This study investigates the distribution and magnitudes of the global sources and sinks of OCS and CS2. From an analysis of the correlations between measured emission rates and environmental parameters, the sources of OCS and CS2 are estimated to be 1.23 (0.83−1.71) Tg(OCS) yr−1 and 0.57 (0.34−0.82) Tg(CS2) yr−1, respectively. Our results indicate that 30% of the atmospheric OCS source is derived from the oxidation of CS2, while emissions from the ocean and other natural terrestrial sources contribute 28% and 24%, respectively. In the case of CS2, the major source is from chemical industrial emissions (58%) while the ocean contributes about 34% to the total CS2 source. Our estimate of the OCS and CS2 emission rates suggests that anthropogenic activities contribute approximately 32% to the total OCS source. The major sink for CS2 is oxidation by tropospheric OH, whereas, that for OCS appears to be uptake by vegetation.

This study investigates the importance of carbonyl sulfide (OCS) in the formation of stratospheric background sulfur aerosol. Specific questions examined include the loss rate in the stratosphere, the net flux into the stratosphere, and the contribution of OCS to the stratospheric background sulfur aerosol. From an analysis of current atmospheric measurements of OCS, the total amount of OCS in the atmosphere is evaluated to be 5.2 Tg. Of this total, 4.63 Tg is in the troposphere and 0.57 Tg in the stratosphere. Based on the estimated global OCS source strength of 1.2 Tg yr−1, the global atmospheric lifetime of OCS is estimated to be 4.3 years. Using a one‐dimensional photochemical model, the stratospheric photochemical lifetime of OCS is estimated to be 10 years, more than 2 times longer than its global atmospheric lifetime. These results suggest that most of the OCS transported into the stratosphere returns to the troposphere where it is mainly taken up by surface vegetation. The production of stratospheric background sulfur aerosol from OCS oxidation is calculated to be 3.0×1010 gS yr−1, 2 to 5 times smaller than the most recent estimates of the amount of sulfur required to maintain the stratospheric background aerosol level. Possible explanations for this difference include (1) an overevaluation of the nonvolcanic background aerosol burden; (2) an underevaluation of the lifetime of stratospheric background aerosol; (3) the presence of other sulfur sources such as high‐altitude aircraft emissions; or (4) that the stratospheric OCS database used in our analysis is flawed with a substantial yet unidentified systematic error.

[1] The dispersion and chemical evolution of NO x in ship plumes has been investigated for marine boundary layer (MBL) conditions. This effort has involved combining a plume dispersion algorithm with a time-dependent photochemical box model. The analysis has considered several factors, all of which can influence the net impact of NO x on the background environment. These include the following: season of the year, latitude of point of release, meteorological setting, and ship NO x emission rate. Reaction rates within a plume were shown to be a nonlinear function of the levels of NO x , leading to relative estimates of ship plume NO x lifetimes that were factors of 2.5-10 times shorter than for ambient marine conditions. The shortened ship-plume NO x lifetime reflects both elevated daytime levels of OH and nighttime levels of NO 3 and N 2 O 5 , all of which were estimated to be several times larger than those typical of ambient marine conditions. During daylight hours, elevated ship plume OH resulted in the net photochemical production of O 3 , with peak concentrations being 5-65% higher than background values, depending on latitude. The areal integrated O 3 effect, however, is estimated to be quite small due to further plume dilution. In addition, because of the shorter estimated lifetime for NO x , it would seem reasonable that the integrated O 3 production from the current Lagrangian modeling effort would be significantly lower than that predicted by global 3-D grid models. The current predicted shortened lifetime for NO x is quite significant in terms of assessing a ship plume's impact on background marine levels of NO x . In fact, these results would seem to explain a significant fraction of the overprediction of NO x levels in and near shipping lanes recently estimated using 3-D Eulerian global models.

Abstract. Observations of new particle production recorded near a frontal cloud at-6 km above sea level in a remote marine region are reported. Two distinct locations situated near the cloud were found to have enhanced concentrations of sulfuric acid vapor (H2SO4) and freshly formed 3-4 nm particles. Both were in droplet-free air situated above cloudy regions.No evidence for enhanced H2SO4 or nucleation was observed in clear air far from the cloud.In the nucleating region the aerosol size distribution from 3 nm to 600 gm was observed to be trimodal, with a prominent ultrafine mode, and was qualitatively similar to surface-based measurements recorded in regions of postfrontal subsidence. The measurements support the notion that new particle production in the free troposphere occurs preferentially in clear air near clouds via enhanced photochemical production of nucleation precursor gases and that H2SO4 participates. A model simulation suggested that a doubling of ultraviolet intensities above the cloud due to cloud enhanced up-welling radiation or reasonable enhancements in sulfur dioxide concentrations could account for the higher H2SO4 concentration observed near the cloud. In the nucleation regions, H2SO4 and water vapor concentrations were too low for binary nucleation of sulfuric acid and water, according to current nucleation models. The mechanisms of particle formation and growth remain uncertain. The measurements were part of the first Aerosol Characterization Experiment (ACE 1) conducted in the remote South Pacific Ocean (153øE, 47øS) on November 27, 1995. Models, based on observations, suggested that new particles were formed in these regions via binary H2SO4-H20 nucleation [Hegg et al., 1990; Perry and Hobbs, 1994]. The upper perimeters of convective clouds appear to be ideal sites for new particle production due to enhanced actinic fluxes, high relative humidity fields, cold temperatures, and reduced aerosol surface areas associated with higher altitudes. High actinic fluxes from down-welling and cloud reflected up-welling radiation combined with enhanced water vapor concentration could lead to increased photochemical production of OH, which in turn leads to production of low volatility products, such as H2SO4 production from sulfur dioxide (SO2) oxidation. Measurements of high OH concentrations above 24,107

The first measurements of OH, H2SO4, and MSA performed at the South Pole as part of the Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) study are presented. OH concentrations were found to be quite elevated for such a dry environment, with average values of 2x106 molecule cm−3. Model simulations suggest that much of the observed OH is a result of unexpectedly high NO concentrations. Concentrations of H2SO4 and MSA were generally low with average values of 2.5x105 and 1x105 molecule cm−3, respectively. Major variations in the concentration levels of the above species were found to have a high correlation with changes in the polar mixing layer as estimated from the measured temperature difference from 22 to 2m above the snow surface. Chemical details are discussed.

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Reported here are modeling results based on ISCAT (Investigation of Sulfur Chemistry of Antarctic Troposphere) 2000 observations recorded at the South Pole (SP) during the Austral Summer of 2000. The observations included a comprehensive set of photochemical parameters, e.g., NO, O 3 , and CO. It is worthy to note that not only were OH and HO 2 observed, but also HO x precursor species CH 2 O, H 2 O 2 , and HONO were measured. Previous studies have suggested that HONO is the major source of OH/HO x in the Arctic; however, observed HONO levels at SP induced dramatic model overprediction of both HO x and NO x when used to constrain the model calculations. In contrast, model predictions constrained by observed values of CH 2 O and H 2 O 2 are consistent with the observations of OH and HO 2 (i.e., within 20%) for more than half of the data. Significant model overpredictions of OH, however, were seen at the NO levels lower than 50 pptv or higher than 150 pptv. An analysis of HO x budget at the median NO level suggests that snow emissions of H 2 O 2 and CH 2 O are the single most important primary source of SP HO x , contributing 46% to the total source. Major sinks for HO x are found to be dry deposition of HO 2 NO 2 and HNO 3 as well as their reactions with OH. Although ISCAT 2000 led to a major progress in our understanding of SP HO x chemistry, critical aspects of this chemistry are still in need of further investigation.