Measurements of HONO during OASIS in Barrow, Alaska G. Villena Tapia 1, P. Wiesen 1, F. M. Flocke 2, A. J. Weinheimer 2, D. D. Montzka 2, D. J. Knapp 2, S. R. Hall 2, K. Ullmann 2, L. Mauldin 2, E. Kosciuch 2, J. McGrath 2, H. J. Beine 3, R. M. Staebler 4, J. Kleffmann 1 1 Physical Chemistry Depart., University of Wuppertal, Germany. 2 National Center for Atmospheric Research (NCAR), Colorado, USA. 3 Land, Air and Water Resources, University of California Davis, USA. 4 Environment Canada, Toronto, Canada. kleffman@uni-wuppertal.de
Introduction Nitrous Acid (HONO) direct OH source: HONO + hν (<39 nm) NO + OH OH: "cleaning detergent of the atmosphere" For polar conditions HONO was proposed to be one of the most important OH sources Different heterogeneous formation mechanisms proposed: NO 3- + hν NO 2- ( HONO) + O 2 NO 2 + H 2 O HONO + HNO 3 NO 2 + humic acid + hν HONO + products Still under discussion
Introduction Polar HONO was mostly measured by "wet chemical" instruments (mistchambers, denuders, coil/hplc ) The only polar HONO intercomparison show a factor of 7 overestimation of a mist-chamber/ic compared to the spectroscopic LIF technique (Liao et al., 26) Models indicate strong overestimation of measured polar HONO levels (e.g. Bloss et al., 26) "Wet chemical" HONO measurements may be affected by strong interferences Still under discussion
Aim of the study Interference-free measurement of HONO in polar regions during OASIS in Barrow, Alaska Has [HONO] been overestimated before? Is HONO a net source of OH radicals in polar regions? What is the formation mechanism of HONO in polar regions?
Experimental HONO was measured by the LOPAP instrument - interferences are corrected - no sampling lines - validated (DOAS) - extremely sensitive: DL:.2 pptv
Experimental External sampling unit no sampling lines sampling effluent pumped to the instrument HONO sampled by a fast, selective reaction in a stripping coil instrument in the container
Experimental Other parameters used for evaluation: - OH (NCAR) - NO, NO 2, NO y (NCAR) - J-values (NCAR) - O 3 (NCAR) - meteorological data (Environment Canada)
HONO was measured from 13 th March - 14 th April Very high concentrations often observed 6 HONO [ppt] 5 HONO [pptv] 4 3 2 1 13.3.29 23.3.29 2.4.29 12.4.29 date
High correlation with CO Local emissions from BARC and Barrow 6 5 HONO [ppt] CO ppbv 5 45 HONO [pptv] 4 3 2 4 35 3 25 CO [ppbv] 1 2 15 24.3.29 12: 25.3.29 : 25.3.29 12: 26.3.29 : date
Only "clean" days considered 7 6 HONO [ppt] CO ppbv 2 19 HONO [pptv] 5 4 3 2 1 18 17 16 CO [ppbv] 15 26.3.29 : 26.3.29 12: 27.3.29 : date
"Average" clean day HONO [pptv] 12 1 8 6 4 2 HONO HONO/NOx [%] HONO/NOy [%] 18 15 12 9 6 3 HONO/NO x, HONO/NO y [%] : 6: 12: 18: : time
OASIS average clean day: - HONO/NO x = 5.3±2.8 % - HONO/NO y = 1.±.5 % In excellent agreement with other remote LOPAP data: - Zugspitze: HONO/NO x = 2.5 %, HONO/NO y = 1. % - Jungfraujoch: HONO/NO x = 4.6 %, HONO/NO y = 1.1 % Factor 4 lower compared to most other polar data - HONO/NO x = 2-1 % Was explained by interferences (Kleffmann and Wiesen, 28) Should be corrected for also for other instruments
Theoretical HONO daytime level by photostationary state (PSS) assumption and know gas phase reactions: HONO + hν NO + OH J (HONO) NO + OH + M HONO + M k (NO+OH) HONO + OH NO 2 + H 2 O k (HONO+OH) [HONO] PSS = J k (NO+ OH) (HONO) + k [NO] [OH] (HONO+ OH) [OH] known (IUPAC) measured [HONO] theo. can be calculated during daytime
[HONO] meas. >> [HONO] theo. 12 1 HONO(meas) HONO(PSS) HONO(PSS+khet) HONO [pptv] 8 6 4 2 : 6: 12: 18: : time Extra HONO/OH source: d[hono]/dt extra
Extra daytime HONO/OH source of up to 8 pptv/h Correlates with J(NO 2 ) HONOx1 [pptv], extra HONO/OH source [pptv/h] 12 1 8 6 4 2 HONOx1 d[hono]/dt(extra) J(NO2) : 6: 12: 18: : time.15.12.9.6.3 J(NO 2) [s -1 ]
Extra HONO/OH source >> OH source by O 3 -photolysis HONO [pptv], OH sources [pptv/h] 12 1 8 6 4 HONOx1 d[hono]/dt(extra) doh/dt (O3) J(NO2).15.12.9.6 J(NO 2) [s -1 ] 2.3 : 6: 12: 18: : time
Extra HONO/OH source correlates with J(NO 2 ) -NO 2 +HA+hν correlates with J(NO 2 ) - nitrate-photolysis not 12 d([hono]/dt(extra) [pptv/h] 1 8 6 4 2.2.4.6.8.1.12 J(NO 2 ) [s -1 ]
But correlates also with J(O 1 D) - nitrate photolysis would correlate with J(O 1 D) 12 d([hono]/dt(extra) [pptv/h] 1 8 6 4 2 1E-6 2E-6 3E-6 4E-6 5E-6 6E-6 J(O 1 D) [s -1 ]
Photochemical HONO formation observed But: Mechanism still not clear Can be hopefully further clarified when still missing data ([HA], [nitrate], etc.) is available
Conclusion HONO successfully measured under polar conditions for the first time by the LOPAP technique HONO/NO x and HONO/NO y in excellent agreement with other remote LOPAP data, but much lower compared to previous data from "wet chemical" instruments HONO] was overestimated in the past, in excellent agreement with recent LIF data from South Pole Interferences are a serious problem for "wet chemical" HONO instruments
Conclusion [HONO] exp. >> [HONO] theo. Extra HONO source of up to 8 pptv/h quantified Much more important compared to the O 3 -photolysis Extra HONO source correlates with light intensity Photochemical source proposed in good agreement to other studies Formation mechanism still not clear
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