ARF seasonal pattern is observed at the TOA, a clear seasonal cycle is detected at the SURF with the most negative values occurring in summer. On the other hand, ARFE shows mean values of –62.7 W m–2 τ–1 at the SURF and –14.3 W m–2 τ–1 at the TOA. Results show that ARFE highly depends on the aerosol single scattering albedo, showing opposite behavior at the SURF and at the TOA. The large temporal variability detected in the aerosol properties in the region of study confirms the general need for long time series of measurements to achieve an accurate estimation of the ARF and ARFE.

Wintertime characteristics of aerosols over middle Indo-Gangetic Plain: Vertical profile, transport and radiative forcing

Winter-specific characteristics of airborne particulates over middle Indo-Gangetic Plain (IGP) were evaluated in terms of aerosol chemical and micro-physical properties under three-dimensional domain. Emphases were made for the first time to identify intra-seasonal variations of aerosols sources, horizontal and vertical transport, effects of regional meteorology and estimating composite aerosol short-wave radiative forcing over an urban region (25°10′–25°19′N; 82°54′–83°4′E) at middle-IGP. Space-borne passive (Aqua and Terra MODIS, Aura OMI) and active sensor (CALIPSO-CALIOP) based observations were concurrently used with ground based aerosol mass measurement for entire winter and pre-summer months (December, 1, 2014 to March, 31, 2015). Exceptionally high aerosol mass loading was recorded for both PM10 (267.6±107.0μgm−3) and PM2.5 (150.2±89.4μgm−3) typically exceeding national standard. Aerosol type was mostly dominated by fine particulates (particulate ratio: 0.61) during pre to mid-winter episodes before being converted to mixed aerosol types (ratio: 0.41–0.53). Time series analysis of aerosols mass typically identified three dissimilar aerosol loading episodes with varying attributes, well resemble to that of previous year's observation representing its persisting nature. Black carbon (9.4±3.7μgm−3) was found to constitute significant proportion of fine particulates (2–27%) with a strong diurnal profile. Secondary inorganic ions also accounted a fraction of particulates (PM2.5: 22.5%; PM10: 26.9%) having SO4−2, NO3− and NH4+ constituting major proportion. Satellite retrieved MODIS-AOD (0.01–2.30) and fine mode fractions (FMF: 0.01–1.00) identified intra-seasonal variation with transport of aerosols from upper to middle-IGP through continental westerly. Varying statistical association of columnar and surface aerosol loading both in terms of fine (r; PM2.5: MODIS-AOD: 0.51) and coarse particulates (PM10: MODIS-AOD: 0.53) was found influenced by local meteorology (boundary layer and humidity) and aerosol vertical profile. A gradual increase in aerosol vertical profile (surface to 4.9km) was evident with dominance of polluted continental, polluted dust and smoke at lower altitude. Presence of mineral dusts in higher altitude during later phase was linked with its transboundary transport, originating from western dry regions. Conclusively, winter-specific short-wave aerosol radiative forcing revealed an ATM warming effect (31–47Wm−2) while cooling both at TOA (−20 to −32Wm−2) and SUF (−51 to −80Wm−2) with significant level of intra-seasonal variations in heating rates (0.86–1.32Kday−1).

Satellite‐based shortwave aerosol radiative forcing of dust storm over the Arabian Sea

AbstractDust storm events over the Arabian Sea (AS) have been detected using Moderate Resolution Imaging Spectroradiometer (MODIS) data. Shortwave Aerosol Radiative Forcing (SWARF) due to dust storm is estimated using synchronous observation of Clouds and Earth's Energy System (CERES) and MODIS aerosol optical depth (AOD). Study established a relationship between them as SWARF = −39.12 × AOD − 16.53 (0.4 ≤ AOD ≤ 4.0) with r2 = 0.96. The developed relation can be used for quick, independent estimation of instantaneous SWARF for dust storm over the AS. The relationship can be used to explore the possible effect of dust on climate modulation in this region.

Long term characterization of aerosol optical properties: Implications for radiative forcing over the desert region of Jodhpur, India

AOT data for eight years period (2004–2012) using the MICROTOPS II Sun photometer has been used to study the wavelength dependent optical characteristics of aerosols over Jodhpur, situated in the desert region in NW India. The daily mean AOT at 500 nm for the present study period was 0.66 ± 0.14 with an average Angstrom exponent as 0.71 ± 0.20. Linear regression analysis of monthly AOT and Angstrom Exponent indicated an increasing trend of both. Seasonal variations of daily AOT and α as well as spectral dependence of seasonal mean AOT are presented. Diurnal variation of AOT and α in different season is studied. Impact of dust storm events on the aerosol characteristics over Jodhpur during the study period is studied. AOT values derived from MICROTOPS II were cross checked with Sun Sky Radiometer (Model POM-01, Prede Inc.) data for the period from May 2011 to April 2012 and were found to be in good agreement. Short wave aerosol radiative forcing (ARF) was computed for one year period of May 2011 to April 2012. Spectral variation of AOT, SSA and ASP showed more AOT and ASP during pre monsoon period when SSA was comparatively low; indicating towards more prevalence of coarse size absorbing dust in this period. The ARF at SUF and TOA was negative during all the seasons indicating dominance of scattering type aerosols mainly dust particles whereas that at ATM was positive in all the seasons indicating heating of the atmosphere, especially more during pre monsoon (+40.5 W/m2) than during rest of the year (+19.5 W/m2). A high degree of correlation between ARF at the SUF with AOT (R2 = 0.94) indicated that ARF is a strong function of AOT. The radiative forcing efficiency inferred to scattering nature of aerosols at SUF (−4.2 W/m2/AOD) and TOA (−63.2 W/m2/AOD) indicating cooling at surface and top of the atmosphere whereas, there was warming of the atmosphere in between (+59 W/m2/AOD). The atmospheric heating rates varied from 0.49 K/day in post monsoon to 1.13 K/day in pre monsoon. This study has enabled us to understand the long term nature and physical characteristics of atmospheric aerosols over Jodhpur.

Dust aerosol radiative effects during summer 2012 simulated with a coupled regional aerosol–atmosphere–ocean model over the Mediterranean

Abstract. The present study investigates the radiative effects of dust aerosols in the Mediterranean region during summer 2012 using a coupled regional aerosol–atmosphere–ocean model (CNRM-RCSM5). A prognostic aerosol scheme, including desert dust, sea salt, organic, black-carbon and sulphate particles, has been integrated to CNRM-RCSM5 in addition to the atmosphere, land surface and ocean components. An evaluation of this aerosol scheme of CNRM-RCSM5, and especially of the dust aerosols, has been performed against in situ and satellite measurements, showing its ability to reproduce the spatial and temporal variability of aerosol optical depth (AOD) over the Mediterranean region in summer 2012. The dust vertical and size distributions have also been evaluated against observations from the TRAQA/ChArMEx campaign. Three simulations have been carried out for summer 2012 with CNRM-RCSM5, including the full prognostic aerosol scheme, only monthly-averaged AOD means from the aerosol scheme or no aerosols at all, in order to focus on the radiative effects of dust particles and the role of the prognostic scheme. Surface short-wave aerosol radiative forcing variability is found to be more than twice as high over regions affected by dust aerosols, when using a prognostic aerosol scheme instead of monthly AOD means. In this case downward surface solar radiation is also found to be better reproduced according to a comparison with several stations across the Mediterranean. A composite study over 14 stations across the Mediterranean, designed to identify days with high dust AOD, also reveals the improvement of the representation of surface temperature brought by the use of the prognostic aerosol scheme. Indeed the surface receives less radiation during dusty days, but only the simulation using the prognostic aerosol scheme is found to reproduce the observed intensity of the dimming and warming on dusty days. Moreover, the radiation and temperature averages over summer 2012 are also modified by the use of prognostic aerosols, mainly because of the differences brought in short-wave aerosol radiative forcing variability. Therefore this first comparison over summer 2012 highlights the importance of the choice of the representation of aerosols in climate models.