Seasonal Pattern in Diurnal Variability of Mixing Ratio...

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International Journal of Earth and Atmospheric Science | October-December, 2014 | Vol 1 | Issue 3 | Pages 95-103 © 2014 Jakraya Publications (P) Ltd INTERNATIONAL JOURNAL OF EARTH AND ATMOSPHERIC SCIENCE Journal homepage: www.jakraya.com/journal/ijeas ORIGINAL ARTICLE Seasonal Pattern in Diurnal Variability of Mixing Ratio and δ 13 C of Air CO 2 Observed at an Urban Station Bangalore, India Tania Guha a * and Prosenjit Ghosh b,c,d a Research Center for Environmental Changes, Academia Sinica, Taiwan. b Centre for Earth Sciences, c Centre for Atmospheric and Oceanic Science, d Divecha Centre for Climate Change, Indian Institute of Science, Bangalore, India. *Corresponding Author: Tania Guha Email: [email protected] Received: 26/10/2014 Revised: 30/11/2014 Accepted: 04/12/2014 Abstract We present here the observations on the seasonal variability in the amplitude of diurnal variation of mixing ratios and carbon isotope ratio (δ 13 C) of air-CO 2 , from an urban station-Bangalore (BLR), India. Although the diurnal variability was present for all the months, its amplitude was found to be higher during both the dry-summer and post-monsoon months compared to southwest monsoon months. The average source-CO 2 value obtained for Bangalore was -25.0‰ with minor variability recorded in the source isotopic composition on seasonal time scale. We identified that the seasonal variability in the diurnal variation of air CO 2 was governed by the factors like seasonal variability of the vertical stability of atmosphere. To understand this, the vertical profile of morning time potential temperature was investigated using IMD Radiosonde data. During dry hot summer and post monsoon months CO 2 emitted from anthropogenic and biogenic sources in a polluted atmosphere was trapped at the ground level as a consequence of the lowering of Nocturnal Boundary Layer (NBL) and it also prevents the mixing of CO 2 from the overlying free air. The contribution of CO 2 from overlying free atmosphere was found to be ~6% less during dry hot summer and post monsoon seasons compared to southwest monsoon months. Keywords: Stable isotopes, Nocturnal Boundary Layer, Air-CO 2 , Carbon isotope ratio, Diurnal variation. 1. Introduction Carbon dioxide is one of the major green-house gases present in the global atmosphere and its increase is primarily responsible for the present day Global Warming (IPCC 2001). The rise in global CO 2 is mainly caused by anthropogenic emissions from the combustion of fossil fuel, biomass burning and deforestation etc. Fraction of the total emitted CO 2 is taken up by both the terrestrial and oceanic reservoirs. Thus it is important to identify the contribution of sources and sinks in governing the global air CO 2 trend (Keeling et al., 1989a; Tan et al., 1993; Ciais et al., 1995a; 1995b; Fung et al., 1997). Combined monitoring of mixing ratios (μmol.mol -1 ) and carbon isotope ratios (δ 13 C) of air-CO 2 were used to identify different sources (Keeling et al., 1958) and their contribution. The contribution of these sources to the observed variability of atmospheric CO 2 varies in different seasons depending upon atmospheric stability, boundary layer condition, and atmospheric transport driven mixing etc. This gives rise to both spatial and temporal scale variation of atmospheric CO 2 level. On a temporal scale there are mainly two prominent scales of variation in both mixing ratios (μmol.mol -1 ) and carbon isotope ratios (δ 13 C) of air-CO 2 i.e. long- term/secular variation and short-term variation (Inoue and Sugimura, 1985; Keeling et al., 1989; Mook, 1986). The long-term variation of atmospheric CO 2 is due to fossil fuel combustion, long term change in uptake of CO 2 by the terrestrial biospheric reservoirs and uptake by the ocean (Mook, 1986). The short-term variation in atmospheric CO 2 is also caused by accumulation of CO 2 by plants during photosynthesis and release during respiration (Keeling et al., 1989; Mook, 1986) and gas exchange during atmosphere- ocean interaction (Berner 1999; Clark et al., 2003), fossil fuel combustion etc. The amplitude of short term variation is larger than the long-term variation thus it

Transcript of Seasonal Pattern in Diurnal Variability of Mixing Ratio...

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International Journal of Earth and Atmospheric Science | October-December, 2014 | Vol 1 | Issue 3 | Pages 95-103 © 2014 Jakraya Publications (P) Ltd

INTERNATIONAL JOURNAL OF EARTH AND ATMOSPHERIC SCIENCE Journal homepage: www.jakraya.com/journal/ijeas

ORIGINAL ARTICLE

Seasonal Pattern in Diurnal Variability of Mixing Ratio and δ13C of Air CO2 Observed at an Urban Station Bangalore, India Tania Guhaa* and Prosenjit Ghoshb,c,d aResearch Center for Environmental Changes, Academia Sinica, Taiwan. bCentre for Earth Sciences, cCentre for Atmospheric and Oceanic Science, dDivecha Centre for Climate Change, Indian Institute of Science, Bangalore, India. *Corresponding Author: Tania Guha Email: [email protected] Received: 26/10/2014 Revised: 30/11/2014 Accepted: 04/12/2014

Abstract

We present here the observations on the seasonal variability in the amplitude of diurnal variation of mixing ratios and carbon isotope ratio (δ13C) of air-CO2, from an urban station-Bangalore (BLR), India. Although the diurnal variability was present for all the months, its amplitude was found to be higher during both the dry-summer and post-monsoon months compared to southwest monsoon months. The average source-CO2 value obtained for Bangalore was -25.0‰ with minor variability recorded in the source isotopic composition on seasonal time scale. We identified that the seasonal variability in the diurnal variation of air CO2 was governed by the factors like seasonal variability of the vertical stability of atmosphere. To understand this, the vertical profile of morning time potential temperature was investigated using IMD Radiosonde data. During dry hot summer and post monsoon months CO2 emitted from anthropogenic and biogenic sources in a polluted atmosphere was trapped at the ground level as a consequence of the lowering of Nocturnal Boundary Layer (NBL) and it also prevents the mixing of CO2 from the overlying free air. The contribution of CO2 from overlying free atmosphere was found to be ~6% less during dry hot summer and post monsoon seasons compared to southwest monsoon months.

Keywords: Stable isotopes, Nocturnal Boundary Layer, Air-CO2, Carbon isotope ratio, Diurnal variation.

1. Introduction Carbon dioxide is one of the major green-house

gases present in the global atmosphere and its increase is primarily responsible for the present day Global Warming (IPCC 2001). The rise in global CO2 is mainly caused by anthropogenic emissions from the combustion of fossil fuel, biomass burning and deforestation etc. Fraction of the total emitted CO2 is taken up by both the terrestrial and oceanic reservoirs. Thus it is important to identify the contribution of sources and sinks in governing the global air CO2 trend (Keeling et al., 1989a; Tan et al., 1993; Ciais et al., 1995a; 1995b; Fung et al., 1997). Combined monitoring of mixing ratios (µmol.mol-1) and carbon isotope ratios (δ13C) of air-CO2 were used to identify different sources (Keeling et al., 1958) and their contribution. The contribution of these sources to the observed variability of atmospheric CO2 varies in different seasons depending upon atmospheric stability,

boundary layer condition, and atmospheric transport driven mixing etc. This gives rise to both spatial and temporal scale variation of atmospheric CO2 level. On a temporal scale there are mainly two prominent scales of variation in both mixing ratios (µmol.mol-1) and carbon isotope ratios (δ13C) of air-CO2 i.e. long-term/secular variation and short-term variation (Inoue and Sugimura, 1985; Keeling et al., 1989; Mook, 1986). The long-term variation of atmospheric CO2 is due to fossil fuel combustion, long term change in uptake of CO2 by the terrestrial biospheric reservoirs and uptake by the ocean (Mook, 1986). The short-term variation in atmospheric CO2 is also caused by accumulation of CO2 by plants during photosynthesis and release during respiration (Keeling et al., 1989; Mook, 1986) and gas exchange during atmosphere-ocean interaction (Berner 1999; Clark et al., 2003), fossil fuel combustion etc. The amplitude of short term variation is larger than the long-term variation thus it

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has a masking effect on the long-term variation of atmospheric CO2. That is why the study of short-term variation in atmospheric CO2 is necessary for understanding the fluxes of CO2 exchanged between atmosphere and terrestrial biosphere (Heimann et al., 1989) at shorter time scale. This can also provide understanding on the effect of atmospheric phenomena like boundary layer condition, break/active phase of Indian summer monsoon on the variability of CO2 fluxes from the terrestrial biosphere (Valsala et al., 2013) etc.. One of the primary steps towards understanding all these interactions is the precise monitoring of short-term variation in both mixing ratio and δ13C of air CO2 (Ciasis et al., 1995a, b; Keeling et al., 1995; Francey et al., 1995), especially from a site which is expected to capture higher amplitude of variation. In this context, urban areas are the most suitable locations to study as they generate significant amount of CO2 from anthropogenic emission (Grimmond et al., 2002; Briber et al., 2013) and thus also record higher amplitude of variation in both mixing ratio and δ13C of air CO2.

The short-term variation in atmospheric CO2 is of two type, 1) synoptic scale variation i.e. diurnal variation and 2) seasonal variation. Diurnal variations of CO2 mixing ratio in an urban atmosphere have been well studied and documented (Grimmond et al., 2002; Nasarallah et al., 2003). The combined observation on diurnal variation of both mixing ratio and δ13C of air CO2 revealed the role of photosynthesis and respiration together with the variable contribution of CO2 from the burning of fossil fuels like gasoline and natural gas (Pataki et al., 2006). Recent investigation revealed that the presence or absence of atmospheric inversion plays important factor affecting the diurnal variation of CO2 as observed at an urban location like Salt Lake, Utah City, US (Pataki et al., 2005). Similar effect of Boundary layer on the diurnal variation of CO2 mixing ratio and δ13C of air CO2 has been documented from an urban station, Bangalore (mentioned hereafter as “BLR”) in India during October, 2008 (Guha and Ghosh, 2010). The study revealed the presence of Nocturnal Boundary Layer (Wallace and Hobbs, 2006) in the early morning responsible for the incomplete mixing of CO2 from free atmosphere (mentioned hereafter as “FA”) with the CO2 produced at the ground level (mentioned hereafter as “PA”), whereas the absence of NBL in the afternoon allowed mixing of CO2 from the FA. Afternoon time, the mixing of CO2 from the FA diluted the signature of polluted atmosphere i.e. the atmosphere with more anthropogenic CO2, considered as pollutant (Jacobson, 2008), at ground level. The boundary Layer height varies during different seasons. For example, the NBL lies close to the ground during pre-monsoon whereas

during southwest monsoon atmospheric convection pushes the NBL up and merges with the Planetary Boundary Layer (Raman et al., 1990). This seasonal change in the NBL height might be an important factor affecting the diurnal variation of atmospheric CO2 at a seasonal time scale. In addition , the BLR station experiences four distinct seasons in a year i.e. dry hot summer (March-May), southwest monsoon (mentioned hereafter as “SWM”) (June–September), post monsoon (October-November) and winter (December–February) in a year. Thus it was interesting to explore the diurnal variation of CO2 mixing ratio and δ13C of air CO2 at BLR station. Detail observations on the diurnal variation of both the mixing ratio and δ13C of air CO2 from BLR station has been presented in Guha and Ghosh (2014).

The amplitude of diurnal variation in both mixing ratio and δ13C of air CO2 was found to vary during different seasons of year. This further compelled us to investigate the possible effect of seasonal variability in atmospheric boundary layer condition on the on the seasonal pattern in diurnal variability of air-CO2, as presented in the current study. In the present day scenario, the rapid urbanization and its associated alternation in land-use and land-cover has a significant effect on near surface boundary layer (Jain et al., 2014) which eventually affects the air-CO2 variability at ground level (Guha and Ghosh, 2010). Thus along with its direct effect on green-house gases like CO2, land-use changes may also indirectly affect air-CO2 variability via alternation in atmospheric boundary layer condition. These combined effects must be included in the global carbon cycle model while estimating the future emission of CO2, especially for urban areas. 2. Experimental Design 2.1 Sampling, Extraction and Analysis

The samples were collected from Indian Institute of Science campus, Bangalore, India, (mentioned in the entire study as BLR station) (Fig 1) during 2008 through 2011, covering all the four seasons. Paired air samples were collected from a height of 5m above ground, using the air sampler. During sampling both the glass flask were first flushed with ambient air and then filled at a gauge pressure of ~1.2bar. Immediately after sampling, CO2 was cryogenically extracted from the respective flasks and its concentration was determined. Subsequently, its isotopic ratio was measured using the Dual inlet peripheral of isotope ratio mass spectrometer, IRMS MAT 253. Details about the precision and reproducibility of analysis are given in Guha and Ghosh, 2013 and 2014. During 2008 – 2009, the precision of mixing ratio and δ13C measurement of air

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Fig 1: Positioning of the sampling location, BLR station for the current study CO2 were ±9.3µmol.mol-1 and ±0.09‰ respectively which was improved to ±7µmol.mol-1 and ±0.05‰ during 2009-2011 as reported in. This allows the identification of the isotopic composition of source CO2 for the different months using Keeling plot method (Keeling, 1989) as demonstrated in Guha and Ghosh, 2014. Based on this, the average source CO2 for four different seasons were calculated for BLR station. To further understand the factors responsible for the observed variability in the amplitude of diurnal variation of the CO2 mixing ratio and δ13C of CO2 isotopic ratio for different seasons, the seasonal variation in the vertical stability of the atmosphere were investigated using the radiosonde observations (http://weather.uwyo.edu/upperair/sounding.html). 2.3 Radiosonde Observation for Different Months

In order to understand the seasonal variation on the height of NBL, morning time (05:30 Indian Standard Time, IST) Radiosonde data obtained from Indian Meteorological Department (IMD) were used (http://weather.uwyo.edu/upperair/sounding.html) for the respective months. The detail of the Radiosonde measurement was given in Guha and Ghosh, 2010. Potential temperature (θ) was calculated based on the measured temperature and the pressure readings at different heights. The morning time (05:30 IST) vertical profile of the potential temperature for several days in a month was compared to arrive at a

representative average value and to justify the vertical profile of potential temperature of the atmosphere at a seasonal time scales. 2.4 Modeling Approach

A conceptual vertical mixing model was applied here, where air comprising of CO2 generated from both plant respiration and automobile exhaust (“polluted air”, PA) was allowed to mix with CO2 from the free atmosphere, “FA” (Guha and Ghosh, 2010). A similar modeling approach was used earlier to estimate the contribution of CO2 from the FA (Guha and Ghosh, 2014). The horizontal mixing due to wind was considered to be minimal at the time of sampling (Guha and Ghosh, 2010). Detail of model equation was given in Guha and Ghosh, 2010. In the model, the average δ13C values of source CO2 for the specific months were used as representative value for PA and the long-term average δ13C value (-7.9‰) of CO2 from the Cabo de Rama (Bhattacharya et al., 2009; Guha and Ghosh, 2010), representing marine unpolluted air was adopted as the isotopic composition of CO2 in the FA for estimation. Using the isotopic ratios, the contribution of FA at the ground level was estimated for different months. Further, we estimated the average contribution of CO2 from the FA for the four distinct seasons. The variable contribution of FA during different seasons captures the extent of pollution at the ground level at the time of our measurement.

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3. Results and Discussion The observations on amplitude of diurnal

variability in both CO2 mixing ratio and δ13C of air CO2 from the BLR station were plotted in Fig 2. Table 1: observations on the amplitude of diurnal

variation in mixing ratio and δ13C of air CO2

Seasons Date Difference between morning and afternoon time samples

Mixing Ratio (µmol.mol-1) δ13C

(‰ VPDB)

Dry hot summer

9-03-09 11 0.0 25-03-09 43 1.7 6-04-09 31 0.7 24-04-09 39 1.4 29-04-09 31 0.8

Southwest µonsoon

8-06-09 9 0.4 1-07-09 1 0.2

Post monsoon

28-10-08 2 0.4 29-10-08 62 2.1 30-10-08 62 2.2 31-10-08 38 1.7 1-11-08 43 2.2 2-11-08 41 0.9 3-11-08 43 1.4 4-11-08 78 2.6 5-11-08 1 1.1 6-11-08 24 0.9 16-10-09 41 1.5 16-10-09 20 0.9 17-10-09 36 1.3 17-10-09 46 1.5 18-10-09 18 1.3 18-10-09 23 1.3 28-10-09 20 0.4 29-10-09 15 0.6 5-11-10 12 0.9 5-11-10 38 0.9 6-11-10 52 1.9

winter

28-12-10 15 0.5 28-12-10 12 0.5 29-12-10 10 0.5 30-12-10 8 0.3 30-12-10 15 0.3 19-12-11 27 0.7 20-12-11 11 0.6 20-12-11 11 0.6 14-02-09 21 0.6 14-02-09 26 0.8

The amplitude of diurnal variability was defined

here as the difference between the values of morning and aftertime observations in both CO2 mixing ratio and δ13C of air CO2 (Table 1).

The average amplitude of diurnal variability for four different seasons was shown in Fig 2 with

different color bars. The diurnal variability was found to be higher during dry hot summer months i.e. pre-monsoon time and also during post monsoon time as compared to the southwest monsoon season. This diurnal variation of air CO2 composition for successive days in a month allowed estimation of the δ13C value of end member local source CO2, as discussed in Guha and Ghosh, 2014. Based on this, the average source values for four different seasons were estimated as given in Table 2. Table 2: Average δ13C values of source CO2 for four

different seasons

Years δ13C (‰ VPDB)

Dry hot summer -25.2±3 Southwest monsoon -25.6±4 Post monsoon -24.7±2 Winter -24.4±2

The results suggest that the source values of

CO2 for BLR station at the ground level do not vary much on seasonal scales. Thus to understand the significance of variability in the amplitude of diurnal variation we considered the NBL height. Radiosonde data available for individual days in a month displaying the vertical structure of atmosphere at monthly time scales are shown in Fig 3. The figure shows that during the dry hot summer months (March-May) NBL remain close to ground. As the south west monsoon seasons approaches, the atmospheric convection process favors vertical upliftment of NBL and it merges with the atmospheric boundary layer. Immediately after the SWM, i.e. during the post monsoon seasons (October-November), the ground starts cooling rapidly than the atmosphere above and it give rise to a shift in the height of NBL (Raman et al., 1990). All though during the winter months (December-February) a strong and persistent NBL close to ground was commonly recorded (Alappattu et al., 2009) in other region, here we found that during December and February months of our study NBL was not very prominent was missing, there was a gradual change in potential temperature observed vertically as shown in (Fig 3) the plot. BLR station also receives rain occasionally during the end of northeast monsoon i.e. ~December-January; the weak convection during this period might be a responsible factor, affecting the vertical profile of potential temperature. In summary, during the dry hot summer months and the post monsoon months, the NBL lies close to the ground compared to the southwest monsoon months. As noticed during the previous studies, the presence or absence of NBL affects the diurnal variation in air-CO2 composition at ground -

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Fig 2: Diurnal variability of average mixing ratio and carbon isotopic ratio of atmospheric CO2 (standard deviation

are shown) observed for different seasons over Bangalore during the period covering October, 2008 till December, 2011. The different months coming under the same seasons were plotted together with same colored symbol. There are four different color legends used for four different seasons.

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Fig 3: (a) Seasonal variation of NBL obtained from Radiosonde data. The morning time (05:30 IST) vertical

profiles of Potential temperature were obtained for the respective months as shown in the figure

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level due to differential mixing of CO2 with free atmosphere (Guha and Ghosh, 2010). Thus this variability in NBL height during different seasons might be a responsible factor for the variability in the amplitude of diurnal signals in air CO2 composition as observed during different seasons. To quantify this effect on urban air quality, the extent of CO2 mixing from free atmosphere was estimated for morning time samples using a vertical mixing model approach. 3.1 Seasonal Variation in the Contribution of FA at the Ground Level

The contribution of CO2 from FA to the ground level was found to be higher during SWM and winter time compared to dry hot summer and post-monsoon months as shown in Fig 4. During both the dry hot summer months and the post monsoon months, strongly stratified layer with prominent NBL reduces the vertical mixing of CO2 from FA with CO2 from PA generated at the ground level, whereas during SWM the merging of NBL with Planetary Boundary Layer at the higher altitude allowed mixing of the FA with PA and give rise to higher contribution of FA at ground level. This suggests that during dry hot summer and post monsoon months the ground level CO2 pollution was not diluted with free air i.e. less polluted CO2, whereas the dilution in CO2 pollution takes place during SWM months. During December-February, the

contribution of CO2 from FA was found to be unusually higher. In absence of persistent NBL associated with weak convection, its effect might have been dampened during this period of our study. Further investigation during winter might give insight into this interpretation.

In summary, the seasonal variation in the structure of NBL has an effect on the vertical mixing of locally produce polluted CO2 at ground level with the CO2 from free air (FA) and consequently affects the amplitude of diurnal variation of mixing ratio and δ13C of air CO2 at a seasonal time scale. The contribution of CO2 from FA was found to be ~6% less during the pre and post monsoon period compared to the SWM seasons. Thus NBL height variability during different seasons can be considered as an important factor controlling the CO2 pollution at the ground level, i.e. when NBL remain close to ground it prevents the mixing of polluted CO2 with the FA and consequently give rise to highly polluted air with more anthropogenic CO2. A seasonal pattern in the time bound daily sampling of air CO2 from other cities around the world and India can provide an important measure to access air quality of urban atmosphere.

Some of these results can be used for comparison of pollution level and green house contribution globally.

Fig 4: Seasonal variation of percentage contribution of CO2 from free air (FA) showing the quality of air over urban

region

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4. Conclusions

The seasonal variability of NBL height and its effect on air CO2 was documented based on repeated sampling of air CO2 during different time of year. Our observation suggests that the ground-based air-CO2 is largely affected by the seasonal variability in atmospheric boundary layer condition. During dry hot summer and post monsoon time the presence of NBL close to ground level reduces the chance of mixing of locally produced CO2 with free air CO2, whereas during SWM the NBL was pushed upwards and it allowed mixing to happen. These results in higher amplitude of diurnal signals (both in mixing ratio and δ13C of air-CO2) for the period of dry hot summer and post monsoon months compared to SWM months. This temporal variability is usually considered while estimation the seasonal variability in CO2 fluxes from a region. Thus it is crucial to consider the boundary layer structural variability while estimating and comparing the seasonal fluxes of CO2 using global carbon cycle model. The long term monitoring of CO2 and the vertical structure of temperature of atmosphere using Radiosonde data can provide a new tool to quantify the

contribution of factors like seasonal variability in NBL on air-CO2 flux estimation during different seasons, especially from urban stations which are the primary emitter of CO2. Moreover, observation of similar kind from remote station will provide information on efficiency of NBL induced trapping of CO2 emitted at the ground level as compared with the observation from urban region. Such monitoring exercise will improve our understanding of carbon footprint of mega cities as well. Acknowledgments

This manuscript is a contribution to the Ministry of Earth Science, Government of India and project MoES/ATMOS/PP-IX/09. We take this opportunity to thank anonymous reviewers for important suggestions and comment. We thank the Divecha Centre for Climate Change, IISc for financial support and Department of Science and Technology for funding the project. We also thank to Prof. J.S. Srinivasan, Deivecha Centre for Climate Change, IISc for supporting the activity.

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