Tapas Chakraborty Indian Association for the Cultivation of Science Calcutta, India MJ16, OSU ISMS 2013 Photochemistry of acetone in simulated atmosphere

Photochemical decomposition (46%) Annual atmospheric budget of acetone: Anthropogenic activity: 1.3  0.7 Tg/year (Industrial Solvents and Automobile exhaust) ~95 Tg (10 12 g)/ year flows into atmosphere MJ16, OSU ISMS 2013

Financial Support: Department of Science and Technology, Govt. of India Acknowledgement: Mr. Aparajeo Chattopadhyay Ms. Piyali Chatterjee

CH 3 –CO–CH 3 CH 3 CO  +  CH 3  CH 3 + CO +  CH 3 UV light, λ < 339 nm λ < 339 nm R G W Norrish ΔH 0 = 84.5 kcal /mol ΔH 0 = 95.5 kcal /mol UV light, λ < 299 nm λ < 299 nm Basics of acetone photochemistry Photo-oxidation CH 3 CO  +  CH O 2  CH 3 (CO)OO  + CH 3 OO  Photo-oxidation CH 3 CO  +  CH O 2  CH 3 (CO)OO  + CH 3 OO  0 km 20 km Wavelength (nm) Actinic Flux (Photons cm -2 s -1 nm -1 )

Early works…. JACS, 59, 2092 (1937)

Early works…. CH 3 (CO)OO  + CH 3 OO   CH 3 OH, HCHO, CH 3 O , HO 2 ,  OH, etc. Trans Farad Soc, 49, 1292 (1953) JACS, 75, 5810 (1953) JACS, 76, 1979 (1953)

Early works…. JPC, 65, 1622 (1961) JPC, 67, 1686 (1963)

Whytock et. al, Can. J. Chem, 45, 867 (1967) Queen’s college, Scotland Cohen, J. Chem. Phys, 47, 3828 (1967) University College, London Calvert et. al, J. Phys. Chem, 88, 5069 (1984) NCAR, Boulder, Colorado Ravishankara et. al, Geophys. Res. Lett, 24, 3177 (1997) NOAA, Univ. Colorado, Ravishankara et. al, Chem. Phys, 231, 229, (1998) NOAA, Univ. Colorado, Warneck et. al. J. Phys. Chem, 104, 9436 (2000) Max Planck Institut fur Chemie, Mainz, Germany Francisco et. al, Chem. Phys. Lett, 329, 179 (2000) Purdue University Pilling et. al, Chem. Phys. Lett, 365, 374 (2002) University of Leeds

Globar Source DTGS Detector UV B Lamps Philips TL-01 20W (Emission maximum - 311nm) Sample Vacuum Pump Pressure Gauge Measurement scheme IFS 66, Bruker Optics

Wavenumber (cm -1 ) IR spectroscopy probing of acetone photochemistry in absence of air Excitation wavelength – 311 nm Ro-vibrational transitions of CO

O.D (a.u) Wavenumber (cm -1 ) In synthetic air In absence of air 1033 cm cm -1 O.D (a.u) cm-1 Photo-oxidation in synthetic air yields HCOOH

acetone d6 - acetone Excitation Wavelength = 311 nm 1143 cm cm cm cm DCOOH DCOOD CD 3 OD Wavenumber (cm -1 ) d6 – acetone + O 2 Photo-oxidation of acetone d 6

Time Variation of products concentrations Time (minutes) O.D Acetone:O 2 :N 2 = 1:1:4 (synthetic air) Time (minutes) O.D Acetone:O 2 = 1:5

Longer exposure time (synthetic air) Time (minutes) O.D Acetone:O 2 :N 2 = 1:1:4 Time Variation of products concentrations

Adsorption of HCOOH on cell walls UV light was switched off after 2 hours of exposure O.D (HCOOH) Time (minutes)

Decrease in concentration of pure HCOOH with time O.D (HCOOH) Time (minutes) Adsorption of HCOOH on cell walls

A simple Model for estimation of HCOOH yield An apparent consecutive process CH 3 COCH 3 + O 2 HCOOH HCOOH unadsorbed k1k1 k2k2 Photochemical reaction Adsorption Assumptions: 1.HCOOH formation in step-1 occurs at a constant rate for a particular reaction set 2.Adsorption of HCOOH on cell walls follows 1 st order kinetics Rate of formation of HCOOH, d[HCOOH]/dt = k 1 – k 2 [HCOOH] Solving, [HCOOH] t = (k 1 /k 2 )[1-exp(-k 2 t)] ………. (1) Steps to estimate concentration of HCOOH produced in the reaction: k 2 is estimated following adsorption kinetics of HCOOH Corrected concentration of HCOOH produced at time t, [HCOOH] actual = k 1  t

· · Original data Corrected data HCOOH (molecules cm -3) Time (minutes) Corrected formation rate of HCOOH

Effect of oxygen pressure Ratio of partial pressures (O 2 /Acetone) OD Total pressure kept fixed

CH 3 OH / HCOOH CH 3 OH / HCHO HCOOH / HCHO Ratio of partial pressures (O 2 /Acetone) Relative OD Effect of oxygen pressure

Total Pressure (mbar) OD Varying total pressure keeping Acetone:O 2 fixed Varying total pressure of synthetic air Effect of total pressure

Reaction mechanism Ratio of partial pressures (O 2 /Acetone) OD Total pressure kept fixed CH 3 -CO-CH 3  CH 3 CO  +  CH 3 UV +2O 2 + M  CH 3 (CO)OO  + CH 3 OO  + M 2CH 3 OO   CH 3 OH + HCHO + O 2 2CH 3 OO   2CH 3 O  + O 2 2CH 3 O   CH 3 OH + HCHO CH 3  + O 2  HCHO +  OH  OH + CH 3 OH  HCHO + H 2 O HCO-H  HCO  + H   HO 2  2 HO 2   2OH  + O 2 CH 3 CO-OO    OH

We confirm once again that HCOOH is one of the major photoproducts of acetone photoreaction in atmospheric condition under exposure of UV-B light. The yields of formic acid for several specific set of reaction conditions have been estimated for the first time. The relative yields of CH 3 OH, HCHO and HCOOH vary differently upon increasing oxygen partial pressure in the reactor cell. The observation suggests that HCHO might be produced via more than one channel, which requires further measurements to settle. Summary