In August to September 1995 a field experiment was conducted over the Gulf of Maine and Bay of Fundy to study the radiative forcing of pollution aerosols. The chemical and physical characteristics of two pollution cases were studied in detail in contrast to a clean atmosphere case. In the pollution cases, NH4++SO4= showed a unimodal distribution with a peak at 0.24 mu m diameter. It was dominant among identified chemical components, including inorganic ions and organic compounds. However, the identified components were only about 1/3 of the aerosol mass as determined from the physical measurements. The unidentified mass, with a large accumulation mode, was likely due to unmeasured organic matter. In the clean case, sea salt was the dominant species with a bimodal distribution. The results were used to calculate the direct backscatter coefficient beta(pi) at 0.532 and 1.064 mu m using the Mie theory for comparison with LIDAR observations to determine the contributions by the chemical components. In the dean case, the sea salt aerosols contributed about half of beta(pi). In the pollution cases, NH4++SO4= contributed 20-40% to beta(pi). The unidentified mass had contributions to beta(pi) of>40% and >70% for the two pollution cases. The LIDAR beta(pi) results were inverted to derive optical depths over the 300-2400 m altitude range. Using these optical depths, the direct backscattered fraction of radiative flux for the pollution aerosols was estimated to be about 5 times higher than aerosols in the clean atmosphere.

Nighttime measurements of particle number distribution and mass composition, and concentrations of NO2, O-3, NH3, and HNO3, were made at night at a rural site in Ontario in August of 1992. A simple model of particle growth was constructed to simulate the observed rapid growth of aerosol mass in the 0.2 to 0.5 mu m diameter range. Both measurements and model results indicate that the growth of accumulation mode aerosol mass was due to condensation of the nitrate radical, HNO3, and NH(3 )onto particles, with the formation of particle ammonium nitrate. The results show that the reaction of O(3 )with NO2 in the isolated nocturnal boundary layer can lead to the production of gas-phase nitric acid. When this occurs in the presence of local ammonia emissions and preexisting particles, rapid growth of particle ammonium nitrate takes place. The model results show that most of the observed variations can be accounted for by a coupled system of equations including dynamical and thermodynamic effects. The dynamical approach to equilibrium is sufficiently fast that the gas-phase nitric acid concentrations are more sensitive to the magnitude of the thermodynamic equilibrium concentration than the dynamical time constant. A simple parameterization for the effects of sulphate on particle nitrate formation was developed and shown to provide a good estimate of the equilibrium concentration of gas-phase nitric acid.

In September 1994 the chemical and physical properties of aerosol particles over southern California were characterized. Concentrations of particle chemical species were higher near the surface than at higher altitudes. In these particles, measured organic and inorganic water soluble masses were 7 +/- 1% and 75 +/- 12%, respectively, of the dried total particle mass (TPM). Non-sea-salt (nss) SO4=, NO3-, NH4+, and H+ were the major contributors to the mass and ionic equivalence of the particles, The ratio of the soluble organics (SumOrg) to nss SO4= was found to be a function of the TPM, which was similar to the simple relationship found over the northwestern Atlantic Ocean for altitudes <3 km. When the H+ ion was included, there was a good ionic balance between the cations and anions in the particles, The pH of the particles ranged from -2.4 to 0.20 (averaging -0.85). The particle volume size distribution dV/d log D shows distinct features according to the altitude and location of sample collection, In urban plumes, dV/d log D was significantly higher than at high altitudes and had a consistent accumulation mode peaking at 0.24 mu m. For all samples, nss SO4= and NH4+ comprised 59% of the accumulation mode particle mass on average. Samples showed two groups with high and low NH4+/nss SO4= ratios of 0.85 and 0.01, respectively. For the first group the low ambient relative humidities indicate that NH4+ and nss SO4= were in crystallized ammonium sulfate and letovicite in equilibrium with solution, while the second group was probably close to H2SO4 droplets. On the basis of these compositions the refractive index eta was found to be about 1.5 for the first group and <1.4 for the second group. The water contents of the NH4+ - nss SO4= particles were estimated to be 35 +/- 24%, leading to an average ratio of H2O/nss SO4= = 0.9 +/- 1.2.

Observations made at a ground site east of Vancouver, Canada, were used in a principal component analysis (PCA) to derive (1) the emission ratios (ER) SO2, NOx, HNO2, HNO3, ketones, and aldehydes relative to CO and (2) the photochemical production efficiencies (PPE) of O-3, HNO2, HNO3, PAN, and several ketones and aldehydes relative to the formation of NOz. It is shown that derived ER(SO2) and ER(NOx) are consistent with the mobile emission sources in the Greater Vancouver Regional District (GVRD) emission inventory. Combining the ER data for other species with the GVRD CO emission inventory, the daily emission rates for these species have been estimated, indicating significant sources for these species. The derived PPE values for O-3, HNO2, HNO3, PAN, the ketones, and the aldehydes are as follows: PPE(O-3) is approximately 8-11 depending on the time of day; PPE(acetone) is between 1 and 2; PPE(MVK), PPE(MACR), and PPE(MEK) are approximately the same between 0.1 and 0.2; PPE(HCHO) is between 1 and 2, but PPE(propanal) and PPE(butanal) are lower than PPE(HCHO) by a factor of 2-10. It is shown that the daily photochemical production rates of O-3 and the carbonyl species are approximately linear functions of the NOx daily emission rates. When the photochemical production rates are compared with counterpart daily emission rates, it is shown that for the organic compounds, the contributions from photochemistry were more important than daytime emissions; for HNO2, there is more emission contribution than photochemistry, opposite to the case for HNO3. (C) 1997 Elsevier Science Ltd.

Two aircraft, the National Research Council of Canada (NRCC) Convair 580 (CV580) and NRCC DHC-6 Twin Otter, along with the Yarmouth and Digby Ferries, a ground site near Yarmouth and coordination with satellite overpasses (AVHRR and LANDSAT) provided an exceptionally well rounded compliment of observing platforms to meet the project objectives for the Radiation, Aerosols and Cloud Experiment (RACE) (refer to http://www.on.doe.ca/armp/RACE/ RACE.html for a complete list of instrumentation and investigators involved). The general flight plans involved upwind measurements of a selected target by the CV580 lidar, followed by coincident flights allowing the Twin Otter to perform in-site measurements while the Convair used a variety of remote sensors from above. The CV580 then descended to perform in-site measurements including size segregated samples through the use of a micro-orifice uniform deposit impactor (MOUDI). This paper will focus on the airborne lidar results during RACE and in particular introduce two case studies comparing the lidar with a MOUDI impactor and ASASP particle probe using Mie theory.

A new technique for condensing detailed organic reactions into more compact forms is introduced, and compared to methods which have been used in the ADOM and RADM air quality models. The previous methods made use of integrated reactivity weighting, designed to predict the correct product concentrations on time scales of one to two days or longer. The latter methods are shown to be subject to high errors in the prediction of initial hydrocarbon product concentrations. These errors occur when the hydrocarbon chemistry is the most reactive. The new method is a hybrid scheme which uses the reaction rates at the initial time as a further constraint on the lumping. This results in improved prediction of product concentrations on shorter time scales while retaining the long-term accuracy of the earlier techniques. The hybrid reactivity weighting technique is shown to have higher accuracy than the earlier ADOM or RADM techniques in the regions of the most reactive chemistry, and is therefore very relevant to regional oxidant models.

Daily air measurements at one midcontinental and two coastal sites on the Atlantic Ocean and Pacific Ocean in Canada during April 1992 to June 1993 are used to show the seasonality of methanesulfonate (MSA), non-sea-salt (nss) SO4=, and SO2. At the Atlantic site, Kejimkujik National Park, Nova Scotia (44 degrees 22'N, 65 degrees 12'W), the concentration ranges were <0.002-0.13, 0.2-26, and <0.2-10 mu g m(-3) for MSA, nss SO4=, and SO,, respectively, and the annual means were 0.02, 2.2, and 1.3 mu g m(-3). MSA had a monthly median peak of 0.036 mu g m(-3) in June, versus an SO2 peak of 2.7 mu g m(-3) in February, and no discernible nss SO4= seasonal peak. The MSA/nss SO4= ratios were low compared to ratios in other parts of the world but exhibited a seasonality dominated by that in MSA. At the midcontinental site, Experimental Lake Area, Ontario (49 degrees 39'N, 93 degrees 43'W), the concentrations were usually lower than at the coastal sites. The ranges were <0.002-0.066, 0.04-12.5, and <0.16-16.6 mu g m(-3) for MSA, nss SO4=, and SO2, respectively, and the annual means were 0.01, 1.6, and 0.93 mu g m(-3). The monthly median seasonal peaks at this site were 0.01 mu g m(-3) for MSA in September, 2.1 mu g m for nss SO4= in March, and 2.8 mu g m(-3) for SO2 in January. The MSA/nss SO4= ratio at this site was similar to that found at Kejimkujik. At Saturna Island, British Columbia (48 degrees 47'N, 123 degrees 08'W), the ranges were 0.002-0.2, 0.14-6.4, and 0.12-21 mu g m(-3) for MSA, nss SO4=, and SO2, respectively, and the annual means were 0.06, 1.1, and 3.0 mu g m(-3). The monthly median peaks were 0.12 mu g m(-3) for MSA in August, 5 mu g m(-3) for SO2 in February, and a m nominal peak of 1.5 mu g m(-3) for nss SO4= in August. The MSA/nss SO4= ratio had a seasonal pattern dominated by that of MSA with a peak of 0.1 in August. These data should enhance the existing databases for testing global sulfur models.

Observations of aerosol chemistry and microphysics were made at Chebogue Point, Nova Scotia, from August 16 to September 8, 1993 as part of the North Atlantic Regional Experiment (NAPE) intensive. Most of the aerosols were classified into two groups according to the geometric mean volume diameter (D-gv) of the particles which contributed the greatest volume (sub-0.5 mu m) The group-1 aerosols; representing 33% of the data, are characterized by D-gv of 0.18-0.19 mu m; the group-2 aerosols, representing 50% of the data, are characterized by D-gv of 0.20-0.22 mu m; and the remaining aerosols bear similarities to either groups 1 or 2 but lie outside the D-gv ranges. The differences between these aerosol groups are consistent with the addition of sulfate to the group-2 aerosols via recent processing through cloud. Factors supporting this possibility include the presence of low marine stratus upwind of the site only on days when the group-2 aerosol was observed, the higher D-gv for the group-2 aerosols consistent with the observed size threshold for activation in these clouds, and the association of non-sea-salt SO4= (nssSO(4)(=)) with larger particle sizes for the group-2 aerosols. In general, the masses of the most abundant inorganic and organic ions, nssSO(4)(=) and oxalate, were associated with the main volume of the sub-0.5-mu m particles. Cloud condensation nucleus (CCN) concentrations active at 0.4% supersaturation (CCN0.4) were highly correlated with the concentrations of particles >0.01 mu m and oxalate and moderately correlated with nssSO(4)(=). Concentrations of CCN active at 0.06% supersaturation (CCN0.6) correlate very well with the concentrations of particles >0.19 mu m diameter. In the case of the recently cloud-processed aerosols, for which nssSO(4)(=) is more strongly associated with particles >0.19 mu m, the CCN0.06 also correlate well with nssSO(4)(=) CCN spectra computed using the measured size distributions and aerosol chemistry agree well with the measured CCN.

Airborne observations from 14 flights in marine stratus over the Gulf of Maine and Bay of Fundy in August and September of 1993 are examined for the relationships among the cloud droplet number concentrations (Nd), the out-of-cloud aerosol particle number concentrations (N-a), the major ion concentrations in the cloud water, and turbulence in cloud. There was a wide range of aerosol concentrations, but when low stratus and the main anthropogenic plume from eastern North America were in the same area the plume overrode the cloud. The N-d increased with increasing N-a and cloud water sulfate concentration (cwSO(4)(=)), but the relationships were very weak. The separation of the data between smooth and lightly turbulent air substantially improved the ability to explain the variance in the N-d by either of these two quantities. Also, the relative increase in N-d for increases in N-a and cwSO(4)(=) was greater for lightly turbulent air than for smooth air. The estimated minimum size of particles activated in these clouds ranged from 0.14 mu m to 0.31 mu m, corresponding to average supersaturations of <0.1%. The minimum size tended to be lower for lightly turbulent air and smaller N-a. The results for lightly turbulent air agree well with previously reported parameterizations of the impact of aerosols on N-d, but the results for smooth air do not agree. In general, more knowledge of the physical factors controlling the N-d in stratiform clouds, such as turbulence, is needed to improve not only our ability to represent N-d but also to increase our understanding of the impact of the aerosol particles on the N-d and climate.

Aerosol chemical and physical measurements were made at altitudes from 0.27 to 3 km near the coast of southern Nova Scotia, Canada, during the 1993 North Atlantic Regional Experiment. The volume distributions of aerosol with diameters between 0.005 and 3 mu m were dominated by accumulation mode particles. The mass and volume ratios (R(m) and R(v)) of the sum of soluble organics (SumOrg) to non-sea-salt (nss) SO4= were relatively invariant for estimated total particle mass (TPM) in excess of 13 mu g m(-3) (high TPM) but increased sharply with decreasing TPM below 13 mu g m(-3) (low TPM). Overall, the relationships between TPM and R(m) and R(v) were found to be R(m) = -(0.17 +/- 0.44) + (3.5 +/- 1.0)TPM-((0.69+/-0.39)) and R(v) = -(0.35 +/- 0.70) + (5.7 +/- 1.7)TPM-((0.68+/-0.39)). The high TPM aerosols originated in eastern North America and had average composition of 46% nssSO(4)(=), 8% NH4+, and 8% SumOrg. In contrast, low TPM aerosols were found to be of background continental or marine tropospheric origins and had average composition of 23% nssSO(4)(=), 9% NH4+, and 20% SumOrg. The aerosols in both TPM regimes were separated into two groups based on the mode of the volume distributions. The correlation between the mass of each species and the particle volume distribution was investigated for these groups.

Using a principal component analysis technique and data on atmospheric gases and aerosols at a rural site in Ontario, Canada from the Eulerian model evaluation field study (EMEFS), the Eulerian acid deposition and oxidant model (ADOM) is evaluated. Seventy-nine and 76% of the variances in the data and model output, respectively, are explained by three principal components. They are a chemically aged/transported component, a diurnal cycle component, and an area emission component, all characterized by their ratios of gases and temporal variation patterns. The ADOM component contributions to sulphur species are in general agreement with the EMEFS components, but with notable differences for key photochemical species including O-3. The temporal variations of the ADOM components are close to those of the EMEFS components. The EMEFS chemically aged/transported component shows a high degree of photochemical processing, with the ratios [NOx]/[TNOy]=0.3 and [O-3]/([TNOy]-[NOx])=9+/-1. The corresponding ADOM component predicts lower [NOx]/[TNOy] and [O-3]/([TNOy]-[NOx]) ratios, probably caused by a chemical mechanism in the model that is too fast, and lower contributions to O-3, NO2, TNO3, PAN, TNOy, and HCHO, probably caused by model grid dilution or lower model emissions. The EMEFS diurnal component owes its variance to the daily photochemistry and nighttime dry deposition of the chemical species. In comparison, the matching ADOM component underpredicts the ratio [O-3]/([TNOy]-[NOx]) and the NO2 consumption and O-3 production but overpredicts the contributions to the other species. The EMEFS emission component represents emissions from local/regional area sources. The corresponding ADOM component underpredicts TNOy by 44% and the fraction of TNOy as NOx compared to the EMEFS component, suggesting that the model has lower emissions of NOx and a photochemical mechanism that converts NOx faster than indicated by the EMEFS results.

Based on atmospheric measurements of multiple species at Egbert, a rural site in Ontario, Canada, during summer 1988, the emission ratios of HCHO/CO and HCHO/SIGMANO(y) for area sources and secondary production of HCHO have been estimated using a modified principal component analysis technique. The technique yields three principal components that represent a photochemically aged air mass, a diurnal cycle, and fresh area emissions. The area emission component has an emission ratio CO/SIGMANO(y) = 9 +/- 3 and SO2/CO = 0.005 +/- 0.003, in agreement with NAPAP area emission data for the eastern US [Buhr et al., 1992]. The emission ratios of HCHO/CO and HCHO/SIGMANO(y) in this component are 0.0056 +/- 0.0022 and 0.05 +/- 0.007, respectively. If these ratios are typical of eastern North American area emissions, the total primary HCHO emission for this region will be 8 x 10(9) moles HCHO annually based on the NAPAP CO emission inventories. Evidence of secondary HCHO production can be found in the photochemically aged component which has considerably higher HCHO/CO (0.016 +/- 0.004) and HCHO/SIGMANO(y) (0.29 +/- 0.03) ratios than the emission ratios. It is estimated that for every 1 ppb NO(x) converted to NO(y), 0.4 ppb HCHO are produced for the ratio (1-NO(x)/NO(y))<0.6; after which the relative HCHO production rate becomes smaller. Using this relative rate, the maximum total HCHO production over the eastern North America is estimated to be 1.3 x 10(11) moles year-1, or approximately 16 times that from primary emission.

Eight water-soluble organic anions were measured in 70 aerosol samples and 10 snow samples at Barrow, Alaska in March-April, 1989. The ranking of the ions in aerosols according to total (coarse+fine aerosol) median concentrations was acetate (44 ng m-3), oxalate (27), benzoate (23), formate (22), propionate (6), methanesulfonate (5), lactate (4), and pyruvate (4). When added up, the median organic anion mass was 156 ng m-3. The organic anions/nssSO4= mass ratio had a median of 0.18 and 0.07 in the coarse (>l mum) and fine (<1 mum) size fractions, respectively, but can be very high on occasions. On average, the organic anions made up more than 10% of the water-soluble aerosol mass. A similar ranking in concentration was also found for the organic ions in the snow pack samples. The organic anion/nssSO4= mass ratio in these samples was >0.5, substantially higher than in aerosols.

In six aircraft flights of AGASP-II, 2-15 April 1986, from ca. 1300-8100 m altitude, the most abundant elements measured in size separated aerosol samples were silicon, chlorine, and sulfur. Concentrations were higher than at ground level (G), particularly at highest altitudes (HT, 5600-8100 m, upper troposphere to lower stratosphere) compared to mid troposphere (MT, 1300-4700m), especially for ultrafine particles <0.0625 mu m aerodynamic diameter. HT and MT median and G average concentrations, mu g m(-3) STP, respectively (1) Si = 3.64, 1.30, 0.092; (2) S = 1.44, 0.265, 0.087; (3) Cl=1.62, 0.36, 0.213. The weight ratio Al/Si was less than half that expected for Earth crust material (0.3), evidence against fine silicon originating mainly by dispersion of volcanic debris or other eolian dust particles. Instead, pollution from high rank (mainly bituminous) coal combustion, which can form SiO vapors from quartz in the ash and fine alkaline aerosol with low Al/Si ratio, is a more likely source of apparently widespread aerosol silicon contamination of the Arctic atmosphere. Chlorine and sulfur gases may be scavenged by coarse alkaline dust particles and acidic chlorine and sulfur may be derived from coal combustion processes, thus also accounting for their high concentrations.

Surface ozone, particulate bromine and inorganic and organic gaseous bromine species were measured at Barrow, AK, during March and April 1989 to examine the causes of surface ozone destruction during the arctic spring. Satellite images of the Alaskan Arctic taken during the same period were also studied in conjunction with calculated air mass trajectories to Barrow to investigate the possible origins of the ozone-depleted air. It was found that during major ozone depletion events (O-3 < 25 ppbv) concentrations of particulate bromine and the organic brominated gases bromoform and dibromochloromethane were elevated. Air mass trajectories indicated that the air had crossed areas of the Arctic Ocean where leads had been observed by satellite. The transport time from the leads was typically a day or less, suggesting a fast loss mechanism for ozone. A similarly fast production of particulate bromine was shown by irradiating ambient nighttime air in a chamber with actinic radiation that approximated daylight conditions. Such rapid reactions are not in keeping with gas-phase photolysis of bromoform, but further studies showed evidence for a substantial fraction of organic bromine in the particulate phase; thus heterogeneous reactions may be important in ozone destruction.

Interpretation of simultaneous measurements at three stations in different parts of the Arctic suggests that during winter air masses are forced into the Arctic from Eurasia in a surge towards Alaska and further return over the North Pole towards the European Arctic. On some occasions direct flow of the Eurasian air masses was detected in the European Arctic. Simple statistical methods and dispersion modelling proved useful in studying source-receptor relationships in the Arctic.

In the spring of 1986 and 1989, particle nitrite was measured at Barrow, Alaska, by filter sampling and by ion chromatographic analysis. Particle nitrite concentrations averaged 2.9 +/- 3.4 and 2.6 +/- 2.0 ppt (molar ratio) in 1986 and 1989, respectively. Both seasons showed diurnal variations with higher concentrations during the day which might have been caused by daytime down mixing. In 1989, nitrite was determined in several snow samples with concentrations between 0 and 0.18 mu mol l(-1). Particle nitrite was probably in disequilibrium with gas phase, suggesting a heterogeneous source for gaseous HONO. A relationship between particle nitrite and sodium ions suggests that sea salt could be involved in nitrite formation, perhaps through hydrolysis of nitryl halides.

Measurements in the Arctic troposphere over several years show that MSA concentrations in the atmospheric boundary layer, 0.08-6.1 parts per trillion (ppt, molar mixing ratio), are lower than those over mid-latitude oceans. The seasonal cycle of MSA at Alert, Canada (82.5 degrees N, 62.3 degrees W), has two peaks of 6 ppt in March-April and July-August and minima of 0.3 ppt for the rest of the year. At Dye 3 (65 degrees N, 44 degrees W) on the Greenland Ice Sheet, a similar seasonal MSA cycle is observed although the concentrations are much lower with a maximum of 1 ppt. Around Barrow, Alaska (71.3 degrees N, 156.8 degrees W), MSA is between 1.0 and 25 ppt in July, higher than 1.5+/-1.0 ppt in March-April. The mid-tropospheric MSA level of 0.6-1 ppt in the summer Arctic is much lower than about 6 ppt in the boundary layer. Al Alert, the ratio of MSA to non-sea-salt (nss) SO42- ranges from 0.02 to 1.13 and is about 10 times higher in summer than in spring. The summer ratios are higher than found over mid-latitude regions and, when combined with reported sulfur isotope compositions from the Arctic, suggest that on average a significant fraction (about 16-23%) of Arctic summer boundary layer sulfur is marine biogenic. The measurements show that the summer Arctic boundary layer has a significantly higher MSA/nss-SO42- ratio than aloft.