Smog Chamber Simulation On Secondary Organic Aerosol Formation From Photooxidation Of Xylene

In the last decades atmospheric concentration of PM_2.5 in China has decreased significantly due to the effective control of SO₂ emissions.However,the annual level of PM_2.5 in the country is still 3-5 times higher than the value of 10μg/m~3recommended by the World Health Organization（WHO）,especially in winter.Specifically,the North China Plain（NCP）,the Fenwei Plain and other regions have frequently experienced heavy haze events in winter,indicating that PM_2.5 pollution in many regions of China is still serious.A number of field observations have shown that secondary organic aerosols（SOA）are one of the main components of atmospheric PM_2.5 in China,especially during the winter haze period,and SOA plays an important role in the formation process of the haze episodes.However,the formation mechanism of SOA is still unclear.In this study,SOA derived from xylene,which is an important anthropogenic VOCs in the uran atmosphere of the NCP region,was investigated by using a laboratory smog chamber,focusing on the the production yields,chemical compositions and optical properties.The effects of NO₂,SO₂ and NH₃,as well as relative humidty,on the SOA mass concentration,chemical composition and optical properties were explored.The major findings and conclusions are summarized as follows:（1）Under the low relative humidity（30%RH）conditions,when the initial NO₂concentration increases from 0 ppb to 893 ppb,the increase in NO₂ concentration can promote the photooxidation of xylene to generate SOA by a factor of 2,in which the production of o-xylene SOA was 10-30μg m^-3 higher than that of p-xylene SOA.The increase in the NO₂ concentration promoted the formation of low oxidation state of carbon（OSc）substances through an acid-catalyzed reaction,which subsequently partitioned into the aerosol phase,and thus increased the production of xylene SOA.When the NO₂ concentration increased from 122 ppb to 893 ppb,the relative contents of nitrogen-containing components in o-xylene SOA and p-xylene SOA increased by4.1%and 5.1%,respectively.Such an increase of nitrogen-containing organic components（NOC）also increased the light absorption of the xylene SOA.Compared with the low RH case（30%RH）,the formation of HONO is an important factor for promoting the formation of o-xylene SOA and p-xylene SOA under the high relative humidity（70%RH）conditions,of which the concentration was increased by 1 time and2 times when NO₂concentration was 1000 ppb,respectively.However,higher humidity also led to a greater wall loss,which resulted in the number concentrations of o-xylene SOA and p-xylene SOA decreased to 10.8×10~4 cm^-3 and 15.2×10~4 cm^-3 when NO₂concentration increased from 0 ppb to 1000 ppb,respectively.Under the humid conditions（70%RH）,the increase of NO₂ concentration also increased the MAC_λ=365nm value of the xylene SOA by about 2-3 times through producing light-absorbing NOCs.（2）When NH₃ concentration increased from 0 ppb to 120 ppb,the formation of o-xylene SOA was increased by 4.2 times,and the formation of p-xylene SOA was enhanced by 12.8 times.The reaction of gas-phase oxidation products with NH₃ to form low-volatile substances is an important reason for the increase in xylene SOA mass concentration.NO₂ and NH₃ can jointly promote the formation of xylene SOA,among which the formation of o-xylene SOA and p-xylene SOA can reach 267μg m^-3 and 610μg m^-3,respectively.Since NH₄NO₃ can be formed in the co-existence of NH₃ and NO₂,the gas-phase products such as ammonia,aldehydes and ketones will undergo heterogeneously reactions on hygroscopic NH₄NO₃ particles such as carbonylammonium condensation and Maillard reaction,thereby promoting the production of xylene SOA.The presence of NH₃ with or without NO₂ caused a decrease in the content of organic acids in xylene SOA and an increase in the content of NOCs.The elemental composition and characteristic peaks of AMS Iindicated that sufficient carboxylic acid in o-xylene SOA are enriched in carboxylic acid,which are mainly neutralized by NH₃ to form organic ammonium carboxylate.In contrast,organic acid in p-xylene SOA are much less and thus after the neutralization with the carboxylic acids NH₃ in the chamber further underwent a heterogeneous reaction with carbonyls to form imidazole-like substances.In the solo presence of NH₃,120 ppb NH₃ promoted the light absorbance of o-xylene by about 2 times compared with the absence of NH₃,but did not showed significant enhancing effect on the light absorbance of p-xylene SOA,because the ketones generated from p-xylene are less likely to react heterogeneously with NH₃.After adding 120 ppb NO₂,the MAC_{λ=365 nm} of xylene SOA showed an increasing trend with the increase of NH₃ concentration,and the maximum MAC_{λ=365 nm} of o-xylene SOA and p-xylene SOA reached 0.48 m~2·g^-1and 0.29 m~2·g^-1,respectively.In the presence of both NO₂ and NH₃,the higher content of NOCs is an important reason for the increase of the MAC_{λ=365 nm} value of xylene SOA.（3）In the presence of 50 ppb SO₂,the mass concentration of SOA produced from the xylene photochemical oxidation increased firstly and then decreased along with a continuous increase in NO₂ concentrations,during which the SOA mass concentration showed maxima of 259μg/m~3for o-xylen and 189μg/m~3 for p-xylene at 300 ppb NO₂conditions,respectively,mainly due to the acid-catalyzed reactions.Under the high NO₂ conditions there are two processes causing the decrease in the SOA concentration.One is the increasing production of more volatile SOA produced by the reation of RO₂with NO,and another reason is that the high level of NO₂ consumed most·OH radicals and thus resulted in much less amount of sulfate produced compared to that under low NO₂ condtions.AMS chemical composition analysis results showed that SO₂ not only can activate acid-catalyzed reactions but also can heterogeneously react in aerosol phase by forming sulfur containing organic compounds.However,compared to that in the absence of NO₂,the relative abundance of NOC in the xylene SOA did not increase along with an increasing NO₂ level,indicating that SO₂ is favorable for the xylene SOA formation through acid-catalyzed reactions but does not favor NOC forrmation.In the presence of 50 ppb SO₂,the values of MAC_{λ=365 nm} of o-xylene and p-xylene SOA maximized separately at 0.72 m~2 g^-1and 0.60m~2 g^-1 when NO₂ was 100ppb,which are about one time higher than those in the absence of SO₂ and can be explained by the formation of light-absorbing SOA due to carbonyl condensation and the production of sulfur-containing organic compounds.