Impacts of transboundary air pollution and local emissions on PM_2.5 pollution in the Pearl River Delta region of China and the public health, and the policy implications

In general, primary PM emissions (local + regional) were the major contributor to the PM_2.5 concentration, accounting for roughly 40% of ambient PM_2.5 in the PRD region (figure 3). Primary PM emissions were mainly from local sources and demonstrated clear seasonal variation whereby the highest contribution occurred in January (∼7.0 μg m^−3 or 21%); contributions in April (∼4.3 μg m^−3 or 25%) and October (∼4.4 μg m^−3 or 17%) were similar; and the smallest contribution occurred in July (∼3.75 μg m^−3 or 43%). The regional primary PM contribution (up to ∼26.5%) was smaller than the local contribution for most cities in the PRD region but exhibited a comparable seasonal variation. The sensitivity of PM_2.5 to primary PM was linear (Koo et al 2009). Thus, we did not conduct a sensitivity analysis for primary PM.

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Moreover, because of the nonlinear relationship between secondary aerosols, which are formed through oxidation and neutralization of precursor gases (i.e. NO_x, SO₂, VOC, and NH₃), and their precursor gaseous emissions, the contribution of individual emission species to the PM_2.5 level may vary with emissions and background chemical composition. Thus, from an environment protection perspective, we analyzed the hourly seminormalized first-order and second-order sensitivity coefficients, which reflect the responses of pollutants' concentration to perturbations in parameters such as gaseous emissions (Cohan et al 2005) (figure 4). A positive sensitivity coefficient indicates that pollutant concentration demonstrates positive feedback to a change in emissions, whereas a negative sensitivity coefficient means that pollutant concentration increases or decreases when emissions decrease or increase, respectively.

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The contribution of gaseous emissions to the PM_2.5 level ranged from approximately −4% to 25% (figure 3). Among gaseous emissions, emissions of NH₃—the main base for neutralizing SO₂ and NO_x—contributed to the PM_2.5 concentration more (from ∼2% to ∼11%) than those of other gases (i.e. VOC, SO₂, and NO_x), despite low NH₃ emissions in the PRD region. The index of the gas ratio (GR, less than zero) and the degree of sulfate neutralization (DSN, less than 1.5) indicate that the PRD was a NH₃-poor region (figures S5 and S6 in SI for details).

The sensitivity coefficients of PM_2.5 and its components to NH₃ emissions were positive under various VOC and NO_x ratios (figure 4(a)). The highest sensitivity coefficients were calculated when the ratio of VOC to NO_x was approximately 6. Ammonium concentration reduced in correspondence with reduction in ammonia, as expected. Nitrate concentration was also significantly lower because nitrate formation was highly dependent on the NH₃ emissions in such a NH₃-poor region. Compared with nitrate, sulfate concentration was less sensitive (or exhibited smaller positive sensitivity coefficients) to NH₃ emissions. Ammonia preferred to react with sulfuric acid; thus, sulfate was maintained in its aerosol phase although ammonia was reduced. The effect of NH₃ emission on SOA level is minimal due to the sensitivity coefficient of SOA to NH₃ emission is close to zero. Overall, the level of PM_2.5, and in particular ammonium and nitrate, reduced in response to reduced NH₃ emissions, even though the response of sulfate level was marginal. The results indicate that ammonia should be one of the targeted species to effectively reduce PM_2.5 in the region.

The VOC contribution to PM_2.5 was relatively small (figure 3): 3% in spring. VOC contribution shows a clear spatial difference and was the highest in July, followed by April, on the west side of the Pearl River, but highest in April, followed by January and July, on the east side.

Figure 4(b) depicts that the sensitivity coefficients of nitrate, sulfate, and ammonium to VOC emissions switched from positive to negative at a VOC/NO_x ratio of roughly 6, which meant their concentrations responded positively to a change in VOC emissions but negatively with an increased VOC/NO_x ratio. Notably, previous studies in Europe and United States found that when the VOC/NO_x ratio is around 5.5:1, OH reacts with VOC and NO₂ at an equal rate (Seinfeld and Pandis 2006, Tsimpidi et al 2008, Megaritis et al 2013). This means that our finding is consistent with those reported by previous studies.

Low VOC/NO_x ratios (<6) typically occur in urban areas, which have substantial NO_x emissions from traffic and were more likely a VOC-limited region for photochemistry. At a low VOC/NO_x ratio (VOC-limited regions), the rate of O₃ formation decreased linearly in relation to a reduction in VOC emissions, resulting in a lower hydroxyl radical (OH) concentration. Consequently, the levels of sulfate and nitrate decreased because of a lack of oxidants, and the ammonium concentration was also reduced. However, at a high VOC/NO_x ratio (>6), which typically occurs in rural areas (NO_x-limited regions), a reduction of VOC emissions resulted in an increase of OH and O₃ concentrations (Tsimpidi et al 2008). Consequently, sulfate, nitrate, and ammonium concentrations increased through oxidation and neutralization when VOC emissions were reduced. Moreover, as an important precursor to SOA, it is expected that a reduction (increment) of VOC leads to decreases (increases) in SOA. As the VOC/NO_x ratio increased, the sensitivity coefficient of SOA to VOC went up to equilibrium. Overall, the sensitivity coefficients of PM_2.5 were the highest at a VOC/NO_x ratio of approximately 6, and at higher than that ratio, they were lower because of lower concentrations of nitrate, sulfate, and ammonium.

The contribution of NO_x emissions to the PM_2.5 concentration was negative in most of the PRD cities (figure 3), especially in January and April. A reduction in NO_x emissions might cause an increase in the level of PM_2.5 components (i.e. nitrate). Conversely, the PM_2.5 concentration attributable to NO_x emissions was positive in HZ (∼25%) and ZQ (∼7%). It may be because of their relatively low NO_x emissions. The sensitivity coefficient of PM_2.5 to NO_x emissions explains the different contributions.

As with sensitivity to VOC emissions, a VOC/NO_x ratio of roughly 6 was a turning point for NO_x emissions (figure 4(c)). At a low VOC/NO_x ratio (<6), the sensitivity coefficients of PM_2.5 and its components were negative, which meant their concentration responded inversely to a change in NO_x emissions. O₃ and OH levels increased as a result of reduced NO_x emissions in VOC-limited regions (typically urban areas). Increasing oxidant concentration may lead to increases in nitrate and sulfate levels by accelerating the formation of NO_x and SO₂ through reactions with oxidants (Dong et al 2014, Zhao et al 2015). Overall, the reduction in NO_x emissions caused an increase in the number of OH radicals that were supposed to react with NO_x to form nitric acid, and the excessive OH radicals reacted with SO₂ to form sulfuric acid and thus sulfate. The simultaneous increases in sulfate and nitrate also resulted in an increase in the ammonium level. Therefore, in VOC-limited regions (i.e. most parts of the PRD region), contribution of NO_x emissions to PM_2.5 to NO_x emissions was negative.

At a high VOC/NO_x ratio (>6), PM_2.5, nitrate, and ammonium levels show a positive response to a perturbation in NO_x emissions. Decreasing NO_x emissions resulted in a linear reduction in ambient oxidant levels in NO_x-limited regions (typically rural areas). The simultaneous decreases in both oxidants and NO_x consequently reduced the extent of nitrate formation. The effect of a perturbation in NO_x emissions on nitrate was thus decreased. Furthermore, in rural areas, sulfate level still negatively responded to changes in NO_x emissions at first but then switched to a positive response. When the VOC/NO_x ratio was higher than 6 but below 15, which occurred in transition zones, a reduction in NO_x emissions led to the existence of excessive OH radicals. Reactions between SO₂ and OH led to elevated levels of sulfuric acid and sulfate. However, with an increase in the VOC/NO_x ratio (especially >16), chiefly in NO_x-limited regions, a small NO_x reduction resulted in a reduction in the number of free OH radicals, thereby decreasing the overall oxidant level. The sulfate level consequently reduced along with a reduction in NO_x emissions.

Moreover, the sensitivity of SOA to NO_x emission was all negative which means a reduction in NO_x emissions could lead to an increase in SOA especially in urban areas (low VOC/NO_x ratio). It is because VOC competes with NO_x for an oxidant to form SOA.

In sum, a reduction in NO_x emissions in urban areas may lead to an increase in secondary inorganic aerosol species, resulting in an overall increase in the PM_2.5 concentration (Tsimpidi et al 2008, Zhao et al 2015). An emission control policy that focuses on NO_x emissions alone may thus be an ineffective means of reducing the PM_2.5 level in urban areas in the PRD region. Conversely, the PM_2.5 level decreased in relation to reduced NO_x emissions because of decreased levels of nitrate and ammonium and may thus offer a more promising reduction strategy.

The contribution of SO₂ emissions to the PM_2.5 concentration was marginal (figure 3): up to 3%. The effect was slightly stronger in summer (up to ∼3.3 μg m^−3) than in other seasons (up to ∼0.8 μg m^−3).

The sensitivity coefficients of PM_2.5 and its components to a perturbation in SO₂ emissions were positive except for nitrate (figure 4(d)), regardless of the ratio of VOC to NO_x. Additionally, the sensitivity of SOA to SO₂ emission is close to zero, indicating a negligible impact of SO₂ emissions on SOA formation. Positive sensitivity coefficients indicate that a reduction in SO₂ emissions lowered the sulfate, ammonium, and PM_2.5 levels. The sulfate concentration changed linearly according to a perturbation in SO₂. The response of nitrate to SO₂ emissions was negative, which might be because nitrate formed by combining with NH₃ and was freed up because of a reduction in SO₂ emissions. In such an NH₃-poor region, the formation of nitrate is highly dependent on the availability of ammonia (Huang et al 2014), which is acquired through competing with SO₂. Ammonia reacts preferentially with sulfuric acid, even though it also reacts with nitric acid to form nitrate aerosol when sufficient ammonia is available. Thus, a reduction in SO₂ emissions led to an increase in free ammonia level, and consequently, more nitric acid was transferred to the particulate phase. Overall, when SO₂ emissions were reduced, PM_2.5 increased because of the relatively larger quantities of ammonium and sulfate, even though this was partly offset by a decreased nitrate level. This result also indicates that reducing the PM_2.5 level by controlling SO₂ emissions alone is ineffective because SO₂ control results in an increase in nitrate concentrations.

At low VOC/NO_x ratios (i.e. <15), the cross-sensitivity coefficients of PM_2.5, sulfate, and ammonium were negative (figure 4(e)), indicating that a reduction in the levels of NO_x (or SO₂) species led to an increase in sensitivity of PM_2.5, sulfate, and ammonium to SO₂ (or NO_x). Thus, simultaneously controlling SO₂ and NO_x could maximize the reduction in PM_2.5 concentration. In urban areas, the most effective measure for controlling PM_2.5 is to reduce the amount of SO₂ and NO_x simultaneously. Conversely, at high VOC/NO_x ratios (i.e. >15), the cross-sensitivity coefficient of PM_2.5 was positive, which indicates that a reduction in the levels of NO_x (or SO₂) species could lead to a reduction in the sensitivity of PM_2.5 to SO₂ (or NO_x), causing PM_2.5 level to increase because of a reduction in SO₂ (or NO_x). This result implies that reductions in SO₂ and NO_x in areas with high VOC/NO_x ratios—that is, rural areas—would be unable to control PM_2.5 effectively.