Heterogeneous Reaction Of NO2 Research Articles

PM2.5 acidity at a background site in the Pearl River Delta region in fall-winter of 2007–2012

Based on field observations and thermodynamic model simulation, the annual trend of PM2.5 acidity and its characteristics on non-hazy and hazy days in fall-winter of 2007–2012 in the Pearl River Delta region were investigated. Total acidity ([H+]total) and in-situ acidity ([H+]in-situ) of PM2.5 significantly decreased (F-test, p<0.05) at a rate of −32±1.5nmolm−3year−1 and −9±1.7nmolm−3year−1, respectively. The variation of acidity was mainly caused by the change of the PM2.5 component, i.e., the decreasing rates of [H+]total and [H+]in-situ due to the decrease of sulfate (SO42−) exceeded the increasing rate caused by the growth of nitrate (NO3−). [H+]total, [H+]in-situ and liquid water content on hazy days were 0.9–2.2, 1.2–3.5 and 2.0–3.0 times those on non-hazy days, respectively. On hazy days, the concentration of organic matter (OM) showed significant enhancement when [H+]in-situ increased (t-test, p<0.05), while this was not observed on non-hazy days. Moreover, when the acidity was low (i.e., R=[NH4+]/(2×[SO42−]+[NO3−])>0.6), NH4NO3 was most likely formed via homogenous reaction. When the acidity was high (R≤0.6), the gas-phase formation of NH4NO3 was inhibited, and the proportion of NO3− produced via heterogeneous reaction of N2O5 became significant.

Exploring Possible Missing Sinks of Nitrate and Its Precursors in Current Air Quality Models —A Case Simulation in the Pearl River Delta, China, Using an Observation-Based Box Model

Nitrate and its precursor (gaseous HNO3) in China are generally overestimated by current chemical transport models in comparison to in-situ observations. In this study, we used an observation-based box model and in-situ measurements at a rural site in southern China to investigate possible missing sinks of nitrate. We found that a heterogeneous reaction of HNO3 to NOx on soot can better balance the NOx/HNO3 chemistry in models and improve model performance with regard to nitrate, particularly where an additional HONO source (the heterogeneous reaction of NO2 on soot) is incorporated into current models. Through a series of sensitivity simulations, the uptake coefficients of heterogeneous reactions were suggested to be 3.0 × 10−3 and 1 × 10−4 for HNO3 and HONO over southern China in fall, respectively. A 3-D simulation with the suggested uptake coefficients further confirmed that heterogeneous reactions significantly decreased the nitrate concentrations (PNO3) in southern China, by up to 10 μg m−3 (50%-80% of simulated PNO3) in polluted regions (e.g., the Yangtze River). In contrast to nitrate and HNO3, NOx concentrations in China were enhanced, which partly explained the underestimation of NO2 in current models compared to satellite observations.

Impacts of Additional HONO Sources on Concentrations and Deposition of NO&lt;sub&gt;y&lt;/sub&gt; in the Beijing-Tianjin-Hebei Region of China

This study applies the chemistry version of the Weather Research and Forecasting model (WRF-Chem) to examine impacts of three additional HONO sources, i.e., the reaction of photo-excited NO2 (NO2*) with water vapor (NO2* chemistry), the NO2 heterogeneous reaction on aerosol surfaces and HONO emissions, on concentrations and deposition of individual NOy species over the Beijing-Tianjin-Hebei region (BTH) in summer and winter periods of 2007. The results show that the three additional HONO sources produce a 20%∼40% (> 100%) increase in monthly-mean OH concentrations in many urban areas in August (February), leading to a 10%∼40% (10%∼100%) variation in monthly-mean concentrations of NOx, NO3− and PAN, a 5%∼10% (10%∼40%) increase in the total dry deposition of NOy, and an enhancement of 1.4 Gg N (1.5 Gg N) in the total of dry and wet deposition of NOy over this region in August (February). These results suggest that the additional HONO sources aggravate regional-scale acid deposition, emphasizing the importance of the additional HONO sources in the NOy budget.

Impacts of additional HONO sources on O3 and PM2.5 chemical coupling and control strategies in the Beijing–Tianjin–Hebei region of China

The objective of this work is to examine the impacts of additional HONO sources on the chemical interaction between ozone (O3) and particulate matter with a diameter≤2.5 µm (PM2.5). Three additional HONO sources, i.e. HONO emissions, the reaction of photo-excited nitrogen dioxide (NO2*) with water vapour (H2O), and NO2 heterogeneous reaction on aerosol surfaces, were inserted into the fully coupled Weather Research and Forecasting-Chemistry model to evaluate O3 and PM2.5 concentration enhancements in the Beijing–Tianjin–Hebei (BTH) region during August 2007. Results show that the additional HONO sources significantly increase O3 and PM2.5 concentrations during daytime. Up to 9 ppb enhancements of O3 and 32 µg m−3 increases in PM2.5 are found at seven urban sites over the BTH region. O3 increases are closely connected to PM2.5 increases over urban areas during daytime when the additional HONO sources are taken into account. PM2.5 inorganic components of , and are increased by 5–18, 10–58 and 10–40%, respectively, over urban areas during daytime. The simultaneous increment of O3 and PM2.5 during daytime due to the additional HONO sources is related to the increasing oxidants (OH, H2O2 and O3) that enhance the atmospheric oxidising capacity. The concentration variations of O3 and PM2.5 under a variety of NOx, volatile organic compound and ammonia (NH3) emission control scenarios show that the additional HONO sources increase the sensitivity of O3 and PM2.5 concentrations to the changes of NOx emissions. An increase of the PM2.5 sensitivity to changes in NH3 emissions is also found. This indicates that without considering the additional HONO sources, the effectiveness of emission control strategies in reducing O3 and PM2.5 concentrations would be significantly underestimated.

Heterogeneous reactions of NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; on the surface of MgO particles

氧化镁(MgO)是大气中矿物气溶胶的重要组分之一, 对二次污染物的形成有着重要影响. 本研究采用原位漫反射红外傅里叶变换光谱(DRIFTS)与离子色谱(IC)技术, 研究了二氧化氮(NO 2 )在MgO颗粒表面的非均相反应. 探讨了无光照、紫外光照、臭氧(O 3 )、温度及相对湿度( RH )等对该反应的影响机制, 建立了新的测定摄取系数的方法. 结果表明, 无光照时, NO 2 在MgO颗粒表面生成的主要产物为硝酸盐和亚硝酸盐; 在NO 2 -MgO-O 3 和NO 2 - MgO- h n 两种反应体系中主要产物均为硝酸盐, 生成的硝酸根峰面积分别是无光照条件下的1.54倍和3.04倍, O 3 和紫外光照对硝酸盐的生成均具有促进作用; 在紫外光照条件下, NO 2 在MgO颗粒物表面生成硝酸根的初始速率随温度的升高而呈单峰变化, 40℃时初始速率最大; 影响NO 2 与MgO颗粒物反应的敏感因素为紫外光照和臭氧, 其次为相对湿度和温度. 在25℃, RH 为5%时, 无光照条件和紫外光照条件下反应初始摄取系数分别为9.01 × 10 -4 和5.65 × 10 -3 .

Heterogeneous interaction of SiO2 with N2O5: aerosol flow tube and single particle optical levitation-Raman spectroscopy studies.

Silica (SiO2) is an important mineral present in atmospheric mineral dust particles, and the heterogeneous reaction of N2O5 on atmospheric aerosol is one of the major pathways to remove nitrogen oxides from the atmosphere. The heterogeneous reaction of N2O5 with SiO2 has only been investigated by two studies previously, and the reported uptake coefficients differ by a factor of >10. In this work two complementary laboratory techniques were used to study the heterogeneous reaction of SiO2 particles with N2O5 at room temperature and at different relative humidities (RHs). The uptake coefficients of N2O5, γ(N2O5), were determined to be (7.2 ± 0.6) × 10(-3) (1σ) at 7% RH and (5.3 ± 0.8) × 10(-3) (1σ) at 40% RH for SiO2 particles, using the aerosol flow tube technique. We show that γ(N2O5) determined in this work can be reconciled with the two previous studies by accounting for the difference in geometric and BET derived aerosol surface areas. To probe the particle phase chemistry, individual micrometer sized SiO2 particles were optically levitated and exposed to a continuous flow of N2O5 at different RHs, and the composition of levitated particles was monitored online using Raman spectroscopy. This study represents the first investigation into the heterogeneous reactions of levitated individual SiO2 particles as a surrogate for mineral dust. Relative humidity was found to play a critical role: while no significant change of particle composition was observed by Raman spectroscopy during exposure to N2O5 at RH of <2%, increasing the RH led to the formation of nitrate species on the particle surface which could be completely removed after decreasing the RH back to <2%. This can be explained by the partitioning of HNO3 between the gas and adsorbed phases. The atmospheric implications of this work are discussed.

Heterogeneous reaction of NO2 on α-Al2O3 in the dark and simulated sunlight.

Although there have been a number of publications focused on heterogeneous of NO2 on mineral particles, most of these studies were focused on β-Al2O3 and performed in the dark. Less was known about the reaction process of NO2 on α-Al2O3, especially the effect of sunlight factor. The heterogeneous reaction between NO2 and α-Al2O3 was investigated by using diffuse reflectance infrared Fourier transform spectrometry. The effects of NO2 and O2 concentrations as well as simulated sunlight were examined, and the reaction mechanism including the consumption of surface OH groups, oxidation process of nitrites, and the formation of water was also discussed in detail. It was observed that the formation rates of nitrates and nitrites were sensitive to NO2 concentrations and O2 concentrations. Nitrite was identified to be an intermediate production and disappeared very soon as [NO2] was up to 4.035 × 10(15) molecules/cm(3). Light played an important role in the changes of the electronic configuration of mineral dust, such as electronic donating ability, surface OH groups orientation, as well as the conversion efficiency between proton acid and nonproton acid, all of which could significantly enhance the heterogeneous reaction process. The reaction order for NO2 and O2 was determined to be 0.960 ± 0.111 and 0.620 ± 0.028, respectively. The uptake coefficient of NO2, which dominated the first step of the heterogeneous reaction, was calculated by the infrared absorbance with the use of ion chromatography and determined to be 9.9 × 10(-10) in the dark and varied from 2.54 to 3.33 × 10(-9) under simulated sunlight from 0.45 to 1.35 mW/cm(2). It was also found that γNO2 was independent of [NO2] and sunlight increased the uptake coefficient by three times, indicating that the heterogeneous reaction between NO2 and α-Al2O3 was enhanced under sunlight.

Effects of additional HONO sources on visibility over the North China Plain

The objective of the present study was to better understand the impacts of the additional sources of nitrous acid (HONO) on visibility, which is an aspect not considered in current air quality models. Simulations of HONO contributions to visibility over the North China Plain (NCP) during August 2007 using the fully coupled Weather Research and Forecasting/Chemistry (WRF/Chem) model were performed, including three additional HONO sources: (1) the reaction of photo-excited nitrogen dioxide (NO* 2) with water vapor; (2) the NO2 heterogeneous reaction on aerosol surfaces; and (3) HONO emissions. The model generally reproduced the spatial patterns and diurnal variations of visibility over the NCP well. When the additional HONO sources were included in the simulations, the visibility was occasionally decreased by 20%–30% (3–4 km) in local urban areas of the NCP. Monthly-mean concentrations of NO 3 − , NH 4 + , SO 4 2− and PM2.5 were increased by 20%–52% (3–11 μg m−3), 10%–38%, 6%–10%, and 6%–11% (9–17 μg m−3), respectively; and in urban areas, monthly-mean accumulationmode number concentrations (AMNC) and surface concentrations of aerosols were enhanced by 15%–20% and 10%–20%, respectively. Overall, the results suggest that increases in concentrations of PM2.5, its hydrophilic components, and AMNC, are key factors for visibility degradation. A proposed conceptual model for the impacts of additional HONO sources on visibility also suggests that visibility estimation should consider the heterogeneous reaction on aerosol surfaces and the enhanced atmospheric oxidation capacity due to additional HONO sources, especially in areas with high mass concentrations of NO x and aerosols.

Formation of nitroanthracene and anthraquinone from the heterogeneous reaction between NO2 and anthracene adsorbed on NaCl particles.

Oxidative derivatives of polycyclic aromatic hydrocarbons (PAHs), that is, nitro-PAHs and quinones, are classed as hazardous semivolatile organic compounds but their formation mechanism from the heterogeneous reactions of PAHs adsorbed on atmospheric particles is not well understood. The heterogeneous reaction of NO2 with anthracene adsorbed on NaCl particles under different relative humidity (RH 0-60%) was investigated under dark conditions at 298 K. The formation of the major products, 9,10-anthraquinone (9,10-AQ) and 9-nitroanthracene (9-NANT), were determined to be second-order reactions with respect to NO2 concentration. The rate of formation of 9,10-AQ under low RH (0-20%) increased as the RH increased but decreased when the RH was further increased in high RH (40-60%). In contrast, the rate of formation of 9-NANT across the whole RH range (0-60%) decreased significantly with increasing RH. Two different reaction pathways are discussed for the formation of 9,10-AQ and 9-NANT, respectively, and both are considered to be coupled to the predominant reaction of NO2 with the NaCl substrate. These results suggest that relative humidity, which controls the amount of surface adsorbed water on NaCl particles, plays an important role in the heterogeneous reaction of NO2 with adsorbed PAHs.

Light-induced nitrous acid (HONO) production from NO2 heterogeneous reactions on household chemicals

Nitrous acid (HONO) can be generated in various indoor environments directly during combustion processes or indirectly via heterogeneous NO2 reactions with water adsorbed layers on diverse surfaces. Indoors not only the concentrations of NO2 are higher but the surface to volume (S/V) ratios are larger and therefore the potential of HONO production is significantly elevated compared to outdoors. It has been claimed that the UV solar light is largely attenuated indoors.Here, we show that solar light (λ > 340 nm) penetrates indoors and can influence the heterogeneous reactions of gas-phase NO2 with various household surfaces. The NO2 to HONO conversion mediated by light on surfaces covered with domestic chemicals has been determined at atmospherically relevant conditions i.e. 50 ppb NO2 and 50% RH. The formation rates of HONO were enhanced in presence of light for all the studied surfaces and are determined in the following order: 1.3·109 molecules cm−2 s−1 for borosilicate glass, 1.7·109 molecules cm−2 s−1 for bathroom cleaner, 1.0·1010 molecules cm−2 s−1 on alkaline detergent (floor cleaner), 1.3·1010 molecules cm−2 s−1 for white wall paint and 2.7·1010 molecules cm−2 s−1 for lacquer.These results highlight the potential of household chemicals, used for cleaning purposes to generate HONO indoors through light-enhanced NO2 heterogeneous reactions.The results obtained have been applied to predict the timely evolution of HONO in a real indoor environment using a dynamic mass balance model. A steady state mixing ratio of HONO has been estimated at 1.6 ppb assuming a contribution from glass, paint and lacquer and considering the photolysis of HONO as the most important loss process.

Kinetics of NO Reduction by Coal, Biomass, and Graphitic Chars: Effects of Burnout Level and Conditions

During the combustion of coal and other carbonaceous materials, the heterogeneous reaction of NO with the evolving char created in the combustion process is important for understanding the formation and reduction of NO. This investigation quantifies the effects of char burnout level and conditions on the kinetics of NO reduction by chars made from coals ranging in rank from lignite to low-volatile bituminous (Beulah Zap, Dietz, Utah Blind Canyon, Pittsburgh #8, and Pocahontas #3) as well as graphite and coconut char. Kinetic data were measured in a packed-bed reactor at temperatures between 723 and 1173 K. The rate constant for the NO−char reaction was found to depend upon the extent of burnout/conversion and conditions under which the char was burned out. The NO−char rate constant consistently decreases with increasing burnout when the char burnout levels are accomplished in a drop tube reactor at 1800 K and 3−5% O2. However, the NO−char rate constant increases as char burnout increases (up to 90%) when the char burnout levels result from reacting the char with 3050 ppm of NO in a packed bed at 723−1173 K. For the latter case, the relationship of the NO−char rate constant (on the basis of moisture ash-free char mass) and char burnout is approximately linear with roughly the same upward slope between 20 and 80% burnout for all coal chars studied. The activation energy of the NO−char reaction is apparently independent of both char burnout level and burnout conditions. The CO2/CO ratio in the exhaust stream appears to correlate with the NO−char rate constant for the chars burned out at low-temperature conditions.

Formation of indoor nitrous acid (HONO) by light-induced NO2 heterogeneous reactions with white wall paint.

Gaseous nitrogen dioxide (NO2) represents an oxidant that is present in relatively high concentrations in various indoor settings. Remarkably increased NO2 levels up to 1.5 ppm are associated with homes using gas stoves. The heterogeneous reactions of NO2 with adsorbed water on surfaces lead to the generation of nitrous acid (HONO). Here, we present a HONO source induced by heterogeneous reactions of NO2 with selected indoor paint surfaces in the presence of light (300 nm<λ<400 nm). We demonstrate that the formation of HONO is much more pronounced at elevated relative humidity. In the presence of light (5.5 W m(-2)), an increase of HONO production rate of up to 8.6·10(9) molecules cm(-2) s(-1) was observed at [NO2]=60 ppb and 50% relative humidity (RH). At higher light intensity of 10.6 (W m(-2)), the HONO production rate increased to 2.1·10(10) molecules cm(-2) s(-1). A high NO2 to HONO conversion yield of up to 84% was observed. This result strongly suggests that a light-driven process of indoor HONO production is operational. This work highlights the potential of paint surfaces to generate HONO within indoor environments by light-induced NO2 heterogeneous reactions.

Heterogeneous reaction of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; with illite and Arizona test dust particles

Abstract. The heterogeneous reaction of N2O5 with airborne illite and Arizona test dust (ATD) particles was investigated at room temperature and at different relative humidities using an atmospheric pressure aerosol flow tube. N2O5 at concentrations in the range 8 to 24 × 1012 molecule cm−3 was monitored using thermal-dissociation cavity ring-down spectroscopy at 662 nm. At zero relative humidity a large uptake coefficient of N2O5 to illite was obtained, γ(N2O5) = 0.09, which decreased to 0.04 as relative humidity was increased to 67%. In contrast, the uptake coefficient derived for ATD is much lower (~0.006) and displays a weaker (if any) dependence on relative humidity (0–67%). Potential explanations are given for the significant differences between the uptake behaviour for ATD and illite and the results are compared with uptake coefficients for N2O5 on other mineral surfaces.

Heterogeneous Reaction of NO2 on Al2O3: The Effect of Temperature on the Nitrite and Nitrate Formation

Although recent evidence suggests that the heterogeneous reaction of NO2 on the surface of mineral aerosol plays an important role in the atmospheric chemistry, a fundamental understanding of how temperature influences the rate and extent of nitrate formation processes remains unclear. This work presents the first laboratory study of the effect of temperature on heterogeneous reaction of NO2 on the surface of γ-Al2O3 in the temperature range of 250-318 K at ambient pressure. From the analysis of IR spectra, nitrite was found to be an intermediate product at temperatures between 250 and 318 K. It is proved by our experiments that nitrite would convert to the bidentate nitrate as the reaction proceeded. In addition, it is interesting to find that the rate of conversion increased with decreasing temperature. Along with nitrite decrease, the initial rate of nitrate formation increased while the rate of nitrate formation in the steady region decreased with decreasing temperature. The uptake coefficients at seasonal temperatures were determined for the first time and were found to be sensitive to temperature. Finally, atmospheric implications of the role of temperature on the heterogeneous reaction of NO2 with mineral aerosol are discussed.

Role of Organic Carbon in Heterogeneous Reaction of NO2 with Soot

A large uncertainty among the reported uptake coefficients of NO2 on soot highlights the importance of the composition of soot in this reaction. Soot samples with different fractions of organic carbon (OC) were prepared by combusting n-hexane under controlled conditions. The heterogeneous reaction of NO2 on soot was investigated using a flow tube reactor at ambient pressure. The soot with the highest fuel/oxygen ratio showed the largest uptake coefficient (γ(initial)) of NO2 and yield of HONO (yHONO). Compared to fresh soot samples, preheated samples exhibited a great decrease in uptake coefficient of NO2 and HONO yield due to the removal of OC from soot. Ozonized soot also showed a lower reactivity toward NO2 than fresh soot, which can be ascribed to the consumption of OC with a reduced state (OCR). A linear dependence of the NO2 uptake coefficient and yields of HONO and NO on the OCR content of the soot was established, with γ(initial)(NO2) = (1.54 ± 1.39) × 10(-6) + (1.96 ± 0.35) × 10(-7) × OCR, yHONO = (11.6 ± 16.1) + (1.3 ± 0.40) × OCR, and yNO = (13.1 ± 1.9) - (0.2 ± 0.05) × OCR, respectively.

Heterogeneous reaction of NO2 on the surface of montmorillonite particles

The studies on heterogeneous reactions over montmorillonite, which is a typical 2:1 layered aluminosilicate, will benefit to the understanding of heterogeneous reactions on clay minerals. Montmorillonite can be classified as sodium montmorillonite or calcium montmorillonite depending on the cation presented between the different layers. Using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the heterogeneous reaction mechanism of NO2 on the surface of montmorillonite was firstly investigated. Results showed that the reaction of NO2 on the surface of sodium and calcium montmorillonite fit a first-order kinetics, and the reaction duration of calcium montmorillonite was longer than that of sodium montmorillonite under the dry condition. For either sodium or calcium montmorillonite, the uptake coefficient decreased as humidity increased.

Kinetic Studies of Heterogeneous Reactions of Polycyclic Aromatic Hydrocarbon Aerosols with NO3Radicals

Polycyclic aromatic hydrocarbons (PAHs) and their derivates are mutagenic and carcinogenic substances widely distributed in the atmospheric environment. In this study, effective rate constants for heterogeneous reactions of NO(3) radicals with five 4-ring PAHs [benzo[a]anthracene (BaA), chrysene (Ch), pyrene (Py), 1-nitropyrene (1-NP), and 1-hydroxypyrene (1-OHP)] adsorbed on suspended azelaic acid particles are investigated by a mixed-phase relative rate method with gas-phase isoprene as the reference substance. The concentrations of suspended PAH particles and gas-phase isoprene are monitored concurrently by a real-time vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS) and an online atmospheric gas analysis mass spectrometer, respectively. The obtained effective rate constants for the reactions of BaA, Ch, Py, 1-NP, and 1-OHP particles with NO(3) radicals are 4.3 × 10(-12), 4.0 × 10(-12), 6.4 × 10(-12), 1.3 × 10(-12), and 1.0 × 10(-11) cm(3)·molecule(-1)·s(-1), respectively, and their corresponding atmospheric lifetimes range from several minutes to half an hour at the NO(3) radical concentration of 5 × 10(8) molecules·cm(-3). In addition, the NO(3) uptake coefficients on particulate PAHs are estimated according to the consumption of PAHs under the exposure of NO(3) radicals. The experimental results of these heterogeneous reactions in the aerosol state provide supplementary knowledge for kinetic behaviors of airborne PAHs particles.

Aerosol effects on ozone concentrations in Beijing: A model sensitivity study

AbstractMost previous O3 simulations were based only on gaseous phase photochemistry. However, some aerosol-related processes, namely, heterogeneous reactions occurring on the aerosol surface and photolysis rate alternated by aerosol radiative influence, may affect O3 photochemistry under high aerosol loads. A three-dimensional air quality model, Models-3/Community Multi-scale Air Quality-Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution, was employed to simulate the effects of the above-mentioned processes on O3 formation under typical high O3 episodes in Beijing during summer. Five heterogeneous reactions, i.e., NO2, NO3, N2O5, HO2, and O3, were individually investigated to elucidate their effects on O3 formation. The results showed that the heterogeneous reactions significantly affected O3 formation in the urban plume. NO2 heterogeneous reaction increased O3 to 90 ppb, while HO2 heterogeneous reaction decreased O3 to 33 ppb. In addition, O3 heterogeneous loss decreased O3 to 31 ppb. The effects of NO2, NO3, and N2O5 heterogeneous reactions showed opposite O3 concentration changes between the urban and extra-urban areas because of the response of the reactions to the two types of O3 formation regimes. When the aerosol radiative influence was included, the photolysis rate decreased and O3 decreased significantly to 73 ppb O3. The two aerosol-related processes should be considered in the study of O3 formation because high aerosol concentration is a ubiquitous phenomenon that affects the urban- and regional air quality in China.

Physical and chemical processes of wintertime secondary nitrate aerosol formation

The UCD/CIT model was modified to include a process analysis (PA) scheme for gas and particulate matter (PM) to study the formation of secondary nitrate aerosol during a stagnant wintertime air pollution episode during the California Regional PM2.5/PM10 Air Quality Study (CRPAQS) where detailed measurements of PM components are available at a few sites. Secondary nitrate is formed in the urban areas from near the ground to a few hundred meters above the surface during the day with a maximum modeled net increase rate of 4 μg·m−3·d−1 during the study episode. The secondary nitrate formation rate in rural areas is lower due to lower NO2. In the afternoon hours, near-surface temperature can be high enough to evaporate the particulate nitrate. In the nighttime hours, both the gas phase N2O5 reactions with water vapor and the N2O5 heterogeneous reactions with particle-bound water are important for secondary nitrate formation. The N2O5 reactions are most import near the surface to a few hundred meters above surface with a maximum modeled net secondary nitrate increase rate of 1 μg·m−3·d−1 and are more significant in the rural areas where the O3 concentrations are high at night. In general, vertical transport during the day moves the nitrate formed near the surface to higher elevations. During the stagnant days, process analysis indicates that the nitrate concentration in the upper air builds up and leads to a net downward flux of nitrate through vertical diffusion and a rapid increase of surface nitrate concentration.

Heterogeneous reaction of NO2 with sea salt particles

To understand how NO2 reacts with sea salt particles in the atmosphere of Mega-cities in coastal zones, the heterogeneous reaction of NO2 on the surface of wet sea salt was investigated with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and ion chromatography (IC). Kinetic measurements indicated that nitrate formation on sea salt was second order in NO2 concentration and reactive uptake coefficients were (5.51 ± 0.19) × 10−7 and 1.26 ± 10−6 respectively under 0% and 20% relative humidity (RH) at NO2 molecular concentration of 1.96 × 1015 mol/cm3. The results showed that liquid water was formed at the site of MgCl2·6H2O, CaCl2·2H2O on the surface of sea salt and made the reaction more sustainable by releasing hydrated water and absorbing water from air even under a low RH (⩽ 30%). Therefore, pure NaCl particles should not be used to represent sea salt in studies of the heterogeneous reaction with NO2.