This study reveals aerosol liquid water content (ALWC) in PM2.5 ranged from 2% up to 74%, and the associated secondary inorganic fraction rose from 24% to 55%, while ambient relative humidity (RH) increased from 15% to 83% in the atmosphere over Beijing. Unexpectedly, the secondary inorganic fraction in PM2.5 increased with an increase in the ambient RH, which is a meteorological parameter independent of anthropogenic activities, indicating the presence of a feedback mechanism driven by Henry's law and thermodynamic equilibrium. During haze episodes, simultaneously elevated RH levels and anthropogenic secondary inorganic mass concentrations resulted in an abundant ALWC. The condensed water could act as an efficient medium for multiphase reactions, thereby facilitating the transformation of reactive gaseous pollutants into particles and accelerating the formation of heavy haze. ALWC was well correlated with the mass concentrations of both nitrate and sulfate, indicating both nitrate and sulfate salts play key roles in determining ALWC. Coincident with a significant reduction in SO2 emissions throughout China, nitrates will become a dominant anthropogenic inorganic salt driving ALWC. Thus, the abundance of ALWC and its effects on the aerosol chemistry and climate should be reconsidered.
An intensive field campaign was conducted in Chongqing during the summer of 2015 to explore the formation mechanisms of ozone pollution. The sources of ozone, the local production rates, and the controlling factors, as well as key species of volatile organic compounds (VOCs), were quantified by integrating a local ozone budget analysis, calculations of the relative incremental reactivity, and an empirical kinetic model approach. It was found that the potential for rapid local ozone formation exists in Chongqing. During ozone pollution episodes, the ozone production rates were found to be high at the upwind station Nan Quan, the urban station Chao Zhan, and the downwind station Jin-Yun Shan. The average local ozone production rate was 30x10(-9) V/V h(-1) and the daily integration of the produced ozone was greater than 180x10(-9) V/V. High ozone concentrations were associated with urban and downwind air masses. At most sites, the local ozone production was VOC-limited and the key species were aromatics and alkene, which originated mainly from vehicles and solvent usage. In addition, the air masses at the northwestern rural sites were NO (x) -limited and the local ozone production rates were significantly higher during the pollution episodes due to the increased NO (x) concentrations. In summary, the ozone abatement strategies of Chongqing should be focused on the mitigation of VOCs. Nevertheless, a reduction in NO (x) is also beneficial for reducing the regional ozone peak values in Chongqing and the surrounding areas.
Dinitrogen pentoxide (N2O5) is one of the basic trace gases which plays a key role in nighttime atmosphere. An intercomparison and validation of different N2O5 measurement methods is important for determining the true accuracy of these methods. Cavity ring down spectroscopy (CRDS) and cavity enhanced absorption spectrometer (CEAS) were used to measure N2O5 at the campus of the University of Chinese Academy of Sciences (UCAS) from February 21, 2016 to March 4, 2016. The detection limits were 1.6 ppt (1 sigma) at 30 s intervals for the CEAS instrument and 3.9 ppt (1 sigma) at 10 s time resolution for the CRDS instrument respectively. In this study, a comparison of the 1 min observations from the two instruments was presented. The two data sets showed a good agreement within their uncertainties, with an absolute shift of 15.6 ppt, slope of 0.94 and a correlation coefficient R-2 = 0.97. In general, the difference between the CRDS and CEAS instruments for N2O5 measurement can be explained by their combined measurement uncertainties. However, high relative humidity (> 60%) and high PM2.5 concentration (> 200 mu g/m(3)) may contribute to the discrepancies. The excellent agreement between the measurement by the CRDS and CEAS instruments demonstrates the capability of the two instruments for accurately measuring N2O5 with high sensitivity. (C) 2017 Elsevier B.V. All rights reserved.
Organic nitrates are secondary atmospheric pollutants that play a key role in ozone and aerosol production. This study focused on the simulation of organic nitrates through a box model coupled with RACM2 (Regional Atmospheric Chemistry Mechanism, version 2), based on data from the PRIDE-PRD2006 campaign in Backgarden, China. Our study found that an overestimation of organic nitrate production rate was generated by the model. Furthermore, the effective production ratio (alpha(eff)) of organic nitrates was around 0.033 after optimizing its chemical production module. The chemical impacts of organic nitrates on ozone production were related to VOC-OH reactivity and alpha(eff). We found that VOC-OH reactivity was positively related to alpha(eff), resulting in the suppression of ozone production caused by organic nitrates, which showed that P(O (x) =O-3+NO2) increased initially and subsequently decreased with VOC-OH reactivity. These results highlight the importance of organic nitrate's impact on ozone production in strategies to control ozone pollution, specifically regarding the reduction of low-molecular-weight VOCs in the Pearl River Delta.
Molecular chlorine (Cl-2) and nitryl chloride (GINO(2)) concentrations were measured using chemical ionization mass spectrometry at a rural site over the North China Plain during June 2014. High levels of daytime Cl-2 up to similar to 450 pptv were observed. The average diurnal Cl-2 mixing ratios showed a maximum around noon at pptv. ClNO2 exhibited a strong diurnal variation with early morning maxima reaching ppbv levels and afternoon minima sustained above 60 pptv. A moderate correlation (R-2 = 0.31) between Cl-2 and sulfur dioxide was observed, perhaps indicating a role for power plant emissions in the generation of the observed chlorine. We also observed a strong correlation (R-2 = 0.83) between daytime (10:00-20:00) Cl-2 and ClNO2, which implies that both of them were formed from a similar mechanism. In addition, Cl-2 production is likely associated with a photochemical mechanism as Cl-2 concentrations varied with ozone (O-3) levels. The impact of Cl-2 and ClNO2 as Cl atom sources is investigated using a photochemical box model. We estimated that the produced Cl atoms oxidized slightly more alkanes than OH radicals and enhanced the daily concentrations of peroxy radicals by 15% and the O-3 production rate by 19%.
Heterogeneous reactions of mineral dust aerosol with trace gases in the atmosphere could directly and indirectly affect tropospheric oxidation capacity, in addition to aerosol composition and physicochemical properties. In this article we provide a comprehensive and critical review of laboratory studies of heterogeneous uptake of OH, NO3, O-3, and their directly related species as well (including HO2, H2O2, HCHO, HONO, and N2O5) by mineral dust particles. The atmospheric importance of heterogeneous uptake as sinks for these species is assessed (i) by comparing their lifetimes with respect to heterogeneous reactions with mineral dust to lifetimes with respect to other major loss processes and (ii) by discussing relevant field and modeling studies. We have also outlined major open questions and challenges in laboratory studies of heterogeneous uptake by mineral dust and discussed research strategies to address them in order to better understand the effects of heterogeneous reactions with mineral dust on tropospheric oxidation capacity.
The heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) is important to understanding the formation of particulate nitrate (pNO(3)(-)). Measurements of N2O5 in the surface layer taken at an urban site in Beijing are presented here. N2O5 was observed with large day-to-day variability. High N2O5 concentrations were determined during pollution episodes with the co-presence of large aerosol loads. The maximum value was 1.3 ppbv (5 s average), associated with an air mass characterized by a high level of O-3. N2O5 uptake coefficients were estimated to be in the range of 0.025-0.072 using the steady-state lifetime method. As a consequence, the nocturnal pNO(3)(-) formation potential by N2O5 heterogeneous uptake was calculated to be 24-85 mu g m(-3) per night and, on average, 57 mu g m(-3) during days with pollution. This was comparable to or even higher than that formed by the partitioning of HNO3. The results highlight that N2O5 heterogeneous hydrolysis is vital in pNO(3)(-) formation in Beijing.
A chemical box model was used to study nitrate radical (NO3), dinitrogen pentoxide (N2O5) and nitryl chloride (C1NO(2)) in a rural site during the Campaign of Air Quality Research in Beijing 2006 (CAREBeijing-2006). The model was based on regional atmospheric chemistry mechanism version 2 (RACM(2)) with the heterogeneous uptake of N2O5 and the simplified chloride radical (C1) chemistry mechanism. A high production rate of NO3 with a mean value of 0.8 ppbv/h and low mixing ratios of NO3 and N2O5 (peak values of 17 pptv and 480 pptv, respectively) existed in this site. Budget analysis showed that NO emission suppressed the NO3 chemistry at the surface layer, the reaction of NO3 with VOCs made a similar contribution to NO3 loss as N2O5 heterogeneous uptake. The NO3 chemistry was predominantly controlled by isoprene, and NO3 oxidation produced organic nitrate with a mean value of 0.06 ppbv/h during nighttime. The organic nitrate production initiated by NO3 was equal to that initiated by OH, implying the importance of nighttime chemistry for secondary organic aerosol (SOA) formation. We confirmed that the N2O5 heterogeneous reaction accounted for nighttime particle NO3 enhancement, with a large day to day variability, and made less of a contribution to NOx loss compared to that of OH reacting with NO2. Additionally, abundant C1NO(2), up to 5.0 ppbv, was formed by N2O5 heterogeneous uptake. C1NO(2) was sustained at a high level until noon in spite of the gradually increasing photolysis of C1NO(2) after sunrise. Chlorine activation caused by N2O5 heterogeneous uptake increased primary ROx formation by 5% and accounted for 8% of the net ozone production enhancement in the morning.
To better understand the sources, formation, and the transport of air pollutants over North China Plain (NCP), a four-week intensive campaign during summertime in 2014 was conducted in a central NCP rural site. In this study, particle hygroscopicity and volatility measurements were focused to characterize the thermodynamic properties of nanoparticles and gain insight into chemical composition of nanoparticles during the new particle formation (NPF) events. The water-soluble fractions of 30 and 50 nm newly formed particles were respectively 0.64 +/- 0.06 and 0.61 +/- 0.06, indicating that the water-soluble chemical compounds, most likely ammonium sulfate, dominated the condensational growth of newly formed particles over the NCP. Due to containing higher water-soluble fraction, nanoparticles can be activated as cloud condensation nuclei (CCN) at lower supersaturation in the atmosphere of NCP in contrast to cleaner environments, such as Melpitz (Central European background) and Hyytiala (boreal forest) during the NPF events. Our observations showed that the NPF and subsequent growth significantly resulted in an enhancement in CCN number concentration. The ranges of enhancement factors of CCN number concentration for supersaturation (SS) = 0.2, 0.4, 0.8% were respectively 1.9-7.0, 2.7-8.4, and 3.6-10.1. After being heated to 300 degrees C, the shrink factors for 30 and 50 nm particles were respectively 0.35 and 038. This indicated that non-volatile compounds could be produced during the growth process of newly formed particles. (C) 2017 Elsevier B.V. All rights reserved.
Total OH reactivity measurements were conducted on the Peking University campus (Beijing) in August 2013 and in Heshan (Guangdong province) from October to November 2014. The daily median OH reactivity was 20 +/- 11 s(-1) in Beijing and 31 +/- 20 s(-1) in Heshan, respectively. The data in Beijing showed a distinct diurnal pattern with the maxima over 27 s(-1) in the early morning and minima below 16 s(-1) in the afternoon. The diurnal pattern in Heshan was not as evident as in Beijing. Missing reactivity, defined as the difference between measured and calculated OH reactivity, was observed at both sites, with 21% missing reactivity in Beijing and 32% missing reactivity in Heshan. Unmeasured primary species, such as branched alkenes, could contribute to missing reactivity in Beijing, especially during morning rush hours. An observation-based model with the RACM2 (Regional Atmospheric Chemical Mechanism version 2) was used to understand the daytime missing reactivity in Beijing by adding unmeasured oxygenated volatile organic compounds and simulated intermediates of the degradation from primary volatile organic compounds (VOCs). However, the model could not find a convincing explanation for the missing reactivity in Heshan, where the ambient air was found to be more aged, and the missing reactivity was presumably attributed to oxidized species, such as unmeasured aldehydes, acids and dicarbonyls. The ozone production efficiency was 21% higher in Beijing and 30% higher in Heshan when the model was constrained by the measured reactivity, compared to the calculations with measured and modeled species included, indicating the importance of quantifying the OH reactivity for better understanding ozone chemistry.
In 2014, a large, comprehensive field campaign was conducted in the densely populated North China Plain. The measurement site was located in a botanic garden close to the small town Wangdu, without major industry but influenced by regional transportation of air pollution. The loss rate coefficient of atmospheric hydroxyl radicals (OH) was quantified by direct measurements of the OH reactivity. Values ranged between 10 and 20 s(-1) for most of the daytime. Highest values were reached in the late night with maximum values of around 40 s(-1). OH reactants mainly originated from anthropogenic activities as indicated (1) by a good correlation between measured OH reactivity and carbon monoxide (linear correlation coefficient R-2 = 0 : 33) and (2) by a high contribution of nitrogen oxide species to the OH reactivity (up to 30% in the morning). Total OH reactivity was measured by a laser flash photolysis-laser-induced fluorescence instrument (LP-LIF). Measured values can be explained well by measured trace gas concentrations including organic compounds, oxygenated organic compounds, CO and nitrogen oxides. Significant, unexplained OH reactivity was only observed during nights, when biomass burning of agricultural waste occurred on surrounding fields. OH reactivity measurements also allow investigating the chemical OH budget. During this campaign, the OH destruction rate calculated from measured OH reactivity and measured OH concentration was balanced by the sum of OH production from ozone and nitrous acid photolysis and OH regeneration from hydroperoxy radicals within the uncertainty of measurements. However, a tendency for higher OH destruction compared to OH production at lower concentrations of nitric oxide is also observed, consistent with previous findings in field campaigns in China.
A comprehensive field campaign was carried out in summer 2014 in Wangdu, located in the North China Plain. A month of continuous OH, HO2 and RO2 measurements was achieved. Observations of radicals by the laser-induced fluorescence (LIF) technique revealed daily maximum concentrations between (5-15) x 10(6) cm(-3), (3-14) x 10(8) cm(-3) and (3-15) x 10(8) cm 3 for OH, HO2 and RO2, respectively. Measured OH reactivities (inverse OH lifetime) were 10 to 20 s(-1) during daytime. The chemical box model RACM 2, including the Leuven isoprene mechanism (LIM), was used to interpret the observed radical concentrations. As in previous field campaigns in China, modeled and measured OH concentrations agree for NO mixing ratios higher than 1 ppbv, but systematic discrepancies are observed in the afternoon for NO mixing ratios of less than 300 pptv (the model-measurement ratio is between 1.4 and 2 in this case). If additional OH recycling equivalent to 100 pptv NO is assumed, the model is capable of reproducing the observed OH, HO2 and RO2 concentrations for conditions of high volatile organic compound (VOC) and low NOx concentrations. For HO2, good agreement is found between modeled and observed concentrations during day and night. In the case of RO2, the agreement between model calculations and measurements is good in the late afternoon when NO concentrations are below 0.3 ppbv. A significant model underprediction of RO2 by a factor of 3 to 5 is found in the morning at NO concentrations higher than 1 ppbv, which can be explained by a missing RO2 source of 2 ppbvh(-1). As a consequence, the model underpredicts the photochemical net ozone production by 20 ppbv per day, which is a significant portion of the daily integrated ozone production (110 ppbv) derived from the measured HO2 and RO2. The additional RO2 production from the photolysis of ClNO2 and missing reactivity can explain about 10% and 20% of the discrepancy, respectively. The underprediction of the photochemical ozone production at high NOx found in this study is consistent with the results from other field campaigns in urban environments, which underlines the need for better understanding of the peroxy radical chemistry for high NOx conditions.
Simultaneous measurements of meteorological data, trace gases, and volatile organic compounds were made in two regional sites, viz. Backgarden and Kaiping, in the Pearl River Delta (PRD) during summer and autumn, respectively. The strong deviations from the NO-NO2-O-3 Photostationary State, quantified by the leighton ratios, are carefully deduced through a comprehensive data set consist of the high-quality measurements of NO, NO2, O-3 and JNoz as well as the peroxy radical measurements. This is the first report of the Leighton ratio in China, with relatively high recorded values of 2.3 +/- 0.4 (Backgarden) and 3.1 +/- 1.4 (Kaiping), suggesting a strongly oxidising atmosphere in the PRD, typical of the ozone pollution season. A sensitivity analysis using a zero-dimensional chemical box model based on the regional atmospheric chemistry mechanism, version 2 (RACM2) constrained by the experimental measurements, indicated that peroxy radicals account for 70 (Backgarden) and 66% (Kaiping) of the observed positive deviations from the NOx photostationary state (characterized by a Leighton ratio of 1) on average. We consider that the remaining deviations result from neglecting the effects of chlorine chemistry, so We introduced a Cl chemistry module into RACM2, and the modelled results for Cl were as follows: 4.7 x 10(-4) pptv in Backgarden and 1.3 x 10(-3) pptv in Kaiping; these results are lower than the CI concentration derived from the NOx photostationary state. More work is required to confirm the role of additional peroxy radical sources at both high and low NOx regimes, as well as that of the halogen radicals, in perturbing the NO-NOx-O-3 cycle, which would significantly enhance trace gas removal and photochemical ozone production. (C) 2017 Elsevier Ltd. All rights reserved.
A small and portable incoherent broadband cavityenhanced absorption spectrometer (IBBCEAS) for NO3 and N2O5 measurement has been developed. The instrument features a mechanically aligned non-adjustable optical mounting system, and the novel design of the optical mounting system enables a fast setup and stable operation in field applications. To remove the influence of the strong nonlinear absorption by water vapour, a dynamic reference spectrum through NO titration is used for the spectrum analysis. The wall loss effects of the sample system were extensively studied, and the total transmission efficiencies were determined to be 85 and 55% for N2O5 and NO3, respectively, for our experimental setup. The limit of detection (LOD) was estimated to be 2.4 pptv (1 sigma) and 2.7 pptv (1 sigma) at 1 s intervals for NO3 and N2O5, respectively. The associated uncertainty of the field measurement was estimated to be 19% for NO3 and 22-36% for N2O5 measurements from the uncertainties of transmission efficiency, absorption cross section, effective cavity length, and mirror reflectivity. The instrument was successfully deployed in two comprehensive field campaigns conducted in the winter and summer of 2016 in Beijing. Up to 1.0 ppb NO3 C N2O5 was observed with the presence of high aerosol loadings, which indicates an active night-time chemistry in Beijing.
In the last four decades, various techniques including spectroscopic, wet chemical and mass spectrometric methods, have been developed and applied for the detection of ambient nitrous acid (HONO). We developed a HONO detection system based on long path photometry which consists of three independent modules i.e., sampling module, fluid propulsion module and detection module. In the propulsion module, solenoid pumps are applied. With solenoid pumps the pulsed flow can be computer controlled both in terms of pump stroke volume and pulse frequency, which enables the attainment of a very stable flow rate. In the detection module, a customized Liquid Waveguide Capillary Cell (LWCC) is used. The customized LWCC pre-sets the optical fiber in-coupling with the liquid wave guide, providing the option of fast startup and easy maintenance of the absorption photometry. In summer 2014, our system was deployed in a comprehensive campaign at a rural site in the North China Plain. More than one month of high quality HONO data spanning from the limit of detection to 5 ppb were collected. Intercomparison of our system with another established system from Forschungszentrum Juelich is presented and discussed. In conclusion, our instrument achieved a detection limit of 10 pptV within 2 min and a measurement uncertainty of 7%, which is well suited for investigation of the HONO budget from urban to rural conditions in China. (C) 2016 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
Particulate pollution is a major air pollution problem in Chinese mega-cities. Under such conditions, the atmospheric gas-phase chemistry is strongly influenced by heterogeneous reactions, of which to quantify the heterogeneous reaction processes of N2O5 is essential for the understanding of the nighttime oxidation capacity, regional NOx budget, photochemical ozone prodution, etc. In this paper, we extensively review the research progress of the N2O5 heterogeneous reaction processes such as its reaction mechanism, measurement techniques of the corresponding uptake coefficient (gamma(N2O5)) and the measurement results on different aerosol substrates. The heterogeneous reaction processes of N2O5 is a typical reactive uptake process which can be ideally studied by the aerosol flow tube system. The corresponding laboratory kinetic studies are started from model aerosols (sulfate), and evolved to be more realistic aerosols according to the accumulated knowledges on the aerosol properties obtained in field studies. It is found that the gamma(N2O5) varied from 0. 001 to 0. 2 on different aerosol substrates, more than two orders of magnitude. The variation is influenced by the ambient temperature, relative humidity, mixing state, phase state, aerosol chemical compositions like NO3-, Cl-, SO42-, liquid water content (LWC), organics, etc., of which the uptake coefficient is higher with higher LWC, Cl-, SO42- while lower with higher NO3- and organics. The avaiable field studies in the United States and Europe showed that, to describe gamma(N2O5), these impact factors can' t be independently expressed; and the dependence seems to be very complicated and cross correlated. Therefore the state of art parameterization methods of gamma(N2O5) developed from lab kinetic studies are still not able to describe the field observations. Since high aerosol loading and high N2O5 are always co-located at urban aeras, more field observations and sucessful parameterization of gamma(N2O5) is proposed to be conducted in typical urban conditions including Chinese megacity regions.
The photolysis frequency of NO2, j(NO2), is an important analytical parameter in the study of tropospheric chemistry. A chemical actinometer (CA) was built to measure the ambient j(NO2) based on a high precision NOx instrument with 1 min time resolution. Parallel measurements of the ambient j(NO2) by using the CA and a commercial spectroradiometer (SR) were conducted at a typical urban site (Peking University Urban Environmental Monitoring Station) in Beijing. In general, good agreement was achieved between the CA and SR data with a high linear correlation coefficient (R-2 = 0.977) and a regression slope of 1.12. The regression offset was negligible compared to the measured signal level. The j(NO2) data were calculated using the tropospheric ultraviolet visible radiation (TUV) model, which was constrained to observe aerosol optical properties. The calculated j(NO2) was intermediate between the results obtained with CA and SR, demonstrating the consistency of all the parameters observed at this site. The good agreement between the CA and SR data, and the consistency with the TUV model results, demonstrate the good performance of the installed SR instrument. Since a drift of the SR sensitivity is expected by the manufacturer, we propose a regular check of the data acquired via SR against those obtained by CA for long-term delivery of a high quality series of j(NO2) data. Establishing such a time series will be invaluable for analyzing the long-term atmospheric oxidation capacity trends as well as O-3 pollution for urban Beijing. (C) Higher Education Press and Springer-Verlag Berlin Heidelberg 2016
Nitrate radical (NO3) and dinitrogen pentoxide (N2O5) are key species of the tropospheric chemistry, that play a central role in the tropospheric chemical issues such as atmospheric self cleansing capacity, secondary aerosol formations, reactive halogen chemistry, global sulfur cycles, etc. Nevertheless, the accurate and precise determination of both NO3 and N2O5 is still a challenging task due to their low ambient concentrations, high reactivity and short life time. In this paper, we summarize all kinds of measurement techniques used in the field observations of NO3 and N2O5, including differential optical absorption spectroscopy (DOAS), cavity ring-down spectroscopy (CRDS), cavity enhance absorption spectroscopy (CEAS), laser-induced fluorescence (LIF), matrix isolation electron spin resonance spectroscopy (MIESR), and chemical ionization mass spectrometry(CIMS). The advantages and disadvantages of those techniques are reviewed on the aspects of measurement accuracy, precision, time resolution, interference, calibration and operation stability. The absorption spectroscopy is the best technical approach, especially the subcategories-CRDS and CEAS developed in the last decade are the techniques with high potential of good performance in field applications. However, because high aerosol loadings are always presented in the atmosphere of the mega-city regions in China, the aerosol extinction could be a significant barrier to come over for the techniques based on absorption spectroscopy. Moreover, the observed NO3 and N2O5 concentrations and the major scientific findings of corresponding measurement campaigns conducted in typical tropospheric conditions as urban, forest, free troposphere and marine environments, etc. are outlined. Finally, we discuss the unresolved issues of the NO3 and N2O5 chemistry and possible new directions for future studies in chemically complex environments.
Nitrous acid (HONO), as a primary precursor of OH radicals, has been considered one of the most important nitrogen-containing species in the atmosphere. Up to 30% of primary OH radical production is attributed to the photolysis of HONO. However, the major HONO formation mechanisms are still under discussion. During the Campaigns of Air Quality Research in Beijing and Surrounding Region (CAREBeijing2006) campaign, comprehensive measurements were carried out in the megacity Beijing, where the chemical budget of HONO was fully constrained. The average diurnal HONO concentration varied from 0.33 to 1.2 ppbv. The net OH production rate from HONO, P (OH)(HONO)(net), was on average (from 05:00 to 19:00 h) 7.1 x 10(6) molecule/(cm(3) s), 2.7 times higher than from O-3 photolysis. This production rate demonstrates the important role of HONO in the atmospheric chemistry of megacity Beijing. An unknown HONO source (P (unknown)) with an average of 7.3 x 10(6) molecule/(cm(3) s) was derived from the budget analysis during daytime. P (unknown) provided four times more HONO than the reaction of NO with OH did. The diurnal variation of P (unknown) showed an apparent photo-enhanced feature with a maximum around 12:00 h, which was consistent with previous studies at forest and rural sites. Laboratory studies proposed new mechanisms to recruit NO2 and J(NO2) in order to explain a photo-enhancement of of P (unknown). In this study, these mechanisms were validated against the observation-constraint P (unknown). The reaction of exited NO2 accounted for only 6% of P (unknown), and P (unknown) poorly correlated with [NO2] (R = 0.26) and J(NO2)[NO2] (R = 0.35). These results challenged the role of NO2 as a major precursor of the missing HONO source.
The removal of trace gases from the troposphere is, in most cases, initialized by reactions with hydroxyl radicals, and the products of these reactions are eventually deposited on the Earth's surface. The concentration of these hydroxyl radicals is therefore a measure of atmospheric self-cleansing. In theory, hydroxyl-radical concentrations can be enhanced by the recycling of some of the reaction products. The only known efficient recycling process involves nitrogen oxide and leads to production of ozone, yet observations in regions with high hydrocarbon and low nitrogen oxide concentrations show substantially elevated hydroxyl-radical concentrations, up to ten times higher than expected. If we normalize observed hydroxyl-radical concentrations to the maximum achievable in model calculations with variable nitrogen oxide concentrations, this photochemical coordinate system uncovers a common feature in almost all of these observations: even in the presence of inadequate amounts of nitrogen oxides, hydroxyl-radical concentrations are enhanced to the theoretical maximum obtainable at very much higher nitrogen oxide concentrations. This means that this important part of the self-cleansing capability of the atmosphere is working at maximum efficiency even in regions with a high burden of biogenic hydrocarbons and low nitrogen oxide concentration. Since these processes do not involve nitrogen oxides, tropospheric ozone production is greatly reduced compared with the expectation from current theory.