Comment on acp-2022-477

Characteristics of the chemical processes of organic aerosols by time-of-flight aerosol chemical speciation monitor (TOF-ACSM) in winter in a site of Fenhe Valley, northern China

Sources and processes of organic aerosol in non-refractory PM1 and PM2.5 during foggy and haze episodes in an urban environment of the Yangtze River Delta, China

Brown carbon from biomass burning imposes strong circum-Arctic warming

Abstract. 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.

The moving of high emission for biomass burning in China: View from multi-year emission estimation and human-driven forces

The refined source apportionment of ammonium (NH4+), especially for biomass burning emissions, is unclear. Hourly resolution δ15N‐NH4+ values for three winter haze episodes (EP1–EP3) were determined in the North China Plain (NCP) in 2019 to identify and quantify the contribution of biomass burning. A reasonable source‐resolved structure of NH4+ was obtained after using the corrected source signatures of 15N and considering biomass burning emissions. The time‐series variation in biomass burning and vehicle sources identified by δ15N was more reasonable and matched better with their tracers (e.g., levoglucosan). The non‐negligible contribution of biomass burning in the NCP was found and contributed 13.0% ± 11.4% to NH4+ in haze episodes, but in special stages was as high as 29% ± 11% in local emission of EP1 and 23% ± 15% in southwest transportation of EP2. The redistribution of NH4+ sources further emphasize the contribution of biomass burning to haze episodes, as its contribution to PM2.5 increased up to 5% (without considering SO42− and NO3−). Considering NH4+ uniqueness in the formation of new particles, the role of biomass burning during haze episodes should be considered. This study indicates that further studies need to be conducted to reduce biomass burning emissions in the NCP.

<p>Cloud Condensation Nuclei (CCN) and other aerosol properties were investigated in Delhi, India, from Feb. to Mar. 2018. The high anthropogenic influence on aerosol was studied with size-resolved CCN measurements (supersaturation (<em>S</em>) between 0.13 to 0.66% and selected diameters from 10 to 300 nm). Furthermore the chemical composition (Aerosol Chemical Speciation Monitor and Aethalometer AE33) of the particles was measured. The aerosol number size distribution was derived by size data inversion of Differential Mobility Particle Sizer (DMPS) from size-resolved CCN measurements. Based on multi-year back trajectory (BT) data, a spatial clustering analysis was done for the actual campaign period and two distinct clusters were identified: northwest- west northwest-long range transport (NW-LRT) and south-southeast-east southeast (SE).</p><p>There was preponderant organic mass fraction (<em>f</em><sub>org)</sub> in the aerosols throughout the campaign, with prominent diurnal variation except during the SE period. Pronounced diurnal variation was observed also in black carbon (BC) with an average concentration of 16 <em>µ</em>g/m<sup>3</sup> during NW-LRT, in contrast to a weak diurnal cycle with lower average concentration of 8 <em>µ</em>g/m<sup>3</sup> during SE. During the NW-LRT cluster the air masses traversed over agriculture fields with biomass burning (BB) activities identified using the fire radiative power (FRP) observations of Copernicus Atmosphere Monitoring Service (CAMS) Global Fire Assimilation System (GFAS). So it can be speculated that the BB emissions from the fields have contributed to enhanced BC concentrations during this period over Delhi. The remaining period, showing a mixture of local and long-range transported emissions also had a BC concentration higher than SE period when only local/regional emissions were observed. This is an important insight into the air pollution apocalypse in Delhi.</p><p>The overall average values of critical dry diameter (<em>D</em><sub>c</sub>) for CCN activation varied from 54 ± 8 nm at <em>S</em> = 0.66% to 139 ± 12 nm at <em>S</em> = 0.13%.The hygroscopicity parameter derived from CCN data (<em>к</em><sub>CCN</sub>) was in the range from 0.1 to 0.9 with an arithmetic mean of 0.27 ± 0.10, which is close to that of Beijing, another polluted continental region (0.31 ± 0.08, Gunthe et al., 2011). <em>к</em><sub>CCN</sub> also shows good agreement with the hygroscopicity parameter derived from the chemical composition measurements. A linear fit (Gunthe et al., 2009) applied to the relationship between refractory/non-refractory organic mass fraction and <em>к</em><sub>CCN </sub>at <em>S</em> = 0.13%, gives an effective hygroscopicity parameter <em>к</em><sub>org</sub> = 0.17 ± 0.09 and <em>к</em><sub>inorg</sub> = 0.80 ± 0.09, when extrapolated to <em>f</em><sub>org</sub> = 1 and <em>f</em><sub>org</sub> = 0, respectively. The presence of externally mixed inactive CCN particles is indicated by an average maximum activated fraction (<em>MAF</em>) of 0.82 ± 0.17 at <em>S</em> = 0.13%. The overall average <em>D</em><sub>c</sub>, <em>к</em><sub>CCN</sub>, and <em>MAF </em>did not vary much between NW-LRT and SE periods, although the particle number concentration was higher during NW-LRT. Moreover, high CCN efficiency was observed during NW-LRT, in spite of its enhanced BC concentration, indicating the presence of aged internally mixed aerosols. Further details will be presented.</p>

Optical properties and possible sources of brown carbon in PM2.5 over Xi'an, China

Abstract. Organic aerosol (OA) represents a large fraction of submicron aerosols in the megacity of Beijing, yet long-term characterization of its sources and variations is very limited. Here we present an analysis of in situ measurements of OA in submicrometer particles with an aerosol chemical speciation monitor (ACSM) for 2 years from July 2011 to May 2013. The sources of OA are analyzed with a multilinear engine (ME-2) by constraining three primary OA factors including fossil-fuel-related OA (FFOA), cooking OA (COA), and biomass burning OA (BBOA). Two secondary OAs (SOA), representing a less oxidized oxygenated OA (LO-OOA) and a more oxidized (MO-OOA), are identified during all seasons. The monthly average concentration OA varied from 13.6 to 46.7 µg m−3 with a strong seasonal pattern that is usually highest in winter and lowest in summer. FFOA and BBOA show similarly pronounced seasonal variations with much higher concentrations and contributions in winter due to enhanced coal combustion and biomass burning emissions. The contribution of COA to OA, however, is relatively stable (10–15 %) across different seasons, yet presents significantly higher values at low relative humidity levels (RH < 30 %), highlighting the important role of COA during clean periods. The two SOA factors present very different seasonal variations. The pronounced enhancement of LO-OOA concentrations in winter indicates that emissions from combustion-related primary emissions could be a considerable source of SOA under low-temperature (T) conditions. Comparatively, MO-OOA shows high concentrations consistently at high RH levels across different T levels, and the contribution of MO-OOA to OA is different seasonally with lower values occurring more in winter (30–34 %) than other seasons (47–64 %). Overall, SOA (= LO-OOA + MO-OOA) dominates OA composition during all seasons by contributing 52–64 % of the total OA mass in the heating season and 65–75 % in non-heating seasons. The variations in OA composition as a function of OA mass loading further illustrate the dominant role of SOA in OA across different mass loading scenarios during all seasons. However, we also observed a large increase in FFOA associated with a corresponding decrease in MO-OOA during periods with high OA mass loadings in the heating season, illustrating an enhanced role of coal combustion emissions during highly polluted episodes. Potential source contribution function analysis further shows that the transport from the regions located to the south and southwest of Beijing within ∼ 250 km can contribute substantially to high FFOA and BBOA concentrations in the heating season.

Abstract. Severe regional haze pollution events occurred in eastern and central China in January 2013, which had adverse effects on the environment and public health. Extremely high levels of particulate matter with aerodynamic diameter of 2.5 μm or less (PM2.5) with dominant components of sulfate and nitrate are responsible for the haze pollution. Although heterogeneous chemistry is thought to play an important role in the production of sulfate and nitrate during haze episodes, few studies have comprehensively evaluated the effect of heterogeneous chemistry on haze formation in China by using the 3-D models due to of a lack of treatments for heterogeneous reactions in most climate and chemical transport models. In this work, the WRF-CMAQ model with newly added heterogeneous reactions is applied to East Asia to evaluate the impacts of heterogeneous chemistry and the meteorological anomaly during January 2013 on regional haze formation. As the parameterization of heterogeneous reactions on different types of particles is not well established yet, we arbitrarily selected the uptake coefficients from reactions on dust particles and then conducted several sensitivity runs to find the value that can best match observations. The revised CMAQ with heterogeneous chemistry not only captures the magnitude and temporal variation of sulfate and nitrate, but also reproduces the enhancement of relative contribution of sulfate and nitrate to PM2.5 mass from clean days to polluted haze days. These results indicate the significant role of heterogeneous chemistry in regional haze formation and improve the understanding of the haze formation mechanisms during the January 2013 episode.

The increased prevalence of severe biomass burning due to forest fires in Southeast ASEAN countries (Malaysia, Indonesia, Brunei, Singapore, and Timor-Leste) is strongly linked with human-induced land-use changes, primarily related to agricultural expansion, mainly conversion of forests to oil palm plantations. Large-scale biomass burning that recurs almost annually in this region plays a vital role in regional climate change. Biomass burning emissions are recognized as an essential source of greenhouse gases and atmospheric pollutants. In Malaysia, most biomass burning emissions are due to Indonesia's transboundary pollution, mostly between July and October. Seasonal monsoons and transboundary pollution greatly influence the spatial and temporal variations of long-term air quality in Malaysia. Extreme weather conditions in Malaysia are always associated with long periods of a dry atmosphere, which frequently occurs during El Niño events, though the strength of El Niño was not the primary factor for the large and intense biomass burning in Southeast ASEAN. In this study, we review the impact of biomass burning on the local environment in Malaysia, including air pollution and haze, atmospheric acidity and neutralization, and plant productivity, including respiratory and health issues.

This work evaluates the sensitivity of tropospheric ozone (O3) and its radiative forcing (RF) from 1890 to 1990 to different biomass burning (BB) emissions using a global tropospheric climate‐chemistry model (CCM) with a modified chemical mechanism for anthropogenic volatile organic compounds (AVOC). The use of the most efficient simplified chemical scheme among three chemical mechanisms with different degrees of complexity is acceptable for global scale simulations of the radiative effects of tropospheric O3 changes in the CCM, since the differences in the results are small. The CCM model with simplified chemistry is implemented to study various aspects of the impact of BB emissions on tropospheric O3 and its RF. The backward emission model results are in good agreement with the present‐day observations for regions downwind of BB sources, while the forward emission model may reasonably produce the distributions of regional emissions in the preindustrial period. The global mean RF due to tropospheric O3 increase from 1890 to 1990 is 0.41 W m−2 on a global average. When no anthropogenic emissions in the preindustrial period are considered, this forcing reaches 0.47 W m−2. We find that the global mean BB forcing due to tropospheric O3 changes from 1890 to 1990 is 0.15 W m−2 on a global average. The preindustrial BB emissions need to be represented realistically when evaluating controls on the emissions of trace gases and aerosols for a sustainable society, because there are significant open biomass burning emissions in the preindustrial period. The significant regional differences in the surface O3 concentrations among three different BB data sets are found in the United States for 1890 and in Brazil for 1990. In these regions during the periods, the most common land uses in the forests are logging and conversion of primary or secondary forests to cattle pasture or shifting cultivation. Improvement in the data sets of historical land use changes and accurate representation of carbon dynamics are needed for improving the model calculation of biomass burning emissions.

Although many considerable efforts have been done to reveal the driving factors on haze aggravation, however, the roles of aerosol liquid water (ALW) in SIAs formation were mainly focused on the condition of aerosol liquid water content (ALWC)<100 μg/m3. Based on the in-situ high-resolution field observation, this work studied the decisive roles and the shifting of secondary inorganic aerosols formation mechanism during haze aggravation, revealing the different roles of ALWC in a broader scale (~ 500 μg/m3) in nitrate and sulfate formation induced by aqueous chemistry in ammonia-rich atmosphere. The results showed that chemical domains of perturbation gas limiting the generation of secondary particulate matters presented obvious shifts from HNO3 sensitive to HNO3 and NH3 co-sensitive regime with the haze aggravation, indicating the powerful driving effects of ammonia in ammonia-rich atmosphere. When ALWC<75 μg/m3, the sulfate generation was preferentially triggered by the high ammonia utilization, then accelerated by nitrogen oxide oxidation from Clean to Moderate pollution stages, characterizing as NOR<0.3, SOR<0.4, NTR<0.7 and the moral ratio of NO3-:SO42-=2:1. While ALWC>75 μg/m3, aqueous-phase chemistry reaction of SO2 and NH3 in ALW became the prerequisite for SIAs formation driven by Henry’s law in the ammonia-rich atmosphere during Heavy and Serious stages, characterizing as high SOR (0.5–0.9), NOR (0.3–0.5), NTR (>0.7) and the moral ratio of NO3-:SO42-=1:1. A positive feedback of sulfate on nitrate production was also observed in this work. Our results provided the evidence for the response of the transition ALWC with seasonal variability and climate change. It implies the target controlling of haze should not simply focus on SO2 and NO2, more attention should be paid on gaseous precursors (e.g., SO2, NO2, NH3) and aerosol chemical constitution during different haze stages.

Although many considerable efforts have been done to reveal the driving factors on haze aggravation, however, the roles of aerosol liquid water (ALW) in SIAs formation were mainly focused on the condition of aerosol liquid water content (ALWC)<100 μg/m3. Based on the in-situ high-resolution field observation, this work studied the decisive roles and the shifting of secondary inorganic aerosols formation mechanism during haze aggravation, revealing the different roles of ALWC in a broader scale (~ 500 μg/m3) in nitrate and sulfate formation induced by aqueous chemistry in ammonia-rich atmosphere. The results showed that chemical domains of perturbation gas limiting the generation of secondary particulate matters presented obvious shifts from HNO3 sensitive to HNO3 and NH3 co-sensitive regime with the haze aggravation, indicating the powerful driving effects of ammonia in ammonia-rich atmosphere. When ALWC<75 μg/m3, the sulfate generation was preferentially triggered by the high ammonia utilization, then accelerated by nitrogen oxide oxidation from Clean to Moderate pollution stages, characterizing as NOR<0.3, SOR<0.4, NTR<0.7 and the moral ratio of NO3-:SO42-=2:1. While ALWC>75 μg/m3, aqueous-phase chemistry reaction of SO2 and NH3 in ALW became the prerequisite for SIAs formation driven by Henry’s law in the ammonia-rich atmosphere during Heavy and Serious stages, characterizing as high SOR (0.5–0.9), NOR (0.3–0.5), NTR (>0.7) and the moral ratio of NO3-:SO42-=1:1. A positive feedback of sulfate on nitrate production was also observed in this work. Our results provided the evidence for the response of the transition ALWC with seasonal variability and climate change. It implies the target controlling of haze should not simply focus on SO2 and NO2, more attention should be paid on gaseous precursors (e.g., SO2, NO2, NH3) and aerosol chemical constitution during different haze stages.

Particulate matter from biomass burning emissions affects air quality, ecosystems and climate; however, quantifying these effects requires that the connection between primary emissions and secondary aerosol production is firmly established. We performed atmospheric simulation chamber experiments on the chemical oxidation of residential biomass burning emissions under dark conditions. Biomass burning organic aerosol was found to age under dark conditions, with its oxygen-to-carbon ratio increasing by 7–34% and producing 1–38 μg m−3 of secondary organic aerosol (5–80% increase over the fresh organic aerosol) after 30 min of exposure to NO3 radicals in the chamber (corresponding to 1–3 h of exposure to typical nighttime NO3 radical concentrations in an urban environment). The average mass concentration of SOA formed under dark-oxidation conditions was comparable to the mass concentration formed after 3 h (equivalent to 7–10 h of ambient exposure) under ultraviolet lights (6 μg m−3 or a 47% increase over the emitted organic aerosol concentration). The dark-aging experiments showed a substantial increase in secondary nitrate aerosol (0.12–3.8 μg m−3), 46–100% of which is in the form of organic nitrates. The biomass burning aerosol pH remained practically constant at 2.8 throughout the experiment. This value promotes inorganic nitrate partitioning to the particulate phase, potentially contributing to the buildup of nitrate aerosol in the boundary layer and enhancing long-range transport. These results suggest that oxidation through reactions with the NO3 radical is an additional secondary aerosol formation pathway in biomass burning emission plumes that should be accounted for in atmospheric chemical-transport models.