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Abstract
Abstract. The effects of acidic seed aerosols on the formation of secondary organic aerosol (SOA) have been examined in a number of previous studies, several of which have observed strong linear correlations between the aerosol acidity (measured as nmol H+ m−3 air sample volume) and the percent change in secondary organic carbon (SOC). The measurements have used several precursor compounds representative of different classes of biogenic hydrocarbons including isoprene, monoterpenes, and sesquiterpenes. To date, isoprene has displayed the most pronounced increase in SOC, although few measurements have been conducted with anthropogenic hydrocarbons. In the present study, we examine several aspects of the effect of aerosol acidity on the secondary organic carbon formation from the photooxidation of 1,3-butadiene, and extend the previous analysis of isoprene. The photooxidation products measured in the absence and presence of acidic sulfate aerosols were generated either through photochemical oxidation of SO2 or by nebulizing mixtures of ammonium sulfate and sulfuric acid into a 14.5 m3 smog chamber system. The results showed that, like isoprene and β-caryophyllene, 1,3-butadiene SOC yields linearly correlate with increasing acidic sulfate aerosol. The observed acid sensitivity of 0.11% SOC increase per nmol m−3 increase in H+ was approximately a factor of 3 less than that measured for isoprene. The results also showed that the aerosol yield decreased with increasing humidity for both isoprene and 1,3-butadiene, although to different degrees. Increasing the absolute humidity from 2 to 12 g m−3 reduced the 1,3-butadiene yield by 45% and the isoprene yield by 85%.
Highlights
The role of aerosol acidity in increasing the formation of secondary organic aerosol (SOA) in the atmosphere continues to be a topic of considerable debate
1,3-butadiene, a chemically similar compound released from primarily anthropogenic sources, was demonstrated to produce higher concentrations of SOA under acidic conditions
1,3-butadiene, a chemically similar compound released from primarily anthropogenic sources, was demonstrated to produce higher concentrations of SOA under acidic conditions, albeit to a lesser extent than was seen with isoprene
Summary
The role of aerosol acidity in increasing the formation of secondary organic aerosol (SOA) in the atmosphere continues to be a topic of considerable debate. In the previous studies by Surratt et al (2007) and Offenberg et al (2009), all measurements were conducted at a single humidity level (30 % relative humidity), while Zhang et al (2012) examined only dry conditions Extending these studies to a wider range of hydrocarbons and across a more realistic range of humidities should provide data of greater atmospheric relevance and contribute to further development of acidity-influenced SOA chemistry in air quality models. Each hydrocarbon/NO system was tested at multiple humidity levels using only a low concentration (1 μg m−3) ammonium sulfate seed aerosol (ER666 for 1,3-butadiene, ER667 for isoprene) This provided a base case for exploring the changes in SOC formation and aerosol yield in the absence of significant aerosol acidity. For 1,3-butadiene, a more acidic inorganic aerosol, generated using a solution incorporating a higher fraction of sulfuric acid solution to ammonium sulfate solution, was employed (ER444)
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