The Potential Role of Criegee Intermediates in Nighttime Atmospheric Chemistry. A Modeling Study

We evaluate the role of Criegee intermediates (CI) from ozonolysis of alkenes on nighttime chemistry in areas impacted by ozone and high emissions of biogenic volatile organic compounds, for example, the Southeast United States, using the Master Chemical Mechanism. Criegee reactions with NO₂ may be an alternate source of NO₃. Reactions of CI with NO₃ have not been investigated but could influence NO_<i>x</i> recycling. Evaluation of these reactions depends on recently measured rate constants for CI reactions with water vapor, NO₂, and other trace gases. We vary the CI rate coefficients with NO₂ and H₂O and explore a range of initial conditions. We find that the CI production has the largest effects at low NO₂ (<1 ppbv), high isoprene (10 ppbv) and low RH (<50%). At higher RH that is characteristic of the southeast U.S. and other high biogenic emitting regions, the effects are negligible using current literature values for CI rate constants with H₂O. Under conditions for which CI reactions affect nighttime NO₃ chemistry in the model, NO₃ production from CI reaction with NO₂ increases by up to 70% and 47% at 10% and 50% relative humidity (RH), respectively, but are negligible (2%) at 80% RH. Organic nitrate formation is highly correlated with NO₃ production and increases up to 75%, 34%, and 1% at 10%, 50%, and 80% RH, respectively. Including CI chemistry increases formaldehyde production under all conditions. In addition to the rate constants for CI radicals with water vapor, model simulations are also sensitive to the rate constant for CI decomposition and to the yield of NO₃ from CI reaction with NO₂, which are uncertain

Evolution of the Complex Refractive Index of Secondary Organic Aerosols during Atmospheric Aging

The wavelength-dependence of the complex refractive indices (RI) in the visible spectral range of secondary organic aerosols (SOA) are rarely studied, and the evolution of the RI with atmospheric aging is largely unknown. In this study, we applied a novel white light-broadband cavity enhanced spectroscopy to measure the changes in the RI (400–650 nm) of β-pinene and <i>p</i>-xylene SOA produced and aged in an oxidation flow reactor, simulating daytime aging under NO_<i>x</i>-free conditions. It was found that these SOA are not absorbing in the visible range, and that the real part of the RI, <i>n</i>, shows a slight spectral dependence in the visible range. With increased OH exposure, <i>n</i> first increased and then decreased, possibly due to an increase in aerosol density and chemical mean polarizability for SOA produced at low OH exposures, and a decrease in chemical mean polarizability for SOA produced at high OH exposures, respectively. A simple radiative forcing calculation suggests that atmospheric aging can introduce more than 40% uncertainty due to the changes in the RI for aged SOA