Ozone Decomposition on Kaolinite as a Function of Monoterpene Exposure and Relative Humidity

Abstract

Atmospheric processing of mineral aerosol by trace gases results in the formation of surface-adsorbed products that have the capacity to alter the chemical and physical properties of these airborne particulates. To investigate one potential impact of aerosol processing by biogenic volatile organic compounds (BVOCs), we investigated the heterogeneous decomposition of ozone on pure and monoterpene-processed kaolinite. We used a laminar flow reactor to measure O 3 reactive uptake coefficients on kaolinitecoated tubes as a function of relative humidity, O 3 concentration, and pre-exposure to gaseous limonene and αpinene. At 26% RH, kaolinite has a near equivalent of a monolayer of adsorbed water, and the ozone steady-state uptake coefficient was γ av = 2.9 × 10 −9 assuming the BET surface area. Pre-exposing kaolinite to limonene and α-pinene increased O 3 uptake coefficients by nearly 2 orders of magnitude to 2.1 × 10 −7 and 2.5 × 10 −7 , respectively. At all humidities studied (10−50% RH), O 3 uptake was at least 1 order of magnitude higher for monoterpene-processed kaolinite compared to that of pure kaolinite. This dramatic increase in O 3 reactivity is attributed to surface-adsorbed organics, namely limonenediol and α-terpineol, which contain alkene functionalities susceptible to ozonolysis. Increasing relative humidity decreased O 3 uptake for monoterpene-processed kaolinite consistent with competitive adsorption of water resulting in lower organic surface concentrations. These results demonstrate the significant impact adsorbed organics can have on O 3 uptake coefficients on mineral aerosol, which should be accounted for in atmospheric modeling studies.

Ozone Chemistry and Photochemistry at the Surface of Icelandic Volcanic Dust: Insights from Elemental Speciation Analysis

“…Although previous investigations of water uptake by clays, , dust, , and volcanic ash have demonstrated the equivalent of monolayer water coverage at <30% RH, we find that the reactivity of our sample toward ozone persists even at 50% RH. This observation, which again agrees with previous studies of ozone uptake by mineral dust and mineral dust proxies, ,,,, may reflect uneven surface water coverage, which would leave surface sites available for ozone uptake at RH values higher than those associated with apparent monolayer coverage . Alternatively, it may reflect the reaction of ozone with soluble species (e.g., chloride and bromide) present in a surface water film under elevated RH conditions …”

“…As shown in Figure , the highest γ BET values (50 ppb ozone) were found under dry conditions, whereas the lowest value was observed at 50% RH. This trend, previously observed for clays , and mineral dust proxies, ,, has been attributed to competition by water vapor for reactive surface sites. This explanation is consistent with our results, and in particular our observation that γ BET values for ozone determined under dry conditions were highly variable, as under these conditions, any variation in the amount of sorbed water remaining in the dust films after overnight drying would not be mitigated by contributions from the humidified air flow.…”

“…In all subsequent sections, therefore, we report γ BET , which accounts for the interaction of ozone with the entire coating volume Here, m is the dust mass (g) and S BET is the specific surface area of the dust (BET; m 2 g –1 ), as determined for bulk samples. Importantly, because this approach may overestimate the surface area presented by solid particle coatings, the γ BET values we report here represent lower limits for the ambient reactivity of volcanic dust toward ozone.…”

Ozone uptake by commercial brake pads and brake pad components: assessing the potential indirect air quality impacts of non-exhaust emissions

“…The γ BET values for ozone by brake pads that we report are at the upper end of those determined for atmospheric PM components; [23][24][25][26]36,37 in addition, because the BET approach employed here may overestimate the accessible surface area of the particles when present as coatings in the flow tube, 37 these γ BET values likely underestimate the ambient reactivity of this PM class. In the following paragraphs, we note additional experimental choices that may result in differences between the γ BET values reported here and the reactivity of "real-world" brake wear.…”

“…Time-dependent ozone uptake has been observed for many atmospheric PM types, and has been attributed to saturation of reactive surface sites. 23,25,26,36,37 Unlike γ BET,SS , which are similar for all brake pads, γ BET, t shows large sample-to-sample variation, with the ceramic and organic PBR samples having notably larger ratios of γ BET,30min to γ BET,SS (29× and 15×) than the other samples (5–8×; see Table S11† for all ratios). As shown in Fig.…”