Grantee Research Project Results

Overview:  It now is widely accepted that aqueous oxidation of water-soluble carbonyl compounds (e.g., glyoxal and methylglyoxal) by OH radical in clouds and wet aerosols results in the formation of secondary organic aerosol (SOA). Reactions during cloud droplet evaporation (Loeffler, et al., ES&T 2006), with inorganic constituents in wet aerosols (Noziere, et al., JPC 2009; Perri, et al., AE 2010; Yasmeen, et al., ACP 2010), and with other oxidants (e.g., ozone; Grgic, et al., PCCP 2010) also contribute. The potential importance of SOA formation through aqueous chemistry is all the more apparent when one realizes that liquid water is ubiquitous and abundant. Model estimates suggest that globally, the mass of aerosol water is more than twice the dry particle mass (Liao and Seinfeld, personal communication). Efforts currently are under way to add SOA formation through aqueous chemistry to several regional and global chemical transport models. Although current modeling estimates have large uncertainties, SOA formed through aqueous chemistry appears to be comparable in magnitude to “traditional” SOA formed through gas phase chemistry and vapor-pressure driven partitioning (Carlton, et al., ES&T 2008; Fu, et al., JGR 2008, AE 2009). Based on the addition of this chemistry to a cloud parcel model, it appears that “aqueous” SOA formation from the biogenic hydrocarbon, isoprene, is enhanced at high NO_x because the gas phase yields of water-soluble carbonyl compounds are higher at high NO_x (Ervens, et al., GRL 2008). This suggests that SOA formed from biogenic hydrocarbons through aqueous chemistry might be reduced through controls on anthropogenic emissions.

 

Anthropogenic atmospheric wet deposition of secondary organic nitrogen is implicated in the eutrophication of coastal estuaries (Seitzinger and Sanders, Mar Ecol Progr Ser 1997). Smog chamber experiments suggest that water-soluble organic nitrogen compounds are formed through gas photochemistry at high NO_x. We hypothesize that organic nitrogen also is formed from reactions in atmospheric waters. Our analyses of New Jersey rainwater samples (Altieri, et al., ACP 2009, ES&T 2009) provide some insights as to what reactions and products might be important. Our experiments, reported below, suggest that organic nitrogen formation from nitric acid and from ammonium sulfate is negligible at cloud-relevant concentrations in the presence of OH radical. Further work is under way to determine whether organic nitrogen formation is important in wet aerosols.

 

Our most recent aqueous laboratory experiments with glyoxal/methylglyoxal and OH radical (plus control experiments) verify the formation of oxalate, other organic acids, and humic-like substances (HULIS) including oligomers from OH radical reactions (Altieri, et al., AE 2008; Lim, et al., ACP 2010; Tan, et al., ES&T 2009, AE 2010). These are common constituents of atmospheric particles. For example, HULIS account for roughly one-third of atmospheric organic aerosol mass and are not formed through atmospheric gas-phase chemistry (Kiss, et al., 2002; Lin, et al., JAS 2010; Zappoli, et al., 1999). SOA formation through aqueous photooxidation (e.g., cloud processing) was proposed by Blando and Turpin (AE 2000). It is the only identified process that can explain the atmospheric abundance/temporal dynamics of oxalic acid; it has different precursors than “traditional” SOA and could be a substantial contributor to total SOA globally and regionally. We hypothesized that similar chemistry also occurs in aerosol water. In fact, our most recent experiments and modeling suggest that oligomers/HULIS are the main SOA products in wet aerosols, whereas organic acids/salts are the major SOA-forming products in clouds (Lim, et al., ACP 2010). Evidence for SOA formation via OH radical reactions in wet aerosol is provided by high relative humidity smog chamber experiments (Volkamer, et al., ACP 2009). The Volkamer, et al., experiments demonstrate that SOA formation from glyoxal at high RH is dramatically faster in the presence of OH radical. Recent atmospheric evidence for “aqueous” SOA formation is provided by ambient measurements showing an increase in SOA at relative humidities above 70% (Hennigan, et al., GRL 2008 and ACP 2009; Sorooshian, et al., GRL 2010).

 

One limitation of our previous work is that experiments were conducted at concentrations about 100 times greater than those found in clouds (and 1000 times lower than concentrations in aerosol water). Our current STAR grant included support (50%) for the purchase of an ion chromatography (IC) system that has allowed us to conduct experiments at cloud-relevant (and higher) concentrations (Tan, et al., ES&T 2009). Two additional analytical improvements have been made. We now can conduct experiments with real-time analysis by electrospray ionization (ESI) mass spectrometry (Perri, et al., AE 2009). We also can analyze samples by ESI-MS after IC pre-separation (Tan, et al., AE 2010). These techniques have improved our mechanistic understanding in substantial ways. The postdoctoral fellow recruited to work on this grant has made substantial progress understanding the detailed radical chemistry, extending the range of concentrations for which we can make accurate predictions (Lim, et al., ACP 2010). Also, in a logical extension of our past work, we now have conducted experiments to study the cloud chemistry of glyoxal in the presence and absence of nitric acid (HNO₃) and ammonium sulfate (NH₄)₂SO₄. Finally, we are collaborating with EPA scientists and others to facilitate incorporation of in-cloud SOA formation into CMAQ and other models.

 

Objective 1:  With support from this grant and Rutgers University, we purchased a Dionex Ion Chromatography System (ICS-3000), which has allowed us to conduct aqueous photooxidation experiments at lower concentrations than previously possible. In year 1, we conducted experiments with glyoxal + ·OH (10^-12 M) at 30 μM, 300 μM and 3000 μM (Tan, et al., ES&T 2009). In year 2, we conducted the same suite of experiments with methylglyoxal (Tan, et al., AE 2010) and in year 3, with acetic acid (Tan, in preparation). A subset of these experiments has been repeated with the addition of cloud-relevant concentrations of H₂SO₄ (840 μM), HNO₃ (840 μM), or (NH₄)₂SO₄ (1.68 mM). In all cases, OH radical was formed from UV photolysis of H₂O₂. Therefore, control experiments were conducted with UV and with H₂O₂, with the precursor, and with the precursor and a mix of expected products. Our previous experiments were conducted only at 1000-3000 μM. The 30 μM experiments are representative of cloud concentrations. Concentrations in aerosol water could be as high as 1-10 M. Acetic acid is an intermediate aqueous-phase product of methylglyoxal plus OH radical. Acetic acid experiments were conducted to investigate the hypothesis that methylglyoxal oligomers are oligoesters formed through an acetic acid reaction pathway via condensation reactions (Altieri, et al., AE 2008). Some experiments of each type were analyzed in real time by ESI-MS. Discrete samples were collected and analyzed for organic acids by ion chromatography (IC), and their mass spectra were obtained by ESI-MS with and without pre-separation by IC. Ultra-high resolution Fourier transform ion cyclotron resonance electrospray ionization mass spectroscopy (FTICR-MS) was conducted on selected samples. Total carbon in each sample was measured using a total organic carbon analyzer. Sulfate and nitrate were measured by IC. Dissolved oxygen and pH were measured at the beginning and end of each experiment. The chemistry in the reaction vessel also was modeled using our aqueous phase chemistry model (Lim model). In this way we could use the experiments to validate and refine our chemistry model. The rate constants in the model are reasonably well known, but in many cases the products had not been verified previously.

 

The Lim model did a good job of capturing the magnitude and concentration dynamics of oxalic acid and total carbon for both glyoxal and methylglyoxal when the experiments were conducted at cloud-relevant concentrations in the presence and absence of inorganic constituents. This suggests that the Lim model is adequate for use in predicting glyoxal and methylglyoxal cloud chemistry leading to SOA formation and that the presence of sulfate, nitrate and ammonium at cloud-relevant concentrations has little effect on this chemistry.

 

By examining changes with increasing concentrations, we are developing a better understanding of the chemistry in wet aerosols. As the initial concentrations of the organic precursor increased, the predictive capability of the model degraded, and the measured organic acids accounted for a smaller and smaller portion of the total carbon in the reaction vessel. We found that increasing precursor concentrations resulted in samples with increasingly complex mass spectra and resulted in the formation of organic acids and oligomers with larger carbon numbers. Because higher carbon number products form and form only in the presence of OH radical, as well as because they only form at high organic concentrations, we hypothesized that these products form through radical-radical reactions.

 

To investigate this possibility, we expanded the glyoxal mechanism in the original Lim model to explicitly account for radical-radical reactions. Model performance for 30 μM experiments changed little; model performance was much improved for 3000 μM glyoxal experiments (Lim, et al., ACP 2010). Specifically, the oxalic acid concentration dynamics were better captured, and the model captured the concentration dynamics of the most abundant quantified higher carbon number organic acid (tartaric acid). We ran this model for glyoxal concentrations from 10 μM to 10 M (OH radical concentrations of 10^-12 M) and concluded that oxalate is the major SOA-forming product at cloud-relevant concentrations, and oligomers are the major SOA products in wet aerosols. We expect OH radical chemistry will be the dominant daytime pathway for “aqueous” SOA formation from water-soluble aldehydes; our OH radical experiments and control experiments provide support for this.

 

We previously proposed that oligomers forming from methylglyoxal and from pyruvic acid in the presence (but not absence) of OH radical were oligoesters formed via condensation reactions involving a hydroxy acid and one of several major organic acid products (Altieri, et al., AE 2008). At that time, we proposed that hydroxy acids formed because succinic acid formed from the OH radical attack of acetic acid. While the formation of at least some succinic acid was confirmed by FT-ICR MS analysis of methylglyoxal experimental samples, analysis of acetic acid experiments by IC-ESI-MS showed that succinic acid was not a product of acetic acid plus OH radical (Tan, et al., in preparation). A complex spectrum of high molecular weight products observed in methylglyoxal plus OH experiments (but not controls) was not observed in acetic acid experiments conducted at identical concentrations. These results, our new insights regarding the chemistry of glyoxal, and the formation of these products only in the presence of OH radical but not in mixtures containing a hydroxy acid (i.e., lactic acid), lead us to conclude that oligomers from methylglyoxal plus OH radical form through radical-radical reactions (Tan, et al., in preparation). As with glyoxal, we expect oligomers to be the major SOA products of OH radical chemistry in wet aerosols and small organic acids (oxalate, pyruvate) to be the main SOA-forming products in clouds.

 

Sulfur-containing organic products were seen in the ultra high resolution FT-ICR MS analyses of samples containing inorganic constituents but not in controls. These might be important products in wet aerosols. To investigate this possibility, we developed a model to investigate the radical chemistry of glycolaldehyde, OH radical and H₂SO₄. The model suggests that organosulfates could be a measureable product (as high as 10-30% of the total organic carbon) in wet acidic aerosols, but they form in negligible quantities at concentrations found in clouds (less than 1% of organic carbon; Perri, et al., AE 2010). Samples from 1 mM glyoxal experiments conducted in the presence of nitrogen (HNO₃, (NH₄)₂SO₄) have identical IC-ESI mass spectra to those conducted in the absence of nitrogen, and the concentrations of major products were not affected by the addition of nitrogen (Kirkland, et al., in preparation). This suggests that the addition of cloud-relevant concentrations of nitrate and ammonium had a negligible effect on the formation of SOA-forming products. We currently are examining FT-ICR mass spectra from these samples to see if organic nitrogen products are present in small quantities in these samples. If so, they could be significant products in wet aerosols, as we found for organosulfate. In analyses of New Jersey rainwater samples, we found a series of compounds whose elemental compositions were identical to those formed through oligomerization reactions in methylglyoxal+·OH experiments performed earlier in our laboratory. We also found a wide variety of organic nitrogen products in these rainwater samples (Altieri, et al., ACP 2009, ES&T 2009).

 

Objective 2:  Based on the experiments, modeling and collaborative research made possible by this EPA STAR grant we conclude that increased NO_x results in higher yields of SOA from the aqueous chemistry of isoprene reaction products. Our experiments discussed above suggest that nitrate has little effect on the aqueous chemistry leading to SOA formation, at least in clouds (Kirkland, et al., in preparation). However, the yields of water soluble carbonyls from gas-phase isoprene chemistry are higher at high NO_x (Ervens, et al., GRL 2008). This suggests that more “aqueous” SOA will form from a biogenic hydrocarbon-like isoprene in polluted conditions than in clean conditions.

 

Objective 3:  In-cloud production of SOA from glyoxal and methylglyoxal has been added to the CMAQ model using a yield-based approach similar to that used for other SOA formation processes. The model was used to predict organic carbon (OC) concentrations measured on an airplane during the ICARTT experiment. The addition of cloud processing to CMAQ improved agreement between modeled OC and measured water-soluble organic carbon (WSOC) for all flights, most notably aloft. SOA formation through cloud processing was negligible in certain areas and substantial in others. This work and similar modeling for the Northeastern United States and the globe conducted by others using GEOSChem (Fu, et al., JGR 2008, AE 2009), suggest that SOA formation through aqueous chemistry is comparable in magnitude to “traditional” SOA, although the uncertainties are still large. We are cooperating with a number of modelers who are working toward adding SOA formation through aqueous chemistry into their models.

 

To further improve the treatment of cloud chemistry in CMAQ, a Rosenbrock solver (ROS3) has been incorporated into the full-scale version of CMAQ v.4.7. The current ROS3 aqueous chemical mechanism is the same as in the base model version and will be made publicly available via CMAS during the next (FY2011) public release. The ROS3 version implements the same aqueous chemical mechanism as the base model, with the sole exception being that gas-to-water partitioning is described kinetically, rather than assuming instantaneous thermodynamic equilibrium.

We expect to publish two to three additional papers to complete this STAR grant. One will report on experiments conducted with and without the addition of nitrogen (nitric acid and ammonium sulfate). Another paper will explain new insights regarding the aqueous chemistry of methylglyoxal. This paper will make use of acetic acid experiments. A third ongoing effort involves the use of our chemical model to further the implementation of “aqueous” SOA schemes in chemical transport models.