Temporal assessment of volatile organic compounds at a site with high atmospheric variability in the North-West Province

Transcription

1 Temporal assessment of volatile organic compounds at a site with high atmospheric variability in the North-West Province K. Jaars B.Sc. Industrial Science Thesis submitted in the fulfillment of the requirements for the degree Master of Science in Chemistry at the Potchefstroom Campus of the North-West University Supervisor: Co-supervisor: Dr. P.G. van Zyl Dr. J.P. Beukes November 2012 Potchefstroom

2 Acknowledgements I would like to acknowledge and thank the following people, institutions and organisations that have aided me in the completion of this study. First and foremost, I want to thank my Heavenly Father, and by doing so quoting this prayer from the movie Book of Eli. Dear Lord, thank you for giving me the strength and conviction to complete the task (my M.Sc.) you entrusted to me. Thank you for guiding me straight and true through the many obstacles in my path. And for keeping me resolute when all around seemed lost. Thank you for your protection and for your many signs along the way. Thank you for the good that I may have done. I am so sorry about the bad. Thank you for the friends I made during my studies. Please watch over them as you watched over me. I fought the good fight. I finished the race. I kept the faith. My sincerest gratitude to my supervisors, Pieter van Zyl and Paul Beukes, who have supported me throughout my post graduate studies. Their encouragement, supervision and insights enabled me to develop an in-depth understanding of the subject. This thesis would have been impossible without them. Many scientists and professionals have in one way or another contributed to my scientific development and results presented in this thesis. Dr. Heidi Hellén, Dr. Hannele Hakola, Hermanni Aaltonen of the Finnish Meteorological Institute assisted with the VOC analyses and contributed to my development during my visit in Finland, Helsinki. I am especially grateful to Dr. Lauri Laakso and Ville Vakkari for their advice, patience, support, feedbacks and useful comments. I would like to thank Heikki Laakso for the development of the VOC auto sampler. The University of Helsinki and the Finnish Meteorological Institute have been sources of friendships, as well as good advice and collaboration. Sincere thanks to all my colleagues and friends in the Atmospheric Research and Chromium Technology Research Group for their support, interest and encouragement. In particular, I would like to thank Andrew Venter, Dr. Micky Josipovic and Dr. Petri Tiitta for all the time they dedicated to helping me with my VOC measurements. Also, to Wanda Booyens for taking care of my measurements during the time I was in Finland. I am grateful for Cecile van Zyl for the time and effort she put into proofreading my thesis. Acknowledgements Page i

3 The following institutions are gratefully acknowledged for their financial support: The Atmospheric Research and Chromium Technology Research Group at the North-West University, The Finnish Meteorological institute, the University of Helsinki and the National Research Foundation (NRF). Finally, my sincerest appreciation to my family. Not only for their constant encouragement, but also for their never-ending love, prayers, support, encouragement and understanding during this period. My deepest gratitude goes to my parents, Isak and Els Jaars, for their everlasting love to me. I am indebted to my parents for their care and love. Special thanks go to my brothers and sisters for their support. It is difficult to find words to express my gratitude to them. Without their encouragements, I would not have finished this degree. It is also to my family I would like to dedicate this work. Thank you Kerneels Jaars Acknowledgements Page ii

4 Abstract Volatile organic compounds (VOCs) are emitted into the atmosphere from biogenic and anthropogenic sources with atmospheric lifetimes ranging from minutes to months, depending on the specific VOC compound considered. It is estimated that biogenic VOCs (BVOCs) (e.g. isoprenes, terpenes) make up 90% of the global atmospheric VOC budget. However, in highly industrialised regions, anthropogenic VOCs (e.g. benzene, toluene, ethylbenzene and xylene, combined abbreviated as BTEX) might dominate. VOCs have various reversible and irreversible effects on human health. They also have environmental impacts that range from changes in the population of terrestrial and aquatic ecosystems to the extinction of vulnerable species. VOCs are precursors for the formation of ozone (O 3 ) during solar radiation initiated reactions in the presence of NO x. Tropospheric O 3 is considered a pollutant, with negative impacts on human health, ecosystems and food security. O 3 is also a short-lived greenhouse gas. Through reactions with radical species, VOCs lead to the formation of higher molecular weight organic compounds, which produce carbon monoxide (CO), peroxyacytyl nitrate (PAN) and ultimately secondary organic aerosol (SOA) particles. SOA particles impact directly on air quality and visibility, as well as directly and indirectly on the radiation balance of the earth that contributes to the regulation of climate. Notwithstanding the importance of atmospheric VOCs, limited data is available for VOCs in South Africa. In this study, a comprehensive dataset of BVOC and anthropogenic VOC species was obtained at the Welgegund measurement station in the North West Province, South Africa. Measurements were conducted from 9 February 2011 to 4 February Samples were collected on Tenax-TA and Carbopack-B adsorption tubes twice a week for two hours during day time and two hours during night time. The first 1.25m of the stainless steel sampling inlet was heated to 120ºC to remove O 3 that could lead to sample degradation. Analyses of the sampled adsorption tubes were conducted by thermal desorption, cryofocusing, re-desorption, followed by gas chromatography separation and analysis with a mass selective detector (GC-MS). The results indicated that toluene was the most abundant aromatic hydrocarbon and heptane the most abundant alkane. Benzene is currently the only VOC listed as a criteria pollutant in the South African Air Quality Act with an annual average standard of 1.6ppb. The annual Abstract Page iii

5 median benzene concentration was 0.13 ppb, while the highest daily benzene concentration measured was 8.7 ppb. No distinct seasonal cycles were identified for anthropogenic VOC species measured, i.e. aromatic hydrocarbons and alkanes. However, air mass history analysis indicated that air masses that passed over the Mpumalanga Highveld, the Vaal Triangle and the Johannesburg-Pretoria conurbation (collectively referred to as Area I) had significantly higher concentrations of these anthropogenic VOCs compared to air masses that passed over the western and eastern Bushveld Igneous Complex, and a region over which air masses typically followed an anti-cyclonic movement pattern (collectively referred to as Area II). Anthropogenic VOC levels in air masses that passed over the regional background (areas with no large point sources) had levels similar to air masses that had passed over Area II. Relatively good interspecies correlations (r > 0.8) between most of the aromatic hydrocarbons in air masses that had passed over Area I, with the exception of benzene, indicated that these species had common sources. Benzene, however, correlated well with CO, indicating that sources associated with incomplete combustion were most likely the origin of benzene in air masses that had passed over Area I. The interspecies concentration ratios for plumes passing over Area I indicated that this source region is relatively close to the Welgegund monitoring station and air masses that passed over this source region were substantially influenced by anthropogenic activities. The concentration ratios for plumes that passed over Area II and the Regional Background indicated that these were aged air masses. Furthermore, the concentration ratios of toluene, ethylbenzene and o,m,p-xylene (TEX) to the total aromatic concentration for air masses that passed over the various source regions showed a greater contribution to the total VOC concentration during periods of higher temperature, i.e. summer. This proved that the evaporation of solvents contributes significantly to VOC levels during the months with higher temperatures. The relative contribution of aromatic hydrocarbons to photochemical O 3 formation in air masses that passed over the various source regions indicated the highest contribution was observed for air masses that passed over Area I, with Area II and the Regional Background in the same order of magnitude. The annual temporal variations of the measured BVOCs indicated that 2-methyl-3-buten-2-ol (MBO) and isoprene exhibited distinct seasonal patterns, i.e. higher values in summer and lower values in winter. The monoterpenes (MT) and the sesquiterpenes (SQT) did not follow Abstract Page iv

6 distinct seasonal patterns. BVOC concentrations correlated relatively well to seasonal variations in temperature, photosynthetically active radiation (PAR), rainfall, relative humidity (RH) and CO 2 flux. This proved that biogenic activity is responsible for BVOCs emitted. The most abundant MT was -pinene, while -caryophyllene was the most abundant SQT with annual median concentrations of ppb and ppb, respectively. Pollution roses for isoprene showed a dominance of sources from the north-west to the north-east, as well as the south-east. These directions correlated to areas where pockets of the savannah biome are located. Keywords: Volatile organic compounds (VOCs), aromatic hydrocarbons, biogenic VOCs, BTEX, Welgegund Abstract Page v

7 Table of contents Acknowledgement Abstract i iii Graphical layout of Thesis 1 Introduction 1.1 Background Significance of the study Objectives Literature survey Graphical layout of Chapter Introduction Defining air pollution Types of pollutant species Volatile Organic Compounds (VOCs) Definition of VOCs Sources of VOCs Anthropogenic sources Biogenic sources Chemistry of atmospheric VOCs Reaction of alkanes Reaction of aromatic hydrocarbons Reaction of BVOCs Tropospheric lifetimes of VOCs The role of reaction products of VOCs in the troposphere The role in O 3 formation Secondary organic aerosol formation...27 Table of contents Page vi

8 2.4.5 Impacts of VOCs A global perspective of atmospheric VOCs VOC measurements Sampling techniques VOC sampling with adsorbent tubes Meteorology of the South African interior Conclusion Experimental procedures Graphical layout of Chapter Air mass back trajectory analysis Site location Sampling and analysis VOC sampling Analytical procedure Supporting measurements Quality assurance Results and discussion Graphical layout of Chapter Compiled VOC dataset Aromatic hydrocarbons Temporal variations of aromatic hydrocarbons Contextualising the measured aromatic hydrocarbon concentrations Spatial assessment of aromatic hydrocarbons according to air mass origin Correlation analysis of aromatic hydrocarbons Plumes passing over Area I Plumes passing over Area II Plumes passing over the Regional Background...71 Table of contents Page vii

9 4.2.5 Aromatic hydrocarbon concentration ratios Ozone formation potential of aromatic hydrocarbons according to air mass origin Effect of photochemical activity on aromatic hydrocarbon concentrations Alkanes Temporal variations of alkanes Spatial distribution of alkane according to air mass origin Biogenic VOCs Meteorological parameters Temporal variations of BVOCs The influence of meteorology on BVOC concentrations at Welgegund The impact of temperature The influence of radiation Spatial distribution of BVOCs Conclusion Project Evaluation and Future Perspectives 5.1 Project evaluation Future perspectives...97 References...98 Appendix A Table of contents Page viii

10 Graphical layout of Study Chapter 1: Introduction Chapter 5: Project evaluation and future perspectives 1.1 Background 1.2 Significance of the study 1.3 Objectives 5.1 Project evaluation 5.2 Future perspectives Chapter 2: Literature survey 2.1 Introduction 2.2 Defining air pollution 2.3 Types of pollutant species 2.4 Volatile organic compounds 2.5 VOC measurements 2.6 Meteorology of South African interior 2.7 Conclusion Chapter 3: Experimental procedures Chapter 4: Results and discussion 3.1 Air mass back trajectory analysis 3.2 Site location 3.3 Sampling and analyis 3.4 Supporting measurements 3.5 Quality assurance and control 4.1 Compiled VOC dataset 4.2 Aromatic hydrocarbons 4.3 Alkanes 3.4 Biogenic VOCs