Aerosol effects on the photochemistry in Mexico City during MCMA-2006/MILAGRO campaign

In the present study, the impact of aerosols on the photochemistry in Mexico City is evaluated using the WRF-CHEM model for the period from 24 to 29 March during the MCMA-2006/MILAGRO campaign. An aerosol radiative module has been developed with detailed consideration of aerosol size, composition, and mixing. The module has been coupled into the WRF-CHEM model to calculate the aerosol optical properties, including optical depth, single scattering albedo, and asymmetry factor. Calculated aerosol optical properties are in good agreement with the surface observations and aircraft and satellite measurements during daytime. In general, the photolysis rates are reduced due to the absorption by carbonaceous aerosols, particularly in the early morning and late afternoon hours with a long aerosol optical path. However, with the growth of aerosol particles and the decrease of the solar zenith angle around noontime, aerosols can slightly enhance photolysis rates when ultraviolet (UV) radiation scattering dominates UV absorption by aerosols at the lower-most model layer. The changes in photolysis rates due to aerosols lead to about 2–17 % surface ozone reduction during daytime in the urban area in Mexico City with generally larger reductions during early morning hours near the city center, resulting in a decrease of OH level by about 9 %, as well as a decrease in the daytime concentrations of nitrate and secondary organic aerosols by 5–6 % on average. In addition, the rapid aging of black carbon aerosols and the enhanced absorption of UV radiation by organic aerosols contribute substantially to the reduction of photolysis rates.National Science Foundation (U.S.). Atmospheric Chemistry Program (ATM-0528227)National Science Foundation (U.S.). Atmospheric Chemistry Program (ATM-0810931)Molina Center for Energy and the Environmen

Impacts of sea-land and mountain-valley circulations on the air pollution in Beijing-Tianjin-Hebei (BTH): A case study

In the study, observational data analyses and the WRF-CHEM model simulations are used to investigate the role of sea-land and mountain-valley breeze circulations in a severe air pollution event occurred in Beijing-Tianjin-Hebei (BTH) during August 9-10, 2013. Both the wind observations and the model simulations have clearly indicated the evolution of the sea-land and mountain-valley breeze circulations during the event. The WRF-CHEM model generally reproduces the local meteorological circulations and also performs well in simulating temporal variations and spatial distributions of fine particulate matters (PM2.5) and ozone (O-3) concentrations compared to observations in BTH. The model results have shown that the offshore land breeze transports the pollutants formed in Shandong province to the Bohai Gulf in the morning, causing the formation of high O-3 and PM2.5 concentrations over the gulf. The onshore sea breeze not only causes the formation of a convergence zone to induce upward movement, mitigating the surface pollution to some degree, also recirculates the pollutants over the gulf to deteriorate the air quality in the coastal area. The upward valley breeze brings the pollutants in the urban area of Beijing to the mountain area in the afternoon, and the downward mountain breeze transports the pollutants back during nighttime. The intensity of the mountain-valley breeze circulation is weak compared to the land-sea breeze circulation in BTH. It is worth noting that the local circulations play an important role when the large-scale meteorological conditions are relatively weak. (C) 2017 Elsevier Ltd. All rights reserved

Evaluation of WRF mesoscale simulations and particle trajectory analysis for the MILAGRO field campaign

Accurate numerical simulations of the complex wind flows in the Mexico City Metropolitan Area (MCMA) can be an invaluable tool for interpreting the MILAGRO field campaign results. This paper uses three methods to evaluate numerical simulations of basin meteorology using the MM5 and WRF models: statistical comparisons with observations, "Concentration Field Analysis" (CFA) using measured air pollutant concentrations, and comparison of flow features using cluster analysis. CFA is shown to be a better indication of simulation quality than statistical metrics, and WRF simulations are shown to be an improvement on the MM5 ones. Comparisons with clusters identifies an under-representation of the drainage flows into the basin and an over-representation of wind shear in the boundary layer. Particle trajectories simulated with WRF-FLEXPART are then used to analyse the transport of the urban plume and show rapid venting and limited recirculation during MILAGRO. Lagrangian impacts were identified at the campaign supersites, and age spectra of the pollutants evaluated at those same sites. The evaluation presented in the paper show that mesoscale meteorological simulations are of sufficient accuracy to be useful for MILAGRO data analysis.National Science Foundation (U.S.) (Award ATM-0511803)National Science Foundation (U.S.) (Award ATM-0810950)National Science Foundation (U.S.) (Award ATM-0810931)Molina Center for Energy and the Environmen

Sources and production of organic aerosol in Mexico City: insights from the combination of a chemical transport model (PMCAMx-2008) and measurements

Urban areas are large sources of organic aerosols and their precursors. Nevertheless, the contributions of primary (POA) and secondary organic aerosol (SOA) to the observed particulate matter levels have been difficult to quantify. In this study the three-dimensional chemical transport model PMCAMx-2008 is used to investigate the temporal and geographic variability of organic aerosol in the Mexico City Metropolitan Area (MCMA) during the MILAGRO campaign that took place in the spring of 2006. The organic module of PMCAMx-2008 includes the recently developed volatility basis-set framework in which both primary and secondary organic components are assumed to be semi-volatile and photochemically reactive and are distributed in logarithmically spaced volatility bins. The MCMA emission inventory is modified and the POA emissions are distributed by volatility based on dilution experiments. The model predictions are compared with observations from four different types of sites, an urban (T0), a suburban (T1), a rural (T2), and an elevated site in Pico de Tres Padres (PTP). The performance of the model in reproducing organic mass concentrations in these sites is encouraging. The average predicted PM[subscript 1] organic aerosol (OA) concentration in T0, T1, and T2 is 18 μg m[superscript −3], 11.7 μg m[superscript −3], and 10.5 μg m[superscript −3] respectively, while the corresponding measured values are 17.2 μg m[superscript −3], 11 μg m[superscript −3], and 9 μg m[superscript −3]. The average predicted locally-emitted primary OA concentrations, 4.4 μg m[superscript −3] at T0, 1.2 μg m[superscript −3] at T1 and 1.7 μg m[superscript −3] at PTP, are in reasonably good agreement with the corresponding PMF analysis estimates based on the Aerosol Mass Spectrometer (AMS) observations of 4.5, 1.3, and 2.9 μg m[superscript −3] respectively. The model reproduces reasonably well the average oxygenated OA (OOA) levels in T0 (7.5 μg m[superscript −3] predicted versus 7.5 μg m[superscript −3] measured), in T1 (6.3 μg m[superscript −3] predicted versus 4.6 μg m[superscript −3] measured) and in PTP (6.6 μg m[superscript −3] predicted versus 5.9 μg m[superscript −3] measured). The rest of the OA mass (6.1 μg m[superscript −3] and 4.2 μg m[superscript −3] in T0 and T1 respectively) is assumed to originate from biomass burning activities and is introduced to the model as part of the boundary conditions. Inside Mexico City (at T0), the locally-produced OA is predicted to be on average 60 % locally-emitted primary (POA), 6 % semi-volatile (S-SOA) and intermediate volatile (I-SOA) organic aerosol, and 34 % traditional SOA from the oxidation of VOCs (V-SOA). The average contributions of the OA components to the locally-produced OA for the entire modelling domain are predicted to be 32 % POA, 10 % S-SOA and I-SOA, and 58 % V-SOA. The long range transport from biomass burning activities and other sources in Mexico is predicted to contribute on average almost as much as the local sources during the MILAGRO period.European UnionSeventh Framework Programme (European Commission) (Grant agreement no.: 212520)National Science Foundation (U.S.) (ATM 0732598)Molina Center for Energy and the EnvironmentNational Science Foundation (U.S.) (ATM 0528227)National Science Foundation (U.S.) (ATM 0810931

Ozone response to emission changes: a modeling study during the MCMA-2006/MILAGRO Campaign

The sensitivity of ozone production to precursor emissions was investigated under five different meteorological conditions in the Mexico City Metropolitan Area (MCMA) during the MCMA-2006/MILAGRO field campaign using the gridded photochemical model CAMx driven by observation-nudged WRF meteorology. Precursor emissions were constrained by the comprehensive data from the field campaign and the routine ambient air quality monitoring network. Simulated plume mixing and transport were examined by comparing with measurements from the G-1 aircraft during the campaign. The observed concentrations of ozone precursors and ozone were reasonably well reproduced by the model. The effects of reducing precursor emissions on urban ozone production were performed for three representative emission control scenarios. A 50% reduction in VOC emissions led to 7 to 22 ppb decrease in daily maximum ozone concentrations, while a 50% reduction in NOx [NO subscript x] emissions leads to 4 to 21 ppb increase, and 50% reductions in both NOx [NO subscript x] and VOC emission decrease the daily maximum ozone concentrations up to 10 ppb. These results along with a chemical indicator analysis using the chemical production ratios of H2O2 [H subscript 2 O subscript 2] to HNO3 [HNO subscript 3] demonstrate that the MCMA urban core region is VOC-limited for all meteorological episodes, which is consistent with the results from MCMA-2003 field campaign; however the degree of the VOC-sensitivity is higher during MCMA-2006 due to lower VOCs, lower VOC reactivity and moderately higher NOx [NO subscript x] emissions. Ozone formation in the surrounding mountain/rural area is mostly NOx-limited [NO subscript x - limited], but can be VOC-limited, and the range of the NOx-limited [NO subscript x - limited] or VOC-limited areas depends on meteorology.United States. Dept. of Energy. Office of Biological and Environmental Research. Atmospheric Science Program (DE-FG02-05ER63980)National Science Foundation (U.S.). Atmospheric Chemistry Program (ATM-0528227)National Science Foundation (U.S.). Atmospheric Chemistry Program (ATM-810931)Mexico. Comisión Ambiental MetropolitanaMolina Center for Energy and the Environmen

A possible pathway for rapid growth of sulfate during haze days in China

Rapid industrialization and urbanization have caused frequent occurrence of haze in China during wintertime in recent years. The sulfate aerosol is one of the most important components of fine particles (PM[subscript 2. 5]) in the atmosphere, contributing significantly to the haze formation. However, the heterogeneous formation mechanism of sulfate remains poorly characterized. The relationships of the observed sulfate with PM[subscript 2. 5], iron, and relative humidity in Xi'an, China have been employed to evaluate the mechanism and to develop a parameterization of the sulfate heterogeneous formation involving aerosol water for incorporation into atmospheric chemical transport models. Model simulations with the proposed parameterization can successfully reproduce the observed sulfate rapid growth and diurnal variations in Xi'an and Beijing, China. Reasonable representation of sulfate heterogeneous formation in chemical transport models considerably improves the PM2. 5 simulations, providing the underlying basis for better understanding the haze formation and supporting the design and implementation of emission control strategies

Hit from both sides: tracking industrial and volcanic plumes in Mexico City with surface measurements and OMI SO2 retrievals during the MILAGRO field campaign

Large sulfur dioxide plumes were measured in the Mexico City Metropolitan Area (MCMA) during the MILAGRO field campaign. This paper seeks to identify the sources of these plumes and the meteorological processes that affect their dispersion in a complex mountain basin. Surface measurements of SO2 and winds are analysed in combination with radar wind profiler data to identify transport directions. Satellite retrievals of vertical SO2 columns from the Ozone Monitoring Instrument (OMI) reveal the dispersion from both the Tula industrial complex and the Popocatepetl volcano. Oversampling the OMI swath data to a fine grid (3 by 3 km) and averaging over the field campaign yielded a high resolution image of the average plume transport. Numerical simulations are used to identify possible transport scenarios. The analysis suggests that both Tula and Popocatepetl contribute to SO2 levels in the MCMA, sometimes on the same day due to strong vertical wind shear. During the field campaign, model estimates suggest that the volcano accounts for about one tenth of the SO2 in the MCMA, with a roughly equal split for the rest between urban sources and the Tula industrial complex. The evaluation of simulations with known sources and pollutants suggests that the combination of observations and meteorological models will be useful in identifying sources and transport processes of other plumes observed during MILAGRO.National Science Foundation (U.S.) (award ATM-0810931)National Science Foundation (U.S.) (ATM-0810950)Molina Center for Energy and the Environmen

Mitigating NO_x emissions does not help alleviate wintertime particulate pollution in Beijing-Tianjin-Hebei (BTH), China

Stringent mitigation measures have reduced wintertime PM_(2.5) concentrations by 42.2% from 2013 to 2018 in the BTH. The observed nitrate aerosols have not exhibited a significant decreasing trend and constituted a major fraction (about 20%) of the total PM_(2.5), although the surface-measured NO₂ level has decreased by over 20%. It still remains elusive about contributions of nitrogen oxides (NO_x) emissions mitigation to the nitrate and PM_(2.5) level. The WRF-Chem model simulations of a persistent haze episode in January 2019 in the BTH reveal that NO_x emissions mitigation does not help lower wintertime nitrate and PM_(2.5) concentrations under current conditions in the BTH, because the substantial O₃ increase induced by NO_x mitigation offsets the HNO₃ loss and enhances sulfate and secondary organic aerosols formation. Our results are further consolidated by occurrence of the severe PM pollution in the BTH during the COVID-19 outbreak with a significant reduction of NO₂

Mitigating NO_x emissions does not help alleviate wintertime particulate pollution in Beijing-Tianjin-Hebei (BTH), China

Stringent mitigation measures have reduced wintertime PM_(2.5) concentrations by 42.2% from 2013 to 2018 in the BTH. The observed nitrate aerosols have not exhibited a significant decreasing trend and constituted a major fraction (about 20%) of the total PM_(2.5), although the surface-measured NO₂ level has decreased by over 20%. It still remains elusive about contributions of nitrogen oxides (NO_x) emissions mitigation to the nitrate and PM_(2.5) level. The WRF-Chem model simulations of a persistent haze episode in January 2019 in the BTH reveal that NO_x emissions mitigation does not help lower wintertime nitrate and PM_(2.5) concentrations under current conditions in the BTH, because the substantial O₃ increase induced by NO_x mitigation offsets the HNO₃ loss and enhances sulfate and secondary organic aerosols formation. Our results are further consolidated by occurrence of the severe PM pollution in the BTH during the COVID-19 outbreak with a significant reduction of NO₂

<p>Worsening summertime ozone pollution in the Guanzhong Basin, China from 2014 to 2018: Impacts of synoptic conditions and anthropogenic emissions</p>

The ozone (O-3) pollution in the Guanzhong Basin (GZB), China has progressively deteriorated from 2014 to 2018, with the summertime near-surface maximum daily 8 h average (MDA8) O-3 concentration ([O-3]) increasing from about 104 to 141 mu g m(-3), although the emission mitigation strategies have been carried out since 2013. However, it remains elusive about whether anthropogenic emissions or meteorological conditions contribute to the worsening of O-3 pollution in the GZB. A subjective classification method is first used to classify the synoptic patterns influencing the GZB into favorable and unfavorable conditions. The unfavorable synoptic conditions generally correspond to less precipitation, more solar radiation and higher near-surface temperature, which facilitate formation of the O-3 pollution, and it is opposite for the favorable conditions. The increasing trend of MDA8 [O-3] is correlated well with the increase in occurrence of unfavorable synoptic conditions and near-surface temperature from 2014 to 2018. Sensitivity experiments using the WRF-Chem model reveals that the emission mitigation strategies from 2014 to 2018 enhance MDA8 [O-3] by 19.2% on average in the GZB, which counts about 54% of the observed MDA8 O-3 increasing trend. Particularly, the particulate matter decrease due to the emission mitigation contributes about 11.5% of the observed trend, caused by the reduced HO2 heterogeneous uptake and enhanced photolysis. The rest 46% of the observed MDA8 O-3 increasing trend can be mainly contributed by the variation of occurrence days of unfavorable synoptic situations