Environmental Pollution 184 (2014) 563e569
Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Chemical characteristics of PM2.5 at a source region of biomass burning emissions: Evidence for secondary aerosol formation N. Rastogi a, *, A. Singh b, D. Singh b, M.M. Sarin a a b
Geosciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380009, India Department of Physics, Punjabi University, Patiala, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2013 Received in revised form 24 September 2013 Accepted 26 September 2013
A systematic study on the chemical characteristics of ambient PM2.5, collected during October-2011 to March-2012 from a source region (Patiala: 30.2 N, 76.3 E; 250 m amsl) of biomass burning emissions in the Indo-Gangetic Plain (IGP), exhibit pronounced diurnal variability in mass concentrations of PM2.5, NO3 þ , NHþ 4 , K , OC, and EC with w30e300% higher concentrations in the nighttime samples. The average WSOC/OC and SO2 4 /PM2.5 ratios for the daytime (w0.65, and 0.18, respectively) and nighttime (0.45, and 0.12, respectively) samples provide evidence for secondary organic and SO2 4 aerosol formation during the daytime. Formation of secondary NO 3 is also evident from higher NH4NO3 concentrations associated with lower temperature and higher relative humidity conditions. The scattering species (SO2 4 þ NO3 þ OC) contribute w50% to PM2.5 mass during OctobereMarch whereas absorbing species (EC) contribute only w4% in OctobereFebruary and subsequently increases to w10% in March, indicating significance of these species in regional radiative forcing. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Aerosol chemistry Haze Paddy-residue burning Transport Indo-Gangetic Plain India
1. Introduction In present-day scenario of growing anthropogenic activities in unprecedented way, atmospheric aerosols are considered to be of immense potential in influencing the air quality, Earth’s radiation budget and climate on scales ranging from local to regional and global. Direct and indirect impacts of aerosols on EartheAtmosphereeOcean system have been documented in literature (Ramanathan et al., 2001, 2007; Rosenfeld et al., 2008; Ramana et al., 2010; Okin et al., 2011). Aerosols can absorb or scatter the incoming solar radiation thus, contribute to Earths’ radiation budget. Aerosols act as cloud condensation nuclei and thus relate to cloud formation, which affect hydrological cycle as well as indirect radiative forcing. Long-range transport of continental aerosols to remote oceans affects the chemistry of marine atmospheric boundary layer (MABL), and atmospheric deposition of aerosols is among major pathways of nutrients/toxicants supply to oceanic phytoplanktons (Duce et al., 1991; Okin et al., 2011). The emissions from biomass and fossil fuel burning produce large amount of particulate and gaseous pollutants on regional and global scale. Primary particulate pollutants are compounded by the emission of high levels of secondary aerosol precursors, including
* Corresponding author. E-mail addresses: [email protected], [email protected] (N. Rastogi). 0269-7491/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2013.09.037
oxides of nitrogen and sulphur, volatile carbon species, and ammonia, resulting in the production of large amounts of secondary inorganic and organic aerosols. Secondary aerosols are mainly formed through multi-phase chemistry, and through condensation of organic and inorganic gases at the surface of pre-existing atmospheric particles and their subsequent oxidation in the atmosphere. On regional scale, Indo-Gangetic Plain (IGP) spread over northern India has large amount of air pollutants due to emissions from vehicles, industries, large-scale post-harvest biomass burning, and bio-fuel burning. These emissions result into deterioration of air quality, increase in vulnerability of people to cardiopulmonary diseases, reduction in crop yield, reduction in visibility degradation, and change in Earth’s radiation balance (Ramanathan et al., 2001; Jethva et al., 2005; Auffhammer et al., 2012). Further, the transport of these anthropogenic aerosols from IGP to the Bay of Bengal in northern Indian Ocean affect the chemistry of MABL, and their dry and wet deposition act as a supply of nutrients/toxicants to oceanic phytoplanktons (Sarin et al., 2010; Srinivas and Sarin, 2013). However, the particulate chemical composition is poorly characterized over IGP, which is essential to assess the effects of pollutants on regional air quality and climate, and plan appropriate mitigation strategies. Present study reports the comprehensive chemical composition of PM2.5 and their temporal characteristics during October-2011 to March-2012 at a site (Patiala, 30.2 N, 76.3 E; 250 m amsl) located in the source region of biomass
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burning in IGP. The major objectives were (a) to characterize the PM2.5 composition and their diurnal behaviour, (b) to understand the role of sources and meteorology in the formation of secondary organic and inorganic aerosols over the study region, and (c) to study the temporal variability of climatically important carbonaceous and inorganic species. 2. Material and methods The study site Patiala (30.2 N, 76.3 E; 250 m amsl) is located in the Punjab province in the IGP. The IGP is surrounded by the unique topography with the Himalayan range of mountains to the north and hills to the south. Patiala is a semi-urban city surrounded by agricultural area. Big industries and power plants are located in and around Ludhiana, Amritsar, and Lahore (Pakistan) in the northwest direction (within w50e200 km) of Patiala. This study was carried out during the months of October 2011 to March 2012. Jethva et al. (2005) documented that spatial variability in aerosol optical depth over IGP is quite low during winter; it implicates that Patiala can be considered as the representative of IGP during the study period. The emission sources and meteorological conditions were not uniform during the study period. Emissions from industries, power plants, and burning of cow-dung cakes as bio-fuel are the perennial sources. However, the period of middle of October to mid November is dominated by emissions from large scale post harvest paddy-residue burning by local farmers to change the crop from paddy to wheat, fertilize the soil and control pests. Fig. 1 presents the true-colour image captured on November 9, 2012 by Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Aqua satellite (image taken from http://modis.gsfc.nasa.gov) depicting haze over IGP, which is common every year. Most of the fires (red dots) are burning in the state of Punjab (Fig. 1). Although the state covers less than 2% of India’s land surface, it produces about one-fifth of the country’s food grains. About 70 to 80 million tons of rice-straw are burnt every year during OctobereNovember over Punjab region (Badarinath et al., 2006). During paddy-residue burning period, the average daytime and nighttime temperature and relative humidity were 28 and 17 C, and 50 and 88%, respectively (Table 1), and winds were weak (95% of the total water-soluble mass (Fig. 3c). However for the nighttime samples, contributions of WSOC (42%), SO2 4 þ þ (16%), NO 3 (22%) to WSS were different, NH4 (8%) and K (8%) were similar, and Cl (2%) was noticeable (Fig. 3d). The contribution of WSOC and SO2 4 to WSS was decreased by 20e30%; however, the contribution of NO 3 was increased by almost a factor of three (from 8 to 22%) during nighttime. These observations suggest the sec ondary formation of SO2 4 and OC during daytime and NO3 during nighttime. Rastogi et al. (2011) documented that the presence of NO 3 precursors and favourable meteorological conditions i.e., temperature w80% lead to high secondary NO 3 aerosol formation event with more effect of lower temperature than that of higher humidity. In the ambience of large scale biomass burning, plenty of NO 3 precursors are expected in the atmosphere, and Table 1 shows that meteorology was also favourable (average temperature: 17 C; average RH: 88%) that lead to huge secondary NO 3 aerosol formation during nighttime (Fig 2g). Further, since daytime temperature was higher (average: 28 C) and relative humidity was lower (average: 50%), NO 3 (volatile species) concentration decreased significantly during daytime (Fig. 2g). During P2, since both daytime and nighttime meteorological parameters were favourable for secondary NO 3 formation (Table 1), daytime versus nighttime differences in NO 3 concentrations were not significant (Fig. 2g). The contributions of WSOC and SO2 4 to WSS were higher during daytime, suggesting the role of photochemistry in production of secondary SO2 4 and WSOC (Weber et al., 2003, 2007). Further, the composition of WSS was somewhat similar for daytime and nighttime samples during P3 (Fig. 3c, d). The average WSOC/OC ratios for daytime samples were w0.60, 0.69 and 0.68, and for and nighttime samples were 0.44, 0.53 and 0.47, during P1, P2 and P3, respectively (Table 1). Higher daytime WSOC/OC ratio further indicates the enhanced daytime formation of photochemically derived secondary organic aerosols in all the three seasons (Weber et al., 2007). Similarly, Fig. 5a and b show the evidence of secondary NO 3 formation under favourable meteoroþ logical conditions. Fig 5a depicts the relationship of NO 3 /NH4 ratio with ambient temperature (T) and relative humidity (RH) with higher ratios associated with lower T and higher RH conditions and the ratio decreases with increasing T. There were a few samples þ þ with high NO 3 /NH4 ratio at higher T but, Fig 5b indicate that the K concentration was also high in those samples, indicating NO was 3 predominantly associated with Kþ in those samples.
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þ Fig. 5. The NO 3 /NH4 weight ratio as a function of temperature with corresponding (a) relative humidity (RH) and (b) Kþ concentrations, provide evidence that secondary NO 3 form under higher humidity and lower temperature conditions.
The SO2 4 /NO3 weight ratio varied from 0.68 to 11.5 for daytime samples and from 0.40 to 3.8 for nighttime samples during the study period. Lower nighttime SO2 4 /NO3 ratios are attributed to nighttime secondary formation of NO3 aerosol as discussed earlier. Higher daytime SO2 4 /NO3 ratios are ascribed to secondary photochemical formation of SO2 aerosols that increases SO2 4 4 , and higher T volatizes the NO3 aerosols, and both the processes lead to 2 higher SO2 4 /NO3 ratio. The average SO4 /NO3 ratios during P1, P2 and P3 were 3.9, 1.6, 4.2, respectively for the daytime samples, and 0.77, 1.0 and 2.3, respectively for nighttime samples (Table 1). These ratios also suggest that NO 3 aerosols over IGP are significant in comparison to SO2 4 , and shall be taken into consideration while estimating aerosols climatic effects over this region.
3.4. Temporal variability in scattering versus absorbing types of aerosols Relative contribution of scattering vis-à-vis absorbing type of aerosols to total aerosol loading decides the aerosol’s net effect on Earth’s radiative forcing over a particular region (Khan et al., 2010; Ramana et al., 2010). The uncertainty in radiative forcing calculation is large because of high spatio-temporal variability in aerosol concentration and composition with limited experimental studies especially over Indian region. It is important to consider real-time aerosol composition for forcing estimation.
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PM2.5 consists of absorbing (EC), and scattering (OC, SO2 4 , and NO 3 ) types of species. Mass ratio of OC to EC (OC/EC) is often used to understand the relative dominance of biomass vis-à-vis fossil fuel burning sources in producing ambient carbonaceous aerosols with higher ratio when biomass burning prevails and lower ratio when fossil fuel dominates. As far as the radiative effects are concerned, higher OC/EC ratio indicates more scattering type (brighter) aerosols whereas lower ratio suggests more absorbing type (darker) aerosols. Further, Ramana et al. (2010) reported the observations from the Cheju atmospheric brown cloud (ABC) Plume Monsoon Experiment (CAPMEX) conducted during summer 2008, and suggested that the warming (or cooling) effect of ABCs depend on the BC-to-SO2 ratios, presuming BC and SO2 are major 4 4 absorbing and scattering type species, respectively, and ignored the OC and other species. As discussed in earlier sections that in addition to SO2 4 and EC, contributions of OC and NO3 (both scattering type species) to PM2.5 are significant over IGP and therefore, it would be appropriate to consider these species also while assessing the effects of absorbing and scattering types of aerosols over this region. Zhang et al. (2012) reported that NO 3 is a strongly scattering aerosol and in some spectral regions its scattering properties are even greater than those of SO2 4 aerosols and thus, cannot be ignored. Table 1 depicts the mass ratios of OC/EC, SO2 4 /EC and (OC þ SO2 4 þ NO3 )/EC for daytime and nighttime samples collected during all the three periods. The OC/EC ratio for the daytime samples ranged from 2.5 to 15 with the average value of w8.7, 6.5 and 3.1, and for nighttime samples varied from 3.2 to 22 with average values of w13, 7.3, and 3.9 during P1, P2 and P3, respectively. The observations indicate the decreasing trend of biomass burning emissions sources and increasing trend of fossil fuel emissions sources from P1 to P3. The daytime and nighttime average OC/EC mass ratios were not drastically different except during P1, however, daytime averages were higher in all the three periods (Table 1). The SO2 4 /EC ratio for daytime samples ranged from 1.2 to 8.8 with the average value of w2.1, 4.8 and 1.7, and for nighttime samples varied from 1.0 to 9.4 with average values of w2.2, 3.2, and 1.4 during P1, P2 and P3, respectively. Here, the average daytime and nighttime SO2 4 /EC were not too different (Table 1). Further, these averaged SO2 4 /EC ratio values were much higher than those observed by Ramana et al. (2010) over China, suggesting aerosols over IGP are relatively brighter. The (OC þ SO2 4 þ NO3 )/EC ratio for daytime samples ranged from 4.1 to 24 with the average value of w12, 15 and 5.5, and for nighttime samples from 5 to 36 with average values of w18, 14, and 6 during P1, P2 and P3, respectively. Further the diurnal differences of (OC þ SO2 4 þ NO3 )/EC were noticeably different during P1; however, not significant during P2 and P3 (Table 1). Our observations contradict one of the conclusions by Jethva et al. (2005), at least for PM2.5, that the ratio of absorbing to non-absorbing fraction is more or less constant with respect to the season. Temporal variability in the relative contributions of EC, SO2 4 , and (OC þ SO2 4 þ NO3 ) to PM2.5 mass is depicted in Fig. 6aec. As discussed earlier, EC fraction to PM2.5 was w4% during P1 and P2 and increased to w10% in P3. The SO2 4 /PM2.5 ratios were w0.18 in daytime and 0.12 in nighttime samples during the whole study period (except OctobereNovember), further indicating daytime secondary SO2 4 formation. The contribution of scattering species (OC þ SO2 4 þ NO3 ) to PM2.5 mass was significant (w50%) and more or less uniform during the whole study period, suggesting that the brighter aerosols always dominate over IGP whereas, absorbing species increases after February. These observations also suggest that ignoring the OC and NO 3 would lead to considerable overestimation of darker aerosols over IGP. Seasonal and diurnal variability in absorbing and scattering type species also suggest that
P2
P1
P3
(c) (OC + SO42- + NO3-)/PM2.5
0.8 0.6 0.4 0.2 0.0
(b) SO42-/PM2.5
0.3 0.2 0.1 0.0
(a) EC/PM2.5
0.12
Daytime Nighttime
0.08 0.04 0.00 Nov
Dec
Jan
Feb
Mar
Apr
2011 − 2012 Fig. 6. Temporal variability in the contribution of absorbing and scattering type species to PM2.5 mass. Contribution of scattering species (including OC and NO 3 ) to PM2.5 remains always dominant (w50%) and that of absorbing species (EC) increases from w4% to 10% from October to March suggesting their roles in estimating radiative forcing over IGP.
considering uniform PM2.5 composition over IGP would lead to erroneous results related to aerosols climatic effects. 4. Conclusions This study presents the comprehensive chemical composition of PM2.5, their diurnal patterns, evidence of secondary organic and inorganic aerosol formation, and temporal variability in relative abundance of scattering and absorbing type species over Patiala, a site located in the middle of source region of biomass burning emissions over IGP, during October-2011 to March 2012. This time span covers periods dominated by emissions from post harvest paddy-residue burning (OctobereNovember, P1), from fossil, wood and bio-fuel burning (DecembereFebruary, P2) and from various regional sources (March, P3) with different meteorological conditions. The PM2.5 composition over the study region was always dominated by OM (40e60%) followed by WSIS (20e40%) whereas, EC contribution was low (w4%) except during P3 (10%). Strong diurnal variability in PM2.5 mass and in the concentrations of þ þ several important chemical species (e.g., SO2 4 , NO3 , NH4 , K , EC, OC, and WSOC) was observed during P1 with w30e300% higher nighttime concentrations. On average, OC/EC ratios for the daytime and nighttime samples were w8.7, 6.5 and 3.1, and w13, 7.3, and 3.9 during the P1, P2 and P3, respectively; suggesting EC contribution to carbonaceous species was relatively low during P1 and increased subsequently. The averaged WSOC/OC ratios for daytime samples were w0.60, 0.69 and 0.68, and for nighttime samples were 0.44, 0.53 and 0.47 during P1, P2 and P3, respectively, suggesting the enhanced secondary organic aerosols formation during daytime in all periods. Secondary NO 3 formation was evident through higher þ NO 3 /NH4 ratios under lower temperature and higher RH
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conditions. The SO2 4 /PM2.5 ratios were w0.18 in daytime and 0.12 in nighttime samples during the whole study period (except OctobereNovember), indicating daytime secondary SO2 4 forma2 tion. Further, the NO 3 was comparable to SO4 whereas OC was significantly higher than SO2 4 during the study period, suggesting the importance of these species as scatterer in estimating net radiative forcing over IGP region. The average (OC þ SO2 4 þ NO3 )/ EC ratios for the daytime (w12, 15 and 5.5) and nighttime samples (w18, 14, and 6) during P1, P2 and P3, respectively, and the average contribution of (OC þ SO2 4 þ NO3 ) to PM2.5 > 50% throughout the study period, both the observations tend to suggest the dominance of scattering type species in all periods and noticeable diurnal difference during P1. A strong linear correlation (r2 ¼ 0.86) has been observed between all daytime and nighttime OC and Kþ over the study site, suggesting that the Kþ can be used as a tracer for biomass burning emissions over the IGP with the OC/Kþ characteristic ratio of w16. Further, the water-soluble species (hygroscopic in nature) were dominant (55%) in PM2.5 during winter, and could be the major contributor to fog formation over IGP under favourable meteorological conditions. These results have implications in understanding the impact of biomass burning emissions on regional air quality and climate over IGP, and designing appropriate mitigation strategies. Acknowledgement This study was partially funded by the ISRO-GeosphereBiosphere Programme office (Bangaluru, India). We thank India Meteorological Department (Punjabi University, Patiala) for providing the relevant meteorological data for the study period. We would also like to thank both the Reviewers of this manuscript for their comments. References Andreae, M.O., Merlet, P., 2001. Emission of trace gases and aerosols from biomass burning. Global Biogeochem. Cycles 15, 955e966. Auffhammer, M., Ramanathan, V., Vincent, J.R., 2012. Observation-based evidence that climate change has reduced Indian rice harvests. Clim. Change 111, 411e 424. Badarinath, K.V.S., Chand, T.R.K., Prasad, V.K., 2006. Agricultural crop residue burning in the Indo-Gangetic Plains e a study using IRS-P6 A WiFS satellite data. Curr. Sci. 91, 1085e1089. Birch, M.E., Cary, R.A., 1996. Elemental carbon based for monitoring occupational exposure to particulate diesel exhaust. Aerosol Sci. Tech. 25, 221e241. Duan, F., Liu, X., Yu, T., Cachier, H., 2004. Identification and estimate of biomass burning contribution to the urban aerosol organic carbon concentrations in Beijing. Atmos. Environ. 38, 1275e1282. Duce, R.A., et al., 1991. The atmospheric input of trace species to the world ocean. Global Biogeochem. Cycles 5, 193e259. Echalar, F., Gaudichet, A., Cachier, H., Artaxo, P., 1995. Aerosol emissions by tropical forest and savanna biomass burning: characteristic trace elements and fluxes. Geophys. Res. Lett. 22, 3034e3042. Feng, J.L., Guo, Z.G., Zhang, T.R., Yao, X.H., Chan, C.K., Fang, M., 2012. Source and formation of secondary particulate matter in PM2.5 in Asian continental outflow. J. Geophys. Res. 117, D03302. http://dx.doi.org/10.1029/2011JD016400.
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Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Chemical characteristics of PM2.5 at a source region of biomass burning emissions: Evidence for secondary aerosol formation N. Rastogi a, *, A. Singh b, D. Singh b, M.M. Sarin a a b
Geosciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380009, India Department of Physics, Punjabi University, Patiala, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2013 Received in revised form 24 September 2013 Accepted 26 September 2013
A systematic study on the chemical characteristics of ambient PM2.5, collected during October-2011 to March-2012 from a source region (Patiala: 30.2 N, 76.3 E; 250 m amsl) of biomass burning emissions in the Indo-Gangetic Plain (IGP), exhibit pronounced diurnal variability in mass concentrations of PM2.5, NO3 þ , NHþ 4 , K , OC, and EC with w30e300% higher concentrations in the nighttime samples. The average WSOC/OC and SO2 4 /PM2.5 ratios for the daytime (w0.65, and 0.18, respectively) and nighttime (0.45, and 0.12, respectively) samples provide evidence for secondary organic and SO2 4 aerosol formation during the daytime. Formation of secondary NO 3 is also evident from higher NH4NO3 concentrations associated with lower temperature and higher relative humidity conditions. The scattering species (SO2 4 þ NO3 þ OC) contribute w50% to PM2.5 mass during OctobereMarch whereas absorbing species (EC) contribute only w4% in OctobereFebruary and subsequently increases to w10% in March, indicating significance of these species in regional radiative forcing. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Aerosol chemistry Haze Paddy-residue burning Transport Indo-Gangetic Plain India
1. Introduction In present-day scenario of growing anthropogenic activities in unprecedented way, atmospheric aerosols are considered to be of immense potential in influencing the air quality, Earth’s radiation budget and climate on scales ranging from local to regional and global. Direct and indirect impacts of aerosols on EartheAtmosphereeOcean system have been documented in literature (Ramanathan et al., 2001, 2007; Rosenfeld et al., 2008; Ramana et al., 2010; Okin et al., 2011). Aerosols can absorb or scatter the incoming solar radiation thus, contribute to Earths’ radiation budget. Aerosols act as cloud condensation nuclei and thus relate to cloud formation, which affect hydrological cycle as well as indirect radiative forcing. Long-range transport of continental aerosols to remote oceans affects the chemistry of marine atmospheric boundary layer (MABL), and atmospheric deposition of aerosols is among major pathways of nutrients/toxicants supply to oceanic phytoplanktons (Duce et al., 1991; Okin et al., 2011). The emissions from biomass and fossil fuel burning produce large amount of particulate and gaseous pollutants on regional and global scale. Primary particulate pollutants are compounded by the emission of high levels of secondary aerosol precursors, including
* Corresponding author. E-mail addresses: [email protected], [email protected] (N. Rastogi). 0269-7491/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2013.09.037
oxides of nitrogen and sulphur, volatile carbon species, and ammonia, resulting in the production of large amounts of secondary inorganic and organic aerosols. Secondary aerosols are mainly formed through multi-phase chemistry, and through condensation of organic and inorganic gases at the surface of pre-existing atmospheric particles and their subsequent oxidation in the atmosphere. On regional scale, Indo-Gangetic Plain (IGP) spread over northern India has large amount of air pollutants due to emissions from vehicles, industries, large-scale post-harvest biomass burning, and bio-fuel burning. These emissions result into deterioration of air quality, increase in vulnerability of people to cardiopulmonary diseases, reduction in crop yield, reduction in visibility degradation, and change in Earth’s radiation balance (Ramanathan et al., 2001; Jethva et al., 2005; Auffhammer et al., 2012). Further, the transport of these anthropogenic aerosols from IGP to the Bay of Bengal in northern Indian Ocean affect the chemistry of MABL, and their dry and wet deposition act as a supply of nutrients/toxicants to oceanic phytoplanktons (Sarin et al., 2010; Srinivas and Sarin, 2013). However, the particulate chemical composition is poorly characterized over IGP, which is essential to assess the effects of pollutants on regional air quality and climate, and plan appropriate mitigation strategies. Present study reports the comprehensive chemical composition of PM2.5 and their temporal characteristics during October-2011 to March-2012 at a site (Patiala, 30.2 N, 76.3 E; 250 m amsl) located in the source region of biomass
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burning in IGP. The major objectives were (a) to characterize the PM2.5 composition and their diurnal behaviour, (b) to understand the role of sources and meteorology in the formation of secondary organic and inorganic aerosols over the study region, and (c) to study the temporal variability of climatically important carbonaceous and inorganic species. 2. Material and methods The study site Patiala (30.2 N, 76.3 E; 250 m amsl) is located in the Punjab province in the IGP. The IGP is surrounded by the unique topography with the Himalayan range of mountains to the north and hills to the south. Patiala is a semi-urban city surrounded by agricultural area. Big industries and power plants are located in and around Ludhiana, Amritsar, and Lahore (Pakistan) in the northwest direction (within w50e200 km) of Patiala. This study was carried out during the months of October 2011 to March 2012. Jethva et al. (2005) documented that spatial variability in aerosol optical depth over IGP is quite low during winter; it implicates that Patiala can be considered as the representative of IGP during the study period. The emission sources and meteorological conditions were not uniform during the study period. Emissions from industries, power plants, and burning of cow-dung cakes as bio-fuel are the perennial sources. However, the period of middle of October to mid November is dominated by emissions from large scale post harvest paddy-residue burning by local farmers to change the crop from paddy to wheat, fertilize the soil and control pests. Fig. 1 presents the true-colour image captured on November 9, 2012 by Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Aqua satellite (image taken from http://modis.gsfc.nasa.gov) depicting haze over IGP, which is common every year. Most of the fires (red dots) are burning in the state of Punjab (Fig. 1). Although the state covers less than 2% of India’s land surface, it produces about one-fifth of the country’s food grains. About 70 to 80 million tons of rice-straw are burnt every year during OctobereNovember over Punjab region (Badarinath et al., 2006). During paddy-residue burning period, the average daytime and nighttime temperature and relative humidity were 28 and 17 C, and 50 and 88%, respectively (Table 1), and winds were weak (95% of the total water-soluble mass (Fig. 3c). However for the nighttime samples, contributions of WSOC (42%), SO2 4 þ þ (16%), NO 3 (22%) to WSS were different, NH4 (8%) and K (8%) were similar, and Cl (2%) was noticeable (Fig. 3d). The contribution of WSOC and SO2 4 to WSS was decreased by 20e30%; however, the contribution of NO 3 was increased by almost a factor of three (from 8 to 22%) during nighttime. These observations suggest the sec ondary formation of SO2 4 and OC during daytime and NO3 during nighttime. Rastogi et al. (2011) documented that the presence of NO 3 precursors and favourable meteorological conditions i.e., temperature w80% lead to high secondary NO 3 aerosol formation event with more effect of lower temperature than that of higher humidity. In the ambience of large scale biomass burning, plenty of NO 3 precursors are expected in the atmosphere, and Table 1 shows that meteorology was also favourable (average temperature: 17 C; average RH: 88%) that lead to huge secondary NO 3 aerosol formation during nighttime (Fig 2g). Further, since daytime temperature was higher (average: 28 C) and relative humidity was lower (average: 50%), NO 3 (volatile species) concentration decreased significantly during daytime (Fig. 2g). During P2, since both daytime and nighttime meteorological parameters were favourable for secondary NO 3 formation (Table 1), daytime versus nighttime differences in NO 3 concentrations were not significant (Fig. 2g). The contributions of WSOC and SO2 4 to WSS were higher during daytime, suggesting the role of photochemistry in production of secondary SO2 4 and WSOC (Weber et al., 2003, 2007). Further, the composition of WSS was somewhat similar for daytime and nighttime samples during P3 (Fig. 3c, d). The average WSOC/OC ratios for daytime samples were w0.60, 0.69 and 0.68, and for and nighttime samples were 0.44, 0.53 and 0.47, during P1, P2 and P3, respectively (Table 1). Higher daytime WSOC/OC ratio further indicates the enhanced daytime formation of photochemically derived secondary organic aerosols in all the three seasons (Weber et al., 2007). Similarly, Fig. 5a and b show the evidence of secondary NO 3 formation under favourable meteoroþ logical conditions. Fig 5a depicts the relationship of NO 3 /NH4 ratio with ambient temperature (T) and relative humidity (RH) with higher ratios associated with lower T and higher RH conditions and the ratio decreases with increasing T. There were a few samples þ þ with high NO 3 /NH4 ratio at higher T but, Fig 5b indicate that the K concentration was also high in those samples, indicating NO was 3 predominantly associated with Kþ in those samples.
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þ Fig. 5. The NO 3 /NH4 weight ratio as a function of temperature with corresponding (a) relative humidity (RH) and (b) Kþ concentrations, provide evidence that secondary NO 3 form under higher humidity and lower temperature conditions.
The SO2 4 /NO3 weight ratio varied from 0.68 to 11.5 for daytime samples and from 0.40 to 3.8 for nighttime samples during the study period. Lower nighttime SO2 4 /NO3 ratios are attributed to nighttime secondary formation of NO3 aerosol as discussed earlier. Higher daytime SO2 4 /NO3 ratios are ascribed to secondary photochemical formation of SO2 aerosols that increases SO2 4 4 , and higher T volatizes the NO3 aerosols, and both the processes lead to 2 higher SO2 4 /NO3 ratio. The average SO4 /NO3 ratios during P1, P2 and P3 were 3.9, 1.6, 4.2, respectively for the daytime samples, and 0.77, 1.0 and 2.3, respectively for nighttime samples (Table 1). These ratios also suggest that NO 3 aerosols over IGP are significant in comparison to SO2 4 , and shall be taken into consideration while estimating aerosols climatic effects over this region.
3.4. Temporal variability in scattering versus absorbing types of aerosols Relative contribution of scattering vis-à-vis absorbing type of aerosols to total aerosol loading decides the aerosol’s net effect on Earth’s radiative forcing over a particular region (Khan et al., 2010; Ramana et al., 2010). The uncertainty in radiative forcing calculation is large because of high spatio-temporal variability in aerosol concentration and composition with limited experimental studies especially over Indian region. It is important to consider real-time aerosol composition for forcing estimation.
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PM2.5 consists of absorbing (EC), and scattering (OC, SO2 4 , and NO 3 ) types of species. Mass ratio of OC to EC (OC/EC) is often used to understand the relative dominance of biomass vis-à-vis fossil fuel burning sources in producing ambient carbonaceous aerosols with higher ratio when biomass burning prevails and lower ratio when fossil fuel dominates. As far as the radiative effects are concerned, higher OC/EC ratio indicates more scattering type (brighter) aerosols whereas lower ratio suggests more absorbing type (darker) aerosols. Further, Ramana et al. (2010) reported the observations from the Cheju atmospheric brown cloud (ABC) Plume Monsoon Experiment (CAPMEX) conducted during summer 2008, and suggested that the warming (or cooling) effect of ABCs depend on the BC-to-SO2 ratios, presuming BC and SO2 are major 4 4 absorbing and scattering type species, respectively, and ignored the OC and other species. As discussed in earlier sections that in addition to SO2 4 and EC, contributions of OC and NO3 (both scattering type species) to PM2.5 are significant over IGP and therefore, it would be appropriate to consider these species also while assessing the effects of absorbing and scattering types of aerosols over this region. Zhang et al. (2012) reported that NO 3 is a strongly scattering aerosol and in some spectral regions its scattering properties are even greater than those of SO2 4 aerosols and thus, cannot be ignored. Table 1 depicts the mass ratios of OC/EC, SO2 4 /EC and (OC þ SO2 4 þ NO3 )/EC for daytime and nighttime samples collected during all the three periods. The OC/EC ratio for the daytime samples ranged from 2.5 to 15 with the average value of w8.7, 6.5 and 3.1, and for nighttime samples varied from 3.2 to 22 with average values of w13, 7.3, and 3.9 during P1, P2 and P3, respectively. The observations indicate the decreasing trend of biomass burning emissions sources and increasing trend of fossil fuel emissions sources from P1 to P3. The daytime and nighttime average OC/EC mass ratios were not drastically different except during P1, however, daytime averages were higher in all the three periods (Table 1). The SO2 4 /EC ratio for daytime samples ranged from 1.2 to 8.8 with the average value of w2.1, 4.8 and 1.7, and for nighttime samples varied from 1.0 to 9.4 with average values of w2.2, 3.2, and 1.4 during P1, P2 and P3, respectively. Here, the average daytime and nighttime SO2 4 /EC were not too different (Table 1). Further, these averaged SO2 4 /EC ratio values were much higher than those observed by Ramana et al. (2010) over China, suggesting aerosols over IGP are relatively brighter. The (OC þ SO2 4 þ NO3 )/EC ratio for daytime samples ranged from 4.1 to 24 with the average value of w12, 15 and 5.5, and for nighttime samples from 5 to 36 with average values of w18, 14, and 6 during P1, P2 and P3, respectively. Further the diurnal differences of (OC þ SO2 4 þ NO3 )/EC were noticeably different during P1; however, not significant during P2 and P3 (Table 1). Our observations contradict one of the conclusions by Jethva et al. (2005), at least for PM2.5, that the ratio of absorbing to non-absorbing fraction is more or less constant with respect to the season. Temporal variability in the relative contributions of EC, SO2 4 , and (OC þ SO2 4 þ NO3 ) to PM2.5 mass is depicted in Fig. 6aec. As discussed earlier, EC fraction to PM2.5 was w4% during P1 and P2 and increased to w10% in P3. The SO2 4 /PM2.5 ratios were w0.18 in daytime and 0.12 in nighttime samples during the whole study period (except OctobereNovember), further indicating daytime secondary SO2 4 formation. The contribution of scattering species (OC þ SO2 4 þ NO3 ) to PM2.5 mass was significant (w50%) and more or less uniform during the whole study period, suggesting that the brighter aerosols always dominate over IGP whereas, absorbing species increases after February. These observations also suggest that ignoring the OC and NO 3 would lead to considerable overestimation of darker aerosols over IGP. Seasonal and diurnal variability in absorbing and scattering type species also suggest that
P2
P1
P3
(c) (OC + SO42- + NO3-)/PM2.5
0.8 0.6 0.4 0.2 0.0
(b) SO42-/PM2.5
0.3 0.2 0.1 0.0
(a) EC/PM2.5
0.12
Daytime Nighttime
0.08 0.04 0.00 Nov
Dec
Jan
Feb
Mar
Apr
2011 − 2012 Fig. 6. Temporal variability in the contribution of absorbing and scattering type species to PM2.5 mass. Contribution of scattering species (including OC and NO 3 ) to PM2.5 remains always dominant (w50%) and that of absorbing species (EC) increases from w4% to 10% from October to March suggesting their roles in estimating radiative forcing over IGP.
considering uniform PM2.5 composition over IGP would lead to erroneous results related to aerosols climatic effects. 4. Conclusions This study presents the comprehensive chemical composition of PM2.5, their diurnal patterns, evidence of secondary organic and inorganic aerosol formation, and temporal variability in relative abundance of scattering and absorbing type species over Patiala, a site located in the middle of source region of biomass burning emissions over IGP, during October-2011 to March 2012. This time span covers periods dominated by emissions from post harvest paddy-residue burning (OctobereNovember, P1), from fossil, wood and bio-fuel burning (DecembereFebruary, P2) and from various regional sources (March, P3) with different meteorological conditions. The PM2.5 composition over the study region was always dominated by OM (40e60%) followed by WSIS (20e40%) whereas, EC contribution was low (w4%) except during P3 (10%). Strong diurnal variability in PM2.5 mass and in the concentrations of þ þ several important chemical species (e.g., SO2 4 , NO3 , NH4 , K , EC, OC, and WSOC) was observed during P1 with w30e300% higher nighttime concentrations. On average, OC/EC ratios for the daytime and nighttime samples were w8.7, 6.5 and 3.1, and w13, 7.3, and 3.9 during the P1, P2 and P3, respectively; suggesting EC contribution to carbonaceous species was relatively low during P1 and increased subsequently. The averaged WSOC/OC ratios for daytime samples were w0.60, 0.69 and 0.68, and for nighttime samples were 0.44, 0.53 and 0.47 during P1, P2 and P3, respectively, suggesting the enhanced secondary organic aerosols formation during daytime in all periods. Secondary NO 3 formation was evident through higher þ NO 3 /NH4 ratios under lower temperature and higher RH
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conditions. The SO2 4 /PM2.5 ratios were w0.18 in daytime and 0.12 in nighttime samples during the whole study period (except OctobereNovember), indicating daytime secondary SO2 4 forma2 tion. Further, the NO 3 was comparable to SO4 whereas OC was significantly higher than SO2 4 during the study period, suggesting the importance of these species as scatterer in estimating net radiative forcing over IGP region. The average (OC þ SO2 4 þ NO3 )/ EC ratios for the daytime (w12, 15 and 5.5) and nighttime samples (w18, 14, and 6) during P1, P2 and P3, respectively, and the average contribution of (OC þ SO2 4 þ NO3 ) to PM2.5 > 50% throughout the study period, both the observations tend to suggest the dominance of scattering type species in all periods and noticeable diurnal difference during P1. A strong linear correlation (r2 ¼ 0.86) has been observed between all daytime and nighttime OC and Kþ over the study site, suggesting that the Kþ can be used as a tracer for biomass burning emissions over the IGP with the OC/Kþ characteristic ratio of w16. Further, the water-soluble species (hygroscopic in nature) were dominant (55%) in PM2.5 during winter, and could be the major contributor to fog formation over IGP under favourable meteorological conditions. These results have implications in understanding the impact of biomass burning emissions on regional air quality and climate over IGP, and designing appropriate mitigation strategies. Acknowledgement This study was partially funded by the ISRO-GeosphereBiosphere Programme office (Bangaluru, India). We thank India Meteorological Department (Punjabi University, Patiala) for providing the relevant meteorological data for the study period. We would also like to thank both the Reviewers of this manuscript for their comments. References Andreae, M.O., Merlet, P., 2001. Emission of trace gases and aerosols from biomass burning. Global Biogeochem. Cycles 15, 955e966. Auffhammer, M., Ramanathan, V., Vincent, J.R., 2012. Observation-based evidence that climate change has reduced Indian rice harvests. Clim. Change 111, 411e 424. Badarinath, K.V.S., Chand, T.R.K., Prasad, V.K., 2006. Agricultural crop residue burning in the Indo-Gangetic Plains e a study using IRS-P6 A WiFS satellite data. Curr. Sci. 91, 1085e1089. Birch, M.E., Cary, R.A., 1996. Elemental carbon based for monitoring occupational exposure to particulate diesel exhaust. Aerosol Sci. Tech. 25, 221e241. Duan, F., Liu, X., Yu, T., Cachier, H., 2004. Identification and estimate of biomass burning contribution to the urban aerosol organic carbon concentrations in Beijing. Atmos. Environ. 38, 1275e1282. Duce, R.A., et al., 1991. The atmospheric input of trace species to the world ocean. Global Biogeochem. Cycles 5, 193e259. Echalar, F., Gaudichet, A., Cachier, H., Artaxo, P., 1995. Aerosol emissions by tropical forest and savanna biomass burning: characteristic trace elements and fluxes. Geophys. Res. Lett. 22, 3034e3042. Feng, J.L., Guo, Z.G., Zhang, T.R., Yao, X.H., Chan, C.K., Fang, M., 2012. Source and formation of secondary particulate matter in PM2.5 in Asian continental outflow. J. Geophys. Res. 117, D03302. http://dx.doi.org/10.1029/2011JD016400.
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