Biomass burning in mainland Southeast Asia frequently produces smoke plumes that are transported across national borders, deteriorating air quality in downwind regions. To elucidate the monsoon-related impacts of transboundary biomass burning from Southeast Asia, we conducted a year-long investigation of molecular characteristics of PM2.5-bound organic aerosols at a tropical rainforest site in Southwest China. The results showed distinct seasonal variations in all measured chemical components, with a marked pollution enhancement (February to April) during the late dry season, coinciding with extensive upwind fires and prevailing southwesterly air mass transport. Sugars dominated the quantified organics, and levoglucosan was consistently the most abundant single compound. It exhibited pronounced increases during the late dry season, reflecting intensified smoke influence during the peak burning period. Source apportionment results revealed that direct plant emissions were the major source of organic aerosols in early dry season (46%) and wet season (58%), indicating a limited role of biomass burning during these periods. In contrast, biomass burning became the dominant contributor (54%) in late dry season, largely associated with transboundary transport from Southeast Asia. These results demonstrate that episodic transboundary biomass burning can dominate organic aerosol loading in southwestern China, highlighting the importance of coordinated regional mitigation during the peak dry-season burning period.

Brown carbon (BrC) is an important type of organic aerosol that can significantly affect the climate and air quality because of its important role in radiation balance and visibility impairment. Despite its importance, the optical properties and sources of BrC remain poorly characterized, especially in tropical rainforest regions. Here, we collected yearlong PM2.5 samples from a typical tropical forest site in southwest China. We combined optical measurements and chemical analyses to investigate the optical properties and sources of water-soluble BrC. We show that the average absorbance coefficient of BrC (Abs(365)) was higher in the dry season than wet season, coinciding with increased water-soluble organic carbon concentrations during the dry season. Correlation analyses revealed a strong association between Abs(365) and biomass-burning (BB) organic markers (R-2 > 0.6), along with a moderate correlation with secondary organic aerosol (SOA) markers (R-2 > 0.3) in the dry season, indicating that enhanced biomass-burning activities substantially increased BrC levels. However, such a clear association was not observed in the wet season. These results suggest that biomass burning was the major source of BrC in the dry season, followed by secondary formation, while no single dominant source of BrC was apparent in the wet season. The estimation of the contribution of solar absorption by BrC relative to elemental carbon shows that BrC contributed over 29 % of light absorption in the near-UV range in both seasons, indicating an important role of BrC in solar absorption. Overall, our work gains insights into the optical properties and possible sources of BrC in tropical forest regions.

Atmospheric secondary organic aerosols (SOAs) play a significant role in climate change, air quality, and human health, yet their formation mechanisms and influencing factors in the field environment are not fully understood. This study conducted a one-year-long observation of specific organic tracers of SOAs derived from isoprene (SOAI), α/β-pinene (SOAP), β-caryophyllene (SOAC), and aromatics (SOAA) oxidation in PM2.5 at a tropical rainforest site in Xishuangbanna, Southwest China. All the SOA tracers presented obviously higher concentrations in dry season than in wet season. The ratio of 2-methylglyceric acid (2-MGA) to 2-methyltetrols (2-MTs) in dry season (0.49) was around 3 times higher than that in wet season (0.17), because the higher NOx concentrations (36.18 µg m−3 in dry season vs. 22.96 µg m−3 in wet seasons) enhanced the 2-MGA formation in dry season. The ratios of 3-methyl-1,2,3-butanetricarboxylic acid to the sum of cis-pinonic and pinic acids (M/P) were above 2 in both seasons, suggesting that the organic aerosols in the tropical rainforest region have undergone a significant degree of oxidation. The concentration of β-caryophyllene was approximately 18 times higher in the dry season (9.18 ± 10.23 ngm−3) than in the wet season (0.49 ± 0.35 ng m−3), which is related to the seasonal activity of biomass burning. In dry season, levoglucosan exhibited significant correlations with SOAP, SOAC, and SOAA, indicating that they were likely influenced by biomass burning. Backward trajectory analysis and potential source contribution factor analyses revealed that the sources of all the SOA tracers are likely influenced by biomass burning from the south and northwest directions in dry season, whereas in wet season they are likely affected by the transport from the southwest and northeast directions. This study highlights the important contribution of biomass burning in Southeast Asia on the SOA formation in the tropical rainforest region of Southwest China.

The gas-particle partitioning of carbonyl compounds through reversible and irreversible pathways constitutes a critical route for secondary organic aerosol formation. This study simultaneously observed distribution of formaldehyde (FA), acetaldehyde (AA), glyoxal (GLY), and methylglyoxal (MGLY) in both gas and particle phases, as well as concentration and stable carbon isotope (δ¹³C) of oxalic acid. The results show that in the reversible partitioning process, the carbonyl compounds during clean period are mainly partitioned into organic phase, and the salt-in effect exhibits kinetic limitations. However, they are mainly partitioned into aerosol liquid water during haze periods with lower partitioning coefficients, and there is a significant salt-in effect. The carbonyls present different irreversible partitioning pathways to contribute oxalic acid formation in haze periods. In Haze I (O3: 24 ± 11 ppb), the carbonyl compounds are mainly partitioned into aqueous phase and reacted with water to form polymers or undergoing self-polymerization. The large multifunctional compounds decomposed to yield oxalic acid, leading to progressive enrichment of δ¹³C. In Haze II (O3: 41 ± 13 ppb), the elevated atmospheric oxidation capacity promoted aqueous-phase oxidation of the carbonyls to generate intermediate products and ultimately forming oxalic acid. Under these conditions, oxalic acid-C exhibited a depleted δ¹³C signal.

The gas-particle partitioning of carbonyl compounds through reversible and irreversible pathways constitutes a critical route for secondary organic aerosol formation. This study simultaneously observed distribution of formaldehyde (FA), acetaldehyde (AA), glyoxal (GLY), and methylglyoxal (MGLY) in both gas and particle phases, as well as concentration and stable carbon isotope (δ¹³C) of oxalic acid. The results show that in the reversible partitioning process, the carbonyl compounds during clean period are mainly partitioned into organic phase, and the salt-in effect exhibits kinetic limitations. However, they are mainly partitioned into aerosol liquid water during haze periods with lower partitioning coefficients, and there is a significant salt-in effect. The carbonyls present different irreversible partitioning pathways to contribute oxalic acid formation in haze periods. In Haze I (O3: 24 ± 11 ppb), the carbonyl compounds are mainly partitioned into aqueous phase and reacted with water to form polymers or undergoing self-polymerization. The large multifunctional compounds decomposed to yield oxalic acid, leading to progressive enrichment of δ¹ ³C. In Haze II (O3: 41 ± 13 ppb), the elevated atmospheric oxidation capacity promoted aqueous-phase oxidation of the carbonyls to generate intermediate products and ultimately forming oxalic acid. Under these conditions, oxalic acid-C exhibited a depleted δ¹ ³C signal.

Dust transport significantly affects downwind aerosol formation and regional climate, yet the evolutionary mechanisms of SOA during this process remain poorly understood. Here, we conducted vertical observations of PM2.5 and size-segregated aerosols at the foot and top of Mount Hua, focusing on C2 formation and its δ13C signatures influenced by dust transport. Under non-dust conditions, PM2.5 and diacid concentrations at the foot were 4.5 and 2.1 times higher than those at the top, indicating stronger anthropogenic influence at lower elevations. Aerosols at the top revealed enhanced photochemical aging, with higher C2 / C4 (5.84 vs. 4.74), C3 / C4 ratios (1.04 vs. 0.56), and more positive δ13C values (−21.5 ‰ vs. −27.6 ‰). The positive correlation of C2 with ALWC and its consistent size distribution with precursors confirm aqueous-phase oxidation as the dominant formation pathway. During dust events, although PM2.5 concentrations increased, C2 concentrations in PM2.5 decreased by 59 % at the foot and 25 % at the top. Concurrently, the δ13C values of C2 showed a positive shift, particularly at the top (from −21.5 ‰ to −13.2 ‰), suggesting that alkaline dust catalyzes the formation of 13C-enriched oxalate. Size-segregated data revealed a shift of C2 from the fine to the coarse mode, with the coarse-to-fine ratio increasing from 0.3–0.4 to 0.6–1.1. These findings demonstrate that under dust influence, the primary formation pathway of C2 shifts from aqueous-phase oxidation in fine particles to heterogeneous reactions on coarse-particle surfaces. Moreover, this shift is accompanied by a positive shift in the δ13C signature of C2 and is more pronounced at higher altitudes.

To investigate the impact of urban emission reduction on particulate matter (PM2.5) pollution characteristics, field observations were conducted near a megacity in the Guanzhong Plain, China, during three periods: COVID-19 lockdown ("lockdown"), pre-lockdown ("normal"), and post-lockdown ("festival"). The observation showed that despite reduced NO2 and SO2, PM2.5 increased significantly during "lockdown." Molecular characteristics and PMF source apportionment revealed that biomass burning contributions to PM2.5 increased by over 70% compared with "normal." Meanwhile, secondary aerosol formation (primarily through liquid-phase oxidation) accounted for more than 50% of PM2.5 levels during "lockdown." Additionally, metal ions released by fireworks burning accelerated the liquid-phase formation of sulfate, resulting in secondary sulfate-related sources contributing about 33% of PM2.5 during "festival." The study demonstrates synergistic effects between biomass/fireworks burning and liquid-phase oxidation, indicating that unbalanced emission reductions may exacerbate pollution through atmospheric aging and regional transport. Effective air quality management requires coordinated multi-pollutant control strategies.

Aerosol and cloud acidity are essential to human health, ecosystem health and productivity, as well as climate effects. The main chemical composition of cloud water greatly varies in different regions, resulting in substantial differences in the pH of cloud water. However, the influences of the anthropogenic emissions of acidic gases and substances, alkaline dust components, and dicarboxylic acids (diacids) on the ground concerning the acidity of cloud water in the free troposphere of the Guanzhong Plain, China, remain clear. In this study, cloud water and PM 2.5 samples were simultaneously collected in the troposphere (Mt. Hua, 2060 m a.s.l). The results indicated that the cloud water was alkaline (pH = 7.6) and PM 2.5 was acidic (pH = 3.2). These results showed the neutral property of clouds collected in the heavily polluted Guanzhong Plain, although most previous studies always considered acidity as a marker of pollution. The sulfate (SO 4 2- ), nitrate (NO 3 - ), and ammonium (NH 4 + ) (SNA) of particulate matter and cloud water in the same period were compared. SO 4 2- was dominant in particulate matters (accounting for 63.4 % of the total SNA) but substantially decreased in cloud water (only 30.1 % of the total SNA), whereas NO 3 - and NH 4 + increased from 28.5 % and 8.2 % to 39.8 % and 30.2 %, respectively. This could be attributed to the complex formation mechanism and sources of SO 4 2- and NO 3 - in the cloud. The results of ion balance indicated that a significant deficit of inorganic anion equivalents was observed in the cloud water samples. The high concentration of diacids in the cloud phase (1237.4 mu g L -1 ) may facilitate the formation of salt complexes with NH 4 + , thus influencing the acidity of the cloud water. The pH of cloud water has increased in recent decades due to the sustained reduction of sulfur dioxide, which may also affect the acidity of future precipitation.

The concentration of PM2.5 has considerably reduced in recent years, but remains relatively high in China. In particular, the increasing contribution of organic compounds to PM2.5 generates popular pressure for further reductions, resulting in an urgent need to study organic aerosol (OA). To investigate the molecular composition and source contribution of OA in the rural area of the Guanzhong Plain, Northwest China, PM2.5 samples were collected during 3-23 August 2016 and 5-20 January 2017 and studied for more than 100 organic tracer compounds. The mean concentration of total measured organic compounds is 662 ± 296 ng/m3 in summer and 3258 ± 1925 ng/m3 in winter. Levoglucosan is the most abundant single compound found throughout the sampling period, which is a crucial tracer for biomass burning emissions, preliminary suggesting that biomass burning is an essential source of OA. In summer, organic compounds such as lipid compounds, sugar compounds, and polycyclic aromatic hydrocarbons (PAHs), more come from higher plants, wood burning, vehicle exhausts, plastic waste, and other direct emission sources. oxygenated PAHs (OPAHs), nitrophenols, and phthalic acids more come from the atmosphere through the oxidation reaction of aromatic precursors, especially photochemical oxidation. However, in winter, most of the increases in concentrations of organic compounds are attributed to biomass burning. The analysis of a haze event (14-19 January 2017) during the winter sampling period shows that the increases in the concentration of organic compounds are unaccompanied by strong secondary formation under lower relative humidity (49.1% ± 13.5%). The main reason for the growth of OA in this haze event is the accumulation of primary OA (POA). The source apportionment by the positive matrix factorization (PMF) model shows that biomass burning (37.1%) is the primary source of OA in the rural regions of the Guanzhong Plain, especially in winter (40.6%). The contribution of secondary formation decreases from 26.0% in summer to 16.9% in winter, and the contribution of fossil fuel emissions is comparable across both seasons.

Water-soluble inorganic ions (WSIIs) play a pivotal role in atmospheric chemical reactions, particularly influencing the formation of secondary particulate matter. A comprehensive grasp of the vertical distribution of atmospheric pollutants holds immense significance in understanding the diffusion and transportation of these pollutants. This study investigates the WSIIs of PM2.5 and size-segregated particles at the top (∼2060 m a.s.l) and foot of Mt. Hua during the winter of 2020. All the measured ions present significant higher concentrations (1.9∼6.9 times) at the foot than the top. Cl− and K+ at the foot are more than 4 times of those at the top, whereas Ca2+ and Mg2+ are only 1.3-1.9 times higher. The particle size distribution of NO3−, SO42−, K+ and Cl− demonstrate a single peak distribution (0.7-1.1 μm) at the foot, but with a bimodal distribution (0.7-1.1 μm and 4.7-5.8 μm) at the top. These differences suggest that the aerosol at the alpine region is mainly transported via long-distance from Northwest/North China, but limited influenced by vertical transport through valley breeze. The changes of concentration and size distribution of WSIIs in dust event and non-dust period indicate that the effects of dust event on aerosols at ground surface were weaker than that of the free troposphere of Guanzhong Plain. Notably, our study underscores the dominant influence of NO3− in shaping the gas-particle distribution of ammonia within the winter free troposphere. Our results highlight the significant role of long-range transport on aerosols in the free troposphere in Guanzhong Plain, Northwest China.

Ultrafine particles play a crucial role in understanding climate change, mitigating adverse health effects, and developing strategies for air pollution control. However, the factors influencing the occurrence and development of new particle formation (NPF) events, as well as the underlying chemical mechanisms, remain inadequately explained. This study compared number concentrations and size distributions of atmospheric ultrafine particles at Xi'an (urban area) and the summit of Mt. Hua (alpine region) in summer to investigate the NPF mechanism and particle growth in both clean and polluted areas of the Guanzhong Plain. The average particle number concentration in Xi'an was significantly higher than that at Mt. Hua. The diurnal variation of total particle number concentration differed between Xi'an and Mt. Hua indicating a divergence in influencing factors. The size distributions in Xi'an varied across different timescales and weather conditions, whereas Mt. Hua exhibited little variation. This stability at Mt. Hua is attributed to its cleaner background atmosphere and the steady influx of aging particles with larger diameters transported from the free atmosphere. In both areas, geometric mean diameters (GMDs) were inversely proportional to particle number concentrations suggesting that increase in particle numbers were primarily due to the generation of smaller particles. The potential governing factors for NPF events differed somewhat between the urban and mountainous stations. In the urban area, intense local stationary and mobile emission sources promoted the growth of newly formed nanoparticles, with ozone-oxidized condensable vapors serving as key precursors. In contrast, at the mountainous station, NPF process were significantly influenced by anthropogenic precursors from long-range transport and locally emitted biogenic organics. The rapid increase in ultrafine particle concentrations primarily poses serious health risks and degrades air quality in urban areas, while also contributing to climate-related effects in alpine regions.

Atmospheric polycyclic aromatic hydrocarbons (PAHs) are potentially carcinogenic and mutagenic to human beings, and thus have attracted significant attention in recent decades. Compound-specific stable carbon isotopes (813C) of PAHs are indicative for source identification, but have not been extensively applied in tracing atmo-spheric oxidation processes. In this study, we examined the molecular compositions and 813C values of PAHs in wintertime PM2.5 in a highly polluted urban region in Norwest China. The average concentrations of total parent-PAHs (pPAHs) and oxygenated-PAHs (OPAHs) were measured as 70 & PLUSMN; 29 and 36 & PLUSMN; 17 ng m- 3, respectively. The results obtained from the incremental lifetime cancer risk (ILCR) model suggested a high potential cancer risk of PAHs in the urban region. The temperoal variations of PAHs and carbonaceous fractions implied that the contribution of secondary organic compounds was limited during the severe pollution period (PM2.5 > 115 & mu;g m-3). Both molecular diagnostic ratios and 813C values indicated that the major emission sources of the pPAHs were coal and/or biomass burning. The 813C value of fluorene became heavier as the OPAHs concentration increased, while the concentrations of fluorene was negatively correlated with OPAHs, indicating the secondary formation of OPAHs from fluorene oxidation in the cold season. Our results also confirmed that 813C values of pPAHs are indeed indicative of atmospheric oxidation processes.

Glyoxal and methylglyoxal are important volatile organic compounds in the atmosphere. The gas–particle partitioning of these carbonyl compounds makes significant contributions to O3 formation. In this study, both the gas- and particle-phase glyoxal and methylglyoxal concentrations at the foot and top of Mount Hua were determined simultaneously. The results showed that the gaseous-phase glyoxal and methylglyoxal concentrations at the top were higher than those at the foot of the mountain. However, the concentrations for the particle phase showed the opposite trend. The average theoretical values of the gas–particle partitioning coefficients of the glyoxal and methylglyoxal concentrations (4.57 × 10−10 and 9.63 × 10−10 m3 μg−1, respectively) were lower than the observed values (3.79 × 10−3 and 6.79 × 10−3 m3 μg−1, respectively). The effective Henry’s law constants (eff.KH) of the glyoxal and methylglyoxal were in the order of 108 to 109 mol/kgH2O/atm, and they were lower at the foot than they were at the top. The particle/gas ratios (P/G ratios) of the glyoxal and methylglyoxal were 0.039 and 0.055, respectively, indicating more glyoxal and methylglyoxal existed in the gas phase. The factors influencing the partitioning coefficients of the glyoxal and methylglyoxal were positively correlated with the relative humidity (RH) and negatively correlated with the PM2.5 value. Moreover, the partitioning coefficient of the glyoxal and methylglyoxal was more significant at the top than at the foot of Mount Hua.

The particle number size distribution (PNSD) and hygroscopicity on alpine sites can directly reflect the interaction between aerosol and cloud in the free troposphere, and narrow the errors caused by extrapolation from the ground observation. In this study, PNSD and cloud condensation nuclei (CCN) concentration ( N CCN ) were observed at the summit of Mt. Hua from December 16, 2020, to January 23, 2021. The result showed that the modes and hygroscopicity parameters ( κ ) of aerosols were significantly affected by the height and direction of the originated air masses. The particle in nucleation mode from new particle formation erupted frequently in 13:00–18:00 local time due to the intense photochemistry and had a potential contribution to CCN through the process of the growth with the ratio of 0.83 nm·h −1 , which was affected by air masses, meteorological conditions, and gaseous pollutants. Subsequently, the hygroscopicity κ varied from 0.22 to 0.13 with the supersaturation from 0.2% to 1.0% as more components that were difficult to dissolve or had low hygroscopicity. The comprehensive analysis of PNSD and hygroscopicity showed that smaller k from two‐parameter power model corresponded to particles with higher hygroscopicity or larger diameter, while larger k matched to the hydrophobic or ultrafine particles. In addition, the interactions between aerosol and cloud were estimated more accurately and the uncertainty of indirect effect was shrunk. Overall, we provided insights into PNSD and hygroscopicity of alpine aerosol particles by direct observation on Mt. Hua, which may be important to other elevated regions as well.

Dicarboxylic acids are strong hygroscopic organic compounds in the atmosphere, and thus significantly affect the cloud formation process and radiative forcing on a regional scale. So far, the evolution of dicarboxylic acids during vertical transport from the surface to the mountaintop has yet to be explicitly understood. In this study, the molecular distribution and stable carbon isotopic (δ13C) compositions of dicarboxylic acids and related organic compounds (DCRCs) in PM2.5 were measured simultaneously at the top (c. 2060 m a.s.l.) and foot (c. 400 m a.s.l.) of Mount (Mt.) Hua during the summer of 2020. Due to the strong anthropogenic emissions at ground level, the concentrations of DCRCs at foot of Mt. Hua were generally higher than those at the top. Oxalic acid (C2) was the predominant diacid in both sites, whose concentrations at foot and top of Mt. Hua were 87–852 and 40–398 ng m−3, respectively. Ratios of adipic acid to azelaic acid (C6/C9), phthalic aid to azelaic acid (pH/C9), glyoxal to methylglyoxal (Gly/mGly), and lower δ13C values (−21.0 ± 2.3 ‰ and − 21.9 ± 2.7 ‰) of C2 indicated that the contributions of anthropogenic sources to DCRCs in PM2.5 in the mountain region are more significant than biogenic sources. Aerosols from the foot of Mt. Hua could affect the atmosphere on the top of the mountain via vertical transport under the influence of daytime valley wind, even though the altitude of Mt. Hua is beyond the boundary layer most of time. The value δ13C of C2 is linearly correlated with C2/mGly, C2/pyruvic acid (Pyr), C2/glyoxylic acid (ωC2) at the top of the mountain, and C2/Gly, C2/ωC2 at the foot of the mountain, indicating that the formation pathway of C2 is mGly-Pyr-ωC2-C2 at the top of Mt. Hua and Gly-ωC2-C2 at the foot of Mt. Hua.

Net primary production (NPP) serves as a crucial indicator of the ecosystem’s capacity to capture atmospheric CO2. Gaining insights into the dynamics of NPP and its driving mechanisms is pivotal for optimizing ecosystem carbon sink resource management. Since the implementation of the Grain-for-Green Program (GFGP) in 1999, the Yellow River Basin (YRB) has been one of the most significant areas for ecological restoration in China. However, our knowledge regarding the interannual variability (IAV) of NPP and the underlying driving forces in this region remains incomplete. In this study, we utilized a light use efficiency model to assess the spatiotemporal dynamics, IAV, and driving factors of NPP in the YRB during the period from 1999 to 2018. Our findings revealed that the average annual NPP in the YRB approximated 189.81 Tg C. Over the study duration, NPP significantly increased in 79.63% of the basin with an overall increasing rate of 6.76 g C m−2 yr−1. The most prominent increase was observed in the key GFGP implementation area, predominantly in the semi-humid region. Notably, the middle altitude region (1–1.5 km), semi-humid region, and grassland emerged as the primary contributors to the basin’s total vegetation carbon sequestration. However, it is worth emphasizing that there was substantial IAV in the temporal trends of NPP, with the semi-humid region being the most influential contributor (62.66%) to the overall NPP IAV in the YRB. Further analysis of the driving mechanisms unveiled precipitation as the primary driver of NPP IAV in the YRB with a contribution of 62.9%, followed by temperature (23.07%) and radiation (14.03%). Overall, this study deepened our understanding of the IAV and driving mechanisms of NPP in the YRB under ecological restoration, and provided scientific support for optimizing the management of regional carbon sequestration resources.

Aerosol pH is not only a diagnostic indicator of secondary aerosol formation, but also a key factor in the specific chemical reaction routes that produce sulfate and nitrate. To understand the characteristics of aerosol acidity in the Mt. Hua, the chemical fractions of water-soluble inorganic ions in the atmospheric PM2.5 and size-resolved particle at the top and foot of Mt. Hua in summer 2020 were studied. The results showed the mass concentrations of PM2.5 and water-soluble ions at the foot were 2.0–2.6 times higher than those at the top. The secondary inorganic ions, i.e., SO42−, NO3−, and NH4+ (SNA) were 56 %–61 % higher by day than by night. SO42− was mainly distributed in the fine particles (Dp < 2.1 μm). NO3− showed a unimodal size distribution (peaking at 0.7–1.1 μm) at the foot and a bimodal (0.7–1.1 μm and 4.7–5.8 μm) size distribution at the top. At the top site, the distribution of NO3− in coarse particles (> 2.1 μm) was mainly attributed to the gaseous HNO3 volatilized from fine particles reacting with cations in coarse particles to form non-volatile salts (such as Ca(NO3)2). The pH values of PM2.5 were 2.7 ± 1.3 and 3.3 ± 0.42 at the top and foot, respectively. NH4+/NH3(g) plays a decisive role in stabilizing aerosol acidity. In addition, the increase of the liquid water content (LWC) at the foot facilitates the gas-particle conversion of NH3, while the H+ concentration was diluted, resulting in a decrease in acidity at the foot. NH4+/NH3 had good linear correlations with SO42−, NO3−, and LWC during the daytime at both sites, indicating that SO42−, NO3−, and LWC together affect the gas-particle distribution of ammonia by day: however, the effect of LWC at night was not evident.

In this study, the characteristics and formation mechanism of summertime isoprene, monoterpene, and toluene-derived secondary organic aerosols (SOAs) were investigated in a rural area of Guanzhong Plain, Northwest China. The variations in key indicators of primary sources indicated a significant influence of biomass burning on PM2.5 during the observation period. The concentrations of total measured SOA tracers from isoprene, monoterpene, and toluene were 40.85 ± 17.31, 24.27 ± 7.50, and 10.61 ± 0.33 ng/m3, respectively. The average ratio of cis-pinonic and pinic acids to 3-Methyl-1,2,3-butanetricarboxylic acid (MBTCA)(P/M) were 0.45 and 0.85 by day and by night, respectively. The low ratio in the daytime was mainly due to the stronger photo-degradation and particle-to-gas distribution of semi-volatile cis-pinonic and pinic acids. The monoterpene SOA tracers were significantly correlated with levoglucosan at night (R2 = 0.51, p < 0.01), as were toluene SOA tracers and levoglucosan (R2 > 0.67, p < 0.01), indicating the significant contribution of biomass combustion to these SOAs. The mass concentration of isoprene-, monoterpenes-, and toluene-derived SOC was estimated by using the tracer yield method. The total calculated SOCs by day and by night were 0.25–0.71 (average: 0.46) and 0.26–0.78 (average: 0.42) µgC/m3, accounting for 3.35–10.58% and 3.87–13.51% of OC by day and by night, respectively.