Long-term Trends in PM_2.5 Chemical Composition and Its Impact on Aerosol Properties: Field Observations from 2007 to 2020 in Pearl River Delta, South China

<p>Long-term data on PM<span class="inline-formula">_2.5</span> chemical composition provide essential information for evaluating the effectiveness of air pollution control measures and understanding the evolving mechanisms of secondary species formation in the real atmosphere. This study presented field measurements of PM<span class="inline-formula">_2.5</span> and its chemical composition at a regional background site in the Pearl River Delta (PRD) from 2007 to 2020. PM<span class="inline-formula">_2.5</span> concentration declined significantly from 87.1 <span class="inline-formula">±</span> 15.5 to 34.0 <span class="inline-formula">±</span> 11.3 <span class="inline-formula">µ</span>g m<span class="inline-formula">⁻³</span> (<span class="inline-formula">−</span>4.0 <span class="inline-formula">µ</span>g m<span class="inline-formula">⁻³</span> yr<span class="inline-formula">⁻¹</span>). The proportion of secondary species increased from 57 % to 73 % with the improvement in air quality. Among these species, sulfate (SO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="13pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="9e5c3b810d685753e2e31321de9aeed4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-13729-2025-ie00001.svg" width="13pt" height="17pt" src="acp-25-13729-2025-ie00001.png"/></svg:svg></span></span>) showed a sharp decline, while nitrate (NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="9f81e901bf06635e082f559a787da68a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-13729-2025-ie00002.svg" width="9pt" height="16pt" src="acp-25-13729-2025-ie00002.png"/></svg:svg></span></span>) exhibited a moderate decrease. Consequently, the proportion of NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="06954914259a113e7faaa0d01a8ee756"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-13729-2025-ie00003.svg" width="9pt" height="16pt" src="acp-25-13729-2025-ie00003.png"/></svg:svg></span></span> in 2020 doubled relative to 2007. In addition, we further found that SO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="13pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="31655fb078684da776f2b5262b6b028b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-13729-2025-ie00004.svg" width="13pt" height="17pt" src="acp-25-13729-2025-ie00004.png"/></svg:svg></span></span> reduction (<span class="inline-formula">−</span>10 % yr<span class="inline-formula">⁻¹</span>) lagged behind SO<span class="inline-formula">₂</span> reduction (<span class="inline-formula">−</span>13 % yr<span class="inline-formula">⁻¹</span>), while NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="d4917cb251612ae03efebb0a66479930"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-13729-2025-ie00005.svg" width="9pt" height="16pt" src="acp-25-13729-2025-ie00005.png"/></svg:svg></span></span> reduction (<span class="inline-formula">−</span>6 % yr<span class="inline-formula">⁻¹</span>) outpaced that of NO<span class="inline-formula">₂</span> (<span class="inline-formula">−</span>3 % yr<span class="inline-formula">⁻¹</span>). These contrasting trends were associated with an increase in sulfur oxidation rate (SOR) and a decrease in nitrogen oxidation rate (NOR). Changes in PM<span class="inline-formula">_2.5</span> chemical composition also influenced aerosol physicochemical properties, such as aerosol pH (0.04 yr<span class="inline-formula">⁻¹</span>), aerosol liquid water content (ALWC, <span class="inline-formula">−</span>1.1 <span class="inline-formula">µ</span>g m<span class="inline-formula">⁻³</span> yr<span class="inline-formula">⁻¹</span>), and the light extinction coefficient (<span class="inline-formula">−</span>21.44 Mm<span class="inline-formula">⁻¹</span> yr<span class="inline-formula">⁻¹</span>). Given important roles of aerosol acidity and ALWC in the heterogeneous reactions, these changes may further inhibit the formation of secondary species in the atmosphere, particularly secondary organic aerosols.</p>