Effect of oxidant concentration, exposure time, and seed particles on secondary organic aerosol chemical composition and yield

Abstract

Abstract. We performed a systematic intercomparison study of the chemistry and yields of secondary organic aerosol (SOA) generated from OH oxidation of a common set of gasphase precursors in a Potential Aerosol Mass (PAM) continuous flow reactor and several environmental chambers. In the flow reactor, SOA precursors were oxidized using OH concentrations ranging from 2.0 × 10 8 to 2.2 × 10 10 molec cm −3 over exposure times of 100 s. In the environmental chambers, precursors were oxidized using OH concentrations ranging from 2 × 10 6 to 2 × 10 7 molec cm −3 over exposure times of several hours. The OH concentration in the chamber experiments is close to that found in the atmosphere, but the integrated OH exposure in the flow reactor can simulate atmospheric exposure times of multiple days compared to chamber exposure times of only a day or so. In most cases, for a specific SOA type the most-oxidized chamber SOA and the least-oxidized flow reactor SOA have similar mass spectra, oxygen-to-carbon and hydrogen-to-carbon ratios, and carbon oxidation states at integrated OH exposures between approximately 1 × 10 11 and 2 × 10 11 molec cm −3 s, or about 1-2 days of equivalent atmospheric oxidation. This observation suggests that in the range of available OH exposure overlap for the flow reactor and chambers, SOA elemental composition as measured by an aerosol mass spectrometer is similar whether the precursor is exposed to low OH concentrations over long exposure times or high OH concentrations over short exposure times. This similarity in turn suggests that both in the flow reactor and in chambers, SOA chemical composition at low OH exposure is governed primarily by gas-phase OH oxidation of the precursors rather than heterogeneous oxidation of the condensed particles. In general, SOA yields measured in the flow reactor are lower than measured in chambers for the range of equivalent OH exposures that can be measured in both the flow reactor and chambers. The influence of sulfate seed particles on isoprene SOA yield measurements was examined in the flow reactor. The studies show that seed particles increase the yield of SOA produced in flow reactors by a factor of 3 to 5 and may also account in part for higher SOA yields obtained in the chambers, where seed particles are routinely used.

The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization

“…ber has yet to be reported (Lambe et al, 2015). Moreover, under the steady-state operating conditions, it is possible to accumulate sufficient products for detailed analytical evaluation.…”

“…Since the concept of potential aerosol mass (PAM) was proposed, the PAM reactor, operated as a flow tube reactor, has been widely used in laboratory and field studies of SOA formation (Chen et al, 2013;Kang et al, 2007Kang et al, , 2011Keller and Burtscher, 2012;Kroll et al, 2009;Lambe et al, 2011aLambe et al, , 2012Lambe et al, , 2015Ortega et al, 2016Ortega et al, , 2013Palm et al, 2016;Slowik et al, 2012;Smith et al, 2009). A powerful attribute of the PAM and subsequent flow reactors is the capability to generate hydroxyl radical (OH) levels that lead to integrated OH exposure ranging as high as ∼ 10 12 molecules cm −3 s, at which it is possible to simulate atmospheric oxidation conditions comparable to those occurring over ∼ 1 week.…”

“…Such reactors comprise both chambers and flow reactors. The flow tube reactor has emerged as a widely used platform (Bruns et al, 2015;Chen et al, 2013;Ezell et al, 2010;Kang et al, 2007Kang et al, , 2011Karjalainen et al, 2016;Keller and Burtscher, 2012;Khalizov et al, 2006;Lambe et al, 2011aLambe et al, , b, 2012Lambe et al, , 2015Li et al, 2015;Ortega et al, 2013Ortega et al, , 2016Palm et al, 2016;Peng et al, 2015Peng et al, , 2016Simonen et al, 2016;Tkacik et al, 2014). The flow tube reactor is generally operated under steadystate conditions.…”

Assessing the degree of plug flow in oxidation flow reactors (OFRs): a study on a potential aerosol mass (PAM) reactor

“…Because the mechanism of exposure between traditional chambers OFRs was different, validating the OFR concept began by replicating data obtained from traditional chambers Lambe et al, 2015;Liu et al, 2015), and assessing whether the chemistry was realistic McNeill et al, 2008;Peng et al, 2015;Renbaum and Smith, 2011). Consequently, much modeling work has focused on pure chemical reactions and comparison of SOA yields between the two (Bruns et al, 2015;Lambe et al, 2015;Li et al, 2015;Ortega et al, 2016;Peng et al, 2015). However, little modeling work has been done on understanding hydrodynamics or flow fields inside OFRs so that the flow patterns can be improved.…”

“…This application therefore altered the study of SOA formation, previously dominated by the traditional large, well-mixed reactors (Kroll and Seinfeld, 2008;Rudich et al, 2007;Turpin et al, 2000), by allowing the generation of laboratory data beyond first simulated day of exposure. Because the mechanism of exposure between traditional chambers OFRs was different, validating the OFR concept began by replicating data obtained from traditional chambers Lambe et al, 2015;Liu et al, 2015), and assessing whether the chemistry was realistic McNeill et al, 2008;Peng et al, 2015;Renbaum and Smith, 2011). Consequently, much modeling work has focused on pure chemical reactions and comparison of SOA yields between the two (Bruns et al, 2015;Lambe et al, 2015;Li et al, 2015;Ortega et al, 2016;Peng et al, 2015).…”