Salt aerosol concentration

Furthermore, measurement of the gross sea-salt aerosol concentration is not sufficient. Because the aerosol residence time depends on the particle size, the magnitude of the hysteresis effect will be sensitive to the particle size. Thus, the instantaneous particle-size distribution and the history of this distribution must be known. 

At the opposite extreme, the long-term wind velocity must be considered because it controls the movement of the air mass being sampled. The occurrence of recent precipitation events alters the sea-salt aerosol concentration, as does the passage of air over a large land mass. 

Dependence of Sea-Salt Aerosol Concentration on Various Environmental Parameters... 

During a February 1981 oceanographic cruise in the western South Atlantic Ocean, we used our system aboard the USNS Hayes to sample marine aerosols automatically for 16 days at 1.5-h intervals along the cruise track shown in Figure 1. The sea-salt aerosol concentrations were determined for correlation with wind speed and other relevant meteorological parameters. This chapter identifies the most important variables and the extent to which they affect modeling of salt loads in the marine atmosphere. 

Salt Aerosol Observations. The average salt aerosol concentration for the entire cruise was 21.7 30.9 /xg/m (arithmetic mean and standard deviation, respectively, for 253 samples from the ARCAS). Salt aerosol concentrations were also measured by Prospero 16) from ships in the South Atlantic Ocean at an elevation of 15 m. His overall statistics for 35 samples taken in the tropical and central South Atlantic (5° S to 35° S) are 9.1 5.3 jLtg/m. When he includes five additional samples collected near the Cape of Good Hope, these values become 11.3 9.2 /xg/m. A higher salt... 

Figure 4. Time-series plot of sea-salt aerosol concentration and local wind speed from Julian Day 41 to Julian Day 47, 1981.

Data Interpretation. The wind speed was correlated to the natural log of the salt load with the equation. In 0 = aw + fc, where 6 is the sea-salt aerosol concentration in micrograms per SCM, a is the slope of the linear-regression line, b is the intercept of the linear-regression line, and u is the wind speed in meters per second (i, 5). 

Table II lists slope and intercept values for the linear-regression equations and Gaussian statistics, both for the full data set and for subsets categorized by various sampling, meteorological, or oceanographic conditions. The overall statistics for the cruise (Case I in Table II) indicate that the geometric mean salt aerosol concentration was 11.5 3.0 jug/SCM, and the arithmetic mean wind speed was 9.8 3.9 m/s. Generally, data are included in the table to show that condensation processes, hysteresis effects, and advection impose difficulties when trying to match the time series of local wind speed and salt aerosol concentrations.

Sea Surface Temperature Effects. The cruise track was divided into four intervals according to sea surface temperature. Each interval spanned approximately 6 °C with decreasing sea surface temperatures (SST) from Stations 1-8. Linear-regression parameters for the four intervals are presented in Cases VIII-XI they show that the relationship between local wind speed and sea-salt aerosol concentration varies drastically. Linear-regression slopes range from near zero to approximately 0.17 the respective correlation coefficients vary from near zero to 0.6. The lowest correlation coefficients and slope values relating salt aerosol to wind speed are associated with the coldest SST values. 

Our data set definitely shows high variability for any simple relationship between local wind speed and sea-salt aerosol concentration. This occurrence may only reflect unique meteorological conditions during one summer season because frequency plots of our wind-speed data show values outside the ranges of the expected seasonal averages prevailing in this region of the South Atlantic (21). The data set was also filtered (Table II, Case XII) to exclude samples collected at wind speeds less than 8 m/s. This trial exercise results in an increase in the slope when compared to Case I, but the correlation coefficient is unexpectedly lower. However, this manipulation does not represent true winter conditions because the temperature difference between the sea and the air would be different in the winter than it was for our February cruise. 

Stability Effects. Although wind speed is the driving force for salt generation, other factors complicate a direct link between anemometer readings and salt aerosol concentrations. These factors include condensa-... 

The distributions can be compared by ordering the data values and computing the correlations between equally ranked pairs from the ordered data. Various functions relating the distributions of salt aerosol concentration to wind speed were tested to determine the best fit of the data. Our baseline correlation coefficient (0.79) was for a direct linear relationship between the ordered pairs. Relating salt concentration to the square of... 

A simple physical relationship does not exist between synoptic measurements of wind speed and sea-salt aerosol concentrations in the marine atmosphere because of advection, hysteresis, condensation processes, and the varying stability of the marine boundary layer. In the region of the South Atlantic Ocean discussed in this chapter, the low correlation between the time series for sea-salt aerosol concentration and local wind speed is attributed to the high variability of the effects just mentioned. Removing the temporal constraint by ordering both data sets results in an extremely high (r = 0.99) correlation coefficient. This result provides promise for the... 

Systematic differences appear to exist in sea-salt aerosol concentrations related to locations having significantly different sea surface temperatures. The predominant effect of sea surface temperature may pertain to the probability of formation of stable or unstable conditions in the marine boundary layer. 

Wave features may be useful for enhancing the accuracy of real-time estimates of the salt aerosol concentration because the composite wave spectrum contains a history of the previous wind and reflects the degree of coupling of the sea surface with the local winds. 

More research in diverse regions is necessary to obtain quantitative information on the variability in sea-salt aerosol concentration and related parameters before reliable predictive models can be formulated. For the near future, the prediction of salt aerosol concentration should be modeled on a statistical basis with available data. This exercise may lead to reliable generalizations for easily categorized oceanic regimes. 

A real-time prediction capability for salt aerosol concentration is expected to take longer to develop. Eventually, such a capability could be based on regional statistical models whose parameters are modified from synoptic satellite observations. 

We have incorporated reactions (l)-(7) into a photochemical box model of the MBL [23]. The model treats chemical reactions in the gas phase and deliquesced sea-salt particles, as well as exchange between the two phases standard 03-NOx-HOx-S chemistry [23] is used. The sea-salt aerosol concentration was 3x10 m ... 

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