Effect of relative humidity on the mechanism of new particle formation from monoterpene oxidation

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1 Effect of relative humidity on the mechanism of new particle formation from monoterpene oxidation James N. Smith, Sabrina Chee (UC Irvine) Xiaoxiao Li, Jiming Hao, Jingkun Jiang (Tsinghua Univ.) Jonathan Abbatt (Univ. Toronto) Atmospheric Chemistry Mechanisms Conference 2018 December 7, 2018 Davis, CA

2 Motivation: Why is continental New Particle Formation (NPF) reduced during periods of high Relative Humidity (RH)? Indirect: High RH is accompanied by increased in condensation sink and decrease in solar radiation, both of which decrease NPF Direct: H 2 (g) may directly influence the formation of NPF precursors or clusters Extremely low volatility organic compounds (ELVC) are important compounds for NPF Formation of the observed Highly xidized Molecules (HM) via R 2 autoxidation Ehn, et al. 2014, Nature Ehn, et al. 2014, Nature Barsanti, 2017, JPCL Is the formation of NPF related HM suppressed by H 2 (g)?

3 New Instrument for measuring gas phase HM: Transverse Ionization Chemical Ionization Mass Spectrometer (TI CIMS) (4 10 lpm) No sheath air required simpler for experiments Based on the Cluster CIMS inlet designed by Zhao and Eisele, adapted for TFWERK ToF mass specs Position of the ion source relative to inlet orifice is adjustable to change reaction time Curtain gas passed over inlet orifice reduces water clustering For this study, N 3 analyte and mass analysis by a LTF MS. Zhao, Eisele et al., JGR, 2010 Hanson, et al., EST, 2011 Li et al., ACPD, 2018

4 TI inlet sensitivity to H 2 S 4 is higher than the Aerodyne CIMS inlet based on the design by Eisele and Tanner H 2 S 4 calibrator (Kürten et al., 2012) Li et al., ACPD, 2018

5 Instrument Setup and Experimental Conditions Precursor [precursor] (ppbv) [ 3 ](ppbv) [C 6 H 12 ] (ppmv) limonene α pinene 3 carene ± ± ± ± ± ± ± ± ±5 0 RH: 1 90% High and low monoterpene concentration to study impact of new particle formation. 3 with / without C 6 H 12 H scavenger Low Nx Li et al., ACPD, 2018

6 SA number concentration decrease with increasing RH, however, detected HM do not change! Limonene consumption: constant SA number/ NPF intensity: decreasing SA mass: relatively constant HM: constant! Li et al., ACPD, 2018 Li et al., ACPD, 2018

7 Decrease of number concentration at high RH is more consistent than change in mass concentration SA Number Concentration: decreasing by 2~3 factors SA Mass Concentration: changing within 2 factors divided divided Consistent with the result from Bonn et al., (2002); Water s influence on NPF (ELVCs) is much larger than it s influence on SA mass (LVCs/SVCs) generation. Li et al., ACPD, 2018

8 RH have no significant influence on the detected HM >400 peaks identified in each spectrum none of which changed with RH! Sum of Monomers Sum of Dimers Li et al., ACPD, 2018

9 Discussion: Why don t HM monomers change with RH? divided divided R 2 +R 2 chemistry dominates over R 2 +H 2 in our system. R2 channel H-shift

10 Discussion: Why don t HM dimers change with RH? It s reasonable that dimers are not influenced by H 2 (g) R2 channel H-shift

11 Discussion: sci accretion products may explain the decreasing SA number concentration and observed constant HM formation 0.44 C 10 H H 0.4 * PZ C 10 H 16 3 eci C 10 H 16 3 H ò Hydroperoxide Channel H shift C 10 H 17 2m+1 m th /n th generation R 2 C 10 H 15 2n+2 + M sci Channel ñ sci C 10 H 16 3 ô R R' SZ(sCI-C 10 ) R' R C10 + H 2 /alcohol/carboxlic acid õ C 10 R H ó H 2 H H a-hydroxy hydroperoxide C 10 H 18 4 R Anhydride (sci-c 10 ) H 2 Pinonic Acid C 10 H 16 3 H + H More xidized Products Conditions R 2 forms HM sci further oxidized by H sci forms SZ (sci C 10 ) sci forms anhydride (sci C 10 ) Formed in the gas phase Contributes to nucleation/early growth Suppressed by water vapor NT detected by nitrate CIMS Another possibility: Water vapor plays a role in hindering cluster growth and/or causing cluster fragmentation and evaporation

12 Conclusions High RH suppresses NPF from monoterpene oxidation, but not by decreasing R 2 autoxidation products. Cannot rule out the contribution of sci accretion products, which are not visible using N 3 CIMS. Also possible that HM clusters may react with water vapor in such as way as to suppress further growth Acknowledgements DE NSF AGS

13 Supplementary N 2 curtain gas significantly removes H 2 influence on detection, making the quantification of HMs at different RH comparable. RH>90%, N 2 =0 LPM RH>90%, N 2 =1 LPM The existence of water clusters in high RH largely complicates peak identification and accurate quantification.

14 Supplementary Prediction of vapor saturation mass concentration (C*) of the H and 3 generated C 10 and C 20 Predicted Volatility of the major C 10 and C 20 closed shell products SIMPL.1 (open points) A group contribution method (Pankow and Asher, 2008) Molecular Corridor method (filled points) Molecular formula based method (Li et al., 2016) The difference of C* predicted from the two methods was within one order of magnitude for C 20 HMs, and 3~4 orders of magnitude for C 10 HMs. Despite these differences, nearly all of the C 20 HMs can be classified as ELVCs, while C 10 products are mostly LVCs. Typically, the volatilities of 3 derived C 20 HMs were less than H related HMs.