1. NO X AND NO Y IN THE TROPICAL MARINE BOUNDARY LAYER C. Reed, M. J. Evans, J. D. Lee, L. J. Carpenter, K. A. Read, L. Mendes

2. Observatory established in 2006 as global GAW station. Only GAW global station continuously measuring NO & NO 2 in the tropics & at a background site. Also measure… O 3 – Thermo 49i (2) CO – Picarro CRDS (Aero-lazer AL5001) NMHCs, o-VOCs – Agilent GC-FID + pre-concentrator Halocarbons – Agilent GC-MS + pre-concentrator Total gaseous mercury(TGM) – Tekran 2537A Meteorology – 10m Campbell, 30m Davis Spec-RAD (jNO 2, jO (1D) etc.) – Ocean Optics QE65000 + 2pi collector Our partners: Max Planck Institute Jena & Leibnitz-IFT. CH 4, CO 2, N 2 O, Aerosol (Online and offline) The Cape Verde Atmospheric Observatory (CVO)

3. The Cape Verde archipelago Tropical North Atlantic (16 o 51' 49 N, 24 o 52' 02 W)

4. The Cape Verde archipelago Constant strong North Easterly ~40 o, ~ 5 m s -1

5. Air masses sampled at Cape Verde Air from N-Atlantic, Europe, N-America, and Sahara

6. NO x measurements at Cape Verde Custom dual channel NO chemiluminescence instrument. Channel 1: NO and NO x by selective photolytic dissociation of NO 2 @ 395nm or 385nm (since March 2015). Nafion dryer on sample. Channel 2: NO y 4 channel thermal dissociation NO y (ΣPANs, ΣANs, HNO 3, ΣNO y ) LODs; NO ~0.3 ppt, NO 2 ~ 0.35 ppt, NOy ~ 5 ppt / hour. Calibrated:Sensitivity, NO 2 converter efficiency, artifact NO/NO 2 signals on both channels. Pre-reactor zero signal every 5 minutes NO x measuring near continuously since October 2006. NO y until 2009, Speciated NO y since 2015 Original single channel instrument from 2006 – 2009, upgraded to dual channel 2009 – present. Air Quality Design, inc. Golden, Colorado, USA

7. Cape Verde NO x Typically low mixing ratios of NO x ; 10 – 50 pptv leading to net ozone destruction --- NO 2 --- NO --- O 3

8. Sources of NOx at Cape Verde NO y inlet mounted on 10m tower 3 heated quartz furnaces 1 molybdenum catalyst Switchable cyclone 1. http://www.leos.le.ac.uk/group/mpb/images/trop5.png NO y species a reservoir for NO x Different compound classes in gas and particulate phase necessitates speciation measurement. NO y decomposition believed to be source of NO x

9. NO y speciation measurement technique NO x from thermal decomposition of NO y species detected quantitatively. NO y = NO + NO 2 + + PANs+ XONO 2 + NO 3 + N 2 O 5 + HONO + HO 2 NO 2 + RO 2 NO 2 + CH 3 ONO 2 + C 2 H 5 ONO 2 + …+ RONO2 + HNO 3 + p-NO 3 − + … ∑PANs ∑ANs HNO 3 NPN HNO 3 NO and NO 2 observed Inter-conversion conserves NO x Need to measure both

10. Surprisingly high labile NO y Particulate nitrate measurements agree with filter sample record; 300-400 ppt 1 Gas phase nitric acid agrees with previous wet measurements; 30-50 ppt 2 Alkyl nitrates consistent with other published observations; 0-30 ppt 3 1.Fomba et al., Atmos. Chem. Phys., 14, 8883-8904, 2014 2.Lawler et al., Geophys. Res. Lett. 36, L08810, 2009 3.Chuck et al., Science, 297, 1151-1154, 2002

11. NO y at Cape Verde Winter-time peak in NO y observed, not replicated by models Model Data NOy seasonality driven by abundant thermally labile compounds…. Possibly water soluble organic nitrates (WSON) – is a current area of investigation

12. NO x production in the Marine Boundary Layer NO x peaks during daylight hours, indicating a source from photolysis. Models predict thermal decomposition of NO y as source of NO x ….. Lost by reaction of NO 2 + OH to HNO 3 and deposited.

13. Daytime source of NO x ‘Renoxification’ by nitrate photolysis on aerosol. 1,2 Nitrate photolysis found to be up to 100x faster than HNO 3. 3 Constrain model to observed IO and BrO 4, also p-Nitrate 5 NO x O3O3 OH Preliminary modelling suggests that nitrate photolysis reproduces observed NO x well Little effect on ozone or OH of new NO x source, observations replicated ---- Measured ---- Model w/o p-NO 3 photolysis ---- Model with p-NO 3 photolysis 1.Ndour et al., Geophys. Res. Lett., 36, L05816, 2009 2.Scharko et al., Environ. Sci. Tech., 48, 11991-20001, 2014 3.Ye et al., Nature, 2016 4.Read et al., Nature, 453, 1232-1235, 2008 5.Fomba et al., Atmos. Chem. Phys., 14, 8883-8904, 2014

14. Conclusions and outlook Abundant NO y species observed at Cape Verde Possibly from WSON Not main source of NO x however Rapid nitrate photolysis drives NO x production Source of daytime peak of NO x and absence of night-time build up. Incorporating fast nitrate photolysis into our box model Apply fast nitrate photolysis to a global CTM model Conduct supporting measurements of HONO Acknowledgements: Dr Wadinga Fomba, Dr Marty Buhr, Dr Stephan Bauguitte, Helder Timas, Gisella Duarte, NCAS, NERC, INMG.