Atmospheric Environment (v.41, #20)

Bay Region Atmospheric Chemistry Experiment (BRACE) by Thomas Atkeson; Holly Greening; Noreen Poor (4163-4164).

The National Oceanic and Atmospheric Administration's (NOAA's) Air Resources Laboratory (ARL), using the NOAA Twin Otter aircraft, made meteorological and chemical measurements during 21 flights in May 2002 in and around the Tampa Bay, Florida area as part of the Bay Region Atmospheric Chemistry Experiment (BRACE). One or more vertical profiles were flown during each flight both over land and over the Gulf of Mexico. NOAA's Environmental Technology Laboratory (ETL; now part of NOAA's Earth System Research Laboratory (ESRL)) deployed three surface-based 915-MHz radar wind profilers equipped with radio acoustic sounding systems (RASS) at Ruskin, Sydney, and St. Petersburg. The National Weather Service Office in Tampa (NWS/TBW) released rawinsondes twice daily from the Ruskin site. The measurements of temperature, dew point, potential temperature, ozone, and condensation nuclei acquired during the aircraft profiles are analyzed, and in combination with the profiler and sounding data, are used to determine the structure of the boundary layer over the Tampa Bay region and the temporal and spatial changes that occurred in that structure during representative flights.
Keywords: Boundary layer; Vertical profile; Temperature;

The NOAA Twin Otter and its role in BRACE: Platform description by Winston T. Luke; Jeffrey R. Arnold; R. Laureen Gunter; Thomas B. Watson; Dennis L. Wellman; Purnendu K. Dasgupta; Jianzhong Li; Daniel Riemer; Paul Tate (4177-4189).
This paper describes in detail the platform and equipment used to make airborne measurements as part of the Bay Regional Atmospheric Chemistry Experiment (BRACE). A De Havilland DHC-6 Twin Otter, operated by the National Oceanic and Atmospheric Administration's Aircraft Operations Center, was used to measure chemical and meteorological parameters during the BRACE field intensive in May, 2002. The Twin Otter flew more than 90 h on 24 missions during BRACE, measuring a suite of positional (latitude, longitude, altitude, azimuth, roll, pitch, velocity), meteorological (temperatures, pressure, 2D wind fields, dew point, UV radiation) and chemical/physical (O3, CO, SO2, CO2, NO, NO2, NO Y , HNO3, CN, HCHO, H2O2, PAN, NMHCs, aerosol ionic composition, aerosol size) parameters. This manuscript describes the chemical and meteorological measurement systems, calibration procedures, and instrument performance specifications. Companion papers in this special issue present an overview and summary of results from the aircraft flights.
Keywords: Trace gas measurement; Aircraft; Oxides of nitrogen; Ozone; Aerosols; Air quality;

The NOAA Twin Otter and its role in BRACE: A comparison of aircraft and surface trace gas measurements by Winston T. Luke; Jeffrey R. Arnold; Thomas B. Watson; Purnendu K. Dasgupta; Jianzhong Li; Keith Kronmiller; Benjamin E. Hartsell; Thomas Tamanini; Clemente Lopez; Clark King (4190-4209).
A DeHavilland DHC-6 Twin Otter, operated by the National Oceanic and Atmospheric Administration, was deployed in Tampa, FL to measure aerosols and primary and secondary trace gases in support of the Bay Regional Atmospheric Chemistry Experiment (BRACE). The Twin Otter repeatedly overflew the surface chemistry monitoring super site near Sydney, FL to assess the comparability of surface and airborne datasets and the spatial representativeness of the surface measurements. Prior to comparing the chemical datasets, we evaluated the comparability of the standards used to calibrate surface and airborne detectors, as well as the uniformity of wind fields aloft and at the surface. Under easterly flow, when the dearth of significant upwind emission sources promoted chemical homogeneity at Sydney, trace gas concentrations at the surface and aloft were generally well correlated; R 2 ranged from 0.4396 for H2O2 to 0.9738 for O3, and was typically better than 0.70 for NO, NO2, NO Y , HNO3, HCHO, and SO2. Mean ratios of aircraft-to-surface concentrations during 10 overflights of Sydney were as follows: 1.002±0.265 (NO), 0.948±0.183 (NO2), 1.010±0.214 (NO Y ), 0.941±0.263 (HCHO), and 0.952±0.046 (O3). Poorer agreement and larger variability in measured ratios were noted for SO2 (1.764±0.559), HNO3 (1.291±0.391), and H2O2 (1.200±0.657). Under easterly flow, surface measurements at Sydney were representative of conditions over horizontal scales as large as 50 km and agreed well with airborne values throughout the depth of the turbulently mixed boundary layer at mid-day. Westerly flow advected the Tampa urban plume over the site; under these conditions, as well as during transitional periods associated with the development of the land–sea breeze, surface conditions were representative of smaller spatial scales. Finally, we estimate possible errors in future measurement-model comparisons likely to arise from fine scale (or subgrid;<2 km) variability of trace gas concentrations. Large subgrid variations in concentration fields were observed downwind of large emission point sources, and persisted across multiple model grid cells (distances>4 km) in coherent plumes. Variability at the edges of the well-mixed urban plume, and at the interface of the land–sea breeze circulation, was significantly smaller. This suggests that even a failure of modeled wind fields to resolve the sea breeze return can induce moderate, but not overwhelming, errors in simulated concentration fields and dependent chemical processes.
Keywords: Oxides of nitrogen; Ozone; Air quality; Atmospheric chemistry; Aircraft; Meteorology;

Field test of four methods for gas-phase ambient nitric acid by J.R. Arnold; Benjamin E. Hartsell; Winston T. Luke; S.M. Rahmat Ullah; Purnendu K. Dasgupta; L. Greg Huey; Paul Tate (4210-4226).
Three semi-continuous methods for detecting nitric acid (HNO3) were tested against the annular denuder + filter pack (ADS) integrated collection technique at the Tampa Bay Regional Atmospheric Chemistry Experiment (BRACE) Sydney research station ∼ 20 km downwind of the Tampa, Florida, urban core. The semi-continuous instruments included: two slightly differing implementations of the NO Y - NO Y * (total oxides of nitrogen minus that total denuded of HNO3) denuder difference technique, one from the NOAA Air Resources Lab (ARL), and one from Atmospheric Research and Analysis, Inc. (ARA); the parallel plate wet diffusion scrubber + online ion chromatography technique from Texas Tech University (TTU); and the chemical ionization mass spectrometer from the Georgia Institute of Technology (GIT). Twelve hour ADS samples were collected by the University of South Florida (USF). Results for 10 min samples computed from the various higher sampling frequencies of each semi-continuous instrument showed good agreement ( R 2 > 0.7 ) for afternoon periods of the highest production and accumulation of HNO3. Further, agreement was within ± 30 % for these instruments even at HNO3 concentrations < 0.30 ppb . The USF ADS results were biased low, however, by 44%, on average, compared to the corporate 12 h aggregated means from the semi-continuous methods, and by > 60 % for the nighttime samples; ADS results were below the corporate mean maximum HNO3 concentration by > 30 % as well. The four instruments using semi-continuous methods, by contrast, were all within 10% of each other's 12 h mean mixing ratios. While only ARA employed a formal minimum detection limit at 0.050 ppb, error analysis with the other techniques established that at the same level of precision, TTU's effective limit was approximately the same as ARA's and that ARL's limit was 0.030 ppb; analysis for GIT showed no apparent effective limit at the levels of HNO3 encountered in this field study. The importance of sample inlet height for HNO3 measurements was indirectly shown through comparison to previous field work at this site when sample inlet heights ranged from 1.5–10 m and produced systematic discrepancies in HNO3 concentrations correlated with height of more than a factor of 2.
Keywords: Instrument intercomparison; NO Y denuder difference; Parallel plate diffusion scrubber; Chemical ionization mass spectrometry; Nitrogen dioxide interference;

As part of the BRACE 2002 May field intensive, the NOAA Twin Otter flew 21 missions over terrestrial, marine, and mixed terrestrial and marine sites in the greater Tampa, Florida, airshed including over Tampa Bay and the Gulf of Mexico. Aerosols were collected with filter packs and their inorganic fractions analyzed post hoc with ion chromatography. Anion mass dominated both the fine- (particle diameters ⩽2.5 μm) and coarse-mode (particle diameters 10.0–2.5 μm) inorganic fractions: SO4 2−in the fine fraction, 3.7 μg m−3 on average and Cl and NO3 in the coarse fraction, 0.6 μg m−3 on average and 1.4 μg m−3 on average, respectively. Ammonium ion dominated the inorganic fine-mode cation mass, averaging 1.2 μg m−3, presumably in association with SO4 2. Coarse-mode cation mass was dominated by Na+, but the concentrations of Ca2+ and K+ together often equaled or exceeded the Na+ mass which was, on average, 0.6 μg m−3. Nitrate appeared predominantly in the coarse rather than the fine fraction, as expected, and the fine fraction never contributed >15% of the total NO3 concentration. Nitric acid dominated the NO3 contribution from both aerosol size fractions, and constituted at least 45% of the total NO3 in all samples. Coarse-mode Cl depletion, and hence NO3 replacement, reached 100% within the first 4 h of plume travel from the urban core in some samples, although it was most often less than 100% and slightly below the expected 1:1 ratio with coarse-mode NO3 concentration: the slope of the regression line of NO3 concentration to Cl depletion was 0.9 in the coarse fraction. In addition, terrestrial samples were markedly lower in Cl depletion, and thus in substituted NO3 , than were marine and mixed samples: 15–25% depletion in terrestrial samples vs. 50–65% in marine samples with the same air mass age. Thus, we conclude that NO3 and its progenitor compound HNO3 were present in the Tampa airshed in insufficient amounts to titrate fully the slightly alkaline coarse-mode particles there, and to replace completely the Cl from the coarse-mode NaCl.
Keywords: Sea salt; Chloride depletion; Nitrate limitation; Aerosol constituents; Coastal ecosystems;

Conversion of sea salt aerosol to NaNO3 and the production of HCl: Analysis of temporal behavior of aerosol chloride/nitrate and gaseous HCl/HNO3 concentrations with AIM by Purnendu K. Dasgupta; Scott W. Campbell; Rida S. Al-Horr; S.M. Rahmat Ullah; Jianzhong Li; Carlo Amalfitano; Noreen D. Poor (4242-4257).
Equilibrium modeling predicts that atmospheric sea salt can partition gas-phase HNO3 to solid- or aqueous-phase NaNO3. One month of semi-continuous and simultaneous measurements of particulate chloride and nitrate and gaseous HCl and HNO3 concentrations were made in the Tampa, Florida, as part of the Bay Region Atmospheric Chemistry Experiment (BRACE). Tampa's proximity to coastal and bay waters enriches its atmosphere with sea salt. To help explain and interpret the observed time-dependent concentration and gas-to-particle phase partitioning behavior for the NaCl–HNO3 reaction, we applied the Aerosol Inorganics Model III (AIM) to the measurement data. Good agreement between model predictions and observations was found. Measurement and modeling results suggested that coarse-mode sea salt particles from the Atlantic Ocean arrived in the morning at the monitoring site when relative humidity (RH) was high and the nature of the equilibrium least favored the outgassing of HCl from the particles. As the RH dropped in the afternoon, the equilibrium favored outgassing of HCl and the particulate nitrate concentration increased even as the concentration of coarse particles decreased. This effect was tied to the change in the ratio of nitrate to chloride activity coefficients γ NO 3 - / γ Cl - with RH. AIM simulations indicated that this ratio approached unity at high RH but could take on small values (∼0.05) at the lowest RH observed here. Thus, the particle phase slightly favored nitrate over chloride at high RH and greatly favored it at lower RH. Modeling revealed how diurnal changes in RH can rapidly shift HNO3 concentrations from gas- to particle-phase and thus affect the distance over which nitrogen is transported.
Keywords: Parallel plate wetted denuder; Gas-to-particle phase partitioning; Thermodynamic modeling; Supermicron particles;

Atmospheric production of oxalic acid/oxalate and nitric acid/nitrate in the Tampa Bay airshed: Parallel pathways by P. Kalyani Martinelango; Purnendu K. Dasgupta; Rida S. Al-Horr (4258-4269).
Oxalic acid is the dominant dicarboxylic acid (DCA), and it constitutes up to 50% of total atmospheric DCAs, especially in non-urban and marine atmospheres. A significant amount of particulate H2Ox/oxalate (Ox) occurred in the coarse particle fraction of a dichotomous sampler, the ratio of oxalate concentrations in the PM10 to PM2.5 fractions ranged from 1 to 2, with mean±sd being 1.4±0.2. These results suggest that oxalate does not solely originate in the gas phase and condense into particles. Gaseous H2Ox concentrations are much lower than particulate Ox concentrations and are well correlated with HNO3, HCHO, and O3, supporting a photochemical origin. Of special relevance to the Bay Region Atmospheric Chemistry Experiment (BRACE) is the extent of nitrogen deposition in the Tampa Bay estuary. Hydroxyl radical is primarily responsible for the conversion of NO2 to HNO3, the latter being much more easily deposited. Hydroxyl radical is also responsible for the aqueous phase formation of oxalic acid from alkenes. Hence, we propose that an estimate of •OH can be obtained from H2Ox/Ox production rate and we accordingly show that the product of total oxalate concentration and NO2 concentration approximately predicts the total nitrate concentration during the same period.
Keywords: Oxalate; Oxalic acid; BRACE; Nitrogen deposition; Photochemical production;

Application of CALINE4 to roadside NO/NO2 transformations by Kerstin L. Kenty; Noreen D. Poor; Keith G. Kronmiller; William McClenny; Clark King; Thomas Atkeson; Scott W. Campbell (4270-4280).
The CALINE4 roadway dispersion model has been applied to concentrations of NO x and NO2 measured near Gandy Boulevard in Tampa, FL (USA) during May 2002. A NO x emission factor of 0.86 gr mi−1 was estimated by treating NO+NO2 (NO x ) as a conserved species and minimizing the differences between measured and calculated NO x concentrations. This emission factor was then used to calculate NO2 concentrations using the NO/NO2 transformation reactions built into CALINE4. A comparison of measured and calculated NO2 concentrations indicates that for ambient O3 concentrations less than 40 ppb the model under-predicts the chemical transformation of NO. The enhanced transformation of NO may be due to reactions of NO with oxidants such as peroxy radicals that are present either in the atmosphere or in vehicle exhaust.
Keywords: NO x ; Emissions; Roadway; CALINE4;

Estimation of the particle and gas scavenging contributions to wet deposition of organic nitrogen by Silvia M. Calderón; Noreen D. Poor; Scott W. Campbell (4281-4290).
We estimated the wet deposition flux of dissolved organic nitrogen (DON) to Tampa Bay, Florida, using 24-h integrated aerosol and rainwater samples collected simultaneously on days with rainfall between July and September 2005. In rainwater, dissolved inorganic nitrogen (DIN) and DON average concentrations were 54.7±44.0 and 4.7±2.7 μM-N, respectively, and DON represented 8.9±5.8% of the total dissolved nitrogen (TDN=DIN+DON). Our estimates of wet deposition fluxes for NH 4 + , NO 3 - and DON were 1.40, 3.18 and 0.34 kg-N ha−1  yr−1, respectively. In aerosols (PM10), DIN and DON concentrations were 78.5±56.2 and 6.3±2.6 nmol m−3, respectively, and DON represented 10.3±7.3% of TDN. Particle scavenging rates were calculated assuming a lognormal size distribution for particles and lognormal, gamma and Marshall–Palmer size distributions for raindrops. For the range of precipitation rates and measured aerosol concentrations, below-cloud scavenging of aerosol-phase DON contributed only 1±0.7% to rainwater N concentrations. Dimethylamine (DMA) was observed in aqueous extracts of fine (PM2.5) and coarse (PM10−2.5) aerosol samples, but could be quantitatively measured only in fine particles with an average concentration of 688±615 pmol-N m−3, representing an average contribution of 12.8±6.7% of the total DON concentration measured in the same particle fraction. AIM 2 vapor–liquid equilibrium modeling predicted an average gas-phase DMA concentration of 107.4±176.9 pmol-N m−3. Although DMA concentrations were below our analytical detection limit on all rainwater samples, the average modeled gas and particle DMA contribution to DON concentration in rainwater was 0.4±0.7%.
Keywords: Aliphatic amines; Ammonia; Nitric acid; Dimethylamine; Size distribution;

Modeling air/sea flux parameters in a coastal area: A comparative study of results from the TOGA COARE model and the NOAA Buoy model by Kristin Sopkin; Connie Mizak; Sherryl Gilbert; Vembu Subramanian; Mark Luther; Noreen Poor (4291-4303).
Because estuaries and coastal regions are particularly susceptible to nutrient over-enrichment due to their close proximity to source-rich regions, a goal of the BRACE study was to improve estimates of nitrogen air/sea transfer rates in the Tampa Bay Estuary. Our objective was to critically evaluate two air/sea gas exchange models to determine their efficacy for use in a coastal region, with the ultimate goal of improving nitrogen exchange estimates in Tampa Bay. We used meteorological data and oceanographic parameters collected hourly at an instrumented tower located in Middle Tampa Bay, Florida. The data was used to determine the friction velocity and the turbulent flux of heat and moisture across the air/sea interface and then compared with modeled parameters at the same offshore site. On average both models underpredicted sensible heat flux and there was considerable scatter in the data during stable conditions, indicating that nitrogen gas exchange rates may also be underestimated. Model improvement, however, was observed with friction velocity comparisons. Model inter-comparisons of sensible heat flux and friction velocity suggest excellent agreement between the TOGA COARE and the NOAA Buoy models, but model estimated heat transfer coefficients and latent heat fluxes did not agree as well. Based on our analysis, we conclude that both models are suitable for use in a coastal environment to estimate nitrogen air/sea gas exchange, although the NOAA Buoy model requires fewer meteorological inputs. However, if the purpose is to conduct more sophisticated microscale modeling of air/sea interactions, we recommend the TOGA COARE model.
Keywords: Air/sea interface; Monin–Obukhov similarity theory; Tampa Bay; Turbulent fluxes; Ultrasonic anemometer;

Effect of shoreline meteorological measurements on NOAA Buoy model prediction of coastal air–sea gas transfer by Connie Mizak; Scott Campbell; Kristin Sopkin; Sherryl Gilbert; Mark Luther; Noreen Poor (4304-4309).
The NOAA Buoy model is currently used to estimate the air–sea transfer rates of highly soluble gases over coastal water bodies, such as Tampa Bay, using offshore meteorological measurements. Since a goal of the BRACE study was to improve estimates of nitrogen deposition over Tampa Bay, our objective was to investigate if the model accurately predicts gas transfer when shoreline input data are used in lieu of offshore measurements. To accomplish this objective, we compared over-water measurements of sensible heat with NOAA Buoy model predictions using both offshore and shoreline meteorology. In the summer months, the apparent daytime influence of land surface heating on air temperature produces a higher air than water temperature at the shoreline. For the NOAA Buoy model, this yields stable atmospheric conditions and thus under-predicts the over-water exchange rates for a shallow estuary. If the data records are removed from the model for periods when air temperature is 4.8 K greater than the water temperature, the shoreline and over-water transfer rates are in reasonable agreement.
Keywords: Bulk transfer theory; Coastal waters; Dry deposition; Gas exchange; Shoreline measurements;

Influence of air mass origin on the wet deposition of nitrogen to Tampa Bay, Florida—An eight-year study by Hillary Strayer; Ronald Smith; Connie Mizak; Noreen Poor (4310-4322).
Rainfall delivers on the average ∼ 10 % of the total annual nitrogen load directly to Tampa Bay, based on precipitation monitoring at a National Atmospheric Deposition Program (NADP) Atmospheric Integrated Research Monitoring Network (AIRMoN) site located adjacent to Tampa Bay in urban Tampa. We coupled the chemical analyses for 606 daily precipitation samples collected from 1996 to 2004 with corresponding air mass trajectory information to investigate if wet-deposited nitrogen originated from near versus removed source regions. Air mass trajectories were obtained using the National Oceanic and Atmospheric Administration (NOAA) HYbrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model, and were classified into six trajectory categories by the direction of their approach to Tampa Bay. Rainfall nitrate and ammonium concentrations were significantly lower for over-water air mass trajectories than for over-land trajectories as expected, but contributed to 40% of the total wet-deposited nitrogen, a likely consequence of the higher frequency of rain events for these trajectories. Average rainfall nitrate concentrations were significantly higher for air masses that stagnated over the urbanized bay region. We estimated that local sources contributed 1 kg N ha - 1 yr - 1 or 25% of the total inorganic nitrogen wet-deposited to Tampa Bay.
Keywords: Nitrate; Ammonium; Precipitation; Trajectory; HYSPLIT; Tampa Bay Estuary;

PM2.5 and size-segregated aerosols were collected in May 2002 as part of the Bay Regional Atmospheric Chemistry Experiment (BRACE), Florida, USA. Aerosol organic composition was used to estimate sources of a series of alkanes and polycyclic aromatic hydrocarbons (PAHs) using chemical indices, hierarchical cluster analysis (HCA) and a chemical mass balance receptor model (CMB). Aerosols were collected on quartz fiber filters (QFF) using a PM2.5 high volume sampler and on aluminum foil discs using a Micro-Orifice Uniform Deposit Impactor (MOUDI, 50% aerodynamic cut diameters were 18, 10, 5.6, 3.2, 1.8, 1.0, 0.56, 0.315 and 0.171 μm). Target compounds included alkanes and PAHs and were solvent extracted using a mixture of dichloromethane, acetone and hexane, concentrated and then analyzed using a gas chromatograph/mass spectrometer (GC/MS). The target compounds in PM2.5 were dominated by six sources during the study period: mobile sources (39±5%), coal burning (33±5%), biogenic primary emission (20±2%), oil combustion (5±2%), biomass burning (1.0±0.3%) and an unidentified source (3±2%). Results obtained from the chemical indices, HCA and CMB were in very good agreement with each other. PAH size distributions are presented for days dominated by a same source. Seventy-five percent and 50% of the PAH were found below 1.8 and 0.56 μm, respectively (monthly PAH geometric diameters averaged 0.43 μm). Coarse size PAHs were observed on 1 day (15 May) and were correlated with nitrate and sodium size distribution. It is hypothesized that the PAHs, sodium and nitrate were internally mixed and that the PAHs deposited onto a pre-existing marine aerosol. This transfer process has significant implications for PAH deposition and lifetime and warrants further study.
Keywords: MOUDI; Gas chromatograph; Mass spectrometer; PAH; CMB;