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

Atmospheric Environment 114 (2015) 66e74

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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv

Long term characterization of aerosol optical properties: Implications for radiative forcing over the desert region of Jodhpur, India V. Vizaya Bhaskar a, P.D. Safai b, *, M.P. Raju b a b

India Meteorological Department, Pune, India Indian Institute of Tropical Meteorology, Pune, India

h i g h l i g h t s
Long term continuous data on aerosol optical properties at a desert location in India.
Dominance of coarse absorbing dust particles in AOT, especially during pre-monsoon.
Pronounced diurnal variation in AOT and a during all seasons.
ARF indicated cooling at SUF and TOA and warming of ATM.
ARF at ATM twice more in pre monsoon than rest of the year.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 December 2014 Received in revised form 6 February 2015 Accepted 21 May 2015 Available online xxx

AOT data for eight years period (2004e2012) 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 a as well as spectral dependence of seasonal mean AOT are presented. Diurnal variation of AOT and a 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 (þ40.5 W/m2) than during rest of the year (þ19.5 W/m2). A high degree of correlation between ARF at the SUF with AOT (R2 ¼ 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 (þ59 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. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Aerosol optical thickness € m exponent Angstro Temporal variation Dust storm Radiative forcing Heating rates

1. Introduction

* Corresponding author. E-mail address: [email protected] (P.D. Safai). http://dx.doi.org/10.1016/j.atmosenv.2015.05.043 1352-2310/© 2015 Elsevier Ltd. All rights reserved.

The effect of aerosols on climate is one of the largest uncertainties in current global climate models and lack of extensive and reliable measurements in most regions, makes it difficult to quantify the global impact of aerosols on Earth's climate (Hansen

V.V. Bhaskar et al. / Atmospheric Environment 114 (2015) 66e74

et al., 1997). Aerosols interact with solar and earth radiation directly and indirectly leading to the respective aerosol radiative forcing (ARF). The magnitude and sign of ARF is determined by nature, size distribution and chemical characteristics of aerosol species. Mineral dust aerosols significantly alter the magnitude and sign of the ARF by altering the aerosol single scattering albedo. Several studies and field experiments have documented the characteristics and impact of mineral dust aerosols (Duce, 1995; Miller and Tegen, 1998; Prospero et al., 2002; Tegen et al., 2004; Zhu et al., 2007). Such studies over Indian subcontinent are rather few and limited to short term observations only (Dey et al., 2004; Deepshikha et al., 2006; Moorthy et al., 2007; Pandithurai et al., 2008; Singh et al., 2010; Sharma et al., 2012; Harikishan et al., 2015) whereas studies with large data sets conducted for longer periods are almost non existent. Dust storms are common in the northwestern part of the Indian subcontinent which is one of the primary sources of mineral dust aerosols in south Asia (Washington et al., 2003; Badarinath et al., 2007; Gautam et al., 2009). Dust aerosols from these source regions are driven by winds to Indo Gangetic Plains and further as a land atmosphere interaction (Tegen et al., 1996; Ginoux et al., 2001). During their traverse, dust aerosols encounter with anthropogenic aerosols of different size and nature over different regions and mix with them and as a result the aerosol optical characteristics, earth's atmosphere radiation energy balance and ambient air quality of the respective regions get modified (Nastos et al., 2011). Therefore the study of aerosol optical characteristics over desert area in the north/western India will give important information on the anthropogenic, natural as well as mixed aerosol characteristics. The present investigation, for the first time, reports the temporal variation of aerosol optical thickness (AOT) along with other optical characteristics during different seasons over Jodhpur, located to the east of Thar Desert in western Indian region for the period during October 2004 to April 2012 using hand held MICROTOPS-II sun photometer. The station joined the SKYNETWORK from 2011. The optical properties of aerosols such as AOT, single scattering albedo (SSA) and asymmetry parameter (ASP) obtained from Sun Sky Radiometer during May 2011 to April 2012 are used in SBDART model for the estimation of aerosol radiative forcing and atmospheric heating rates. 2. Details on location and methodology Jodhpur (26180 N, 73 010 E, 217m) is located in Rajasthan in the Great Indian Desert also known as Thar Desert in the north western India and experiences dry and hot weather and scanty rainfall. Summer temperature peak goes beyond 41  C during daytime and the lowest temperature in winter season reaches 5  C in the night. The rainy season for Jodhpur is during July and August when higher humidity levels exist with annual precipitation of about 30 cm and annual relative humidity about 46%. During the AprileJune period, frequent dust/sand storms from western and north-western desert regions cause large scale loading of dust aerosols over Jodhpur. This causes considerable reduction in visibility and in the radiation flux reaching the surface (Singh et al., 2005). In the summer monsoon season, western parts of Rajasthan are convergent region of pollution from peninsular India and marine influx from the Arabian Sea and with the onset of monsoon rains, heavy dust loading significantly diminishes due to aerosol washout from the atmosphere. The sources of aerosols over Jodhpur are therefore dust from deserts in the west and north of Jodhpur, sea salt from Arabian Sea as well as anthropogenic particles from local as well as distant sources. Thus this region provides an excellent setting for studying the optical and radiative characteristics of anthropogenic as well as natural aerosols over the region.

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AOT data for the period of 87 months for the eight year period during October 2004eApril 2012 at Jodhpur is used in the present study. Instantaneous cloud free observations inferred from hand held MICROTOPS II Sun photometer (Solar Light Co., USA) at around 03, 05, 06, 07, 09, 11 & 12 UTC are used to study the wavelength dependent optical characteristics of aerosols. The cloud free day implies that the sun was completely visible in the whole or intervals of the day and conducive for taking MICROTOPS II observations. MICROTOPS II sun photometer instrument measures the total columnar AOT from the direct sun radiation centered at 368, 500, 675, 778 and 1028 nm and subtracts the Rayleigh scattering component. Angstrom exponent (a) in the UVevIS range is calculated from optical thickness values at wavelengths 368 and 675 nm (Angstrom, 1929). As the MICROTOPS-II instrument measured AOT values are susceptible to occasional spurious values due to erratic behavior of filter and various other components, extreme outliers are rejected through a simple statistical technique. Daily data points outside three times of standard deviation, on either side of long term mean for AOT at 500 nm and a were excluded barring dust storm event days data. The accuracy of measurements for precision and consistency of the MICROTOPS-II instruments are discussed in detail by Srivastava et al. (2006). The present dataset is unique not only, because the AOT measurements in the desert regions are very sparse (Holben et al., 2001), but also because of its relatively long-term continuous observations. AOT and a values obtained from Sun Sky Radiometer (Model POM-01, Prede Inc.) for the period from May 2011 to April 2012 have been compared with values derived from MICROTOPS II for validation of the data. The sun/sky radiometer (Model: POM-01 of Prede, Japan), installed at Jodhpur is capable for measuring direct solar and diffuse sky radiance at seven spectral channels from visible to near-infrared spectral regions (340, 380, 400, 500, 675, 870 and 1020 nm), with a half band width of 3 nm for 340 nm wavelength and 10 nm for other wavelengths. The aerosol optical parameters such as AOT, SSA and ASP are derived using Skyrad.Pack (version 4.2) radiative transfer code (Nakajima et al., 1996). Sky radiometer measures diffuse radiation at various scattering angles from the Sun at different wavelengths in narrow band. The measured sky spectral radiances can be used to obtain different optical and size related properties of aerosols in the total atmospheric column. The operational details and errors associated with the instrument are already described elsewhere (Pandithurai et al., 2008; Ningombam et al., 2014). 3. Results and discussion 3.1. Monthly variation of AOT and Angstrom Exponent The daily mean AOT at 500 nm for the present study period (Oct 2004 to Apr 2012) was 0.66 ± 0.14 with an average value of a as 0.71 ± 0.20 (Since AOT varies with season/month, only the years that contain all months data were considered). Excluding the dust storm events, the above figures were 0.62 ± 0.19 and 0.72 ± 0.25, respectively. Daily maximum AOT and minimum a for the study period were 3.17 and 0.21 whereas; the instantaneous maximum AOT and minimum a values were 7.51 and 0.15, respectively indicating significant contribution of coarse mode aerosols. Instantaneous AOT at 500 nm showed fluctuation from 7.51 to 0.37, whereas daily and monthly average values were between 3.17 to 0.33 and 1.11 to 0.27, respectively. Broad range of instantaneous a value (1.49 to 0.15) over Jodhpur suggests that accumulation mode aerosols either from local sources or from distant sources through transportation are combined with coarse mode dust particles from surrounding desert area. The monthly mean AOT averaged for the period of Oct 2004eApr 2012 (Fig. 1) showed an

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V.V. Bhaskar et al. / Atmospheric Environment 114 (2015) 66e74 1.2 AOT

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Fig. 1. Mean monthly variation of AOT at 500 nm and Angstrom Exponent during 2004e12.

increase from January (0.47) to July (0.88) and later on decreased till January while the mean monthly a for the same period exhibited almost an inverse trend with decrease from January (0.78) to May (0.40) and an increase there onwards till December (0.91). Inverse relation between AOT and a has been reported by Alam et al. (2012) Lahore and Karachi; Pandithurai et al. (2008) over Delhi and Verma et al. (2013) over Jaipur. The high values of AOT and low values of a in pre-monsoon month infer to the atmospheric loading of natural desert dust aerosols with dominance of coarse mode particulate. Monthly mean AOT and a value in the individual years are plotted in Fig. 2aeb. Both AOT & a displayed an increasing trend, indicating enhanced aerosol load over Jodhpur with increase in anthropogenic fine mode particles in the atmosphere along with prevailing coarse dust particles. This considerable variation in AOT and a throughout the year is expected to show impact on the average aerosol radiation forcing during different seasons at Jodhpur.

variation of daily AOT and a revealed three characteristic features of aerosols over Jodhpur. In post monsoon to winter seasons, highest frequency of low AOT values was associated with highest frequency of high values of a (0.8e1 & > 1.0) indicating dominant anthropogenic aerosols in the atmosphere. In the pre monsoon season highest frequency of high AOT values (0.8e1 & >1.0) corresponded with lowest frequency of high a indicating dominance of coarse mode desert dust aerosols. In the monsoon season, moderate frequency of high AOT was associated with moderate frequency of high a indicating mixed aerosols over Jodhpur as the coarse particles get partially settled due to occasional rains. Soni et al. (2012) reported highest AOT values over Jodhpur during pre-monsoon months. Ranjan et al. (2007) have also reported enhanced AOT values during pre monsoon months over Rajkot which lies in the south of Thar desert. 3.3. Spectral variation of AOT, SSA and ASP

3.2. Seasonal variation of AOT and Angstrom Exponent Seasonal variation of daily AOT and a is shown in Fig. 3. The four seasons are categorized as pre-monsoon (March to July), monsoon (AugusteSeptember), post-monsoon (OctobereDecember) and winter (JanuaryeFebruary). As per India Meteorological Department convention, the monsoon season is June to September. But for Jodhpur, the seasonal rains occur during July and August and the region continue to be affected by dust storms till July therefore the first half of monsoon i.e. June and July months are included in premonsoon whereas, August and September are considered to represent monsoon at Jodhpur in the present study. Moorthy et al. (2007) have also reported strong dust raising winds from April to July over Jodhpur that are responsible for the exhibition of desert/ arid region characteristics. AOT values at 500 nm showed decreasing trend from pre-monsoon to winter season whereas Angstrom Exponent showed increasing trend from pre-monsoon to winter. As already stated in methodology section, we have considered June and July as part of pre-monsoon for Jodhpur, due to prevalence of dust storms in these months and also due to the more occurrence of seasonal rains during August and September. Subsequently, the AOT values are also high during June and July. On the other hand with the commencement of rains in late July/August months, aerosols start getting washed out from the atmosphere and as consequence the AOT values are on decreasing trend. Considering the fact that the divergent AOT/a characters in two half's of the monsoon season get muffled if AOT/a is studied for the entire four months of monsoon season; our present classification of pre-monsoon and monsoon is justified. Analysis of seasonal

Fig. 4 shows the spectral variation of seasonal mean AOT for 30 seasons during the study period. Spectral variation of AOT for any given season was found to be relatively low compared to its inter seasonal variation which is highly dependent on the coarse-to-fine ratio, the particle size, the columnar size distribution, mixing processes with anthropogenic aerosols in the atmosphere and intensity of various phenomena like dust storms, volcano etc that influence the region of study. The maximum and minimum variation of AOT across the 368e1028 nm spectrum for 30 seasons was 0.36 and 0.16 where as the above figures across different seasons were 0.88 and 0.29, respectively. Among the seasons, higher variation of AOT was found for monsoon and minimum for winter season. Spectral variation of AOT, SSA and ASP at seven wavelengths (340, 380, 400, 500, 675, 870 and 1020 nm) as obtained from Sunsky radiometer observations during the one year period of May 2011 to April 2012 is shown in Fig. 5. SSA is a measure of the reflectivity of a particle and is an important optical characteristic of aerosols in assessing their radiative effects. SSA helps determine whether aerosols have a heating or cooling effect on climate in their interaction with solar radiation. ASP is defined as the cosineweighted average of the phase function, where the phase function is the probability of radiation being scattered in a given direction. Values of ASP can range from 1 for 180-degrees backwards scattering to þ1 for complete forward scattering. As seen from Fig. 5, the respective spectral variation of each of these optical parameters followed similar trends in all the months. AOT and ASP was more whereas SSA was less in all wavelengths during

V.V. Bhaskar et al. / Atmospheric Environment 114 (2015) 66e74 1.2

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Fig. 2. Variation of (a) AOT at 500 nm and (b) Angstrom Exponent during 2004e12.

March to July. At the same time, AOT and ASP were less whereas SSA was more in all wavelengths during August to February. For example at 500 nm, values of AOT, ASP and SSA were 0.593, 0.738 and 0.872, respectively during March to July; but these values were 0.376, 0.702 and 0.912 during August to February. Less AOT and ASP values along with high SSA indicates towards the more presence of scattering type aerosols like sulfate, non absorbing dust and sea salt, especially during monsoon through winter period (August to February). Whereas, more AOT and ASP values and low SSA during pre monsoon (March to June) signifies the presence of more absorbing type aerosols such as hematite dust. Angstrom Exponent showed higher values during August to February (0.993) than during March to July (0.570) which shows more prevalence of coarse size aerosols during pre monsoon season. Similar results have been reported by Ranjan et al. (2007) and Alam et al. (2011). 3.4. Diurnal variation of AOT and Angstrom Exponent Expressing the instantaneous AOT and a value in a day as a percentage difference of the daily mean and averaging the computed percentages season wise for each hour the diurnal variation of AOT and a for each season is studied as suggested by Wang et al. (2004). Diurnal variation of AOT and a for four seasons is shown in Fig. 6aeb. As in the case of seasonal variation of daily AOT, the first two months of monsoon season (June and July) are clubbed with pre-monsoon

season of March to May for the reasons explained earlier. Relatively higher values of AOT were found during 10 AM to 3 PM. The AOT was low in the morning (8 AM) and gradually increased and reached peak in the afternoon (1230 PM) and decreased thereafter to reach lowest value in the evening (0530 PM) in all the seasons (Fig. 6a). Smirnov et al. (2002) have reported low AOT values in morning and high values during afternoon at the sites close to desert dust sources in Saudi Arabia and Mongolia. However in the monsoon season, slightly different feature was observed with AOT showing peak at around 10 AM and then gradually decreasing to reach lowest value at 05.30 PM. Increased aerosol input through lifting loose soil and other particulates and formation of clouds, caused by surface heating and vertical convective mixing and increase in local anthropogenic activity are attributed to increase of AOT from morning to afternoon. Whereas reduction on surface heating and consequent less vertical mixing during afternoon to evening hours might have led to decrease in AOT. Diurnal variation of a indicated high values in the morning (8 AM) which gradually decreased throughout the day and reached lowest at 0530 PM during post monsoon and winter seasons whereas during pre monsoon and monsoon seasons, a values increased from morning (8 AM) and peaked around 12.30 PM in the noon and then decreased gradually till 0530 PM. Overall these diurnal features indicate towards dominance of fine size aerosols in total AOT during morning till noon hours in all the seasons whereas coarse size aerosols contributed more to AOT during afternoon to evening hours,

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are shown in Fig. 7. Mean AOT for these 117 dust storm days was 0.84. Highest day average and highest instantaneous AOT were observed to be 3.174 and 7.505, respectively. Similarly mean a and lowest day average and lowest instantaneous a were 0.514, 0.046 and 0.153, respectively. The higher AOT and negative a indicate coarse mode dominance associated with desert dust aerosols. Similar results related with aerosol optical characteristics during dust storms have been reported by several studies (Pandithurai et al., 2008; Sharma et al., 2012; Verma et al., 2013; Alam et al., 2014).

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Fig. 3. Seasonal variation of (a) AOT at 500 nm and (b) Angstrom Exponent during 2004e12.

especially more during evening. This feature might be related with variations in the local meteorological parameters (Verma et al., 2013). 3.5. Dust strom events Surface synoptic eye observations of 117 dust/sand storms in and around Jodhpur have been picked up from the archives of India Meteorological Department (IMD). Aerosol optical characteristics have been studied for these dust storm days. Maximum instantaneous AOT and instantaneous minimum Angstrom Exponent values

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The AOT values obtained from the Sun Sky Radiometer (Model POM-01, Prede Inc.) for the period from May 2011 to April 2012 have been compared with AOT values derived from MICROTOPS II sun photometer. In the 639 instantaneous AOT observations that are compared, the AOT values of MICROTOPS II were found to be higher than Sun Sky Radiometer values in all but only two cases and in 511 (80%) cases the difference was within 0.4. The maximum and minimum difference values were 1.052 and 0.016, respectively. The scatter plot shown in Fig. 8 indicates a correlation of 0.81 at 5% level of significance, suggesting that aerosol measurements with MICROTOP II and Sun Sky Radiometer are in good agreement.

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Fig. 4. Spectral variation of AOT (MICROTOP e derived) during 2004e12. (PM: Post Monsoon, MN: Monsoon, WI: Winter and PRM: Pre Monsoon).

Aerosol radiative forcing (ARF) is defined as the difference in the net solar flux (Wm2), with and without aerosols. The direct ARF comprises both scattering and absorption of solar radiation by atmospheric aerosols that may show negative or positive radiative forcing depending upon the nature of aerosols, their sources and distribution, and surface albedo. A negative value of TOA forcing implies cooling of the atmosphere due to the loss of radiation to space by enhanced backscattering due to aerosols. Whereas, the absorbing aerosols result in positive radiative forcing that promotes warming of the atmosphere. ARF at SUF is always negative because of the attenuation of solar radiation by aerosols. The difference between the radiative forcing at the TOA and at the SUF defines the atmospheric radiative forcing (ATM) which represents the amount of energy trapped by aerosols within the atmosphere that heats up the atmosphere (Sharma et al., 2012). In the present study, ARF calculations are computed in the shortwave spectrum (0.2e4.0 mm) separately for SUF, ATM and TOA using the Santa Barbara Discrete-ordinate Atmospheric Radiative Transfer (SBDART) model developed at University of California, Santa Barbara (Ricchiazzi et al., 1998). SBDART computes planeparallel radiative transfer calculations both in clear and cloudy sky conditions within the Earth's atmosphere and at the surface. Several studies have extensively used this model for ARF calculations over India (Moorthy et al., 2005; Pandithurai et al., 2008; Singh et al., 2010; Srivastava et al., 2014). ARF is strongly dependent upon spectral AOT, SSA and ASP as well as surface albedo and meteorological conditions. For the estimation of shortwave ARF, along with the observed spectral AOT, SSA and Asymmetry parameter at 500 nm; satellite derived ozone and water vapor from OMI/AURA level 2 and MODIS level 3 data respectively were used inputs for the radiative transfer model. These input parameters were used to run the model for every change of 5 solar zenith angles initially from 0 to 89 once with aerosol condition and another without aerosol condition. Hence the output of the radiative transfer model is the monthly average of solar flux at TOA, SUF and ATM. The uncertainties in shortwave ARF calculations could arise from various assumptions, such as model atmosphere as well as

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Fig. 5. Month wise spectral variation of Sun-sky radiometer derived AOT, SSA and ASP during 2011e12.

uncertainties in surface albedo, molecular scattering absorption, and errors in measured optical parameters. The overall uncertainty in ARF calculations does not exceed 20% (Pant et al., 2006). The seasonally averaged values of ARF at TOA, SUF, and ATM were estimated for the one year periods (May 2011eApril 2012). As seen from Fig. 9a, the SUF and TOA forcing was negative during all the seasons indicating dominance of scattering type aerosols mainly dust particles. However, the negative forcing at TOA in premonsoon was comparatively less which denotes possible presence of absorbing dust particles as well as impact of combustion activities in the local surroundings. The ATM forcing was positive in all the seasons indicating heating of the atmosphere, especially more during pre-monsoon (March to July). The SW ARF respectively for TOA, SUF and ATM over Jodhpur was 3.65, 44.15 and 40.50 W/m2 during pre-monsoon; 7.79, 31.75 and 23.95 W/m2 during monsoon; 6.78, 24.24 and 17.46 W/m2 during post-monsoon

and 5.85, 24.06 and 18.20 W/m2 during winter. Fig. 9b shows the monthly variation of SW ARF at TOA, SUF and ATM over Jodhpur during the study period. It can be seen that ARF at ATM was nearly two times more during March to July (Mean 40.5 W/m2) than that during August to February months (Mean 19.5 W/m2). Variation of monthly mean AOT at 500 nm closely followed the variation of monthly mean ARF at ATM as seen from Fig. 10 and both showed significantly good correlation with each other (R2 ¼ 0.82, p ¼ 0.0001). Similarly, AOT at 500 nm correlated well with ARF at SUF (R2 ¼ 0.94). A high degree of correlation between ARF and AOT indicates that ARF is a strong function of AOT. The slope of the linear regressions defines the aerosol forcing efficiency that is the rate of forcing per unit AOT and represents the effectiveness of the composite aerosols in perturbing the radiative balance (Santos et al., 2008). In the present study, as shown in Fig. 11, the radiative forcing efficiencies were found to be 4.2, 63.2 and þ 59 W/m2/AOD

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Fig. 8. Scatter plot of MICROTOPS & Sun Sky Radiometer AOT at 500 nm.

March 2012 over Lahore, Karachi, Delhi and Jodhpur. They have reported a small heating at the TOA at Delhi and Jodhpur during dust event days indicating absorbing nature of dust aerosols over the station. Further, a significant cooling was observed at the surface at all the stations which were more pronounced during dust event. Also, significant heating in the atmosphere was observed at all the stations. Alam et al. (2014) reported maximum AOT values during March 2012 at Kuwait, Bahrain, Qatar, and Saudi Arabia with AOT values of 4.9, 4.4, 4.3, and 4.9, respectively. Whereas, in Oman, the Arabian Sea, and Iran; maximum AOT values occurred reaching 4.5, 5, and 5, respectively in this period. The dust storm then spread across Pakistan, passing through Multan, Faisalabad, and Lahore where maximum AOT values of 2.1, 2.6, and 2.7, respectively, were attained. The shortwave ARF values at Lahore on dusty and non-

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for TOA, SUF and ATM, respectively. The radiative forcing efficiency indicates scattering nature of aerosols at SUF and TOA which indicates cooling at surface and top of the atmosphere whereas, there is warming of the atmosphere in between. Pandithurai et al. (2008) reported consistent increase in cooling at surface from 39 W/m2 in March to 99 W/m2 in June during premonsoon season of 2006 over Delhi, while the atmospheric heating ranged from 27 Wm2 (March) to 123 W/m2 (June) since dust events lead to enhanced heating rates in the lower atmosphere. Srivastava et al. (2014) studied the impact of dust storm in

2

Fig. 6. Season wise diurnal variation of (a) AOT at 500 nm and (b) Angstrom Exponent during 2004e12.

-60 -80

Fig. 9. Seasonal (a) and monthly (b) variation of SW ARF at TOA, SUF and ATM during 2011e12.

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2

50

ARF --0-- AOT500 nm

0.9

45

ARF at ATM: AOT500 nm

0.8

40

r = 0.89, p = 0.0001 0.7

35

0.6

30

AOT500 nm

1.0

55

ARF at ATM (W/m )

73

0.5

25

0.4

20 0.3 15 0.2 May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr

Fig. 10. Monthly variation of ARF at ATM and AOT at 500 nm during 2011e12.

dusty days respectively ranged between50 W/m2 and194 W/m2 at the earth's surface, and between 31 W/m2 and 105 W/m2 at the top of the atmosphere (TOA). The average heating rate of the atmosphere due to aerosols is defined as

vT=vt ¼ g=Cp DF=DP where vT/vt is the heating rate (K/day), Cp is the specific heat capacity of air at constant pressure, g is the acceleration due to gravity and DP is the atmospheric pressure difference between top and bottom boundary of each layer (Liou, 2002). In this study, we calculated the aerosol heating rate for the whole atmospheric column, which is the difference in heating rates between an aerosolladen and an aerosol-free atmosphere. The mean heating rates during pre monsoon, monsoon, post monsoon and winter seasons were 1.13, 0.67, 0.49 and 0.51 K/day, respectively. Heating rates showed good positive correlation with AOT during all the seasons (R2 ¼ 0.79, p ¼ 0.0001) whereas Angstrom Exponent showed significant negative correlation with heating rates (R2 ¼ 0.59, p ¼ 0.003). This feature indicates towards impact of coarse size aerosols mainly absorbing dust particles on the warming of the atmosphere over Jodhpur. Srivastava et al. (2014) have reported significant heating of the atmosphere over Jodhpur during dust storm in March 2012 (ATM forcing of 71.6 W/m2) corresponding to heating rate of 2 K/day. Pandithurai et al. (2008) have reported higher atmospheric heating (0.6e2.5 k/day) at Delhi during pre monsoon season attributed to enhanced atmospheric absorption in

dry season. Moorthy et al. (2007) have Estimated that the dust absorption over Indian desert of Thar in Rajasthan would lead to lower atmospheric warming of 0.7e1.2 K/day. Such large heating of the atmosphere due to dust aerosols have implications for regional climate and monsoon circulation (Ramanathan et al., 2001). 4. Conclusions Aerosol optical characteristics have been characterized using the MICROTOPS II Sun photometer observations over Jodhpur for the years 2004e2012. Also, the impact of optical parameters on radiative properties and atmospheric heating were studied for 2011-12 data using Sun Sky Radiometer data. The main findings of the investigation are: The daily mean AOT at 500 nm for 2004e2012 was 0.66 ± 0.14 and the mean Angstrom exponent was 0.71 ± 0.20. Seasonal variation of daily AOT and Angstrom exponent showed dominance of anthropogenic aerosols in post monsoon to winter season and that of coarse mode desert aerosols in the pre monsoon. The enhanced AOT and negative a infers to the abundance of large size desert dust aerosols in the atmosphere during intense dust storm periods. Diurnal variation of AOT and a showed dominance of fine size aerosols in total AOT during morning till noon hours in all the seasons whereas coarse size aerosols contributed more to AOT during afternoon to evening hours. Spectral variation of optical properties showed less AOT and ASP along with high SSA and a during monsoon through winter indicating more presence of

Fig. 11. Correlation between AOT at 500 nm with ARF values at TOA, SRF and ATM. (The slope of the regressions shows the radiative forcing efficiency).

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scattering type coarse aerosols whereas during pre monsoon, more AOT and ASP and less SSA and a showed dominance of fine size absorbing aerosols. The ARF at SUF and TOA was negative during all the seasons indicating dominance of scattering type aerosols mainly dust particles. However, the negative forcing at TOA in premonsoon was comparatively less inferring to possible presence of absorbing dust particles as well as impact of combustion activities in the local surroundings. The ATM forcing was positive in all the seasons indicating heating of the atmosphere, especially more during premonsoon. ARF at ATM was nearly two times more during pre monsoon than that during rest of the year which reflected in the atmospheric heating rate that was 1.13 K/day during pre monsoon as compared to around 0.55 K/day during other seasons. Acknowledgments The authors are thankful to the Additional Director General of Meteorology (Research), India Meteorological Department, Pune for supplying the required sun photometer data and for guiding, encouraging and providing the required facilities for the study. Thanks are also due to Director, Indian Institute of Tropical Meteorology, Pune for encouragement in carrying out this study. References Alam, K., Trautmann, T., Blaschke, T., 2011. Aerosol optical properties and radiative forcing over mega city Karachi. Atmos. Res. 101, 773e782. Alam, K., Trautmann, T., Blaschke, T., Majid, H., 2012. Aerosol optical and radiative properties during summer and Winter season over lahore and karachi. Atmos. Environ. 50, 234e245. Alam, K., Trautmann, T., Blaschke, T., Subhan, F., 2014. Changes in aerosol optical properties due to dust storms in the Middle East and Southwest Asia. Remote Sens. Environ. 143, 216e227. Angstrom, A.K., 1929. On the atmospheric transmission of sun radiation and on the dust on the air. Geogr. Ann. 12, 130e159. Badarinath, K.V.S., Kharol, S.K., Kaskaoutis, D.G., Kambezidis, H.D., 2007. Dust storm over Indian region and its impact on the ground reaching solar radiationda case study using multi-satellite data and ground measurements. Sci. Total Environ. 384, 316e332. Deepshikha, S., Satheesh, S.K., Srinivasan, J., 2006. Dust aerosols over India and adjacent continents retrieved using METEOSAT infrared radiance. Part I: sources and regional distribution. Ann. Geophys. 24 (1), 37e61. Dey, S., Tripathi, S.N., Singh, R.P., 2004. Influence of dust storms on the aerosol optical properties over the Indo-Gangetic basin. J. Geophys. Res. 109, D20211,. http://dx.doi.org/10.1029/2004JD004924. Duce, R.A., 1995. Sources, distributions, and fluxes of mineral aerosols and their relationship to climate. In: Charlson, R.J., Heintzenberg, J. (Eds.), Aerosol Forcing of Climate. Wiley, New York, pp. 43e72. Gautam, R., Liu, Z., Singh, R.P., Hsu, N.C., 2009. Two contrasting dust-dominant periods over India observed from MODIS and CALIPSO data. J. Geophys. Res. Lett. 36 (6). Article ID L06813. Ginoux, P., Chin, M., Tegen, I., 2001. Sources and distributions of dust aerosols simulated with the GOCART model. J. Geophys. Res. 106 (D17), 20255e20273. Hansen, J., Sato, M., Ruedy, R., 1997. Radiative forcing and climate response. J. Geophys. Res. 102, 6831e6864. Harikishan, G., Padmakumari, B., Maheskumar, R.S., Kulkarni, J.R., 2015. Radiative effect of dust aerosols on cloud microphysics and meso-scale dynamics during monsoon breaks over Arabian sea. Atmos. Environ. 105, 22e31. Holben, B.N., et al., 2001. An emerging ground-based aerosol climatology: aerosol optical depth from AERONET. J. Geophys. Res. 106, 12,067e12,097. Liou, K.N., 2002. An Introduction to Atmospheric Radiation. Elsevier, New York. Miller, R.L., Tegen, I., 1998. Climate response to soil dust aerosols. J. Clim. 11 (12), 3247e3267. Moorthy, K.K., Babu, S.S., Satheesh, S.K., 2005. Aerosol characteristics and radiative impacts over the Arabian Sea during the intermonsoon season: results from ARMEX field campaign. J. Atmos. Sci. 62 (1), 192e206. Moorthy, K.K., Babu, S.S., Satheesh, S.K., Srinivasan, J., Dutt, C.B.S., 2007. Dust

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