Reply on RC3

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.

Aerosol liquid water content (ALWC) and pH significantly influence secondary inorganic aerosol (SIA) formation. However, the spatiotemporal variations in ALWC and aerosol pH, along with their impacts on SIA formation, remain poorly understood. Based on a‐year‐long field observations of aerosol chemical components, ALWC and aerosol pH for Lanzhou and Beijing were calculated using ISORROPIA II. Aerosol liquid water content in Lanzhou was highest in winter and lowest in summer (17.2 vs. 3.30 μg m −3 ), opposite to that of Beijing (10.6 vs. 22.9 μg m −3 ). Aerosol pH in both cities was highest in winter and lowest in summer (4.68 vs. 2.96 in Lanzhou, 5.34 vs. 2.88 in Beijing). Machine learning identified TNH 3 (NH 4 + + NH 3 ), SO 4 2− , NO 3 − , relative humidity (RH), and temperature as crucial factors influencing ALWC and aerosol pH in both cities, while Mg 2+ and Cl − were unique factors influencing pH in Lanzhou during summer and winter, respectively. During pollution periods, the effect of ALWC on enhancing heterogeneous and aqueous‐phase reactions for SIA formation was more pronounced in Beijing than in Lanzhou, as elevated ALWC levels provided more reaction medium and facilitated solid‐to‐liquid phase transitions. Additionally, elevated pH notably enhanced aqueous‐phase sulfate production via the O 3 oxidation pathway in winter in both cities and via the H 2 O 2 oxidation pathway in summer in Beijing. This study highlights the heterogeneity of ALWC and pH, along with their distinct impacts on SIA formation, which should be considered in atmospheric models to improve predictions of secondary aerosol formation and better assess the associated environmental and climatic effects.

<strong class="journal-contentHeaderColor">Abstract.</strong> Haze pollution is a severe environmental problem, caused by elevation of fine particles (aerodynamic diameter <span class="inline-formula">&lt;2.5</span> <span class="inline-formula">µm</span>, PM<span class="inline-formula">_2.5</span>), which is related to secondary aerosol formation, unfavourable synoptic conditions and regional transport, etc. The regional haze formation in basin areas, along with intensive emission of precursors, high relative humidity and poor dispersion conditions, is still limitedly understood. In this study, a field campaign was conducted to investigate the factors resulting in haze formation in the Sichuan Basin (SCB) during winter in 2021. The fine aerosol chemical composition was characterised using a time-of-flight aerosol chemical speciation monitor (ToF-ACSM), which also provided detailed information on the sources for organic aerosols (OAs). The average concentration of non-refractory fine particles (NR-PM<span class="inline-formula">_2.5</span>) was <span class="inline-formula">98.5±38.7</span> <span class="inline-formula">µg m⁻³</span>, and organics aerosols, nitrate, sulfate, ammonium and chloride took up 40.3 %, 28.8 %, 10.6 %, 15.3 % and 5.1 % of PM<span class="inline-formula">_2.5</span>. Three factors, including a hydrocarbon-like OA (HOA), a biomass burning OA (BBOA) and an oxygenated OA (OOA), were identified by applying the positive matrix factorisation (PMF) analysis, and they constituted 24.2 %, 24.2 % and 51.6 % of OA on average, respectively. Nitrate formation was promoted by gas-phase and aqueous-phase oxidation, while sulfate was mainly formed through aqueous-phase process. OOA showed strong dependence on O<span class="inline-formula">_<i>x</i></span>, demonstrating the contribution of photooxidation to OOA formation. OOA concentration increased as aerosol liquid water content (ALWC) increased within 200 <span class="inline-formula">µg m⁻³</span> and kept relatively constant when ALWC <span class="inline-formula">&gt;200</span> <span class="inline-formula">µg m⁻³</span>, suggesting the insignificant effect of aqueous-phase reactions on OOA formation. Among the three haze episodes identified during the whole campaign, the driving factors were different: the first haze episode (H1) was driven by nitrate formation through photochemical and aqueous-phase reactions, and the second haze episode (H2) was mainly driven by the intense emission of primary organic aerosols from biomass burning and vehicle exhaust, while the third haze episode (H3) was mainly driven by reactions involving nitrate formation and biomass burning emission. HOA and BBOA were scavenged, while OOA, nitrate and sulfate formation was enhanced by aqueous-phase reactions during fog periods, which resulted in the increase of <span class="inline-formula">O:C</span> from pre-fog to post-fog periods. This study revealed the factors driving severe haze formation in the SCB and implied the benefit of controlling nitrate as well as intense biomass burning and vehicle exhaust emission for the mitigation of heavy aerosol pollution in this region.

Abstract. Particle acidity (aerosol pH) is an important driver of atmospheric chemical processes and the resulting effects on human and environmental health. Understanding the factors that control aerosol pH is critical when enacting control strategies targeting specific outcomes. This study characterizes aerosol pH at a land–water transition site near Baltimore, MD, during summer 2018 as part of the second Ozone Water-Land Environmental Transition Study (OWLETS-2) field campaign. Inorganic fine-mode aerosol composition, gas-phase NH3 measurements, and all relevant meteorological parameters were used to characterize the effects of temperature, aerosol liquid water (ALW), and composition on predictions of aerosol pH. Temperature, the factor linked to the control of NH3 partitioning, was found to have the most significant effect on aerosol pH during OWLETS-2. Overall, pH varied with temperature at a rate of −0.047 K−1 across all observations, though the sensitivity was −0.085 K−1 for temperatures &gt; 293 K. ALW had a minor effect on pH, except at the lowest ALW levels (&lt; 1 µg m−3), which caused a significant increase in aerosol acidity (decrease in pH). Aerosol pH was generally insensitive to composition (SO42-, SO42-:NH4+, total NH3 (Tot-NH3) = NH3 + NH4+), consistent with recent studies in other locations. In a companion paper, the sources of episodic NH3 events (95th percentile concentrations, NH3 &gt; 7.96 µg m−3) during the study are analyzed; aerosol pH was higher by only ∼ 0.1–0.2 pH units during these events compared to the study mean. A case study was analyzed to characterize the response of aerosol pH to nonvolatile cations (NVCs) during a period strongly influenced by primary Chesapeake Bay emissions. Depending on the method used, aerosol pH was estimated to be either weakly (∼ 0.1 pH unit change based on NH3 partitioning calculation) or strongly (∼ 1.4 pH unit change based on ISORROPIA thermodynamic model predictions) affected by NVCs. The case study suggests a strong pH gradient with size during the event and underscores the need to evaluate assumptions of aerosol mixing state applied to pH calculations. Unique features of this study, including the urban land–water transition site and the strong influence of NH3 emissions from both agricultural and industrial sources, add to the understanding of aerosol pH and its controlling factors in diverse environments.

Abstract. Nitrogen oxides (NOx) and ammonia (NH3) from anthropogenic and biogenic emissions are central contributors to particulate matter (PM) concentrations worldwide. The response of PM to changes in the emissions of both compounds is typically studied on a case-by-case basis, owing in part to the complex thermodynamic interactions of these aerosol precursors with other PM constituents. Here we present a simple but thermodynamically consistent approach that expresses the chemical domains of sensitivity of aerosol particulate matter to NH3 and HNO3 availability in terms of aerosol pH and liquid water content. From our analysis, four policy-relevant regimes emerge in terms of sensitivity: (i) NH3 sensitive, (ii) HNO3 sensitive, (iii) NH3 and HNO3 sensitive, and (iv) insensitive to NH3 or HNO3. For all regimes, the PM remains sensitive to nonvolatile precursors, such as nonvolatile cations and sulfate. When this framework is applied to ambient measurements or predictions of PM and gaseous precursors, the “chemical regime” of PM sensitivity to NH3 and HNO3 availability is directly determined. The use of these regimes allows for novel insights, and this framework is an important tool to evaluate chemical transport models. With this extended understanding, aerosol pH and associated liquid water content naturally emerge as previously ignored state parameters that drive PM formation.

For aerosol related studies, the size distribution and hygroscopicity of chemical components are very important information. In order to characterize the distributions of chemical compositions and the water absorption ability for ambient aerosols of Beijing, a MOUDI-120 sampler was used to collected size-resolved samples in three seasons. All the samples were analyzed in the laboratory for water-soluble inorganic ions and carbon fractions. The size-resolved aerosol liquid water content (ALWC) of the sampled particles was modeled by the ISORROPIA II. The distributions of the chemical compositions, the ALWC, and the charge balance conditions were all discussed for three different pollution levels. During the sampling, the aerosols in stages 6–10 (< 1.0 μm) were under relatively dry conditions due to the significant pressure drops. Three modes (condensation mode, droplet mode, and coarse mode) could be identified from the distributions of the main chemical components and the ALWC. For the droplet mode, the ammonium was not enough to balance NO3– and SO42– during the heavily polluted period. The influence of relative humidity on ALWC is greater than that of the chemical compositions.

Metals are an important atmospheric aerosol component; their impacts on health and the environment depend also on their solubility, dissolution kinetics and chemical form in which they are present in the aerosol (e.g., oxidation state, inorganic salt or oxide/hydroxide, organic complex). In this study, we investigated the impact of fog processing on the solubility and dissolution of metals in PM2.5 samples collected in an urban background site in Padova (Italy). For each sample, we determined the solubility and dissolution kinetics of 17 elements in a solution simulating fog water in the winter season in the Po Valley (pH 4.7, T 5 °C, and water content ∼0.5 g m-3). We also determined water-soluble inorganic and organic compounds having ligand properties. We used the model E-AIM IV to calculate the aerosol liquid water (ALW) content and pH, and we used the model Visual MinteQ to determine the speciation picture of the most important elements under conditions of both deliquescent aerosol (ALW and pH calculated using E-AIM IV, ambient temperature) and simulated fog. We found that the dissolution of Al, Cu, and Fe metal ions, predicted to be largely coordinated with organic compounds under fog conditions, was either immediate or considerably faster in samples collected on days with observed fog events compared with those collected on days having drier conditions. For readily soluble elements, such as As, Cd, Cr, Sr, and Zn, such an effect was not observed. Our study highlights the importance of coordination chemistry in atmospheric aerosol and fog in determining the bioavailability of particle-bound metals.

Abstract. Heterogeneous uptake of hydroperoxyl radicals (HO2) onto aerosols has been proposed to be a significant sink of HOx, hence impacting the atmospheric oxidation capacity. Accurate calculation of the HO2 uptake coefficient γHO2 is key to quantifying the potential impact of this atmospheric process. Laboratory studies show that γHO2 can vary by orders of magnitude due to changes in aerosol properties, especially aerosol soluble copper (Cu) concentration and aerosol liquid water content (ALWC). In this study we present a state-of-the-art model called MARK to simulate both gas- and aerosol-phase chemistry for the uptake of HO2 onto Cu-doped aerosols. Moreover, a novel parameterization of HO2 uptake was developed that considers changes in relative humidity (RH) and condensed-phase Cu ion concentrations and which is based on a model optimization using previously published and new laboratory data included in this work. This new parameterization will be applicable to wet aerosols, and it will complement current IUPAC recommendations. The new parameterization is as follows (the explanations for symbols are in the Appendix): 1γHO2=1αHO2+3×υHO24×106×RdHcorrRT×(5.87+3.2×ln⁡(ALWC/[PM]+0.067))×[PM]-0.2×Cu2+eff0.65+υHO2l4RTHorgDorgε. All parameters used in the paper are summarized in Table A1. Using this new equation, field data from a field campaign were used to evaluate the impact of the HO2 uptake onto aerosols on the ROx (= OH + HO2 + RO2) budget. Highly variable values for HO2 uptake were obtained for the North China Plain (median value &lt; 0.1).

To investigate the chemical compositions, sources, and aging processes of submicron particles (PM1), a comprehensive field campaign was conducted in Guangzhou urban area of China during the autumn (October–November) of 2018. The average mass concentration of PM1 was 35.6 ± 20.8 µg m−3, which was mainly contributed by organic aerosols (OA, 42%), then followed by sulfate (25%) and inorganic nitrate (11%). The inorganic nitrate was found to be the main driving component (up to ∼50%) to account for the fast increase of PM1 during the polluted periods of Guangzhou autumn. The promotion effects of sulfate, aerosol liquid water content, and particles acidity on nitrate formation were systematically discussed. Source apportionment results showed 72% of OA in Guangzhou autumn was contributed by secondary OA (SOA), and 28% of primary OA (POA), including vehicle emission related hydrocarbon‐like OA (HOA, 16%), nitrogen‐containing OA (NOA, 3%) and cooking OA (COA, 8%). To explore the aging processes of OA, the dynamic variations of OA and its oxidation level as a function of ambient photochemical age are shown. Using an in situ field‐deployed oxidation flow reactor, the heterogeneous reaction rate coefficients of ambient POA with OH radicals () were estimated to be 4.0–5.4 × 10−13 cm3 molecules−1 s−1, which is equivalent to a lifetime of POA &gt;2 weeks. The long heterogeneous lifetime of POA supports gas phase oxidation was the major pathway for ambient OA aging. The OH uptake coefficient () was estimated to be 0.76–0.84, underlining that OH radicals can be taken up efficiently on ambient aerosols.

During the past decade, the air quality has been greatly improved in China since the implementation of the “Clean Air Act”. However, haze events are still being reported in some regions of China, and the pollution mechanism remains unclear. In this study, we investigate the chemical characteristics of the pollution mechanism of the PM2.5 composition in Suzhou from October 18 to December 15, 2020. A notable declining trend in temperature was observed from 18 to 27 November, which indicates the seasonal transition from fall to the winter season. Four representative periods were identified based on meteorological parameters and the PM2.5 mass concentrations. The heavy pollution period had the typical characteristics of a relatively low temperature, a high relative humidity, and mass loadings of atmospheric pollutants; nitrate was the dominant contributor to the haze pollution during this period. The nitrate formation mechanism was driven by the planetary boundary layer dynamics. The potential source contribution function model (PSCF) showed that the major PM2.5 composition originated from the northwest direction of the sampling site. The aerosol liquid water content presented increasing trends with an increasing relative humidity. The pH was the highest during the heavy pollution period, which was influenced by the aerosol liquid water content and the mass loadings of NO3−, SO42−, NH4+, and Cl−. The comprehensive analysis in this paper could improve our understanding of the nitrate pollution mechanism and environmental effects in this region.

Until now, there has been a lack of knowledge regarding the vertical profiles of nitrate formation in the urban boundary layer (BL) based on triple oxygen isotopes. Here, we conducted vertical measurements of the oxygen anomaly of nitrate (Δ17O-NO3-) on a 325 m meteorological tower in urban Beijing during the winter and summer. The simultaneous vertical measurements suggested different formation mechanisms of nitrate aerosols at ground level and 120 and 260 m in the winter due to the less efficient vertical mixing under stable atmospheric conditions. Particularly, different chemical processes of nitrate aerosols at the three heights were found between clean days and polluted days in the winter. On clean days, nocturnal chemistry (NO3 + HC and N2O5 uptake) contributed to nitrate production equally with OH/H2O + NO2 at ground level, while it dominated aloft (contributing 80% of nitrate production at 260 m), due to the higher aerosol liquid water content and O3 concentration there. On polluted days, nocturnal reactions dominated the formation of nitrate at the three heights. Particularly, the contribution of the OH/H2O + NO2 pathway to nitrate production increased from the ground level to 120 m might be attributed to the hydrolysis of NO2 to HONO and then further photolysis to OH radicals in the day. In contrast, the proportion of N2O5 + H2O decreased at 260 m, likely due to the low relative humidity aloft that inhibited the N2O5 hydrolysis reactions in the residual layer. Our results highlighted that the differences between meteorology and gaseous precursors could largely affect particulate nitrate formation at different heights within the polluted urban BL.

Supplementary material to &amp;quot;Urban aerosol chemistry at a land-water transition site during summer – Part 2: Aerosol pH and liquid water content&amp;quot;

Polar nitrated aromatic compounds (pNACs) are key ambient brown carbon chromophores; however, their formation mechanisms, especially in the aqueous phase, remain unclear. We developed an advanced technique for pNACs and measured 1764 compounds in atmospheric fine particulate matter sampled in urban Beijing, China. Molecular formulas were derived for 433 compounds, of which 17 were confirmed using reference standards. Potential novel species with up to four aromatic rings and a maximum of five functional groups were found. Higher concentrations were detected in the heating season, with a median of 82.6 ng m-3 for Σ17pNACs. Non-negative matrix factorization analysis indicated that primary emissions particularly coal combustion were dominant in the heating season. While in the non-heating season, aqueous-phase nitration could generate abundant pNACs with the carboxyl group, which was confirmed by their significant association with the aerosol liquid water content. Aqueous-phase formation of 3- and 5-nitrosalicylic acids instead of their isomer of 4-hydroxy-3-nitrobenzoic acid suggests the existence of an intermediate where the intramolecular hydrogen bond favors kinetics-controlled NO2• nitration. This study provides not only a promising technique for the pNAC measurement but also evidence for their atmospheric aqueous-phase formation, facilitating further evaluation of pNACs' climatic effects.

The influence of chemical composition, aerosol acidity, and metal dissolution on the oxidative potential of fine particulate matter and redox potential of the lung lining fluid

Semicontinuous ambient aerosol composition measurements performed during the PMTACS-NY Summer 2001 field campaign in Queens/New York with an aerosol mass spectrometer (AMS, developed by Aerodyne Research Inc.) are described. The measurements include 10 min averages of the nonrefractory sulfate, nitrate, ammonium, chloride, and organic mass concentrations in the particle size range of 50 to approximately 1000 nm. Particle-bound water concentrations (i.e., aerosol liquid water content) were estimated from the mass spectral information and local meteorological data. Aggregate semicontinuous AMS mass measurements were compared with those from a TEOM mass monitor that was also deployed at the PMTACS-NY 2001 site. On average, the AMS observed 64% of the total particulate matter mass measured by the TEOM Monitor. Filter and additional semicontinuous particulate sulfate measurements performed simultaneously at the site suggest that the observed discrepancy in mass balance between the two instruments is attributable to a combination of large particles (≥1 μm) lost in the AMS inlet system and the refractory aerosol components not measured by the AMS. Measured diurnal patterns of sulfate, nitrate, organics, and total nonrefractory mass concentrations indicate that elevated PM levels measured during this campaign were due to regional transport as well as local production of particulate matter.