•Correlations between the personal sensors and more expensive research instruments were higher than with the government monitors.•The sensors were able to detect high and low air pollution levels in agreement with expectations (e.g., high levels on or near busy roadways and lower levels in background residential areas and parks).•Our finding suggests that the low cost, personal sensors have potential to reduce exposure measurement error in epidemiological studies and provide valid data for citizen science studies.Low cost, personal air pollution sensors may reduce exposure measurement errors in epidemiological investigations and contribute to citizen science initiatives. Here we assess the validity of a low cost personal air pollution sensor. Study participants were drawn from two ongoing epidemiological projects in Barcelona, Spain. Participants repeatedly wore the pollution sensor − which measured carbon monoxide (CO), nitric oxide (NO), and nitrogen dioxide (NO2). We also compared personal sensor measurements to those from more expensive instruments. Our personal sensors had moderate to high correlations with government monitors with averaging times of 1-h and 30-min epochs (r ~ 0.38–0.8) for NO and CO, but had low to moderate correlations with NO2 (~0.04–0.67). Correlations between the personal sensors and more expensive research instruments were higher than with the government monitors. The sensors were able to detect high and low air pollution levels in agreement with expectations (e.g., high levels on or near busy roadways and lower levels in background residential areas and parks). Our findings suggest that the low cost, personal sensors have potential to reduce exposure measurement error in epidemiological studies and provide valid data for citizen science studies.

NOx (NOx ≡ NO + NO2) regulates O3 and HOx (HOx ≡ OH + HO2) concentrations in the upper troposphere. In the laboratory, it is difficult to measure rates and branching ratios of the chemical reactions affecting NOx at the low temperatures and pressures characteristic of the upper troposphere, making direct measurements in the atmosphere especially useful. We report quasi-Lagrangian observations of the chemical evolution of an air parcel following a lightning event that results in high NOx concentrations. These quasi-Lagrangian measurements obtained during the Deep Convective Clouds and Chemistry experiment are used to characterize the daytime rates for conversion of NOx to different peroxy nitrates, the sum of alkyl and multifunctional nitrates, and HNO3. We infer the following production rate constants [in (cm3/molecule)/s] at 225 K and 230 hPa: 7.2(±5.7) × 10–12 (CH3O2NO2), 5.1(±3.1) × 10–13 (HO2NO2), 1.3(±0.8) × 10–11 (PAN), 7.3(±3.4) × 10–12 (PPN), and 6.2(±2.9) × 10–12 (HNO3). The HNO3 and HO2NO2 rates are ∼30–50% lower than currently recommended whereas the other rates are consistent with current recommendations to within ±30%. The analysis indicates that HNO3 production from the HO2 and NO reaction (if any) must be accompanied by a slower rate for the reaction of OH with NO2, keeping the total combined rate for the two processes at the rate reported for HNO3 production above.

Recent observations suggest a large and unknown daytime source of nitrous acid (HONO) to the atmosphere. Multiple mechanisms have been proposed, many of which involve chemistry that reduces nitrogen dioxide (NO2) on some time scale. To examine the NO2 dependence of the daytime HONO source, we compare weekday and weekend measurements of NO2 and HONO in two U.S. cities. We find that daytime HONO does not increase proportionally to increases in same-day NO2, i.e., the local NO2 concentration at that time and several hours earlier. We discuss various published HONO formation pathways in the context of this constraint.

Eight distinct hydroxy nitrates are stable products of the first step in the atmospheric oxidation of isoprene by OH. The subsequent chemical fate of these molecules affects global and regional production of ozone and aerosol as well as the location of nitrogen deposition. We synthesized and purified 3 of the 8 isoprene hydroxy nitrate isomers: (E/Z)-2-methyl-4-nitrooxybut-2-ene-1-ol and 3-methyl-2-nitrooxybut-3-ene-1-ol. Oxidation of these molecules by OH and ozone was studied using both chemical ionization mass spectrometry and thermo-dissociation laser induced fluorescence. The OH reaction rate constants at 300 K measured relative to propene at 745 Torr are (1.1 ± 0.2) × 10–10 cm3 molecule–1 s–1 for both the E and Z isomers and (4.2 ± 0.7) × 10–11 cm3 molecule–1 s–1 for the third isomer. The ozone reaction rate constants for (E/Z)-2-methyl-4-nitrooxybut-2-ene-1-ol are (2.7 ± 0.5) × 10–17 and (2.9 ± 0.5) × 10–17 cm3 molecule–1 s–1, respectively. 3-Methyl-2-nitrooxybut-3-ene-1-ol reacts with ozone very slowly, within the range of (2.5–5) × 10–19 cm3 molecule–1 s–1. Reaction pathways, product yields, and implications for atmospheric chemistry are discussed. A condensed mechanism suitable for use in atmospheric chemistry models is presented.

The reactive uptake coefficients γ, for nitrate radical, NO3, on ∼100 nm diameter squalane and squalene aerosol were measured (1 atm pressure of N2 and 293 K). For squalane, a branched alkane, γNO3 of 2.8 × 10−3 was estimated. For squalene which contains 6 double bonds, γNO3 was found to be a function of degree of oxidation with an initial value of 0.18 ± 0.03 on fresh particles increasing to 0.82 ± 0.11 on average of over 3 NO3 reactions per squalene molecule in the aerosol. Synchrotron VUV-ionization aerosol mass spectrometry was used to detect the particle phase oxidation products that include as many as 3 NO3 subunits added to the squalene backbone. The fraction of squalene remaining in the aerosol follows first order kinetics under oxidation, even at very high oxidation equivalents, which suggests that the matrix remains a liquid upon oxidation. Our calculation indicates a much shorter chemical lifetime for squalene-like particle with respect to NO3 than its atmospheric lifetime to deposition or wet removal.

The presence of organic surfactants in atmospheric aerosol may lead to a depression of cloud droplet growth and evaporation rates affecting the radiative properties and lifetime of clouds. Both the magnitude and mechanism of this effect, however, remain poorly constrained. We have used Raman thermometry measurements of freely evaporating micro-droplets to determine evaporation coefficients for several concentrations of acetic acid, which is ubiquitous in atmospheric aerosol and has been shown to adsorb strongly to the air–water interface. We find no suppression of the evaporation kinetics over the concentration range studied (1–5 M). The evaporation coefficient determined for 2 M acetic acid is 0.53 ± 0.12, indistinguishable from that of pure water (0.62 ± 0.09).

A novel instrument is described that quantifies total particle-phase organic nitrates in real time with a detection limit of 0.11 μg m−3 min−1, 45 ppt min−1 (−ONO2). Aerosol nitrates are separated from gas-phase nitrates with a short residence time activated carbon denuder. Detection of organic molecules containing −ONO2 subunits is accomplished using thermal dissociation coupled to laser induced fluorescence detection of NO2. This instrument is capable of high time resolution (seconds) measurements of particle-phase organic nitrates, without interference from inorganic nitrate. Here we use it to quantify organic nitrates in secondary organic aerosol generated from high-NOx photooxidation of limonene, α-pinene, Δ-3-carene, and tridecane. In these experiments the organic nitrate moiety is observed to be 6−15% of the total SOA mass.

We describe ground and space-based measurements of spatial and temporal variation of NO2 in four California metropolitan regions. The measurements of weekly cycles and trends over the years 2005−2008 observed both from the surface and from space are nearly identical to each other. Observed decreases in Los Angeles and the surrounding cities are 46% on weekends and 9%/year from 2005−2008. Similar decreases are observed in the San Francisco Bay area and in Sacramento. In the San Joaquin Valley cities of Fresno and Bakersfield weekend decreases are much smaller, only 27%, and the decreasing trend is only 4%/year. We describe evidence that the satellite observations provide a uniquely complete view of changes in spatial patterns over time. For example, we observe variations in the spatial pattern of weekday−weekend concentrations in the Los Angeles basin with much steeper weekend decreases at the eastern edge of the basin. We also observe that the spatial extent of high NO2 in the San Joaquin Valley has not receded as much as it has for other regions in the state. Analysis of these measurements is used to describe observational constraints on temporal trends in emission sources in the different regions.

Current understanding of the vapor−liquid exchange kinetics of liquid water is incomplete, leading to uncertainties in modeling the climatic effects of clouds and aerosol. Initial studies of atmospherically relevant solutes (ammonium sulfate, sodium chloride) indicate that their effect on the evaporation kinetics of water is minimal, but all those constituent ions are also expected to be depleted in concentration at the air−water interface. We present measurements of the evaporation kinetics of water from 4 M sodium perchlorate solution, which is expected to have an enhanced concentration of perchlorate in the surface layer, using Raman thermometry of liquid microdroplets in a free evaporation regime. We determine the evaporation coefficient γe to be 0.47 ± 0.02, ca. 25% smaller than our measured value for pure water (0.62 ± 0.09). This change, while small, indicates that direct interactions between perchlorate ions and evaporating water molecules are affecting the evaporation mechanism and kinetics and suggests that other solutes with high surface affinities may also produce a similar influence in the atmosphere and elsewhere.

Aqueous evaporation and condensation kinetics are poorly understood, and uncertainties in their rates affect predictions of cloud behavior and therefore climate. We measured the cooling rate of 3 M ammonium sulfate droplets undergoing free evaporation via Raman thermometry. Analysis of the measurements yields a value of 0.58 ± 0.05 for the evaporation coefficient, identical to that previously determined for pure water. These results imply that subsaturated aqueous ammonium sulfate, which is the most abundant inorganic component of atmospheric aerosol, does not affect the vapor–liquid exchange mechanism for cloud droplets, despite reducing the saturation vapor pressure of water significantly.

The reactive uptake coefficients γ, for nitrate radical, NO3, on ∼100 nm diameter squalane and squalene aerosol were measured (1 atm pressure of N2 and 293 K). For squalane, a branched alkane, γNO3 of 2.8 × 10−3 was estimated. For squalene which contains 6 double bonds, γNO3 was found to be a function of degree of oxidation with an initial value of 0.18 ± 0.03 on fresh particles increasing to 0.82 ± 0.11 on average of over 3 NO3 reactions per squalene molecule in the aerosol. Synchrotron VUV-ionization aerosol mass spectrometry was used to detect the particle phase oxidation products that include as many as 3 NO3 subunits added to the squalene backbone. The fraction of squalene remaining in the aerosol follows first order kinetics under oxidation, even at very high oxidation equivalents, which suggests that the matrix remains a liquid upon oxidation. Our calculation indicates a much shorter chemical lifetime for squalene-like particle with respect to NO3 than its atmospheric lifetime to deposition or wet removal.

The presence of organic surfactants in atmospheric aerosol may lead to a depression of cloud droplet growth and evaporation rates affecting the radiative properties and lifetime of clouds. Both the magnitude and mechanism of this effect, however, remain poorly constrained. We have used Raman thermometry measurements of freely evaporating micro-droplets to determine evaporation coefficients for several concentrations of acetic acid, which is ubiquitous in atmospheric aerosol and has been shown to adsorb strongly to the air–water interface. We find no suppression of the evaporation kinetics over the concentration range studied (1–5 M). The evaporation coefficient determined for 2 M acetic acid is 0.53 ± 0.12, indistinguishable from that of pure water (0.62 ± 0.09).