Abstract. Organic aerosol (OA) concentrations are simulated over the Beijing-Tianjin-Hebei (BTH) region from 9 to 26 January, 2014 using the Weather Research and Forecasting model coupled with chemistry (WRF-CHEM), with the goal of examining the impact of heterogeneous HONO sources on SOA formation and the SOA formation from different pathways during wintertime haze days. The model generally performs well in simulating air pollutants and organic aerosols against measurements in BTH. Model results show that heterogeneous HONO sources substantially enhance the near-surface SOA formation, increasing regional average near-surface SOA concentration by about 46.3 % during the episode. Oxidation and partitioning of primary organic aerosols treated as semi-volatile dominate the SOA formation, contributing 58.9 % of the near-surface SOA mass in BTH. Irreversible uptake of glyoxal and methylglyoxal on aerosol surfaces constitutes the second most important SOA formation pathway during the episode, with SOA contribution increasing from 8.5 % in non-haze conditions to 30.2 % in haze conditions. Additionally, direct emissions of glyoxal and methylglyoxal from residential living sources contribute about 25.5 % to the total SOA mass on average in BTH. Our study highlights the importance of heterogeneous HONO sources and primary residential emissions of glyoxal and methylglyoxal to SOA formation in winter over BTH.
Abstract. PM2.5, particulate matter with a diameter of 2.5 µm or less, is one of the major components of air pollution in eastern China. In the past few years, China's government has made strong efforts to reduce PM2.5 pollution. However, another important pollutant (ozone) is becoming a problem in eastern China. Ozone (O3) is produced by photochemistry, which requires solar radiation for the formation of O3. Under heavy PM2.5 pollution, solar radiation is often depressed, and the photochemical production of O3 is prohibited. This study shows that during late spring and early fall in eastern China, under heavy PM2.5 pollution, there was often strong O3 photochemical production, causing a co-occurrence of high PM2.5 and O3 concentrations. This co-occurrence of high PM2.5 and O3 is unusual and is the main focus of this study. Recent measurements show that there were often high HONO surface concentrations in major Chinese megacities, especially during daytime, with maximum concentrations ranging from 0.5 to 2 ppbv. It is also interesting to note that high HONO concentrations occurred during high aerosol concentration periods, suggesting that there were additional HONO surface sources in eastern China. Under high daytime HONO concentrations, HONO can be photodissociated to OH radicals, which enhance the photochemical production of O3. In order to study the above scientific issues, a radiative transfer model (TUV; tropospheric ultraviolet–visible) is used in this study, and a chemical steady-state model is established to calculate OH radical concentrations. The calculations show that by including the OH production of photodissociated HONO, the calculated OH concentrations are significantly higher than the values without including this production. For example, by including HONO production, the maximum OH concentration under high aerosol conditions (AOD = 2.5) is similar to the value under low aerosol conditions (AOD = 0.25) in the no-HONO case. This result suggests that even under high aerosol conditions, the chemical oxidizing process for O3 production can occur, which explains the co-occurrence of high PM2.5 and high O3 in late spring and early fall in eastern China. However, the O3 concentrations were not significantly affected by the appearance of HONO in winter. This study shows that the seasonal variation of solar radiation plays important roles for controlling the OH production in winter. Because solar radiation is at a very low level in winter, adding the photolysis of HONO has a smaller effect in winter than in other seasons, and OH remains at low values by including the HONO production term. This study provides some important scientific insight to better understand O3 pollution in eastern China.
Abstract. High temporal resolution measurements of black carbon (BC) and organic carbon (OC) covering the time period of 1956–2006 in an ice core over the southeastern Tibetan Plateau show a distinct seasonal dependence of OC / BC ratio with higher values in the non-monsoon season than during the summer monsoon. We use a global aerosol-climate model, in which BC emitted from different source regions can be explicitly tracked, to quantify BC source-receptor relationships between four Asian source regions and the southeastern Tibetan Plateau as a receptor. The model results show that South Asia is a primary contributor during the non-monsoon season (October to May) (81%) and on an annual basis (74%), followed by East Asia (14% and 21%, respectively). The ice-core record also indicates stable and relatively low BC and OC deposition fluxes from late 1950s to 1980, followed by an overall increase to recent years. This trend is consistent with the BC and OC emission inventories and the fuel consumption of South Asia as the primary contributor. Moreover, the increasing trend of OC / BC ratio since the early 1990s indicates a growing contribution of coal combustion and biomass burning to the emissions. The estimated radiative forcing induced by BC and OC impurities in snow has increased since 1980, suggesting an increasing influence of carbonaceous aerosols on the Tibetan glacier melting and the availability of water resources in the surrounding regions. Our study indicates that more attention to OC is merited because of its non-negligible light absorption and the recent rapid increases evident in the ice core record.
An episodic simulation is conducted to characterize ozone (O 3 ) formation and to investigate the dependence of O 3 formation on precursors in the Houston‐Galveston (HG) area using a regional chemical transport model (CTM). The simulated net photochemical O 3 production rates, P (O 3 ), in the Houston area are higher than those in most other U.S. urban cities, reaching 20–40 ppb hr −1 for the daytime ground NO x levels of 5–30 ppb. The NO x turnaround value (i.e., the NO x concentration at which P (O 3 ) reaches a maximum) is also larger than those observed in most other U.S. cities. The large abundance and high reactivity of anthropogenic volatile organic compounds (AVOCs) and the coexistence of abundant AVOCs and NO x in this area are responsible for the high O 3 production rates and the NO x turnaround value. The simulated O 3 production efficiency is typically 3–8 O 3 molecules per NO x molecule oxidized during the midday hours. The simulation reveals a RO 2 peak up to 70 ppt at night, and the reactions of alkene‐NO 3 and alkene‐O 3 are responsible for more than 80% of the nighttime RO 2 in the residual layer, contributing to over 70% and about 10%, respectively. Isoprene accounts for about 40% of the nighttime RO 2 peak concentration. The nighttime RO 2 level is limited by the availability of alkenes. Hydrolysis of N 2 O 5 on sulfate aerosols leads to an increase of HNO 3 by as much as 30–60% but to a decrease of NO x by 20–50% during the night in the lower troposphere. Heterogeneous conversion of NO 2 to HONO on the surfaces of soot aerosol accelerates the O 3 production by about 1 hour in the morning and leads to a noticeable increase of 7 ppb on average in the daytime O 3 level. The sensitivity study suggests that the near‐surface chemistry over most of the Houston metropolitan area is in or close to the NO x ‐VOC transition regime on the basis of the current emission inventory. Doubling AVOC emissions leads to the NO x sensitive chemistry. Biogenic VOCs contribute about 5% on the average to the total near‐surface O 3 in the Houston area.
Abstract. Water vapor has been proposed to amplify the severe haze pollution in China by enhancing the aerosol-radiation feedback (ARF). Observations have revealed that the near-surface PM2.5 concentrations ([PM2.5]) generally exhibits an increasing trend with relative humidity (RH) in North China Plain (NCP) during 2015 wintertime, indicating that the aerosol liquid water (ALW) caused by hygroscopic growth could play an important role in the PM2.5 formation and accumulation. Simulations during a persistent and heavy haze pollution episode from 05 December 2015 to 04 January 2016 in NCP were conducted using the WRF-CHEM model to comprehensively quantify contributions of the ALW effect to near-surface [PM2.5]. The WRF-CHEM model generally performs reasonably well in simulating the temporal variations of RH against measurements in NCP. The factor separation approach (FSA) was used to evaluate the contribution of the ALW effect on the ARF, photochemistry, and heterogeneous reactions to [PM2.5]. The ALW not only augments particle sizes to enhance aerosol backward scattering, but also increases the effective radius to favor aerosol forward scattering. The contribution of the ALW effect on the ARF and photochemistry to near [PM2.5] is not significant, generally within 1.0 μg m−3 on average in NCP during the episode. Serving as an excellent substrate for heterogeneous reactions, the ALW substantially enhances the secondary aerosol (SA) formation, with an average contribution of 71 %, 10 %, 26 %, and 48 % to near-surface sulfate, nitrate, ammonium, and secondary organic aerosol concentrations. Nevertheless, the SA enhancement due to the ALW decreases the aerosol optical depth and increases the effective radius to weaken the ARF, reducing near-surface primary aerosols. The contribution of the ALW total effect to near-surface [PM2.5] is 17.5 % on average, which is overwhelmingly dominated by enhanced SA. Model sensitivities also show that when the RH is less than 80 %, the ALW progressively increases near-surface [PM2.5], but commences to decrease when the RH exceeding 80% due to the high occurrence frequencies of precipitation.
Abstract. The NO-NO2 system was analyzed in different chemical regimes/air masses based on observations of reactive nitrogen species and peroxy radicals made during the intensive field campaign MIRAGE-Mex (4 to 29 March 2006). The air masses were categorized into 5 groups based on combinations of macroscopic observations, geographical location, meteorological parameters, models, and observations of trace gases: boundary layer (labeled as "BL"), biomass burning ("BB"), free troposphere (continental, "FTCO" and marine, "FTMA"), and Tula industrial complex ("TIC"). In general, NO2/NO ratios in different air masses are near photostationary state. Analysis of this ratio can be useful for testing current understanding of tropospheric chemistry. The ozone production efficiency (OPE) for the 5 air mass categories ranged from 4.5 (TIC) to 8.5 (FTMA), consistent with photochemical aging of air masses exiting the Mexico City Metropolitan Area.
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