Bruce G. Doddridge

A tropical Atlantic paradox: Shipboard and satellite views of a tropospheric ozone maximum and wave-one in January-February 1999

During the Aerosols99 trans-Atlantic cruise from Norfolk, VA, to Cape Town, South Africa, daily ozonesondes were launched from the R/V Ronald H Brown between 17 January and 6 February 1999. A composite of tropospheric ozone profiles along the latitudinal transect shows 4 zones, nearly identical to the ozone distribution during a January-February 1993 trans-Atlantic cruise [Weller et al., 1996]. Sondes from the cruise and Ascension Island (8S, 14.5W), as well as the Earth-Probe (EP)/TOMS satellite instrument, show elevated tropospheric ozone (> 35 Dobson Units) throughout the south Atlantic in January 1999. Ozone layers associated with biomass burning north of the ITCZ (Intertropical Convergence Zone) are prominent at 0-5 km from 10-ON, but even higher ozone (100 ppbv, 5-15 km) occurred south of the ITCZ, where it was not burning - an ozone paradox that contributes to a wave-one zonal pattern in tropospheric ozone. Back trajectories, satellite observations and shipboard tracers suggest that the south Atlantic ozone results from a combination of interhemispheric transport, aged stratospheric-upper tropospheric air, and possibly from ozone supplied by lightning nitric oxide.

Seasonal variations in elemental carbon aerosol, carbon monoxide and sulfur dioxide: Implications for sources

As part of Maryland Aerosol Research and CHaracterization (MARCH-Atlantic) study, measurements of 24-hr average elemental carbon (EC) aerosol concentration were made at Fort Meade, Maryland, USA, a suburban site within the Baltimore-Washington corridor during July 1999, October 1999, January 2000, April 2000 and July 2000. Carbon monoxide (CO) and sulfur dioxide (SO2) were also measured nearly continuously over the period. Tight correlation between EC and CO in every month suggests common or proximate sources, likely traffic emissions. The EC versus CO slope varies in different seasons and generally increases with ambient temperature. The temperature dependence of EC/CO ratios suggests that EC source strength peaks in summer. By using the well established emission inventory for CO, and EC/CO ratio found in this study, EC emission over North America is estimated at 0.31±0.12 Tg yr−1, on the low end but in reasonable agreement with prior inventories based on emission factors and fuel consumption.

Climatologies of NOx and NOy: A comparison of data and models

Abstract Climatologies of tropospheric NOx (NO + NO2) and NOy (total reactive nitrogen: NOx + N03 + 2 × N2O5 + HNO2 + HNO3 + HNO4 + ClONO2 + PAN (peroxyacetylnitrate) + other organic ni trates) have been compiled from data previously published and, in most cases, publicly archived. Emphasis has been on non-urban measurements, including rural and remote ground sites, as well as aircraft data. Although the distribution of data is sparse, a compilation in this manner can begin to provide an understanding of the spatial and temporal distributions of these reactive nitrogen species. The cleanest measurements in the boundary layer are in Alaska, northern Canada and the eastern Pacific, with median NO mixing ratios below 10 pptv, NOx below 50 pptv, and NOy below 300 pptv. The highest NO values (greater than 1 ppbv) were found in eastern North America and Europe, with correspondingly high NOy (∼ 5 ppbv). A significantly narrower range of concentrations is seen in the free troposphere, particularly at 3–6 km, with NO typically about 10 pptv in the boreal summer. NO increases with altitude to ∼ 100 pptv at 9–12 km, whereas NOy does not show a trend with altitude, but varies between 100 and 1000 pptv. Decreasing mixing ratios eastward of the Asian and North American continents are seen in all three species at all altitudes. Model-generated climatologies of NOx and NOy from six chemical transport models are also presented and are compared with observations in the boundary layer and the middle troposphere for summer and winter. These comparisons test our understanding of the chemical and transport processes responsible for these species distributions. Although the model results show differences between them, and disagreement with observations, none are systematically different for all seasons and altitudes. Some of the differences between the observations and model results may likely be attributed to the specific meteorological conditions at the time that measurements were made differing from the model meteorology, which is either climatological flow from GCMs or actual meteorology for an arbitrary year. Differences in emission inventories, and convection and washout schemes in the models will also affect the calculated NOα and NOy distributions.

Origins of fine aerosol mass in the Baltimore-Washington corridor: implications from observation, factor analysis, and ensemble air parcel back trajectories

Chemically speciated fine particulate matter (PM2.5) and trace gases (including NH3, HNO3, CO, SO2 ,N O y) have been sampled at Fort Meade (FME: 39.101N, 76.741W; elevation 46 m MSL), Maryland, since July 1999. FME is suburban, located in the middle of the Baltimore–Washington corridor, and generally downwind of the highly industrialized Midwest. The PM2.5 at FME is expected to be of both local and regional sources. Measurements over a 2year period include eight seasonally representative months. The PM2.5 shows an annual mean of 13m gm � 3 and primarily consists of sulfate, nitrate, ammonium, and carbonaceous material. Day-to-day and seasonal variations in the PM2.5 chemical composition reflect changes of contribution from various sources. UNMIX, an innovative receptor model, is used to retrieve potential sources of the PM2.5. A six-factor model, including regional sulfate, local sulfate, wood smoke, copper/iron processing industry, mobile, and secondary nitrate, is constructed and compared with reported source emission profiles. The six factors are studied further using an ensemble back trajectory method to identify possible source locations. Sources of local sulfate, mobile, and secondary nitrate are more localized around the receptor than those of other factors. Regional sulfate and wood smoke are more regional and associated with westerly and southerly transport, respectively. This study suggests that the local contribution to PM2.5 mass can vary from o30% in summer to >60% in winter. r 2002 Elsevier Science Ltd. All rights reserved.

Transport-induced interannual variability of carbon monoxide determined using a chemistry and transport model

Transport-induced interannual variability of carbon monoxide (CO) is studied during 1989–1993 using the Goddard chemistry and transport model (GCTM) driven by assimilated data. Seasonal changes in the latitudinal distribution of CO near the surface and at 500 hPa are captured by the model. The annual cycle of CO is reasonably well simulated at sites of widely varying character. Day to day fluctuations in CO due to synoptic waves are reproduced accurately at remote North Atlantic locations. By fixing the location and magnitude of chemical sources and sinks, the importance of transport-induced variability is investigated at CO-monitoring sites. Transport-induced variability can explain 1991–1993 decreases in CO at Mace Head, Ireland, and St. Davids Head, Bermuda, as well as 1991–1993 increases in CO at Key Biscayne, Florida. Transport-induced variability does not explain decreases in CO at southern hemisphere locations. The model calculation explains 80–90% of interannual variability in seasonal CO residuals at Mace Head, St. Davids Head, and Key Biscayne and at least 50% of variability in detrended seasonal residuals at Ascension Island and Guam. Upper tropospheric interannual variability during October is less than 8% in the GCTM. Exceptions occur off the western coast of South America, where mixing ratios are sensitive to the strength of an upper tropospheric high, and just north of Madagascar, where concentrations are influenced by the strength of offshore flow from Africa.

Large‐scale pollution of the atmosphere over the remote Atlantic Ocean: Evidence from Bermuda

Ozone acts as a greenhouse gas and controls much of the oxidizing capacity of the atmosphere. Photochemical production of ozone in urban areas (smog) is a serious environmental problem, but how far this process extends on regional or global scales remains a major unanswered question in atmospheric science. In summer, Bermuda basks in pristine marine air, but in spring, episodes of high ozone are common. From meteorological analyses and observation of ozone, carbon monoxide, and reactive nitrogen compounds, the authors conclude that half or more of the excess ozone in Bermuda originates from air pollution over eastern North America. 50 refs., 7 figs., 2 tabs.

Pollutant Transport During a Regional O3 Episode in the Mid-Atlantic States

Ozone (O3) concentrations in the Baltimore-Washington (B-W) metropolitan area frequently exceed the National Ambient Air Quality Standard (NAAQS) in the summer months. The most extreme O3 events occur in multi-day high O3 episodes.1 These events can be regional in scale, with O3 concentrations exceeding the NAAQS at numerous locations along the eastern U.S. seaboard, and are typically associated with slow-moving or stagnant high pressure systems.2-5 In the B-W region, the most extreme events typically occur with surface high pressure overhead or just west of the region and an upper air high-pressure area (ridge) to the west or northwest.1 Besides providing conditions conducive to local O3 production (subsidence and strong low-level inversions, weak horizontal winds, little cloud cover), this weather pattern may also result in transport of O3 and its precursors from heavily industrialized areas west and north of the B-W region. In this paper, observations and back trajectories made during the severe regional O3 event of July 12-15, 1995, are used to confirm the hypothesis that significant regional-scale transport of O3 and its precursors occur during extreme O3 events of the standard type in the B-W area.

Analysis of a summertime PM2.5 and haze episode in the mid-Atlantic region.

Abstract Observations of the mass and chemical composition of particles less than 2.5 μm in aerodynamic diameter (PM2.5), light extinction, and meteorology in the urban Baltimore-Washington corridor during July 1999 and July 2000 are presented and analyzed to study summertime haze formation in the mid-Atlantic region. The mass fraction of ammoniated sulfate (SO4 2-) and carbonaceous material in PM2.5 were each ∼50% for cleaner air (PM2.5 < 10 μg/m3) but changed to ∼60% and ∼20%, respectively, for more polluted air (PM2.5 > 30 μg/m3). This signifies the role of SO4 2- in haze formation. Comparisons of data from this study with the Interagency Monitoring of Protected Visual Environments network suggest that SO4 2− is more regional than carbonaceous material and originates in part from upwind source regions. The light extinction coefficient is well correlated to PM2.5 mass plus water associated with inorganic salt, leading to a mass extinction efficiency of 7.6 ± 1.7 m2/g for hydrated aerosol. The most serious haze episode occurring between July 15 and 19, 1999, was characterized by westerly transport and recirculation slowing removal of pollutants. At the peak of this episode, 1-hr PM2.5 concentration reached ∼45 μg/m3, visual range dropped to ∼5 km, and aerosol water likely contributed to ∼40% of the light extinction coefficient.

Trace gas concentrations and meteorology in rural Virginia: 1. Ozone and carbon monoxide

Carbon monoxide (CO) and ozone (O3) play a central role in the oxidizing capacity of the atmosphere. Standard meteorological parameters and concentrations of these trace gases at Big Meadows, Shenandoah National Park, Virginia, were monitored almost continuously from October 1988 to October 1989. The National Park Service has been measuring O3 at this and two other sites in the park since 1983. Seasonal, monthly, and diurnal variations of hourly averages are examined. In the winter, dry deposition dominates; ozone values are relatively low with CO and O3 negatively correlated. In the summer, photochemistry dominates; ozone values are relatively high, and CO and O3 are positively correlated. Ozone shows a yearly mean mixing ratio of 33 (σ = 12) ppbv and did not exceed the ambient air quality standard during this year. CO mixing ratios averaged 204 (σ = 51) ppbv with no discernible diurnal or seasonal variation. Histograms of hourly means of O3 and CO appear lognormal, but the chi-square tests for goodness of fit reject the hypotheses. Several lines of evidence suggest that the data are little affected by local sources and are reasonably representative of the regional air quality. The summer of 1989 was cooler than normal, and the average ozone concentration was lower than the 7-year mean, although an analysis of the full record illustrates no statistically significant trend.

Stratosphere‐troposphere exchange in a midlatitude mesoscale convective complex: 2. Numerical simulations

Mixing across the tropopause due to intense convective events may significantly influence the atmospheric chemical balance. Stratosphere-troposphere exchange acts as an important natural source of O3 in the troposphere, and a source of H2O, HCs, CFCs, HCFCs, and reactive nitrogen in the stratosphere. The redistribution of atmospheric trace gases produces secondary radiative, dynamical and climate effects, influencing lower stratospheric temperatures and the tropopause height. During the 1989 North Dakota Thunderstorm Project, a severe storm which evolved into a mesoscale convective complex (MCC) on June 28–29 showed the unusual feature of an anvil formed well within the stratosphere and produced strong vertical mixing of atmospheric trace gases including H2O, CO, O3 and NOy as discussed by Poulida et al. [this issue] in Part 1 of this paper. In this paper the two-dimensional NASA Goddard Cumulus Ensemble (GCE) model was employed to simulate this convective storm using observed initial and boundary conditions. The sensitivity to the domain size, initial and boundary conditions, stability, and time resolution are evaluated. Synoptic-scale moisture convergence, simulated by moist boundary inflow, influences significantly the storm intensity, spatial structure, and trace gas transport, and produces a storm that reintensifies after the initial decay, mimicking the observed behavior of the MCC. The deformation of the tropopause documented with aircraft observations was qualitatively reproduced along with transport of stratospheric ozone downward into the troposphere, and the transport of trace species from the boundary layer upward into the stratosphere. If the chemistry and dynamics of this storm are typical of the roughly 100 MCCs occurring annually over midlatitudes, then this mechanism plays an important role in CO, NOy, and O3 budgets and could be the dominant source of H2O in the lower stratosphere and upper troposphere over midlatitudes.