The North China Plain (NCP) frequently experiences heavy haze pollution, particularly during wintertime. In winter 2015–2016, the NCP region suffered several extremely severe haze episodes with air ...pollution red alerts issued in many cities. We have investigated the sources and aerosol evolution processes of the severe pollution episodes in Handan, a typical industrialized city in the NCP region, using real-time measurements from an intensive field campaign during the winter of 2015–2016. The average (±1σ) concentration of submicron aerosol (PM1) during 3 December 2015–5 February 2016 was 187.6 (±137.5) µg m−3, with the hourly maximum reaching 700.8 µg m−3. Organic was the most abundant component, on average accounting for 45 % of total PM1 mass, followed by sulfate (15 %), nitrate (14 %), ammonium (12 %), chloride (9 %) and black carbon (BC, 5 %). Positive matrix factorization (PMF) with the multilinear engine (ME-2) algorithm identified four major organic aerosol (OA) sources, including traffic emissions represented by a hydrocarbon-like OA (HOA, 7 % of total OA), industrial and residential burning of coal represented by a coal combustion OA (CCOA, 29 % of total OA), open and domestic combustion of wood and crop residuals represented by a biomass burning OA (BBOA, 25 % of total OA), and formation of secondary OA (SOA) in the atmosphere represented by an oxygenated OA (OOA, 39 % of total OA). Emissions of primary OA (POA), which together accounted for 61 % of total OA and 27 % of PM1, are a major cause of air pollution during the winter. Our analysis further uncovered that primary emissions from coal combustion and biomass burning together with secondary formation of sulfate (mainly from SO2 emitted by coal combustion) are important driving factors for haze evolution. However, the bulk composition of PM1 showed comparatively small variations between less polluted periods (daily PM2. 5 ≤ 75 µg m−3) and severely polluted periods (daily PM2. 5 > 75 µg m−3), indicating relatively synchronous increases of all aerosol species during haze formation. The case study of a severe haze episode, which lasted 8 days starting with a steady buildup of aerosol pollution followed by a persistently high level of PM1 (326.7–700.8 µg m−3), revealed the significant influence of stagnant meteorological conditions which acerbate air pollution in the Handan region. The haze episode ended with a shift of wind which brought in cleaner air masses from the northwest of Handan and gradually reduced PM1 concentration to < 50 µg m−3 after 12 h. Aqueous-phase reactions under higher relative humidity (RH) were found to significantly promote the production of secondary inorganic species (especially sulfate) but showed little influence on SOA.
This study discusses an analysis of combined airborne and ground observations of particulate nitrate (NO
) concentrations made during the wintertime DISCOVER-AQ study at one of the most polluted ...cities in the United States, Fresno, CA in the San Joaquin Valley (SJV) and focuses on development of understanding of the various processes that impact surface nitrate concentrations during pollution events. The results provide an explicit case-study illustration of how nighttime chemistry can influence daytime surface-level NO
concentrations, complementing previous studies in the SJV. The observations exemplify the critical role that nocturnal chemical production of NO
aloft in the residual layer (RL) can play in determining daytime surface-level NO
concentrations. Further, they indicate that nocturnal production of NO
in the RL, along with daytime photochemical production, can contribute substantially to the build-up and sustaining of severe pollution episodes. The exceptionally shallow nocturnal boundary layer heights characteristic of wintertime pollution events in the SJV intensifies the importance of nocturnal production aloft in the residual layer to daytime surface concentrations. The observations also demonstrate that dynamics within the RL can influence the early-morning vertical distribution of NO
, despite low wintertime wind speeds. This overnight reshaping of the vertical distribution above the city plays an important role in determining the net impact of nocturnal chemical production on local and regional surface-level NO
concentrations. Entrainment of clean free tropospheric air into the boundary layer in the afternoon is identified as an important process that reduces surface-level NO
and limits build-up during pollution episodes. The influence of dry deposition of HNO
gas to the surface on daytime particulate nitrate concentrations is important but limited by an excess of ammonia in the region, which leads to only a small fraction of nitrate existing in the gas-phase even during the warmer daytime. However, in late afternoon, when diminishing solar heating leads to a rapid fall in the mixed boundary layer height, the impact of surface deposition is temporarily enhanced and can lead to a substantial decline in surface-level particulate nitrate concentrations; this enhanced deposition is quickly arrested by a decrease in surface temperature, which drops the gas-phase fraction to near zero. The overall importance of enhanced late afternoon gas-phase loss to the multiday build-up of pollution events is limited by the very shallow nocturnal boundary layer. The case study here demonstrates that mixing down of NO
from the RL can contribute a majority of the surface-level NO
in the morning (here, ~80%), and a strong influence can persist into the afternoon even when photochemical production is maximum. The particular day-to-day contribution of aloft nocturnal NO
production to surface concentrations will depend on prevailing chemical and meteorological conditions. Although specific to the SJV, the observations and conceptual framework further developed here provide general insights into the evolution of pollution episodes in wintertime environments.
The composition and concentrations of water‐soluble gases and ionic aerosol components were measured from January to February 2013 in Fresno, CA, with a particle‐into‐liquid sampler with ion ...chromatography and annular denuders. The average (±1σ) ionic aerosol mass concentration was 15.0 (±9.4) µg m−3, and dominated by nitrate (61%), followed by ammonium, sulfate, chloride, potassium, nitrite, and sodium. Aerosol‐phase organic acids, including formate and glycolate, and amines including methylaminium, triethanolaminium, ethanolaminium, dimethylaminium, and ethylaminium were also detected. Although the dominant species all came from secondary aerosol formation, there were primary sources of ionic aerosols as well, including biomass burning for potassium and glycolate, sea spray for sodium, chloride, and dimethylamine, and vehicles for formate. Particulate methanesulfonic acid was also detected and mainly associated with terrestrial sources. On average, the molar concentration of ammonia was 49 times greater than nitric acid, indicating that ammonium nitrate formation was limited by nitric acid availability. Particle water was calculated based on the Extended Aerosol Inorganics Model (E‐AIM) thermodynamic prediction of inorganic particle water and κ‐Köhler theory approximation of organic particle water. The average (±1σ) particle water concentration was 19.2 (±18.6) µg m−3, of which 90% was attributed to inorganic species. The fractional contribution of particle water to total fine particle mass averaged at 36% during this study and was greatest during early morning and night and least during the day. Based on aqueous‐phase concentrations of ions calculated by using E‐AIM, the average (±1σ) pH of particles in Fresno during the winter was estimated to be 4.2 (±0.2).
Key Points
Ionic aerosol mass in Fresno, CA, during the winter is dominated by NH4NO3, and NH4NO3 production is limited by HNO3 availability
Particulate amines and organic acids were quantified, and their sources were discussed
Particle water is an important component of ambient fine PM, and the concentration is primarily controlled by inorganic composition and RH
In this study the long-term trends of non-refractory submicrometer aerosol (NR-PM1) composition and mass concentration measured by an Aerosol Chemical Speciation Monitor (ACSM) at the Atmospheric ...Radiation Measurement (ARM) program's Southern Great Plains (SGP) site are discussed. NR-PM1 data was recorded at ∼30 min intervals over a period of 19 months between November 2010 and June 2012. Positive Matrix Factorization (PMF) was performed on the measured organic mass spectral matrix using a rolling window technique to derive factors associated with distinct sources, evolution processes, and physiochemical properties. The rolling window approach also allows us to capture the dynamic variations of the chemical properties in the organic aerosol (OA) factors over time. Three OA factors were obtained including two oxygenated OA (OOA) factors, differing in degrees of oxidation, and a biomass burning OA (BBOA) factor. Back trajectory analyses were performed to investigate possible sources of major NR-PM1 species at the SGP site. Organics dominated NR-PM1 mass concentration for the majority of the study with the exception of winter, when ammonium nitrate increases due to transport of precursor species from surrounding urban and agricultural areas and also due to cooler temperatures. Sulfate mass concentrations have little seasonal variation with mixed regional and local sources. In the spring BBOA emissions increase and are mainly associated with local fires. Isoprene and carbon monoxide emission rates were obtained by the Model of Emissions of Gases and Aerosols from Nature (MEGAN) and the 2011 U.S. National Emissions Inventory to represent the spatial distribution of biogenic and anthropogenic sources, respectively. The combined spatial distribution of isoprene emissions and air mass trajectories suggest that biogenic emissions from the southeast contribute to SOA formation at the SGP site during the summer.
•Realtime measurements of non-refractory submicron aerosols were conducted at SGP.•Diurnal, weekly, monthly, and seasonal variations of aerosol composition are reported.•Two types of oxygenated organic aerosols and biomass burning OA were determined.•Enhanced nitrate during winter was due to transport of NOx and NH3 combined with cooler temperatures.
The optical properties, composition and sources of the wintertime aerosols in the San Joaquin Valley (SJV) were characterized through measurements made in Fresno, CA during the 2013 DISCOVER-AQ ...campaign. PM2.5 extinction and absorption coefficients were measured at 405, 532, and 870 nm along with refractory black carbon (rBC) size distributions and concentrations. BC absorption enhancements (E abs) were measured using two methods, a thermodenuder and mass absorption coefficient method, which agreed well. Relatively large diurnal variations in the E abs at 405 nm were observed, likely reflecting substantial nighttime emissions of wood burning organic aerosols (OA) from local residential heating. Comparably small diurnal variations and absolute nighttime values of E abs were observed at the other wavelengths, suggesting limited mixing-driven enhancement. Positive matrix factorization analysis of OA mass spectra from an aerosol mass spectrometer resolved two types of biomass burning OA, which appeared to have different chemical composition and absorptivity. Brown carbon (BrC) absorption was estimated to contribute up to 30% to the total absorption at 405 nm at night but was negligible (<10%) during the day. Quantitative understanding of retrieved BrC optical properties could be improved with more explicit knowledge of the BC mixing state and the distribution of coating thicknesses.
Wildfires are important contributors to atmospheric aerosols and a large source of emissions that impact regional air quality and global climate. In this study, the regional and nearfield influences ...of wildfire emissions on ambient aerosol concentration and chemical properties in the Pacific Northwest region of the United States were studied using real-time measurements from a fixed ground site located in Central Oregon at the Mt. Bachelor Observatory (∼2700 m a.s.l.) as well as near their sources using an aircraft. The regional characteristics of biomass burning aerosols were found to depend strongly on the modified combustion efficiency (MCE), an index of the combustion processes of a fire. Organic aerosol emissions had negative correlations with MCE, whereas the oxidation state of organic aerosol increased with MCE and plume aging. The relationships between the aerosol properties and MCE were consistent between fresh emissions (∼1 h old) and emissions sampled after atmospheric transport (6–45 h), suggesting that biomass burning organic aerosol concentration and chemical properties were strongly influenced by combustion processes at the source and conserved to a significant extent during regional transport. These results suggest that MCE can be a useful metric for describing aerosol properties of wildfire emissions and their impacts on regional air quality and global climate.
The Alpha Jet Atmospheric eXperiment (AJAX) flew
scientific flights between 2011 and 2018 providing measurements of trace gas
species and meteorological parameters over California and Nevada, USA. ...This
paper describes the observations made by the AJAX program over 229 flights
and approximately 450 h of flying. AJAX was a multi-year,
multi-objective, multi-instrument program with a variety of sampling
strategies resulting in an extensive dataset of interest to a wide variety
of users. Some of the more common flight objectives include satellite
calibration/validation (GOSAT, OCO-2, TROPOMI) at Railroad Valley and other
locations and long-term observations of free-tropospheric and boundary layer
ozone allowing for studies of stratosphere-to-troposphere transport and
long-range transport to the western United States. AJAX also performed
topical studies such as sampling wildfire emissions, urban outflow and
atmospheric rivers. Airborne measurements of carbon dioxide, methane, ozone,
formaldehyde, water vapor, temperature, pressure and 3-D winds made by the
AJAX program have been published at NASA's Airborne Science Data Center (https://asdc.larc.nasa.gov/project/AJAXTS9 (last access: 1 November 2022), https://doi.org/10.5067/ASDC/SUBORBITAL/AJAX/DATA001,
Iraci et al., 2021a).
Biomass burning is an important source of trace gases and particles, and can influence air quality on local, regional, and global scales. With the threat of wildfire events increasing due to changes ...in land use, increasing population, and climate change, the importance of characterizing wildfire emissions is vital. In this work we characterize trace gas emissions from 12 wildfires and 1 prescribed fire in California between 2013 and 2017, in some cases with multiple measurements performed during different burn periods of a specific fire. Airborne measurements of carbon dioxide, methane, ozone, formaldehyde, water vapor, temperature, and three‐dimensional winds were made by the Alpha Jet Atmospheric eXperiment and have been published at NASA's Atmospheric Science Data Center (https://doi.org/10.5067/ASDC/AJAX/wildfire, Iraci et al., 2021). The majority of these measurements were made as close as possible to each fire and represent fresh emissions from known fire sources. This set of observations from 13 different fires offers the opportunity to explore trace gas emissions over a range of meteorology, fire conditions, and to a lesser extent, vegetation type and drought, and adds to the body of knowledge collected by other investigators and field campaigns.
Plain Language Summary
Biomass burning is an important source of trace gases and particles which can influence air quality on local, regional, and global scales. This set of airborne observations of trace pollutants and winds from 13 different fires broadens the range of fire types and conditions for which observations have been made near the top of visible smoke plumes. Calibrated and quality‐controlled observations of methane, formaldehyde, ozone, carbon dioxide, water vapor, and 3‐D winds near to a variety of active fires offer the opportunity to explore emissions over a range of meteorology, fire conditions, and to a lesser extent, vegetation type, and drought. This data set can also be an asset for bridging the gap of knowledge between models and in situ data collected by other investigators and field campaigns.
Key Points
Trace gas observations near/around 12 wildfires and one prescribed fire (2013–2017) highlight variability of many fires over multiple years
Five flights to Soberanes fire, providing trace gas and wind measurements over the lifetime of the megafire
Several downwind legs and regional transits also included, making possible contributions to studies of human exposure at regional scales
The 1,2-HX elimination reaction (where X = F, Cl, Br, OH) has been established as an important reaction in the degradation of compounds introduced into the upper atmosphere, including common CFC ...replacement compounds. By analyzing the electron densities of the transition state geometries of these reactions using QTAIM, we see that we can divide these reactions into two types. For HF and HOH elimination, the transition state is a complete ring of bonds, and neither the C–H nor the C–X bonds have been broken at the maximum of energy. There is very little accumulation of electron density on the X atom, with the majority of charge being lost by the hydrogen atom undergoing elimination, being transferred on to the two carbon atoms. In HCl and HBr elimination, a similar loss of electron density of the hydrogen atom is accompanied by significant accumulation of electron density on the X atom and a smaller change in electron density on the carbon atoms. The C–X bond is broken in the transition state geometry, with no ring critical point being present. This may explain the relative stabilities of halohydrocarbons and haloalcohols with respect to loss of H–X.
The San Joaquin Valley (SJV) of California has one of the nation's most severe wintertime PM2.5 pollution problems. The DISCOVER-AQ (Deriving Information on Surface Conditions from Column and ...Vertically Resolved Observations Relevant to Air Quality) field campaign took place in the SJV from January 16 to February 6, 2013. It captured two PM2.5 pollution episodes with peak 24-h concentrations approaching 70 μg/m3. Using meteorological fields generated from WRFv3.6, CMAQv5.0.2 was applied to simulate PM2.5 formation in the SJV from January 10 through February 10, 2013. Overall, the model was able to capture the observed accumulation of PM2.5 within the simulation period. The model was able to produce increased concentrations of ammonium nitrate and organic carbon, which are two major components of wintertime PM2.5 in the SJV. Comparison to measurements made by aircraft showed that there was general agreement between observed and modeled daytime vertical distributions of selected gas and particulate species, reflecting the adequacy of modeled daytime mixing layer heights. Excess ammonia predicted by the model implied that ammonium nitrate formation was limited by the availability of nitric acid, consistent with observations. Evaluation of the ammonium nitrate diurnal profile revealed that the observed morning increase of ammonium nitrate was also evident from the model. This paper demonstrates that the CMAQ model is able to simulate elevated wintertime PM2.5 formation observed in the SJV during the DISCOVER-AQ 2013 period, which featured both climatic (i.e., 2011–2014 California Drought) and emissions differences compared to a previous large air quality field campaign in the SJV during 1999–2000.
•A field campaign DISCOVER-AQ took place in the SJV during Jan–Feb 2013•Ammonium nitrate and organic carbon are the two key PM2.5 components.•Ammonium nitrate formation is limited by the availability of nitric acid.•CMAQ captured the accumulation of elevated PM2.5 in the SJV during the campaign.•CMAQ generally captured average diurnal cycle of ammonium nitrate in Fresno.
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