In situ secondary organic aerosol formation from ambient pine forest air using an oxidation flow reactor

Abstract

Abstract. Ambient air was oxidized by OH radicals in an oxidation flow reactor (OFR) located in a montane pine forest during the BEACHON-RoMBAS campaign to study biogenic secondary organic aerosol (SOA) formation and aging. High OH concentrations and short residence times allowed for semi-continuous cycling through a large range of OH exposures ranging from hours to weeks of equivalent (eq.) atmospheric aging. A simple model is derived and used to account for the relative time scales of condensation of low volatility organic compounds (LVOCs) onto particles, condensational loss to the walls, and further reaction to produce volatile, non-condensing fragmentation products. More SOA production was observed in the OFR at nighttime (average 4 μg m-3 when LVOC fate corrected) compared to daytime (average 1 μg m-3 when LVOC fate corrected), with maximum formation observed at 0.4–1.5 eq. days of photochemical aging. SOA formation followed a similar diurnal pattern to monoterpenes, sesquiterpenes, and toluene + p-cymene concentrations, including a substantial increase just after sunrise at 07:00 LT. Higher photochemical aging (> 10 eq. days) led to a decrease in new SOA formation and a loss of preexisting OA due to heterogeneous oxidation followed by fragmentation and volatilization. When comparing two different commonly used methods of OH production in OFRs (OFR185 and OFR254), similar amounts of SOA formation were observed. We recommend the OFR185 mode for future forest studies. Concurrent gas-phase measurements of air after OH oxidation illustrate the decay of primary VOCs, production of small oxidized organic compounds, and net production at lower ages followed by net consumption of terpenoid oxidation products as photochemical age increased. New particle formation was observed in the reactor after oxidation, especially during times when precursor gas concentrations and SOA formation were largest. Approximately 6 times more SOA was formed in the reactor from OH oxidation than could be explained by the VOCs measured in ambient air. Several recently-developed instruments quantified ambient semi- and intermediate-volatility organic compounds (S/IVOCs) that were not detected by a PTR-TOF-MS. An SOA yield of 24–80 % from those compounds can explain the observed SOA, suggesting that these typically unmeasured S/IVOCs play a substantial role in ambient SOA formation. Our results allow ruling out condensation sticking coefficients much lower than 1. Our measurements help clarify the magnitude of SOA formation in forested environments, and demonstrate methods for interpretation of ambient OFR measurements.

Comparison of secondary organic aerosol formation from toluene on initially wet and dry ammonium sulfate particles at moderate relative humidity

“…This trend may be due to the transition of functionalization reactions to fragmentation ones (Kroll et al, 2009;Lambe et al, 2011a). Previous oxidation flow reactor studies suggest that gas-phase chemistry dominates over heterogeneous OH oxidation at OH levels below 1.0 × 10 12 molecules cm −3 s Palm et al, 2016). In this study, the highest OH exposure was 5.28 × 10 11 molecules cm −3 s, and heterogeneous oxidation of SOA may not play an important role in reducing the mass of SOA, although we cannot exclude that it plays a role.…”

Comprehensive characterization of atmospheric organic carbon at a forested site

“…This correction is thus applied in order to relate the OFR measurements to what would occur in the atmosphere. This model was verified using sulfate aerosol formation from SO2 during this study 11 . The average overall correction factor is 1.8x.…”

“…A detailed description of the OFR measurements during this field study can be found in Palm et al 11 ; a brief summary is presented here. In situ SOA formation was measured by the AMS and a (similar to chamber wall losses) 16 , being oxidized again prior to condensation leading to volatile fragmentation products, or exiting the OFR to condense rapidly on sampling line walls.…”

“…Here we describe the first attempt at comprehensive characterization of atmospheric organic carbon, by integrating measurements taken by multiple state-of-the-art mass spectrometric instruments co-located at a montane ponderosa pine forest site. This work expands on earlier compilations of organic carbon measurements [9][10][11] by including data from several new analytical instruments that target known gaps in analytical measurements (multifunctional species, S/IVOCs), and examining not only the amount of atmospheric organic carbon but also its key properties (volatility, carbon number, and carbon oxidation state). We present measurements of organic species spanning the entire volatility range found in the atmosphere, from VOCs to low-volatility organic aerosol components, over a range of oxidation states, from reduced to highly oxidized.…”

Different roles of water in secondary organic aerosol formation from toluene and isoprene

“…Thus, the reduction of SCI based oligomers is the major reason for the decrease in SOA yields from isoprene-NO 2 photooxidations. It is noted that the wall loss of semi-volatile organic compounds (SVOCs) can lead to the underestimation of the yield of SOA (Matsunaga and Ziemann, 2010;Loza et al, 2010;Zhang et al, 2014;Yeh and Ziemann, 2015;Ye et al, 2016;Palm et al, 2016;Krechmer et al, 2016;La et al, 2016;Nah et al, 2017). Since SCI-derived oligomers are the major products of SOA from isoprene-NO 2 irradiations, a question arises about which process is dominant for the reduction of SOA production under humid condition, wall loss of SCI related oligomers (in gas phase), or the reaction between SCI and H 2 O.…”