Aerosol NO3 Research Articles

Regional aerosol chemistry of the Amazon Basin during the dry season

We report here a set of measurements which describe the distribution and chemical composition of the atmospheric aerosol over the tropical rain forest of the Amazon Basin, in Brazil, during the 1985 July–August dry season. Our data suggest that there is remarkable compositional and spatial homogeneity of the atmospheric aerosol on an extensive regional scale. We describe the aerosol regime associated with emissions from the moist tropical forest during early to middle dry season conditions. The moist forest ecosystem is a source of aerosols, composed of material directly released by vegetation, together with species originating from biogenic emissions of various reduced atmospheric gases. Particulate organic carbon is the dominant component of the atmospheric aerosol, exhibiting an average concentration of 740±190 nmol C m−3 in the mixed layer and 220±60 nmol C m−3 in free tropospheric air. On the average the atmospheric concentration of aerosol constituents was enhanced in mixed layer air fivefold over free tropospheric levels. Oxalate and SO42− exhibited the greatest enrichment in the mixed layer, while Cl− showed essentially no enrichment in mixed layer aerosols. For many constituents their enhancement in mixed layer air was greatest in the aerosol coarse fraction, exceeding a factor of 10 in some cases. Approximately 75% of total aerosol NO3− in the mixed layer was contained in the coarse atmospheric aerosol. The atmospheric aerosol in the Amazonian atmosphere is essentially acid‐base neutral, primarily as a result of incorporation Of NH4+, presumably derived from NH3 released by the forest ecosystem. Ammonium ion is the principal ionic constituent of the atmospheric aerosol, being present at a concentration typically more than twice that of other major ionic species. The biospheric‐atmospheric coupling of emissions of NH3 and reduced sulfur gases by the forest ecosystem, and subsequent incorporation of atmospheric NH3 into acidic aerosols produced by oxidation of reduced atmospheric sulfur gases, is an important aspect of biogeochemical cycling to be pursued in future studies.

Vertical distribution of dimethylsulfide, sulfur dioxide, aerosol ions, and radon over the Northeast Pacific Ocean

Dimethylsulfide (DMS), sulfur dioxide (SO2), methanesulfonate (MSA), nonsea-salt sulfate (nss-SO4 2−), sodium (Na+), ammonium (NH4 +), and nitrate (NO3 −) were determined in samples collected by aircraft over the open ocean in postfrontal maritime air masses off the northwest coast of the United States (3–12 May 1985). Measurements of radon daughter concentrations and isentropic trajectory calculations suggested that these air masses had been over the Pacific for 4–8 days since leaving the Asian continent. The DMS and MSA profiles showed very similar structures, with typical concentrations of 0.3–1.2 and 0.25–0.31 nmol m−3 (STP) respectively in the mixed layer, decreasing to 0.01–0.12 and 0.03–0.13 nmol m−3 (STP) at 3.6 km. These low atmospheric DMS concentrations are consistent with low levels of DMS measured in the surface waters of the northeastern Pacific during the study period. The atmospheric SO2 concentrations always increased with altitude from <0.16–0.25 to 0.44–1.31 nmol m−3 (STP). The nonsea-salt sulfate (ns-SO4 2−) concentrations decreased with altitude in the boundary layer and increased again in the free troposphere. These data suggest that, at least under the conditions prevailing during our flights, the production of SO2 and nss-SO4 2− from DMS oxidation was significant only within the boundary layer and that transport from Asia dominated the sulfur cycle in the free troposphere. The existence of a ‘sea-salt inversion layer’ was reflected in the profiles of those aerosol components, e.g., Na+ and NO3 −, which were predominantly present as coarse particles. Our results show that long-range transport at mid-tropospheric levels plays an important role in determining the chemical composition of the atmosphere even in apparently ‘remote’ northern hemispheric regions.

Seasonal and diurnal variability of nitric acid vapor and ionic aerosol species in the remote free troposphere at Mauna Loa, Hawaii

Measurements of nitric acid vapor and aerosol nitrate, sulfate, and ammonium were made at the Mauna Loa Observatory on Hawaii between October of 1984 and January of 1986 during 10‐day periods every 2 months. Nitric acid exhibited both a diurnal cycle (higher concentrations during the upslope wind hours of 0900 to 2100 LT) and a seasonal modulation (highest concentrations during the late summer). In August the daytime/nighttime average HNO3 concentrations were 97/79 parts per trillion by volume (pptv), while in February they were only 18/8 pptv. The ratio of summer/winter average HNO3 concentrations was 8. Dry deposition at the surface may be a major factor causing the diurnal variation, causing the measured surface values to be lower than the concentrations in the free troposphere above the surface. The long‐range transport of anthropogenic odd nitrogen from the west coast of North America is a likely cause of the higher summertime nitric acid concentrations. This suggests that there may be regular periods during which apparently “short‐lived” anthropogenic trace gases may be transported to remote parts of the atmosphere with relatively high efficiency. Aerosol NO3−, SO42−, and NH4+ also showed seasonal variations: summer/winter ratios were 3, 2, and 0.5, respectively.

Chemical and microphysical studies of nonprecipitating summer cloud in Ontario, Canada

During the fall of 1981 and the summer of 1982, airborne and ground‐level field studies were conducted in southern and central Ontario to investigate the processes leading to cloud water acidification. Aircraft measurements were made of aerosol particles, cloud droplets, and cloud liquid water content. Cloud water samples were collected in nonprecipitating cumulus and stratocumulus clouds and analyzed for the concentrations of SO4=, NO3−, Cl−, Na+, NH4+, Ca++, Mg++, and H+. Aerosol composition was studied at a rural location on the ground. The ground‐level aerosol composition was compared with the cloud water composition. Observations of the vertical variation of the aerosol particle size distribution were used to justify this procedure, but it was also assumed that the relative fraction of submicron aerosol SO4= did not vary between the ground and cloud base during individual flights. In this way the cloud water SO4= was explained by the nucleation of water droplets on sulphate aerosol particles in at least nine of 11 cases studied. In several of the cases the concentration of NO3− measured in cloud water samples was much higher than could be accounted for by the measurement of below‐cloud aerosol NO3− plus gaseous HNO3. Similarly, concentrations of Ca++ found in the cloud water far exceeded concentrations measured in the aerosol.

Particulate sulfate and nitrate in the boundary layer over the North Pacific Ocean

Concentrations of aerosol NO3− and non‐sea‐salt (nss) SO4= have been measured in samples collected weekly between January 1981 and March 1982 at seven stations in the Sea‐Air Exchange Program Asian Dust Network in the North Pacific between 4°N and 53°N. Mean NO3− concentrations were relatively uniform, ranging between 0.18 and 0.35 μg/m3 STP; mean nss SO4= ranged from 0.37 to 1.20 μg/m3 STP. At most stations there was no consistent correlation between NO3− and nss SO4=or between these species and mineral aerosol. At Midway these species did covary in a clear and consistent manner; this correlation appears to be forced by large‐scale meteorological factors. In some cases, high nss SO4= concentrations were associated with high mineral aerosol concentrations; this suggests that the SO4= in these episodes was transported from the continents. A particularly impressive event occurred in May 1981, when much of the North Pacific region was affected by extremely high nss SO4= concentrations; evidence suggests that the SO4= was not derived from the soil material itself. At Fanning Island, NO3− concentrations showed a strong seasonal variation; there is evidence that the high NO3− concentrations are attributable to transport from the continents. In contrast, nss SO4= concentrations at Fanning were relatively constant throughout the year, and they were consistent with recent estimates of the oceanic emission rates of sulfur as dimethyl sulfide. The NO3− data from the tropics suggest that there is a relatively uniform background concentration of about 0.10 μg/m3 STP; this concentration is close to that recently predicted by models that assume only lightning and the stratosphere as sources for NOx.