Flow patterns influencing the seasonal behavior of surface ozone and carbon monoxide at a coastal site near Hong Kong

Surface O₃ and CO were measured at Cape D’Aguilar, Hong Kong during the period of January 1994 to December1996 in order to understand the temporal variations of surface O₃ and CO in East Asia–West Pacific region. The isentropic backward trajectories were used to isolate different air masses reaching the site and to analyze the long-range transport and photochemical buildup of O₃ on a regional scale. The results show that the diurnal variation of surface O₃ was significant in all seasons with daily O₃ production being about 20 ppbv in fall and 10 ppbv in winter, indicating more active photochemical processes in the subtropical region. The distinct seasonal cycles of O₃ and CO were found with a summer minimum (16 ppbv)–fall maximum (41 ppbv) for O₃ and a summer minimum (116 ppbv)–winter maximum (489 ppbv) for CO. The isentropic backward trajectory cluster analyses suggest that the air masses (associated with regional characteristics) to the site can be categorized into five groups, which are governed by the movement of synoptic weather systems under the influence of the Asian monsoon. For marine-originated air masses (M-SW, M-SE and M-E, standing for marine-southwest, marine-southeast and marine-east, respectively) which always appear in summer and spring, the surface O₃ and CO have relatively lower mixing ratios (18, 16 and 30 ppbv for O₃, 127, 134 and 213 ppbv for CO), while the continental air masses (C-E and C-N, standing for continent-east and continent-north, respectively) usually arrive at the site in winter and fall seasons with higher O₃ (43 and 48 ppbv) and CO (286 and 329 ppbv). The 43 ppbv O₃ and 286 ppbv CO are representative of the regionally polluted continental outflow air mass due to the anthropogenic activity in East Asia, while 17 ppbv O₃ and 131 ppbv CO can be considered as the signature of the approximately clean marine background of South China Sea. The very high CO values (461–508 ppbv) during winter indicate that the long-range transport of air pollutants from China continent is important at the monitoring site. The fall maximum (35–46 ppbv) of surface O₃ was believed to be caused by the effects of the weak slowly moving high-pressure systems which underlie favorable photochemical production conditions and the long-range transport of aged air masses with higher O₃ and its precursors.     

An examination of oxidant amounts on secondary organic aerosol formation and aging

The effect of HO_x radicals (OH and HO₂) and ozone (O₃) on aerosol formation and aging has been studied. Experiments were performed in presence as well as in absence of oxygen in a flow-through chamber at 299 K for three organic precursor gases, isoprene, α-pinene and m-xylene. The HO_x source was the UV photolysis of humidified air or nitrogen and was measured with a GTHOS (Ground-based Tropospheric Hydrogen Oxides Sensor). The precursor gases concentration was monitored with an online GC-FID. The aerosol mass was then quantified by a Tapered Element Oscillating Microbalance (TEOM). Typical oxidant mixing ratios were (0–4.5) ppm for O₃, 200 pptv for OH and 3 ppbv for HO₂. A simple kinetics model is used to infer the aerosol production mechanism. In the present of O₃ (or O₂), the SOA yields were 0.46, 0.036 and 0.12 for α-pinene with an initial concentration of 100 ppbv (RH = 37%), isoprene with an initial concentration of 177 ppbv (RH = 50%) and m-xylene with an initial concentration of 100 ppbv (RH = 37%), respectively. When the chosen precursor gases reacted with HO_x in the absence of O₃, the maximum SOA yields were significantly increased by factors of 1.6 for isoprene 1.1 for α-pinene, and 3 for m-xylene respectively. The comparison of the calculated and measured potential aerosol mass concentrations as function of time shows that presence of ozone or oxygen can influence the aerosol yield and the absence of ozone or oxygen in the system resulted in high concentrations of its organic aerosol products.     

Determination of PAN,PPN, PnBN and selected pentyl nitrates in Athens,Greece

Quasi-continuous measurements of PAN, PPN, PnBN and the alkyl nitrates—2-methyl-2-butyl nitrate, 3-pentyl nitrate and 2-pentyl nitrate were carried out in Athens using a simple cryoconcentration technique. The maximum mixing ratios measured were 6.6, 1.0 and 0.07 ppbv for PAN, PPN and PnBN, respectively, for the peroxyacyl nitrates, and 0.3, 0.09 and 0.03 ppb for 2-methyl-2-butyl nitrate, 2-pentyl nitrate and 3-pentyl nitrate, respectively. Mean ratios of PPN/PAN mixing ratios were 0.102 and of PnBN/PAN 0.012. 2PN/3PN mean ratios were 1.8 near the theoretical value of 1.6. All maximum values of measured nitrogenous compounds were associated with maximum mixing ratios of ozone and NO_x and occurred when southwestern winds prevailed in association with a temperature inversion.     

Ozone response to precursor controls: comparison of data analysis methods with the predictions of photochemical air quality simulation models

Comparisons were made between the predictions of six photochemical air quality simulation models (PAQSMs) and three indicators of ozone response to emission reductions: the ratios of O₃/NO_z and O₃/NO_y and the extent of reaction. The values of the two indicator ratios and the extent of reaction were computed from the model-predicted mixing ratios of ozone and oxidized nitrogen species and were compared to the changes in peak 1 and 8 h ozone mixing ratios predicted by the PAQSMs. The ozone changes were determined from the ozone levels predicted for base-case emission levels and for reduced emissions of volatile organic compounds (VOCs) and oxides of nitrogen (NO_x). For all simulations, the model-predicted responses of peak 1 and 8 h ozone mixing ratios to VOC or NO_x emission reductions were correlated with the base-case extent of reaction and ratios of O₃/NO_z and O₃/NO_y. Peak ozone values increased following NO_x control in 95% (median over all simulations) of the high-ozone (>80 ppbv hourly mixing ratio in the base-case) grid cells having mean afternoon O₃/NO_z ratios less than 5 : 1, O₃/NO_y less than 4 : 1, or extent less than 0.6. Peak ozone levels decreased in response to NO_x reductions in 95% (median over all simulations) of the grid cells having peak hourly ozone mixing ratios greater than 80 ppbv and where mean afternoon O₃/NO_z exceeded 10 : 1, O₃/NO_y was greater than 8 : 1, or extent exceeded 0.8. Ozone responses varied in grid cells where O₃/NO_z was between 5 : 1 and 10 : 1, O₃/NO_y was between 4 : 1 and 8 : 1, or extent was between 0.6 and 0.8. The responses in such grid cells were affected by ozone responses in upwind grid cells and by the changes in ozone levels along the upwind boundaries of the modeling domains.     

Effects of Land Use Data on Dry Deposition in a Regional Photochemical Model for Eastern Texas

ABSTRACT

Land use data are among the inputs used to determine dry deposition velocities for photochemical grid models such as the Comprehensive Air Quality Model with extensions (CAMx) that is currently used for attainment demonstrations and air quality planning by the state of Texas. The sensitivity of dry deposition and O₃ mixing ratios to land use classification was investigated by comparing predictions based on default U.S. Geological Survey (USGS) land use data to predictions based on recently compiled land use data that were collected to improve biogenic emissions estimates. Dry deposition of O₃ decreased throughout much of eastern Texas, especially in urban areas, with the new land use data. Predicted 1-hr averaged O₃ mixing ratios with the new land use data were as much as 11 ppbv greater and 6 ppbv less than predictions based on USGS land use data during the late afternoon. In addition, the area with peak O₃ mixing ratios in excess of 100 ppbv increased significantly in urban areas when deposition velocities were calculated based on the new land use data. Finally, more detailed data on land use within urban areas resulted in peak changes in O₃ mixing ratios of ~2 ppbv. These results indicate the importance of establishing accurate, internally consistent land use data for photochemical modeling in urban areas in Texas. They also indicate the need for field validation of deposition rates in areas experiencing changing land use patterns, such as during urban reforestation programs or residential and commercial development.     

In situ measurements of NO,NO2, NOy,and O3 in Dinghushan (112°E, 23°N), China during autumn 2008

Measurements of O₃, NO, NO₂, and NO_y mixing ratios were carried out at a station-Dinghushan in Guangdong province of China from Oct. 18th, 2008 to Nov. 7th, 2008. This research shows that under conditions of a strong subtropical high (temperature high, relative humidity low), on Oct. 29th, 2008 the Dinghushan station observed severe photochemical pollution. The Maximum hour average concentration of O₃ reached 128 ppbv, and the serious photochemical pollution is caused by superposition of local photochemical reaction and regional transport. The observation that NO_x ozone production efficiency (OPE) values for high O₃ pollution on Oct. 29–30th, 2008 were 10.5 and 15, which were more than the values of the city source region and lower than that of the surrounding clean areas. It means the sensitivity of O₃ generated was transitioning from VOCs limited condition to NOx-limited regime. By applying a Smog Production Model, the results show that the extent of reaction values less than 0.6 were occurred on 17 days during campaign, and 13 days for the extents of reactions more than 0.6. However, there were no data with values over 0.8, which indicates that the observation station represent a VOCs sensitive system during campaign. Analysis of the extents of reactions and wind data show that the pollution is mostly subject to a southeasterly airflow influence.     

The evolution of photochemical smog in a power plant plume

The evolution of photochemical smog in a plant plume was investigated with the aid of an instrumented helicopter. Air samples were taken in the plume of the Cumberland Power Plant, located in central Tennessee, during the afternoon of 16 July 1995 as part of the Southern Oxidants Study – Nashville Middle Tennessee Ozone Study. Twelve cross-wind air sampling traverses were made at six distance groups from 35 to 116 km from the source. During the sampling period the winds were from the west–northwest and the plume drifted towards the city of Nashville TN. Ten of the traverses were made upwind of the city, where the power plant plume was isolated, and two traverses downwind of the city when the plumes were possibly mixed. The results revealed that even six hours after the release, excess ozone production was limited to the edges of the plume. Only when the plume was sufficiently dispersed, but still upwind of Nashville, was excess ozone (up to 109 ppbv, 50–60 ppbv above background levels) produced in the center of the plume. The concentrations image of the plume and a Lagrangian particle model suggests that portions of the power plant plume mixed with the urban plume. The mixed urban power plant plume began to regenerate O₃ that peaked at 120 ppbv at a short distance (15–25 km) downwind of Nashville. Ozone productivity (the ratio of excess O₃ to NO_y and NO_z) in the isolated plume was significantly lower compared with that found in the city plume. The production of nitrate, a chain termination product, was significantly higher in the power plant plume compared to the mixed plume, indicating shorter chain length of the photochemical smog chain reaction mechanism.     

A Lagrangian model investigation of chemico-microphysical evolution of northeast Asian pollution plumes within the MBL during TRACE-P

In this work, we determine the major channels through which air pollutants, mainly originating in Northeast Asian mega-cities, flow out into the Northwestern Pacific atmosphere. For this purpose, comprehensive backward/forward trajectory analyses are conducted. Two important channels along which pollutants from the Northeast Asian mega-cities flow out are defined, and are labeled as “DC8 transport path” and “P3B transport path”. We then comprehensively examine the chemico-microphysical transformations of the anthropogenic pollutants from the Northeast Asian mega-cities along the two major transport paths, using a new Lagrangian forward-trajectory photochemical model. In the newly developed model, state-of-the-science parameterizations for considering chemico-microphysical aging processes and atmospheric aerosol processes are incorporated. As air masses travel toward low latitudes through the marine boundary layer (MBL), the temperature increases along the trajectories and large amounts of PAN experience thermal decomposition. By this process, PAN can be an important supplier of NO₂ in the remote MBL. The O₃ productions in the remote Northwestern Pacific MBL are fueled and maintained by NO_x provided from the PAN decomposition. High O₃ levels (>50 ppb) are observed within the remote MBL of the Northwestern Pacific Oceans from several TRACE-P DC8 and P3B measurements under the continental outflow situations. Gas-phase SO₂ is continuously converted into nss-sulfate via heterogeneous oxidation reaction with H₂O₂ at a particle pH of 2–5. The Lagrangian-trajectory modeling studies also indicate that in the remote MBL of Northwestern Pacific Ocean under continental outflow situations, conditions are unfavorable for nucleation events, because of the depletion of SO₂, the large aerosol surface areas available for H₂SO₄ sink, and high temperatures.     

Ambient formaldehyde and its contributing factor to ozone and OH radical in a rural area

Formaldehyde (HCHO), as well as correlative pollutants was measured from 1 to 31 July in 2007 at Mazhuang, a rural site located in the east of China. Gaseous HCHO was scrubbed from the air with an acidic 2,4-dinitrophenylhydrazine (DNPH) solution, which leaded to the reaction of HCHO with DNPH and produced a stable product, 2,4-dinitrophenylhydrazone, followed by online analysis by high-performance liquid chromatography (HPLC) coupled with Ultraviolet detector. During the observation period, mixing ratios of HCHO ranged from 0.2 ppbv to 6.2 ppbv, with an average of 1.5 ± 0.67 ppbv. HCHO shows an evident diurnal variation, the maximum appeared during 12:00–14:00. The average concentration diurnal variations of measured HCHO, ozone (O₃), Methylhydroperoxides (MHP, CH₃OOH), hydrogen peroxide (H₂O₂), nitrogen oxides (NO_x) and meteorological parameters were compared. The similar variations of HCHO, O₃ and radiation imply that photo-oxidation of hydrocarbons might be the major source for HCHO. Based on the maximum incremental reactivity (MIR) coefficient of HCHO, the calculation shows that HCHO contributes about 20% to total observed O₃ during the study period. In order to compare the contributions of O₃, HCHO and HONO to OH radical, photolysis rate parameters (J-values) of the three compounds were calculated by the Tropospheric Ultraviolet and Visible (TUV) Radiation Model (4.4 version). Based on the comparison, this study reaches the conclusion that O₃ is the dominant source of OH radical at Mazhuang. This study also uses P(HCHO)/P(O₃) which represents the ratio of contrbutions of HCHO and O₃ to OH radical, to discuss the action of HCHO in OH radical soucers. The result shows that P(HCHO)/P(O₃) is 12.5% on average, with the maximum of 21.0% at 13:00_P.M. and minimum of 7.5% before 9:00_A.M. and after 17:00_P.M..Therefore HCHO is also an important source of OH radical and cannot be ignored.     

Variability of tropospheric hydroperoxides at a coastal surface site in Antarctica

The annual cycles of hydrogen peroxide (H₂O₂) and methylhydroperoxide (MHP) have been investigated at a remote site in Antarctica in order to study seasonal variations as well as chemical processes in the troposphere. The measurements have been performed from March 1997 to January 1998 and in February 1999 at the German Antarctic research station Neumayer which is located at 70°39′S, 8°15′W. The obtained time series for hydrogen peroxide and methylhydroperoxide in near-surface air represents the first all-year measurements in Antarctica and indicates clearly the occurrence of seasonal variations. During polar night mean values of 0.054±0.046 ppbv (range<0.03–0.11 ppbv) for hydrogen peroxide and 0.089±0.052 ppbv (range<0.05–0.14 ppbv) for methylhydroperoxide were detected. At the sunlit period higher Mixing ratios were found, 0.20±0.13 ppbv (range<0.03–0.91 ppbv) for hydrogen peroxide and 0.19±0.10 ppbv (range<0.05–0.89 ppbv) for methylhydroperoxide. Occasional long-range transport of air masses from mid-latitudes caused enhanced peroxide concentrations at polar night. During the period of stratospheric ozone depletion we observed peroxide mixing ratios comparable to typical winter levels.     

Shouldn’t snowpacks be sources of monocarboxylic acids?

We report the first measurements of the mixing ratios of acetic (CH₃COOH) and formic (HCOOH) acids in the air filling the pore spaces of the snowpacks (firn air) at Summit, Greenland and South Pole. Both monocarboxylic acids were present at levels well above 1 ppbv throughout the upper 35 cm of the snowpack at Summit. Maximum mixing ratios in Summit firn air reached nearly 8 ppbv CH₃COOH and 6 ppbv HCOOH. At South Pole the mixing ratios of these acids in the top 35 cm of firn air were also generally >1 ppbv, though their maximums barely exceeded 2.5 ppbv of CH₃COOH and 2.0 ppbv of HCOOH. Mixing ratios of the monocarboxylic acids in firn air did not consistently respond to diel and experimental (fast) variations in light intensity, unlike the case for N oxides in the same experiments. Air-to-snow fluxes of CH₃COOH and HCOOH apparently support high mixing ratios (means of (CH₃COOH/HCOOH) 445/460 and 310/159 pptv at Summit and South Pole, respectively) in air just above the snow during the summer sampling seasons at these sites. We hypothesize that oxidation of carbonyls and alkenes (that are produced by photo- and OH-oxidation of ubiquitous organic compounds) within the snowpack is the source of the monocarboxylic acids.     

Aircraft measurements of O3, NOx,CO, VOCs,and SO2 in the Yangtze River Delta region

In this study, air pollutants, including ozone (O₃), nitrogen oxides (NO_x = NO + NO₂), carbon monoxides (CO), sulfur dioxide (SO₂), and volatile organic compounds (VOCs) measured in the Yangtze River Delta (YRD) region during several air flights between September/30 and October/11 are analyzed. This measurement provides horizontal and vertical distributions of air pollutants in the YRD region. The analysis of the result shows that the measured O₃ concentrations range from 20 to 60 ppbv. These values are generally below the US national standard (84 ppbv), suggesting that at the present, the O₃ pollutions are modest in this region. The NO_x concentrations have strong spatial and temporal variations, ranging from 3 to 40 ppbv. The SO₂ concentrations also have large spatial and temporal variations, ranging from 1 to 35 ppbv. The high concentrations of CO are measured with small variations, ranging from 3 to 7 ppmv. The concentrations of VOCs are relatively low, with the total VOC concentrations of less than 6 ppbv. The relative small VOC concentrations and the relative large NO_x concentrations suggest that the O₃ chemical formation is under a strong VOC-limited regime in the YRD region. The measured O₃ and NO_x concentrations are strongly anti-correlated, indicating that enhancement in NO_x concentrations leads to decrease in O₃ concentrations. Moreover, the O₃ concentrations are more sensitive to NO_x concentrations in the rural region than in the city region. The ratios of Δ[O₃]/Δ[NO_x] are ?2.3 and ?0.25 in the rural and in the city region, respectively. In addition, the measured NO_x and SO₂ concentrations are strongly correlated, highlighting that the NO_x and SO₂ are probably originated from same emission sources. Because SO₂ emissions are significantly originated from coal burnings, the strong correlation between SO₂ and NO_x concentrations suggests that the NO_x emission sources are mostly from coal burned sources. As a result, the future automobile increases could lead to rapid enhancements in O₃ concentrations in the YRD region.     

Mixing ratios of volatile organic compounds (VOCs) in the atmosphere of Karachi,Pakistan

Mixing ratios of carbon monoxide (CO), methane (CH₄), non-methane hydrocarbons, halocarbons and alkyl nitrates (a total of 72 species) were determined for 78 whole air samples collected during the winter of 1998–1999 in Karachi, Pakistan. This is the first time that volatile organic compound (VOC) levels in Karachi have been extensively characterized. The overall air quality of the urban environment was determined using air samples collected at six locations throughout Karachi. Methane (6.3 ppmv) and ethane (93 ppbv) levels in Karachi were found to be much higher than in other cities that have been studied. The very high CH₄ levels highlight the importance of natural gas leakage in Karachi. The leakage of liquefied petroleum gas contributes to elevated propane and butane levels in Karachi, although the propane and butane burdens were lower than in other cities (e.g., Mexico City, Santiago). High levels of benzene (0.3–19 ppbv) also appear to be of concern in the Karachi urban area. Vehicular emissions were characterized using air samples collected along the busiest thoroughfare of the city (M.A. Jinnah Road). Emissions from vehicular exhaust were found to be the main source of many of the hydrocarbons reported here. Significant levels of isoprene (1.2 ppbv) were detected at the roadside, and vehicular exhaust is estimated to account for about 20% of the isoprene observed in Karachi. 1,2-Dichloroethane, a lead scavenger added to leaded fuel, was also emitted by cars. The photochemical production of ozone (O₃) was calculated for CO and the various VOCs using the Maximum Incremental Reactivity (MIR) scale. Based on the MIR scale, the leading contributors to O₃ production in Karachi are ethene, CO, propene, m-xylene and toluene.     

An analysis on abnormally low ozone in the upper troposphere over subtropical East Asia in spring 2004

Abnormally low ozone (O₃) mixing ratios were observed by electrochemical concentration cell (ECC) ozonesondes in the upper troposphere over subtropical East Asia in spring 2004, a season when high tropospheric O₃ is usually observed in the region. Low O₃ with a lowest mixing ratio of 13 ppbv, less than a fourth of the respective seasonal average of 60–100 ppbv, was observed at 11–18 km above ground over Hong Kong (22.31°N, 114.17°E), Sanya (18.23°N, 109.52°E) and Taipei (24.98°N, 121.43°E). The origin of the low O₃ was investigated using meteorological evidence, satellite imagery and three-dimensional backward air trajectory. We found for the first time that the low O₃ resulted from deep convective pumping of low O₃ maritime air masses near the center of typhoon Sudal from the boundary layer of the tropical region to the east of the Philippines to the upper troposphere. The low O₃ air masses were then transported to the higher latitudes far ahead of the typhoon following the long-range transport driven by the circulations associated with the typhoon and the northern Hadley cell. The findings of this study highlight that more research efforts are needed to understand the effect of the circulation associated with tropical cyclones on the distribution and budget of O₃ and other trace gases in the troposphere.     

Peroxyacetyl nitrate (PAN) concentrations in the Antarctic troposphere measured during the photochemical experiment at Neumayer (PEAN'99)

Because investigations of PAN at higher southern latitudes are very scarce, we measured surface PAN concentrations for the first time in Antarctica. During the Photochemical Experiment at Neumayer (PEAN'99) campaign mean surface PAN mixing ratios of 13±7 pptv and maximum values of 48 pptv were found. When these PAN mixing ratios were compared to the sum of NO_x and inorganic nitrate they were found to be equal or higher. Low ambient air temperatures and low PAN concentrations caused a slow homogeneous PAN decomposition rate of approximately 5×10⁻² pptv h⁻¹. These slow decay rates were not sufficient to firmly establish the simultaneously observed NO_x concentrations. In addition, low concentration ratios of [HNO₃]/[NO_x] imply that the photochemical production of NO_x within the snow pack can influence surface NO_x mixing ratios in Antarctica. Alternate measurements of PAN mixing ratios at two different heights above the snow surface were performed to derive fluxes between the lower troposphere and the underlying snow pack using calculated friction velocities. Most of the concentration differences were below the precision of the measurements. Therefore, only an upper limit for the PAN flux of ±1×10¹³ molecules m⁻² s⁻¹ without a predominant direction can be estimated. However, PAN fluxes below this limit can still influence both the transfer of nitrogen compounds between atmosphere and ice, and the PAN budget in higher southern latitudes.     

Variations and sources of ambient formaldehyde for the 2008 Beijing Olympic games

As the host city of the 2008 Olympic games, Beijing implemented a series of air pollution control measures before and during the Olympic games. Ambient formaldehyde (HCHO) concentrations were measured using a fluorometric instrument based on a diffusion scrubber and the Hantzsch reaction; hydrocarbons were simultaneously measured using gas chromatography–mass spectrometry (GC–MS). Meteorological parameters, CO, O₃, and NO₂ concentrations were measured by standard commercial instrumentation. In four separate periods: (a) before the vehicle plate number control (3–19 July); (b) during the Olympic Games (8–24 August); (c) during the Paralympic Games (6–17 September) and (d) after the vehicle control was ceased (21–28 September), the average HCHO mixing ratios were 7.31 ± 2.67 ppbv, 5.54 ± 2.41 ppbv, 8.72 ± 2.48 ppbv, and 6.42 ± 2.79 ppbv, while the total non-methane hydrocarbons (NMHCs) measured were 30.41 ± 18.08 ppbv, 18.12 ± 9.38 ppbv, 30.50 ± 13.37 ppbv, and 33.33 ± 15.85 ppbv, respectively. Both HCHO and NMHC levels were the lowest during the Olympic games, and increased again during the Paralympic games even with the same vehicle control measures operative. Similar diurnal HCHO and O₃ patterns indicated that photo-oxidation of NMHCs may be the major source of HCHO. The diurnal profile of total NMHCs was very similar to that of NO₂ and CO: morning and evening peaks appeared in rush hours, indicating even after strict vehicle control, automobile emission may still be the dominant source of the HCHO precursors. The contributions of HCHO, alkanes, alkenes, and aromatics to OH loss rates were also calculated. HCHO contributed 22 ± 3% to the total VOCs and 24 ± 1% to the total OH loss rate. HCHO was not only important in term of abundance, but also important in chemical reactivity in the air.     

Analysis of ozone and VOCs measured in Shanghai: A case study

Shanghai Meteorological Administration has established a volatile organic compounds (VOCs) laboratory and an observational network for VOCs and ozone (O₃) measurements in the city of Shanghai. In this study, the measured VOCs and O₃ concentrations from 15 November (15-Nov) to 26 November (26-Nov) of 2005 in Shanghai show that there are strong day-to-day and diurnal variations. The measured O₃ and VOCs concentrations have very different characterizations between the two periods. During 15-Nov to 21-Nov (defined as the first period), VOCs and O₃ concentrations are lower than the values during 22-Nov to 28-Nov (defined as the second period). There is a strong diurnal variation of O₃ during the second period with maximum concentrations of 40–80 ppbv at noontime, and minimum concentrations at nighttime. By contrast, during the first period, the diurnal variation of O₃ is in an irregular pattern with maximum concentrations of only 20–30 ppbv. The VOC concentrations change rapidly from 30–50 ppbv during the first period to 80–100 ppbv during the second period. Two chemical models are applied to interpret the measurements. One model is a regional chemical/dynamical model (WRF-Chem) and another is a detailed chemical mechanism model (NCAR MM). Model analysis shows that the meteorological conditions are very different between the two periods, and are mainly responsible for the different chemical characterizations of O₃ and VOCs between the two periods. During the first period, meteorological conditions are characterized by cloudy sky and high-surface winds in Shanghai, resulting in a higher nighttime planetary boundary layer (PBL) and faster transport of air pollutants. By contrast, during the second period, the meteorological conditions are characterized by clear sky and weak surface winds, resulting in a lower nighttime PBL and slower transport of air pollutants. The chemical mechanism model calculation shows that different VOC species has very different contributions to the high-ozone concentrations during the second period. Alkane (40 ppbv) and aromatic (30 ppbv) are among the highest VOC concentrations observed in Shanghai. The analysis suggests that the aromatic is a main contributor for the O₃ chemical production in Shanghai, with approximately 79% of the O₃ being produced by aromatic. This analysis implies that future increase in VOC (especially aromatic) emissions could lead to significant increase in O₃ concentrations in Shanghai.     

An evaluation of the interaction of morning residual layer and afternoon mixed layer ozone in Houston using ozonesonde data

The Tropospheric Ozone Pollution Project (TOPP) launched >220 ozonesondes in Houston (July 2004–June 2008) providing examples of pollution transported into, re-circulated within, and exported from the Houston area. Fifty-one launches occurred during the Texas Air Quality Study (TexAQS) II and the summer portion of IONS-06 (INTEX [Intercontinental Transport Experiment] Ozonesonde Network Study). On 11 days during TexAQS II and on 8 other occasions, ozonesondes were launched both at dawn and in the afternoon. Analysis of these “intensive” launch sequences shows that morning residual layer (RL) ozone concentrations ([O₃]) explained 60–70% of the variability found in the afternoon mixed layer (ML). Furthermore, maximum RL [O₃] is nearly identical to the mean ML [O₃] from the previous afternoon (morning minus afternoon = ?1.6 ± 8.4 ppbv). During TexAQS II, mean [O₃] below 1.3 km (the mean ML height from ozonesonde data) increased from 37 ± 22 ppbv in the morning to 74 ± 18 ppbv in the afternoon, suggesting an average net local daily O₃ production of ～500–900 tons over the metropolitan Houston area.     

The influence of south foehn on the ozone mixing ratios at the high alpine site Arosa

Within 2 years of trace gas measurements performed at Arosa (Switzerland, 2030 m above sea level), enhanced ozone mixing ratios were observed during south foehn events during summer and spring (5–10 ppb above the median value). The enhancements can be traced back to ozone produced in the strongly industrialized Po basin as confirmed by various analyses. Backward trajectories clearly show advection from this region during foehn. NO_y versus O₃ correlation and comparison of O₃ mixing ratios between Arosa and Mt. Cimone (Italy, 2165 m asl) suggest that ozone is the result of recent photochemical production (+5.6 ppb on average), either directly formed during the transport or via mixing of air processed in the Po basin boundary layer. The absence of a correlation between air parcel residence times over Europe and ozone mixing ratios at Arosa during foehn events is in contrast to a previous analysis, which suggested such correlation without reference to the origin of the air. In the case of south foehn, the continental scale influence of pollutants emission on ozone at Arosa appears to be far less important than the direct influence of the Po basin emissions. In contrast, winter time displays a different situation, with mean ozone reductions of about 4 ppb for air parcels passing the Po basin, probably caused by mixing with ozone-poor air from the Po basin boundary layer.     

Examination of the impact of photoexcited NO2 chemistry on regional air quality

Impact of the excited nitrogen dioxide (NO₂¹) chemistry on air quality in the U.S. is examined using the Community Multiscale Air Quality (CMAQ) model for a summer month. Model simulations were conducted with and without the NO₂¹ chemistry. The largest impact of the NO₂¹ chemistry in the eastern U.S. occurred in the northeast and in the western U.S. occurred in Los Angeles. While the single largest daily maximum 8-h ozone (O₃) increased by 9 ppbv in eastern U.S. and 6 ppbv in western U.S., increases on most days were much lower. No appreciable change in model performance statistics for surface-level O₃ predictions relative to measurements is noted between simulations with and without the NO₂¹ chemistry. Based on model calculations using current estimates of tropospheric emission burden, the NO₂¹ chemistry can increase the monthly mean daytime hydroxyl radicals (OH) and nitrous acid (HONO) by a maximum of 28% and 100 pptv, respectively.