Polycyclic Aromatic. atmosphere of Guangzhou,

Transcription

1 Size distributions of Polycyclic Aromatic Hydrocarbons in the urban atmosphere of Guangzhou, China Huan Yu and Jian Zhen Yu Dept of Chemistry Hong Kong University of Science and Technology

2 Target pollutants fractal soot d m = 6 nm near-spherical soot d m = 47 nm

3 Nano-MOUDI and inversion method Nano-MOUDI collection efficiency curves NanoMoudi-IITM 5A.5 Mode Mode Mode 3 Mode 4 histogram dc C/Cdlog(Dp).5.. 3

4 Thermal desorption-gcms method for PAHs analysis in aerosol samples Aerosol-laden filter inserted into GC injector liner PAH vaporize in GC liner and enter GC-MS system No solvent extraction Save time Lower MDL 4

5 Modal characteristics of PAHs with various volatilities (urban GZ, July 6).4. PHE =.5ng/m 3.4. CHR=3.ng/m 3.4. FLA =.ng/m 3 dc C/Cdlog(Dp) dlog(dp) dc/c /Cdlog(Dp) dc/ BaP=.3ng/m 3.4. IND=9.3ng/m 3.4. COR=6.ng/m 3 dc/c Cdlog(Dp) log(dp) dc/cdl dc/cdlo og(dp)

6 Mean mode concentrations of thirteen PAHs in Guangzhou urban samples 6

7 PAH partitioning and size distribution in fresh soot particles Equilibrium timescale τ K ppm soot ρ sootd eq = < πnd D ( + K PM ) πnd D = D PM p g p soot p g g p soot. PAHs quickly reach an adsorption equilibration (<s). Most of freshly emitted PAHs are in particle phase upon exiting the exhaust pipe 3. Size distribution ib ti patterns of all PAH compounds on fresh soot particles are the same as the surface area size distribution of soot particles. 7

8 In the atmosphere, sorption mechanisms of organic vapors remain ambiguous in the literature The importance of gas/particle partitioning. The transport and fate of organic pollutants in the atmosphere. SOA yield (Pankow 994a,b; Odum et al. 996) 8

9 Studies arguing EC adsorption is important. K p values of PCBs and PCNs are faithfully predicted from KOA, but under-predict K p values of PAHs, often by to 5 times. Harner and Bildleman 998. Absorption in OM accounted for less than % of the total PAHs sorbed on the aerosols in Baltimore. Dachs and Eisenreich 3. Unusually high solubilities in particles are observed for PAHs, but not PCBs. Cousins and Mackay 4. Inclusion of adsorption onto BC increased the fraction on particles, resulting in a closer approximate to the measured logk p. Lohmann and Lammel 4 Studies arguing OM absorption is important. In the atmosphere, EC surfaces become rapidly attenuated with OM and salts, which immediately lowers their importance as a sorbing phase. Hans Peter H. Arp et al. 8. K p estimation based on Poly-Parameter Linear Free Energy Relationships (PP- LFERs) methods showed that absorption is the dominating mechanism. Roth et al. 5a,b 9

10 Our evidence supporting EC adsorption Indication size distribution of EC, OC, and PAHs Indication High abundance of EC at GZ μg m -3 EC OC Chicago a Philadelphia a Baltimore a Guangzhou a. USEPA PHE =.5 ng m PYR =.5 ng m BkF=. ng m PER=.7 ng m COR=6. ng m ANT = ng m CHR=3. ng m BeP=9. ng m IND=9.3 ng m EC=5.6 μg m -3.. d FLA =. ng m BbF=3. ng m BaP=.3 ng m BghiP=.6 ng m OC=5.7 μg m -3..

11 Our evidence supporting EC adsorption Indication 3 Correlations of PAHs and ECOC in size modes of all samples n=35 EC OC phenanthrene.69.4 Anthracene fluoranthene Pyrene.7.3 Chrysene.69. benzo[b]fluoranthene benzo[k]fluoranthene. benzo[e]pyrene.48. benzo[a]pyrene.49. Perylene 5. indeno[,,3-cd]pyrene.5 benzo[ghi]perylene.46.3 Coronene.4.4

12 Our evidences supporting EC adsorption Indication 4 Predicted K p from absorption/adsorption vs. measured K p in urban GZ in summer 5 (Yang et al. ) m3 μg-) log Kp ( measured Kp range by Yang et al. predicted Kp from OM absorption+ EC adsorption predicted Kp from OM absorption only PHE FLU PYR CHR BaP BghiP

13 Size-resolved theoretical partition coefficients K p = ( Q QV ) / RT f EC N sasootte f 76RT + 6 MW γ om om L MW om p mode represented by D p PHE FLA PYR CHR BaP BghiP COR nuclei mode.4 μm condensation mode I. μm condensation mode II.4 μm droplet mode μm coarse mode 4 μm TSP Percentage contributions to K p due to EC adsorption mode represented by D p PHE FLA PYR CHR BaP BghiP COR nuclei limode 4μ.4 μm 94.3% 9.5% 97.% 98.6% 99.4% 99.% 86.% condensation mode I. μm 98.% 97.6% 99.% 99.6% 99.8% 99.7% 95.3% condensation mode II.4 μm 93.6% 9.6% 96.9% 98.4% 99.3% 99.% 84.4% droplet mode μm 86.9% 83.% 93.4% 96.6% 98.5% 97.8% 7.% coarse mode 4 μm 76.8% 7.% 87.6% 93.5% 97.% 95.7% 55.% TSP 85.% 8.3% 9.% 96.% 98.% 97.3% 69.4% 6 3

14 The equilibration timescales of PAHs with particles of five size modes in GZ urban atmosphere Equilibrium timescale.e+5 τ eq = PM * πnd p K p D g (hour) τeq (.E+4.E+3.E+.E+.E+.E-.E-.E-3.E-4.E-5.E-6 coarse mode accumulation mode III accumulation mode II accumulation mode I nuclei mode 4 d 4. d hr hr 6 min 36 s 3.6 s.36 s 36 ms 3.6 ms.e-7 Phe, Ant Flu Pyr Chr BaP, BeP, BbF, BkF, Per BghiP, Ind Cor 4

15 Implications. Timescale estimation based on theoretical partition coefficients is consistent with PAH size distribution observations and predicts that measured 5-6 ring PAHs in urban aerosols are lower than the theoretical ti values.. PAH partition behaviors are different in different particle sizes and should be examined on a size-resolved basis. 5

16 Can sorption mechanisms of urban scenario be extrapolated to remote location scenarios?.5.5 PHE=6. ng m ANT=.98 ng m FLA=8.9 ng m PYR=6.9 ng m -3 The role of adsorption will decrease, but CHR=5 9ngm BbF=6.6 6 ng m -3 dc/cd dlogdp BeP=3. ng m -3.. EC=4.8 μg m -3.. dc/cd logdp PER=.8 ng m -3.. OC=6.83 μg m -3.. dc/cd dlogdp BkF=.7 ng m IND=.9 ng m -3.. SO 4 - =6.5 μg m -3.. dc/cdlogd Dp dc/cd dlogdp BaP=.8 ng m -3.. BghiP=.3 ng m -3.. K + =.9 μg m -3.. Size distributions of PAHs, EC, OC and sulfate after aerosol aging (PRD receptor sites: HKUST and Qingyuan) Sulfate aqueous coating on EC could trap PAHs inside particles, preventing volatilization from particle phase and oxidation by gaseous oxidants. Theoretical partition coefficients may underestimate aerosol phase PAHs due to internal mixing of EC and OM/salts. 6

17 Can PAH sorption mechanisms be extrapolated to other organic vapors? Organic vapor Water-Insoluble Aerosol Fraction Water-soluble Aerosol Fraction EC WIOM minerals Salts WSOM Aqueous phase apolar combustion process (PAHs,PCDD/Fs) non-combustion process (PCB,PCNs) polar ionizable. PAHs are produced as precursors of soot during the combustion processes.. Salt/organic coating trap PAHs inside the soot core, but prevent non-combustion original organic vapors from adsorption by the soot core. 3. Adsorption may be important for combustion generated PAHs, but not for noncombustion generated vapors. 7

18 Conclusions. Accumulation modes with MMAD at.4 μm and. μm accounted for 49%~% of PAHs. Coarse mode was only important for 3-4 ring PAHs (6%-3%).. Timescale estimation based on theoretical partition coefficients is consistent ste t with PAH size distribution observations. o s 3. EC adsorption contributed significantly to aerosol phase PAHs in GZ urban atmosphere. But theoretical partition coefficients overestimate 5~7 ring PAHs in urban aerosols. 8

19 Acknowledgement Prof. Jianzhen Yu for her supervision of this study. Peking University for organizing PRIDE-PRD6 campaign. This work was partially supported by the Research Council of Hong Kong, China (645). 9

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