Abstract
Abstract. Direct measurements of N2O5 reactivity on ambient aerosol particles were made during September 2009 at the Scripps Institution of Oceanography (SIO) Pier facility located in La Jolla, CA. N2O5 reactivity measurements were made using a custom flow reactor and the particle modulation technique alongside measurements of aerosol particle size distributions and non-refractory composition. The pseudo-first order rate coefficients derived from the particle modulation technique and the particle surface area concentrations were used to determine the population average N2O5 reaction probability, γ(N2O5), approximately every 50 min. Insufficient environmental controls within the instrumentation trailer led us to restrict our analysis primarily to nighttime measurements. Within this subset of data, γ(N2O5) ranged from <0.001 to 0.029 and showed significant day-to-day variations. We compare these data to a recent parameterization that utilizes aerosol composition measurements and an aerosol thermodynamics model. The parameterization captures several aspects of the measurements with similar general trends over the time series. However, the parameterization persistently overestimates the measurements by a factor of 1.5–3 and does not illustrate the same extent of variability. Assuming chloride is internally mixed across the particle population leads to the largest overestimates. Removing this assumption only partially reduces the discrepancies, suggesting that other particle characteristics that can suppress γ(N2O5) are important, such as organic coatings or non-aqueous particles. The largest apparent driver of day-to-day variability in the measured γ(N2O5) at this site was the particle nitrate loading, as inferred from both the measured particle composition and the parameterizations. The relative change in measured γ(N2O5) as a function of particle nitrate loading appears to be consistent with expectations based on laboratory data, providing direct support for the atmospheric importance of the so-called "nitrate effect".
Highlights
Nitrogen oxides (NOx ≡NO + NO2) influence air quality and climate by regulating tropospheric ozone (O3) production and hydroxyl radical (OH) abundances which in turn affect aerosol mass formation and the lifetime of the greenhouse gases such as methane (Jacob, 2000; Logan et al, 1981; Shindell et al, 2009)
The nitrate radical can react with a variety of different volatile organic compounds (VOC) or again with NO2 to form dinitrogen pentoxide, N2O5, which can react on aerosol particles
A high time resolution temperature measurement taken in the instrument trailer adjacent to the flow reactor apparatus was used with ambient relative humidity (RH) and T to calculate a flow reactor RH
Summary
Nitrogen oxides (NOx ≡NO + NO2) influence air quality and climate by regulating tropospheric ozone (O3) production and hydroxyl radical (OH) abundances which in turn affect aerosol mass formation and the lifetime of the greenhouse gases such as methane (Jacob, 2000; Logan et al, 1981; Shindell et al, 2009). 20–80 % of NOx removal in polluted regions (Alexander et al, 2009; Brown et al, 2004; Dentener and Crutzen, 1993) These previous studies have assumed that N2O5 reactions on particles proceed solely via a hydrolysis channel to form two HNO3 molecules. The presence of particulate chloride provides an additional reaction channel for the solvated N2O5 intermediate (R4) (Behnke et al, 1997; Finlayson-Pitts et al, 1989; Roberts et al, 2009) This additional nucleophile can change the product distributions and increase the rate of N2O5 uptake in particles by overcoming the nitrate effect (Bertram and Thornton, 2009). Other mechanisms of suppression are apparent, but remain to be conclusively identified
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