ALBERT

Description: Using data collected at the Pierre Auger Observatory during the past 3.7 years, we demonstrated a correlation between the arrival directions of cosmic rays with energy above 6 x 10(19) electron volts and the positions of active galactic nuclei (AGN) lying within approximately 75 megaparsecs. We rejected the hypothesis of an isotropic distribution of these cosmic rays with at least a 99% confidence level from a prescribed a priori test. The correlation we observed is compatible with the hypothesis that the highest-energy particles originate from nearby extragalactic sources whose flux has not been substantially reduced by interaction with the cosmic background radiation. AGN or objects having a similar spatial distribution are possible sources.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Pierre Auger Collaboration -- Abraham, J -- Abreu, P -- Aglietta, M -- Aguirre, C -- Allard, D -- Allekotte, I -- Allen, J -- Allison, P -- Alvarez, C -- Alvarez-Muniz, J -- Ambrosio, M -- Anchordoqui, L -- Andringa, S -- Anzalone, A -- Aramo, C -- Argiro, S -- Arisaka, K -- Armengaud, E -- Arneodo, F -- Arqueros, F -- Asch, T -- Asorey, H -- Assis, P -- Atulugama, B S -- Aublin, J -- Ave, M -- Avila, G -- Backer, T -- Badagnani, D -- Barbosa, A F -- Barnhill, D -- Barroso, S L C -- Bauleo, P -- Beatty, J -- Beau, T -- Becker, B R -- Becker, K H -- Bellido, J A -- Benzvi, S -- Berat, C -- Bergmann, T -- Bernardini, P -- Bertou, X -- Biermann, P L -- Billoir, P -- Blanch-Bigas, O -- Blanco, F -- Blasi, P -- Bleve, C -- Blumer, H -- Bohacova, M -- Bonifazi, C -- Bonino, R -- Boratav, M -- Brack, J -- Brogueira, P -- Brown, W C -- Buchholz, P -- Bueno, A -- Busca, N G -- Caballero-Mora, K S -- Cai, B -- Camin, D V -- Caruso, R -- Carvalho, W -- Castellina, A -- Catalano, O -- Cataldi, G -- Cazon-Boado, L -- Cester, R -- Chauvin, J -- Chiavassa, A -- Chinellato, J A -- Chou, A -- Chye, J -- Clark, P D J -- Clay, R W -- Colombo, E -- Conceicao, R -- Connolly, B -- Contreras, F -- Coppens, J -- Cordier, A -- Cotti, U -- Coutu, S -- Covault, C E -- Creusot, A -- Cronin, J -- Dagoret-Campagne, S -- Daumiller, K -- Dawson, B R -- de Almeida, R M -- De Donato, C -- de Jong, S J -- De La Vega, G -- de Mello Junior, W J M -- de Mello Neto, J R T -- De Mitri, I -- de Souza, V -- Del Peral, L -- Deligny, O -- Selva, A Della -- Fratte, C Delle -- Dembinski, H -- Di Giulio, C -- Diaz, J C -- Dobrigkeit, C -- D'Olivo, J C -- Dornic, D -- Dorofeev, A -- Dos Anjos, J C -- Dova, M T -- D'Urso, D -- Duvernois, M A -- Engel, R -- Epele, L -- Erdmann, M -- Escobar, C O -- Etchegoyen, A -- Facal San Luis, P -- Falcke, H -- Farrar, G -- Fauth, A C -- Fazzini, N -- Fernandez, A -- Ferrer, F -- Ferry, S -- Fick, B -- Filevich, A -- Filipcic, A -- Fleck, I -- Fonte, R -- Fracchiolla, C E -- Fulgione, W -- Garcia, B -- Garcia Gamez, D -- Garcia-Pinto, D -- Garrido, X -- Geenen, H -- Gelmini, G -- Gemmeke, H -- Ghia, P L -- Giller, M -- Glass, H -- Gold, M S -- Golup, G -- Albarracin, F Gomez -- Berisso, M Gomez -- Herrero, R Gomez -- Goncalves, P -- Goncalves do Amaral, M -- Gonzalez, D -- Gonzalez, J G -- Gonzalez, M -- Gora, D -- Gorgi, A -- Gouffon, P -- Grassi, V -- Grillo, A -- Grunfeld, C -- Guardincerri, Y -- Guarino, F -- Guedes, G P -- Gutierrez, J -- Hague, J D -- Hamilton, J C -- Hansen, P -- Harari, D -- Harmsma, S -- Harton, J L -- Haungs, A -- Hauschildt, T -- Healy, M D -- Hebbeker, T -- Heck, D -- Hojvat, C -- Holmes, V C -- Homola, P -- Horandel, J -- Horneffer, A -- Horvat, M -- Hrabovsky, M -- Huege, T -- Iarlori, M -- Insolia, A -- Ionita, F -- Italiano, A -- Kaducak, M -- Kampert, K H -- Keilhauer, B -- Kemp, E -- Kieckhafer, R M -- Klages, H O -- Kleifges, M -- Kleinfeller, J -- Knapik, R -- Knapp, J -- Koang, D-H -- Kopmann, A -- Krieger, A -- Kromer, O -- Kumpel, D -- Kunka, N -- Kusenko, A -- La Rosa, G -- Lachaud, C -- Lago, B L -- Lebrun, D -- Lebrun, P -- Lee, J -- Leigui de Oliveira, M A -- Letessier-Selvon, A -- Leuthold, M -- Lhenry-Yvon, I -- Lopez, R -- Lopez Aguera, A -- Lozano Bahilo, J -- Maccarone, M C -- Macolino, C -- Maldera, S -- Malek, M -- Mancarella, G -- Mancenido, M E -- Mandat, D -- Mantsch, P -- Mariazzi, A G -- Maris, I C -- Martello, D -- Martinez, J -- Martinez Bravo, O -- Mathes, H J -- Matthews, J -- Matthews, J A J -- Matthiae, G -- Maurizio, D -- Mazur, P O -- McCauley, T -- McEwen, M -- McNeil, R R -- Medina, M C -- Medina-Tanco, G -- Meli, A -- Melo, D -- Menichetti, E -- Menschikov, A -- Meurer, Chr -- Meyhandan, R -- Micheletti, M I -- Miele, G -- Miller, W -- Mollerach, S -- Monasor, M -- Monnier Ragaigne, D -- Montanet, F -- Morales, B -- Morello, C -- Moreno, E -- Moreno, J C -- Morris, C -- Mostafa, M -- Muller, M A -- Mussa, R -- Navarra, G -- Navarro, J L -- Navas, S -- Nellen, L -- Newman-Holmes, C -- Newton, D -- Thi, T Nguyen -- Nierstenhofer, N -- Nitz, D -- Nosek, D -- Nozka, L -- Oehlschlager, J -- Ohnuki, T -- Olinto, A -- Olmos-Gilbaja, V M -- Ortiz, M -- Ostapchenko, S -- Otero, L -- Pakk Selmi-Dei, D -- Palatka, M -- Pallotta, J -- Parente, G -- Parizot, E -- Parlati, S -- Pastor, S -- Patel, M -- Paul, T -- Pavlidou, V -- Payet, K -- Pech, M -- Pekala, J -- Pelayo, R -- Pepe, I M -- Perrone, L -- Petrera, S -- Petrinca, P -- Petrov, Y -- Ngoc, Dieppham -- Ngoc, Dongpham -- Pham Thi, T N -- Pichel, A -- Piegaia, R -- Pierog, T -- Pimenta, M -- Pinto, T -- Pirronello, V -- Pisanti, O -- Platino, M -- Pochon, J -- Porter, T A -- Privitera, P -- Prouza, M -- Quel, E J -- Rautenberg, J -- Reucroft, S -- Revenu, B -- Rezende, F A S -- Ridky, J -- Riggi, S -- Risse, M -- Riviere, C -- Rizi, V -- Roberts, M -- Robledo, C -- Rodriguez, G -- Rodriguez Frias, D -- Rodriguez Martino, J -- Rodriguez Rojo, J -- Rodriguez-Cabo, I -- Ros, G -- Rosado, J -- Roth, M -- Rouille-d'Orfeuil, B -- Roulet, E -- Rovero, A C -- Salamida, F -- Salazar, H -- Salina, G -- Sanchez, F -- Santander, M -- Santo, C E -- Santos, E M -- Sarazin, F -- Sarkar, S -- Sato, R -- Scherini, V -- Schieler, H -- Schmidt, F -- Schmidt, T -- Scholten, O -- Schovanek, P -- Schussler, F -- Sciutto, S J -- Scuderi, M -- Segreto, A -- Semikoz, D -- Settimo, M -- Shellard, R C -- Sidelnik, I -- Siffert, B B -- Sigl, G -- De Grande, N Smetniansky -- Smialkowski, A -- Smida, R -- Smith, A G K -- Smith, B E -- Snow, G R -- Sokolsky, P -- Sommers, P -- Sorokin, J -- Spinka, H -- Squartini, R -- Strazzeri, E -- Stutz, A -- Suarez, F -- Suomijarvi, T -- Supanitsky, A D -- Sutherland, M S -- Swain, J -- Szadkowski, Z -- Takahashi, J -- Tamashiro, A -- Tamburro, A -- Tascau, O -- Tcaciuc, R -- Thomas, D -- Ticona, R -- Tiffenberg, J -- Timmermans, C -- Tkaczyk, W -- Todero Peixoto, C J -- Tome, B -- Tonachini, A -- Torresi, D -- Travnicek, P -- Tripathi, A -- Tristram, G -- Tscherniakhovski, D -- Tueros, M -- Tunnicliffe, V -- Ulrich, R -- Unger, M -- Urban, M -- Valdes Galicia, J F -- Valino, I -- Valore, L -- van den Berg, A M -- van Elewyck, V -- Vazquez, R A -- Veberic, D -- Veiga, A -- Velarde, A -- Venters, T -- Verzi, V -- Videla, M -- Villasenor, L -- Vorobiov, S -- Voyvodic, L -- Wahlberg, H -- Wainberg, O -- Waldenmaier, T -- Walker, P -- Warner, D -- Watson, A A -- Westerhoff, S -- Wieczorek, G -- Wiencke, L -- Wilczynska, B -- Wilczynski, H -- Wileman, C -- Winnick, M G -- Wu, H -- Wundheiler, B -- Xu, J -- Yamamoto, T -- Younk, P -- Zas, E -- Zavrtanik, D -- Zavrtanik, M -- Zech, A -- Zepeda, A -- Ziolkowski, M -- Kegl, B -- New York, N.Y. -- Science. 2007 Nov 9;318(5852):938-43.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17991855" target="_blank"〉PubMed〈/a〉

Description: Cold molecular clouds are the birthplaces of stars and planets, where dense cores of gas collapse to form protostars. The dust mixed in these clouds is thought to be made of grains of an average size of 0.1 micrometer. We report the widespread detection of the coreshine effect as a direct sign of the existence of grown, micrometer-sized dust grains. This effect is seen in half of the cores we have analyzed in our survey, spanning all Galactic longitudes, and is dominated by changes in the internal properties and local environment of the cores, implying that the coreshine effect can be used to constrain fundamental core properties such as the three-dimensional density structure and ages and also the grain characteristics themselves.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Pagani, Laurent -- Steinacker, Jurgen -- Bacmann, Aurore -- Stutz, Amelia -- Henning, Thomas -- New York, N.Y. -- Science. 2010 Sep 24;329(5999):1622-4. doi: 10.1126/science.1193211.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Laboratoire d'Etude du Rayonnement et de la Matiere en Astrophysique (LERMA) and UMR 8112 du CNRS, Observatoire de Paris, 61 Avenue de l'Observatoire, 75014 Paris, France. laurent.pagani@obspm.fr〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/20929841" target="_blank"〉PubMed〈/a〉

Description: Nitrous acid (HONO) often plays an important role in tropospheric photochemistry as a major precursor of the hydroxyl radical (OH) in early morning hours and potentially during the day. However, the processes leading to formation of HONO and its vertical distribution at night, which can have a considerable impact on daytime ozone formation, are currently poorly characterized by observations and models. Long-path differential optical absorption spectroscopy (LP-DOAS) measurements of HONO during the 2006 TexAQS II Radical and Aerosol Measurement Project (TRAMP), near downtown Houston, TX, show nocturnal vertical profiles of HONO, with mixing ratios of up to 2.2 ppb near the surface and below 100 ppt aloft. Three nighttime periods of HONO, NO2 and O3 observations during TRAMP were used to perform model simulations of vertical mixing ratio profiles. By adjusting vertical mixing and NOx emissions the modeled NO2 and O3 mixing ratios showed very good agreement with the observations. Using a simple conversion of NO2 to HONO on the ground, direct HONO emissions, as well as HONO loss at the ground and on aerosol, the observed HONO profiles were reproduced well by the model. The unobserved increase of HONO to NO2 ratio (HONO/NO2) with altitude that was simulated by the initial model runs was found to be due to HONO uptake being too small on aerosol and too large on the ground. Refined model runs, with adjusted HONO uptake coefficients, showed much better agreement of HONO and HONO/NO2 for two typical nights, except during morning rush hour, when other HONO formation pathways are most likely active. One of the nights analyzed showed increase of HONO mixing ratios together with decreasing NO2 mixing ratios that the model was unable to reproduce, most likely due to the impact of weak precipitation during this night. HONO formation and removal rates averaged over the lowest 300 m of the atmosphere showed that NO2 to HONO conversion on the ground was the dominant source of HONO, followed by traffic emission. Aerosol did not play an important role in HONO formation. Although ground deposition was also a major removal pathway of HONO, net HONO production at the ground was the main source of HONO in our model studies. Sensitivity studies showed that in the stable NBL, net HONO production at the ground tends to increase with faster vertical mixing and stronger emission. Vertical transport was found to be the dominant source of HONO aloft.

Description: Sun-lit snow is increasingly recognized as a chemical reactor that plays an active role in uptake, transformation, and release of atmospheric trace gases. Snow is known to influence boundary layer air on a local scale, and given the large global surface coverage of snow may also be significant on regional and global scales. We present a new detailed one-dimensional snow chemistry module that has been coupled to the 1-D atmospheric boundary layer model MISTRA, we refer to the coupled model as MISTRA-SNOW. The new 1-D snow module, which is dynamically coupled to the overlaying atmospheric model, includes heat transport in the snowpack, molecular diffusion, and wind pumping of gases in the interstitial air. The model includes gas phase photochemistry and chemical reactions both in the interstitial air and the atmosphere. Heterogeneous and multiphase chemistry on atmospheric aerosol is considered explicitly. The chemical interaction of interstitial air with snow grains is simulated assuming chemistry in a liquid (aqueous) layer on the grain surface. The model was used to investigate snow as the source of nitrogen oxides (NOx) and gas phase reactive bromine in the atmospheric boundary layer in the remote snow covered Arctic (over the Greenland ice sheet) as well as to investigate the link between halogen cycling and ozone depletion that has been observed in interstitial air. The model is validated using data taken 10 June–13 June, 2008 as part of the Greenland Summit Halogen-HOx experiment (GSHOX). The model predicts that reactions involving bromide and nitrate impurities in the surface snow at Summit can sustain atmospheric NO and BrO mixing ratios measured at Summit during this period.

Description: The Greenland Summit Halogen-HOx (GSHOX) Campaign was performed in spring 2007 and summer 2008 to investigate the impact of halogens on HOx (= OH + HO2) cycling above the Greenland Ice Sheet. Chemical species including hydroxyl and peroxy radicals (OH and HO2 + RO2), ozone (O3), nitrogen oxide (NO), nitric acid (HNO3), nitrous acid (HONO), reactive gaseous mercury (RGM), and bromine oxide (BrO) were measured during the campaign. The median midday values of HO2 + RO2 and OH concentrations observed by chemical ionization mass spectrometry (CIMS) were 2.7 × 108 molec cm−3 and 3.0 × 106 molec cm−3 in spring 2007, and 4.2 × 108 molec cm−3 and 4.1 × 106 molec cm−3 in summer 2008. A basic photochemical 0-D box model highly constrained by observations of H2O, O3, CO, CH4, NO, and J values predicted HO2 + RO2 (R = 0.90, slope = 0.87 in 2007; R = 0.79, slope = 0.96 in 2008) reasonably well and under predicted OH (R = 0.83, slope = 0.72 in 2007; R = 0.76, slope = 0.54 in 2008). Constraining the model to HONO observations did not significantly improve the ratio of OH to HO2 + RO2 and the correlation between predictions and observations. Including bromine chemistry in the model constrained by observations of BrO improved the correlation between observed and predicted HO2 + RO2 and OH, and brought the average hourly OH and HO2 + RO2 predictions closer to the observations. These model comparisons confirmed our understanding of the dominant HOx sources and sinks in this environment and indicated that BrO impacted the OH levels at Summit. Although, significant discrepancies between observed and predicted OH could not be explained by the measured BrO. Finally, observations of enhanced RGM were found to be coincident with under prediction of OH.

Description: Nitrous acid (HONO) often plays an important role in tropospheric photochemistry as a major precursor of the hydroxyl radical (OH) in early morning hours and potentially during the day. However, the processes leading to formation of HONO and its vertical distribution at night, which can have a considerable impact on daytime ozone formation, are currently poorly characterized by observations and models. Long-path differential optical absorption spectroscopy (LP-DOAS) measurements of HONO during the 2006 TexAQS II Radical and Aerosol Measurement Project (TRAMP), near downtown Houston, TX, show nocturnal vertical profiles of HONO, with mixing ratios of up to 2.2 ppb near the surface and below 100 ppt aloft. Three nighttime periods of HONO, NO2 and O3 observations during TRAMP were used to perform model simulations of vertical mixing ratio profiles. By adjusting vertical mixing and NOx emissions the modeled NO2 and O3 mixing ratios showed very good agreement with the observations. Using a simple conversion of NO2 to HONO on the ground, direct HONO emissions, as well as HONO loss at the ground and on aerosol, the observed HONO profiles were reproduced by the model for 1–2 and 7–8 September in the nocturnal boundary layer (NBL). The unobserved increase of HONO to NO2 ratio (HONO/NO2) with altitude that was simulated by the initial model runs was found to be due to HONO uptake being too small on aerosol and too large on the ground. Refined model runs, with adjusted HONO uptake coefficients, showed much better agreement of HONO and HONO/NO2 for two typical nights, except during morning rush hour, when other HONO formation pathways are most likely active. One of the nights analyzed showed an increase of HONO mixing ratios together with decreasing NO2 mixing ratios that the model was unable to reproduce, most likely due to the impact of weak precipitation during this night. HONO formation and removal rates averaged over the lowest 300 m of the atmosphere showed that NO2 to HONO conversion on the ground was the dominant source of HONO, followed by traffic emission. Aerosol did not play an important role in HONO formation. Although ground deposition was also a major removal pathway of HONO, net HONO production at the ground was the main source of HONO in our model studies. Sensitivity studies showed that in the stable NBL, net HONO production at the ground tends to increase with faster vertical mixing and stronger NOx emission. Vertical transport was found to be the dominant source of HONO aloft.

Description: The chemical composition of the boundary layer in snow covered regions is impacted by chemistry in the snowpack via uptake, processing, and emission of atmospheric trace gases. We use the coupled one-dimensional (1-D) snow chemistry and atmospheric boundary layer model MISTRA-SNOW to study the impact of snowpack chemistry on the oxidation capacity of the boundary layer. The model includes gas phase photochemistry and chemical reactions both in the interstitial air and the atmosphere. While it is acknowledged that the chemistry occurring at ice surfaces may consist of a true quasi-liquid layer and/or a concentrated brine layer, lack of additional knowledge requires that this chemistry be modeled as primarily aqueous chemistry occurring in a liquid-like layer (LLL) on snow grains. The model has been recently compared with BrO and NO data taken on 10 June–13 June 2008 as part of the Greenland Summit Halogen-HOx experiment (GSHOX). In the present study, we use the same focus period to investigate the influence of snowpack derived chemistry on OH and HOx + RO2 in the boundary layer. We compare model results with chemical ionization mass spectrometry (CIMS) measurements of the hydroxyl radical (OH) and of the hydroperoxyl radical (HO2) plus the sum of all organic peroxy radicals (RO2) taken at Summit during summer 2008. Using sensitivity runs we show that snowpack influenced nitrogen cycling and bromine chemistry both increase the oxidation capacity of the boundary layer and that together they increase the mid-day OH concentrations. Bromine chemistry increases the OH concentration by 10–18% (10% at noon LT), while snow sourced NOx increases OH concentrations by 20–50% (27% at noon LT). We show for the first time, using a coupled one-dimensional snowpack-boundary layer model, that air-snow interactions impact the oxidation capacity of the boundary layer and that it is not possible to match measured OH levels without snowpack NOx and halogen emissions. Model predicted HONO compared with mistchamber measurements suggests there may be an unknown HONO source at Summit. Other model predicted HOx precursors, H2O2 and HCHO, compare well with measurements taken in summer 2000, which had lower levels than other years. Over 3 days, snow sourced NOx contributes an additional 2 ppb to boundary layer ozone production, while snow sourced bromine has the opposite effect and contributes 1 ppb to boundary layer ozone loss.

Description: Nitrous acid (HONO) acts as a major precursor of the hydroxyl radical (OH) in the urban atmospheric boundary layer in the morning and throughout the day. Despite its importance, HONO formation mechanisms are not yet completely understood. It is generally accepted that conversion of NO2 on surfaces in the presence of water is responsible for the formation of HONO in the nocturnal boundary layer, although the type of surface on which the mechanism occurs is still under debate. Recent observations of higher than expected daytime HONO concentrations in both urban and rural areas indicate the presence of unknown daytime HONO source(s). Various formation pathways in the gas phase, and on aerosol and ground surfaces have been proposed to explain the presence of daytime HONO. However, it is unclear which mechanism dominates and, in the cases of heterogeneous mechanisms, on which surfaces they occur. Vertical concentration profiles of HONO and its precursors can help in identifying the dominant HONO formation pathways. In this study, daytime HONO and NO2 vertical profiles, measured in three different height intervals (20–70, 70–130, and 130–300 m) in Houston, TX, during the 2009 Study of Houston Atmospheric Radical Precursors (SHARP) are analyzed using a one-dimensional (1-D) chemistry and transport model. Model results with various HONO formation pathways suggested in the literature are compared to the the daytime HONO and HONO/NO2 ratios observed during SHARP. The best agreement of HONO and HONO/NO2 ratios between model and observations is achieved by including both a photolytic source of HONO at the ground and on the aerosol. Model sensitivity studies show that the observed diurnal variations of the HONO/NO2 ratio are not reproduced by the model if there is only a photolytic HONO source on aerosol or in the gas phase from NO2* + H2O. Further analysis of the formation and loss pathways of HONO shows a vertical dependence of HONO chemistry during the day. Photolytic HONO formation at the ground is the major formation pathway in the lowest 20 m, while a combination of gas-phase, photolytic formation on aerosol, and vertical transport is responsible for daytime HONO between 200–300 m a.g.l. HONO removal is dominated by vertical transport below 20 m and photolysis between 200–300 m a.g.l.

Description: Nocturnal chemistry in the atmospheric boundary layer plays a key role in determining the initial chemical conditions for photochemistry during the following morning as well as influencing the budgets of O3 and NO2. Despite its importance, chemistry in the nocturnal boundary layer (NBL), especially in heavily polluted urban areas, has received little attention so far, which greatly limits the current understanding of the processes involved. In particular, the influence of vertical mixing on chemical processes gives rise to complex vertical profiles of various reactive trace gases and makes nocturnal chemistry altitude-dependent. The processing of pollutants is thus driven by a complicated, and not well quantified, interplay between chemistry and vertical mixing. In order to gain a better understanding of the altitude-dependent nocturnal chemistry in the polluted urban environment, a field study was carried out in the downtown area of Phoenix, AZ, in summer 2001. Vertical profiles of reactive species, such as O3, NO2, and NO3, were observed in the lowest 140 m of the troposphere throughout the night. The disappearance of these trace gas vertical gradients during the morning coincided with the morning transition from a stable NBL to a well-mixed convective layer. The vertical gradients of trace gas levels were found to be dependent on both surface NOx emission strength and the vertical stability of the NBL. The vertical gradients of Ox, the sum of O3 and NO2, were found to be much smaller than those of O3 and NO2, revealing the dominant role of NO emissions followed by the O3+NO reaction for the altitude-dependence of nocturnal chemistry in urban areas. Dry deposition, direct emissions, and other chemical production pathways of NO2 also play a role for the Ox distribution. Strong positive vertical gradients of NO3, that are predominantly determined by NO3 loss near the ground, were observed. The vertical profiles of NO3 and the calculated vertical profiles of its reservoir species (N2O5) confirm earlier model results suggesting complex vertical distributions of atmospheric denoxification processes during the night.

Description: Nitrous Acid (HONO) plays an important role in tropospheric chemistry as a precursor of the hydroxyl radical (OH), the most important oxidizing agent in the atmosphere. Nevertheless, the formation mechanisms of HONO are still not completely understood. Recent field observations found unexpectedly high daytime HONO concentrations in both urban and rural areas, which point to unrecognized, most likely photolytically enhanced HONO sources. Several gas-phase, aerosol, and ground surface chemistry mechanisms have been proposed to explain elevated daytime HONO, but atmospheric evidence to favor one over the others is still weak. New information on whether HONO formation occurs in the gas-phase, on aerosol, or at the ground may be derived from observations of the vertical distribution of HONO and its precursor nitrogen dioxide, NO2, as well as from its dependence on solar irradiance or actinic flux. Here we present field observations of HONO, NO2 and other trace gases in three altitude intervals (30–70 m, 70–130 m and 130–300 m) using UCLA's long path DOAS instrument, as well as in situ measurements of OH, NO, photolysis frequencies and solar irradiance, made in Houston, TX, during the Study of Houston Atmospheric Radical Precursor (SHARP) experiment from 20 April to 30 May 2009. The observed HONO mixing ratios were often ten times larger than the expected photostationary state with OH and NO. Larger HONO mixing ratios observed near the ground than aloft imply, but do not clearly prove, that the daytime source of HONO was located at or near the ground. Using a pseudo steady-state (PSS) approach, we calculated the missing daytime HONO formation rates, Punknown, on four sunny days. The NO2-normalized Punknown, Pnorm, showed a clear symmetrical diurnal variation with a maximum around noontime, which was well correlated with actinic flux (NO2 photolysis frequency) and solar irradiance. This behavior, which was found on all clear days in Houston, is a strong indication of a photolytic HONO source. [HONO]/[NO2] ratios also showed a clear diurnal profile, with maxima of 2–3% around noon. PSS calculations show that this behavior cannot be explained by the proposed gas-phase reaction of photoexcited NO2 (NO2*) or any other gas-phase or aerosol photolytic process occurring at similar or longer wavelengths than that of HONO photolysis. HONO formation by aerosol nitrate photolysis in the UV also seems to be unlikely. Pnorm correlated better with solar irradiance (average R2 = 0.85/0.87 for visible/UV) than with actinic flux (R2 = 0.76) on the four sunny days, clearly pointing to HONO being formed at the ground rather than on the aerosol or in the gas-phase. In addition, the observed [HONO]/[NO2] diurnal variation can be explained if the formation of HONO depends on solar irradiance, but not if it depends on the actinic flux. The vertical mixing ratio profiles, together with the stronger correlation with solar irradiance, support the idea that photolytically enhanced NO2 to HONO conversion on the ground was the dominant source of HONO in Houston.