A simplified description of the evolution of organic aerosol composition in the atmosphere

Abstract

Organic aerosol (OA) in the atmosphere consists of a multitude of organic species which are either directly emitted or the products of a variety of chemical reactions. This complexity challenges our ability to explicitly characterize the chemical composition of these particles. We find that the bulk composition of OA from a variety of environments (laboratory and field) occupies a narrow range in the space of a Van Krevelen diagram (H:C versus O:C), characterized by a slope of ∼−1. The data show that atmospheric aging, involving processes such as volatilization, oxidation, mixing of air masses or condensation of further products, is consistent with movement along this line, producing a more oxidized aerosol. This finding has implications for our understanding of the evolution of atmospheric OA and representation of these processes in models.

OH-Initiated Heterogeneous Aging of Highly Oxidized Organic Aerosol

“…On the other hand, if the oxidation reaction involves fragmentation-the cleavage of C-C bonds-and subsequent loss of carbon to the gas phase, then the resulting slope can take on a range of different values, depending on the chemical characteristics of the volatilized fragment. While previous studies have observed that measured ratios and trajectories of reduced and lightly oxidized organic aerosol follow a line with a slope of -1 19 , the data presented here have slopes ranging from -0.6 to -0.3 (Table 1). This tendency of 9 more highly oxidized systems to acquire oxygen with a smaller net loss of hydrogen is consistent with recent studies of the aging of oxidized organic aerosol 20 .…”

“…Atmospheric oxidation of the most oxidized organic aerosol is therefore unlikely to be a significant sink of aerosol mass, though it is still capable of transforming the chemical composition-and therefore key properties such as hygroscopicity 1,29,30 oxygen-to-carbon (O/C) at all points during the oxidation of the four systems in this study. The solid black line represents observations of ambient aerosol at moderate levels of oxidation 19 . The dashed black line represents measurements of ambient OOA at higher oxidation levels 20 .…”

“…In order to compare the results with similar studies of ambient organic aerosol 19,20 , we apply the method of Aiken et al 15,16 with no additional correction factor. The measured H/C and O/C values of the pure compounds tend to be lower than their exact values; however, the overall trends observed in the trajectories remain unchanged, regardless of the correction factor used.…”

“…The slope of the trajectory is indicative of the average chemical transformation of particulate organics 19 . For example, a slope of 0 corresponds to the addition of hydroxyl groups (oxygen addition with no net hydrogen loss), -2 corresponds to carbonyl addition, and steeper slopes denote the conversion of functional groups (e.g.…”

Estimating the contribution of organic acids to northern hemispheric continental organic aerosol

“…Oxidation of VOCs is thought to be a key source of organic acids with recent studies providing new insight into their formation pathways and properties [Paulot et al, 2011;Sommariva et al, 2011;Andrews et al, 2012;Millet et al, 2015;Yuan et al, 2015]. Recent studies indicate organic acids can be a large fraction of SOA at locations highly influenced by biogenic emissions [Vogel et al, 2013], and it has been hypothesized that their fractional contribution to OA increases with increasing photochemical age [Heald et al, 2010;Levin et al, 2014]. However, quantitative determination of the fraction of submicron OA that is composed of organic acids is still limited despite indications that they can be a substantial fraction of OA [Russell et al, 2011].…”