ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-3883-2012 The IPAC-NC field campaign: a pollution and oxidization pool in the lower atmosphere over Huabei, China Ma J. Z. 1 Wang W. 2 8 Chen Y. 1 Liu H. J. 2 Yan P. 1 6 Ding G. A. 1 Wang M. L. 1 Sun J. 1 7 Lelieveld J. 3 4 5 Chinese Academy of Meteorological Sciences, Beijing, China Chinese Research Academy of Environmental Sciences, Beijing, China Max Planck Institute for Chemistry, Mainz, Germany Cyprus Institute, Nicosia, Cyprus King Saud University, Riyadh, Saudi Arabia now at: CMA Meteorological Observation Centre, Beijing, China now at: CMA Numerical Prediction Centre, Beijing, China deceased, March 2010 03 05 2012 12 9 3883 3908 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/3883/2012/acp-12-3883-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/3883/2012/acp-12-3883-2012.pdf

In the past decades, regional air pollution characterized by photochemical smog and grey haze-fog has become a severe environmental problem in China. To investigate this, a field measurement campaign was performed in the Huabei region, located between 32–42° N latitude in eastern China, during the period 2 April–16 May 2006 as part of the project "Influence of Pollution on Aerosols and Cloud Microphysics in North China" (IPAC-NC). It appeared that strong pollution emissions from urban and industrial centers tend to accumulate in the lower atmosphere over the central area of Huabei. We observed widespread, very high SO<sub>2</sub> mixing ratios, about 20–40 ppbv at 0.5–1.5 km altitude and 10–30 ppbv at 1.5–3.0 km altitude. Average CO mixing ratios were 0.65–0.7 ppmv at 0.5–1.5 km altitude, and very high CO around 1 ppmv was observed during some flights, and even higher levels at the surface. We find the high pollution concentrations to be associated with enhanced levels of OH and HO<sub>2</sub> radicals, calculated with a chemical box model constrained by the measurements. In the upper part of the boundary layer and in the lower free troposphere, high CO and SO<sub>2</sub> compete with relatively less NO<sub>2</sub> in reacting with OH, being efficiently recycled through HO<sub>2</sub>, preventing a net loss of HO<sub>x</sub> radicals. In addition to reactive hydrocarbons and CO, the oxidation of SO<sub>2</sub> causes significant ozone production over Huabei (up to ~13% or 2.0 ppbv h<sup>−1</sup> at 0.8 km altitude). Our results indicate that the lower atmosphere over Huabei is not only strongly polluted but also acts as an oxidation pool, with pollutants undergoing very active photochemistry over this part of China.

 References Akimoto, H.: Global air quality and pollution, Science, 302, 1716–1719, http://dx.doi.org/10.1126/science.1092666doi:10.1126/science.1092666, 2003. Apel, E. C., Emmons, L. K., Karl, T., Flocke, F., Hills, A. J., Madronich, S., Lee-Taylor, J., Fried, A., Weibring, P., Walega, J., Richter, D., Tie, X., Mauldin, L., Campos, T., Weinheimer, A., Knapp, D., Sive, B., Kleinman, L., Springston, S., Zaveri, R., Ortega, J., Voss, P., Blake, D., Baker, A., Warneke, C., Welsh-Bon, D., de Gouw, J., Zheng, J., Zhang, R., Rudolph, J., Junkermann, W., and Riemer, D. D.: Chemical evolution of volatile organic compounds in the outflow of the Mexico City Metropolitan area, Atmos. Chem. Phys., 10, 2353–2375, http://dx.doi.org/10.5194/acp-10-2353-2010doi:10.5194/acp-10-2353-2010, 2010. Atkinson, R. and Arey, J.: Atmospheric degradation of volatile organic compounds, Chem. Rev., 103, 4605–4638, 2003. Barletta, B., Meinardi, S., Simpson, I. J., Atlas, E. L., Beyersdorf, A. J., Baker, A. K., Blake, N. J., Yang, M., Midyett, J. R., Novak, B. J., McKeachie, R. J., Fuelberg, H. E., Sachse, G. W., Avery, M. A., Campos, T., Weinheimer, A. J., Rowland, F. S., and Blake, D. R.: Characterization of volatile organic compounds (VOCs) in Asian and north American pollution plumes during INTEX-B: identification of specific Chinese air mass tracers, Atmos. Chem. Phys., 9, 5371–5388, http://dx.doi.org/10.5194/acp-9-5371-2009doi:10.5194/acp-9-5371-2009, 2009. Boersma, K. F., Eskes, H. J., Veefkind, J. P., Brinksma, E. J., van der A, R. J., Sneep, M., van den Oord, G. H. J., Levelt, P. F., Stammes, P., Gleason, J. F., and Bucsela, E. J.: Near-real time retrieval of tropospheric NO<sub>2</sub> from OMI, Atmos. Chem. Phys., 7, 2103–2118, http://dx.doi.org/10.5194/acp-7-2103-2007doi:10.5194/acp-7-2103-2007, 2007. Chan, C. K. and Yao, X.: Air pollution in mega cities in China, Atmos. Environ., 42, 1–42, 2008. Chen, D. and Shen, X.: Recent progress on GRAPES research and application, Q. J. Appl. Meteorol., 17, 773–777, 2006. Chen, Y., Zhao, C., Zhang, Q., Deng, Z., Huang, M., and Ma, X.: Aircraft study of Mountain Chimney Effect of Beijing, China, J. Geophys. Res., 114, D08306, http://dx.doi.org/10.1029/2008jd010610doi:10.1029/2008jd010610, 2009. Cheng, H. and Wang, M.: Difference in nonmethane hydrocarbons (NMHCs) between urban and rural atmospheres, Meteorol. Sci. Technol., 38, 668–672, 2010. Commane, R., Floquet, C. F. A., Ingham, T., Stone, D., Evans, M. J., and Heard, D. E.: Observations of OH and HO<sub>2</sub> radicals over West Africa, Atmos. Chem. Phys., 10, 8783–8801, http://dx.doi.org/10.5194/acp-10-8783-2010doi:10.5194/acp-10-8783-2010, 2010. Cooper, O. R., Parrish, D. D., Stohl, A., Trainer, M., Nedelec, P., Thouret, V., Cammas, J. P., Oltmans, S. J., Johnson, B. J., Tarasick, D., Leblanc, T., McDermid, I. S., Jaffe, D., Gao, R., Stith, J., Ryerson, T., Aikin, K., Campos, T., Weinheimer, A., and Avery, M. A.: Increasing springtime ozone mixing ratios in the free troposphere over western North America, Nature, 463, 344–348, 2010. 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, 13607-13613, 2011. Crutzen, P. R. and Andreae, M. O.: Biomass burning in the tropics: Impact on atmospheric chemistry and biogeochemical cycles, Science, 250, 1669–1678, 1990. de Reus, M., Fischer, H., Sander, R., Gros, V., Kormann, R., Salisbury, G., Van Dingenen, R., Williams, J., Zöllner, M., and Lelieveld, J.: Observations and model calculations of trace gas scavenging in a dense Saharan dust plume during MINATROC, Atmos. Chem. Phys., 5, 1787–1803, http://dx.doi.org/10.5194/acp-5-1787-2005doi:10.5194/acp-5-1787-2005, 2005. Dickerson, R. R., Li, C., Li, Z., Marufu, L. T., Stehr, J. W., McClure, B., Krotkov, N., Chen, H., Wang, P., Xia, X., Ban, X., Gong, F., Yuan, J., and Yang, J.: Aircraft observations of dust and pollutants over northeast China: Insight into the meteorological mechanisms of transport, J. Geophys. Res., 112, D24S90, http://dx.doi.org/10.1029/2007jd008999doi:10.1029/2007jd008999, 2007. Ding, A. J., Wang, T., Thouret, V., Cammas, J.-P., and Nédélec, P.: Tropospheric ozone climatology over Beijing: analysis of aircraft data from the MOZAIC program, Atmos. Chem. Phys., 8, 1–13, http://dx.doi.org/10.5194/acp-8-1-2008doi:10.5194/acp-8-1-2008, 2008. Ehhalt, D. H.: Photooxidation of trace gases in the troposphere Plenary Lecture, Phys. Chem. Chem. Phys., 1, 5401–5408, 1999. Farina, S. C., Adams, P. J., and Pandis, S. N.: Modeling global secondary organic aerosol formation and processing with the volatility basis set: Implications for anthropogenic secondary organic aerosol, J. Geophys. Res., 115, D09202, http://dx.doi.org/10.1029/2009jd013046doi:10.1029/2009jd013046, 2010. Fiedler, V., Nau, R., Ludmann, S., Arnold, F., Schlager, H., and Stohl, A.: East Asian SO<sub>2</sub> pollution plume over Europe – Part 1: Airborne trace gas measurements and source identification by particle dispersion model simulations, Atmos. Chem. Phys., 9, 4717–4728, http://dx.doi.org/10.5194/acp-9-4717-2009doi:10.5194/acp-9-4717-2009, 2009. Geng, F. H., Tie, X. X., Xu, J. M., Zhou, G. Q., Peng, L., Gao, W., Tang, X., and Zhao, C. S.: Characterizations of ozone, NO$_\textx$, and VOCs measured in Shanghai, China, Atmos. Environ., 42, 6873–6883, http://dx.doi.org/10.1016/j.atmosenv.2008.05.045doi:10.1016/j.atmosenv.2008.05.045, 2008. Gurjar, B. R., Butler, T. M., Lawrence, M. G., and Lelieveld, J.: Evaluation of emissions and air quality in megacities, Atmos. Environ., 42, 1593–1606, 2008. Hatakeyama, S., Takami, A., Wang, W., and Tang, D.: Aerial observation of air pollutants and aerosols over Bo Hai, China, Atmos. Environ., 39, 5893–5898, 2005. Hocking, W. K., Carey-Smith, T., Tarasick, D. W., Argall, P. S., Strong, K., Rochon, Y., Zawadzki, I., and Taylor, P. A.: Detection of stratospheric ozone intrusions by windprofiler radars, Nature, 450, 281–284, http://www.nature.com/nature/journal/v450/n7167/suppinfo/nature06312_S1.html, 2007. 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, http://dx.doi.org/10.1126/science.1164566doi:10.1126/science.1164566, 2009. Jacob, D. J., Crawford, J. H., Kleb, M. M., Connors, V. S., Bendura, R. J., Raper, J. L., Sachse, G. W., Gille, J. C., Emmons, L., and Heald, C. L.: Transport and Chemical Evolution over the Pacific (TRACE-P) aircraft mission: Design, execution, and first results, J. Geophys. Res., 108, 9000, http://dx.doi.org/10.1029/2002jd003276doi:10.1029/2002jd003276, 2003. Jimenez, J. L., Canagaratna, M. R., Donahue, N. M., Prevot, A. S. H., Zhang, Q., Kroll, J. H., DeCarlo, P. F., Allan, J. D., Coe, H., Ng, N. L., Aiken, A. C., Docherty, K. S., Ulbrich, I. M., Grieshop, A. P., Robinson, A. L., Duplissy, J., Smith, J. D., Wilson, K. R., Lanz, V. A., Hueglin, C., Sun, Y. L., Tian, J., Laaksonen, A., Raatikainen, T., Rautiainen, J., Vaattovaara, P., Ehn, M., Kulmala, M., Tomlinson, J. M., Collins, D. R., Cubison, M. J., E., Dunlea, J., Huffman, J. A., Onasch, T. B., Alfarra, M. R., Williams, P. I., Bower, K., Kondo, Y., Schneider, J., Drewnick, F., Borrmann, S., Weimer, S., Demerjian, K., Salcedo, D., Cottrell, L., Griffin, R., Takami, A., Miyoshi, T., Hatakeyama, S., Shimono, A., Sun, J. Y., Zhang, Y. M., Dzepina, K., Kimmel, J. R., Sueper, D., Jayne, J. T., Herndon, S. C., Trimborn, A. M., Williams, L. R., Wood, E. C., Middlebrook, A. M., Kolb, C. E., Baltensperger, U., and Worsnop, D. R.: Evolution of organic aerosols in the atmosphere, Science, 326, 1525–1529, http://dx.doi.org/10.1126/science.1180353doi:10.1126/science.1180353, 2009. Jöckel, P., Tost, H., Pozzer, A., Brühl, C., Buchholz, J., Ganzeveld, L., Hoor, P., Kerkweg, A., Lawrence, M. G., Sander, R., Steil, B., Stiller, G., Tanarhte, M., Taraborrelli, D., van Aardenne, J., and Lelieveld, J.: The atmospheric chemistry general circulation model ECHAM5/MESSy1: consistent simulation of ozone from the surface to the mesosphere, Atmos. Chem. Phys., 6, 5067–5104, http://dx.doi.org/10.5194/acp-6-5067-2006doi:10.5194/acp-6-5067-2006, 2006. Kanakidou, M., Mihalopoulos, N., Kindap, T., Im, U., Vrekoussis, M., Gerasopoulos, E., Dermitzaki, E., Unal, A., Koçak, M., Markakis, K., Melas, D., Kouvarakis, G., Youssef, A. F., Richter, A., Hatzianastassiou, N., Hilboll, A., Ebojie, F., Wittrock, F., von Savigny, C., Burrows, J. P., Ladstaetter-Weissenmayer, A., and Moubasher, H.: Megacities as hot spots of air pollution in the East Mediterranean, Atmos. Environ., 45, 1223–1235, 2011. Kanaya, Y., Pochanart, P., Liu, Y., Li, J., Tanimoto, H., Kato, S., Suthawaree, J., Inomata, S., Taketani, F., Okuzawa, K., Kawamura, K., Akimoto, H., and Wang, Z. F.: Rates and regimes of photochemical ozone production over Central East China in June 2006: a box model analysis using comprehensive measurements of ozone precursors, Atmos. Chem. Phys., 9, 7711–7723, http://dx.doi.org/10.5194/acp-9-7711-2009doi:10.5194/acp-9-7711-2009, 2009. Kleinman, L. I., Daum, P. H., Lee, Y. N., Nunnermacker, L. J., Springston, S. R., Weinstein-Lloyd, J., and Rudolph, J.: A comparative study of ozone production in five U.S. metropolitan areas, J. Geophys. Res., 110, D02301, http://dx.doi.org/10.1029/2004jd005096doi:10.1029/2004jd005096, 2005. Kubistin, D., Harder, H., Martinez, M., Rudolf, M., Sander, R., Bozem, H., Eerdekens, G., Fischer, H., Gurk, C., Klüpfel, T., Königstedt, R., Parchatka, U., Schiller, C. L., Stickler, A., Taraborrelli, D., Williams, J., and Lelieveld, J.: Hydroxyl radicals in the tropical troposphere over the Suriname rainforest: comparison of measurements with the box model MECCA, Atmos. Chem. Phys., 10, 9705–9728, http://dx.doi.org/10.5194/acp-10-9705-2010doi:10.5194/acp-10-9705-2010, 2010. Kuhn, U., Ganzeveld, L., Thielmann, A., Dindorf, T., Schebeske, G., Welling, M., Sciare, J., Roberts, G., Meixner, F. X., Kesselmeier, J., Lelieveld, J., Kolle, O., Ciccioli, P., Lloyd, J., Trentmann, J., Artaxo, P., and Andreae, M. O.: Impact of Manaus City on the Amazon Green Ocean atmosphere: ozone production, precursor sensitivity and aerosol load, Atmos. Chem. Phys., 10, 9251–9282, http://dx.doi.org/10.5194/acp-10-9251-2010doi:10.5194/acp-10-9251-2010, 2010. Kulmala, M.: Atmospheric science: How particles nucleate and grow, Science, 302, 1000–1001, http://dx.doi.org/10.1126/science.1090848doi:10.1126/science.1090848, 2003. Lawrence, M. G. and Lelieveld, J.: Atmospheric pollutant outflow from southern Asia: a review, Atmos. Chem. Phys., 10, 11017–11096, http://dx.doi.org/10.5194/acp-10-11017-2010doi:10.5194/acp-10-11017-2010, 2010. Lawrence, M. G., Butler, T. M., Steinkamp, J., Gurjar, B. R., and Lelieveld, J.: Regional pollution potentials of megacities and other major population centers, Atmos. Chem. Phys., 7, 3969–3987, http://dx.doi.org/10.5194/acp-7-3969-2007doi:10.5194/acp-7-3969-2007, 2007. Lelieveld, J.: Atmospheric chemistry: A missing sink for radicals, Nature, 466, 925–926, 2010. Lelieveld, J., Berresheim, H., Borrmann, S., Crutzen, P. J., Dentener, F. J., Fischer, H., Feichter, J., Flatau, P. J., Heland, J., Holzinger, R., Korrmann, R., Lawrence, M. G., Levin, Z., Markowicz, K. M., Mihalopoulos, N., Minikin, A., Ramanathan, V., de Reus, M., Roelofs, G. J., Scheeren, H. A., Sciare, J., Schlager, H., Schultz, M., Siegmund, P., Steil, B., Stephanou, E. G., Stier, P., Traub, M., Warneke, C., Williams, J., and Ziereis, H.: Global air pollution crossroads over the Mediterranean, Science, 298, 794–799, http://dx.doi.org/10.1126/science.1075457doi:10.1126/science.1075457, 2002a. Lelieveld, J., Peters, W., Dentener, F. J., and Krol, M. C.: Stability of tropospheric hydroxyl chemistry, J. Geophys. Res., 107, 4715, http://dx.doi.org/10.1029/2002jd002272doi:10.1029/2002jd002272, 2002b. Lelieveld, J., Butler, T. M., Crowley, J. N., Dillon, T. J., Fischer, H., Ganzeveld, L., Harder, H., Lawrence, M. G., Martinez, M., Taraborrelli, D., and Williams, J.: Atmospheric oxidation capacity sustained by a tropical forest, Nature, 452, 737–740, 2008. Lelieveld, J., Hoor, P., Jöckel, P., Pozzer, A., Hadjinicolaou, P., Cammas, J.-P., and Beirle, S.: Severe ozone air pollution in the Persian Gulf region, Atmos. Chem. Phys., 9, 1393–1406, http://dx.doi.org/10.5194/acp-9-1393-2009doi:10.5194/acp-9-1393-2009, 2009. Levy, H. I.: Normal atmosphere: large radical and formaldehyde concentrations predicted, Science, 173, 141–143, 1971. Li, C., Marufu, L. T., Dickerson, R. R., Li, Z., Wen, T., Wang, Y., Wang, P., Chen, H., and Stehr, J. W.: In situ measurements of trace gases and aerosol optical properties at a rural site in northern China during East Asian Study of Tropospheric Aerosols: An International Regional Experiment 2005, J. Geophys. Res., 112, D22S04, http://dx.doi.org/10.1029/2006jd007592doi:10.1029/2006jd007592, 2007. Lin, W., Xu, X., Zhang, X., and Tang, J.: Contributions of pollutants from North China Plain to surface ozone at the Shangdianzi GAW Station, Atmos. Chem. Phys., 8, 5889–5898, http://dx.doi.org/10.5194/acp-8-5889-2008doi:10.5194/acp-8-5889-2008, 2008. Lin, W., Xu, X., Ge, B., and Zhang, X.: Characteristics of gaseous pollutants at Gucheng, a rural site southwest of Beijing, J. Geophys. Res., 114, D00G14, http://dx.doi.org/10.1029/2008jd010339doi:10.1029/2008jd010339, 2009. Liu, S., Trainer, M., Fehsenfeld, F., Parrish, D., Williams, E., Fahey, D., Hübler, G., and Murphy, P.: Ozone Production in the Rural Troposphere and the Implications for Regional and Global Ozone Distributions J. Geophys. Res., 92, 4191–4207, 1987. Liu, Y., Shao, M., Zhang, J., Fu, L. L., and Lu, S. H.: Distributions and source apportionment of ambient volatile organic compounds in Beijing city, China, J. Environ. Sci. Heal. A, 40, 1843–1860, 2005. Liu, Ying, Shao, Min, Lu, Sihua, Chang, Chih-chung, Wang, Jia-Lin, and Chen, Gao: Volatile Organic Compound (VOC) measurements in the Pearl River Delta (PRD) region, China, Atmos. Chem. Phys., 8, 1531–-1545, http://dx.doi.org/10.5194/acp-8-1531-2008doi:10.5194/acp-8-1531-2008, 2008. Lou, S., Holland, F., Rohrer, F., Lu, K., Bohn, B., Brauers, T., Chang, C.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, http://dx.doi.org/10.5194/acp-10-11243-2010doi:10.5194/acp-10-11243-2010, 2010. Lu, K., Zhang, Y., Su, H., Brauers, T., Chou, C. C., Hofzumahaus, A., Liu, S. C., Kita, K., Kondo, Y., Shao, M., Wahner, A., Wang, J., Wang, X., and Zhu, T.: Oxidant (O<sub>3</sub> + NO<sub>2</sub>) production processes and formation regimes in Beijing, J. Geophys. Res., 115, D07303, http://dx.doi.org/10.1029/2009jd012714doi:10.1029/2009jd012714, 2010. Ma, J. and van Weele, M.: Effect of stratospheric ozone depletion on the net production of ozone in polluted rural areas, Chemosphere – Global Change Science, 2, 23–37, 2000. Ma, J., Liu, H., and Hauglustaine, D.: Summertime tropospheric ozone over China simulated with a regional chemical transport model 1. Model description and evaluation, J. Geophys. Res., 107, 4660, http://dx.doi.org/10.1029/2001jd001354doi:10.1029/2001jd001354, 2002a. Ma, J., Tang, J., Zhou, X., and Zhang, X.: Estimates of the chemical budget for ozone at Waliguan Observatory, J. Atmos. Chem., 41, 21–48, http://dx.doi.org/10.1023/A:1013892308983doi:10.1023/A:1013892308983, 2002b. Ma, J., Zhou, X., and Hauglustaine, D.: Summertime tropospheric ozone over China simulated with a regional chemical transport model 2. Source contributions and budget, J. Geophys. Res., 107, 4612, http://dx.doi.org/10.1029/2001jd001355doi:10.1029/2001jd001355, 2002c. Ma, J., Chen, Y., Wang, W., Yan, P., Liu, H., Yang, S., Hu, Z., and Lelieveld, J.: Strong air pollution causes widespread haze-clouds over China, J. Geophys. Res., 115, D18204, http://dx.doi.org/10.1029/2009jd013065doi:10.1029/2009jd013065, 2010. Madronich, S.: Photodissociation in the atmosphere: 1. Actinic flux and the effects of ground reflections and clouds, J. Geophys. Res., 92, 9740–9752, 1987. Madronich, S. and Calvert, J. G.: The NCAR master mechanism of the gas-phase chemistry-Version 2.0, The National Center for Atmospheric Research, Boulder, Colorado. Rep. NCAR/TN-333+STR, 1989. Madronich, S. and Calvert, J. G.: Permutation reactions of organic peroxy radicals in the troposphere, J. Geophys. Res., 95, 5697–5715, 1990. Mao, J., Ren, X., Brune, W. H., Olson, J. R., Crawford, J. H., Fried, A., Huey, L. G., Cohen, R. C., Heikes, B., Singh, H. B., Blake, D. R., Sachse, G. W., Diskin, G. S., Hall, S. R., and Shetter, R. E.: Airborne measurement of OH reactivity during INTEX-B, Atmos. Chem. Phys., 9, 163–173, http://dx.doi.org/10.5194/acp-9-163-2009doi:10.5194/acp-9-163-2009, 2009. Mao, J., Jacob, D. J., Evans, M. J., Olson, J. R., Ren, X., Brune, W. H., Clair, J. M. St., Crounse, J. D., Spencer, K. M., Beaver, M. R., Wennberg, P. O., Cubison, M. J., Jimenez, J. L., Fried, A., Weibring, P., Walega, J. G., Hall, S. R., Weinheimer, A. J., Cohen, R. C., Chen, G., Crawford, J. H., McNaughton, C., Clarke, A. D., Jaeglé, L., Fisher, J. A., Yantosca, R. M., Le Sager, P., and Carouge, C.: Chemistry of hydrogen oxide radicals (HO$_\rm x$) in the Arctic troposphere in spring, Atmos. Chem. Phys., 10, 5823–5838, http://dx.doi.org/10.5194/acp-10-5823-2010doi:10.5194/acp-10-5823-2010, 2010. Martinez, M., Harder, H., Kubistin, D., Rudolf, M., Bozem, H., Eerdekens, G., Fischer, H., Klüpfel, T., Gurk, C., Königstedt, R., Parchatka, U., Schiller, C. L., Stickler, A., Williams, J., and Lelieveld, J.: Hydroxyl radicals in the tropical troposphere over the Suriname rainforest: airborne measurements, Atmos. Chem. Phys., 10, 3759–3773, http://dx.doi.org/10.5194/acp-10-3759-2010doi:10.5194/acp-10-3759-2010, 2010. Meng, Z. Y., Xu, X. B., Yan, P., Ding, G. A., Tang, J., Lin, W. L., Xu, X. D., and Wang, S. F.: Characteristics of trace gaseous pollutants at a regional background station in Northern China, Atmos. Chem. Phys., 9, 927–936, http://dx.doi.org/10.5194/acp-9-927-2009doi:10.5194/acp-9-927-2009, 2009. Molina, L. T., Kolb, C. E., de Foy, B., Lamb, B. K., Brune, W. H., Jimenez, J. L., Ramos-Villegas, R., Sarmiento, J., Paramo-Figueroa, V. H., Cardenas, B., Gutierrez-Avedoy, V., and Molina, M. J.: Air quality in North America's most populous city – overview of the MCMA-2003 campaign, Atmos. Chem. Phys., 7, 2447–2473, http://dx.doi.org/10.5194/acp-7-2447-2007doi:10.5194/acp-7-2447-2007, 2007. Molina, L. T., Madronich, S., Gaffney, J. S., Apel, E., de Foy, B., Fast, J., Ferrare, R., Herndon, S., Jimenez, J. L., Lamb, B., Osornio-Vargas, A. R., Russell, P., Schauer, J. J., Stevens, P. S., Volkamer, R., and Zavala, M.: An overview of the MILAGRO 2006 Campaign: Mexico City emissions and their transport and transformation, Atmos. Chem. Phys., 10, 8697–8760, http://dx.doi.org/10.5194/acp-10-8697-2010doi:10.5194/acp-10-8697-2010, 2010. Molina, M. J. and Molina, L. T.: Megacities and atmospheric pollution, J. Air Waste Manage. Assoc., 54, 644–680, 2004. Pan, X. L., Yan, P., Tang, J., Ma, J. Z., Wang, Z. F., Gbaguidi, A., and Sun, Y. L.: Observational study of influence of aerosol hygroscopic growth on scattering coefficient over rural area near Beijing mega-city, Atmos. Chem. Phys., 9, 7519–7530, http://dx.doi.org/10.5194/acp-9-7519-2009doi:10.5194/acp-9-7519-2009, 2009. Peeters, J. and Muller, J.-F.: HO<sub>x</sub> radical regeneration in isoprene oxidation via peroxy radical isomerisations. II: experimental evidence and global impact, Phys. Chem. Chem. Phys., 12, 14227–14235, 2010. Pilling, M. J.: Atmospheric chemistry: Update on Amazonian atmosphere, Nature Geosci., 5, 168–169, 2012. Pope, C. A. and Dockery, D. W.: Health effects of fine particulate air pollution: lines that connect, J. Air Waste Manage. Assoc., 56, 709–742, 2006. Ramanathan, V., Chung, C., Kim, D., Bettge, T., Buja, L., Kiehl, J. T., Washington, W. M., Fu, Q., Sikka, D. R., and Wild, M.: Atmospheric brown clouds: impacts on South Asian climate and hydrological cycle, P. Natl. Acad. Sci. USA, 102, 5326–5333, http://dx.doi.org/10.1073/pnas.0500656102doi:10.1073/pnas.0500656102, 2005. Ren, X., 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, 2003. Robinson, A. L., Donahue, N. M., Shrivastava, M. K., Weitkamp, E. A., Sage, A. M., Grieshop, A. P., Lane, T. E., Pierce, J. R., and Pandis, S. N.: Rethinking organic aerosols: Semivolatile emissions and photochemical aging, Science, 315, 1259–1262, http://dx.doi.org/10.1126/science.1133061doi:10.1126/science.1133061, 2007. Rohrer, F. and Berresheim, H.: Strong correlation between levels of tropospheric hydroxyl radicals and solar ultraviolet radiation, Nature, 442, 184–187, 2006. Russo, R. S., Talbot, R. W., Dibb, J. E., Scheuer, E., Seid, G., Jordan, C. E., Fuelberg, H. E., Sachse, G. W., Avery, M. A., Vay, S. A., Blake, D. R., Blake, N. J., Atlas, E., Fried, A., Sandholm, S. T., Tan, D., Singh, H. B., Snow, J., and Heikes, B. G.: Chemical composition of Asian continental outflow over the western Pacific: Results from Transport and Chemical Evolution over the Pacific (TRACE-P), J. Geophys. Res., 108, 8804, http://dx.doi.org/10.1029/2002jd003184doi:10.1029/2002jd003184, 2003. Seinfeld, J. H. and Pandis, S. N.: Atmospheric Chemistry and Physics, John Wiley and Sons, New York, 1998. Shao, M., Lu, S., Liu, Y., Xie, X., Chang, C., Huang, S., and Chen, Z.: Volatile organic compounds measured in summer in Beijing and their role in ground level ozone formation, J. Geophys. Res., 114, D00G06, http://dx.doi.org/10.1029/2008jd010863doi:10.1029/2008jd010863, 2009. Sheehy, P. M., Volkamer, R., Molina, L. T., and Molina, M. J.: Oxidative capacity of the Mexico City atmosphere – Part 2: A RO<sub>x</sub> radical cycling perspective, Atmos. Chem. Phys., 10, 6993–7008, http://dx.doi.org/10.5194/acp-10-6993-2010doi:10.5194/acp-10-6993-2010, 2010. 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, http://dx.doi.org/10.5194/acp-6-2753-2006doi:10.5194/acp-6-2753-2006, 2006. Sillman, S.: The relation between ozone, NO<sub>x</sub> and hydrocarbons in urban and polluted rural environments, Atmos. Environ., 33, 1821–1845, 1999. Singh, H. B., Brune, W. H., Crawford, J. H., Flocke, F., and Jacob, D. J.: Chemistry and transport of pollution over the Gulf of Mexico and the Pacific: spring 2006 INTEX-B campaign overview and first results, Atmos. Chem. Phys., 9, 2301–2318, http://dx.doi.org/10.5194/acp-9-2301-2009doi:10.5194/acp-9-2301-2009, 2009. Sinha, V., Williams, J., Lelieveld, J., Ruuskanen, T. M., Kajos, M. K., Patokoski, J., Hellen, H., Hakola, H., Mogensen, D., Boy, M., Rinne, J., and Kulmala, M.: OH reactivity measurements within a boreal forest: Evidence for unknown reactive emissions, Environ. Sci. Technol., 44, 6614–6620, http://dx.doi.org/10.1021/es101780bdoi:10.1021/es101780b, 2010. Solomon, P., Cowling, E., Hidy, G., and Furiness, C.: Comparison of scientific findings from major ozone field studies in North America and Europe, Atmos. Environ., 34, 1885–1920, 2000. Stockwell, W. R., Kirchner, F., Kuhn, M., and Seefeld, S.: A new mechanism for regional atmospheric chemistry modeling, J. Geophys. Res., 102, 25847–25879, http://dx.doi.org/10.1029/97jd00849doi:10.1029/97jd00849, 1997. Taraborrelli, D., Lawrence, M. G., Crowley, J. N., Dillon, T. J., Gromov, S., Grosz, C. B. M., Vereecken, L., and Lelieveld, J.: Hydroxyl radical buffered by isoprene oxidation over tropical forests, Nature Geosci., 5, 190–193, 2012. Volkamer, R., Sheehy, P., Molina, L. T., and Molina, M. J.: Oxidative capacity of the Mexico City atmosphere – Part 1: A radical source perspective, Atmos. Chem. Phys., 10, 6969–6991, http://dx.doi.org/10.5194/acp-10-6969-2010doi:10.5194/acp-10-6969-2010, 2010. Wang, T., Nie, W., Gao, J., Xue, L. K., Gao, X. M., Wang, X. F., Qiu, J., Poon, C. N., Meinardi, S., Blake, D., Wang, S. L., Ding, A. J., Chai, F. H., Zhang, Q. Z., and Wang, W. X.: Air quality during the 2008 Beijing Olympics: secondary pollutants and regional impact, Atmos. Chem. Phys., 10, 7603–7615, http://dx.doi.org/10.5194/acp-10-7603-2010doi:10.5194/acp-10-7603-2010, 2010. Wang, W., Ma, J., Hatakeyama, S., Liu, X., Chen, Y., Takami, A., Ren, L., and Geng, C.: Aircraft measurements of vertical ultrafine particles profiles over Northern China coastal areas during dust storms in 2006, Atmos. Environ., 42, 5715–5720, 2008. Wang, Y., Hao, J., McElroy, M. B., Munger, J. W., Ma, H., Chen, D., and Nielsen, C. P.: Ozone air quality during the 2008 Beijing Olympics: effectiveness of emission restrictions, Atmos. Chem. Phys., 9, 5237–5251, http://dx.doi.org/10.5194/acp-9-5237-2009doi:10.5194/acp-9-5237-2009, 2009. Wu, X. J., Jin, Z. Y., Huang, L. P., and Chen, D. H.: The software framework and application of GRAPES model, J. Appl. Meterol. Sci., 16, 539–546, 2005. Xu, J., Ma, J. Z., Zhang, X. L., Xu, X. B., Xu, X. F., Lin, W. L., Wang, Y., Meng, W., and Ma, Z. Q.: Measurements of ozone and its precursors in Beijing during summertime: impact of urban plumes on ozone pollution in downwind rural areas, Atmos. Chem. Phys., 11, 12241–12252, http://dx.doi.org/10.5194/acp-11-12241-2011doi:10.5194/acp-11-12241-2011, 2011. Xu, X. D., Zhou, X. J., and Shi, X. H.: Spatial structure and scale feature of the atmospheric pollution source impact of city agglomeration, Science in China, Ser. D Earth Sciences, 48, Supp. II, 1–24, 2005. Xue, J. S., Zhuang, S. Y., Zhu, G. F., Zhang, H., Liu, Z. Q., Yan, L., and Zhuang, Z. R.: Scientific design and preliminary results of three-dimensional variational data assimilation system of GRAPES, Chin. Sci. Bull., 53, 3446–3457, http://dx.doi.org/10.1007/s11434-008-0416-0doi:10.1007/s11434-008-0416-0, 2008. Yan, P., Tang, J., Huang, J., Mao, J. T., Zhou, X.J., Liu, Q., Wang, Z. F., and Zhou, H. G.: The measurement of aerosol optical properties at a rural site in Northern China, Atmos. Chem. Phys., 8, 2229–2242, http://dx.doi.org/10.5194/acp-8-2229-2008doi:10.5194/acp-8-2229-2008, 2008. Zhang, L., Jacob, D. J., Boersma, K. F., Jaffe, D. A., Olson, J. R., Bowman, K. W., Worden, J. R., Thompson, A. M., Avery, M. A., Cohen, R. C., Dibb, J. E., Flock, F. M., Fuelberg, H. E., Huey, L. G., McMillan, W. W., Singh, H. B., and Weinheimer, A. J.: Transpacific transport of ozone pollution and the effect of recent Asian emission increases on air quality in North America: an integrated analysis using satellite, aircraft, ozonesonde, and surface observations, Atmos. Chem. Phys., 8, 6117–6136, http://dx.doi.org/10.5194/acp-8-6117-2008doi:10.5194/acp-8-6117-2008, 2008a. Zhang, Q., Ma, X. C., Tie, X. X., Huang, M. Y., and Zhao, C. S.: Vertical distributions of aerosols under different weather conditions: Analysis of in-situ aircraft measurements in Beijing, China, Atmos. Environ., 43, 5526–5535, http://dx.doi.org/10.1016/j.atmosenv.2009.05.037doi:10.1016/j.atmosenv.2009.05.037, 2009a. Zhang, Q., Streets, D. G., Carmichael, G. R., He, K. B., Huo, H., Kannari, A., Klimont, Z., Park, I. S., Reddy, S., Fu, J. S., Chen, D., Duan, L., Lei, Y., Wang, L. T., and Yao, Z. L.: Asian emissions in 2006 for the NASA INTEX-B mission, Atmos. Chem. Phys., 9, 5131–5153, http://dx.doi.org/10.5194/acp-9-5131-2009doi:10.5194/acp-9-5131-2009, 2009b. Zhang, Y. H., Hu, M., Zhong, L. J., Wiedensohler, A., Liu, S. C., Andreae, M. O., Wang, W., and Fan, S. J.: Regional Integrated Experiments on Air Quality over Pearl River Delta 2004 (PRIDE-PRD2004): Overview, Atmos. Environ., 42, 6157–6173, http://dx.doi.org/10.1016/j.atmosenv.2008.03.025doi:10.1016/j.atmosenv.2008.03.025, 2008b. Zhao, B., Wang, P., Ma, J. Z., Zhu, S., Pozzer, A., and Li, W.: A high-resolution emission inventory of primary pollutants for the Huabei region, China, Atmos. Chem. Phys., 12, 481–501, http://dx.doi.org/10.5194/acp-12-481-2012doi:10.5194/acp-12-481-2012, 2012. Zhu, S., Butler, T., Sander, R., Ma, J., and Lawrence, M. G.: Impact of dust on tropospheric chemistry over polluted regions: a case study of the Beijing megacity, Atmos. Chem. Phys., 10, 3855–3873, http://dx.doi.org/10.5194/acp-10-3855-2010doi:10.5194/acp-10-3855-2010, 2010.