Iodic acid formation and yield from iodine photolysis at the CERN CLOUD chamber

<p>Iodine oxoacids are key species involved in the cycling of iodine between the gas- and aerosol phases. Iodic acid (HIO<sub>3</sub>) nucleates particles more efficiently than sulfuric acid and ammonia at comparable concentrations, and grows them at comparable rates, but the formation mechanism of HIO<sub>3</sub> is essentially unknown. As a result, atmospheric models of iodine chemistry are currently incomplete. Proposed precursors for iodine oxoacids include iodine atoms and higher iodine oxides (e.g., I<sub>2</sub>O<sub>2</sub>, I<sub>2</sub>O<sub>3</sub>, I<sub>2</sub>O<sub>4</sub>), but theoretical predictions have not currently been assessed under experimental conditions that approximate the open ocean marine atmosphere. We present results from laboratory experiments at the CLOUD chamber that observe rapid oxoacid formation from photolysis of iodine (I<sub>2</sub>) at green wavelengths, in the presence of ozone and variable relative humidity (0-80%). Under these (soft) experimental conditions iodine oxide (IO) radical concentrations closely approximate those found in the remote marine boundary layer. A chemical box model is constrained by measurements of I<sub>2</sub>, ozone, RH, photolysis frequencies (i.e., I<sub>2</sub>, IO, OIO, HOI, I<sub>x</sub>O<sub>y</sub>) and known losses of gases to particles and the chamber walls, and evaluated using time resolved measurements of IO, OIO, and I<sub>x</sub>O<sub>y</sub> species in the chamber. Hypothesized mechanisms for HIO<sub>3</sub> formation - either proposed in the literature or motivated from our observations - are then discussed in terms of their ability to explain the observed amounts (yield), and the temporal evolution of HIO<sub>3</sub>. Finally, the atmospheric relevance of the laboratory findings is assessed in context of unique field measurements at the Maido Observatory, La Reunion, during spring 2018, where IO radicals and HIO<sub>3</sub> were measured simultaneously in the remote free troposphere.</p>

Constraining remote oxidation capacity with ATom observations

The global oxidation capacity, defined as the tropospheric mean concentration of the hydroxyl radical (OH), controls the lifetime of reactive trace gases in the atmosphere such as methane and carbon monoxide (CO). Models tend to underestimate the methane lifetime and CO concentrations throughout the troposphere, which is consistent with excessive OH. Approximately half of the oxidation of methane and non-methane volatile organic compounds (VOCs) is thought to occur over the oceans where oxidant chemistry has received little validation due to a lack of observational constraints. We use observations from the first two deployments of the NASA ATom aircraft campaign during July–August 2016 and January–February 2017 to evaluate the oxidation capacity over the remote oceans and its representation by the GEOS-Chem chemical transport model. The model successfully simulates the magnitude and vertical profile of remote OH within the measurement uncertainties. Comparisons against the drivers of OH production (water vapor, ozone, and NOy concentrations, ozone photolysis frequencies) also show minimal bias, with the exception of wintertime NOy. The severe model overestimate of NOy during this period may indicate insufficient wet scavenging and/or missing loss on sea-salt aerosols. Large uncertainties in these processes require further study to improve simulated NOy partitioning and removal in the troposphere, but preliminary tests suggest that their overall impact could marginally reduce the model bias in tropospheric OH. During the ATom-1 deployment, OH reactivity (OHR) below 3 km is significantly enhanced, and this is not captured by the sum of its measured components (cOHRobs) or by the model (cOHRmod). This enhancement could suggest missing reactive VOCs but cannot be explained by a comprehensive simulation of both biotic and abiotic ocean sources of VOCs. Additional sources of VOC reactivity in this region are difficult to reconcile with the full suite of ATom measurement constraints. The model generally reproduces the magnitude and seasonality of cOHRobs but underestimates the contribution of oxygenated VOCs, mainly acetaldehyde, which is severely underestimated throughout the troposphere despite its calculated lifetime of less than a day. Missing model acetaldehyde in previous studies was attributed to measurement uncertainties that have been largely resolved. Observations of peroxyacetic acid (PAA) provide new support for remote levels of acetaldehyde. The underestimate in both model acetaldehyde and PAA is present throughout the year in both hemispheres and peaks during Northern Hemisphere summer. The addition of ocean sources of VOCs in the model increases cOHRmod by 3 % to 9 % and improves model–measurement agreement for acetaldehyde, particularly in winter, but cannot resolve the model summertime bias. Doing so would require 100 Tg yr−1 of a long-lived unknown precursor throughout the year with significant additional emissions in the Northern Hemisphere summer. Improving the model bias for remote acetaldehyde and PAA is unlikely to fully resolve previously reported model global biases in OH and methane lifetime, suggesting that future work should examine the sources and sinks of OH over land.

Aircraft measurements of nitrous acid in excess of model predictions in the boundary layer and free troposphere

<p>Middle and long-term  photo-chemical effects of local and regional pollution are not well quantified and are an area of active study. NO<sub>x</sub> (here defined as NO, NO<sub>2</sub>, and HONO) is a regional pollutant, which influences atmospheric oxidation capacity and ozone formation. Airborne measurements of atmospheric trace gases from the HALO (High Altitude Long Range) aircraft, particularly of NO, NO<sub>2</sub>, and HONO were performed as part of the EMeRGe (Effect of Megacities on the Transport and Transformation of Pollutants on the Regional to Global Scales) campaign over continental Europe and southeast Asia in July 2017 and April 2018, respectively. NO (and NO<sub>Y</sub>), O<sub>3</sub>, and the photolysis frequencies of NO<sub>2</sub> and HONO were measured in-situ. NO<sub>2</sub> and HONO were inferred from Limb measurements of the mini-DOAS (Differential Optical Absorption Spectroscopy) instrument, using the novel scaling method (Hüneke et al., 2017). These measurements were compared with simulations of the MECO/EMAC models. In relatively polluted air-masses in the boundary layer and free troposphere, HONO measured in excess of model predictions (and previous measurements) suggests an in-situ formation and a significant source of OH as well as a pathway for re-noxification. Aerosol composition simultaneously measured  by the C-Tof-AMS instrument may reveal potential reaction mechanisms to explain the discrepancy. </p>

Sources and Sinks of Nitrated Phenols: Application of an Observation-based Model

<p><strong>Sources and Sinks of Nitrated Phenols: </strong><strong>Application of an Observation-based Model </strong></p><p>Yuwen Peng<sup>1</sup>, Sihang Wang<sup>1</sup>, Caihong Wu<sup>1</sup>, Jipeng Qi<sup>1</sup>, Chaomin Wang<sup>1</sup>, <br>Wei Song<sup>2</sup>, Xinmin Wang<sup>2</sup>, Bin Yuan<sup>1,</sup><sup>*</sup>, Min Shao<sup>1,</sup><sup>**</sup></p><p><sup>1</sup> Institute for Environmental and Climate Research, Jinan University, 511443 Guangzhou, China</p><p><sup>2</sup> Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China</p><p><sup>*</sup> [email protected]</p><p><sup>**</sup> [email protected]</p><p><strong> </strong></p><p><strong>Abstract: </strong>Nitrated phenols are one of the intermediate products of aromatics oxidation that has been proved to be phytotoxic, mutagenic and important components of brown carbon and SOA in the atmosphere. Although its sources and sinks have been reported, high-time-resolution measurements of nitrophenols and the evaluation of reported rate constants insufficient. In this paper, we measured the concentration of nitrated phenols at an urban site in Guangzhou, then we use an observation-based model to compare different photolysis frequencies of nitrophenol and analyze its budget. The primary emission of traffic seems to be the dominant factor when considering its diurnal profile. Photolysis has proved to be the dominant sink of nitrophenol in the atmosphere.</p>

Photolysis and oxidation by OH radicals of two carbonyl nitrates: 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone

Multifunctional organic nitrates, including carbonyl nitrates, are important species formed in NOx-rich atmospheres by the degradation of volatile organic compounds (VOCs). These compounds have been shown to play a key role in the transport of reactive nitrogen and, consequently, in the ozone budget; they are also known to be important components of the total organic aerosol. However, very little is known about their reactivity in both the gas and condensed phases. Following a previous study that we published on the gas-phase reactivity of α-nitrooxy ketones, the photolysis and reaction with OH radicals of 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone (which are a β-nitrooxy ketone and γ-nitrooxy ketone, respectively) were investigated for the first time in simulation chambers. The photolysis frequencies were directly measured in the CESAM chamber, which is equipped with a very realistic irradiation system. The jnitrate/jNO2 ratios were found to be (5.9±0.9)×10-3 for 4-nitrooxy-2-butanone and (3.2±0.9)×10-3 for 5-nitrooxy-2-pentanone under our experimental conditions. From these results, it was estimated that ambient photolysis frequencies calculated for typical tropospheric irradiation conditions corresponding to the 1 July at noon at 40∘ N (overhead ozone column of 300 and albedo of 0.1) are (6.1±0.9)×10-5 s−1 and (3.3±0.9)×10-5 s−1 for 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone, respectively. These results demonstrate that photolysis is a very efficient sink for these compounds with atmospheric lifetimes of few hours. They also suggest that, similarly to α-nitrooxy ketones, β-nitrooxy ketones have enhanced UV absorption cross sections and quantum yields equal to or close to unity and that γ-nitrooxy ketones have a lower enhancement of cross sections, which can easily be explained by the larger distance between the two chromophore groups. Thanks to a product study, the branching ratio between the two possible photodissociation pathways is also proposed. Rate constants for the reaction with OH radicals were found to be (2.9±1.0)×10-12 and (3.3±0.9)×10-12 cm3 molecule−1 s−1, respectively. These experimental data are in good agreement with rate constants estimated by the structure–activity relationship (SAR) of Kwok and Atkinson (1995) when using the parametrization proposed by Suarez-Bertoa et al. (2012) for carbonyl nitrates. Comparison with photolysis rates suggests that the OH-initiated oxidation of carbonyl nitrates is a less efficient sink than photodissociation but is not negligible in polluted areas.

Constraining remote oxidation capacity with ATom observations

The global oxidation capacity, defined as the tropospheric mean concentration of the hydroxyl radical (OH), controls the lifetime of reactive trace gases in the atmosphere such as methane and carbon monoxide (CO). Models tend to underestimate the methane lifetime and CO concentrations throughout the troposphere, which is consistent with excessive OH. Approximately half the oxidation of methane and non-methane volatile organic compounds (VOCs) is thought to occur over the oceans where oxidant chemistry has received little validation due to a lack of observational constraints. We use observations from the first two deployments of the NASA ATom aircraft campaign during July–August 2016 and January–February 2017 to evaluate the oxidation capacity over the remote oceans and its representation in the GEOS-Chem chemical transport model. The model successfully simulates the magnitude and vertical profile of remote OH within the measurement uncertainties. Comparisons against the drivers of OH production (water vapor, ozone, and NOy concentrations, ozone photolysis frequencies) also show minimal bias with the exception of wintertime NOy, for which a model overestimate may indicate insufficient wet scavenging and/or missing loss on seasalt aerosol but large uncertainties remain that require further studies of NOy partitioning and removal in the troposphere. During the ATom-1 deployment, OH reactivity (OHR) below 3 km is significantly enhanced, and this is not captured by the sum of its measured components (cOHRobs) or by the model (cOHRmod). This enhancement could suggest missing reactive VOCs but cannot be explained by new estimates of ocean VOC sources and additional modeled reactivity in this region would be difficult to reconcile with the full suite of ATom measurement constraints. The model generally reproduces the magnitude and seasonality of cOHRobs but underestimates the contribution of oxygenated VOC, mainly acetaldehyde, which is severely underestimated throughout the troposphere despite its calculated lifetime of less than a day. Missing model acetaldehyde in previous studies was attributed to measurement uncertainties that have been largely resolved. Observations of peroxyacetic acid (PAA) provide new support for remote levels of acetaldehyde. The underestimate in modeled acetaldehyde and PAA is present throughout the year in both hemispheres and peaks during Northern Hemisphere summer. The addition of ocean VOC sources in the model increases annual surface cOHRmod by 10 % and improves model-measurement agreement for acetaldehyde particularly in winter but cannot resolve the model summertime bias. Doing so would require a 100 Tg yr−1 source of a long-lived unknown precursor throughout the year with significant additional emissions in the Northern Hemisphere summer. Improving the model bias for remote acetaldehyde and PAA is unlikely to fully resolve previously reported model global biases in OH and methane lifetime, suggesting that future work should examine the sources and sinks of OH over land.

Photolysis and oxidation by OH radicals of two carbonyl nitrates: 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone

Multifunctional organic nitrates, including carbonyl nitrates, are important species formed in NOx rich atmospheres by the degradation of VOCs. These compounds have been shown to play a key role in the transport of reactive nitrogen and consequently in the ozone budget, but also to be important components of the total organic aerosol. However, very little is known about their reactivity in both gas and condensed phases. Following a previous study we published on the gas-phase reactivity of β-nitrooxy ketones, the photolysis and the reaction with OH radicals of 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone, respectively a β-nitrooxy ketone and a γ-nitrooxy ketone, were investigated for the first time in simulation chambers. Ambient photolysis frequencies calculated for 40° latitude North were found to be (4.2 ± 0.6) × 10−5 s−1 and (2.2 ± 0.7) × 10−5 s−1 for 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone, respectively. These results demonstrate that photolysis is a very efficient sink for these compounds with atmospheric lifetimes of few hours. It was also concluded that, similarly to α-nitrooxy ketones, β-nitrooxy ketones have enhanced UV absorption cross sections and quantum yields equal or close to unity. γ-nitrooxy ketones have been shown to have lower enhancement of cross sections which can easily be explained by the increasing distance between the two chromophore groups. Thanks to a products study, branching ratio between the two possible photodissociation pathways are also proposed. Rate constants for the reaction with OH radicals were found to be (2.9 ± 1.0) × 10−12 cm3 molecule−1 s−1 and (3.3 ± 0.9) × 10−12 cm3 molecule−1 s−1, respectively. These experimental data are in good agreement with rate constants estimated by the SAR of Kwok and Atkinson (1995) when using the parametrization proposed by Suarez-Bertoa et al. (2012) for carbonyl nitrates. Comparison with photolysis rates suggests that OH-initiated oxidation of carbonyl nitrates is a less efficient sink that photodissociation but is not negligible in polluted area.

The impact of aerosols on photolysis frequencies and ozone production in Beijing during the 4-year period 2012–2015

During the period 2012–2015, photolysis frequencies were measured at the Peking University site (PKUERS), a site representative of Beijing. We present a study of the effects of aerosols on two key photolysis frequencies, j(O1D) and j(NO2). Both j(O1D) and j(NO2) display significant dependence on aerosol optical depth (AOD; 380 nm) with a non-linear negative correlation. With the increase in AOD, the slopes of photolysis frequencies vs. AOD decrease, which indicates that the capacity of aerosols to reduce the actinic flux decreases with AOD. The absolute values of slopes are equal to 4.2–6.9×10-6 and 3.4×10-3 s−1 per AOD unit for j(O1D) and j(NO2) respectively at a solar zenith angle (SZA) of 60∘ and AOD smaller than 0.7, both of which are larger than those observed in a similar, previous study in the Mediterranean. This indicates that the aerosols in Beijing have a stronger extinction effect on actinic flux than absorptive dust aerosols in the Mediterranean. Since the photolysis frequencies strongly depended on the AOD and the SZA, we established a parametric equation to quantitatively evaluate the effect of aerosols on photolysis frequencies in Beijing. According to the parametric equation, aerosols lead to a decrease in seasonal mean j(NO2) by 24 % and 30 % for summer and winter, respectively, and a corresponding decrease in seasonal mean j(O1D) by 27 % and 33 %, respectively, compared to an aerosol-free atmosphere (AOD =0). Based on an observation campaign in August 2012, we used a photochemical box model to simulate the ozone production rate (P(O3)). The simulation results shows that the monthly mean daytime net ozone production rate is reduced by up to 25 % due to the light extinction of aerosols. Through further in-depth analysis, it was found that particulate matter concentrations maintain a high level under the condition of high concentrations of ozone precursors (volatile organic compounds, VOCs, and NOx), which inhibits the production of ozone to a large extent. This phenomenon implies a negative feedback mechanism in the atmospheric environment of Beijing.

The impact of aerosols on photolysis frequencies and ozone production in urban Beijing during the four-year period 2012–2015

During the period 2012–2015, the photolysis frequencies were measured at the Peking University site (PKUERS), a representative site of urban Beijing. We present a study of the effects of aerosols on two key photolysis frequencies, j(O1D) and j(NO2). Both j(O1D) and j(NO2) display significant dependence on AOD with a nonlinear negative correlation. With the increase in AOD, the slopes of photolysis frequencies vs AOD decrease, which indicates that the capacity of aerosols to reduce the actinic flux decreases with AOD. In addition, the slopes are equal to 4.21–6.93 × 10−6 s−1 and 3.20 × 10−3 s−1 per AOD unit for j(O1D) and j(NO2) respectively at SZA of 60°, both of which are larger than those observed in the Mediterranean. This indicates that the aerosols in urban Beijing have a stronger extinction on actinic flux than absorptive dust aerosols in the Mediterranean. Since the photolysis frequencies strongly depended on the AOD and the solar zenith angle (SZA), we established a parametric equation to quantitatively evaluate the effect of aerosols on photolysis frequencies in Beijing. According to the parametric equation, aerosols lead to a decrease in j(NO2) by 24.2 % and 30.4 % for summer and winter, respectively, and the corresponding decrease in j(O1D) by 27.3 % and 32.6 % respectively, compared to an aerosol-free atmosphere. Based on an observation campaign in August 2012, we used the photochemical box model to simulate the ozone production rate (P(O3)). The simulation results shows that the monthly average net ozone production rate is reduced by up to 25 % due to the light extinction of aerosols. Through further in-depth analysis, it was found that particulate matter concentrations maintain high level under the condition of high concentrations of ozone precursors (VOCs and NOx), which inhibits the production of ozone to a large extent. This phenomenon implies a negative feedback mechanism in the atmospheric environment of urban Beijing.