Jennie L. Moody

Atmospheric deposition of nutrients to the North Atlantic Basin

Atmospheric chemical models are used to estimate the deposition rate of various inorganic oxides of nitrogen (NOy), reduced nitrogen species (NHx) and mineral dust to the North Atlantic Ocean (NAO). The estimated deposition of NOy to the NAO (excluding the coastal ocean) and the Caribbean is 360 × 109 Moles-N m-2 yr-1 (5.0 Tg N); this is equivalent to about 13% of the estimated global emission rate (natural and anthropogenic) and a quarter of the emission rate from sources in North America and Europe. In the case of NHx, 258 Moles-N m-2 yr-1 (3.6 Tg N) are deposited to the NAO and the Caribbean; this is about 6% of the global continental emissions. There is relatively little data on the deposition rate of organic nitrogen species; nonetheless, this evidence suggests that concentrations and deposition rates are comparable to those for inorganic nitrogen.

Relationships between ozone and carbon monoxide at surface sites in the North Atlantic region

As part of the North Atlantic Regional Experiment (NARE), measurements of O3 and CO at five surface sites were made from July 1991 to January 1995. The investigation of the variabilities and correlation of O3 and CO presented here indicates that the seasonal cycles of the medians and the means of O3 and CO are qualitatively similar to the cycles observed at other northern midlatitude sites. The signature of O3 produced from anthropogenic precursors is clearest in the spring at the Azores and in the summer at Sable Island. The influence of the natural stratospheric O3 source is apparent at Sable Island, particularly in the spring. At all sites the variability of CO throughout the year is dominated by episodes of pollution transport. The slopes of the monthly O3-CO correlations in the summer in Atlantic Canada and the spring in the Azores are quite uniform at 0.3 to 0.4. However, individual pollution transport events often have larger (≤1.0) slopes, which indicate significantly different net O3 production efficiencies between episodes. The average slope of O3 versus CO at Sable Island in the winter for moderate pollution transport events (CO ≤180 ppbv) is −0.28, which indicates that the titration of ambient O3 by emitted NO with little if any photochemical O3 production dominates the O3 chemistry over eastern North America in winter. Diurnal cycles driven by photochemistry are observed in the summer for both O3 and CO at the Azores (net loss) and Sable Island (net production.) These observations are consistent with the work of Duderstadt et al. [this issue] who find positive net photochemical O3 production at Sable Island, and with the modeling of Atherton et al. [1996] who find a region dominated by photochemical loss of O3 and CO in the central Atlantic.

Sources of nitrogen in wet deposition to the Chesapeake Bay region

Nitrogen-containing compounds in wet deposition can provide significant sources of nutrients to phytoplankton and potentially contribute to eutrophication in estuaries and coastal waters. Quantifying both inorganic and organic forms of nitrogen in wet deposition as well as determining their sources is important for understanding how to control eutrophication. Stable nitrogen isotope data can provide information regarding what source processes produced nitrogen in precipitation and air mass trajectories can predict where the air mass which produced the precipitation was geographically located before the event occurred. In this study, the wet deposition concentrations, fluxes, and δ15N values of ammonium, nitrate, and dissolved organic nitrogen were determined for 60 precipitation events collected from May, 1993 to December 1994 at a site near the Chesapeake Bay, an estuary currently experiencing eutrophication. Grouping the concentration data according to season showed a peak in ammonium coupled with depleted δ15N values in the spring which were indicative of agricultural emissions. A peak in nitrate in the spring seemed to indicate greater soil emissions at that time, but concentrations were also high at other times of the year. No trend was observed for the dissolved organic nitrogen with season. Back trajectories were calculated for each precipitation event and grouped into five major transport patterns. Combining the flux and isotopic composition data with the air flow history revealed that dominant sources of ammonium in precipitation to the region are probably fertilizers, soil, and animal excreta emissions which have the highest fluxes in air masses originating from the southwest and west. The dominant source of nitrate to the region is probably fossil-fuel combustion and the highest fluxes originate from the northwest and west. Speculation on the dissolved organic nitrogen sources is probably premature, but its flux pattern is similar to the nitrate pattern, suggesting that their sources may be similar.

Transport climatology of tropospheric ozone: Bermuda, 1988–1991

We determined the major transport patterns for Bermuda and quantified the degree to which they influenced variability in ozone concentrations by applying cluster analysis to isentropic trajectories from September 1988, through September 1991. Concentration distributions of ozone associated with these transport patterns were significantly different. The highest concentrations of ozone in each season were associated with transport off the North American continent; the lowest concentrations were during low-level maritime transport around the Bermuda high. Using the vertical component of the isentropic trajectories, we also showed that the most extreme concentrations of ozone occurred with rapidly descending air from midtropospheric levels. This pattern was most pronounced in April and May when more than 50% of the O3 variability was related to transport differences. We conclude that this relatively remote marine site, which normally experienced low maritime ozone levels (∼30 parts per billion by volume (ppbv)), periodically entrained dry, ozone-rich (∼55 ppbv) midtropospheric air in association with strong subsidence in high pressure behind spring low-pressure systems. Although the ultimate source of these midtroposphere, midlatitude, elevated-ozone concentrations is still being investigated, the synoptic meteorology associated with these transport patterns supports a significant contribution from the upper troposphere and lower stratosphere.

Trace gas signatures of the airstreams within North Atlantic cyclones: Case studies from the North Atlantic Regional Experiment (NARE ’97) aircraft intensive

This study reveals how airstreams within midlatitude cyclones draw and export trace gases from the polluted continental boundary layer, the midtroposphere, and the stratosphere of North America to the troposphere of the North Atlantic Ocean. The North Atlantic Regional Experiment (NARE) produced aircraft-based trace gas measurements from eight midlatitude cyclones during the autumn of 1997. Meteorological and back trajectory analysis identified the various component airstreams of several cyclones, including a cold conveyor belt, two warm conveyor belts, a dry airstream, a previously undefined post cold front air-stream, and a streamer fragment that originated in a dry airstream off the coast of California. O3, CO, and NOy mixing ratio distributions and relationships were determined for each airstream. Airstream chemical composition was related to the origin and transport history of the associated air mass. The lowest O3 values were associated with airstreams originating in Canada or the Atlantic Ocean marine boundary layer; the highest O3 values were associated with airstreams with a recent stratospheric component. The highest CO values were associated with lower tropospheric outflow from New England and a warm conveyor belt that advected boundary layer CO from the southeast United States to the mid and upper troposphere. The highest NOy values were also the result of lower troposphere polluted outflow from New England. Most NOy was removed from the airstreams that transported polluted boundary layer air to the free troposphere. A steep and positive O3/NOy slope was found for all airstreams in the free troposphere regardless of air mass origin.

Summer and spring ozone profiles over the North Atlantic from ozonesonde measurements

Ozone profiles obtained by near-daily ozonesonde observations during campaigns at several sites in the North Atlantic are used to construct time-height cross sections of ozone concentration through the troposphere. Strong day-to-day ozone variability on the scale of synoptic meteorological disturbances is found both in the spring and in the summer throughout much of the troposphere. Layers of high ozone concentration (∼100 ppb) are frequently seen in the middle and upper troposphere and are invariably associated with transport characteristics that strongly support a stratospheric source for these layers. Regions of low ozone (<40 ppb) are seen in the middle and upper troposphere associated with higher relative humidity. The connection of these events with low surface mixing ratios suggests that convective processes mix air low in ozone up through the troposphere. Vertical layering of ozone mixing ratio, which is seen at all of the observing locations, is a result of differing sources of air in the different layers.

Trace gas composition of midlatitude cyclones over the western North Atlantic Ocean: A conceptual model

[1] Midlatitude cyclones provide the energy necessary for most of the trace gas transport from North America to the western North Atlantic Ocean (WNAO). These cyclones are composed of four primary airstreams: warm conveyor belt (WCB), cold conveyor belt (CCB), dry airstream (DA), and post cold front (PCF) airstream. This study is the first to present a conceptual model of the chemical composition of a midlatitude cyclone tracking from North America to the WNAO. The model, a composite of chemical measurements from several cyclones, establishes the fundamental relationships between large-scale chemical transport and midlatitude cyclone structure. It also separates the meteorological influences on airstream trace gas signatures from the influence of surface emissions heterogeneity and presents characteristic mixing ratios of ozone, CO, NOx, and NOy within the four types of airstream during late summer/early autumn. While cyclone track and surface emissions heterogeneity impact the median trace gas values within airstreams, the O3/CO and O3/NOy slopes remain fairly constant. Several characteristics of the conceptual model impact trace gas signatures, regardless of cyclone track: (1) the DA always advects stratospheric ozone into the middle and upper troposphere; (2) the WCB is a more favorable location for photochemical ozone production than the CCB or PCF; (3) the PCF originates to the northwest, is unaffected by wet deposition, and the sunny conditions may allow for some photochemical ozone production; (4) the CCB is generally cloudy and does not show signs of significant photochemical ozone production; (5) both the CCB and the WCB experience wet deposition resulting in little NOy export from the lower troposphere. INDEX TERMS: 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 3364 Meteorology and Atmospheric Dynamics: Synoptic-scale meteorology;

Synoptic meteorology and transport during the North Atlantic Regional Experiment (NARE) intensive: Overview

Analyses of meteorological conditions during the North Atlantic Regional Experiment (NARE) intensive period, defined here as August 1 through September 13, 1993, indicate that transport in the study region in Nova Scotia was influenced by several well-defined synoptic disturbances (weak frontal passages or troughs) and by intervening periods of slowly varying transport conditions. Using trajectory analysis, synoptic charts, and other meteorological products, we have characterized these conditions and indicated the transitions in transport in relation to the synoptic scale meteorological situation. The ideal meteorological scenario for delivering pollution plumes from the U.S. East Coast urban areas over the Gulf of Maine to the Maritime Provinces of Canada is shown to be warm sector flow ahead of an advancing cold front. In addition, for two of these frontal passages we have discussed some of the smaller-scale effects which appear to have influenced the transport and or the composition of air masses reaching the NARE intensive region. Finally, we compare the conditions actually encountered during the field campaign with our idealized conceptual model of warm sector transport.

Precipitation composition and its variability in the southern Indian Ocean: Amsterdam Island, 1980–1987

Event precipitation samples have been collected on Amsterdam Island (37° 47′ S, 77° 31′ E) from May 1980 through January 1987 and analyzed for SO4=, NO3−, Cl−, Na+, Mg++, K+, Ca++, H+, HCOOt, and CH3COOt. The objective of this paper is to assess the processes which influence variability in the chemical composition of precipitation at this remote marine site. Back trajectories and gas phase 222Rn measurements were used to identify source regions and their relative contributions to precipitation composition. The technique of cluster analysis was applied to trajectory data as a method for determining to what degree different atmospheric flow patterns influence variability in the observed composition. The dominant source regions for chemical deposition were found to be Madagascar and/or S.E. Africa and the ocean region north and east of Amsterdam Island. A strong seasonal signal in the precipitation composition is illustrated. Processes which influence variability in the chemical composition include the following: seasonality in the source strength of biogenic precursors for non-sea-salt (nss) SO4=, NH4+ and carboxylic acids; seasonality in biomass-burning continental sources for the concentration of NO3− and a portion of nss Cl−; and variations in source region driven by daily changes in meteorology, as well as seasonal and annual differences in transport and removal patterns. In addition, interannual differences in nss SO4= concentration appear to be related to fluctuations in large-scale circulation patterns as defined by the Southern Oscillation.

Observations of reactive oxidized nitrogen and speciation of NO y during the PROPHET summer 1998 intensive

Measurements of NO y , NO x , PAN, PPN, MPAN, C 3 -C 5 alkyl nitrates, total isoprene nitrates, HNO 3 , HONO, and aerosol NO 3 - and NO 2 - were made during the summer 1998 intensive of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET). Mixing ratios of NO y , NO x , PAN, PPN, and alkyl nitrates were observed to have a strong dependence on the direction of transport to the site as was expected from the distribution of major urban and industrial sources. A peak in NO x and NO x /NO y during the morning in southerly flow provides evidence for the transport of relatively unprocessed emissions in layers above the nocturnal boundary layer. The difference in PAN and PPN levels between north and south flow directions indicates a net transport of reactive nitrogen to regions farther north. Isoprene nitrates were observed to typically comprise less than 1% of NO y in spite of the dominance of isoprene in local VOC chemistry, although due to the rapid losses of isoprene nitrates through reaction, vertical mixing, and deposition, their role in the processing of NO y may be significant. General agreement was observed between NO y and the sum of the individually measured constituents during ten 24 hour periods, although a linear regression indicates the potential for an interference in the individual measurements and a possible shortfall in NO y . Measurements indicate that HONO may play a larger role in the reactive nitrogen budget than previously expected for a rural site. HONO/NO 2 for a 24 hour period was observed to be 0.09-0.25 and suggests the likelihood of a significant heterogeneous production pathway or pathways.