Eighth Conference on Atmospheric Chemistry


Analysis of particle formation processes through the effect of meteorological conditions on organic and inorganic nitrogen concentrations in atmospheric aerosols

Silvia Margarita Calderón, University of South Florida, Tampa, FL; and S. W. Campbell and N. Poor

Dissolved organic nitrogen (DON) species have shown to be present in rainwater and aerosol samples collected on the bay and represent ~10% of the total dissolved nitrogen (TDN). However, DON has not been included in estimates for dry and wet deposition fluxes and many sources could be missing from current inventories. Aerosols collected at Tampa Bay contain dissolved inorganic nitrogen (DIN) species (NH4+, NO2-, NO3-)and DON in fine (PM2.5) and coarse particles (PM10-2.5) under 10 μm of aerodynamic diameter. DIN in fine particles is 97 ± 3 % NH4+. DON is 13 ± 3 % dimethylamine (DMA). DIN in coarse particles is 80 ± 11 % NO3- and DON represents 13 ± 13 % of the TDN concentration. DMA is under detection levels. Approximately ~80% of the total DON in PM10 is in the fine particles. In Tampa Bay, concentrations of DON in fine and coarse particles are 5.3 ± 2.6 and 2.1 ± 1.6 nmol -3(n=55) respectively. Our research goal in this study was to improve our knowledge about DON species by revealing how they integrate into atmospheric particles. Meteorological parameters, such as relative humidity, temperature, wind speed and direction, drive the formation, interaction, size and deposition of particles as well as gas absorption rates into droplets on the atmosphere. To evaluate this idea, 24-hr integrated DIN and DON concentrations in PM10 samples and average meteorological conditions for two different periods (dry and wet) were analyzed using multilinear regression techniques. The dry period was characterized by lower temperature, humidity and daily precipitation (few and short rain events) with higher wind speeds. The wet period can be considered as isothermal due to the small standard deviations of the dry bulb temperature. Another important characteristic of this period is the constantly high water content of the air. It is necessary to say that even when statistical analyses could reveal very important variable dependencies, the information obtained from them is not conclusive. They do not substitute experimental or modeled data. Tendencies in variable dependences were used here as indicators for possible processes. A natural logarithmic transformation was applied to all concentration values to correct deviations in the data distribution, improve fit and correct wedge patterns in residual vs. fitted plots seen in the first regressions. All regressions were done using the stepwise method (SAS). Outliers were identified and extracted from the data set when they had higher values than critical values (e.g.R-Student) and probability-probability plots were used to check normality of the model residuals. Collinearity diagnostics were also performed on each data set to avoid unstable estimates and high standard errors due to linear dependence between predictor variables. DIN-NH4+ concentrations in the dry and wet periods showed that higher temperatures and lower wind speeds (more stable atmosphere) lead to higher particle contents of DIN-NH4+. The higher the temperature, the higher are the gas absorption rates, the reagents concentrations and the rate of chemical reactions leading to the formation of ammonium salts. The higher the wind speed the lower the DIN-NH4+ concentration. This suggests that ammonium is diluted not delivered but stronger winds. During the dry period, the variation of relative humidity also helped to explain the variance in ammonium concentrations. At high water contents the gaseous precursors of ammonium sulfate, ammonia and sulfuric acid, have more absorptive media and are effectively removed from the atmosphere. Particles covered by water films and/or water droplets more effectively scavenge inorganic nitrogen gases, such as ammonium and nitric acid, at higher relative humidity values. For example, the higher the ammonium and sulfate contents in liquid phase, the higher the rate formation of their salt. This could not be seen in the wet period due to the small variance of this variable. In the case of DIN-NO3- concentrations, the regression coefficients for both regressions showed a negative effect of the wind direction. Winds blowing from north and south west were associated with decreases in the nitrate particle contents. For the dry period nitrate concentrations in particles showed a very strong positive correlation with temperature. The temperature effect must be related then with the increase in the rate of chemical reactions leading to the nitric acid production from NOx in the atmosphere. This effect could not be caught for the wet period because the variance in the dry bulb temperature was very small. A more interesting behavior is seen for organic nitrogen in fine particles. The variance in DON concentrations for both periods can be well explained using only their DIN contents. Two situations are likely to occur. First, it is possible that DIN and DON are fed into particles by the same processes, or second, they come from similar sources; therefore the higher the DIN the higher the DON. The DIN behavior in fine particles led to assumption of absorption of gases as the main process involved in particle formation; then DON could come from absorption of organic nitrogen gases in the atmosphere. This is the most likely process that can explain the DMA levels seen on fine particles. On the other hand, it is possible that inorganic nitrogen species and water present in particles could develop a film with a high chemical affinity for organic nitrogen species. With a high fine particle concentration, more surface area for the DON-absorption became available and DON concentration increases. For DON in coarse particles the optimal linear combinations of the predictor variables could not catch as much data variance as the other models. It is important to say that after including DON concentration in coarse particles, the good correlation with meteorological conditions for the fine particles was lost. This could indicate that DON sources for coarse particles are different from the ones for fine particles. External effects, not related with the chosen meteorological set, such as vegetation debris or pollen concentrations could be responsible for this. In conclusion, results from multi-linear regression of DON and DIN concentrations with meteorological data suggested as expected that gas-to-particle conversions are the main source of nitrogen in particles collected over Tampa Bay.

extended abstract  Extended Abstract (304K)

Session 1, Aerosols—Radiative Impacts and Visibility Reduction
Monday, 30 January 2006, 9:00 AM-11:45 AM, A408

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