International Consortium For Atmospheric Research On Transport And Transformation Research Articles

Meteorological Influences on Trace Gas Transport along the North Atlantic Coast during ICARTT 2004

An analysis of coastal meteorological mechanisms facilitating the transit pollution plumes emitted from sources in the Northeastern U.S. was based on observations from the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) 2004 field campaign. Particular attention was given to the relation of these plumes to coastal transport patterns in lower tropospheric layers throughout the Gulf of Maine (GOM), and their contribution to large-scale pollution outflow from the North American continent. Using measurements obtained during a series of flights of the National Oceanic & Atmospheric Administration (NOAA) WP-3D and the National Aeronautics and Space Administration (NASA) DC-8, a unique quasi-Lagrangian case study was conducted for a freshly emitted plume emanating from the New York City source region in late July 2004. The development of this plume stemmed from the accumulation of boundary layer pollutants within a coastal residual layer, where weak synoptic conditions allowed for its advection into the marine troposphere and transport by a mean southwesterly flow. Upon entering the GOM, analysis showed that the plume layer vertical structure evolved into an internal boundary layer form, with signatures of steep vertical gradients in temperature, moisture and wind speed often resulting in periodic turbulence. This structure remained well-defined during the plume study, allowing for the detachment of the plume layer from the surface and minimal plume-sea surface exchange. In contrast, shear driven turbulence within the plume layer facilitated lateral mixing with other low-level plumes during its transit. This turbulence was periodic and further contributed to the high spatial variability in trace gas mixing ratios. Further influences of the turbulent mixing were observed in the impact of the plume inland as observed by the Atmospheric Investigation, Regional Modeling, Analysis and Prediction (AIRMAP) air quality network. This impact was seen as extreme elevations of surface ozone and CO levels, equaling the highest observed that summer.

Calibration and intercomparison of acetic acid measurements using proton-transfer-reaction mass spectrometry (PTR-MS)

Abstract. Acetic acid is one of the most abundant organic acids in the ambient atmosphere, with maximum mixing ratios reaching into the tens of parts per billion by volume (ppbv) range. The identities and associated magnitudes of the major sources and sinks for acetic acid are poorly characterized, due in part to the limitations of available measurement techniques. This paper demonstrates that, when properly calibrated, proton-transfer-reaction mass spectrometry (PTR-MS) can be a valuable technique for fast response, accurate quantification of acetic acid in ambient air. Three different PTR-MS configurations were calibrated at low ppbv mixing ratios using permeation tubes, which yielded calibration factors between 7.0 and 10.9 normalized counts per second per ppbv (ncps ppbv−1) at a drift tube field strength of 132 Townsend (Td). Detection limits ranged from 0.06 to 0.32 ppbv with dwell times of 5 s. These calibration factors showed negligible humidity dependence. Acetic acid was measured with PTR-MS on Appledore B Island, ME, during the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) campaign and validated based on acetic acid measured in parallel using tandem mist chambers coupled with ion chromatography (MC/IC). Mixing ratios ranged from a minimum of 0.075 ± 0.004 ppbv to 3.555 ± 0.171 ppbv, with a median mixing ratio of 0.530 ± 0.025 ppbv. An orthogonal least squares linear regression of paired data yielded a slope of 1.14 ± 0.06 (2σ), an intercept of 0.049 ± 0.020 (2σ) ppbv, and an R2 of 0.78.

Modeling ozone plumes observed downwind of New York City over the North Atlantic Ocean during the ICARTT field campaign

Abstract. Transport and chemical transformation of well-defined New York City (NYC) urban plumes over the North Atlantic Ocean were studied using aircraft measurements collected on 20–21 July 2004 during the ICARTT (International Consortium for Atmospheric Research on Transport and Transformation) field campaign and WRF-Chem (Weather Research and Forecasting-Chemistry) model simulations. The strong NYC urban plumes were characterized by carbon monoxide (CO) mixing ratios of 350–400 parts per billion by volume (ppbv) and ozone (O3) levels of about 100 ppbv near New York City on 20 July in the WP-3D in-situ and DC-3 lidar aircraft measurements. On 21 July, the two aircraft captured strong urban plumes with about 350 ppbv CO and over 150 ppbv O3 (~160 ppbv maximum) about 600 km downwind of NYC over the North Atlantic Ocean. The measured urban plumes extended vertically up to about 2 km near New York City, but shrank to 1–1.5 km over the stable marine boundary layer (MBL) over the North Atlantic Ocean. The WRF-Chem model reproduced ozone formation processes, chemical characteristics, and meteorology of the measured urban plumes near New York City (20 July) and in the far downwind region over the North Atlantic Ocean (21 July). The quasi-Lagrangian analysis of transport and chemical transformation of the simulated NYC urban plumes using WRF-Chem results showed that the pollutants can be efficiently transported in (isentropic) layers in the lower atmosphere (<2–3 km) over the North Atlantic Ocean while maintaining a dynamic vertical decoupling by cessation of turbulence in the stable MBL. The O3 mixing ratio in the NYC urban plumes remained at 80–90 ppbv during nocturnal transport over the stable MBL, then grew to over 100 ppbv by daytime oxidation of nitrogen oxides (NOx = NO + NO2) with mixing ratios on the order of 1 ppbv. Efficient transport of reactive nitrogen species (NOy), specifically nitric acid (HNO3), was confirmed through the comparison of the CO/NOy ratio in photochemically fresh and aged NYC plumes, implying the possibility of long-range transport of O3 over the stable MBL over the North Atlantic Ocean in association with NOx regeneration mechanism. The impact of chemical initial and boundary conditions (IC/BCs) on modelled O3 urban plumes was investigated in terms of the background O3 level and the vertical structure of the urban plumes. Simulations with dynamic ("time-variant") chemical IC/BCs enhanced the O3 level by 2–12 ppbv on average in the atmospheric layer below 3 km, showing better agreement with the observed NYC plumes and biomass-burning plumes than the simulation with prescribed static IC/BCs. The simulation including MOZART-4 chemical IC/BCs and Alaskan/Canadian wildfire emissions compared better to the observed O3 profiles in the upper atmospheric layer (>~3 km) than models that only accounted for North American anthropogenic/biogenic and wildfire contributions to background ozone. The comparison between models and observations show that chemical IC/BCs must be properly specified to achieve accurate model results.

Eta-CMAQ air quality forecasts for O<sub>3</sub> and related species using three different photochemical mechanisms (CB4, CB05, SAPRC-99): comparisons with measurements during the 2004 ICARTT study

Abstract. A critical module of air quality models is the photochemical mechanism. In this study, the impact of the three photochemical mechanisms (CB4, CB05, SAPRC-99) on the Eta-Community Multiscale Air Quality (CMAQ) model's forecast performance for O3, and its related precursors has been assessed over the eastern United States with observations obtained by aircraft (NOAA P-3 and NASA DC-8) flights, ship and two surface networks (AIRNow and AIRMAP) during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) study. The results show that overall none of the mechanisms performs systematically better than the others. On the other hand, at the AIRNow surface sites, CB05 has the best performance with the normalized mean bias (NMB) of 3.9%, followed by CB4 (NMB=−5.7%) and SAPRC-99 (NMB=10.6%) for observed O3≥75 ppb, whereas CB4 has the best performance with the least overestimation for observed O3<75 ppb. On the basis of comparisons with aircraft P-3 measurements, there were consistent overestimations of O3, NOz, PAN and NOy and consistent underestimations of CO, HNO3, NO2, NO, SO2 and terpenes for all three mechanisms although the NMB values for each species and mechanisms were different. The results of aircraft DC-8 show that CB05 predicts the H2O2 mixing ratios most closely to the observations (NMB=10.8%), whereas CB4 and SAPRC-99 overestimated (NMB=74.7%) and underestimated (NMB=−25.5%) H2O2 mixing ratios significantly, respectively. For different air mass flows over the Gulf of Maine on the basis of the ship data, the three mechanisms have relatively better performance for O3, isoprene and SO2 for the clean marine or continental flows but relatively better performance for CO, NO2 and NO for southwesterly/westerly offshore flows. The results of the O3-NOz slopes over the ocean indicate that SAPRC-99 has the highest upper limits of the ozone production efficiency (εN) (5.8), followed by CB05 (4.5) and CB4 (4.0) although they are much lower than that inferred from the observation (11.8), being consistent with the fact that on average, SAPRC-99 produces the highest O3, followed by CB05 and CB4, across all O3 mixing ratio ranges

North American influence on tropospheric ozone and the effects of recent emission reductions: Constraints from ICARTT observations

We use observations from the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) campaign over eastern North America in summer 2004, interpreted with a global 3‐D model of tropospheric chemistry (GEOS‐Chem), to improve and update estimates of North American influence on global tropospheric ozone and the effect of recent U.S. anthropogenic reductions on surface ozone pollution. We find that the 50% decrease in U.S. stationary NOx sources since 1999 has decreased mean U.S. boundary layer ozone concentrations by 6–8 ppbv in the southeast and 4–6 ppbv in the Midwest. The observed dO3/dCO molar enhancement ratio in the U.S. boundary layer during ICARTT was 0.46 mol mol−1, larger than the range of 0.3–0.4 from studies in the early 1990s, possibly reflecting the decrease in the NOx/CO emission ratio as well as an increase in the ozone production efficiency per unit NOx. North American NOx emissions during summer 2004 as constrained by the ICARTT observations (0.72 Tg N fossil fuel, 0.11 Tg N biomass burning, 0.28 Tg N lightning for 1 July to 15 August) enhanced the hemispheric tropospheric ozone burden by 12.4%, with comparable contributions from fossil fuel and lightning (5–6%), but only 1% from biomass burning emissions despite 2004 being a record fire year over Alaska and western Canada.

A study of the ozone formation by ensemble back trajectory-process analysis using the Eta–CMAQ forecast model over the northeastern U.S. during the 2004 ICARTT period

The integrated process rates (IPRs) estimated by the Eta–CMAQ model at grid cells along the trajectory of the air mass transport path were analyzed to quantitatively investigate the relative importance of physical and chemical processes for O 3 formation and evolution over the northeastern U.S. during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) period. The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model is used to determine the back trajectory of air masses reaching the northeast by linking a downwind receptor to upwind source areas. The process analysis is applied to a high O 3 episode occurring on July 22, 2004 at three selected sites in the northeastern U.S. The process analysis at the location of the site shows that during the daytime, the O 3 concentrations in the surface layer are mainly enhanced by the vertical diffusion of O 3-rich air from aloft, followed by horizontal advection (HADV) and chemical production (CHEM), whereas dry deposition (DDEP) and vertical advection (ZADV) mainly deplete O 3 concentrations at the sites of Valley Central (VC), NY and Castle Spring (CS), NH. By integrating the effects of each process over the depth of the daytime planetary boundary layer (PBL), it was found that at the VC site, CHEM and HADV contributed about 53% and 41%, respectively, to O 3 levels within the PBL. This confirms the significance of regional transport of O 3 from the industrialized areas into the Northeast. On the other hand, the process analysis results for O 3 formation in moving air masses indicate that on July 22, large chemical production of O 3 along the transport path over the polluted urban regions leads to significant increase in O 3 in the air mass reaching the VC site, whereas the low chemical production of O 3 along the transport path over the low emission regions leads to the low O 3 concentration at the site of Belleayre Mountain (BM), NY. The dramatic buildup of O 3 concentration from 50 ppb to 102 ppb in the air masses before reaching the VC site after 12:00 EST on 7/22 indicates the significant impact of pollution from the northeastern urban corridor at this site. On the basis of the results at the CS site, it was found that high NO x emissions along the transport path led to large chemical production of O 3 in the air mass reaching the CS site on July 22. In contrast, the low chemical production of O 3 associated with low emission (relatively clean conditions) along the transport path over the northern portions of the domain is responsible for the low O 3 concentration at the CS site on July 26.

AIRS views transport from 12 to 22 July 2004 Alaskan/Canadian fires: Correlation of AIRS CO and MODIS AOD with forward trajectories and comparison of AIRS CO retrievals with DC‐8 in situ measurements during INTEX‐A/ICARTT

We present observations of transport of tropospheric carbon monoxide (CO) obtained by the Atmospheric Infrared Sounder (AIRS) on board NASA's Aqua satellite during the Intercontinental Chemical Transport Experiment–North America (INTEX‐A) field campaign in the summer of 2004, part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). In situ measurements from NASA's DC‐8 provide crucial assessment of AIRS midtropospheric (400–500 mbar) CO retrievals. For the nine best INTEX‐A profiles, convolution of the in situ profiles with AIRS verticality functions demonstrates version 4 AIRS CO retrievals between 400 and 500 mbar are biased high by approximately 8% with a standard deviation slightly less than 5%. The 400–500 mbar region is the only portion of the version 4 AIRS CO retrievals that can be validated as presented here. In some cases, AIRS CO retrievals may be sensitive to CO in the lower to midtroposphere (800–500 mbar). Focusing on one major episode, we investigate transport of CO from a large fire outbreak in the Alaskan/Canadian Yukon region from 11 to 14 July 2004 and follow it downwind to the southeastern United States and Europe by 22 July 2004. Comparison of AIRS CO maps and forward trajectories from fire locations reveals substantial variations in fire emissions especially emission injection height. Any useful forecast model must control for such fire emission variabilities to predict correctly the downwind impact. To match the forward trajectory analyses with AIRS CO observations requires some fires to have directly injected emissions to at least 500 mbar and perhaps as high as 300 mbar. Ground‐based lidar observations show smoke plume altitudes from 3 to 11 km over Wisconsin and from 1 to 4 km over Maryland in agreement with the forward trajectories. The Wisconsin lidar profiles on the afternoon of 18 July 2004 constrain the altitude of CO‐rich smoke observed by AIRS and MODIS to lie between 2 and 5.5 km above the surface, roughly 800 to 500 mbar. We find that changes in the correlation between AIRS CO and MODIS AOD reflect changes in the CO vertical distribution during this event. This finding is confirmed by in situ measurements, meteorological analyses, and forward trajectory analyses.

Sources of particulate matter in the northeastern United States in summer: 1. Direct emissions and secondary formation of organic matter in urban plumes

Ship and aircraft measurements of aerosol organic matter (OM) and water‐soluble organic carbon (WSOC) were made in fresh and aged pollution plumes from major urban areas in the northeastern United States in the framework of the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) study. A large part of the variability in the data was quantitatively described by a simple parameterization from a previous study that uses measured mixing ratios of CO and either the transport age or the photochemical age of the sampled air masses. The results suggest that OM was mostly due to secondary formation from anthropogenic volatile organic compound (VOC) precursors in urban plumes. Approximately 37% of the secondary formation can be accounted for by the removal of aromatic precursors using newly published particulate mass yields for low‐NOx conditions, which are significantly higher than previous results. Of the secondary formation, 63% remains unexplained and is possibly due to semivolatile precursors that are not measurable by standard gas chromatographic methods. The observed secondary OM in urban plumes may account for 35% of the total source of OM in the United States and 8.5% of the global OM source. OM is an important factor in climate and air quality issues, but its sources and formation mechanisms remain poorly quantified.

Volatile organic compounds in northern New England marine and continental environments during the ICARTT 2004 campaign

Volatile organic compound (VOC) measurements were made during the summer 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) at Thompson Farm (TF), a continental site 25 km from the New Hampshire coast, and Appledore Island (AI), a marine site 10 km off the Maine coast. The 24 h mean total hydroxyl radical (OH) reactivity (±1σ) for the suite of VOCs was 4.15 (±2.64) s−1 at TF and 2.57 (±1.10) s−1 at AI. The larger range of reactivity at TF was dominated by isoprene and the monoterpenes (mean combined reactivity = 2.01 (±2.57) s−1). The impact of local anthropogenic hydrocarbon sources such as liquefied petroleum gas (LPG) leakage and fossil fuel evaporation was evident at both sites. During the campaign, a propane flux of 9 (±2) × 109 molecules cm−2 s−1 was calculated from the linear regression of the mean 0100–0400 local time mixing ratios at TF. This is consistent with fluxes observed in 2003 at sites spread throughout the coastal area of New Hampshire indicating that LPG tank leakage is a major hydrocarbon source throughout the region. Net monoterpene fluxes during ICARTT at TF were 6 (±2), 1.8 (±0.4), 1.2 (±0.6), and 0.4 (±0.5) × 109 molecules cm−2 s−1 for α‐pinene, β‐pinene, camphene, and limonene, respectively. Comparison to estimated NO3 and O3 loss rates indicate that gross monoterpene emission rates were approximately double the observed net fluxes at TF and comparable to current monoterpene nighttime emission inventory estimates for the northeast.

Characteristics of the atmospheric CO2 signal as observed over the conterminous United States during INTEX‐NA

High resolution in situ measurements of atmospheric CO2 were made from the NASA DC‐8 aircraft during the Intercontinental Chemical Transport Experiment–North America (INTEX‐NA) campaign, part of the wider International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). During the summer of 2004, eighteen flights comprising 160 h of measurements were conducted within a region bounded by 27 to 53°N and 36 to 139°W over an altitude range of 0.15 to 12 km. These large‐scale surveys provided the opportunity to examine the characteristics of the atmospheric CO2 signal over sparsely sampled areas of North America and adjacent ocean basins. The observations showed a high degree of variability (≤18%) due to the myriad source and sink processes influencing the air masses intercepted over the INTEX‐NA sampling domain. Surface fluxes had strong effects on continental scale concentration gradients. Clear signatures of CO2 uptake were seen east of the Mississippi River, notably a persistent CO2 deficit in the lowest 2–3 km. When combining the airborne CO2 measurements with LANDSAT and MODIS data products, the lowest CO2 mixing ratios observed during the campaign (337 ppm) were tied to mid‐continental agricultural fields planted in corn and soybeans. We used simultaneous measurements of CO, O3, C2Cl4, C2H6, C2H2 and other unique chemical tracers to differentiate air mass types. Coupling these distinct air mass chemical signatures with transport history permitted identification of convection, stratosphere‐troposphere exchange, long‐range transport from Eastern Asia, boreal wildfires, and continental outflow as competing processes at multiple scales influencing the observed concentrations. Our results suggest these are important factors contributing to the large‐scale distribution in CO2 mixing ratios thus these observations offer new constraints in the computation of the North American carbon budget.

Evaluation of real‐time PM2.5 forecasts and process analysis for PM2.5 formation over the eastern United States using the Eta‐CMAQ forecast model during the 2004 ICARTT study

The performance of the Eta‐Community Multiscale Air Quality (CMAQ) modeling system in forecasting PM2.5 and chemical species is assessed over the eastern United States with the observations obtained by aircraft (NOAA P‐3 and NASA DC‐8) and four surface monitoring networks (AIRNOW, IMPROVE, CASTNet and STN) during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) study. The results of the statistical analysis at the AIRNOW sites show that the model was able to reproduce the day‐to‐day and spatial variations of observed PM2.5 and captured a majority (73%) of PM2.5 observations within a factor of 2, with normalized mean bias of −21%. The consistent underestimations in regional PM2.5 forecast at other networks (IMPROVE and STN) were mainly due to the underestimation of total carbonaceous aerosols at both urban and rural sites. The significant underestimation of the “other” category, which predominantly is composed of primary emitted trace elements in the current model configuration, is also one of the reasons leading to the underestimation of PM2.5 at rural sites. The systematic overestimations of SO42− both at the surface sites and aloft, in part, suggest too much SO2 cloud oxidation due to the overestimation of SO2 and H2O2 in the model. The underestimation of NH4+ at the rural sites and aloft may be attributed to the exclusion of some sources of NH3 in the emission inventory. The systematic underestimations of NO3− may result from the general overestimations of SO42−. Note that there are compensating errors among the underestimation of PM2.5 species (such as total carbonaceous aerosols) and overestimation of PM2.5 species (such as SO42−), leading to generally better performance of PM2.5 mass. The systematic underestimation of biogenic isoprene (by ∼30%) and terpene (by a factor of 4) suggests that their biogenic emissions may have been biased low, whereas the consistent overestimations of toluene by the model under the different conditions suggest that its anthropogenic emissions might be too high. The contributions of various physical and chemical processes governing the distribution of PM2.5 during this period are investigated through detailed analysis of model process budgets using the integrated process rate (IPR) analysis along back trajectories at five selected locations in Pennsylvania and Georgia. The results show that the dominant processes for PM2.5 formation and removal vary from the site to site, indicating significant spatial variability.

Nighttime nitrate radical chemistry at Appledore Island, Maine during the 2004 International Consortium for Atmospheric Research on Transport and Transformation

Trace gases including nitrogen dioxide (NO2), nitrate radical (NO3), ozone (O3), and a suite of volatile organic compounds (VOCs) were measured within the New England coastal marine boundary layer on Appledore Island (AI), Maine, USA as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign. These measurements, together with local meteorological records and published kinetic data were used to investigate nighttime NO3 chemistry at AI during the period of 8–28 July 2004. Among the VOCs, isoprene, monoterpenes and dimethylsulfide (DMS) were the dominant NO3 reactants; on average, DMS accounted for 51 ± 34% of the total reactivity. For three case studies, NO3 mixing ratios were calculated from measured parameters with resultant uncertainties of ≤30%. Discrepancies with measured NO3 appeared to result primarily from input parameter variability and exclusion of heterogeneous dinitrogen pentoxide (N2O5) chemistry. We indirectly determined that nighttime NO3 and NOx (=NO + NO2) removal via N2O5 chemistry (gas‐phase + heterogeneous) was on average 51–54% and 63–66% of the total respectively. Our analysis suggested that the minimum average NO3 and NOx removal via heterogeneous N2O5 chemistry was ∼10% of the total. Reducing gas‐phase N2O5 reactivity in accord with Brown et al. (2006a) increased the importance of heterogeneous N2O5 chemistry substantially. It is plausible that the latter pathway was often comparable to gas‐phase removal of NO3 and NOx. Overall, 24 h‐averaged NOx removal was ∼11 ppbv, with nighttime chemical pathways contributing ∼50%.

Four‐dimensional data assimilation experiments with International Consortium for Atmospheric Research on Transport and Transformation ozone measurements

Ozone measurements by various platforms during the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) operations in the summer of 2004 are assimilated into the STEM regional chemical transport model (CTM). Under the four‐dimensional variational data assimilation (4D‐Var) framework, the model forecast (background) error covariance matrix is constructed using both the so‐called NMC (National Meteorological Center, now National Centers for Environmental Prediction) method and the observational (Hollingworth‐Lönnberg) method. The inversion of the covariance matrix is implemented using truncated singular value decomposition (TSVD) approach. The TSVD approach is numerically stable even with severely ill conditioned vertical correlation covariance matrix and large horizontal correlation distances. Ozone observations by different platforms (aircraft, surface, and ozonesondes) are first assimilated separately. The impacts of the various measurements are evaluated on their ability to improve the predictions, defined as the information content of the observations under the current framework. In the end, all observations are assimilated into the CTM. The final analysis matches well with observations from all platforms. Assessed with all the observations throughout the boundary layer and midtroposphere, the model bias is reduced from 11.3 ppbv for the base case to −1.5 ppbv. A reduction of 10.3 ppbv in root mean square error is also seen. In addition, the potential of improving air quality forecasts by chemical data assimilation is demonstrated. The effect of assimilating ozone observations on model predictions of other species is also shown.

A detailed evaluation of the Eta‐CMAQ forecast model performance for O3, its related precursors, and meteorological parameters during the 2004 ICARTT study

The Eta‐Community Multiscale Air Quality (CMAQ) model's forecast performance for ozone (O3), its precursors, and meteorological parameters has been assessed over the eastern United States with the observations obtained by aircraft, ship, ozonesonde, and lidar and two surface networks (AIRNOW and AIRMAP) during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) study. The results at the AIRNOW sites show that the model was able to reproduce the day‐to‐day variations of observed daily maximum 8‐hour O3 and captured the majority (73%) of observed daily maximum 8‐hour O3 within a factor of 1.5 with normalized mean bias of 22%. The model in general reproduced O3 vertical distributions on most of the days at low altitudes, but consistent overestimations above ∼6 km are evident because of a combination of effects related to the specifications of lateral boundary conditions from the Global Forecast System (GFS) as well as the model's coarse vertical resolution in the upper free troposphere. The model captured the vertical variation patterns of the observed values for other parameters (HNO3, SO2, NO2, HCHO, and NOy_sum (NOy_sum = NO + NO2 + HNO3 + PAN)) with some exceptions, depending on the studied areas and air mass characteristics. The consistent underestimation of CO by ∼30% from surface to high altitudes is partly attributed to the inadequate representation of the transport of pollution associated with Alaska forest fires from outside the domain. The model exhibited good performance for marine or continental clear airflows from the east/north/northwest/south and southwest flows influenced only by Boston city plumes but overestimation for southeast flows influenced by the long‐range transport of urban plumes from both New York City and Boston.

Evaluation of the MOCAGE chemistry transport model during the ICARTT/ITOP experiment

Intercontinental Transport of Ozone and Precursors (ITOP), part of International Consortium for Atmospheric Research on Transport and Transformation (ICARTT), was a large experimental campaign designed to improve our understanding of the chemical transformations within plumes during long‐range transport (LRT) of pollution from North America to Europe. This campaign took place in July and August 2004, when a strong fire season occurred in North America. Burning by‐products were transported over large distances, sometimes reaching Europe. A chemical transport model, Modélisation de la Chimie Atmosphérique Grande Echelle (MOCAGE), with a high grid resolution (0.5° × 0.5°) over the North Atlantic area and a daily inventory of biomass burning emissions over the United States, has been used to simulate the period. By comparing our results with available aircraft in situ measurements and satellite data (MOPITT CO and SCIAMACHY NO2), we show that MOCAGE is capable of representing the main characteristics of the tropospheric ozone‐NOx‐hydrocarbon chemistry during the ITOP experiment. In particular, high resolution allows the accurate representation of the pathway of exported pollution over the Atlantic, where plumes were transported preferentially at 6 km altitude. The model overestimates OH mixing ratios up to a factor of 2 in the lower troposphere, which results in a global overestimation of hydrocarbons oxidation by‐products (PAN and ketones) and an excess of O3 (30–50 ppbv) in the planetary boundary layer (PBL) over the continental United States. Sensitivity study revealed that lightning NO emissions contributed significantly to the NOx budget in the upper troposphere of northeast America during the summer 2004.

Regional variation of organic functional groups in aerosol particles on four U.S. east coast platforms during the International Consortium for Atmospheric Research on Transport and Transformation 2004 campaign

Submicron atmospheric aerosol samples were collected during the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) 2004 campaign on four platforms: Chebogue Point (Nova Scotia, Canada), Appledore Island (Maine), the CIRPAS Twin Otter over Ohio, and the NOAA R/V Ronald H. Brown in the Gulf of Maine. Saturated aliphatic C‐C‐H, unsaturated aliphatic C=C−H, aromatic C=C−H, organosulfur C‐O‐S, carbonyl C=O, and organic hydroxyl C‐OH functional groups were measured by calibrated Fourier Transform Infrared (FTIR) spectroscopy at all four sampling platforms. The ratio of molar concentrations of carbonyl C=O to saturated aliphatic C‐C‐H groups was nearly constant at each sampling platform, with the Twin Otter samples having the lowest ratio at 0.1 and the three more coastal platforms having ratios of 0.4 and 0.5. Organic mass (OM) to organic carbon (OC) ratios follow similar trends for the four platforms, with the Twin Otter having the lowest ratio of 1.4 and the coastal platforms having slightly higher values typically between 1.5 and 1.6. Organosulfur compounds were occasionally observed. Collocated organic aerosol sampling with two Aerodyne aerosol mass spectrometers for OM, a Sunset Laboratory thermo‐optical analysis instrument for OC, and an ion chromatography‐particle into liquid sampler (IC‐PILS) for speciated carboxylic acids provided comparable results for most of the project, tracking the time series of FTIR OM, OC, and carbonyl groups, respectively, and showing simultaneous peaks of similar magnitude during most of the project. The FTIR/IC‐PILS comparison suggests that about 9% of the carbonyl groups found in submicron organic particles on the Twin Otter are typically associated with low molecular weight carboxylic acids.

Ammonia sources, transport, transformation, and deposition in coastal New England during summer

During summer 2004, NH3, size‐resolved particulate NH4+, and associated characteristics of surface air were measured continuously on Appledore Island, off the southern Maine coast as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). NH3 concentrations ranged from <0.6 to 123 nmol m−3 with maxima around local noon and minima near dawn. Particulate NH4+ ranged from 10.3 to 191 nmol m−3. The transport of emissions from intensive agricultural activities in the eastern United States was an important source of total NH3 (NH3 + NH4+) over the Gulf of Maine during summer. Under cleaner northwest flow, total NH3 concentrations were relatively low (median = 50.0 nmol m−3) and partitioned roughly equally between phases; under the more polluted midwest flow, total NH3 concentrations were substantially higher (median = 171 nmol m−3) but dominated by particulate NH4+. Because particulate NH4+ was associated primarily with the highly acidic sub‐μm size fractions with low deposition velocities (median flux = 1.5 μmol m–2 day–1), dry‐deposition fluxes were dominated by the gas phase (median = 6.2 μmol m−2 day−1). Consequently, phase partitioning with pollutant‐derived sulfur aerosol substantially increased both the atmospheric lifetime of total NH3 against dry deposition and the relative importance of removal via wet‐ versus dry‐deposition pathways. Total NH3 accounted for 32% of the dry‐deposition flux of inorganic N to the Gulf of Maine during summer. The combined dry deposition of total NH3 and wet deposition of NH4+ via precipitation contributed 40% of the corresponding total atmospheric N flux.

Air mass classification in coastal New England and its relationship to meteorological conditions

The dominant air mass types along coastal New England during summer 2004 were classified and their relationship to synoptic meteorological conditions investigated as a component of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) campaign. Representative air mass types were defined on the basis of variability in the occurrence of volatile organic compounds (VOCs), and they were further grouped into distinct source‐related categories. Emissions from nearby anthropogenic sources were found to govern the total variance of the data set (58% at Appledore Island (AI) and 56% at Thompson Farm (TF)), whereas long‐distance transport was much less dominant (15% on AI and 17% at TF). This result indicates frequent influence from recent anthropogenic emissions at TF and AI, with periodic inputs of aged/processed pollutants. The fractional time periods dominated by anthropogenic (recent and aged/processed) and marine biogenic air masses were almost the same at TF and AI (36% versus 30% and 14% versus 16% respectively), clearly showing the strong impact of continental outflow on the nearshore marine atmosphere and a surprising persistent influence of marine biogenic emissions at an inland site. Empirical orthogonal function (EOF) analysis suggested that the most important circulation pattern during summer 2004 was characterized by a low trough positioned along coastal New England, leaving this region under the control of northwesterly flow from central Canada. This result helps explain why New England in summer 2004 was less polluted compared to previous years.

Inorganic chlorine and bromine in coastal New England air during summer

During summer 2004, a comprehensive suite of reactive trace gases (including halogen radicals and precursors, ozone, reactive N, soluble acids, and hydrocarbons), the chemical and physical characteristics of size‐resolved aerosols, actinic flux, and related physical conditions were measured at Appledore Island, Maine, as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). Sea‐salt mass averaged 4 to 8 times lower than that over the open North Atlantic Ocean. Production in association with sea salt was the primary source for inorganic Cl and Br. Acid displacement of sea‐salt Cl− primarily by HNO3 sustained high HCl mixing ratios (often >2000 pptv) during daytime. Median pHs for the larger sea‐salt size fractions (geometric mean diameters, GMDs ≥ 2.9 μm) ranged from 3.1 to 3.4; median pHs for sub‐μm size fractions were ≤ 1.6. Cl* (including HOCl and Cl2) ranged from <20 to 421 pptv Cl but was less than the detection limit (DL) during most sampling intervals. Periods during which Cl* was consistently detectable corresponded to relatively clean conditions, multiday transport over water, and relatively low actinic flux. At high HCl mixing ratios (>1000 pptv), HCl + OH sustained steady state Cl‐atom concentrations in the range of 104 cm−3. When detectable, photolysis of Cl* was generally the dominant source of atomic Cl; steady state concentrations of Cl atoms were frequently in the range of 104 to 105 cm−3. At these concentrations, Cl played an important role in the chemical evolution of polluted coastal air. Br radical chemistry was relatively unimportant.

Observational constraints on the chemistry of isoprene nitrates over the eastern United States

The formation of organic nitrates during the oxidation of the biogenic hydrocarbon isoprene can strongly affect boundary layer concentrations of ozone and nitrogen oxides (NOx = NO + NO2). We constrain uncertainties in the chemistry of these isoprene nitrates using chemical transport model simulations in conjunction with observations over the eastern United States from the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign during summer 2004. The model best captures the observed boundary layer concentrations of organic nitrates and their correlation with ozone using a 4% yield of isoprene nitrate production from the reaction of isoprene hydroxyperoxy radicals with NO, a recycling of 40% NOx when isoprene nitrates react with OH and ozone, and a fast dry deposition rate of isoprene nitrates. Simulated boundary layer concentrations are only weakly sensitive to the rate of photochemical loss of the isoprene nitrates. An 8% yield of isoprene nitrates degrades agreement with the observations somewhat, but concentrations are still within 50% of observations and thus cannot be ruled out by this study. Our results indicate that complete recycling of NOx from the reactions of isoprene nitrates and slow rates of isoprene nitrate deposition are incompatible with the observations. We find that ∼50% of the isoprene nitrate production in the model occurs via reactions of isoprene (or its oxidation products) with the NO3 radical, but note that the isoprene nitrate yield from this pathway is highly uncertain. Using recent estimates of rapid reaction rates with ozone, 20–24% of isoprene nitrates are lost via this pathway, implying that ozonolysis is an important loss process for isoprene nitrates. Isoprene nitrates are shown to have a major impact on the nitrogen oxide (NOx = NO + NO2) budget in the summertime U.S. continental boundary layer, consuming 15–19% of the emitted NOx, of which 4–6% is recycled back to NOx and the remainder is exported as isoprene nitrates (2–3%) or deposited (8–10%). Our constraints on reaction rates, branching ratios, and deposition rates need to be confirmed through further laboratory and field measurements. The model systematically underestimates free tropospheric concentrations of organic nitrates, indicating a need for future investigation of the processes controlling the observed distribution.