Date of Award

Winter 1997

Project Type

Dissertation

Program or Major

Earth Sciences

Degree Name

Doctor of Philosophy

First Advisor

Robert Talbot

Abstract

Measurements of N gas (HNO$\sb3$, NH$\sb3$) and aerosol (NO$\sb3\sp-,$ NH$\sb4\sp+$) species were made between 1991-1995 to examine the nature of atmospheric N chemistry and to estimate the importance of N dry deposition to the Harvard Forest (Petersham, MA). This U.S. site was influenced by aged rural air masses advected from the northwest (NW) and fresh industrial emissions from the southwest (SW). Mean midday HNO$\sb3$ and aerosol N mixing ratios were four times higher in SW surface winds.

Diel cycles provided evidence of the entrainment of HNO$\sb3$ and aerosol NO$\sb3\sp-$ from aloft as the nocturnal inversion broke down. HNO$\sb3$ made up about 20% of NO$\sb{\rm Y}$ at midday, while the sum of measured NO$\sb{\rm Y}$ species accounted for 60-80% of NO$\sb{\rm Y}$ suggesting that PAN and other organic nitrates were significant at this predominantly oak site. The deposition velocity (V$\sb{\rm d}$ of HNO$\sb3$ was estimated using the modified-Bowen ratio (MBR) and an inferential method. Hourly averaged V$\sb{\rm d}$ for HNO$\sb3$ ranged from $\approx$1 cm s$\sp{-1}$ at night to $\approx$6 cm s$\sp{-1}$ at midday. HNO$\sb3$ deposition was typically 3-4 times higher than the measured NO$\sb{\rm Y}$ flux. Measurement bias, storage effects, and the flux of other NO$\sb{\rm Y}$ species probably contributed to this discrepancy.

NH$\sb3$ levels were suppressed by atmospheric SO$\sb4\sp{2-}$ to mixing ratios of 200-300 pptv, below the NH$\sb3$ compensation point of the canopy. The SO$\sb4\sp{2-}$ regulation of NH$\sb{\rm X}$ (NH$\sb3$ + NH$\sb4\sp+)$ partitioning changed exponentially as a function of air temperature. The bulk aerosol was as a mixture of submicron ammonium (bi)sulfate aerosols with smaller amounts of soil particles. Aerosols from the SW were rarely neutralized, especially when SO$\sb4\sp{2-}$ concentrations were greater than $\approx$100 nmol m$\sp{-3}$, suggesting an upper limit for NH$\sb{\rm X}$ emissions from this region. Aerosol NO$\sb3\sp-$ was 4-8 times lower than NH$\sb4\sp+,$ and associated with supermicron Ca$\sp{2+}.$ The higher V$\sb{\rm d}$ of coarse mode NO$\sb3\sp-$ resulted in similar dry deposition fluxes of 1 kg N ha$\sp{-1}$ yr$\sp{-1}$ for both N aerosol species. These aerosol deposition fluxes were considerably smaller than measured N (NO$\sb3\sp{-}$ + NH$\sb4\sp+$) wet deposition ($\approx$8 kg N ha$\sp{-1}$ yr$\sp{-1}$) and estimates of HNO$\sb3$ inputs (1-7 kg N ha$\sp{-1}$ yr$\sp{-1}$) to this forest ecosystem.

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