Sink For O3 Research Articles

How sensitive is tropospheric oxidation to anthropogenic emissions?

We use a global chemistry transport model to explore how changes in anthropogenic emissions alter ozone production and tropospheric oxidizing capacity over decadal (1990–2010) and centennial timescales (1900–2100). We find that the spatial extent of O3 production and loss in the troposphere changes very little despite large projected increases in precursor emissions. While tropospheric OH shows a long‐term decrease of only 20% between 1900 and 2100, there are widespread changes in distribution which alter regional oxidation capacity substantially. We show that the remote marine boundary layer remains an important net sink of O3, as greater production related to increased continental NOx emissions is outweighed by greater O3 destruction. The critical NOx level required to support net O3 production doubles between 1900 and 2100, from 28 to 55 pptv on average, preventing any large‐scale shift in production regime.

Seasonal variation of ozone deposition to a tropical rain forest in southwest Amazonia

Abstract. Within the project EUropean Studies on Trace gases and Atmospheric CHemistry as a contribution to Large-scale Biosphere-atmosphere experiment in Amazonia (LBA-EUSTACH), we performed tower-based eddy covariance measurements of O3 flux above an Amazonian primary rain forest at the end of the wet and dry season. Ozone deposition revealed distinct seasonal differences in the magnitude and diel variation. In the wet season, the rain forest was an effective O3 sink with a mean daytime (midday) maximum deposition velocity of 2.3 cm s−1, and a corresponding O3 flux of −11 nmol m−2 s−1. At the end of the dry season, the ozone mixing ratio was about four times higher (up to maximum values of 80 ppb) than in the wet season, as a consequence of strong regional biomass burning activity. However, the typical maximum daytime deposition flux was very similar to the wet season. This results from a strong limitation of daytime O3 deposition due to reduced plant stomatal aperture as a response to large values of the specific humidity deficit. As a result, the average midday deposition velocity in the dry burning season was only 0.5 cm s−1. The large diel ozone variation caused large canopy storage effects that masked the true diel variation of ozone deposition mechanisms in the measured eddy covariance flux, and for which corrections had to be made. In general, stomatal aperture was sufficient to explain the largest part of daytime ozone deposition. However, during nighttime, chemical reaction with nitrogen monoxide (NO) was found to contribute substantially to the O3 sink in the rain forest canopy. Further contributions were from non-stomatal plant uptake and other processes that could not be clearly identified. Measurements, made simultaneously on a 22 years old cattle pasture enabled the spatially and temporally direct comparison of O3 dry deposition values from this site with typical vegetation cover of deforested land in southwest Amazonia to the results from the primary rain forest. The mean ozone deposition to the pasture was found to be systematically lower than that to the forest by 30% in the wet and 18% in the dry season.

Measurements of ozone concentrations on a commercial vessel in the marine boundary layer over the northern North Pacific Ocean

The O3 concentration in the marine boundary layer over the northern North Pacific Ocean was measured on a commercial vessel. No significant longitudinal gradient was seen over the area between 170°E and 135°W. A spring O3 maximum and summer minimum were observed. A diurnal cycle of O3 was found, with a high nighttime concentration and a low daytime concentration. Throughout the year an O3 decrease was clearly observed just after sunrise, which is known as sunrise ozone destruction (SOD). SOD may act as an important O3 sink in the marine boundary layer. Dramatic O3 destruction (2.5–3 ppb hour−1) just after sunrise was found in spring 2001. The cause of this phenomenon is discussed.

Halogen cycling and aerosol pH in the Hawaiian marine boundary layer

Abstract. Halogen species (HCl* (primarily HCl), Cl* (including Cl2 and HOCl), BrO, total gaseous inorganic Br and size-resolved particulate Cl- and Br -) and related chemical and physical parameters were measured in surface air at Oahu, Hawaii during September 1999. Aerosol pH as a function of particle size was inferred from phase partitioning and thermodynamic properties of HCl. Mixing ratios of halogen compounds and aerosol pHs were simulated with a new version of the photochemical box model MOCCA that considers multiple aerosol size bins. Inferred aerosol pHs ranged from 4.5 to 5.4 (median 5.1, n=22) for super-mm (primarily sea-salt) size fractions and 2.6 to 5.3 (median 4.6) for sub-mm (primarily sulphate) fractions. Inferred daytime pHs tended to be slightly lower than those at night, although daytime median values did not differ statistically from nighttime medians. Simulated pHs for most sea-salt size bins were within the range of inferred values. However, simulated pHs for the largest size fraction in the model were somewhat higher (oscillating around 5.9) due to the rapid turnover rates and relatively larger infusions of sea-salt alkalinity associated with fresh aerosols. Measured mixing ratios of HCl* ranged from <30 to 250 pmol mol-1 and those for Cl* from <6 to 38 pmol mol-1. Simulated HCl and Cl* (Cl+ClO+HOCl+Cl2) mixing ratios ranged between 20 and 70 pmol mol-1 and 0.5 and 6 pmol mol-1, respectively. Afternoon HCl* maxima occurred on some days but consistent diel cycles for HCl* and Cl* were not observed. Simulated HCl did vary diurnally, peaking before dusk and reaching a minimum at dawn. While individual components of Cl* varied diurnally in the simulations, their sum did not, consistent with the lack of a diel cycle in observed Cl*. Mixing ratios of total gaseous inorganic Br varied from <1.5 to 9 pmol mol-1 and particulate Br - deficits varied from 1 to 6 pmol mol-1 with values for both tending to be greater during daytime. Simulated Brt and Br - mixing ratios and enrichment factors (EFBr) were similar to those observed, with early morning maxima and dusk minima. However, the diel cycles differed in detail among the various simulations. In low-salt simulations, halogen cycling was less intense, Br - accumulated and Brt and EFBr increased slowly overnight. In higher-salt simulations with more intense halogen cycling, Br - and EFBr decreased and Brt increased rapidly after dusk. Cloud processing, which is not considered in this version of MOCCA, may also affect these diel cycles (von Glasow et al., 2003). Measured BrO was never above detection limit (~2 pmol mol-1) during the experiment, however relative changes in the BrO signal during the 3-hour period ending at 11:00 local time were mostly negative, averaging -0.3 pmol mol-1. Both of these results are consistent with MOCCA simulations of BrO mixing ratios. Increasing the sea-salt mixing ratio in MOCCA by ~25% (within observed range) led to a decrease in O3 of ~16%. The chemistry leading to this decrease is complex and is tied to NOx removal by heterogeneous reactions of BrNO3 and ClNO3. The sink of O3 due to the catalytic Cl-ClO and Br-BrO cycles was estimated at -1.0 to -1.5 nmol mol-1 day-1, a range similar to that due to O3 photolysis in the MOCCA simulations.

An ozone budget for the UK: using measurements from the national ozone monitoring network; measured and modelled meteorological data, and a [formula omitted] resistance analogy model of dry deposition

Data from the UK national air-quality monitoring network are used to calculate an annual mass budget for ozone (O3) production and loss in the UK boundary layer during 1996. Monthly losses by dry deposition are quantified from 1 km×1 km scale maps of O3 concentration and O3 deposition velocities based on a ‘big-leaf’ resistance analogy. The quantity of O3 deposition varies from ∼50 Gg-O3 month−1 in the winter to over 200 Gg-O3 month−1 in the summer when vegetation is actively absorbing O3. The net O3 production or loss in the UK boundary layer is found by selecting days when the UK is receiving “clean” Atlantic air from the SW to NW. In these conditions, the difference in O3 concentration observed at Mace Head and a rural site on the east coast of the UK indicates the net O3 production or loss within the UK boundary layer. A simple box model is then used to convert the concentration difference into a mass. The final budget shows that for most of the year the UK is a net sink for O3 (−25 to −800 Gg-O3 month−1) with production only exceeding losses in the photochemically active summer months (+45 Gg-O3 month−1).

Summertime tropospheric ozone over China simulated with a regional chemical transport model 2. Source contributions and budget

A regional three‐dimensional chemical transport model is used to simulate the contributions of various sources to the NOx and O3 distributions in the troposphere over China during a summertime period. O3 tracers are added in the model to simulate the distribution of O3 that originates from stratosphere‐troposphere exchange (STE), lateral boundary import (LBI) and in situ photochemistry via NO, respectively. Tracers for reactive nitrogen species are further incorporated into the model to simulate the distributions of NOx that originate from surface emissions, aviation, lightning, STE and LBI, respectively. Meanwhile, the O3 formed by chemical reactions via the NO tracer originating from these sources is also tagged. The model results indicate that surface emissions are the dominant contributor to the NOx and hence O3 concentrations in the polluted boundary layer of eastern China. At higher altitudes the O3 produced via surface‐emitted NOx accounts for 10–20% of the O3 concentrations over these polluted regions. The impact of aircraft NOx emissions on tropospheric O3 over China is small, contributing up to 2% of the O3 concentrations at cruise altitudes. The net O3 production via surface‐emitted NOx over the region is estimated to be 132 Gg d−1 in the boundary layer and 33 Gg d−1 in the free troposphere. Because there is much more O3 transported into than out of the region and high relative humidity, in situ photochemistry acts as a sink for O3 in the troposphere over the region.

Anthropogenic forcing on tropospheric ozone and OH since preindustrial times

A global three‐dimensional model of tropospheric chemistry is used to investigate the changes in tropospheric O3 and OH since preindustrial times as a result of fuel combustion and industry, biomass burning, and growth in atmospheric CH4. Model results indicate a 63% increase of the global tropospheric O3 burden from preindustrial times to present (80% and 50% in the northern and southern hemispheres, respectively). Anthropogenic emissions of NOx and of CO and hydrocarbons make comparable contributions to the global O3 increase (60% and 40% respectively), even though the local rate of tropospheric O3 production is generally NOχ limited. The rise in O3 production parallels closely the rise in the emissions of CO and hydrocarbon because the O3 yield per mole of CO or hydrocarbon oxidized has remained constant at 0.7–0.8 mol/mol since preindustrial times. In contrast, the O3 production efficiency per mole of NOχ emitted has decreased globally by a factor of 2. We find a 9% decrease in the global mean OH concentration (mass‐weighted) since preindustrial times. A linear relationship is found in the model between the global mean OH concentration and the SN/SC3/2 ratio, where SN and SC are the sources of NOχ and of CO and hydrocarbons, respectively. The relative constancy of the global mean OH concentration since preindustrial times reflects the conservation of the SN/SC3/2 ratio despite large increases in both SN and SC. Comparisons of model results with reconstructed nineteenth century observations of O3 at continental sites indicate a systematic overestimate of about 5 ppbv. Correcting this overestimate would require either a large missing chemical sink for O3 or a downward revision of the natural NOχ source from lightning (3 Tg N yr−1 in our model). The nineteenth century observations of O3 over France show no vertical gradient between the boundary layer and the free troposphere, which is inconsistent with our current understanding of tropospheric O3. The model underestimates preindustrial CO concentrations derived from polar ice cores; these measurements are difficult to reconcile with any reasonable CO emission inventories.

Evidence of inorganic chlorine gases other than hydrogen chloride in marine surface air

We report the first measurements of inorganic chlorine gases in the marine atmosphere using a new tandem mist chamber method. Surface air was sampled during four days including one diel cycle in January, 1992, at Virginia Key, Florida. Concentrations of HCl* (including HCl, ClNO3, ClNO2, and NOCl) were in the range 40 to 268 pptv and concentrations of Cl2* (including Cl2 and any HOCl not trapped in the acidic mist chamber) were in the range <26 to 254 pptv Cl. Concentrations of Cl2* increased during the night, and decreased after sunrise as HCl* concentrations increased by similar amounts. The measurements suggest an unknown source of either HOCl or Cl2 to the marine atmosphere. Photochemical model calculations indicate that photolysis of the observed Cl2* would yield a chlorine atom (Cl•) concentration of order 104–105 cm−3. Oxidation by Cl• would then represent a significant sink for alkanes and dimethylsulfide (DMS) in the marine boundary layer. The cycling of Cl• could provide either a source or a sink for O3, depending on NOX levels.

The role of clouds in tropospheric photochemistry

We show that photochemical processes in the lower half of the troposphere are strongly affected by the presence of liquid water clouds. Especially CH2O, an important intermediate of CH4 (and of other hydrocarbon) oxidation, is subject to enhanced breakdown in the aqueous phase. This reduces the formation of HO x -radicals via photodissociation of CH2O in the gas phase. In the droplets, the hydrated form of CH2O, its oxidation product HCO2 −, and H2O2 recycle O2 − radicals which, in turn, react with ozone. We show that the latter reaction is a significant sink for O3. Further O3 concentrations are reduced as a result of decreased formation of O3 during periods with clouds. Additionally, NO x , which acts as a catalyst in the photochemical formation of O3, is depleted by clouds during the night via scavenging of N2O5. This significantly reduces NO x -concentrations during subsequent daylight hours, so that less NO x is available for O3 production. Clouds thus directly reduce the concentrations of O3, CH2O, NO x , and HO x . Indirectly, this also affects the budgets of other trace gases, such as H2O2, CO, and H2.

Elevational and spatial variation in daytime ozone concentrations in the Virginia Blue Ridge Mountains: implications for forest exposure

Ozone (O3) concentrations before and during the early growing season were monitored continuously at two closely located areas in the Virginia Blue Ridge Mountains: a deciduous forest watershed (Shaver Hollow) and a predominantly open grassy meadow (Big Meadows). In addition, O3 concentrations and canopy development (percent leaf emergence) were measured simultaneously at three sites along the elevational gradient of the Shaver Hollow watershed. When the canopy at Shaver Hollow was leafless, patterns of O3 concentration were similar between the forest and meadow, increasing steadily toward early spring, with low variability of hourly daytime measurements. Greater differences in O3 concentration between forest and meadow and higher variability occurred during the period May-June, a time when the canopy changed from 15 to 100% leaf emergence. Several factors varying along the elevational gradient, related to both canopy structure and meteorological conditions, appeared to be important in affecting absolute O3 concentrations as well as the variability of daytime means in Shaver Hollow. The high metabolic activity of new leaves may act as a sink for O3; the canopy itself may act as a physical barrier to O3 transport, restricting mixing of O3 to intermittent bursts. The coincidence of seasonal increases in O3 concentration in late spring and early summer with the development of new leaves of dominant deciduous trees in this watershed suggests that O3 may represent a potential problem for eastern hardwood forests.

Tropospheric NOx and O3 budgets in the equatorial Pacific

By combining the results of model calculations and simultaneous measurements of O3, NO, HNO3, particulate NO3−, and meteorological parameters in the mid‐Pacific, we have investigated the budget and photochemistry of near surface atmospheric ozone and odd nitrogen. The observed decrease of O3 toward the Intertropical Convergence Zone is consistent with the transport of O3 from higher latitude and a photochemical sink for O3 by the reaction between H2O and O(1D). The total deposition flux of odd nitrogen deduced from the data is about 1.3×109 cm−2 s−1, or 5 MT(N)/yr averaged globally. Most of the deposition flux is due to HNO3 and particulate NO3−. This deposition flux is probably representative of values over most of the Pacific because the HNO3 and particulate NO3− densities measured over the Pacific during the GAMETAG experiments are consistent with our values. Since the residence time of odd nitrogen in the lower troposphere is short, a widely distributed, diffuse source of odd nitrogen is required to balance the deposition flux. It seems that the NO formed by cloud to cloud lightning in the tropical upper troposphere is the most likely source.

On the relationship between the greenhouse effect, atmospheric photochemistry, and species distribution

A radiative‐convective‐photochemical model that extends from 0 to 53 km is used to examine the effect on atmospheric constituents and thermal structure of changes in the atmospheric levels of CO2, CFM's, CO, N2O, and combinations of these species. Calculations were carried out for two reference atmospheres, one with high (HINOX) and one with low (LONOX) levels of NOx. The HINOX atmosphere has a vertical distribution of NOx similar to that resulting from recently published measurements. Such a distribution requires an unrealistically large value for the integrated tropospheric source of NOx. This suggests that the HINOX profile, and the measurements from which it was drawn, are more characteristic of contaminated, rather than clean, air masses. The HINOX troposphere provides a net photochemical source of O3 and is not greatly sensitive to downward transport of O3 from the stratosphere. The LONOX troposphere is a photochemical sink for O3 and, as a result, is sensitive to such variations in downward transport. Results of this study suggest that (1) infrared opacity changes due to CO2 and CFM increases can cause significant changes in tropospheric O3, OH, CH4, CO, and other species through changes in tropospheric water vapor; (2) perturbation or sensitivity studies conducted with tropospheric photochemical models may be subject to significant errors due to the absence of modulating effects provided by the stratosphere; (3) the response to given perturbations is significantly dependent upon the tropospheric NOx distribution present in the model; (4) CO2 growth rates must be included in the calculation of CFM‐O3 time scenarios and must account for surface temperature changes, especially in the steady state; and (5) that thermal or chemical perturbations introduce a variety of coupling or interactive mechanisms which are significant and which may provide compensating effects.