ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-14-4979-2014 Nighttime observation and chemistry of HO<sub>x</sub> in the Pearl River Delta and Beijing in summer 2006 Lu K. D. https://orcid.org/0000-0001-9425-9520 1 2 Rohrer F. 2 Holland F. 2 Fuchs H. https://orcid.org/0000-0003-1263-0061 2 Brauers T. 2 Oebel A. 2 5 Dlugi R. 3 Hu M. 1 Li X. https://orcid.org/0000-0003-2322-4069 1 2 Lou S. R. 2 4 6 Shao M. 1 Zhu T. https://orcid.org/0000-0002-2752-7924 1 Wahner A. https://orcid.org/0000-0001-8948-1928 2 Zhang Y. H. 1 Hofzumahaus A. https://orcid.org/0000-0003-2876-0880 2 State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, China Institut für Energie und Klimaforschung: Troposphäre, Forschungszentrum Jülich, Jülich, Germany Arbeitsgruppe Atmosphärische Prozesse (AGAP), Munich, Germany School of Environmental Science and Technology, Shanghai Jiao Tong University, Shanghai, China now at: Carl Zeiss SMS GmbH, Jena, Germany now at: Shanghai Academy Of Environmental Sciences, Shanghai, China 21 05 2014 14 10 4979 4999 Copyright: © 2014 K. D. Lu et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/14/4979/2014/acp-14-4979-2014.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/14/4979/2014/acp-14-4979-2014.pdf

Nighttime HO<sub>x</sub> chemistry was investigated in two ground-based field campaigns (PRIDE-PRD2006 and CAREBEIJING2006) in summer 2006 in China by comparison of measured and modeled concentration data of OH and HO<sub>2</sub>. The measurement sites were located in a rural environment in the Pearl River Delta (PRD) under urban influence and in a suburban area close to Beijing, respectively. In both locations, significant nighttime concentrations of radicals were observed under conditions with high total OH reactivities of about 40–50 s<sup>−1</sup> in PRD and 25 s<sup>−1</sup> near Beijing. For OH, the nocturnal concentrations were within the range of (0.5–3) × 10<sup>6</sup> cm<sup>−3</sup>, implying a significant nighttime oxidation rate of pollutants on the order of several ppb per hour. The measured nighttime concentration of HO<sub>2</sub> was about (0.2–5) × 10<sup>8</sup> cm<sup>−3</sup>, containing a significant, model-estimated contribution from RO<sub>2</sub> as an interference. A chemical box model based on an established chemical mechanism is capable of reproducing the measured nighttime values of the measured peroxy radicals and $k_{\text{OH}}$, but underestimates in both field campaigns the observed OH by about 1 order of magnitude. Sensitivity studies with the box model demonstrate that the OH discrepancy between measured and modeled nighttime OH can be resolved, if an additional RO<sub>x</sub> production process (about 1 ppb h<sup>−1</sup>) and additional recycling (RO<sub>2</sub> → HO<sub>2</sub> → OH) with an efficiency equivalent to 1 ppb NO is assumed. The additional recycling mechanism was also needed to reproduce the OH observations at the same locations during daytime for conditions with NO mixing ratios below 1 ppb. This could be an indication that the same missing process operates at day and night. In principle, the required primary RO<sub>x</sub> source can be explained by ozonolysis of terpenoids, which react faster with ozone than with OH in the nighttime atmosphere. However, the amount of these highly reactive biogenic volatile organic compounds (VOCs) would require a strong local source, for which there is no direct evidence. A more likely explanation for an additional RO<sub>x</sub> source is the vertical downward transport of radical reservoir species in the stable nocturnal boundary layer. Using a simplified one-dimensional two-box model, it can be shown that ground-based NO emissions could generate a large vertical gradient causing a downward flux of peroxy acetic nitrate (PAN) and peroxymethacryloyl nitrate (MPAN). The downward transport and the following thermal decomposition of these compounds can produce up to 0.3 ppb h<sup>−1</sup> radicals in the atmospheric layer near the ground. Although this rate is not sufficient to explain the complete OH discrepancy, it indicates the potentially important role of vertical transport in the lower nighttime atmosphere.

 References Atkinson, R. and Arey, J.: Gas-phase tropospheric chemistry of biogenic volatile organic compounds: a review, Atmos. Environ., 37, S197–S219, 2003. Brown, S. S., Ryerson, T. B., Wollny, A. G., Brock, C. A., Peltier, R., Sullivan, A. P., Weber, R. J., Dube, W. P., Trainer, M., Meagher, J. F., Fehsenfeld, F. C., and Ravishankara, A. R.: Variability in nocturnal nitrogen oxide processing and its role in regional air quality, Science, 311, 67–70, 2006. Crounse, J. D., Paulot, F., Kjaergaard, H. G., and Wennberg, P. O.: Peroxy radical isomerization in the oxidation of isoprene, Phys. Chem. Chem. Phys., 13, 13 607–13 613, <a href="http://dx.doi.org/10.1039/c1cp21330j">https://doi.org/10.1039/c1cp21330j</a>, 2011. Di Carlo, P., Brune, W. H., Martinez, M., Harder, H., Lesher, R., Ren, X., Thornberry, T., Carroll, M. A., Young, V., Shepson, P. B., Riemer, D., Apel, E., and Campbell, C.: Missing OH Reactivity in a Forest: Evidence for Unknown Reactive Biogenic VOCs, Science, 304, 722–724, 2004. Ehhalt, D. H.: Photooxidation of trace gases in the troposphere, Phys. Chem. Chem. Phys., 1, 5401–5408, 1999. Eisele, F. L., Mount, G. H., Tanner, D., Jefferson, A., Shetter, R., Harder, J. W., and Williams, E. J.: Understanding the production and interconversion of the hydroxyl radical during the Tropospheric OH Photochemistry Experiment, J. Geophys. Res., 102, 6457–6465, 1997. Emmerson, K. M. and Carslaw, N.: Night-time radical chemistry during the TORCH campaign, Atmos. Environ., 43, 3220–3226, 2009. Emmerson, K. M., Carslaw, N., Carpenter, L. J., Heard, D. E., Lee, J. D., and Pilling, M. J.: Urban atmospheric chemistry during the PUMA campaign 1: Comparison of modelled OH and HO<sub>2</sub> concentrations with measurements, J. Atmos. Chem., 52, 143–164, 2005. Faloona, I., Tan, D., Brune, W., Hurst, J., Barket, D., Couch, T. L., Shepson, P., Apel, E., Riemer, D., Thornberry, T., Carroll, M. A., Sillman, S., Keeler, G. J., Sagady, J., Hooper, D., and Paterson, K.: Nighttime observations of anomalously high levels of hydroxyl radicals above a deciduous forest canopy, J. Geophys. Res., 106, 24315–24333, 2001. Fan, S. J., Fan, Q., Yu, W., Luo, X. Y., Wang, B. M., Song, L. L., and Leong, K. L.: Atmospheric boundary layer characteristics over the Pearl River Delta, China, during the summer of 2006: measurement and model results, Atmos. Chem. Phys., 11, 6297–6310, <a href="http://dx.doi.org/10.5194/acp-11-6297-2011">https://doi.org/10.5194/acp-11-6297-2011</a>, 2011. Finlayson-Pitts, B. J. and Pitts Jr., J. N.: Chemistry of the upper and lower atmosphere: Theory, experiments and applications, Academic Press, San Diego, 183–185, 2000. Fuchs, H., Bohn, B., Hofzumahaus, A., Holland, F., Lu, K. D., Nehr, S., Rohrer, F., and Wahner, A.: Detection of HO<sub>2</sub> by laser-induced fluorescence: calibration and interferences from RO<sub>2</sub> radicals, Atmos. Meas. Tech., 4, 1209–1225, <a href="http://dx.doi.org/10.5194/amt-4-1209-2011">https://doi.org/10.5194/amt-4-1209-2011</a>, 2011. Fuchs, H., Dorn, H.-P., Bachner, M., Bohn, B., Brauers, T., Gomm, S., Hofzumahaus, A., Holland, F., Nehr, S., Rohrer, F., Tillmann, R., and Wahner, A.: Comparison of OH concentration measurements by DOAS and LIF during SAPHIR chamber experiments at high OH reactivity and low NO concentration, Atmos. Meas. Tech., 5, 1611–1626, <a href="http://dx.doi.org/10.5194/amt-5-1611-2012">https://doi.org/10.5194/amt-5-1611-2012</a>, 2012. Fuchs, H., Hofzumahaus, A., Rohrer, F., Bohn, B., Brauers, T., Dorn, H.-P., Häseler, R., Holland, F., Kaminski, M., Li, X., Lu, K., Nehr, S., Tillmann, R., Wegener, R., and Wahner, A.: Experimental evidence for efficient hydroxyl radical regeneration in isoprene oxidation, Nature Geosci., 2013. Garland, R. M., Yang, H., Schmid, O., Rose, D., Nowak, A., Achtert, P., Wiedensohler, A., Takegawa, N., Kita, K., Miyazaki, Y., Kondo, Y., Hu, M., Shao, M., Zeng, L. M., Zhang, Y. H., Andreae, M. O., and Pöschl, U.: Aerosol optical properties in a rural environment near the mega-city Guangzhou, China: implications for regional air pollution, radiative forcing and remote sensing, Atmos. Chem. Phys., 8, 5161–5186, <a href="http://dx.doi.org/10.5194/acp-8-5161-2008">https://doi.org/10.5194/acp-8-5161-2008</a>, 2008. Garland, R. M., Schmid, O., Nowak, A., Achtert, P., Wiedensohler, A., Gunthe, S. S., Takegawa, N., Kita, K., Kondo, Y., Hu, M., Shao, M., Zeng, L. M., Zhu, T., Andreae, M. O., and Poschl, U.: Aerosol optical properties observed during Campaign of Air Quality Research in Beijing 2006 (CAREBeijing-2006): Characteristic differences between the inflow and outflow of Beijing city air, J. Geophys. Res., 114, D00G04, <a href="http://dx.doi.org/10.1029/2008JD010780">https://doi.org/10.1029/2008JD010780</a>, 2009. Geiger, H., Barnes, I., Bejan, I., Benter, T., and Spittler, M.: The tropospheric degradation of isoprene: an updated module for the regional atmospheric chemistry mechanism, Atmos. Environ., 37, 1503–1519, 2003. Geyer, A. and Stutz, J.: The vertical structure of OH-HO<sub>2</sub>-RO<sub>2</sub> chemistry in the nocturnal boundary layer: A one-dimensional model study, J. Geophys. Res., 109, D16301, <a href="http://dx.doi.org/10.1029/2003JD004425">https://doi.org/10.1029/2003JD004425</a>, 2004. Geyer, A., Bachmann, K., Hofzumahaus, A., Holland, F., Konrad, S., Klupfel, T., Patz, H. W., Perner, D., Mihelcic, D., Schafer, H. J., Volz-Thomas, A., and Platt, U.: Nighttime formation of peroxy and hydroxyl radicals during the BERLIOZ campaign: Observations and modeling studies, J. Geophys. Res., 108, 8249, <a href="http://dx.doi.org/10.1029/2001JD000656">https://doi.org/10.1029/2001JD000656</a>, 2003. Guenther, A.: Modeling biogenic volatile organic compound emissions to the atmosphere, in Reactive Hydrocarbons in the Atmosphere, edited by C. N. Hewitt, chap. 3, 97–118, Academic Press, 1999. Hofzumahaus, A., Aschmutat, U., Brandenburger, U., Brauers, T., Dorn, H.-P., Hausmann, M., Hessling, M., Holland, F., Plass-Dülmer, C., and Ehhalt, D. H.: Intercomparison of tropospheric OH measurements by different laser techniques during the POPCORN Campaign 1994, J. Atmos. Chem., 31, 227–246, 1998. Hofzumahaus, A., Rohrer, F., Lu, K., Bohn, B., Brauers, T., Chang, C. C., Fuchs, H., Holland, F., Kita, K., Kondo, Y., Li, X., Lou, S., Shao, M., Zeng, L., Wahner, A., and Zhang, Y.: Amplified Trace Gas Removal in the Troposphere, Science, 324, 1702–1704, 2009. Holland, F., Aschmutat, U., Heß ling, M., Hofzumahaus, A., and Ehhalt, D. H.: Highly time resolved measurements of OH during POPCORN using laser-induced fluorescence spectroscopy, J. Atmos. Chem., 31, 205–225, 1998. Holland, F., Hofzumahaus, A., Schäfer, J., Kraus, A., and Pätz, H.-W.: Measurements of OH and HO<sub>2</sub> radical concentrations and photolysis frequencies during BERLIOZ, J. Geophys. Res., 108, 8246, <a href="http://dx.doi.org/10.1029/2001JD001393">https://doi.org/10.1029/2001JD001393</a>, 2003. Kanaya, Y., Cao, R. Q., Akimoto, H., Fukuda, M., Komazaki, Y., Yokouchi, Y., Koike, M., Tanimoto, H., Takegawa, N., and Kondo, Y.: Urban photochemistry in central Tokyo: 1. Observed and modeled OH and HO<sub>2</sub> radical concentrations during the winter and summer of 2004, J. Geophys. Res., 112, D21312, <a href="http://dx.doi.org/10.1029/2007JD008670">https://doi.org/10.1029/2007JD008670</a>, 2007. Kanaya, Y., Hofzumahaus, A., Dorn, H.-P., Brauers, T., Fuchs, H., Holland, F., Rohrer, F., Bohn, B., Tillmann, R., Wegener, R., Wahner, A., Kajii, Y., Miyamoto, K., Nishida, S., Watanabe, K., Yoshino, A., Kubistin, D., Martinez, M., Rudolf, M., Harder, H., Berresheim, H., Elste, T., Plass-Dülmer, C., Stange, G., Kleffmann, J., Elshorbany, Y., and Schurath, U.: Comparisons of observed and modeled OH and HO<sub>2</sub> concentrations during the ambient measurement period of the HO<sub>x</sub> Comp field campaign, Atmos. Chem. Phys., 12, 2567–2585, <a href="http://dx.doi.org/10.5194/acp-12-2567-2012">https://doi.org/10.5194/acp-12-2567-2012</a>, 2012. Karl, M., Dorn, H.-P., Holland, F., Koppmann, R., Poppe, D., Rupp, L., Schaub, A., and Wahner, A.: Product study of the reaction of OH radicals with isoprene in the atmosphere simulation chamber SAPHIR, J. Atmos. Chem., 55, 167–187, 2006. Lee, B. S. and Wang, J. L.: Concentration variation of isoprene and its implications for peak ozone concentration, Atmos. Environ., 40, 5486–5495, 2006. Lelieveld, J., Butler, T. M., Crowley, J. N., Dillon, T. J., Fischer, H., Ganzeveld, L., Harder, H., Lawrence, M. G., Martinez, M., Taraborelli, D., and Williams, J.: Atmospheric oxidation capacity sustained by a tropical forest, Nature, 452, 737–740, 2008. Li, X., Brauers, T., Häseler, R., Bohn, B., Fuchs, H., Hofzumahaus, A., Holland, F., Lou, S., Lu, K. D., Rohrer, F., Hu, M., Zeng, L. M., Zhang, Y. H., Garland, R. M., Su, H., Nowak, A., Wiedensohler, A., Takegawa, N., Shao, M., and Wahner, A.: Exploring the atmospheric chemistry of nitrous acid (HONO) at a rural site in Southern China, Atmos. Chem. Phys., 12, 1497–1513, <a href="http://dx.doi.org/10.5194/acp-12-1497-2012">https://doi.org/10.5194/acp-12-1497-2012</a>, 2012. Liu, Y., Shao, M., Fu, L., Lu, S., Zen, L., and Tang, D.: Source profiles of volatile organic compounds (VOCs) measured in China: Part I, Atmos. Environ., 42, 6247–6260, 2008. Lou, S., Holland, F., Rohrer, F., Lu, K., Bohn, B., Brauers, T., Chang, C., Fuchs, H., Häseler, R., Kita, K., Kondo, Y., Li, X., Shao, M., Zeng, L., Wahner, A., Zhang, Y., Wang, W., and Hofzumahaus, A.: Atmospheric OH reactivities in the Pearl River Delta – China in summer 2006: measurement and model results, Atmos. Chem. Phys., 10, 11243–11260, <a href="http://dx.doi.org/10.5194/acp-10-11243-2010">https://doi.org/10.5194/acp-10-11243-2010</a>, 2010. Lu, K. D. and Zhang, Y. H.: Observations of HO<sub>x</sub> Radical in Field Studies and the Analysis of Its Chemical Mechanism, Prog. Chem., 22, 500–514, 2010. Lu, K. D., Zhang, Y. H., Su, H., Brauers, T., Chou, C. C., Hofzumahaus, A., Liu, S. C., Kita, K., Kondo, Y., Shao, M., Wahner, A., Wang, J. L., Wang, X. S., and Zhu, T.: Oxidant (O<sub>3</sub> + NO<sub>2</sub>) production processes and formation regimes in Beijing, J. Geophys. Res., 115, D07 303, <a href="http://dx.doi.org/10.1029/2009JD012714">https://doi.org/10.1029/2009JD012714</a>, 2010a. Lu, K. D., Zhang, Y. H., Su, H., Shao, M., Zeng, L. M., Zhong, L. J., Xiang, Y. R., Chang, C. C., Chou, C. K. C., and Wahner, A.: Regional ozone pollution and key controlling factors of photochemical ozone production in Pearl River Delta during summer time, Sci. China Ser. B, 53, 651–663, 2010b. Lu, K. D., Rohrer, F., Holland, F., Fuchs, H., Bohn, B., Brauers, T., Chang, C. C., Häseler, R., Hu, M., Kita, K., Kondo, Y., Li, X., Lou, S. R., Nehr, S., Shao, M., Zeng, L. M., Wahner, A., Zhang, Y. H., and Hofzumahaus, A.: Observation and modelling of OH and HO<sub>2</sub> concentrations in the Pearl River Delta 2006: a missing OH source in a VOC rich atmosphere, Atmos. Chem. Phys., 12, 1541–1569, <a href="http://dx.doi.org/10.5194/acp-12-1541-2012">https://doi.org/10.5194/acp-12-1541-2012</a>, 2012. Lu, K. D., Hofzumahaus, A., Holland, F., Bohn, B., Brauers, T., Fuchs, H., Hu, M., Häseler, R., Kita, K., Kondo, Y.and Li, X., Lou, S. R., Oebel, A., Shao, M., Zeng, L. M., Wahner, A., Zhu, T., Zhang, Y. H., and Rohrer, F.: Missing OH source in a suburban environment near Beijing: observed and modelled OH and HO<sub>2</sub> concentrations in summer 2006, Atmos. Chem. Phys., 13, 1057–1080, <a href="http://dx.doi.org/10.5194/acp-13-1057-2013">https://doi.org/10.5194/acp-13-1057-2013</a>, 2013. Makar, P. A., D.Fuentes, J., Wang, D., Staebler, R. M., and Wiebe, H. A.: Chemical processing of biogenic hydrocarbons within and above a temperate deciduous forest, J. Geophys. Res., 104, 3581–3603, 1999. Mao, J., Ren, X., Chen, S., Brune, W. H., Chen, Z., Martinez, M., Harder, H., Lefer, B., Rappenglück, B., Flynn, J., and Leuchner, M.: Atmospheric oxidation capacity in the summer of Houston 2006: Comparison with summer measurements in other metropolitan studies, Atmos. Environ., 44, 4107–4115, <a href="http://dx.doi.org/10.1016/j.atmosenv.2009.01.013">https://doi.org/10.1016/j.atmosenv.2009.01.013</a>, 2010. Mao, J., Ren, X., Zhang, L., Van Duin, D. M., Cohen, R. C., Park, J.-H., Goldstein, A. H., Paulot, F., Beaver, M. R., Crounse, J. D., Wennberg, P. O., DiGangi, J. P., Henry, S. B., Keutsch, F. N., Park, C., Schade, G. W., Wolfe, G. M., Thornton, J. A., and Brune, W. H.: Insights into hydroxyl measurements and atmospheric oxidation in a California forest, Atmos. Chem. Phys., 12, 8009–8020, <a href="http://dx.doi.org/10.5194/acp-12-8009-2012">https://doi.org/10.5194/acp-12-8009-2012</a>, 2012. Martinez, M., Harder, H., Kovacs, T. A., Simpas, J. B., Bassis, J., Lesher, R., Brune, W. H., Frost, G. J., Williams, E. J., Stroud, C. A., Jobson, B. T., Roberts, J. M., Hall, S. R., Shetter, R. E., Wert, B., Fried, A., Alicke, B., Stutz, J., Young, V. L., White, A. B., and Zamora, R. J.: OH and HO<sub>2</sub> concentrations, sources, and loss rates during the Southern Oxidants Study in Nashville, Tennessee, summer 1999, J. Geophys. Res., 108, 4617, <a href="http://dx.doi.org/10.1029/2003JD003551">https://doi.org/10.1029/2003JD003551</a>, 2003. Matsui, H., Koike, M., Kondo, Y., Takegawa, N., Kita, K., Miyazaki, Y., Hu, M., Chang, S. Y., Blake, D. R., Fast, J. D., Zaveri, R. A., Streets, D. G., Zhang, Q., and Zhu, T.: Spatial and temporal variations of aerosols around Beijing in summer 2006: Model evaluation and source apportionment, J. Geophys. Res., 114, D00G13, <a href="http://dx.doi.org/10.1029/2008JD010906">https://doi.org/10.1029/2008JD010906</a>, 2009. Mihelcic, D., Klemp, D., Musgen, P., Patz, H. W., and Volzthomas, A.: Simultaneous Measurements of Peroxy and Nitrate Radicals at Schauinsland, J. Atmos. Chem., 16, 313–335, 1993. Monks, P. S., Granier, C., Fuzzie, S., Stohl, A., Williams, M., Akimoto, H., Ammani, M., Baklanov, A., Baltensperger, U., Bey, I., Blake, N., Blake, R., Carslaw, K., Cooper, O., Dentener, F., Fowler, D., Fragkou, E., Frost, G., Generoso, S., Ginoux, P., Grewe, V., abd H. C. Hansson, A. G., Henne, S., Hjorth, J., Hofzumahaus, A., Huntrieser, H., Isaksen, I. S. A., Jenkin, M. E., Kaiser, J., Kanakidou, M., Klimont, Z., Kulmala, M., Laj, P., Lawrence, M., Lee, J., Liousse, C., Maione, M., McFiggans, G., Metzger, A., Mieville, A., Moussiopoulos, N., Orlando, J., O'Dowd, C., Palmer, P., Parrish, D., Petzold, A., Platt, U., Pöschl, U., Prévôt, A. S. H., Reeves, C. E., Reimann, S., Rudich, Y., Sellegri, K., Steinbrecher, R., Simpson, D., ten Brink, H., Theloke, J., van der Werf, G. R., Vautard, R., Vestreng, V., Vlachokostas, C., and vonGlasow, R.: Atmospheric Composition Change - Global and Regional Air Quality, Atmos. Environ., 43, 5268–5350, 2009. Peeters, J. and Müller, J.-F.: HO<i><sub>x</sub></i> radical regeneration in isoprene oxidation via peroxy radical isomerisations. II: experimental evidence and global impact, Phys. Chem. Chem. Phys., 12, 14227–14235, <a href="http://dx.doi.org/10.1039/c0cp00811g">https://doi.org/10.1039/c0cp00811g</a>, 2010. Peeters, J., Nguyen, T. L., and Vereecken, L.: HO<i><sub>x</sub></i> radical regeneration in the oxidation of isoprene, Phys. Chem. Chem. Phys., 11, 5935–5939, 2009. Platt, U., Rateike, M., Junkermann, W., Rudolph, J., and Ehhalt, D. H.: New tropospheric OH measurements, J. Geophys. Res., 93, 5159–5166, 1988. Platt, U. F., Winer, A. M., Biermann, H. W., Atkinson, R., and Pitts, J. N.: Measurement of Nitrate Radical Concentrations in Continental Air, Environ. Sci. Technol., 18, 365–369, 1984. Ren, X., Harder, H., Martinez, M., Lesher, R. L., Oliger, A., Simpas, J. B., Brune, W. H., Schwab, J. J., Demerjian, K. L., He, Y., Zhou, X., and Gao, H.: OH and HO<sub>2</sub> Chemistry in the urban atmosphere of New York City, Atmos. Environ., 37, 3639–3651, 2003. Ren, X. R., Harder, H., Martinez, M., Lesher, R. L., Oliger, A., Shirley, T., Adams, J., Simpas, J. B., and Brune, W. H.: HO<sub>x</sub> concentrations and OH reactivity observations in New York City during PMTACS-NY2001, Atmos. Environ., 37, 3627–3637, 2003a. Ren, X. R., Harder, H., Martinez, M., Lesher, R. L., Oliger, A., Simpas, J. B., Brune, W. H., Schwab, J. J., Demerjian, K. L., He, Y., Zhou, X. L., and Gao, H. G.: OH and HO<sub>2</sub> chemistry in the urban atmosphere of New York City, Atmos. Environ., 37, 3639–3651, 2003b. Ren, X. R., Harder, H., Martinez, M., Faloona, I. C., Tan, D., Lesher, R. L., Di Carlo, P., Simpas, J. B., and Brune, W. H.: Interference testing for atmospheric HO<sub>x</sub> measurements by laser-induced fluorescence, J. Atmos. Chem., 47, 169–190, 2004. Roberts, J. M., Williams, J., Baumann, K., Buhr, M. P., Goldan, P. D., Holloway, J., Hubler, G., Kuster, W. C., McKeen, S. A., Ryerson, T. B., Trainer, M., Williams, E. J., Fehsenfeld, F. C., Bertman, S. B., Nouaime, G., Seaver, C., Grodzinsky, G., Rodgers, M., and Young, V. L.: Measurements of PAN, PPN, and MPAN made during the 1994 and 1995 Nashville Intensives of the Southern Oxidant Study: Implications for regional ozone production from biogenic hydrocarbons, J. Geophys. Res.-Atmos., 103, 22473–22490, 1998. Sadanaga, Y., Matsumoto, J., and Kajii, Y.: Photochemical reactions in the urban air: Recent understandings of radical chemistry, J. Photochem. Photobiol., 4, 85–104, 2003. Schlosser, E., Bohn, B., Brauers, T., Dorn, H., Fuchs, H., Häseler, R., Hofzumahaus, A., Holland, F., Rohrer, F., Rupp, L. O., Siese, M., Tillmann, R., and Wahner, A.: Intercomparison of Two Hydroxyl Radical Measurement Techniques at the Atmosphere Simulation Chamber SAPHIR, J. Atmos. Chem., 56, 187–205, <a href="http://dx.doi.org/10.1007/s10874-006-9049-3">https://doi.org/10.1007/s10874-006-9049-3</a>, 2007. Schlosser, E., Brauers, T., Dorn, H.-P., Fuchs, H., Häseler, R., Hofzumahaus, A., Holland, F., Wahner, A., Kanaya, Y., Kajii, Y., Miyamoto, K., Nishida, S., Watanabe, K., Yoshino, A., Kubistin, D., Martinez, M., Rudolf, M., Harder, H., Berresheim, H., Elste, T., Plass-Dülmer, C., Stange, G., and Schurath, U.: Technical Note: Formal blind intercomparison of OH measurements: results from the international campaign HOxComp, Atmos. Chem. Phys., 9, 7923–7948, <a href="http://dx.doi.org/10.5194/acp-9-7923-2009">https://doi.org/10.5194/acp-9-7923-2009</a>, 2009. Shao, M., Czapiewski, V., Heiden, A., Kobel, K., Komenda, M., Koppmann, R., and Wildt, J.: Volatile organic compound emissions from Scots pine: Mechanisms and description by algorithms, J. Geophys. Res., 106, 20483–20491, 2001. Shirley, T. R., Brune, W. H., Ren, X., Mao, J., Lesher, R., Cardenas, B., Volkamer, R., Molina, L. T., Molina, M. J., Lamb, B., Velasco, E., Jobson, T., and Alexander, M.: Atmospheric oxidation in the Mexico City Metropolitan Area (MCMA) during April 2003, Atmos. Chem. Phys., 6, 2753–2765, <a href="http://dx.doi.org/10.5194/acp-6-2753-2006">https://doi.org/10.5194/acp-6-2753-2006</a>, 2006. Sillman, S., Carroll, M. A., Thornberry, T., Lamb, B. K., Westberg, H., Brune, W. H., Faloona, I., Tan, D., Shepson, P. B., Sumner, A. L., Hastie, D. R., Mihele, C. M., Apel, E. C., Riemer, D. D., and Zika, R. G.: Loss of isoprene and sources of nighttime OH radicals at a rural site in the United States: Results from photochemical models, J. Geophys. Res., 107, 4043, <a href="http://dx.doi.org/10.1029/2001JD000449">https://doi.org/10.1029/2001JD000449</a>, 2002. Stull, R. B.: An Introduction to Boundary Layer Meteorology, Kluwer Academic Publishers, 440–465, 1988. Takegawa, N., Miyakawa, T., Kondo, Y., Jimenez, J. L., Worsnop, D. R., and Fukuda, M.: Seasonal and diurnal variations of submicron organic aerosol in Tokyo observed using the Aerodyne Aerosol Mass Spectrometer, J. Geophys. Res., 111, D11206, <a href="http://dx.doi.org/10.1029/2005JD006515">https://doi.org/10.1029/2005JD006515</a>, 2006. Tan, D., Faloona, I., Simpas, J. B., Brune, W., and Shepson, P. B.: HO$\rm_x$ budgets in a deciduous forest: Results from the PROPHET summer 1998 campaign, J. Geophys. Res., 106, 24407–24427, 2001. Wang, B., Shao, M., Roberts, J. M., Yang, G., Yang, F., Hu, M., Zeng, L. M., Zhang, Y. H., and Zhang, J. B.: Ground-based on-line measurements of peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) in the Pearl River Delta, China, International J. Environ. Anal. Chem., 90, 548–559, 2010. Wang, J. L., Wang, C. H., Lai, C. H., Chang, C. C., Liu, Y., Zhang, Y., Liu, S., and Shao, M.: Characterization of ozone precursors in the Pearl River Delta by time series observation of non-methane hydrocarbons, Atmos. Environ., 42, 6233–6246, https://doi.org/10.1016/j.atmosenv.2008.01.050, 2008. Whalley, L. K., Edwards, P. M., Furneaux, K. L., Goddard, A., Ingham, T., Evans, M. J., Stone, D., Hopkins, J. R., Jones, C. E., Karunaharan, A., Lee, J. D., Lewis, A. C., Monks, P. S., Moller, S. J., and Heard, D. E.: Quantifying the magnitude of a missing hydroxyl radical source in a tropical rainforest, Atmos. Chem. Phys., 11, 7223–7233, <a href="http://dx.doi.org/10.5194/acp-11-7223-2011">https://doi.org/10.5194/acp-11-7223-2011</a>, 2011. Xiao, R., Takegawa, N., Kondo, Y., Miyazaki, Y., Miyakawa, T., Hu, M., Shao, M., Zeng, L. M., Hofzumahaus, A., Holland, F., Lu, K., Sugimoto, N., Zhao, Y., and Zhang, Y. H.: Formation of submicron sulfate and organic aerosols in the outflow from the urban region of the Pearl River Delta in China, Atmos. Environ., 43, 3754–3763, 2009. Xie, X., Shao, M., Liu, Y., Lu, S. H., Chang, C. C., and Chen, Z. M.: Estimate of initial isoprene contribution to ozone formation potential in Beijing, China, Atmos. Environ., 42, 6000–6010, 2008.