Direct observations of N₂O₅ reactivity on ambient aerosol particles

Abstract

[1] N 2 O 5 reactivity has been measured directly for the first time on ambient aerosol particles using an entrained aerosol flow reactor coupled to a custom-built chemical ionization mass spectrometer at two urban locations during summer. The observed N 2 O 5 reactivity is a strong function of both relative humidity (RH) and particle chemical composition. We show that particulate organic mass loadings, together with ambient relative humidity, play a leading role in determining the reaction rate of N 2 O 5 with particles. Our observed reactivity values are both more variable and, at times, as much as a factor of ten lower than currently implemented large-scale model parameterizations would predict. Such discrepancies have likely consequences for predictions of NO x availability and ozone production, and the sensitivity of these quantities to aerosol particle loadings. Citation:

Estimating N<sub>2</sub>O<sub>5</sub> uptake coefficients using ambient measurements of NO<sub>3</sub>, N<sub>2</sub>O<sub>5</sub>, ClNO<sub>2</sub> and particle-phase nitrate

“…Despite significant variation in both nitrate content (∼ 0.2 to 7 µg m −3 ) and the organic-to-sulfate ratio (0.3-1.2), no trend is seen. The PARADE γ s are nonetheless consistent with the Seattle observations reported by Bertram et al (2009b) in which γ was close to 3 × 10 −2 for organic/sulfate ratios up to 3. The large organic/sulfate ratios up to ∼ 13 observed by Bertram et al were not encountered during the PARADE campaign.…”

“…The directly measured loss rates of N 2 O 5 to ambient aerosol (Bertram et al, 2009b;Riedel et al, 2012b) convert to variable values of γ (∼ 0.005-0.035) in Seattle (RH ∼ 74 ± 13 %), which are comparable to those reported in this work. The largest values of γ were obtained at low organic-to-sulfate ratios, whereas their largest values at a coastal location impacted by chloride containing aerosol were obtained when the molar H 2 O / NO − 3 ratio was high.…”

“…On av- erage, the submicron non-refractory aerosol was (by mass) 55 % organic, 26 % sulfate and nitrate and ammonium were both 9.5 %, which may be considered typical for an anthropogenically influenced, rural region. Particulate sulfate and organic content were correlated, with the particulate sulfateto-organic mass ratio, a parameter that potentially impacts on the N 2 O 5 uptake to particles (Bertram et al, 2009b), varying between ∼ 0.1 and 2.4 with a mean value of ∼ 0.6. The campaign-averaged nitrate particle mass concentration was 0.56 µg m −3 during the day and 0.89 µg m −3 during the night at this site.…”

“…γ values derived from observation of N 2 O 5 uptake to ambient particles in Seattle (Bertram et al, 2009b) have revealed that the uptake coefficient can be dependent on the sulfur-to-organic ratio, with reduced uptake at low ratios. In Fig.…”

Isotopic composition of atmospheric nitrate in a tropical marine boundary layer

“…One of the most striking features of the SSM output was the relative low importance attributed to the N 2 O 5 reaction channel. Recently, questions have emerged regarding the role of N 2 O 5 in models, especially in the clean marine atmosphere (5,6,29). These studies generally point to an overestimation of the uptake coefficient of N 2 O 5 (30).…”