Investigation of HO2 uptake mechanisms onto multiple-component ambient aerosols collected in summer and winter time in Yokohama, Japan

The heterogeneous loss of HO2 radicals onto ambient aerosols plays an important role in tropospheric chemistry. However, sparse investigation of the dominating parameters controlling the HO2 uptake coefficients onto ambient aerosols (γHO2) has largely hindered the application of the measured γHO2 to the global spatial prediction. Here we induced an offline method using LFP-LIF technique to measure the kinetics of HO2 uptake onto ambient aerosols collected in summertime and wintertime in Yokohama city, a regional urban site near Tokyo, Japan. By controlling the dominating parameters which influence γHO2, we were able to investigate the detailed HO2 uptake mechanism. We characterized the chemical composition of the collected ambient aerosols, including organics, inorganics, transition metals ions, etc. and modeled γHO2 using different mechanisms. Results show that γHO2 increased with the increase in RH, and the aerosol states ("dry" or wet/aqueous) have large effects on γHO2. With fixed RH and aerosol chemical composition, γHO2was highly dependent on pH and inversely correlated with [HO2]0. By combing the measured γHO2 values with the modeled ones, we found that both the HO2 self-reaction and transition metal-catalyzed reactions should be accounted for to yield a single parameterization to predict γHO2, and different chemical compositions may have collective effects on γHO2. Results may serve for extending the γHO2 values measured at one observation site to different environmental conditions, which will help us to achieve more accurate modeling results concerning secondary pollutant formation (i.e., ozone).

Reactive aldehyde chemistry explains the missing source of hydroxyl radicals

Hydroxyl radicals (OH) determine the tropospheric self-cleansing capacity, thus regulating air quality and climate. However, the state-of-the-art mechanisms still underestimate OH at low nitrogen oxide and high volatile organic compound regimes even considering the latest isoprene chemistry. Here we propose that the reactive aldehyde chemistry, especially the autoxidation of carbonyl organic peroxy radicals (R(CO)O2) derived from higher aldehydes, is a noteworthy OH regeneration mechanism that overwhelms the contribution of the isoprene autoxidation, the latter has been proved to largely contribute to the missing OH source under high isoprene condition. As diagnosed by the quantum chemical calculations, the R(CO)O2 radicals undergo fast H-migration to produce unsaturated hydroperoxyl-carbonyls that generate OH through rapid photolysis. This chemistry could explain almost all unknown OH sources in areas rich in both natural and anthropogenic emissions in the warm seasons, and may increasingly impact the global self-cleansing capacity in a future low nitrogen oxide society under carbon neutrality scenarios.

Measurement of tropospheric HO2 radical using fluorescence assay by gas expansion with low interferences

An instrument to detect atmospheric HO₂ radicals using fluorescence assay by gas expansion (FAGE) technique has been developed. HO₂ is measured by reaction with NO to form OH and subsequent detection of OH by laser-induced fluorescence at low pressure. The system performance has been improved by optimizing the expansion distance and pressure, the influence factors of HO₂ conversion efficiency are also studied. The interferences of RO₂ radicals were investigated by determining the conversion efficiency of RO₂ to OH during the measurement of HO₂. The dependence of the conversion of HO₂ on NO concentration was investigated, and low HO₂ conversion efficiency was selected to realize the ambient HO₂ measurement, where the conversion efficiency of RO₂ derived by propane, ethene, isoprene and methanol to OH has been reduced to less than 6% in the atmosphere. Furthermore, no significant interferences from PM_2.5 and NO were found in the ambient HO₂ measurement. The detection limits for HO₂ (S/N = 2) are estimated to 4.8 × 10⁵ cm^-3 and 1.1 × 10⁶ cm^-3 ( [Formula: see text] = 20%) under night and noon conditions, with 60 sec signal integration time. The instrument was successfully deployed during STORM-2018 field campaign at Shenzhen graduate school of Peking University. The concentration of atmospheric HOx radical and the good correlation of OH with j(O¹D) was obtained here. The diurnal variation of HOx concentration shows that the OH maximum concentration of those days is about 5.3 × 10⁶ cm^-3 appearing around 12:00, while the HO₂ maximum concentration is about 4.2 × 10⁸ cm^-3 appearing around 13:30.

A hydroxyl radical detection system using gas expansion and fast gating laser-induced fluorescence techniques

An OH radical measurement instrument based on Fluorescence Assay by Gas Expansion (FAGE) has been developed in our laboratory. Ambient air is introduced into a low-pressure fluorescence cell through a pinhole aperture and irradiated by a dye laser at a high repetition rate of 8.5kHz. The OH radical is both excited and detected at 308nm using A-X(0,0) band. To satisfy the high efficiency needs of fluorescence collection and detection, a 4-lens optical system and a self-designed gated photomultiplier (PMT) is used, and gating is actualized by switching the voltage applied on the PMT dynodes. A micro channel photomultiplier (MCP) is also prepared for fluorescence detection. Then the weak signal is accumulated by a photon counter in a specific timing. The OH radical excitation spectrum range in the wavelength of 307.82-308.2nm is detected and the excited line for OH detection is determined to be Q₁(2) line. The calibration of the FAGE system is researched by using simultaneous photolysis of H₂O and O₂. The minimum detection limit of the instrument using gated PMT is determined to be 9.4×10⁵molecules/cm³, and the sensitivity is 9.5×10^-7cps/(OH·cm^-3), with a signal-to-noise ratio of 2 and an integration time of 60sec, while OH detection limit and the detection sensitivity using MCP is calculated to be 1.6×10⁵molecules/cm³ and 2.3×10^-6cps/(OH·cm^-3). The laboratory OH radical measurement is carried out and results show that the proposed system can be used for atmospheric OH radical measurement.

OH production from the photolysis of isoprene-derived peroxy radicals: cross-sections, quantum yields and atmospheric implications

The OH radical production from the near-ultraviolet photolysis of peroxy radicals derived from isoprene has been investigated.

The persistence of pesticides in atmospheric particulate phase: An emerging air quality issue

The persistent organic pollutants (POPs) due to their physicochemical properties can be widely spread all over the globe; as such they represent a serious threat to both humans and wildlife. According to Stockholm convention out of 24 officially recognized POPs, 16 are pesticides. The atmospheric life times of pesticides, up to now were estimated based on their gas-phase reactivity. It has been only speculated that sorption to aerosol particles may increase significantly the half-lives of pesticides in the atmosphere. The results presented here challenge the current view of the half-lives of pesticides in the lower boundary layer of the atmosphere and their impact on air quality and human health. We demonstrate that semivolatile pesticides which are mostly adsorbed on atmospheric aerosol particles are very persistent with respect to the highly reactive hydroxyl radicals (OH) that is the self-cleaning agent of the atmosphere. The half-lives in particulate phase of difenoconazole, tetraconazole, fipronil, oxadiazon, deltamethrin, cyprodinil, permethrin, and pendimethalin are in order of several days and even higher than one month, implying that these pesticides can be transported over long distances, reaching the remote regions all over the world; hence these pesticides shall be further evaluated prior to be confirmed as POPs.

Measurements of the HO2 uptake coefficients onto single component organic aerosols

Measurements of HO2 uptake coefficients (γ) were made onto a variety of organic aerosols derived from glutaric acid, glyoxal, malonic acid, stearic acid, oleic acid, squalene, monoethanol amine sulfate, monomethyl amine sulfate, and two sources of humic acid, for an initial HO2 concentration of 1 × 10(9) molecules cm(-3), room temperature and at atmospheric pressure. Values in the range of γ < 0.004 to γ = 0.008 ± 0.004 were measured for all of the aerosols apart from the aerosols from the two sources of humic acid. For humic acid aerosols, uptake coefficients in the range of γ = 0.007 ± 0.002 to γ = 0.09 ± 0.03 were measured. Elevated concentrations of copper (16 ± 1 and 380 ± 20 ppb) and iron (600 ± 30 and 51 000 ± 3000 ppb) ions were measured in the humic acid atomizer solutions compared to the other organics that can explain the higher uptake values measured. A strong dependence upon relative humidity was also observed for uptake onto humic acid, with larger uptake coefficients seen at higher humidities. Possible hypotheses for the humidity dependence include the changing liquid water content of the aerosol, a change in the mass accommodation coefficient or in the Henry's law constant.

Observational evidence for interhemispheric hydroxyl-radical parity

The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere. The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane. It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4, 7-10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004-2011 the model predicts an annual mean NH-SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 ± 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns. Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 may be overestimated.

Atmospheric and kinetic studies of OH and HO2 by the FAGE technique

A new FAGE setup has recently been built at the University of Lille, France. It permits the quantification of OH and HO2 in the atmosphere with a detection limit of 3 x 105 molecules/(cm3 x min) for OH and 1 x 10(6) molecules/(cm3 x min) for HO2. Its coupling to a photolysis cell enables the measurement of the total reactivity of the hydroxyl radical in ambient air and kinetic studies in laboratory. Two configurations have been considered: one with the photolysis cell at 90 degrees to the FAGE nozzle, the other on line with the FAGE nozzle. The two configurations have been tested and validated by measuring the well known rate constants of OH with CH4, C3H8 and CO. The advantages and drawbacks of each configuration have been evaluated. The "on line" configuration limits losses and permits measurements over a larger reactivity range but is affected by OH formation from the laser beam striking the FAGE nozzle, thus limiting the ability to carry out energy dependence studies which can, in contrast, be successfully performed in the 90 degrees configuration.