Air quality impact and physicochemical aging of biomass burning aerosols during the 2007 San Diego wildfires

Intense wildfires burning >360000 acres in San Diego during October, 2007 provided a unique opportunity to study the impact of wildfires on local air quality and biomass burning aerosol (BBA) aging. The size-resolved mixing state of individual particles was measured in real-time with an aerosol time-of-flight mass spectrometer (ATOFMS) for 10 days after the fires commenced. Particle concentrations were high county-wide due to the wildfires; 84% of 120-400 nm particles by number were identified as BBA, with particles <400 nm contributing to mass concentrations dangerous to public health, up to 148 μg/m(3). Evidence of potassium salts heterogeneously reacting with inorganic acids was observed with continuous high temporal resolution for the first time. Ten distinct chemical types shown as BBA factors were identified through positive matrix factorization coupled to single particle analysis, including particles comprised of potassium chloride and organic nitrogen during the beginning of the wildfires, ammonium nitrate and amines after an increase of relative humidity, and sulfate dominated when the air mass back trajectories passed through the Los Angeles port region. Understanding BBA aging processes and quantifying the size-resolved mass and number concentrations are important in determining the overall impact of wildfires on air quality, health, and climate.

Emission Ratios for Ammonia and Formic Acid and Observations of Peroxy Acetyl Nitrate (PAN) and Ethylene in Biomass Burning Smoke as Seen by the Tropospheric Emission Spectrometer (TES)

We use the Tropospheric Emission Spectrometer (TES) aboard the NASA Aura satellite to determine the concentrations of the trace gases ammonia (NH3) and formic acid (HCOOH) within boreal biomass burning plumes, and present the first detection of peroxy acetyl nitrate (PAN) and ethylene (C2H4) by TES. We focus on two fresh Canadian plumes observed by TES in the summer of 2008 as part of the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS-B) campaign. We use TES retrievals of NH3 and HCOOH within the smoke plumes to calculate their emission ratios (1.0% ± 0.5% and 0.31% ± 0.21%, respectively) relative to CO for these Canadian fires. The TES derived emission ratios for these gases agree well with previous aircraft and satellite estimates, and can complement ground-based studies that have greater surface sensitivity. We find that TES observes PAN mixing ratios of ~2 ppb within these mid-tropospheric boreal biomass burning plumes when the average cloud optical depth is low (<0.1) and that TES can detect C2H4 mixing ratios of ~2 ppb in fresh biomass burning smoke plumes.

Effect of ammonia on secondary organic aerosol formation from alpha-pinene ozonolysis in dry and humid conditions

This study examines the influence of ammonia (NH3) on secondary organic aerosol (SOA) formation from the alpha-pinene/ozone oxidation system for dry and humid conditions. Aerosol yield differed depending on which OH scavenger was used, with the highest yield noted for CO, followed by cyclohexane and 2-butanol. Number and volume concentrations were quickly increased within the reactor by 15 and 8%, respectively, when NH3 was added after the reaction ceased. The increase in number concentration indicated the formation of new particles resulting from gas-to-particle conversion. Moreover, average particle size increased from 242 to 248 nm. The resulting aerosol growth was attributed to ammonium salts formed by the reaction between organic acids and NH3. When NH3 was added to aerosolized cis-pinonic acid in the environmental reactor, a dramatic increase in both number and volume concentrations of cis-pinonic acid was observed. This provides further evidence that NH3 can interact with gas-phase organic acids forming condensable salts and thereby enhancing SOA formation. Initially present NH3 significantly enhanced aerosol yield in alpha-pinene-ozone reactions, regardless of the presence of water vapor. The role of NH3 on SOA formation in the dry and humid conditions is discussed in terms of a theoretical modeling study.

Photochemical Sources of Organic Acids. 2. Formation of C5−C9 Carboxylic Acids from Alkene Ozonolysis under Dry and Humid Conditions

The dependence of organic acid generation by alkene ozonolysis on relative humidity, thermalized Criegee intermediate scavengers, and alkene structure is investigated. Carboxylic acids generated from the ozonolysis of 1-hexene, 1-octene, 1-decene, trans-3-octene, and 1-methylcyclohexene were analyzed as trimethylsilyl (TMS) derivatives. Experiments were performed under dry (relative humidity (RH) < 1%) and humid (RH = 65%) conditions with cyclohexane or n-butyl ether as an OH scavenger. Pentanoic acid is produced from 1-hexene and trans-3-octene with yields 8.5 +/- 2.6 and 5.0 +/- 1.5% under dry conditions and 5.1 +/- 1.5 and 2.8 +/- 0.8% under humid conditions, respectively. Heptanoic acid yields from 1-octene are 8.3 +/- 2.5 and 4.4 +/- 1.3% under dry and humid conditions, respectively. Ozonolysis of 1-methylcyclohexene produced six C5-C7 multifunctional carboxylic acids, with a total yield of 7%. Several other acids and aldehydes were also monitored and quantified. An additional set of experiments with added stabilized Criegee intermediate (SCI) scavengers was performed for 1-octene ozonolysis under dry conditions. The results indicate that SCIs and their reaction with water are minor contributors to acid formation in the atmosphere and suggest that many of the acids are formed directly.

Photochemical Sources of Organic Acids. 1. Reaction of Ozone with Isoprene, Propene, and 2-Butenes under Dry and Humid Conditions Using SPME

Formation of C4 and smaller carboxylic acids from gas-phase ozonolysis of several alkenes under dry (relative humidity (RH) < 1%) and humid (RH = 65%) conditions have been investigated. We have developed a technique based on solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GC/MS) to quantify the acids, as well as other products, and applied it to the reactions of ozone with propene, trans-2-butene, 2,3-dimethyl-2-butene, and isoprene. Acetic acid yields from propene and trans-2-butene ozonolysis in the presence of an OH scavenger were 2.7 +/- 0.6 and 2.9 +/- 0.6%, respectively, under dry conditions and 1.8 +/- 0.4 and 2.3 +/- 0.5% at 65% RH. Isoprene ozonolysis produced methacrylic and propenoic acids with yields of 5.5 +/- 1 and 3.0 +/- 1%, under dry conditions and 4.1 +/- 1 and 1.5 +/- 0.3% under wet conditions, respectively. That water inhibits acid formation indicates that the water reaction with stabilized Criegee intermediates is at most a minor source of acids. Acids that may form as coproducts of the OH radical elimination pathway, acetic acid from 2,3-dimethylbutene and isoprene, and propenoic acid from isoprene were also observed with significant yields (up to 10%), although the production of acetic acid was not a linear function of the alkene reacted. Carbonyl products are also reported.