Atmospheric oxidation chemistry and ozone production: Results from SHARP 2009 in Houston, Texas

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1 JOURNAL OF GEOPHYSICAL RESEARCH: ATMOSPHERES, VOL. 8, , doi:.2/jgrd.5342, 23 Atmospheric oxidation chemistry and ozone production: Results from SHARP 29 in Houston, Texas Xinrong Ren,,2 Diana van Duin, 3 Maria Cazorla, 3,4 Shuang Chen, 3 Jingqiu Mao, 5 Li Zhang, 3 William H. Brune, 3 James H. Flynn, 6 Nicole Grossberg, 6 Barry L. Lefer, 6 Bernhard Rappenglück, 6 Kam W. Wong, 7,8 Catalina Tsai, 7 Jochen Stutz, 7 Jack E. Dibb, 9 B. Thomas Jon, Winston T. Luke, 2 and Paul Kelley 2 Received 3 November 22; revised 5 March 23; accepted 7 March 23; published 7 June 23. [] Ozone (O 3 ) and secondary fine particles come from the atmospheric oxidation chemistry that involves the hydroxyl radical (OH) and hydroperoxyl radical (HO 2 ), which are together called HO x. Radical precursors such as nitrous acid (HONO) and formaldehyde (HCHO) significantly affect the HO x budget in urban environments. These chemical processes connect surface anthropogenic and natural emissions to local and regional air pollution. Using the data collected during the Study of Houston Atmospheric Radical Precursors (SHARP) in spring 29, we examine atmospheric oxidation chemistry and O 3 production in this polluted urban environment. A numerical box el with five different chemical mechanisms was used to simulate the oxidation processes and thus OH and HO 2 in this study. In general, the el reproduced the measured OH and HO 2 with all five chemical mechanisms producing similar levels of OH and HO 2, although midday OH was overpredicted and nighttime OH and HO 2 were underpredicted. The calculated HO x production was dominated by HONO photolysis in the early morning and by the photolysis of O 3 and oxygenated volatile organic compounds (OVOCs) in the midday. On average, the daily HO x production rate was 24.6 ppbv d, of which 3% was from O 3 photolysis, 22% from HONO photolysis, 5% from the photolysis of OVOCs (other than HCHO), 4% from HCHO photolysis, and 3% from O 3 reactions with alkenes. The O 3 production was sensitive to volatile organic compounds (VOCs) in the early morning but was sensitive to NO x for most of afternoon. This is similar to the behavior erved in two previous summertime studies in Houston: the Texas Air Quality Study in 2 (TexAQS 2) and the TexAQS II Radical and Aerosol Measurement Project in 26 (TRAMP 26). Ozone production in SHARP exhibits a longer NO x -sensitive period than TexAQS 2 and TRAMP 26, indicating that NO x control may be an efficient approach for the O 3 control in springtime for Houston. Results from this study provide additional support for regulatory actions to reduce NO x and reactive VOCs in Houston in order to reduce O 3 and other secondary pollutants. Citation: Ren, X., et al. (23), Atmospheric oxidation chemistry and ozone production: Results from SHARP 29 in Houston, Texas, J. Geophys. Res. Atmos., 8, , doi:.2/jgrd Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA. 2 Air Resources Laboratory, National Oceanic and Atmospheric Administration, College Park, Maryland, USA. 3 Department of Meteorology, Pennsylvania State University, University Park, Pennsylvania, USA. Corresponding author: X. Ren, Air Resources Laboratory, National Oceanic and Atmospheric Administration, College Park, MD, USA. (Xinrong.Ren@noaa.gov) 23. American Geophysical Union. All Rights Reserved X/3/.2/jgrd Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. 5 Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, New Jersey, USA. 6 Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas, USA. 7 Department of Atmospheric and Oceanic Sciences, University of California at Los Angeles, Los Angeles, California, USA. 8 Jet Propulsion Laboratory, National Aeronautics and Space Administration Pasadena, California, USA. 9 Climate Change Research Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, New Hampshire, USA. Department of Civil and Environmental Engineering, Washington State University, Seattle, Washington, USA. 577

2 Figure. Asimplified schematic diagram showing atmospheric oxidation chemistry in which OH and HO 2 play central roles in the processes producing secondary pollutants such as ozone and fine particles.. Introduction [2] The chemistry of atmospheric radicals, especially the hydroxyl radical (OH) and hydroperoxyl radical (HO 2 ), collectively called HO x, is deeply involved in the formation of secondary pollutants such as ozone (O 3 ) and fine particles (Figure ). The photolysis of O 3, nitrous acid (HONO), formaldehyde (HCHO), hydrogen peroxide (H 2 O 2 ), and some oxygenated volatile organic compounds (OVOCs) is the initial source for OH and HO 2 radicals. OH initiates most reaction sequences that cycle surface emissions through the atmosphere and react with carbon monoxide (CO) and formaldehyde to produce HO 2. In turn, HO 2 reacts with nitric oxide (NO) to reproduce OH, thus creating a HO x cycle. OH also oxidizes nitrogen dioxide (NO 2 ) and sulfur dioxide (SO 2 ) to produce nitrate and sulfate, two main components of aerosols, and volatile organic compounds (VOCs) to produce organic peroxy radicals, RO 2. Both HO 2 and RO 2 oxidize NO to produce NO 2, without destroying O 3, and the subsequent photolysis of NO 2 produces O 3 (Figure ). [3] Understanding these chemical processes is important in determining the extent and types of emission reductions that are most effective in reducing O 3. Predictive capability for O 3 and its response to regulatory action also requires a firm understanding of HO x sources, sinks, and interactions with anthropogenic hydrocarbons and nitrogen oxides. In urban environments like Houston, radical precursors such as nitrous acid (HONO) and formaldehyde (HCHO) can significantly affect the HO x budget [Olaguer et al., 29; Mao et al., 2]. These chemical processes connect surface emissions, both human and natural, to local and regional pollution. [4] Although portions of the chemistry that lead to the formation of O 3 have been understood for decades, new discoveries have revealed the need to improve scientific understanding and detailed mechanisms of O 3 formation chemistry [Volz-Thomas et al., 23; Texas Air Quality Research Program, 2]. Radical production in Houston and some other urban areas appears to be underestimated by chemical mechanisms [Olaguer et al., 29]. Some gasphase and heterogeneous chemical reactions seem to be missing from the mechanisms, e.g., a missing heterogeneous source of HONO which in turn could be an important OH source. Further, the roles of some radical precursors such as HONO and HCHO in O 3 formation in urban environments have not been well quantified [Texas Air Quality Research Program, 2]. [5] In summer 2, the Texas Air Quality Study campaign (TexAQS 2) was conducted in Eastern Texas. The study revealed that the Greater Houston Area often encountered critical loadings of a variety of species and the rapid O 3 formation processes appeared to be associated with releases of highly reactive VOCs from industrial facilities [Lefer and Rappengluck, 2]. The meteorological conditions in Houston were also found to promote O 3 formation [Berkowitz et al., 24]. In summer 26 within the TexAQS-II efforts, the TexAQS-II Radical and Aerosol Measurement Project (TRAMP 26) was conducted at the Moody Tower site on the campus of the University of Houston. TRAMP 26 found that nitrous acid (HONO) exceeded 2 ppbv close to sunrise and remained at hundreds of pptv during the day and strong vertical gradients indicate ground-level source of HONO [Stutz et al., 2a]. Photolysis of HONO and HCHO was an important HO x source [Mao et al., 2]. Ozone production rates were often greater than 4 ppbv h, and a high OH chain length ( 2) was associated with high VOC abundances in Houston [Mao et al., 2]. [6] Following TexAQS 2 and TRAMP 26, the Study of Houston Atmospheric Radical Precursors (SHARP) in spring 29 aimed to investigate sources of important radical precursors like HONO and HCHO and to reduce uncertainties in photochemical processes and thus to improve our ability to el radicals and ozone formation. In this study, the instrument suite measured the most important contributions to O 3 and particle formation and thus enabled a thorough analysis of the atmospheric chemistry and O 3 formation in Houston. This analysis has the potential to improve the understanding of atmospheric oxidation in Houston and perhaps other urban areas. Such an improved understanding could aid the development of the State Implementation Plan (SIP) for Houston, which is essential for the future primary and secondary National Ambient Air Quality Standards for O 3 proposed by U.S. Environmental Protection Agency (EPA) to be met there. 2. Measurement and Model Description 2.. Site [7] The SHARP campaign (5 April to 3 May 29) was designed to examine the processes involved in the springtime O 3 peak erved in southeast Texas. Chemical and meteorological measurements were made from a height of 7 m above ground level at the top of a m tower on a roof balcony of the north Moody Tower, an 8-story dormitory on the campus of the University of Houston. The campus is located 35 km west of Galveston Bay, 7 km northwest of Galveston, Texas, and the Gulf of Mexico. The north Moody Tower ( N, W) is located in a partially wooded and grass covered land surface approximately 5 km southeast of tall buildings in downtown Houston, km southwest of Interstate 45, and 3.5 km north of the South Interstate 6 Loop. The measurement site is 6 km southwest of the Buffalo Bayou Turning Basin and 25 km 577

3 Table. Meteorological Parameters and Gas-Phase Chemical Species Measured During SHARP Analyte* Instrument Uncertainty (s) Interval Reference Meteorology (T, P, RH, wind, rain, Campbell research meteorological system ~ 5% 6 s Lefer et al. [2] cloud camera) Photolysis rate coefficients Scanning actinic flux spectrometer ~ % 6 s Lefer et al. [2] Basic trace gases (O 3, CO, SO 2, NO, NO 2,NO y ) Thermo 49c, 48c-TLE, 42c-TL with NO xy inlet (Blue Light and Mo converters) ~ 5 % 6 s, 5 min Lefer et al. [2] Luke et al. [2] VOCs (C2-C NMHCs) Perkin-Elmer GC-FID ~ 5 % h Leuchner and Rappenglück [2] HNO 3, HONO, HCl Mist Chamber/IC 5% 5 min Stutz et al. [2a] HONO Liquid coil scrubbing/uv-vis absorption 8% 2 min Ren et al. [2] OH, HO 2, OH reactivity Laser-induced fluorescence 2% 2 min Faloona et al. [24] Mao et al. [29] Ozone production rate Measurement of ozone production sensor ~ 5% min Cazorla et al. [22] O 3,NO 2,SO 2, HCHO, HONO, NO 3 Long-path DOAS 3 5% Variable Stutz et al. [2b] OVOCs, HCHO, isoprene, aromatics PTR-MS 5% 6 s Jon et al. [25] *Only the measurements used in this work are listed here. west-southwest of the San Jacinto Battleground Monument, the western and eastern edges, respectively, of the petrochemical facilities in the Houston Ship Channel. The elevated location of the site is unique because other surface sampling sites are usually much more sensitive to the nearby (i.e., within m) local activities such as traffic, parking lots, delivery trucks, railways, and nocturnal surface drainage Measurements [8] The suite of measurements during SHARP 29 was extensive and included measurements of meteorological parameters, actinic fluxes, inorganic trace gases, VOCs, radicals, and oxygenated species (Table ). All measurements were recorded with synchronized timestamps and were matched with corresponding meteorological parameters. Some key measurements in this study include O 3 production rate measured by the Measurement of Ozone Production Sensor (MOPS) [Cazorla and Brune, 2] and OH and HO 2 radicals measured with laser-induced fluorescence (LIF) spectroscopy at low pressure, often called Fluorescence Assay in a Gas Expansion (FAGE) [Hard et al., 984; Faloona et al. 24]. As described by Mao et al. [22], two approaches were adopted for measuring OH with LIF during SHARP 29: the traditional wavelength ulation method (called OHwave ) and the chemical ulation method (called OHchem ). Because OHwave likely contains certain interferences related to oxidation products of biogenic VOCs [Mao et al., 22], OHchem was used as measured OH in this study. [9] Laboratory studies also found that the HO 2 measurements in some FAGE-type instruments are susceptible to interference from RO 2 species that come from alkenes and aromatics [Fuchs et al., 2; Mao et al., 22]. A laboratory study showed that our LIF instrument was also affected by the same interference. Compared to HO 2, the relative sensitivities for RO 2 are.2 for isoprene,.98 for ethene,.44 for limonene,.4 for cyclohexane,.4 for a-pinene, and.32 for b-pinene. With these measurements, the relative sensitivities for RO 2 derived from other alkenes and aromatics were extrapolated using the el to simulate the conversion of RO 2 to OH in our HO 2 detection cell. The measured HO 2 concentrations were corrected for this artifact by subtracting the product of each RO 2 concentration calculated in the el and its relative sensitivity from the measured HO 2 wave. The corrected measured HO 2 is used in the following analysis. This correction reduces the HO 2 measurements by 6% on average. [] The absolute uncertainty of the GTHOS measurement of OH and HO 2 determined from calibrations is 32% at the 2s confidence level [Faloona et al., 24]. In addition, the uncertainty of the chemical removal method used to measure OHchem is estimated to be about 2 % (2s confidence), so the combined absolute uncertainty for OHchem is about 38% (2s confidence). The correction of alkene-based RO 2 interference increases the HO 2 measurement uncertainty. With a typical midday radical mixing ratio of 2 pptv for measured HO 2 and.3 pptv for an interfering HO 2 level from alkene-based RO 2, the propagation uncertainty of real HO 2 is 34% (2s confidence) for midday conditions. At night, the 2s uncertainty of real HO 2 slightly increases to 36% with an averaged HO 2 mixing ratio of 5.6 pptv and an interfering HO 2 level of.46 pptv Model Description [] A box el was constructed using existing mechanisms to calculate radical formation rates and radical concentrations. Both highly explicit and condensed chemical mechanisms were used in the box el to examine the consistency of these mechanisms with each other and of the mechanisms with measurements. The box el was run with the FACSIMILE software for Windows (MCPA Software), which has been successfully used in the eling efforts for some previous research projects [e.g., Chen et al., 2; Mao et al., 2]. [2] Five photochemical mechanisms were used in this study: the Regional Atmospheric Chemical Mechanism Version 2 (RACM2) [Goliff et al., 23], the Carbon Bond Mechanism Version 25 (CB5) [Yarwood et al., 25], the Statewide Air Pollution Research Center mechanism Version 27 (SAPRC-7) [Carter, 27], the NASA Langley Research Center mechanism (LaRC) [Crawford et al., 999; Olson et al., 24], and the Master Chemical Mechanism (MCM v3.) [Jenkin et al., 23; Saunders et al., 23; Bloss et al., 25]. These mechanisms are well known and have been actively in use in research and regulatory applications. The original mechanisms were used, while kinetic data were updated based on the most recent chemical kinetic data evaluations [e.g., Sander et al., 2]. [3] In order to run the box el with different chemical mechanisms, measurements including measured long-lived 5772

4 .8 OH (pptv) HO 2 (pptv) 4 2 4/3 5/5 5/ 5/5 5/2 5/25 5/3 Day of year (CST) Figure 2. Time series of measured (red) and eled (blue) OH (top) and HO 2 (bottom) mixing ratios during SHARP. Data are averaged in h intervals. The eled OH and HO 2 were the averaged simulations from the five mechanisms. inorganic and organic compounds and meteorological parameters (temperature, pressure, humidity, and photolysis frequencies) were averaged into min values that became the el input. Nitric oxide (NO) was measured during SHARP 29 and was treated as a long-lived inorganic species to constrain the el. Species like HONO and HCHO were measured both locally by individual instruments on Moody Tower and by Long-Path DOAS (LP-DOAS) along the path between the Moody Tower and Downtown Houston. Because most el input parameters were measured on the Moody Tower, the measurements on the Moody Tower were used in the el. The HONO measurements by the liquid coil scrubbing/uv-vis instrument were mainly used in the el because of its better time resolution with some measurement gaps filled by the MC/IC HONO measurements. For each data point, the el was run for 24 h, long enough to allow most calculated reactive intermediates to reach steady state but short enough to prevent the buildup of secondary products. A deposition lifetime of two days was assumed for all calculated species to avoid unexpected accumulation of these species in the el. Model sensitivity runs show that by increasing or decreasing this deposition lifetime by a factor of, i.e., ~5 h and 2 days, the corresponding changes in the eled OH and HO 2 concentrations are less than 3%. At the end of 24 h, the el generated time series of OH, HO 2,RO 2, and other reactive intermediates. [4] It is worth noting that the zero-dimensional (box) el simulations did not include advection and emissions, although advection and emissions are certainly important factors for the air pollution formation. The primary goal of this study is to understand the radical behavior. Most radicals of interest have very short lifetimes of seconds or less, and all of the long-lived radical precursors and O 3 precursors were measured and used to constrain the box el calculations. Thus, advection and emissions can be neglected for this study of radicals and their production and loss rates. [5] Uncertainties in the el calculations were estimated to be 52% for OH and 6% for HO 2 both at the 2s confidence level based on Monte Carlo method by applying uncertainties of kinetic rate coefficients [e.g., Sander et al., 2] and of measurements used to constrain the els [Chen et al., 2]. 3. Results 3.. Comparison of Modeled and Measured OH and HO 2 [6] The measured and eled OH and HO 2 exhibit similar diurnal and day-to-day variations, with maxima in the midday and minima at night (Figures 2 and 3). Both the measured and eled OH peaks occurred at around local solar noon, while the measured and eled HO 2 peaks appeared in the early afternoon (Figure 3). [7] In general, the el reproduced the measured OH and HO 2. All five mechanisms produced similar levels of OH and HO 2, although CB5 produced slightly more HO x than others. The differences among the five mechanisms are mainly due to different treatments of VOCs. Midday OH was overpredicted, while nighttime OH and HO 2 were underpredicted. Comparing measured OH and HO 2 to the averaged el values with the five mechanisms, the median daytime measured-to-eled OH ratio is.9 and the median daytime measured-to-eled HO 2 ratio is.22. The el underpredicted nighttime HO x with a median measured-to-eled OH ratio of 6.34 and a median measured-to-eled HO 2 ratio of.73, indicating that either HO x sources or sinks are incomplete or incorrect in the el mechanisms. [8] Using the composite diurnal values in h bins, independent-sample t-tests (Student s t-tests) were conducted to see if there are significant differences between the measurements and el calculations. A t-test result with a p-value (significance) greater than.5 is considered to be not significantly different between the two samples. 5773

5 OH (pptv) HO 2 (pptv) REN ET AL.: ATMOSPHERIC PHOTOCHEMISTRY IN HOUSTON RACM2 CB5 LaRC SAPRC7 MCM : 6: 2: 8: : Interfering [OH] (pptv) Interfering [HO 2 ] (pptv) : 6: 2: 8: : Figure 3. Median diurnal variations of measured and eled OH (upper-left panel) and HO 2 (bottomleft panel) mixing ratios and interfering OH (top-right panel) and HO 2 (bottom-right) during SHARP. Modeled OH and HO 2 were calculated from five different chemical mechanisms, including RACM2, CB5, LaRC, SAPRC-7, and MCM. Observed OH and HO 2 data are limit to the periods when the eled data are available. Error bars on the left panels represent the absolute uncertainties of the OH and HO 2 measurements. Error bars on the right panels are the standard deviations of the interfering OH and HO 2 in hourly bins. The t-test results for the measured and eled OH show that the p-values are.8 for RACM2,.49 for CB5,.98 for LaRC,.9 for SAPRC-7, and.75 for MCM. The results for the measured and eled HO 2 show the p-values of. for RACM2,.83 for CB5,.2 for LaRC,.52 for SAPRC-7, and.9 for MCM. All these t-tests were conducted at a 95% confidence level and suggest no significant statistical difference between the measurements and el calculations. On the other hand, the nighttime differences for OH are significant Nighttime HO x [9] Studies have found that there are two oxidation pathways that can produce HO x at night: O 3 reactions with alkenes and the nitrate radical (NO 3 ) chemistry [Finlayson- Pitts and Pitts, 2; Monks, 25]. The ozone-alkene chemistry involves the ozone addition to the double carbon bond to form a primary ozonide, which then rapidly decomposes to a vibrationally excited carbonyl oxide (Criegee intermediate) and carbonyl products. The produced Criegee intermediate then further decomposes to produce OH and RO 2 [Monks, 25]. The nitrate radical can react with a few VOCs such as HCHO, unsaturated aldehydes, methacrolein, and glyoxal to produce HO x and RO 2. These processes become important for the nighttime HO x production due to lack of photolytic HO x sources at night. [2] Mean measured nighttime OH was.4 pptv or. 6 molecules cm 3, while the eled nighttime OH concentration (the averaged value of the five mechanisms) was only.7 pptv or.7 5 molecules cm 3 (Figure 3). The estimated OH detection limit was about. pptv, and the measurement uncertainty was about 4% at the 2s confidence level. In our previous studies, OHwave was used, which is now known to have a possible interference in the presence of O 3 and alkenes. In this study, we use OHchem, which appears to have no interference [Mao et al., 22]. On average, OHchem is on average.7 of OHwave during the day and.5 at night during SHARP 29. Further laboratory studies show that the interfering internal OH is made primarily near and in the OH detection cell [Mao et al., 22]. [2] The mean measured nighttime HO 2 was 6.7 pptv, while the mean eled nighttime HO 2 concentration (the averaged value of the five mechanisms) was 3.3 pptv (Figure 3). The el underpredicted nighttime OH significantly (Figure 3), indicating that the importance of OH in the nighttime oxidation chemistry may be underestimated. The median measured-to-eled HO 2 ratio at night was.73, which is marginally greater than the combined uncertainty of measured and eled HO 2 (7%, 2s). The median measured-to-eled OH ratio at night was 6.3, which is significantly beyond the combined uncertainty of measured and eled OH (64%, 2s). These differences indicate that all mechanisms fail to capture the processes that create nighttime OH and HO 2 in this urban environment. [22] Possible reasons for the discrepancy between the erved and eled night HO x include the missing mechanisms that can produce significant nighttime HO x.for example, a recent chamber study found significant OH production from the NO 3 -initiated oxidation of isoprene through RO 2 +HO 2 reactions and oxidation of nitrooxyhydroperoxide [Kwan et al., 22]. A few recent studies also suggested that the photooxidation of isoprene can regenerate OH either through isomerization of isoprene peroxy radicals [Peeters 5774

6 / OH / HO 2 HO 2 /OH ratio... 2 [NO] (ppbv) et al., 29; Peeters and Müller, 2] or the formation of epoxides [Paulot et al., 29; Crounse et al., 2]. Although this later mechanism is OH initiated and mainly proposed for daytime, it can also contribute to nighttime OH production if the oxidation of isoprene through its reaction with OH is significant at night. Apparently further investigation is needed in order to examine possible incomplete or incorrect understanding of atmospheric chemistry in the el that is responsible for the discrepancies Daytime NO Dependence [23] The measured-to-eled OH and HO 2 ratios and their NO dependence can test our understanding of HO x photochemistry. In polluted environments, the cycling between OH and HO 2 is very fast because of existing high levels of NO which reacts with HO 2 to produce OH and NO 2 and thus determine the photochemical equilibrium between OH and HO 2 (Figure ). In order to avoid the confusion of two different effects the poorly known and dominant O 3 + alkene HO x source at night and the NO effect on HO x chemistry during the day for the NO dependence analysis, we limit the data to daytime when O 3 photolysis frequency, J(O D), was greater than. 5 s (corresponding to a period approximately from 8: to ~6:, Central Standard Time) so that the photochemistry is dominant. [24] The el predicted OH generally well when NO is less than ~3 ppbv and slightly underpredicted OH when NO is greater than ~3 ppbv (Figure 4, top). This reasonably good agreement between erved and eled OH at low NO levels is consistent with a few previous studies [Ehhalt, 999; Kanaya et al., 27] in polluted environments but different from some recent studies in VOC-rich and low Figure 4. The ratios of measured-to-eled OH (top), HO 2 (middle), and HO 2 /OH (bottom) as a function of NO mixing ratio. Dots are all min average data with O 3 photolysis frequency, J(O D), greater than. 5 s. Linked symbols show the median values in the log(no) bins. NO x environments [e.g., Ren et al., 28; Lelieveld et al., 28; Hofzumahaus et al., 29; Whalley et al., 2; Lu et al., 22, 23], where biogenic emissions are dominant. This difference is most likely due to the unique chemical conditions in Houston, where VOCs are mainly from anthropogenic emissions. For HO 2, the measured-to-eled ratio is close to and fairly constant when NO is below ppbv, while the ratio then increases as NO increases (Figure 4, middle). This higher-than-expected HO 2 at high NO levels in this study is consistent with results from some previous studies in urban and suburban environments [e.g., Konrad et al., 23; Martinez et al., 23; Ren et al., 23a, 23b; Ren et al., 25; Kanaya et al., 27; Ren et al., 28; Dusanter et al., 29; Lu et al., 22, 23]. [25] Both the measured and eled HO 2 /OH ratios decrease with increasing NO level (Figure 4, bottom) because of the NO reaction with HO 2 to shift HO x into OH by reacting with HO 2. The agreement between measured and eled HO 2 -to-oh ratios is good when NO mixing ratios are less than ppbv, while the difference between measured and eled HO 2 /OH increases as NO further increases. The slope of the measured HO 2 /OH as a function of NO is slightly less than the eled slope. This difference is consistent with the measured HO 2 being greater than the eled HO 2 at high NO. The NO dependence of the measured and eled HO 2 /OH ratios is also consistent with results from several previous studies in urban environments [e.g., Ren et al., 23a; Ren et al., 25; Chen et al., 2; Kanaya et al., 22]. 4. Discussion 4.. HO x Budget [26] A number of urban studies have found significant daytime HONO and OVOCs that can be photolyzed to produce OH and HO 2 radicals [e.g., Ren et al., 23a; Olaguer et al., 29; Mao et al., 2; Liu et al., 22]. Other major processes of primary HO x production includes O 3 photolysis, the reaction of O( D) with H 2 O, and O 3 reactions with alkenes. Major HO x loss processes includes the OH reaction with NO 2 and the reactions among OH, HO 2, and RO 2. [27] During SHARP, the calculated HO x production was dominated by HONO photolysis in the early morning and by the photolysis of O 3 and OVOC in the midday (Figure 5). At night, HO x production was mainly from O 3 reactions with alkenes. On average, the daily HO x production rate was 24.6 ppbv d, of which 3% was from O 3 photolysis, 22% from HONO photolysis, 5% from the photolysis of OVOCs (other than HCHO), 4% from HCHO photolysis, and 3% from O 3 reactions with alkenes. For HO x loss, the clearly dominant process was the OH reaction with NO 2, while the self-reactions among OH, HO 2, and RO 2 became important in the afternoon when these radicals reached their highest values. [28] The importance of HONO and OVOC photolysis to HO x production is consistent with some recent studies in urban and suburban environments [Alicke et al., 23; Dusanter et al., 29; Volkamer et al., 2; Liu et al., 22]. For instance, Dusanter et al. [29] found that HONO photolysis contributed 35% of daytime HO x production in Mexico City during MCMA 26, while Alicke et al. [23] found that HONO photolysis contributed 5775

7 P(HO x ) (ppbv h - ) L(HO x ) (ppbv h - ) : 6: 2: 8: : Figure 5. Diurnal median variations of HO x production (top) and HO x loss (bottom) in the el calculation. HO x production processes include O 3 photolysis followed by the O( D) + H 2 O reaction, the photolysis of OVOCs (other than HCHO), HONO photolysis, HCHO photolysis (the radical producing pathway), and O 3 reactions with alkenes. HO x loss processes include OH reaction with NO x,ho 2 self-reaction, and HO 2 +RO 2 reactions. up to 2% of the total OH formed in a 24 h period during BERLIOZ. Liu et al. [22] found that the photolysis of OVOCs was the primary RO x (= OH + HO 2 +RO 2 ) source with comparable contribution from the HONO photolysis in Beijing during CAREBeijing-27, while Volkamer et al. [2] found that OVOCs contributed about half of the daytime radical production in Mexico City during MCMA 23. [29] As discussed previously, two different pathways can contribute to nighttime HO x production: O 3 reactions with alkenes and NO 3 reactions with VOCs. Typical diurnal variations of HO x production from these two pathways show that HO x production from O 3 + alkene reactions peaked in the midday when O 3 concentrations were the highest, while HO x production from NO 3 chemistry peaked at night because of low NO 3 concentrations during the day due to its fast photolysis (Figure 6). Measurements made by the long-path Differential Optical Absorption Spectrometer (LP-DOAS) during SHARP confirm that there were significant nighttime NO 3 levels away from the surface where low nighttime NO levels were erved. During SHARP, the average erved nighttime (from 6 P.M. to 6 A.M., Central Standard Time) NO 3 mixing ratio was 2.8. pptv and the average eled nighttime NO 3 mixing ratio was 2.6. pptv, indicating good agreement between the erved and eled NO 3 mixing ratios. The average nighttime ozone mixing ratio was 36 9 ppbv. Using the RACM2 mechanism, HO x production rates from O 3 + alkenes and NO 3 chemistry were calculated based on both erved and calculated VOCs that can react with O 3 and NO 3 to produce OH and HO 2. Overall O 3 + alkene reactions contributed about 84 pptv h or 68% to the nighttime HO x production, while NO 3 chemistry contributes about 39 pptv h or 32% (Figure 6) O 3 Production Rate and Its Sensitivity to NO x and VOCs [3] During the day, the photochemical O 3 production rate is essentially the production rate of NO 2 molecules from HO 2 + NO and RO 2 + NO reactions [Finlayson-Pitts and Pitts, 2]. The net instantaneous O 3 production rate, P(O 3 ), can be written approximately as the following equation: PO ð 3 Þ ¼ k HO2þNO½HO 2 Š½NOŠþ X k RO2iþNO½RO 2i Š½NOŠ k OHþNO2þM½OHŠ½NO 2 Š½MŠ PðRONO 2 Þ k HO2þO 3 ½HO 2 Š½O 3 Š k OHþO3 ½OHŠ½O 3 Š k O ð DÞþH O 2O D ½ H 2 OŠ LO ð 3 þ alkenesþ where k terms are the reaction rate coefficients. The negative terms in equation () correspond to the reaction of OH and NO 2 to form nitric acid, the formation of organic nitrates, P(RONO 2 ), the reactions of OH and HO 2 with O 3, the photolysis of O 3 followed by the reaction of O( D) with H 2 O, and O 3 reactions with alkenes. As shown in Figure 7, these negative terms are relatively small compared to the P (O 3 )fromho 2 + NO and RO 2 + NO reactions. Note that only photochemical production and loss terms are included and ozone deposition term is excluded in equation () in order to compare it with the measurement by MOPS, which does not account for ozone deposition. Because we mainly focus on photochemical O 3 production, the advection and dry deposition terms are not included in equation (), although they are important factors affecting ambient O 3 levels but not the O 3 production rate. The estimated uncertainty of P(O 3 ) measured by MOPS is 3% at the 2s confidence level and min integration time [Cazorla et al., 22]. The overall uncertainty of calculated P(O 3 )isabout66%(2s). P(HOx) (ppbv h - ) O 3 + alkenes NO 3 chem : 6: 2: 8: : Figure 6. Median diurnal variations of HO x production in the el calculation from O 3 + alkenes reactions and from NO 3 chemistry. Modeled NO 3 was used in the calculation due to low data coverage of the DOAS-measured NO 3. Shaded areas indicate the nighttime periods. () 5776

8 P(O 3 ) (ppbv hr - ), [NO] (ppbv) P(O 3 ) (ppbv hr - ) : 2: 8: 6: 2: 8: Figure 7. Left: median diurnal variations of total O 3 production rate in the el, P(O 3 ),O 3 production rate calculated from the measured HO 2,O 3 production rate calculated from the eled HO 2,, and O 3 production rate measured by the Measurement of Ozone Production Sensor (MOPS), P(O 3 ) MOPS, as well as NO mixing ratio. Right: median diurnal variation of the eled P(O 3 ) and the major contributions to P(O 3 ) from terms in equation (). [3] During SHARP, the eled P(O 3 ) peaked around noon with the average value of 8 ppbv h (Figure 7), although on a few individual days values as high as ppbv h were erved. Based on the el calculations, the cumulative P(O 3 ) was 26 ppbv d, in which about 68 ppbv d was attributed to the P(O 3 ) from the eled HO 2 reaction with NO, designated as. The cumulative P(O 3 ) from the measured HO 2 alone, designated as, was about 97 ppbv d. The difference between and mainly appeared in the morning, when NO levels were high, while in the afternoon, and agree pretty well (Figure 7). [32] The cumulative difference between and results in a difference of 29 ppbv O 3 per day. Similar results were erved in a few previous studies in urban environments [e.g., Martinez et al., 23; Ren et al., 23a]. Studies also found that in the troposphere, the erved HO 2 -to-ro 2 ratio is roughly constant under a certain environment and has been generally well reproduced by el calculations in various environments [e.g., Cantrell et al., 23; Mihelcic et al., 23; Ren et al., 23b]. So in general, HO 2 and RO 2 both contribute significantly to ozone production through their reactions with NO. If we assume that is 54% of total O 3 production, as derived from the el, then suggests that the actual total O 3 production would be 79 ppbv d, a factor of.4 higher than the cumulated P(O 3 ) in the el. This difference is roughly consistent with the measured-to-eled ratio (.3) of the cumulative O 3 production, where the O 3 production rate was measured directly by the Measurement of Ozone Production Sensor (MOPS), independent of the OH and HO 2 measurements [Cazorla et al., 22]. [33] Ozone production depends directly on NO concentration and P(HO x ) rate [Ren et al., 23a]. In the el, reaches the maximum when NO is around ppbv and then decreases as NO further increases (Figure 8). However, because measured HO 2 does not decrease as much as expected at higher NO levels (Figure 4), does not decrease as much as the el predicted (Figure 8). As a result, the to ratio increases as NO increases at high NO levels. This is roughly consistent with the NO dependence of the ratio of the MOPS measured P(O 3 ) to the eled P(O 3 ), although with less NO dependence (Figure 8). [34] The dependence of O 3 production on NO x and VOCs can be categorized into two typical scenarios: NO x sensitive and VOC sensitive. As in a previous study [Mao et al., 2], we use the method proposed by Kleinman [25] to evaluate the O 3 production sensitivity using the ratio of L N /Q, where L N is the radical loss via the reactions with NO x and Q is the total primary radical production. Because the radical production rate is approximately equal to the radical loss rate, this L N /Q ratio represents the fraction of radical loss due to NO x. It was found that when L N /Q is significantly less than.5, the atmosphere is in a NO x -sensitive regime, and when L N /Q is significantly greater than.5, the atmosphere is in a VOC-sensitive regime [Kleinman et al., 2; Kleinman, 25]. Note that the contribution of organic nitrates impacts the cut-off value for L N /Q to determine the ozone production sensitivity to NO x or VOCs, and this value may vary slightly around.5 in different environments. [35] During the springtime SHARP campaign, the O 3 production sensitivity to NO x or VOCs had a similar behavior as for two previous summertime studies in Houston, TexAQS 2 and TRAMP 26. P(O 3 ) was VOC sensitive in the early morning but became more NO x sensitive throughout the afternoon (Figure 9). These results are independent of the differences between the measured and eled OH and HO 2. Note that in the afternoon, the O 3 production sensitivity during SHARP experienced a longer NO x -sensitive period than TexAQS 2 and TRAMP 26, indicating that NO x control may be a more efficient approach than VOC control for the O 3 control in the 5777

9 P(O 3 ) HO2 (ppb/hr) P(O3 ) HO2 (ppb/hr) / P(O 3 ) HO [NO] (ppbv) Figure 8. Ozone production rate calculated from measured HO 2, (top), O 3 production rate calculated from eled HO 2, (middle), and the PO ð 3Þ HO 2 -to- ratio (bottom) as a function of NO. Blue dots are all min average data. Linked circles show the median values in the log(no) bins. Also shown in the bottom panel is the ratio of the MOPS measured P(O 3 ) to the el P(O 3 ) as a function of NO (linked squares). springtime for Houston. This is confirmed by the cumulated O 3 production from the MOPS measurements, in which 2 ppbv of O 3 was produced in the NO x sensitive regime while only 53 ppbv of O 3 was produced in the VOC sensitive regime. For the days with O 3 mixing ratios greater than 7 ppbv, the transit from VOC sensitive to NO x sensitive appeared in the late morning and early afternoon, about 3 h later than that for the days with O 3 mixing ratios less than 5 ppbv (Figure 9). 5. Summary [36] The measurements performed during SHARP in spring of 29 provided another excellent opportunity to test our understanding of photochemistry in this urban environment. A few highlights from this study are listed below. [37] First, the five photochemical mechanisms (RACM2, CB5, LaRC, SAPRC-7, and MCM) tested in this study exhibited similar diurnal variations of the eled HO x as the measurements, with maxima in the midday and minima at night. Comparing the measured HO x to the averaged eled HO x in the five mechanisms, the measured and eled OH agree quite well with an overall median measured-to-eled ratio of.3. For HO 2, the measurements were consistently higher than that predicted by the box el with an overall median measured-to-eled HO 2 ratio of.45. The el underpredicted both nighttime OH and HO 2, indicating incomplete HO x sources and/or sinks in the el. The NO dependence of measured-toeled OH and HO 2 ratios suggests that the el predicted OH well during the day but underpredicted HO 2 with NO levels greater than a few ppbv, indicating incorrect OH-HO 2 cycling at high NO in the el. [38] Second, the photolysis of HONO was a major HO x source in the early morning. During the midday, O 3 photolysis became a major HO x source, with significant contributions from the photolysis of HONO and OVOCs. Nighttime HO x production was mainly from O 3 reactions with alkenes. OH reaction with NO 2 was a dominant HO x loss process, while the self-reactions among OH, HO 2, and RO 2 became important HO x loss processes in the afternoon when these species reached their peak levels. [39] Third, because the eled HO 2 is less than the measured HO 2 especially at high NO levels, the cumulative is less than the cumulative PO ð 3Þ HO 2 by a factor of.4 on average. This is roughly consistent with the difference in the eled P(O 3 ) and the P(O 3 ) measured by the.8 L N /Q=.5 TexAQS2 TRAMP26 SHARP hr O 3 <5 ppbv 8-hr O 3 >7 ppbv L N /Q.6.4 VOC sensitive NO x sensitive L N /Q : 9: 2: 5: 8: 6: 9: 2: 5: 8: Figure 9. Left: median diurnal profiles of L N /Q in TEXAQS 2, TRAMP 26, and SHARP 29. The dashed line indicates a L N /Q value of.5, which separates the VOC-sensitive and NO x -sensitive regimes. Right: median diurnal profiles of L N /Q in SHARP 29 for high ozone days with 8 h ozone mixing ratios greater than 7 ppbv and low ozone days with 8 h ozone mixing ratios less than 5 ppbv. 5778

10 MOPS, which is completely independent of the OH and HO 2 measurements. The difference indicates possible incomplete chemistry in the chemical mechanisms and thus has implications for the ability of air quality els to accurately predict O 3 production rates. [4] Fourth, similar to the results during TexAQS 2 and TRAMP 26, two summertime studies in Houston, the springtime O 3 production rates during SHARP were VOC sensitive in the morning and NO x sensitive in the afternoon, and experienced a longer NO x -sensitive period than TexAQS 2 and TRAMP 26. The MOPS measurements suggest that during SHARP, the amount of O 3 produced in the NO x -sensitive regime was about twice of what was produced in the VOC-sensitive regime, indicating that NO x control may be an efficient approach for the O 3 control in springtime for Houston. 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